Ground Fault Protection Methods of a Generator Stator

Published by Ahmad Rizal SULTAN, Mohd Wazir bin MUSTAFA, Technology University of Malaysia, Faculty of Electrical Engineering

Abstract. The type of generator grounding method and the system configuration determine the choice of ground fault protection. Ground fault generator stator can cause serious damage to the generator. Therefore, the total area of the generator stator windings must be protected against hazardous condition. Because the conventional methods are unable to separate ground faults close to the neutral point of view, there should be a method to protect entire stator winding. The paper discusses ways to discern ground faults for the generator stator. Some suggestions are proposed that can help in ensuring the generator stator ground fault more accurate.

Streszczenie. Stojan wirnika generatora powinien być szczególnie chroniony przed zagrożeniami. Konwencjonalne metody nie są w stanie oddzielić błąd uziemienia w pobliżu punktu neutralnego. W artykule opisano metody zabezpieczeń stojana generatora przed błędami uziemienia (Zabezpieczenia stojana generatora przed błędami uziemienia)

Keywords: Generator Stator, Ground Fault Protection
Słowa kluczowe: stojan generator, uziemienie, zabezpieczenia

Introduction

Synchronous generators are essential part of the power system. Disruption of the generator stator windings, especially in operation, can stop the generator. The process must be greatly detrimental to the generator companies, as this could result to the termination of plant operations, which means less revenue, as well as very high cost of repairing the damaged generator. Due to the importance of the generator stator winding, a protection system that protects the stator from the ground fault (GF) is a necessity.

The general kinds of the generator fault are the GF [1]. For a single line to ground fault near the neutral, the generated voltage available to drive current to the fault is small. The result is a fault with a low current and also a low neutral voltage displacement. At the most extreme, if the GF happens at a neutral point of its own, where no fault current or voltage displacement.

The importance of detecting GF close to the neutral point of the generator is not dependent on the need to trip because of the fault current magnitude, since it may be negligible and will not, in general, cause immediate damage. If a second ground fault occurs, severe damage may be sustained by the machine because this may result in a short – circuit current not limited by the grounding impedance. This condition may be aggravated if the first GF happens close to, or at the neutral terminal of the generator, because all ground relays operating from the neutral point voltage or current will become inoperative. Furthermore, if the second GF occurs in the same winding, the generator differential relay may also become inoperable since this situation can be categorized as an internal winding fault [2].

This paper reviews the various methods used to discern the GF on the generator stator. The most commonly used protections to discern the occurrences of GF on the generator are overcurrent, overvoltage and undervoltage relay. However, due to the development of protection systems, the protection scheme that utilizes disturbance in the ground, especially for a generator today can be categorized into four main sections, namely the conventional method of stator GF protection, third harmonic protection method, injection protection method and numerical protection method.

Ground Fault Protection of a Generator Stator

An overcurrent / overvoltage generator GF protection should be normally straight forward, safe and reliable. However, it has two insufficiencies. First, it will not detect ground faults close to the generator neutral, and second; it is not self-monitoring. This depends on the open circuit where the platform relay, primary or secondary winding of a current transformer or high resistance cannot be detected before a fault occurs. While grounding faults occur in the systems, every parallel generator has the same voltage, and traditional stator winding grounding fault protections with zero sequence voltage cannot detect which generator was faulted [3, 4].

a. Conventional Method of Stator GF Protection

The overvoltage relay (59G) can detect faults approximately 90 – 95% part of stator winding. These protections are illustrated in Figure 1.

Fig.1. Conventional Stator GF Protection [5]

There is a linear correlation between the voltage identify by the 59G relay and the GF position in the stator winding. For GFs close the neutral (N), the voltage will be detected by the 59G relay. The maximal voltage happens in a GF at the generator terminals (T), where maximum line-to-neutral voltage happens across the neutral grounding transformer. Commonly, the last 5-10% of the winding is not protected by the 59G. GF at the bottom 10% of the stator winding of the generator may not be sensed by the conventional method of stator GF protection (overvoltage or overcurrent relays).

b. Third Harmonic Protection Method

Generally, the output voltage of the generator is not a pure sinusoidal, but distorted by harmonic components. The whole harmonic components generated can be found in triple harmonics as the third harmonic, 9th, 15th and so on. The triple components can be found in all phases and have a large and the same phase angle, which may cause the common point during this phase triple components not to add up to zero for each phase. Therefore, the components appear as triple amount of zero sequences. The third harmonic voltage (THV) is commonly greater than most others triples. Under normal conditions, the characteristics of THV in the stator windings are shown in Figure 2.

Fig.2. Third harmonic voltage at normal condition [6]

When the GF occurs close to neutral at the generator, the THV at the terminal point will be equivalent to the total third harmonic generator yield. While the voltage of harmonics is in the neutral point, the sum reaches down to zero. The model of THV during GF at a neutral point is shown in Figure 3.

Fig.3. Third harmonic voltage during ground fault at neutral [6]

The opposite occurs when a GF develops in terminal point on the generator. The THV in the terminals will drop to zero, while the THV in neutral point increases to a total of all the THV of the generator produced. These characteristics are shown in Figure 4.

Fig.4. Third harmonic voltage during ground fault at a terminal [6]

Based on the above characteristics, when a GF occurs close to the terminal point, the THV at the neutral will decrease, while the terminal will be enlarged. Similarly, the opposite occurs when it crashes near the terminal point.

The following sections describe the five stators of GF protection using the principle of THV method.

b.1 Third-Harmonic Neutral Undervoltage Relay [4]

This protection method is the combination of the conventional (59G & 27TH) and third harmonic neutral voltage (see Figure 5). The third harmonic was measured across the generator neutral grounding resistor. The basic concept of the scheme is that, when the generator stator GF happens close to the generator neutral, the THV reaches to zero. If the third harmonic generator is a sufficiently neutral voltage as long as normal condition, to avert false operation of the relay from energizing, then such generators are candidates for 100% schemes using third-harmonic neutral detection.

The third harmonic undervoltage relay can detect an absence of THV at the generator neutral resulting from a GF close to the neutral. The 27TH and 59G relays must be filtered to prevent fundamental or third harmonic voltages respectively from affecting the operation. The 27TH relay should, if not self-protected, include circuitry to protect its coil from sustained overvoltage. This scheme offers the advantage of not requiring any additional high-voltage equipment, other than those needed for conventional ground-fault detection schemes for single stator generators.

The scheme can also be used for cross-compound and split-winding machines by adding a second VT and third harmonic relay to monitor the voltage at the neutral of the ungrounded stator winding. The scheme provides protection when the main breaker is open, provided that the terminal voltage is above the pickup of the supervisory relay 59.

Supervision is required during the start-up and shutdown either by using a breaker contact or an undervoltage relay so that the relay is disabled when the generator is off-line. Some generators provide very low levels of THV when the generators are lightly loaded. In order to improve the security of this scheme, an underpower relay (device 32) can be used to control the undervoltage relay of third harmonic neutral. The disadvantage with this scheme is the absence of 100% coverage until a relay 59 picks up.

Fig.5. Undervoltage relay scheme of third-harmonic neutral [7]

b.2 Voltage Relay of Third Harmonic at The Generator Terminal [2]

This protection method is supplied by a wye-grounded broken-delta transformer, which can be wye-wye for digital relay. This scheme is shown in Figure 6. Upon the occurrence of a generator neutral ground, the THV available at the line generator terminals becomes elevated. The accompanying overvoltage is used to operate a relay used for this application and must be set so that it will be unresponsive to the maximum THV appearing at this point during normal system operation.

An advantage in this scheme is that it will also detect GF in the bus or in the delta winding when the generator de-connector is open. However, it also has shortcomings due to the need for a three-phase VT on the machine terminals.

b.3 Third Harmonic Voltage Comparator Relay

This method distinguishes the third harmonic in the terminal and a neutral at the generator. This scheme, shown in Figure 7, utilizes the fact that the third harmonic residual voltage in terminals of a machine increase, while the THV at the neutral decreases, for a fault nearby the neutral. The comparison of the third harmonic residual voltage to the neutral third harmonic content may be nearly constant under all load conditions in many un-faulted machines.

Fig. 6. Third harmonic voltage Relay at a generator terminal [2]

Small changes in this ratio may require the reduced sensitivity parameters. The coincidence between the functions of the equipment 59GN and 59D may exist. The settings for both relay should be determined during field testing in conjunction with commissioning. The third harmonic differentials relay 59D detects GFs close to the neutral as well as at the terminal. Relay 59GN, which is used to measure the fundamental frequency neutral voltage, can detect a fault in the upper section of the winding as well as overlapping much of the winding covered by 59D. The (comparator) relay sensitivity is least for a fault in the middle of the windings. At some point in the winding, the difference between the neutral and terminal THVs is equal to the relay setting. Double GFs tend to reduce the sensitivity for the differential relay, and multi-winding machines to offer application difficulties that require careful consideration.

Fig. 7. Third harmonic ratio comparator [2]

b.4 Adaptive third harmonic level detector

The two voltages are applied to derive the third harmonic source voltage at the generator by using the vector combination of the signals (see Figure 8). The THV in the neutral and that of the residual voltage in terminal are continuously compared with the derived source voltage to detect a grounding the first 15% of the windings close to the neutral. The detection scheme then indicates a fault in its zone of coverage if the THV at the neutral is less than 15% of the source voltage and if one-third of the residual third harmonic at the terminals exceed 85%. Ideally, in comparison, the two are equivalent, but in practice, it has been verified that the two are indeed different in a number of ways according to the MW and MVAR typical generator installations.

This approach, in effect, utilizes adaptive undervoltage and overvoltage level detectors, where the setting level adapts to the level available relaying a signal, i.e., the magnitude of source voltage. It is imperative that the detectors will be blocked when the third harmonic source voltage is less than some minimum values, below which the voltage signals are considered unreliable for relaying. A level of 0.75% of nominal phase to neutral voltage on the generator is considered safe.

Fig.8. Adaptive third harmonic level detector [2]

b.5 A Recent Third Harmonic Protection Method

In the protection method, the scheme of THV phasor at the terminals or in the generator neutral are periodically evaluated and stored. Every currently estimated third harmonic magnitude is identified with the saved magnitude before the time of approximately one second. If the difference between magnitudes being compared is greater than the fixed, the verge magnitude as the trigger signal is produced. To date, the application of an existent algorithm using the microcontroller in the system, and preparatory tests using synthesized by voltage signals of GF have been confirmed effective.

Other new methods of fault protection in assuming the differential phase angle of delta THV [8] does not contribute to the THV neutral of the generator, and the generator GF of a stator is equivalent to the terminal, and can be used to discern the GF of the stator of generators.

c. Sub-harmonic Injection Protection Method

The sub-harmonic injection scheme has two main methods, 12.5 Hz and 20 Hz. The signal source of 12.5 Hz is in series with neutral grounding resistance, and the 20 Hz signal source is parallel with the neutral grounding resistance [8]. While a GF happens, the current increases due to the less resistant faulty path, and thus will be detected. The main advantage of the sub-harmonic 64- 100% relays is that, they are quite sensible, regardless of the machine reactive loading.

The scheme of sub-harmonic injection is another main method for unit-connected generator using third-harmonic. The following will explain the principle of stator GF protection by using subharmonic injection method.

c.1 Neutral or Residual Sub-harmonic Voltage Injection (comparator injection and measurement voltages)

This scheme, using a voltage injection at the neutral or residually in the broken-delta VT secondary, can detect GFs anywhere in the part of stator winding, including the neutral point. Full GF protection is available, even when the generator is starting up and during turning gear if the injected voltage source does not originate from the generator. Certain schemes inject a coded signal into a sub-harmonic frequency that can be synchronized to the frequency of a system. When compared with other injection schemes, this coding improves the security within the relay system without sacrificing dependability. For proper relay performance, the scheme is dependent on a reliable subharmonic source.

The use of sub-harmonic frequencies may offer improved sensitivities in relation to the higher-level impedance path of the generator capacitances at these frequencies. Such frequencies are not normally present at the generator’s neutral. This comes with a disadvantage of the economic penalty associated with providing and maintaining a reliable sub-harmonic source and injection equipment.

The major advantage of neutral injection schemes is that they provide 100 GF protections independent of the 95% GF protection schemes [2].

c.2 Neutral sub-harmonic voltage injection (measurement of voltage and current)

This scheme is shown in Figure 9. This scheme uses the sub-harmonic current injection at the generator neutral that can detect the GF’s entire generator stator winding, including the delta windings in a generator step-up transformer (GSU). Full GF protection can be provided without the field being energized, such as during the initial start-up of the generator and turning gear with the independent sub-harmonic voltage supply. Certain schemes inject a coded signal into a sub-harmonic frequency that can be synchronized to the frequency on the system such as for a 50 Hz system used a 12.5 Hz. This coding improves the security within the relay system without sacrificing dependability.

The scheme uses voltage and current measurements as the secondary circuit of grounding transformer of the generator. The voltage and current measurements are derived from the injected signal as they are placed across the generator grounding transformer secondary. In this manner, the reflected impedance of delta winding of the GSU and the generator are measured. If a GF is not present anywhere in the generator zone, the impedance measured is the natural capacitive coupling to be ground of the entire generator zone. If a GF develops, the impedance becomes less than natural capacitive coupling values, and alarm and/or trip set points will be applied.

Fig.9. Subharmonic Voltage Injection Scheme [5]

The major advantage of neutral sub-harmonic injection schemes is that they provide 100 % GF protection even when the generator is not in service and during start-up prior to application within the field [2].

c.3 A Recent Compensated Injection Scheme

In the protection scheme, an additional reactor that connects the compound with the resistance load in the distribution of transformers secondary side is applied to compensate the leakage capacitance from the stator windings of the generator to be grounded. The injection signal frequency is tuned to adapt to the variation of the capacitance caused by temperature varying, insulation aging, etc. Thus, the influence of the capacitance is cleared and high-impedance of GF protection for the entire generator stator winding is taken directly as adjustment measurement the grounding fault resistance and comparison of the session threshold [7].

d. Numerical Protection Method
d.1 Using wavelet transforms.

The THV at a generator neutral and terminals will vary simultaneously when a ground fault occurring in the generator stator, even if it is grounded with a high resistance. Therefore, the signals which be measured by the protective device contain significant transient components. They have much more fault information than the steady component. As wavelets are well suited for the analysis of the non-stationary signals, one will have the ability to extract important information from the noised signals. This information can be used to discern the GF [9].

The scheme (Figure 10) considers the overall produced voltage in the machine, and the use of Wavelet Multi Resolution Analysis (MRA). MRA is an ideal method to the analysis of the transients of the power system [10] and the protection of generators [11]. THV terminal and neutral signals are analyzed using of Wavelet MRA to discern GFs. In this scheme, maximum polarities coefficients are compared to other discrimination grounds transient faults. In this method, the maximum coefficient decreases for the increase in fault impedance. The scheme offers inherent discrimination of sudden load imbalance [12].

