Power Quality Blog

Method of Selecting the Most Important Power Lines in a Transmission and Distribution Network

Published by Dalibor VALEK, Radomir SCUREK,
Technical University of Ostrava, Faculty of Safety Engineering

Abstract: This article describes the method used to graph theory in security analysis. For the pursposes of this study, an environment is referred to here as a network of power lines and devices and a power grid is considered as a system of vertices which combine to make a network. Exploration of Fiedler´s theory will be applied herein to select the most important power lines for the entire network. Components related to these lines are logicly ordered and considered by the author´s modified analysis. This method has been improved and optimalized for risks related with illegal acts. Each power grid component has been connected with a variety of possible attacks and this device was gradually evaluated by five coefficients which takes values from 1 to 10. the level of risk was also assessed on the coefficient basis. In the last phase the most risky electricity network devices have been selected. Security measures have been proposed on the selected devices.

Streszczenie. W artykule opisano wykorzystanie teori grafów w analizie zabezpieczeń. Opracowano model sieciowy z wykorzystaniem teorii Fiedlera. Każdy z elementów sieci połaczono z różnymi formami ataku i opisano odpowiednimi wspolczynnikami. Na tej podstawie wybrano najbardziej ryzykowny wariant. Metoda wyboru najważniejszych linii w sieciach rozdzielczych i przesyłowych.

Key words: Security, matrix, power grid, graph partitioning, analysis.
Słowa kluczowe: bezpiecześtwo, sieci rozdzielcze i przesyłowe, teoria grafów

Introduction

Significant diversification of technical infrastructure from telegraph to internet has been realized within the last fifty years. From a wide point of view, the electric network, traffic network and communications network makeup the foundation of all prosperous companies. Usually these networks are based on a large amount of heterogeneous components characterized by complex dependencies and relationships between them. To ensure the structural integrity, effectivity and overall reliability of the network, illegal acts, security and terrorist protection must be considered.

There has been extreme growth in the consumption of electricity within last two decades. It was caused mainly by the social-economic changes in eastern Europe. Directly proportional to energy consumption has been the rise in newly-built and expanded nuclear power plants and power plants producing electricity from renewable sources.

After the disaster at Fukushima nuclear power plant, discussions started in Europe about the safety of nuclear power plants and the consequences of radioactive substance leaks. In the event of a radioactive leak, for instance, in Europe; there would be reduced production of electricity from nuclear power plants and significant reduction in annual electric energy production. There would, as well, be a very serious impact to energy security. This shortfall would be covered mainly by electricity produced from renewable energy sources and fossil fuel plants. If there would be a massive production of electricity from renewable energy sources, there would be many more opportunities to one-off the impacts to the transmission system.

These hard predictable impacts are caused mainly by renewable sources, mainly caused by wind power plants. Wind power plants work on the opposite principle than conventional power plants because they supply electricity only in case of wind blowing.

During these large fluctuations of electricity transfer, the important thing is to have the robust electricity system, such as electrical wiring, transformers or power plants, be resistant to component damage and to ensure these components are secured against the attacks of different crime or terrorist organizations. These attacks could cause blackout which could mean very severe consequences to human life and country‘s economy. Therefore it will be increasingly important to deal with security measures on devices across our electricity system. With selection of the most important components helps the graph theory which is described in this article.

Power grid

All devices that provide electricity from the production to the final customer are components where failure can result in a threat of the electricity supply to the final consumer. Among the main devices in electricity system is the electrical grid, electrical stations and electrical wiring.

Devices which are considered in the analysis are parts of the electrical grid and the power plants. The Electrical grid is the system of interconnected devices which is used for electricity transmission, transformation, and distribution. Further devices are used for metering, controlling while other devices are used for passive or active protection.

The transmission system is a system of devices which is used to transmit electrical energy with a voltage of 400 kV and 220 kV (depends on the country) from the manufacturers to the power nodes. The Distribution system transmits electrical energy with a voltage of 110 kV and 22 kV (depends on the country) from the transmission system to the customers. Customers in this context are the cities, the factories and the households.

Voltage is transmitted by the different types of wiring. Usage of a specific wiring type which depends on many factors such as quantity of the transmitted voltage, quantity of the transmitted electric current, voltage drop and so on. Wiring is most often outdoors but can also be cabled on the ground or under the ground, the latter option being the more expensive solution. Outdoor wiring has to be resistant to weather changes, extreme weather conditions, humidity and it must have sufficient mechanical solidity to the intention damage. Cable wiring is used in residential areas, industrial areas and in commercial buildings.

Outdoor wiring is carried by electrical pylons. Construction of electrical pylons can be made of wood, reinforced concrete, steel or aluminum alloy. Many types of electrical pylons exist and the difference is mainly it their design or construction.

Electrical pylons are designed from the construction point of view to resist extreme weather conditions and wind power. Instead of main cantilevered pylons, the grid consists of reinforced pylons to ensure stability in case wires break. Without this reinforcement, the main cantilevered pylons would not stay in the right position. These reinforcement pylons are made of special steel which can resists the climatic exposure [1].

Electrical stations [1] belong to electrical grid and are divided into transformations, switch stations and substations. Most of these devices are created by the substations which can be a single building or a bounded area. These substations take care of the input and output electricity flow and they are consisted of conductors, insulators and switch, safety or control devices [4]. In the buildings are mainly situated substations with voltage up to 35 kV and in the outdoor areas are situated substations with very high voltage over 52 kV.

Utilization of graph theory

Electrical wiring, telecommunication, transport infrastructure or others engineering network form system of nodes which make together graphs of different shapes with many degrees of complexity. Many networks are designed on the basis of landscape relief.