Fig.10. Wavelet based Multi Resolution Analysis [12]
Problem of Stator Ground Fault Protection

The most important thing to consider in the use of various stator GF protections is the type of the stator winding of generators, which has an expressive effect upon the efficiency of the GF protection and units of supply, thus should be considered in evaluating the performance of the protection system and analysis for optimization [13]. The generator active load affects the extent of the supply voltage protection element, and is satisfied with the THV [14].

Conclusion

Implementation of GF protection, especially for the 100% coverage scheme, depends on the generator’s neutral, based on the type of stator winding and generator connection. To get the best performance, the characteristics of stator GF protection must be evaluated with pre-fault conditions, fault conditions and various load condition.

REFERENCES

[1] Gilany, M, Malik OP, Generator Stator Winding Protection with 100% Enhanced Sensitivity, Electrical Power and Energy Systems, 2002
[2] IEEE Std C37.101™-2006, IEEE Guide for Generator Ground Protection.
[3] Wang Yuanyuan, A Novel Protection for Stator Winding Grounding Faults in Multi-Generator-Systems, Tencon-IEEE Region 10 Conferences, 2006
[4] C. H. Griffin, Generator Ground Fault Protection using Overcurrent, Overvoltage and Undervoltage Relay, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-101, 1982
[5] Charles J. Mozina, 15 Years of Experience with 100% Generator Stator Ground Fault Protection (What Works, What Doesn’t and Why), 62nd Annual Conference for Protective Relay Engineer, 2009
[6] Blackburn J.Lewis, Protective Relaying: Principles and Application, Marcel Dekker, New York, 1998
[7] IEEE Std C37.101™-1993, IEEE Guide for Generator Ground Protection
[8] NengLing T, Yan D, Stator Ground Fault Protection Based on Phase Angle Differential of Delta Third Harmonic Voltages, Electric Power System Research, 2005
[9] Nengling T, et al, Wavelet-based Ground Fault Protection Scheme for Generator Stator Winding, Electric Power Systems Research, 2002.
[10] Xiangjun Z, et al, Improvement of Subharmonic Injection Schemes for Huge Hydro-generator Stator Ground Protection, International Conference on Power System Technology PowerCon Volume 2, 2002
[11] A.W.Galli, G.T.Heydt, P.F.Ribeiro, Exploring the Power of Wavelet Analysis, IEEE Computer Applications in Power, Oct. 1996, pp. 37-41.
[12] S A Gafoor, P.V. Ramana R., Wavelet-ANN Based Ground Fault Protection Scheme for Turbo Generators, Electric Power Components and Systems, 35:5, 575-590, 2007
[13] M. Zielichowski, Third Harmonic Ground-Fault Protection System of Unit-Connected Generator with Two Parallel Branches Per Phase, Electric Power Systems Research, 78-2008
[14] Fulczyk M, Influence of Generator Load Conditions on Third- Harmonic Voltages in Generator Stator Winding, IEEE Transactions On Energy Conversion, Vol.20, 1, 2005

Authors: Ahmad Rizal Sultan, Faculty of Electrical Engineering, Universiti Teknologi Malaysia (UTM), Skudai, Malaysia 81300, E-mail: rizal.sultan@fkegraduate.utm.my
Mohd Wazir Mustafa, Faculty of Electrical Engineering, Universiti Teknologi Malaysia(UTM), Skudai, Malaysia 81300, E-mail: wazir@fke.utm.my

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 10/2013

Understanding Solar Photovoltaic (PV) Power Generation

Published by Alex Roderick, EE Power – Technical Articles: Understanding Solar Photovoltaic (PV) Power Generation, August 05, 2021.

Learn about grid-connected and off-grid PV system configurations and the basic components involved in each kind.

Solar photovoltaic (PV) power generation is the process of converting energy from the sun into electricity using solar panels. Solar panels, also called PV panels, are combined into arrays in a PV system. PV systems can also be installed in grid-connected or off-grid (stand-alone) configurations. The basic components of these two configurations of PV systems include solar panels, combiner boxes, inverters, optimizers, and disconnects. Grid-connected PV systems also may include meters, batteries, charge controllers, and battery disconnects. There are several advantages and disadvantages to solar PV power generation (see Table 1).

Table.1 Solar Photovoltaic (PV) Power Generation

AdvantagesDisadvantages
• Sunlight is free and readily available in many areas of the country.• PV systems have a high initial investment.
• PV systems do not produce toxic gas emissions, greenhouse gases, or noise.• PV systems require large surface areas for electricity generation.
• PV systems do not have moving parts.• The amount of sunlight can vary.
• PV systems reduce dependence on oil.• PV systems require excess storage of energy or access to other sources, like the utility grid, when systems cannot provide full capacity.
• PV systems have the ability to generate electricity in remote locations that are not linked to a grid.
• Grid-connected PV systems can reduce electric bills.
Table 1. There are advantages and disadvantages to solar PV power generation.
Grid-Connected PV Systems

PV systems are most commonly in the grid-connected configuration because it is easier to design and typically less expensive compared to off-grid PV systems, which rely on batteries. Grid-connected PV systems allow homeowners to consume less power from the grid and supply unused or excess power back to the utility grid (see Figure 2). The application of the system will determine the system configuration and size. For example, residential grid-connected PV systems are rated less than 20 kW, commercial systems are rated from 20 kW to 1MW, and utility energy-storage systems are rated at more than 1MW.

Figure 2. A common configuration for a PV system is a grid-connected PV system without battery backup.
Off-Grid (Stand-Alone) PV Systems

Off-grid (stand-alone) PV systems use arrays of solar panels to charge banks of rechargeable batteries during the day for use at night when energy from the sun is not available. The reasons for using an off-grid PV system include reduced energy costs and power outages, production of clean energy, and energy independence. Off-grid PV systems include battery banks, inverters, charge controllers, battery disconnects, and optional generators.

Solar Panels

Solar panels used in PV systems are assemblies of solar cells, typically composed of silicon and commonly mounted in a rigid flat frame. Solar panels are wired together in series to form strings, and strings of solar panels are wired in parallel to form arrays. Solar panels are rated by the amount of DC that they produce. Solar panels should be inspected periodically to remove dirt, debris, or snow, as well as to check electrical connections.

Since photovoltaics are adversely affected by shade, any shadow can significantly reduce the power output of a solar panel. The performance of a solar panel will vary, but in most cases, guaranteed power output life expectancy is between 10 years and 25 years. Solar panel power output is measured in watts. Power output ratings range from 200 W to 350 W under ideal sunlight and temperature conditions.

Solar Arrays Construction and Mounting

When solar arrays are installed on a property, they must be mounted at an angle to best receive sunlight. Typical solar array mounts include roof, freestanding, and directional tracking mounts (see Figure 4). Roof-mounted solar arrays can blend in with the architecture of a dwelling and will save yard space.

Figure 4. Typical solar array mounts include roof, freestanding, and directional tracking mounts on the roof or on the ground. Image courtesy of Greensarawak

Roof-mounted solar arrays attach to the roof rafters and are engineered to handle the same forces and climate conditions as the rooftop. Composition shingles are considered the easiest roofing on which to mount solar arrays, while slate and tile roofing materials are often considered the most difficult. The main drawback of roof-mounted solar arrays is that they require access for maintenance.

Freestanding solar arrays can be set at heights that allow convenient maintenance. However, freestanding solar arrays usually require a lot of space. Also, freestanding solar arrays should not be mounted on the ground in areas that receive a lot of snow.

Solar array mounts can also be either fixed or tracking. Fixed solar arrays, which are often roof-mounted or freestanding, are preset for height and angle and do not move with the sun. Directional tracking solar arrays move with the sun from east to west and adjust their angle to maintain the maximum exposure as the sun moves. Directional tracking solar arrays can increase the daily energy output of a PV system from 25% to 40%. However, despite the increased power output, directional tracking arrays may not justify the increased cost due to the complexity of the mounting system.

PV Combiner Boxes

A PV combiner box receives the output of several solar panel strings and consolidates this output into one main power feed that connects to an inverter. PV combiner boxes are normally installed close to solar panels and before inverters. PV combiner boxes can include overcurrent protection, surge protection, pre-wired fuse holders, and preconfigured connectors for ease of installation to the inverter. The use of pre-wired connectors saves running wires to the inverter. PV combiner boxes should be inspected periodically for leaks or loose connections. 

PV combiner boxes are not required for every PV system installation. For example, when there are only two or three strings of solar panels, a combiner box may not be required. In these cases, the strings of solar panels are connected directly to the inverter.

PV Inverters

An inverter is a device that receives DC power and converts it to AC power. PV inverters serve three basic functions: they convert DC power from the PV panels to AC power, they ensure that the AC frequency produced remains at 60 cycles per second, and they minimize voltage fluctuations. The most common PV inverters are micro-inverters, string inverters, and power optimizers (See Figure 5).

Figure 5. Microinverters are connected to each solar panel, which are connected in parallel, and convert DC directly to AC. String inverters are used with multiple solar panels connected in series. Power optimizers are installed on each solar panel, which are connected in parallel. Image courtesy of Letsgosolar

A microinverter is a device that converts DC power to AC power and is mounted directly to individual solar panels. Because the DC to AC conversion happens at each solar panel, the microinverters maximize the potential output of a system. For example, if one solar panel is shaded by a tree, it will not affect the output of any other solar panels. Microinverters also eliminate the need for potentially hazardous high-voltage DC wiring.

A string inverter is a device that converts DC power to AC power from several solar panels that are connected in series. However, in a series configuration, if one of the solar panels stops producing electricity, even due to temporary shading, it can decrease the performance of the whole system. String inverters are in the high-voltage range (600 V to 1000 V) and are used with large PV systems with no shading concerns. Usually, only one string inverter is needed for a residential application.

A power optimizer (maximizer) is a hybrid microinverter system that conditions the DC power before sending it to a centralized inverter instead of converting the DC power from the solar panels directly into AC power. Power optimizers, like microinverters, still perform well when one or more panels are shaded or when panels are installed facing different directions. Power optimizer systems tend to cost more than string inverter systems but less than microinverter systems.

PV Disconnects

Automatic and manual safety disconnects protect the wiring and components of PV systems from power surges and other equipment malfunctions. Disconnects ensure that the PV system can be safely shut down and system components can be removed for maintenance or repair. With grid-connected PV systems, safety disconnects ensure that the generating equipment is isolated from the grid for the safety of utility personnel. A disconnect is needed for each source of power or energy storage device in the PV system. An AC disconnect is typically installed inside the home before the main electrical panel. Utilities commonly require an exterior AC disconnect that is lockable and mounted next to the utility meter so that it is accessible to utility personnel.

Author: Alex earned a master’s degree in electrical engineering with major emphasis in Power Systems from California State University, Sacramento, USA, with distinction. He is a seasoned Power Systems expert specializing in system protection, wide-area monitoring, and system stability. Currently, he is working as a Senior Electrical Engineer at a leading power transmission company.

Source URL: https://eepower.com/technical-articles/understanding-solar-photovoltaic-pv-power-generation/

Usability Evaluation of PEM Fuel Cell and Supercapacitors Application in the Emergency Power Backup System

Published by Marek MALINOWSKI, Jacek CHMIELOWIEC, Grzegorz PAŚCIAK, Tymoteusz ŚWIEBODA, Electrotechnical Institute, Division of Electrotechnology and Materials Science

Abstract. Evaluation of FC/SC-based UPS has been done in terms of energy containers usability in the power emergency generation system. The Uninterruptible Power Supply delivers electric support for connected loads basing on hydrogen energy direct conversion into electricity with aid of low-temperature PEM fuel cell and additional energy carriers – supercapacitors. The paper contains description of design and rules of operation, and measurement results that have been determined under various conditions to examine usefulness of FC/SC implementation.

Streszczenie. Na podstawie badań skonstruowanego zasilacza awaryjnego typu UPS bazującego na ogniwie paliwowym typu PEM-FC oraz dodatkowych magazynach energii – superkondensatorach, dokonano oceny sensowności wykorzystania tych zasobników energii w systemie awaryjnej generacji energii elektrycznej. Praca zawiera opis projektu i zasady działania oraz wyniki pomiarowe, otrzymane w różnych warunkach działania zasilacza. (Ocena przydatności zastosowania ogniwa paliwowego typu PEM-FC oraz superkondensatorów w układzie zasilania awaryjnego).

Keywords: UPS, PEM fuel cell, supercapacitor, usability evaluation
Słowa kluczowe: UPS, ogniwa paliwowe PEM, superkondensatory, ocena przydatności

Introduction

Fossil fuels are the best resources intended for energy production discovered by the human race. The advantages such as independence of weather conditions, latitude or seasons of the year, high conversion efficiency and availability of effective technology explain why fossil fuels are still widely used around the world. The supremacy of carbon based fuel resources is due to physical, economic and even historical reasons. The chemical accumulated energy can be oxidize according to the current utilization. The production costs are still fully acceptable. Looking at the history and duration of past energy transitions – from wood to coal and from coal to gas and oil may be predicted that the next transition will be also long, expensive and doubtful process. Advantages of “old” recourses and long path of development the new ones show that it will be severe task to change human habits in terms of hydrocarbon combustion. However, there is no way out [1].

There are various alternatives to traditional carbon fuels. The huge power generation linked with fissions of radioactive atoms delivers CO2-free energy in hundreds of nuclear power plants. However, similarly to fossil fuels the resources will become depleted. The common word properly describing the other alternatives is renewable. The energy sources which are renewable seem to be the only way to keep and cover the growth of energetic utilization. Solar radiation, winds, biomass, tides and hydroelectricity are widespread utilized for energy generation. Unlike fossil fuels the renewables can have a sustainable yield. They are suited both to large scale and to remote or off-grid applications. Due to intermittent character, especially for solar and wind resources, the excess of obtained energy should to be stored in order to utilize in case of the temporary lack of renewables. One of the efficient way in terms of storage is production of the hydrogen by using various method such as electrolysis, photolysis, or thermochemical splitting. After these processes the energy can by restored at any time. For these purposes fuel cells have been designing – the devices that allow high-efficient direct conversion from chemical energy of source fuel to the electricity and even the heat. The water appears as a result of the conversion. Hence, the connection between the hydrogen storage and renewables utilization creates great opportunity to implement inexhaustible, ecologic energy generation processes regardless of scales, areas or applications.

Plenty of applications for fuel cells supplied by the hydrogen can be found [4÷8]. Proton exchange membranes fuel cells (PEMFC) due to their low operation temperature, high electrical efficiency and flexibility have potential to substitute sources of electric energy such as batteries or accumulators. Moreover they are non-polluting low mass and volume power generators which can be used in various application. Typical examples are small-scale power plants, portable devices and electric vehicles.

The other prospective applications are emergency backup systems such as Uninterruptible Power Supplies (UPS) which supply external loads in case of failure of the power network. According to how typical configuration of UPS looks like, above devices generate electricity basing on built-in batteries or various power engines such as internal combustion ones. This configuration involves various well-known disadvantages lowering thereby reliability and lifetime of UPS. The fuel cells (FC) as an excellent power sources can substitute popular sources of the electricity. The aim of this paper was technical description and usability evaluation of a new designed and constructed UPS intended for lab, domestic or office utilities. The FC/SC UPS has been designed as a hybrid device supplied by supercapacitors (SC) and PEM-FC stack with hydrogen as a fuel.

Overall system design

The design and construction of the FC/SC hybrid UPS were done in multistage process. The main tasks have been divided as follow: electrical part, control system, gaseous part. As far as fuel cell plays significant role in terms of electricity generation it’s crucial to show internal structure of the supply which is adjusted mainly due to the stack principle of operation. From this point of view usability can also be evaluated.

Electrical part

Electrical part (fig. 1.) contains double conversion system: AC/DC and DC/AC. An additional converter has been used because of fuel cell and supercapacitors presence. The converter is important for the stack which works under wide voltage range. FC/SC Supply was designed according to technical parameters of commercial 600 W fuel cell stack. This power can not be transfer in whole to the output of UPS due to its different electric equipment’s such as air compressor, electromagnetic valves, control systems or measurement instruments. They require the total power of app. 180 W. Taking into account the conversion efficiency of built-in converters, the rated power of the Supply drops to 400 W which is the first seen limitation of fuel cell application. This loss can be significantly scaled-down by implementation of air pressure vessel instead of electrical compressor that spends considerable amount of the energy or by implementation of the stack which has the blowers both for air compression and cooling.

Fig.1. Fuel cell and supercap – based UPS: schematic diagram of the electric part
Control system

Basically, the control system designed for UPS plays the most important role, so particular attention has been paid to develop that system. During development process three different drivers were tested to meet all requirements according do project assumptions. For the first time, ATmega8-based circuit was implemented [2] to control each particular component built-in the supply. That system, due to small number of peripherals and low frequency working with, had considerable limitations thus DSP-based driver was proposed and then constructed [3]. However, the problems being appeared, that resulted mainly from principle of DSP operation, excluded also this driver. Eventually, control system basing on ATmega128 was designed. The circuit was constructed according to the first control system driver project. It fulfills all of the assumptions mostly as a consequence of control algorithm adjustment. The system allows to work with 11.05911 MHz frequency as well as external quartz-crystal resonator can provide. Various measurements and control components were implemented, thus all-in 8 ADC channels, 20 I/O ports and two PWM channels of the microcontroller with additional communication interfaces are being used to deliver necessary functionality for the UPS. As plenty of parameters have to be captured during back-up system operation, three different noise canceller techniques have been implemented for embedded analog-to-digital controller.

This includes short analog signal path, LC network between analog and digital supplying and usage of ADC channels only as analog ones. By virtue of this the following conclusion can be found. Accurate measurement and management is not only important due to proper operation of the supply but it also plays significant role in terms of scientific investigation. In the other words, measurement of important parameters with high accuracy facilitates evaluation of internal energy sources such as fuel cell stack and supercapacitors in specific condition of UPS operation.

Gaseous part

Figure 2 presents the diagram of gaseous part. The 500 sl metal hydride tank supplies the fuel cell through the pressure regulator to obtain max. 1.5 bar on the anode side. There are electromagnetic valves (V1, V2), mass flow controller and pressure transducers (T1, T2) installed due to managing of hydrogen utilization.

As was proven during investigations the UPS basing on the fuel from the tank is able to supply connected load for 15 minutes. These investigations were performed at the conditions of open V2 valve (0.4 bar initial hydrogen pressure) so the fuel was only spent partially. In order to increase the efficiency of conversion, fuel cell’s dead-end capable system has been implemented. The solution provides great opportunity to spend as much fuel as possible.

Fig.2. Fuel cell – based UPS: gas subsystem connections
Fig.3. Comparison graphs between dead-end-capable configuration (upper) and open H2 outlet of built-in fuel cell (lower), performed at the same level of refilling for hydrogen storage canister

The difference between time of operation is shown in figure 3. At a constant power of 400 W and in case when the hydrogen outlet is open, UPS is able to work for about 15 minutes. During the research hydrogen pressure has been corrected twice to extend the time of operation, possibly in maximal-way. The upper graph shows automatic managing of hydrogen utilization. In this case control system closes V2 valve (dead-end-capability) to increase conversion efficiency thereby the time of working is doubled – approximately to 35 minutes. In order to use dead end capability, outlet valve is periodically being opened for a few seconds mainly due to water steam which mitigates the level of fuel conversion. This is indicated by temporary rise of fuel cell’s voltage resulting from hydrogen purification.

As a supplement of design description, figure 4 and 5 depict configuration of particular components in the UPS.

Fig.4 – Front side of the UPS:
1 – electrical part and: 2 – Converter AC/DC, 3 – Converter DC/DC, 4 – Supercapacitors (SC)
Fig.5 – Rear side of the UPS:
1 – fuel cell (FC), 2 – metal hydride tank, 3 – inverter, 4 – compressor, 5 – electronic μC – based driver
Usability evaluation

Constructions of various applications of fuel cells is not only due to better characteristics and parameters in comparison to the existing products – because it could be often controversial, but by reason of necessity for environmental-friendly technologies’ implementation. Such an opportunity give fuel cells, therefore different reports predict enormous growth in this market. Looking at the advantages of fuel cells two issues appear. The first is efficiency the stack can convert chemical energy with, which is as high as 40% for electricity generation in case of PEM fuel cells. The other is size of the energy that can be produced which is up to the amount of hydrogen being stored. That’s theory. However, some limitations of fuel cell application in the Supply during investigation are being seen. First of all is the principle of fuel cell operation which involved the UPS to be constructed in sophisticated way. So, apart from the fuel cell stack, various devices have to be engaged to conduct chemical conversion of the hydrogen into the electricity. This includes but is not limited to hydrogen containers, air compressors, blowers, electromagnetic valves, flow controller and temperature, pressure, and current sensors. During fuel cell operation the problem may occur with one of the above devices, leads to the failure of the entire system. That sounds particularly in comparison to the classic batteries’ principle of working.

The other issue is the amount of energy that has to be spent to keep the stack on. This results, as above, from implementation of particular elements which require electricity to work. For instance, the power spent for own purposes during UPS operation is as high as 25% of the stack rated power.

Looking at the structure of the typical fuel cell stack various materials can be found that are susceptible to ageing processes. For example typical membrane electrode assembly (MEA) of the fuel cell is made from Nafion-based electrolyte film with carbon/platinum electrodes on each side. It’s common situation the efficiency is partially decreased when fuel cell stack works or is stored in severe conditions which rely on low humidity, over-range temperature or presence of carbon oxide in the fuel. Such a phenomena has been observed since fuel cell stack purchase for the UPS. There are results of voltage-current measurements depicted in figure 6. At the beginning the stack worked as well as fuel cell’s manufacturer declaration. However, after four years we observed approximately 50% reduction of the rated power. The ageing process we predict is connected with inappropriate storage place such as low humidity what permanently scaled-down the ionic conductivity of membrane electrolytes.

Fig.6. The influence of ageing processes on fuel cell stack performance
Fig.7. 3 kF supercapacitors/Pb-acid battery comparison characterized by comparable volume

The second devices built in the UPS intended to store the energy are supercapacitors. Although state-of-the-art supercapacitors have enormous capacitance, typically they can not successfully substitute the batteries in terms of capability of energy storage. As is shown in figure 7 there are two curves made due to comparison of built-in supercapacitors and Pb-acid battery. In spite of comparable volume they have, the battery obtained about 28 Ah of electric charge transferred, while the supercapacitors with capacitance of 3 kF only 2.3 Ah. Thus, it seems that supercapacitors application in stationary systems may be senseless. However, advantages such as huge number of charging-discharging cycles, low internal resistance, long life time and wide range of operating temperature, convince that these energy carriers may find various applications, especially mobile and for transportation ones.

Conclusions

Various tests have been made to evaluate fuel cell and supercapacitors based UPS. The principles of the project excludes typical disadvantages of the classic UPS, such as limited time of operation or long period of charging. This is provided respectively with fuel cell implementation which generates as much energy as amount of hydrogen is stored and with supercapacitors that are characterized by low internal resistance which leads to short time of charging.

Metal hydride tank containing about 500 sl of hydrogen gives an opportunity to supply 400 W load for 15 minutes, however, this value was significantly increased up to 35 minutes by controlling of the fuel utilization using dead-end-capable system. By mounting of the high pressure tank which would have an ability to store more hydrogen or even by replacing of high energy consumption air compressor with a pressure oxygen vessel this value can by further elongated.

The other way is to substitute assembled fuel cell stack by so-called open-cathode stack which is adjusted to use the blowers both for cooling and oxidant delivering at the same time. That solution would eliminate air compressor, bringing additive power to external loads.

As fuel cell stack was used in this project the following issues appeared that have negative influence on usability evaluation: the quantity of the equipment which is required by the stack to operate and which in case of problem with one of them, may lead to the failure of the entire system; the ageing processes that lower the efficiency of fuel cells; capability of supercapacitors to store the energy comparing to the other sources such as Pb-acid battery. In spite of up-to-date disadvantages, the applications of fuel cells in solutions such as uninterruptible power supply will find confirmation due to current enormous development of these energy converters and general hydrogen technology.

Additional question is unavoidable transformation to the hydrogen economy which seems to be closer than we can think about.

The research was supported by Wroclaw Research Centre EIT+ under the project “The Application of Nanotechnology in Advanced Materials” – NanoMat (POIG.01.01.02-002/08) financed from the European Regional Development Fund (Operational Programme Innovative Economy, 1.1.2)

REFERENCES

[1] Kerr R. A.: Do We Have the Energy For the Next Transition?, Science, 329, (2010), 780-781.
[2] Malinowski M,. Paściak G., Bujło P., Chmielowiec J.: Uninterruptible power supply (UPS) as an example of the fuel cell and ultracapacitors application. Proceedings of the Electrotechnical Institute, Pl, 248, (2010).
[3] Malinowski M,. Pasciak G., Debowski L.: Uninterruptible Power Supply Unit with Fuel-Cell as a Backup Energy Source and DSP-based Control System. PCIM Europe 2011, 17-19 May 2011, Nuremberg, Germany, 1181 – 1186
[4] Hwang J. J., Wang D. Y., Shih N. C.: “Development of a Lightweight Fuel Cell Vehicle”, J. Power Sources, 141, (2005), 108-115
[5] Bujło P., Sikora A., Paściak G., Chmielowiec J.: Energy flow monitoring unit for Hy-IEL hybrid (PEM fuel cell-supercapacitor) electric scooter. Electrical Review, 86 (2010), No. 3, 271-273
[6] [2] Weigl J., Saidi H.: “Pios Hydrogen Fuel Cell Tricycle”, J. Hydrogen Energy, 30, (2005), 1035-1036
[7] Bujło P., Bieniecki S., Pasciak G., Chmielowiec J., Mazurek B., Perz J.: “Hybrid Fuel Cell Supercapacitor System for HY-IEL Electric Scooter Drive”, Proceedings of 17-th World Hydrogen Energy Conference, Brisbane, Australia 15 – 19 June 2008
[8] Varkaraki E., Lymberopoulos N., Zachariou A.: Hydrogen based emergency back-up system for telecommunication applications, J. Power Sources, 118, (2003), 14–22

Authors: mgr inż. Marek Malinowski, E-mail: m.mal@iel.wroc.pl, dr inż. Jacek Chmielowiec, E-mail: chmielna@iel.wroc.pl, dr inż. Grzegorz Paściak, E-mail: g.pasciak@iel.wroc.pl, mgr inż. Tymoteusz Świeboda, E-mail: t.swieboda@iel.wroc.pl, Electrotechnical Institute, Renewable Energy Sources Section, M. Sklodowskiej-Curie 55/61, 50-369 Wroclaw, Poland.

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 8/2013

Grounding for Noise Reduction in Electronic Systems

Published by Lorenzo Mari, Master of Science degree in Electric Power Engineering, Rensselaer Polytechnic Institute (RPI), EE Power – Technical Articles: Grounding for Noise Reduction in Electronic Systems, April 06, 2021.

Learn about everyday grounding systems to reduce common-mode noise.

Grounding is the primary method of reducing noise pickup. A good grounding and bonding design can solve a considerable percentage of noise problems.

Isolated Ground (IG) Systems

When disturbances like EMI, RFI, or electrical impulses caused by welders, variable speed drives, appliances, and others, are present in the grounding system, they create common-mode noise between neutral and ground that may affect electronic equipment.

The National Electrical Code (NEC) permits the installation of isolated ground receptacles (IG), i.e., no connection between the yoke and the grounding terminal. Yoke is the metal frame behind the receptacle that is used to fix the device to the outlet box. 

An IG system’s ideal objective is to provide low neutral-to-ground voltages in the electronic equipment’s AC input This prevents the noise from passing through the logic circuitry and corrupting the processed data. 

When properly installed, the neutral to the isolated ground voltage (noise) should be lower than from neutral to the metallic conduit.

But there is nothing isolated in this grounding arrangement. The isolated equipment grounding conductor (EGC) provides a low impedance path for the ground-fault current flow between the receptacle grounding terminal and the neutral in the service equipment or the secondary of a separately derived system (e.g., an isolation transformer). The isolated EGC, and not the conduit, is the safety ground for the electronic equipment – the only route for the fault current to return to the source.  Green insulation with a longitudinal yellow stripe identifies the isolated EGC.

The receptacle’s metal frame must be grounded.  The receptacle’s frame has separate ground connections bonded to the general ground system through the metallic conduit, an insulated – the green wire – or bare equipment grounding conductor run with the circuit conductors, or another wiring method that serves as an EGC.

The NEC allows the isolated EGC to pass through several subpanels without connection to the grounding bus. As a practical matter, the isolated EGC may terminate at the subpanel where the noise attains an acceptable level.

Figure 1 shows an isolated ground system supplying data processing (DP) electronic equipment.

Figure 1. IG system supplying DP equipment.

The metal raceway is grounded, employing a connection to the service equipment enclosure. The isolated EGC, connected to the service equipment’s neutral, passes through the downstream panelboard and terminates in the electronic equipment cabinet. The NEC permits the equipment cabinet’s isolation from the raceway containing the supply conductors, using nonmetallic raceway fittings.

When the electronic equipment manufacturer specifies an isolated ground for the equipment, they usually do not provide an isolated ground terminal for that purpose. There is only one ground terminal attached to the AC power supply, chassis, cabinet, and zero reference. In this case, the safety ground – green or bare wire – should not be connected to that ground terminal. The equipment cabinet should be isolated from the ground and other metals in contact with the safety ground to keep the isolated EGC as the only path for ground-fault current.

Power to electronic equipment should not share a common branch circuit supplying noise-generating devices. For example, feeding a copier – with motors and heaters – from the same circuit will inject high-frequency noise into the grounding system, disrupting the electronic equipment operation. If the copier requires an IG system because it has microprocessor-based circuitry, it should be connected to separate branches. Not to mention powering coffee makers, clocks, radios, vacuum cleaners, electric drills, and other noise-generating devices.

An important fact is that IGs do not always enhance the equipment’s performance.

Shielded Isolation Transformers

Sometimes there is a long separation between the main service equipment and the electronic equipment, resulting in a long EGC between the electronic equipment and the connection to the ground in the power source.  The long wire will have a relatively high impedance, reducing the fault current – needed to open circuit breakers and melt fuses – and increasing the time for fuses and circuit breakers to clear faults. Also, recalling that noise currents circulate through the EGC, the augmented impedance will develop larger noise voltages.

One way of shortening the EGC and reducing the above effects is by installing a shielded isolation transformer near the electronic equipment and its panelboard. The shielded isolation transformer has excellent insulation between its primary and secondary windings, taking the main service equipment grounded neutral out of the picture and restoring the ground at the secondary winding.

Bonding the EGC to the new, closer ground will make a better return path for fault currents and reduce common-mode noise.

Figure 2 shows a diagram of a shielded isolation transformer.

Figure 2. Shielded isolation transformer for electronic equipment.
A Practical Arrangement to Reduce Common-mode Noise

Figure 3 shows a practical arrangement to supply power to plug in electronic equipment, like a set of personal computers and peripherals, using the IG principle. This arrangement should provide an acceptable common-mode noise rejection level.

Figure 3. Grounding using an IG receptacle.

The power source is a shielded isolation transformer close to the panelboard reducing the wire length and noise. The transformer’s shield rejects a lot of high-frequency noise.

According to the NEC, the isolation transformer qualifies as a separately derived system – the neutral is not carried through from the input to the output – requiring a neutral-to-ground bond at the low voltage side. This bond provides a zero-volt reference to the electronic equipment.

The isolated EGC runs from the isolated-ground-type receptacles – powering the electronic equipment – directly to the isolation transformer’s neutral-to-ground bond through the conduit. The EGC may pass through subpanels without connection to the equipment metal frame grounding bus. Bond the metal conduit carrying the isolated EGC to the enclosures at both ends. The NEC requires all wires originating at the secondary winding – phase, neutral, isolated EGC, and green wire when used  – to run in the same conduit.

This arrangement reduces troubles when there is a connection between the peripheral and the computers’ grounding system through the data wire’s shield.

Figure 4. Typical SRG using the raised floor.

The room’s cabinets may be grounded to a single point, typically the grounding bar in the AC panel supplying power to the cabinets. The AC grounding bar is the single point ground for the power wiring and the cabinets.

An excellent option is placing the power source inside the room – e.g., an isolation transformer – grounded within or at the room’s periphery. Computer power centers are complete assemblies to supply branch circuits to data processing equipment, with control, monitoring, and alarm functions.

Short leads connect the isolation transformer’s secondary neutral, all signal grounds, and cabinets to the signal reference grid.

Figure 5 shows typical ground connections in a data processing room.

Figure 5. DP equipment connected to AC power source ground and SRG.
An Overview of Grounding for Noise Reduction in Electronic Systems

The NEC allows an isolated equipment grounding conductor to provide a noise-free, zero-volt reference in electronic systems having microprocessors.

The isolated EGC runs from the neutral/ground junction point (main bonding jumper) on the service equipment, or a separately derived source, to the electronic equipment or isolated receptacles.

The isolated EGC – green wire with yellow stripes – must be installed with the phase, neutral, and safety ground – green wire – conductors in the same conduit and can pass through subpanels without connection to the grounding bus. Any of the approved wiring methods must ground the IG receptacle’s metal enclosure.

Advantageously, an electronic equipment room receives power from a dedicated transformer, like an isolation transformer. The isolation transformer may be shielded and is useful for common-mode noise attenuation. Its primary function is to provide a separate energy source at the closest point to the electronic equipment and isolate it from other energy sources on the premises.

The transformer’s neutral-ground bond serves as a single point for all grounds in the electronic equipment room.

A signal reference grid supplies an equipotential plane for a broad frequency band, providing multiple paths between its parts. If one direction is high impedance due to resonance, other ways of different lengths provide a low impedance route with negligible potential differences between any two points on the grid. The result is a practical equipotential reference for signals from DC to the megahertz range.

About the Author: Lorenzo Mari holds a Master of Science degree in Electric Power Engineering from Rensselaer Polytechnic Institute (RPI). He has been a university professor since 1982, teaching topics as electric circuit analysis, electric machinery, power system analysis, and power system grounding. As such, he has written many articles to be used by students as learning tools. He also created five courses to be taught to electrical engineers in career development programs, i.e., Electrical Installations in Hazardous Locations, National Electrical Code, Electric Machinery, Power and Electronic Grounding Systems and Electric Power Substations Design. As a professional engineer, Mari has written dozens of technical specifications and other documents regarding electrical equipment and installations for major oil, gas and petrochemical capital projects. He has been EPCC Project Manager for some large oil, gas & petrochemical capital projects where he wrote many managerial documents commonly used in this kind of works.

Source URL: https://eepower.com/technical-articles/grounding-for-noise-reduction-in-electronic-systems/

Failure Duration in Distribution Networks

Published by Martin SLIVKA, Radomír GOŇO, Stanislav RUSEK, VŠB-Technical University of Ostrava

Abstract. The paper deals with statistical analysis of data on faults and failures in electrical power distribution. We used the statistical analysis of both MV overhead and cable lines, and electrical power stations of one distribution company. The data were collected from 2000 to 2009 with focus on the duration of failure with respect to different types of equipment. To compare and analyze the data, we used confidence intervals and also statistical distribution of data-sets.

Streszczenie. W artykule analizowano dane statystyczne błędów i defektów w przesyle energii elektrycznej. Wykorzystano dane linii średniego napięcia kablowych i napowietrznych. Dane były zebrane w latach 2000 do 2009. (Analiza okresu trwania uszkodzeń w sieciach przesyłu energii)

Keywords: Mean failure time, confidence interval, exploratory data, distribution analysis.
Słowa kluczowe: uszkodzenia, przesył energy elektrycznej, analiza statystyczna

Introduction

Monitoring the durations of failures is valuable because it gives information on how quickly a distribution company is able to repair a failure. These data can be used as a basis for the maintenance optimisation of distribution network equipment and for the breakdown service operation at distribution companies. Hereby stated data is anonymous and confidential and the selection criteria may not allow a wholly objective assessment. All input data are at the minimum of 3-minute duration [1]. The dataset comprises approximately 7200 values read from 1 January 2000 to 31 December 2009 [4]. The failure data comprises the date of event, its duration and the type of failed equipment. Mean failure time τ for individual months and years was calculated [3]:

.

where: N – number of failures of one equipment type, t – duration of failure (h).

Fig. 1 Box-and-whisker plot (whiskers representing duration in minutes)
Graphical Comparison of Data

The box-and-whisker plot represents the distribution of variables in different datasets. The minimum, the lower and upper quartiles, and the means are not too far from each other; however, the maximum is the furthest. Most failures have low duration as Fig. 1 illustrates.

The bar chart in Fig. 2 represents the trend in failure duration within 2000-2009. Concerning overhead lines, it shows considerable decrease in the failure duration, similarly to the electrical power stations.

Fig. 2 Comparison of different datasets – failure duration
Exploratory Data Analysis (EDA)

First, outliers were excluded by means of z-coordinate. In this case, outliers are those with absolute value of z-coordinate greater than 3. After eliminating outliers, exploratory data analysis was conducted [2].

.

Table 1 – Descriptive data statistics

.
Fig. 3 Histogram of failure duration – cables
Fig. 4 Histogram of failure duration – overhead lines
Fig. 5 Histogram of failure duration – electrical power stations

The EDA shows that all data have pointed distribution. Skewness reflects asymmetry in the distribution of values surrounding the mean – evidently the values above the mean prevail. The mode of electrical power stations is 71 minutes, while cables and overhead lines have 3 minutes. The median of cables is distinctively higher, probably due to the demanding character of repairs.

Frequency histogram

Histograms graphically represent the frequency of occurrence of assessed quantity, in our case failure duration with respect to selected classification. The number of classes is given by the Sturges’ rule. The bar chart depicts the rate in different classes. The chart shows that the failures with the shortest interval are proportionally prevalent. In longer duration the chart shows a steep drop in the rate. The equipment of electrical power stations shows the highest percentage with 92 % value of the first class. The line chart depicts proportional distribution of cumulative frequencies which correspond with proportional data distribution from the shortest failure duration up to a given class. These histograms were devised in MS Office 2010. Figures 3, 4, and 5 illustrate failure duration rate distribution for different datasets.

Distribution Analysis

Distribution analysis of several samples tests hypothesis (H0) which assumes the same original set of the basic probability distribution in comparison with alternative hypothesis (HA) which assumes inequality of mean values of samples (HA: does not hold H0). Distribution analysis can be conducted in the ANOVA table, or with Kruskal-Wallis one-way analysis of variance. The ANOVA table assumes normality of analyzed data. When this assumption is not supported, Kruskal-Wallis one-way analysis of variance can be used, however, at the cost of lower sensitivity compared to the ANOVA table.

Table 2 – Chi-Square a Kolmogorov-Smirnov test results

.
Normality Testing

Out of many methods there are to be used for testing normality, we used chi-square goodness-of-fit and Kolmogorov-Smirnov tests for reasons of good availability in STAGRAPHICS Plus 5.0.

Chi-square test tests the number of frequencies in selected classes from analyzed data. It compares them with the number of frequencies that would occur in case of selected distribution. Only the test results for cable are shown, as the results of the other datasets were similar – also not with the character of normal distribution.

To illustrate normal distribution, we used a Q-Q plot Fig. 6. The blue line in the plot shows congruity of the empirical and the theoretical distribution functions, the latter of which originates in the normal distribution. The empirical distribution function from the distribution we analyzed is represented by the points. The points are not aligned with the blue line, but they more or less deflect from it. The analyzed data do not have the distribution function of normal distribution, therefore they do not originate from it.

Fig. 6 Q-Q Graph

Table 3 – Kruskal-Wallis test results

.
Distribution Analysis

The abnormality of data does not allow distribution analysis by means of table ANOVA, therefore we used Kruskal-Wallis test. The test compares medians of individual samples and tests the null hypothesis that the medians are equal.

The Kruskal-Wallis test

The P-value in the test is zero and therefore there is 95% certainty that the datasets are statistically different. As every dataset is specific in its own way, intuitively we can assume the correctness of such conclusion.

To analyse in more detail differences in datasets we conducted the post – hoc analysis. As the data distribution is abnormal, the Tukey HSD test which is used to find means that are significantly different from each other was used. The test was conducted for 99% confidence interval of data reliability.

Fig. 7 Mean values and intervals of 99% reliability

It is evident that these three independent homogeneous groups have completely different character. The values from the Fig. 7 are in the Tab. 4.

Table 4 – Table of mean values form Tukey HSD test and confidence intervals with 99% reliability

.
Mean failure time

Mean failure time is a significant value that speaks for the condition of given equipment and the demands for its repair. It can be useful to know what the range of interval of mean failure time is.

Table 5 – Confidence intervals for selected reliability intervals

.
Conclusion

The paper deals with statistical representation of reliability data. Altogether, 7186 were analyzed in the monitored period of time. The variables have pointed and abnormal distribution with the prevalence of values above average. The analyzed variables are statistically different, which confirms the intuitive assumption. The mean failure times and reliability confidence intervals are to be found in Tab. 5. It is clear that the distribution of more accurate reliability intervals is closer to the mean value.

This work was supported by the Czech Science Foundation (No. 102/09/1842), by the Grant of SGS VŠB – Technical University of Ostrava (No. SP2013/137) and by the project ENET (No. CZ.1.05/2.1.00/03.0069).

REFERENCES

[1] Provozovatelé d i s t r i bučních soustav : PPDS, Příloha 2: Metodika určování nepřetržitosti distribuce elektřiny a spolehlivosti prvků distribučních sítí. (2011)
[2] Briš R., L i tschmannová M., Statistika I., Sylaby k předmětu VŠB-TU Ostrava (2004)
[3] Martínek, Z., Královacová, V., The Solution for Re-pairable Units. Proceedings of the 11th international Scientific Conference Electric Power Engineering 2010, University of Technology Brno (2010)
[4] Krejčí, P., Santarius, P., Hájovský, R., Velička, R., Čumpelík, R., PQ Monitoring in Selected Networks of Czech Republic. Przeglad Elektrotechniczny, vol. 88, nr. 7b/2012, 183-185

Authors: Ing. Martin Slivka, VŠB–TU Ostrava, Department of Electrical Power Engineering, 17. listopadu 15, Ostrava, martin.slivka.st1@vsb.cz; doc. Ing. Radomír Goňo, Ph.D., radomir.gono@vsb.cz; prof. Ing. Stanislav Rusek, CSc, stanislav.rusek @vsb.cz

Source & Publisher Item Identifier:  PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 11/2013

The Difference Between Class A & Class S

Published by GMC-Instruments

Power Quality: What Does Class A Mean?

Power quality monitoring doesn’t often become an issue until incidents such as system malfunctions, equipment failures, process interruptions, data loss, IT disruptions or even power failure have occurred. As a rule, incidents of this sort are very time-consuming because they cause can’t always pinpointed right away. Beyond this, failures are always associated with costs which could have been avoided.

Power quality is defined in EN 50160, which describes the characteristics of the voltage in electrical power supply networks. However, edition 3 of IEC 61000-4-30 specifies the degree of accuracy required for the measurement of the quality of electrical networks. The standard differentiates amongst different device classes. The measured values obtained from different devices manufactured by various suppliers are rendered comparable in the case of a class A (A = advanced), and class A devices are always used when accurate measurements are required. By means of this standard, reliable, reproducible and comparable results are obtained which can be used for billing purposes.

The following measurements are standardized by edition 3 of IEC 61000-4-30, and are mandatory for class A devices:

• Power frequency
• Magnitude of the supply voltage
• Voltage unbalance
• Voltage dips/swells/interruptions
• Rapid voltage change
• Harmonics/interharmonics, THD
• Flicker
• Mains signaling on the supply voltage

The Difference Between Class A and Class S

Whereas measuring accuracy is very high in the case of class A devices, measuring accuracy requirements specified for class S devices (S = survey) are much lower – data and events are only logged qualitatively and fewer demands are placed upon measuring accuracy. Furthermore, class S devices don’t have to measure as many quantities. The following measurements are mandatory:

• Power frequency
• Magnitude of the supply voltage
• Voltage unbalance
• Voltage dips/swells/interruptions
• Rapid voltage changes

Overview Table – Class A versus Class S

.

Normative Power Quality Monitoring: Voltage Events

Figure 1: Possible Voltage Events

Source URL: https://www.gmc-instruments.de/en/knowledge/power-quality/the-difference-between-class-a-and-class-s/

PV to PEV Power Transfer for Standalone Solar Power System

Published by Burak AKIN, Yıldız Technical University

Abstract. In this paper, efficient PV to PEV’s li–ion battery power transfer is investigated. Dual-interleaved boost topology is applied to the PV to PEV direct power transfer system as DC-DC converter. Proposed dual interleaved boost topology is reached 97 % total efficiency with inversely coupled input boost inductances.

Streszczenie. W artykule opisano metodę transferu energii zed źródła fotowoltaicznego do baterii pojazdu elektrycznego. Zastosowano przekształtnik DC-DC. Osiągnięto 97% sprawności. (Transmisja energii zew źródła fotowoltaicznego do pojazdu elektrycznego)

Keywords: PV – photovoltaic , PEV – plug-in electric vehicle
Słowa kluczowe: bateria słoneczna, pojazd elektryczny

Introduction

It can be difficult to find out grid connected energy, or it is expensive due to first initial costs in rural areas. So, solar power can be a solution as a renewable energy source. For this reason, photovoltaic cell can be used for energy producing from the sun. PV modules can be produced for solar farms by using PV cells. Although PV modules are expensive and relatively low efficient, in near future with the developing technologies it is predicted that the price will be lower and efficiency will be higher. In this paper reliable and high efficient power transfer is investigated from solar power used PV modules to PEV systems.

PEV systems normally demand the related energy from the grid connected energy sources by AC-DC converter. Because the PEV uses high DC voltage li-ion batteries, it is important to use boost converter topology to produce high DC voltage from AC grid with power factor correction (PFC) circuits [1, 2]. However, PV to PEV power transfer has the advantages of direct DC-DC conversion system with boost converter topology without PFC, THDi, reactive power and AC grid interface concerns. Before the efficient power transfer to the PEV system, maximum available power should be consumed from the PV solar energy system.

Solar energy system is renewable but for a limited time effective power source. So, to get high efficiency from the PV to PEV system first maximum power has to be demanded from the PV modules. Maximum power production is possible by controlling the PV modules with maximum power point tracking (MPPT) control systems [3, 4, 5]. This kind of control of the modules can produce the maximum available power instantly. Every switching cycle, efficient DC-DC power transfer charges the li-ion batteries.

MPP is the maximum power of current and voltage rates at one point. MPPT control gets the benefits of the maximum power while daylight condition rapidly changing. There are many cheap and easy MPPT system developed in literature [3, 4, 5]. Some MPPT algorithms are such as perturb and observe, constant voltage, incremental conductance, short circuit pulse, open circuit voltage and temperature method [5].

In this paper the main concern is to get the highest produced power from the PV solar module and transferring it with the highest efficiency to the PEV li-ion battery in standalone solar power system for further energy demand. Also, system complexity and cost problem and dimensions are the other concerns. So after PV cell architecture, PEV li-ion battery should be investigated.

PEV li-ion battery has generally 400 V DC input voltage with approximately 30 kWh energy capacity, so the battery needs efficient and high power charge system. For this reason, CCM working dual interleaved boost converter is investigated. For the calculation, Ii and Vi input current and voltage, Pi and Po input and output power, ηmin predicted minimum efficiency, λ duty rate, ∆IL input current surge, fsw switching frequency, L boost inductance, Vo output voltage and Co output capacitor is represented respectively. The formulas from 1-7 are taken from [2].

.

For DC-DC power conversion, there is no need for input diode bridge. Furthermore the boost converter has the advantage of direct DC-DC distribution system without PFC, THDi, reactive power and AC grid interface concerns.

Proposed dc-dc dual interleaved boost converter topology

To decide which boost topology should be used in DC-DC power conversion, conventional, dual and interleaved boost topologies are investigated. Conventional boost has the advantage of simply control and few components with efficiency disadvantage. Dual boost topology has the advantage of high efficiency with high THDi and current stresses disadvantage. Interleaved boost has the advantage of high efficiency and low current stress with more components disadvantage.

The proposed converter has dual interleaved boost topology to get the benefits of the two converters. To increase the efficiency, inversely coupled inductances are added as boost inductances of the DC-DC dual interleaved boost converter. In Eq. 8 and 9, inversely coupled inductance voltage calculation is represented. In here, L1 and L2 are the main boost inductances and M is the mutual inductance. The proposed converter topology is shown in Fig. 1.

.

In the proposed topology, L1 and L2 are main boost inductances, S1 and S2 are the main power switches and D1 and D2 are the main power diodes. Solar module and li-ion battery are added as input and output power sources. Ci and Co are the input and output capacitors respectively, D3 and D4 are power switches reverse diodes.

Fig.1. Proposed DC-DC dual interleaved boost topology

In the proposed converter, because solar module has limited power source maximum efficiency of the system is important. And also, for economical, power density and dimension problems, the converter has to be simple and cheap design with high switching frequency. Dual interleaved boost topology has two branches with 180o phase delay. Steady state waveforms are shown in Fig. 2. Because solar module output voltage is small and li-ion battery input voltage is high enough, duty ratio is generally bigger than 0.5. Efficiency can be improved by adding inversely coupled inductances mutual effect. Also, current stress is lowered by equally shared input power by two boost converter. This proposed converter can be easily adjusted for high power appliances by adding more boost branches and coupled inductances to the system. In this situation control strategy is important for efficient power transfer.

Fig.2. Steady state waveforms of dual interleaved converter
Control strategy

Generally dual interleaved boost controller senses each boost inductances currents with input and output voltage to generate switching signals. However, proposed converter senses input current, input voltage and output voltage to calculate switching signals. Easy and simply control strategy is developed to control the power switches.

Input current and voltage control is important to use PV modules at maximum power point (MPP). In this paper constant voltage MPP is used to control PV modules at MPPT. At a constant input voltage of PV module, input current is controlled to convert maximum power to the PEV li-ion battery with DC-DC dual interleaved converter. Short circuit protection is added to the system to control PV module at MPP.

Output voltage is sensed with input voltage to calculate duty ratio for the dual interleaved boost converter. Also output voltage regulation is added to the control system to prevent excessive voltage to the PEV li-ion battery. To control the system, calculated duty ratio, input current short circuit protection and output voltage regulator are all generates the S1 switching signal. S2 is controlled with 180o phase delay to S1. However to prevent in rush currents to the output capacitor time delay is added to the control system. Also, for steady state conditions input capacitor Ci and output capacitor Co is added to the system with initial values. For high power density and also fast response of the system high switching frequency is important for the control.

Simulation results

Proposed dual interleaved boost topology POWERSIM circuit schema is shown in Fig. 3 with control circuit. Both interleaved branches working as parallel with 180o phase delay. Control circuit first senses input voltage, input current and output voltage then generates switching signals. First generated signal applied to the first branch power switch S1, afterwards second signal is applied to the second branch power switch S2 with 180o phase delay. As a result both branches shares input current with lower current stress on power switches. Vg1 and Vg2 gating signals is shown in Fig.4 Input current is applied to the inversely coupled inductance to increase the efficiency of the converter.

For steady state response, input capacitor Ci is set to 60V DC and output capacitor Co is set to 400 V DC initial values. All efficiency and other calculations are done according to the steady state condition.

Stand alone solar power system is designed for 3 kW power from beginning to the end resistive load. So, for PV module MPP is set to 60V and 50 A which is for 3 kW maximum power, dual interleaved boost converter is designed for 3 kW output resistive load with 53,34Ω , input and output capacitors are set to 1μ for each watt so 3000μF for total power. PEV li-ion battery has 400V input voltage approximately 30 kWh energy, which is enough at least one week for 3 kWh energy consumption a day.

For better performance, silicone carbide (SIC) semiconductors are used with real specific values in the simulation. S1 and S2 IGBT (IXGH 60N60B2) have lower than 1.8 V saturation voltage with 100 kHz switching capability. Power diodes D1-D2 and reverse power switch diodes D3-D4 (STPSC2006CW) have lower than 1.4 V saturation voltages with better reverse recovery performance.

Fig.3. Proposed converter powersim circuit schema.
Fig.4. Switching signals of the converter for 100 kHz

The proposed dual interleaved DC-DC boost converter has input current short circuit protection and output over voltage protection to provide safety regulations. PV module is controlled by constant voltage MPPT control, so input current short circuit protection is important for the PV system. Also, output over voltage protection is important for the PEV Li-ion battery. PV module output or DC-DC dual interleaved boost converter input current and voltage waveforms are shown in Fig 5 with average measured values.

Fig.5. PV module output current and voltage waveforms

Output voltage waveform is shown in Fig. 6 for PEV Li-ion battery with measured average values. In here, voltage regulation is 0.8% is calculated.

Fig.6. PEV Li-ion battery voltage and current waveforms.

Steady state waveforms of dual interleaved converter in Fig. 2 are also observed in Fig. 7 from the simulation results.

Fig.7. Steady state waveforms of dual interleaved converter POWERSIM simulation

DC-DC dual interleaved boost converter components S1, S2, L1, L2, D1 and D2 power losses are calculated and it is shown in Fig. 8. Dual interleaved boost converter output power is measured 2911 W and input power is measures 3000 W. As a result, maximum efficiency of the PV to PEV power transfer system is calculated and it is reached 97% value at full load. In Fig 9, efficiency of the converter is shown from 10% to 100% load condition.

Fig.8. S1, S2, L1, L2, D1 and D2 power losses
Fig.9. Efficiency of the converter from 10% to 100%
Conclusion

PV to PEV efficient power transfer is investigated in this paper with dual interleaved boost converter. Inversely coupled inductances are added as input boost inductance. Each power switch work with 180o phase delay and shares input current with lower stress. Input current, voltage and output voltage are sensed to generate power switch’s gating signals. High performance DC-DC dual interleaved boost converter is applied to the PV to PEV energy transfer system. The proposed converter has 97% efficiency at full load of 3 kW power at 100 kHz switching frequency. Stand alone PV modules can transfer the maximum energy efficiently to PEV systems by proposed converter. This system can be easily improved to upper power levels. After, end user can use PEV li-ion battery as a power source for a house in rural areas.

REFERENCES

[1] Beltrame, F.; Roggia, L.; Schuch, L.; Pinheiro, J.R.; ,“A comparison of high power single-phase power factor correction pre-regulators” 2010 IEEE International Conference on Industrial Technology (ICIT), pp 625-629, May 2010
[2] Akın, B.; Bodur, H.; , “A New Single-Phase Soft-Switching Power Factor Correction Converter,” Power Electronics, IEEE Transactions on , vol.26, no.2, pp.436-443, Feb. 2011
[3] Jaw-Kuen Shiau; Der-Ming Ma; Pin-Ying Yang; Geng-Feng Wang; Jhij Hua Gong; “Design of a Solar Power Management System for an Experimental UAV,” Aerospace and Electronic Systems, IEEE Transactions on , vol.45, no.4, pp.1350-1360, Oct. 2009
[4] Pastre, M.; Krummenacher, F.; Kazanc, O.; Pour, N.K.; Pace, C.; Rigert, S.; Kayal, M.; , “A solar battery charger with maximum power point tracking,” Electronics, Circuits and Systems (ICECS), 2011 18th IEEE International Conference on , vol., no., pp.394-397, 11-14 Dec. 2011
[5] Faranda R., Leva S., “Energy comparison of MPPT techniques for PV Systems”, WSEAS TRANSACTIONS on POWER SYSTEMS, pp 446-455, Issue 6, Volume 3, June 2008

The correspondence e-mail: bakin@yildiz.edu.tr

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 89 NR 9/2013

Electric Vehicle Fire Extinguishing System

Published by Andrzej ŁEBKOWSKI, Gdynia Maritime University, Department of Ship Automation

Abstract. The paper presents the construction and properties of an electric vehicle fire extinguishing system. Parameters of several electric vehicles are presented, focusing on used traction battery types and system voltages. Various dangers that are possibly present during operation of an electric vehicle are discussed. The advantages of using the fire extinguishing system for electric vehicles are given.

Streszczenie. W pracy przedstawiono konstrukcję oraz właściwości systemu gaśniczego dla pojazdu z napędem elektrycznym. Zaprezentowano parametry wybranych samochodów elektrycznych pod kątem zastosowanych typów akumulatorów trakcyjnych oraz poziomów napięć. Omówiono zagrożenia, jakie mogą wynikać z tytułu eksploatacji pojazdów elektrycznych. Przedstawiono korzyści wynikające z zastosowania systemu gaśniczego dla pojazdów elektrycznych. (System gaśniczy dla pojazdu z napędem elektrycznym).

Słowa kluczowe: pojazdy elektryczne, system przeciwpożarowy, zwarcia elektryczne, akumulatory litowe.
Keywords: electric vehicles, fire extinguishing system, short circuit, lithium batteries.

Introduction

The progress of electric propulsion systems affects the demand of electric vehicles and hybrid vehicles. Regardless the configuration of the vehicle – battery energy storage (EV, BEV, PEV, EVC), battery combined with an ICE (HEV, PHEV), fuel cell powered (FCEV), if one of the vehicles prime movers is an electric motor, then such a vehicle can be regarded as an electric vehicle. Basing on sales statistics of electric vehicles, there are predictions that in the 2040 the market share of EVs could be on the order of 35% [1,2]. In addition to vehicles marketed by automotive companies, there are numerous designs made by research institutes, academies and independent makers. The amount of vehicles converted from conventional propulsion to electric propulsion (EVC) is very high, which can be inferred from number of sale points and general demand for components needed to construct an electric vehicle. These designs, sometimes only single vehicle projects, can pose somewhat greater risk of harboring design flaws which could lead to e.g. a fire hazard. The lack of sufficient experience in design could lead to design flaws which can translate into potential hazards, including fire hazard. At the same time, with growing sales of electric vehicles, independently of applied state of the art driver aid systems, there are unavoidable, statistically determined road accidents in which these vehicles take part. Additionally, as in the case of other machines, e.g. home appliances and conventional cars, as the time passes some elements or systems of a vehicle will unavoidably reach their end of life, resulting in a malfunction.

Fig.1. Fire of the TESLA Model S during fast charging at a Supercharging station, Norway 01.2016 [2]

For the user, in the best case scenario it will mean only, that their vehicle will not work. In the worst case, the failure of one of electric propulsion system’s elements could start a fire ending in a total loss of the vehicle.

The examples of fires in electric vehicles can be found all over the world, in every place and in every type of the vehicle.

Fig.2. An example of improper placement of electric propulsion system elements [11]

The causes of fire – an uncontrolled and spontaneous combustion of matter – can be various, starting from changes in the internal structure of a material (spontaneous battery ignition), powertrain design errors (wrong choice of cable gauge, lack or insufficient protection from overcurrent and short-circuits in the cabling, inadequate protection from cable insulation abrasion damage due to vibrations present during operation, the action of others (arson), force of the nature acts (e.g. moisture short-circuiting the battery pack during flood, physical damage due to hurricane winds), and finally, road accidents (short-circuits, physical damage to cells causing their ignition).

No matter what are the reasons of physical damage of electric propulsion system components, mainly the battery pack, precautions should be taken to mitigate any dangers to life and limb as well as equipment. Moreover, any fire on board of an electric vehicle usually ends up in total loss and substantial negative financial impact on the vehicle owner’s budget. It is considered, that purely electric vehicles are safer than conventional vehicles [3] or hybrid vehicles (combining electric and conventional propulsion). The latter pose even a greater threat to human life and health due to the fact that they have the disadvantages of both propulsion types: possibility of passenger electrocution and possibility of fuel tank fire or explosion.

Dangers present in electric vehicles are associated mostly with the risk of electrocution caused by damage to the electric propulsion system sustained in a crash, as well as the risk of fire, but it’s development is not as rapid as in conventional cars. Even in case of the battery short-circuit, the fire does not appear and spread as quickly as in conventional vehicles (fig. 3), which allows the occupants more time to leave the vehicle.

Fig.3. Causes of crash between electric vehicle and conventional vehicle [4]

The practical experience in the operation of electric vehicles shows that currently used battery types are not 100% safe regarding the possibility of uncontrolled fire. There are numerous examples, where lithium batteries in cell phones, laptop computers, electric vehicles and even passenger jets have spontaneously caught fire. Virtually all types of nickel based batteries (Ni-Cd, Ni-MH) and lithium based batteries (Li-Ion, Li-Polymer) with the exception of lithium iron phosphate batteries (LiFePO4) [11,12,13,14,15,16,17] have a tendency for self-ignition when their terminals become short-circuited (by a physical short-circuit of connected wires, flooding by a conductive liquid like sea water) or by a physical damage to their internal structure (internal short-circuit caused by a damage to the separator between cell electrodes). After a short-circuit on a lithium cell, its temperature rises, the enclosure looses its seal and lets out a mixture of toxic and flammable gases line carbon monoxide or organic electrolyte vapors which results in a fire.

Fig.4. Structure of the battery box of a Nissan Leaf vehicle [7]

The threat to life and health caused by fire can be minimized by following proper guidelines and procedures in case of an accident, set forth in the handouts provided by the manufacturer of the vehicle. The presented information regards mainly the paths of the high voltage traction cables marked with orange colored conduits, as well as places where such cables can be cut after the accident takes place. The voltage on the terminals of traction battery ranges from 48 up to 650 VDC, while the voltage considered safe to humans is 120V for DC, and 60V for AC. It should also be remembered, that the cutoff of cabling powering the inverter from the battery pack does not guarantee complete safety from electrocution. Capacitive elements present in the system can hold a high voltage on the ends of separated cables for several minutes.

The solution which increases the overall safety level is to place the batteries in reinforced, metal, hermetic battery boxes, as shown in figure 4.

A less favorable approach can be displayed in the battery pack made by E4V, housed in a non-hermetic aluminum box, which also contains numerous cable harnesses and electronic components which can be a source of ignition and fire.

Fig.5. Structure of the battery box made by the E4V

The events of 2012 related to the landfall of Sandy hurricane, which caused a flood at the East Coast of the USA, contributed to flooding of 16 Fisker Karma electric vehicles, causing all of them to ignite and burn down completely [6]. A similar fate happened in 2013-2016 to Tesla made cars, which battery packs were physically damaged during operation on roads.

The presently used extinguishing means – fire extinguishers carried in each vehicle in an easily accessible place, cannot provide the vehicle’s operator full control over the spreading fire. Simultaneously, the vehicle’s construction and enclosing all the powertrain components under the hood and inside the body of the vehicle cause that any eventual fire is noticed only after it is in a very developed stage. Therefore it is recommended to install devices which indicate presence of flames or too high a temperature in the vicinity of powertrain and energy storage devices. One of the elements which can be used to fight a fire are fire systems dedicated to electric vehicles. These systems can employ various methods of firefighting including infrasound (30÷60Hz) [5]. An alternative solution is to incorporate in the internal structure of the cells a new generation of TRPS – thermo-responsive polymer switching materials [8], which disconnect a battery from its terminals during a rise in battery’s temperature and reconnect the terminals when the temperature drops. The purpose of TRPS elements is to isolate the car’s wiring from the battery. This solution however does not prevent the fire caused by a physical damage to the cells themselves.

The author is proposing an application for an electric vehicle, which employs an extinguishing agent (CO2, dry powder, etc.) which can be fed directly into the enclosure of the electrical device. This solution is an effective way to suppress any fire before it spreads, therefore saving the whole vehicle from destruction.

System structure

Fire extinguishing system for an electric vehicle consists of: a control unit, sets of sensors placed in the critical points of an electric powertrain and actuators – electromagnetic valves which when energized, release the extinguishing agent into the enclosure or next to the elements which are suspected of being in risk of fire.

Fig.6. Structure of the fire extinguishing system for an electric vehicle

Fig.6. Structure of the fire extinguishing system for an electric vehicle, where:

1-traction motor/motors, 2-power inverter, 3-electrical energy store (battery), 4-AC/DC converter (battery charger), 5-fire and temperature sensors installed on the motor, 6-fire and temperature sensors installed on the power inverter, 7-fire and temperature sensors installed on the battery charger, 9-GSM communication module, 10-ambient temperature sensor, 11-front impact sensor, 12-rear impact sensor, 13-extinguishing agent tank, 14- electromagnetic valve spraying the motor, 15-electromagnetic valve spraying the inverter, 16-electromagnetic valve spraying the battery, 17-electromagnetic valve spraying the battery charger, 18- user interface with alarming function, for communicating the user the actions taken by the fire system, 19-extinguishing agent tank pressure sensor, 20-main contactor connecting the battery to the high voltage vehicle cabling, 21-internal contactor in the battery pack.

In case the batteries in the vehicle are divided into separate battery packs, every pack should be fitted with a set of sensors, electromagnetic valve supplying the extinguishing agents into the box, and contactor which disconnect the high voltage wiring from the batteries. The Control Unit of the fire system can be implemented using a microcontroller (e.g. an ATMEL Atmega324PA). The schematic diagram of the fire system for an electric vehicle structure is presented in the figure 6.

System functions

The main element governing the operation of the fire extinguishing system for an electric vehicle is the Control Unit (CU), into which various sensor signals are supplied, such as: fire and temperature sensors installed on the motor, fire and temperature sensors installed on the power inverter, fire and temperature sensors installed on the battery charger, ambient temperature sensor, rear and front impact sensors. The schematic diagram of the fire extinguishing system for an electric vehicle is presented in the figure 7. In the event, the control algorithm programmed into the control unit memory detects, that the signal value on any sensor measuring the temperature on the battery pack (TB – Battery Temperature), the motor (TM – Motor Temperature), the inverter (TI – Inverter Temperature) or the battery charger (TCh – Charger Temperature) exceeds the threshold of reference signal, or any of the fire sensors (FSB – Battery Fire Sensor, FSM – motor fire sensor, FSI – Inverter Fire Sensor, FSCh – Charger Fire Sensor) or impact sensors (11,12) have been activated, the control unit (CU) first disconnects the high voltage circuit by opening the main contactor (20) (with simultaneous opening of the internal contactor (21)) and opens the corresponding electromagnetic valve of the device which sensor has been activated, therefore spraying the inside of given device with the extinguishing agent. Activation of only the impact sensor(s) will just open the contactors.

Fig.7. Fire system schematic diagram

The reference value threshold, for opening the electromagnetic valves can be independently set for each of the protected powertrain elements: battery temperature TB>90°C, motor temperature TM>150°C, inverter temperature TI>100°C, charger temperature TCh>100°C. The temperature threshold values were selected basing on thermal characteristics of lithium cells [18,19,20]. Each type of chemical battery, including lithium based cells, is subjected to self-heating which can be in simplification treated as proportional to the product of squared current flowing through the cell and the cell’s internal resistance. There is a point, however, where the cell’s temperature will begin to rise on its own, either after reaching a certain temperature due to short-circuit or after a mechanical damage to the electrode separator membrane causing and internal short. For lithium-ion and lithium-polymer cells, at ca. 150°C their safety valve bursts, releasing noxious fumes to the surrounding atmosphere. These gasses can include carbon monoxide (CO), hydrogen fluoride (HF) and phosphorous oxyfluoride (POF3). After exceeding ca. 200°C a thermal runaway process starts, resulting in fast increase of cell’s temperature to about 690°C with presence of flames. Once the runaway starts, the cell’s temperature rises on its own, as the reaction is exothermic. The lithium iron phosphate (LFP) cells reacted slightly different, while they too started to vent gasses at about 150°C, the flames were not present [18].

The fire system, through the user interface, can inform the driver of the actual thermal parameters of the monitored devices, and allows the definition of pre-alarm threshold values. This function allows to alert the user in case the temperature of monitored devices is dangerously high by displaying an appropriate message on the HMI LCD display. Additionally, in case the pressure of the extinguishing agent in the tank drops below the preset value, the fire system can inform the user of this fact. In case the fire breaks out (as indicated by the fire sensor), or the temperature thresholds for powertrain elements are exceeded, the control unit will: turn off the high voltage circuit by opening the contactor (20), isolate the battery by opening the contactor (21), open the appropriate valve releasing the extinguishing agent onto the device in which the fire or elevated temperature was detected. In case only the impact sensors have been activated, the only direct action of the fire system will be deactivating the contactors and displaying a message on the display. In case the impact results in further fire, the system will then react accordingly. The HMI LCD will display a message about the alarm and the measures taken by the fire system. As an extra function, the fire system can remotely warn the user of the fire hazard via an SMS. A short message is sent to the user-defined telephone number, with the information about the event (fire, high temperature or impact) and actions taken by the control unit.

Fig.8. Control algorithm programmed into the control unit’s memory
Summary

The presented fire extinguishing system for an electric vehicle, basing on data from temperature sensors, flame sensors and impact sensors, can alert the vehicle’s driver about a fire in the vehicle and proceed with immediate preventive action. The fire system has an advantage over the other solutions, which are limited to disconnecting the battery, that it can react further, by actively trying to extinguish the present fire.

The fire extinguishing system for an electric vehicle can interact with other diagnostic and monitoring systems including remote notification of emergency services, and remote electric vehicles diagnostics.

The installation of the fire system can minimize the financial loss which could arise from a fire, as well as increasing the safety level for the vehicle occupants and other traffic participants. Application of the fire system for an electric vehicle together with other technologies, like thermal management system, battery management system, battery short-circuit protection, proactive cell design and packing (mechanical crash protection, flood protection), can synergistically increase the level of safety and allows longer life of the battery.

REFERENCES

[1] Randall T., Here’s How Electric Cars Will Cause the Next Oil Crisis, Bloomberg, http://www.bloomberg.com, 02.2016.
[2] Herron D., Model S Catches Fire in Norway at Supercharger, charging system seemingly at fault, The Long Tail Pipe, http://www.longtailpipe.com, 01.2016.
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[7] DeMorro Ch., 99.99% Of Nissan LEAF Batteries Still In Operation, Clean Technica, http://www.cleantechnica.com, 03.2015.
[8] Chen Z., Hsu P.H., Lopez J., Li Y., To J.W.F., Liu N., Wang Ch., Andrews S.C., Liu J., Cui Y., Bao Z., Fast And Reversible Thermoresponsive Polymer Switching Materials For Safer Batteries, Nature Energy, article no. 15009, DOI: 10.1038/NENERGY.2015.9, 01.2016.
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[13] Pereirinha P., Trovao J., Santiago A., Set up and test of a LiFePO4 battery bank for electric vehicle, Przegląd Elektrotechniczny, 01a (2012), p.193-197.
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[15] Fan L., Liu Y., Fuzzy Logic Based Constant Power Control of a Proton Exchange Membrane Fuel Cell, Przegląd Elektrotechniczny, 05b (2012), p.72-75.
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[18] Larsson F., Andersson P., Mellander B.E., Lithium-Ion Battery Aspects on Fires in Electrified Vehicles on the Basis of Experimental Abuse Tests, Batteries 2016, 2, 9; doi:10.3390/batteries2020009, (2016).
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Author: dr inż. Andrzej Łebkowski, Akademia Morska w Gdyni, Katedra Automatyki Okrętowej, ul.Morska 83, 81-225 Gdynia, E-mail: andrzejl@am.gdynia.pl.

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 93 NR 1/2017. doi:10.15199/48.2017.01.77

Electric Vehicles in Mining for the Aspect of Operational Safety

Published by Wiktoria GRYCAN, Wroclaw University of Science and Technology
ORCID: 0000-0001-8121-7612

Abstract. Battery electric vehicles are becoming an opportunity for sustainable development in mining. There are several advantages of electric vehicles in mining, primarily related to improving the safety of miners’ working conditions. It is significant for mining workers that electric vehicles are not a significant additional source of heat, reduce the emission of exhaust gases in the environment of their use, and are characterized by very little or no harmful gases released into the atmosphere. Currently, the mining industry is developing several initiatives for sustainable development, bringing together mining companies, suppliers and research institutions to accelerate technology development and effectively implement BEVs (battery electric vehicles) in the mining industry as soon as possible. These activities are accepted by miners, although, like any modern technology, they raise legitimate concerns and become the beginning of reflection on the safety standards for using electric vehicles that should accompany their development. This article reviews electric vehicles in mining works. Concerns for the development of these technologies have been identified. Particular attention was paid to the concerns about the operational safety of electric vehicles in the mining environment.

Streszczenie. Pojazdy akumulatorowe stają się szansą na zrównoważony rozwój w górnictwie. Zalet pojazdów elektrycznych w górnictwie jest kilka, przede wszystkim związanych z poprawą bezpieczeństwa warunków pracy górników. Dla górników istotne jest, że pojazdy elektryczne nie są znaczącym dodatkowym źródłem ciepła, redukują emisję spalin w środowisku ich użytkowania oraz charakteryzują się bardzo małą emisją szkodliwych gazów lub całkowitym ich brakiem. Obecnie przemysł wydobywczy rozwija kilka inicjatyw na rzecz zrównoważonego rozwoju, skupiając firmy górnicze, dostawców i instytucje badawcze w celu przyspieszenia rozwoju technologii i jak najszybszego wdrożenia pojazdów typu BEV (battery electric vehicles) w górnictwie. Działania te są akceptowane przez górników, choć jak każda nowoczesna technologia budzą uzasadnione obawy i stają się początkiem refleksji nad normami bezpieczeństwa użytkowania pojazdów elektrycznych, które powinny towarzyszyć ich rozwojowi. W artykule przedstawiono przegląd pojazdów elektrycznych w pracach górniczych. Zidentyfikowano obawy dotyczące rozwoju tych technologii. Szczególną uwagę zwrócono na obawy dotyczące bezpieczeństwa eksploatacji pojazdów elektrycznych w środowisku górniczym. (Pojazdy elektryczne w górnictwie w aspekcie bezpieczeństwa ich użytkowania)

Keywords: electric vehicles, operational safety, mining.
Słowa kluczowe: pojazdy elektryczne, bezpieczeństwo, górnictwo.

Introduction

Access to ore in mined mines becomes complex over time. As a result of the exploration, deposits are located more profoundly, and their quality is deteriorating. That brings in the need to employ more and more mining vehicles, which account for an average of 50 per cent of the mine’s total direct emissions [1]. However, traditional diesel vehicles (DV), commonly used in mines, have several disadvantages. [2] indicates that diesel vehicles in mining have low efficiency (35%), low overload capacity, high maintenance cost, require skilled mechanics for maintenance, are noisy (~105 dB), can cause fog formation, and generate high heat. Moreover, in the aspect of development, they are difficult for data gathering, remote monitoring and building autonomously. Also, there is an impediment in fuel transportation for DV in deep mines.

As a result of numbered issues with DV, some sustainability initiatives within the industry, mining companies, vendors and research institutions to support innovation and haste the adoption of new equipment, including BEVs, have been started. For example, in European Union, a project was financed as part of the activities of the Horizon 2020 program. The project’s name was SIMS from Sustainable Intelligent Mining Systems. The project was carried out by a consortium of mining companies, equipment and system suppliers to top-class universities. However, this initiative had a few goals [3]:

● automated planning and reporting of mining progress through ground control to improve event planning (a digital twin), improve results and improve safety in production,
● battery electric vehicles implementation and testing in the operating mines,
● development of training modules for operators and mine workers and educational modules for students and the general public,
● communication and positioning – development and testing in operating mines.

Enterprises involved in the project include KGHM Poland, K + S Germany, Boliden Sweden, Lulea Sweden, Agnico Eagle Finland, LKAB Sweden, ABB, Epiroc, Ericsson and Mobilaris.

Another example of a sustainable mining initiative is International Council on Mining and Metals (ICMM), a global leadership organization for sustainable development. It has 37 association members and 26 Company members (currently). As the official website introduction says [2]: the primary goal of the organization’s Innovation for Cleaner, Safer Vehicles (ICSV) initiative is to enable its members the meeting in a non-competitive space. Such an environment can create conditions for the world’s largest original equipment manufacturers (OEMs) to accelerate the development of a new generation of mining vehicles and improve existing ones. In addition, the program focuses on motivating and encouraging activities directed at project participants to encourage them to find the advantages of the electrification of mining vehicles [4].

The result of such initiatives is electric vehicles being gradually introduced into the mines. Their design and operating conditions depend on the intended use and the mine (open-cast, underground) in which they are used. Notwithstanding the many advantages electric vehicles carry in mines, their emergence creates new risks and concerns for use. The previous pilot projects and the first attempts to use this type of vehicle have allowed identifying potential problems that may arise from introducing electric vehicles to the mine.

Electric Vehicles in mining

Undoubtedly, regardless of the technology used, electric vehicles have numerous advantages over diesel vehicles traditionally used in mining [5]:

● higher energy efficiency (about 90%);
● constant torque (including high torque at low speeds),
● quick response to the load and better overload capacity,
● no exhaust fumes and therefore no mine air pollution and no fog formation,
● generate only a third of the heat emitted by a diesel having the same power
● hourly electric energy cost lower than hourly fuel cost for diesel,
● less maintenance required,
● low noise and vibration level.

The dynamic development and increased interest in battery electric vehicles in mining have been visible for the last ten years. However, other solutions are available for a much more extended period. Nonetheless, vehicle manufacturers already offer a wide range of electric vehicles for various mining activities. As shown in Table 1, mines can choose from: electric rope shovels, Electric Load-Haul-Dump (eLHD) trucks, Electric Haul Trucks, Electric Drills, Electric Service Vehicles, electric crushers, Electric scoops, boomer and smaller trucks, Electric locomotives, Battery-electric explosives charger, Rock bolting rigs and conveyors. However, they differ in the used technology of batteries and charging systems. In addition, design and operational considerations vary for different equipment types. For example, tethered equipment typically requires accommodations for the cable, while trucks might focus more on regenerative braking. [6]

First can be mentioned electric vehicles used in 1975 like electric rope shovels, powered by the power grid. However, the need to power the cable from the mains may cause damage to the cable, electric shock hazards, and vehicle downtime. Moreover, the vehicle’s operating range is limited due to the cable length. Therefore, this technical it is impractical for trucks, so not many vehicles have been commercialized [5]. Since these vehicles do not need refuelling, what makes them time efficient.

Another solution, used in mines for over a hundred years, represented by, e.g. Electric Haul Trucks by ABB and Caterpillar (see Tab.1), is trolley-powered equipment. This technology powers electric rail or trackless vehicles from an overhead cable. It is impractical but creates an attractive alternative for mine trucks, particularly those working on long ramps (up to a few kilometres) [5]. The modernization and development of this technology is BluVein. The system is based on a slotted power rail set down into a public highway. The conductors are not exposed, so it is safe for people, animals and vehicles to pass over it. Electric trucks drive over the rail at 80 or 100 kph, automatically deploying an arm connecting to the rail. The truck can then draw electricity to power itself and charge its battery for later use [7]. The benefits of this solution are eliminating battery replacement, removing downtime for charging, or the possibility of using smaller batteries [8]. However, attention-grabbing, the technology is under development and will be installed in 2022 for a six-month co-trial with the various mines.

Through the current advances in battery technology, since 2016 become possible to manufacture reliable battery electric vehicles (BEVs). These can be used in all mining methods. Also, those requiring regular movement between working levels, such as sublevel stop, cut and fill, small/medium-scale sublevel caving, and room and pillar, can operate on average mine roads. [9] Generally, battery electric vehicles, due to the charging technology, can be divided into [6]:

● On-Board Charging,
● Off-Board Charging of On-Board Batteries,
● Off-Board Charging of Off-Board Batteries (Battery Swapping),
● Hybrid Charging Method,
● Off-Board Proprietary Chargers,
● Alternative Charging Systems and Equipment Types (Overhead Catenary Systems Or Trolley Assist and Charge-While-Operating (Tethered) Electric Equipment

Table 1. Electrically/battery-powered machinery (based on [1], changed with updates)

.

According to research studies [10], BEV makes it possible to reduce the downtime of diesel vehicles despite the need for loading. Also, modern solutions like those used in the Rock bolting rig by Sandwig enable battery charging during, e.g. drilling.[11]

Problems identified for EV development in mines

Problems defined by literature can be generally divided into four main groups: safety, operational, infrastructure, and global issues. From the user’s point of view, operational issues are the most disturbing everyday work. Those make their work inconvenient and inefficient. For example, miners point to the necessity of battery charging as their primary concern with BEV [9]. Also, mining staff for the safe operation of BEV needs additional training and, in some cases, changes in competencies [12]. Skills with growing demand include system evaluation and analysis, mathematics, active listening, instructing, data analysis, data and digital literacy, and judgment and decision-making [12]. Skills with decreasing demand include vehicle operations, materials extraction, operations and control, equipment maintenance and blast-hole drilling [12].

An inconvenience in the extensive use of electric vehicles is the need to adapt the infrastructure. The unique infrastructure like a charging room or a system for battery charging is indispensable for battery charging [4]. Space and infrastructure are needed to test, maintain, discharge, charge, and store batteries [6]. In addition, chargers must meet requirements for operating under the ground, with specific temperature and humidity, and the need for power supply from the electrical system available in the mine. It is necessary to avoid temperatures above 30 degrees [13] and under 0 degrees [14], the same as frequent fast DC high power charging mode [15]. Also, according to [16], battery modules’ voltage should be tested, and used modules should be replaced to ensure long-term and reliable operation. . In systems like BluVein, the whole infrastructure of slotted rails is needed. Therefore, introducing electric vehicles to mines is easier in the case of newly created mines, including electric vehicles already at the design stage.

Of course, BEVs also bring general concerns like materials like lithium, cobalt, and copper accessibility to manufacturing batteries in the future. Another problem is battery utilization. Those problems might become significant with BEV popularisation.

However, a thing of the most significant concern is electrical safety. Safety issues may be connected with a few safety matters. First can be mentioned situations during normal operational conditions. Mining workers designated [9] possible accidents which may occur because of too quiet work of BEV. Miners used to louder conditions may be victims of car hits and similar accidents. They may not be aware of Bevs’ presence in their vicinity. Also, the risk of electrocution raises concerns. People traditionally using diesel mining equipment are often not trained to work with electric vehicles. They are afraid of electrocution, especially periodically performed maintenance operations.

Most safety concerns are connected to batteries. While the charging process. During the charging process, the battery may overcharge, which may damage the battery. Workers are concerned about the risk of electric shock if the battery needs to be replaced. Research [16,17] shows that both, the environmental conditions and the method of charging, are significant for the functioning of the battery. Also, battery systems under the ground are exposed to the traditional hazards of such systems under normal operating conditions. These are identified by [18] as after-accident ignition, electric shock due to the energy remaining in the batteries, and re-ignition due to energy not being discharged. The problem is that those situations under the ground are even more dangerous and hard to handle.

A significant impact on the safety of the charging process also has communication. Regarding safety, communication protocols are exposed to various information security threats, including natural elements (natural disasters, poor communication locations, deterioration of equipment performance) and human elements (destruction, fraud, theft, human lack of skills, malicious network attacks). In the event of communication errors, the electric vehicle may not be charged. Also, the charging process may be interrupted (which affects the efficiency of operation) or overcharged, which may even result in the self-ignition or explosion of electric vehicles and charging devices. [19]

Requirements for BEV users in mining

Due to the novelty of the topic and the lack of widespread use of electric vehicles in mining, unfortunately, there are no clear guidelines regarding the requirements that this equipment should meet. Therefore, it is one of the barriers to the widespread use of EVs in mining. However, the requirements for vehicles should result from the hazards that may occur, and these are difficult to define only under simulation conditions. However, so far, there are only recommendations that should be met by electrical equipment.

Such recommendation is a document “Recommended Practices for Battery Electric Vehicles in Underground Mining” [6] issued by The Global Mining Guidelines Group (GMG). GMG is a network of representatives from mining companies, original equipment manufacturers (OEMs), original technology manufacturers (OTMs), research organizations and academics, consultants, regulators, and industry associations worldwide who collaborate to face the new mining industry challenges. The document defines, among other things, what threats can potentially be generated by the technologies used and what standards theoretically they should meet. Decisions on the admittance of products for use in mining plants in Poland, based on [20], remain in force of the mining plant decision-makers. Decisions regarding the operation of the mining plant should be based on the existing regulations and include natural hazards occurring in mining plants. Due to the lack of applicable standards, making such decisions may be limited and contribute to a slowdown in electrification in the Polish mining industry. The conclusions from the experiences of other mines in the world published, e.g. by GMG, might be helpful for the decision-makers.[6]

Most common safety issues may be generally divided into energy storage – batteries connected ones and those about charging systems. The charger must be well-matched with the energy storage type and chemistry used at the mine, respected for the applicable charging rate (slow or fast), and compatible with different conditions. The charging system enclosure/shell should also have the appropriate environmental protection rating (IP) according to the installation location. Additionally, the system needs to be monitored, so there are no open plugs. Finally, the charger installation should comply with local codes and undergo any necessary approvals or inspections.[6] Document concerning electric vehicles in Poland is [21]. However, it is not applicable because it defines charging stations as an element of the road public transport recharging infrastructure. Also, [22] does not apply to handling electrical equipment in mining, as it refers to the provisions of [20].

Because BEV batteries require frequent charging, exposure to potential hazards often occurs when personnel connect, operate, and disconnect the charging system. Currently, in repairing electric vehicles, the employee must have SEP qualifications up to 1kV. Due to the lack of regulatory requirements for using such vehicles in mining, similar requirements should be expected. Of course, the rights do not guarantee knowledge about handling batteries and electric vehicles with such a specific structure.

Therefore, one should expect the necessity to undergo training to operate the device’s battery during its regular operation. It remains open questions. Who should conduct such training? Should it end with obtaining the appropriate certificate and which employees should be trained – replacing/charging the battery or all vehicle users or working in its vicinity? Due to [20], decisions must be considered within a given mining plant.

Recommendations

Before implementing electric vehicles, the risk assessment should be considered [23]:

● financial risks (e.g., increased infrastructure capital expense, early battery replacement)
● production risks (e.g., discharged vehicle recovery, production rate impact)
● health and safety risks (e.g., fire/explosion, electric shock, arcing fault)
● environmental risks (e.g., worn battery skulls toxic to the environment)

It is important to know why and where BEV will be needed, what is required and what type of equipment is necessary, and who is or should be trained to operate, maintain service and implement emergency procedures. [23]

Protection of equipment and installation should meet appropriate standards responding to the operational conditions (temperature, humidity and others). Excess dust, corrosion, condensation, water or other liquids should be eliminated to reduce the risk of accidents. Behalf of safe working conditions should be specified, required and applied proper maintenance and proper tools (Insulated tools). For the safety of users, battery training should be required. It is required to specify who should be responsible for the replacement of the battery, what conditions should be ensured so that such replacement is also safe underground, what means of protection should be provided to such a person, and what the procedure for admitting to work should be. This task is not easy because technology is still evolving. Training should focus on different user groups, such as Electricians, Service mechanics, and mechanical/electrical specialists. A training checklist for BEV mining vehicles and mining charging systems should also be developed for specific equipment. [23]

Conclusions

At the moment of writing this article, there are no obligatory recommendations for electric vehicle use in mining. The applicable documents concern the general principles of using electric networks, using electric vehicles, and admission to work with voltage. However, the conditions in mines require a risk assessment each time when working with a specific electric vehicle, determination of potential risks and rules for handling the device to avoid electric shock. It is crucial in traditional mines, the existing infrastructure of which is not adapted to using electric vehicles. Nevertheless, as indicated in the literature, despite the potential threats, electric vehicles in mining are its future and an opportunity for sustainable development.

REFERENCES

[1] Ertugrul N., Kani A. P., Davies M., Sbarbaro D., Morán L., Status of Mine Electrification and Future Potentials, Proceedings – 2020 International Conference on Smart Grids and Energy Systems, SGES 2020, 151-156.
[2] International Council on Mining and Metals (ICMM) website https://www.icmm.com/, accessed 14.07.2022.
[3] Sustainable Intelligent Mining Systems (SIMS) project website https://www.simsmining.eu/, accessed 14.07.2022.
[4] Leonida C., Battery-electric Vehicles: Brightening the Mining Industry’s Future, Engineering and Mining Journal, 221 (2020), nr 1, 32-37.
[5] Paraszczak J., Svedlund E., Fytas K., Laflamme M., Electrification of loaders and trucks – A step towards more sustainable underground mining, Renewable Energy and Power Quality Journal, Vol 1 Issue 12, 2014, 81-86.
[6] The Global Mining Guidelines Group (GMG), “Recommended Practices for Battery Electric Vehicles in Underground Mining”, version 3, 2022 https://gmggroup.org/wp-content/uploads/2022/06/2022-06-23_Recommended-Practices-for-Battery-Electric-Vehicles-in- Underground-Mining.pdf, accessed 18.07.2022.
[7] Sprague T., Miners Lead the Charge For Battery-electric Vehicles, Engineering and Mining Journal, 2022, nr 1, 26–30.
[8] https://bluvein.com/about-us/, accessed 17.07.2022.
[9] Halim A., Lööw J., Johansson J., Gustafsson J., van Wageningen A., Kocsis K., Improvement of Working Conditions and Opinions of Mine Workers When Battery Electric Vehicles (BEVs) Are Used Instead of Diesel Machines — Results of Field Trial at the Kittilä Mine, Finland, Mining, Metallurgy and Exploration, Vol 39, (2)2022, 203–219.
[10] Nieto A., Schatz R.S., Dogruoz C., Performance analysis of electric and diesel equipment for battery replacement of tethered LHD vehicles in underground mining. Mining Technology: Transactions of the Institute of Mining and Metallurgy, Vol. 129, (1)2020, 22–29.
[11] DS412iE ROCK SUPPORT DRILL RIG manual file:///C:/Users/wikto/Downloads/ds412ie-specification-sheetenglish.pdf, accessed 17.07.2022.
[12] EY, Will electrification spark the next wave of mining innovation? Survey, 2019 https://assets.ey.com/content/dam/ey-sites/eycom/en_gl/topics/mining-metals/mining-metals-pdfs/eyelectrification-in-mining-survey.pdf, accessed 17.07.2022.
[13] Ren, H. Hsu, R. Li, X. Feng and M. Ouyang, “A comparative investigation of aging effects on thermal runaway behavior of lithium- ion batteries,” eTransportation, Volume 2, 2019.
[14] E. Wikner and T. Thiringer, “Extending Battery Lifetime by Avoiding High SOC,” Applied Sciences 2018, 8, 1825.
[15] Tomaszewska, Z. Chu, X. Feng and S. O’Kane, “Lithium-ion battery fast charging: A review,” eTransportation, Volume 1, 2019, pp. 1-28.
[16] Majek A., Niewczas A., Selected maintenance aspects of traction batteries in electric vehicles, 12th International Science-Technical Conference AUTOMOTIVE SAFETY, AUTOMOTIVE SAFETY 2020. Institute of Electrical and Electronics Engineers Inc.
[17] Huang K., Wang Y., Feng J., Design of lithium-ion battery management system for mine electric vehicle. IOP Conference Series: Earth and Environmental Science, Vol. 680, (1)2021, 1-7.
[18] Bisschop R., Willstrand O., Rosengren M., Handling Lithium- Ion Batteries in Electric Vehicles: Preventing and Recovering from Hazardous Events. Fire Technology, Vol.56, (6)2020, 2671–2694.
[19] Jiang L., Diao X., Zhang Y., Zhang J., Li T., Review of the charging safety and charging safety protection of electric vehicles, World Electric Vehicle Journal, Vol.12, (4)2021, 1-24.
[20] Ustawa z dnia 9 czerwca 2011 r. – Prawo geologiczne i górnicze Dz.U. 2011 nr 163 poz. 981.
[21] Ustawa z dnia 11 stycznia 2018 r. o elektromobilności i paliwach alternatywnych Dz.U. 2018 poz. 317.
[22] ROZPORZĄDZENIE MINISTRA ENERGII 1 z dnia 28 sierpnia 2019 r. w sprawie bezpieczeństwa i higieny pracy przy urządzeniach energetycznych Dz.U. 2019 poz. 1830.
[23] Mayhew Performance Ltd., How to Manage Risks of Implementing BEV in an Underground Environment, Virtual Symposium: Battery Electric Vehicle Safety in Mines, Jan 25 2021. https://youtu.be/0xl1QAndTU4, accessed 18.07.2022.

Authors: dr inż. Wiktoria Grycan, Politechnika Wrocławska, Katedra Energoelektryki, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, E-mail: wiktoria.grycan@pwr.edu.pl;

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 98 NR 12/2022. doi:10.15199/48.2022.12.27

Solar Energy Batteries – A Critical Review

Published by Samhar Saeed Shukir, Electrical Department, Technical Institute- Kut, Middle Technical University, Baghdad, Iraq. Corresponding Author: samharalwandi@gmail.com

ABSTRACT

Solar power has numerous benefits, it is a clean and renewable energy resource that can help us to reduce carbon emissions from fossil fuel use and mitigate climate change. However, solar energy production is limited to daytime hours when sunlight is abundant, and for solving the intermittency problem batteries bank has been used, where it store electricity for later use, so you can keep appliances running during a power outage, and use more of the solar energy that you produce at your home. Solar batteries are a deep cycle batteries, as the current flows from the battery in small quantities and evenly. This article represents; difference between automotive batteries and a solar batteries, a brief explanation of the different types of solar batteries and a comparison between them in terms of price, depth of discharge , service life, charge and discharge temperature, and energy density. The article also introduces an electrical representation of the battery, criteria that are taken into account when choosing the appropriate battery such as battery capacity, battery efficiency, depth of discharge of the battery, the time required for charging the batteries, and connect batteries in series and in parallel, where it is necessary to know this information to choose the appropriate battery for designing the solar systems.

Keywords: Flooded batteries, Gel batteries, AGM batteries, Lithium batteries, Electrical models of batteries, Battery energy, Series and parallel connection of batteries

INTRODUCTION

Automotive batteries also known as starting, lighting, and ignition (SLI) batteries have a very low internal resistance (50 milliohm) to produce a burst of energy. Low internal resistance is achieved by adding extra plates and the lead is applied in a sponge-like form that has the appearance of fine foam for maximum surface area (Figure 1). The plates are thin (1mm), which make the discharge is short

Figure 1: Starting battery

The deep-cycle batteries have an internal resistance that is ten times that of the automotive batteries which is achieved by making the lead plates thick (figure 2). These batteries are characterized by a maximum capacity and a high cycle count, and this makes it ideal for solar energy systems.

Figure 2: Deep-cycle battery

Solar Batteries are a deep cycle batteries used to store the direct current generated by the solar panels, which is converted into alternating current by the inverter to operate the various loads. The battery (12V) generally consists of (6) cells, each of these cells consists of, anode, cathode, and the conductive material (the electrolyte).

There are many types of solar batteries (figure 3), which differ among themselves in the materials from which the anode and cathode are made and the type of electrolyte.

Figure 3: The different types of solar batteries

The most common types of solar batteries are:

1 – lead-acid batteries that include:
• Liquid lead-acid batteries (flooded)
• Gel Batteries
• AGM batteries
2- Lithium batteries

The flooded batteries

It is the oldest type of batteries, the cheapest and the most widespread. It is called “liquid” because the conductive material between the anode and cathode plate is a liquid substance, which is sulfuric acid diluted with water, concentration ratio 3:1. Flooded batteries need maintenance, which includes replacing the acid and adding distilled water once or twice a month to compensate for the water evaporating from the batteries. Figure 4 demonstrates the components of a single cell of a flooded battery, which consists of sponge lead which represents the cathode electrode and a clip of lead, and behind the clip a plate of dioxide Lead, which represents the anode electrode, and these cells are immersed in acid diluted with water.

Figure 4: The liquid lead-acid battery

The gel battery

A gel battery has the same design and functionality as a traditional flooded battery. The gel battery differs from the liquid battery in that the conductive material contains silica in the electrolyte, which creates a gel-like substance. The gel battery is characterized by being suitable for use in many positions due to its stability and absence of any gases emitting from it, and it is a deep cycle battery.

Figure 5: The Jel battery

The AGM battery

A fiberglass material is placed between the anode and the cathode, which absorbs the electrolyte like a sponge and prevents it from leaking or evaporating. An AGM battery is a deep cycle discharge with the provision of mixing the sulfate back into the hydrogen gas, resulting in a reduction of the hydrogen released during the discharge process.

Figure 6: The AGM lead battery

The lithium battery

An anodes consist of graphite-based materials due to the low cost, wide spread, and the stability to accommodate the lithium insertion, but it carbon suffer from a low capacity, so in recent year, the carbon-based anode has been improved, and new types of anode materials, such as silicon, alloy, and metal oxides have been developed, which has improved the lifetime, capacity and performance of lithium batteries. Cathodes consist of a complex lithium compound material, such as LiCoO2 and LiFePO4 . Battery performance significantly differs with different cathodes. Cathode has been fabricated from lithium material blending with conductive material such as carbon due to low impedance because of high diffusion coefficient and high ionic conductivities compared with other materials compound. The electrolyte in lithium batteries includes three types liquid electrolyte, semisolid electrolyte, and solid-state electrolytes. Liquid electrolyte consists of lithium salts such as., LiBF4, LiPF6, LiN(CF3SO2)2, and LiBOB, which are dissolved in organic carbonates such as, ethylene carbonate, propylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and their mixtures. While, the semisolid electrolyte, and solid-state electrolyte are composed of lithium salts as the conducting salts and high-molecular-weight polymer matrices such as, polyvinylidene fluoride and poly(ethylene oxide).

Figure 7: Lithium-ion battery

Table 1: Comparison of different types of batteries

Lead-acid batteryJel batteryAGM batteryLithium battery
MaintenanceNeedNo needNo needNo need
Depth of discharge50%75%50%80%
Lifespan3-5 year6-8 year6-8 year20 year
Cost150$300$250$2000$
Charge temperature-0°C to 50°C-20°C to 50°C0°C to 50°C-0°C to 45°C
Discharge temperature-30°C to 70°C-40°C to 60°C-20°C to 60°C-20°C to 60°C
Storage temperature-20°C to 60°C-40°C to 60°C-20°C to 60°C-20°C to 60°C
Energy density30W .h/kg40W .h/kg50W .h/kg50-260W .h/kg
.

The electrical representation of the battery

To determine the power losses and the terminal voltage of the battery an electrical representation has been achieved by models based on thevenin network. The most simple model consists of a series resistor, RC network to describe basic charge transfer phenomenon, and open circuit voltage (Voc) which dependent on the state of charge (SOC) as obvious in figure 8(a). An enhancement for the batteries simulation can be done by adding a second RC branch as demonstrated in figure 8(b). The first RC branch represents short-term transient behavior, and the second RC branch represents long-term transient behavior

Figure 8: (a) Thevenin-based model, 1RC network (b)Thevenin based model, 2RC network

Voc(soc) = K0 + K1.(soc) + K2.(soc)2 + K3.(soc)3 + K4.(e)K5.SOC
Rcell (soc) = Rcell(0) + K5(e)K6.SOC
R1 (soc) = R1(0) + K7(e)K8.SOC
C1 (soc) = C1(0) + K9(e)K10.SOC
R2 (soc) = R2(0) + K11(e)K12.SOC
C2 (soc) = C2(0) + K13(e)K14.SOC

The coefficients K0, K1, K2, K3,…….,K14 depend on the respective cell type and are subjects of measurements.

For higher accuracy another RC network is proposed, in order to describe finally short- term, mid-term, long-term transient behavior. However, this makes the calculation of the associated capacitors and resistors much more complex, also studies have shown that 2RC model achieves good results, therefore it is proposed in simulations of electrical power grids

The most important information about batteries

The effectivity and performance of the battery depend on the following parameters:

1. Capacity of battery
2. Efficiency of battery
3. Depth of discharge

Battery capacity

The amount of energy that the battery can storage.

If a battery of (12V) has a capacity of (500 A.h) ,the energy can be storage with this battery is:

Energy = Voltage * Current * Time
= 12V * 500A.h
= 6000w.h

Battery efficiency

It is the ratio of the output energy from the battery to the input energy that the battery needs to charge.

If the energy that the battery needs to charge is (6000 wh) and the energy that can be obtained from this battery is (4800 wh), then the efficiency of this battery is:

Battery efficiency = (output energy) / (input energy) * 100%
= (4800 w.h) / (6000 w.h) * 100%
= 80%

Depth of Discharge (DOD)

It is the amount of capacity that can be obtained from the battery capacity.

If the depth of discharge is equal to 50% for a battery whose capacity is (60Ah), then the amount of capacity that can be get it from this battery is:

= 60A.h * 0.5
= 30Ah

The time required for charging the batteries

When the solar panels used to charge the battery, the time required for charging the battery is equal to (capacity of the battery / panel current)

If the panel(9A) used in charging the battery (200A.h) then the time that required to charge the battery = 200A.h. / 9A = 22.22h

Series and parallel connection of batteries

The batteries are connected in parallel or in series to obtain the required current and voltage

Figure 9 shows four batteries each one of (12V , 100Ah) connected in series and figure 10 obvious these four batteries connected in parallel.

Figure 9: Series connection of batteries
Figure 10: Parallel connection of batteries
CONCLUSION

At the present time, due to the rise in temperatures over the past years, it is necessary to take into account the impact of temperatures on the performance of the battery when choosing the appropriate battery for working on design. Temperature, have a significant effect on the performance, and the safety of the solar batteries. As the temperature of the battery increases the chemical reactions inside the battery also quicken, and increased storage capacity of the battery. It was found that an increase in temperature from 25°C to 45°C led to a 20% increase in maximum storage capacity, but an available capacity decreases over time, and the lifecycle of the battery is decreased over time. Lithium battery has better volume and weight, and is relatively cheaper to maintain but the initial cost is higher, and it is more temperature sensitive. Flooded batteries and Gel batteries are the most using in Iraq because they are more cost-effective, it’s price is just 1/4~1/6 of the lithium battery cost with an acceptable limits of the discharge depth (DOD) and it is suitable for high temperature work.

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Source URL & Publisher Item Identifier: https://www.researchgate.net/publication/360685047_Solar_Energy_Batteries-A_Critical_Review
Journal of Instrumentation and Innovation Sciences
e-ISSN: 2456-9860
Volume-6, Issue-3 (September-December, 2021)
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