Graph characteristics

By the graphs we can represent a set of objects which we illustrate the interdependence of the various elements. Objects are assigned as vertices (power plants, transformers etc.) and their connections are called edges (transmission network, distribution network). Graph can be basically represented by simple model of a real network which emphasizes topological properties of objects and neglects their geometric properties. Graphs can be divided as a directed and undirected. Undirected graph is defined as G = (V,E) where V indicates vertices and E indicates edges. In case of undirected graph we do not consider the order of vertices which is used in case electrical network analysis [5].

Graph theory

Given a connected graph G it is possible possible to divide it into two smaller graphs having roughly the same number of edges and vertices and one shared edge. There are many methods to achieve this. One is the Spectral graph Partitioning. This method involves the usage of Laplacian matrix and divide vertices of a connected graph G into two subgraphs by usage of Laplacian matrix eigenvectors [3].

Adjacency matrix

Graph can be represented by the adjacency matrix. It is defined as G, A(G) = (a_i,_j)

This means that the adjacency matrix A represented by the graph G has for i-th row and j-th column value equal to 1 in case there is an edge between node i and j. Otherwise there is assigned value 0 for this position.

Degree matrix

Other matrix defined by graph G is degree matrix D(G) = (d_i,_j)

Degree matrix is a diagonal matrix, which provides us information about degrees of each vertex – number of edges entering or exiting from concrete vertex. This matrix is used together with the adjacency matrix to create a Laplacian matrix.

Laplacian matrix

Laplacian matrix is another way how to represent graph.

Matrix L(G) defined by graph G: L(G) = D(G) – A(G)

Laplacian matrix is difference between the degree matrix and adjacency matrix.

Spectral graph partitioning

Spectral graph partitioning is based on simple principle. For the graph G defined by vertices and edges G = (V,E) which has to be dividend is calculated Laplacian matrix L(G). By the spectral partitioning of Laplacian matrix are calculated eigenvalues and eigenvectors. Such an eigenvector which is related with the second smallest eigenvalue provides us required graph partitioning. This vector is named as a Fiedler´s vector.

Algorithm for finding the most important electrical wirings

Input parameter of the algorithm to calculate the most important electrical wirings is adjacency matrix. Values of single rows and columns of the adjacency matrix are the inputs given by user according to the power lines map. These values are loaded into Excel and then exported to computer program.

Computer program then calculate from adjacency matrix the degree matrix and Laplacian matrix followed by spectral graph partitioning and finding the required Fiedler´s vector. Distribution of final graph is determined by the sign related to Fiedler´s vector. Vertices with positive numbers are assigned to first part of graph and vertices with negative numbers are assigned to second part of graph. As a most important power line is then selected line which connects both parts of graph.

Example of spectral graph partitioning

To illustrate the above theory, a very simple example of spectral graph partitioning is shown.

Given a simple network of ten vertices and eleven edges. To the single vertices are assigned coordinates to display them in program according to its potential deployment in the territory.

According to picture 1 is designed adjacency matrix where single rows and columns are valued on the basis of existence the edges between single vertices.

From adjacency matrix, we can get according to defined equation the Laplacian matrix and Fiedler´s vector related to the second smallest eigenvalue.

Table 1. Adjacency matrix

Table 2. Laplacian matrix and related Fiedler´s vector

Fiedler´s vector divides the network into two approximately equal parts. Positive values of vertices are part of the first portion and negative values of vertices are part of the second portion. Red marked edges displayed on the picture number 2 represent the smallest amount of edges between both network parts. If we bring this situation into the electrical grid environment so we talk about electrical lines in which its failure mean blackout on the largest possible area.

Fig. 2 Selection of the smallest number of edges

Graph theory and its spectral graph partitioning is able to select the most important power lines. Then we can use conventional risk analysis only on the selected components of transmission and distribution network.

Analysis of attack on the electricity system

For consideration of vulnerability of single objects in the electricity system can be used modified method FMEA. This method was modified by the author to be more suitable for consideration of illegal acts. Further in the article, represented analysis named by shortcut FMEAIA (Failure Model and Effect Analysis of Illegal Acts) will be explored.

Classic FMEA method [2] is most commonly used in automotive industry to search and evaluation of potential defects in processes and products. This analysis stands on the subjective evaluation of its author. It means that analysis is highly dependent on the author’s experiences. Level of risk is determined by the multiplying of subjectively evaluated coefficients which are Occurrence, “O“, Severity, “S“, Detection, “D” and in additional in FMEAIA analysis is Appeal, “A” and Accessibility, “AA“.

Level of risk is calculated according to formula:

R = (O · S · D · A) / AA

Values of each coefficient may vary from 1 to 10 according to table 3.

In the evaluation process of the electricity system we should start with the selection of the most important places given from Spectral graph partitioning method described above. Selected devices placed on calculated territory should be considered in the analysis according to above described method of FMEAIA.

Example of analysis procedure according to FMEAIA method can be used as follows:

1. Distribution of system into the devices in terms of production and in term of the electricity wiring.

2. Distribution of devices into the buildings, pylons, wiring and further devices which are fixed with the ground (for example wind power plants, photovoltaic power plant etc.).

3. Distribution into the further smaller technical devices. Example of evaluation of several selected devices is shown in the table 4.

There are many types of attacks which could be considered but for purpose of this this article were used only few attacks to better illustration of FMEAIA method. On the basis of above analysis was found out that the highest value of risk has following attacks:

1. Incision of reinforced and steel pylon in transmission system.

2. Placing the bomb on the reinforced and steel pylon in terms of transmission system.

3. Pylon fusion by the welding machine.

Proposal of security measures

The procedure of risk evaluation is followed by the phase of security measures proposed on the highest risk evaluated devices. Security measures should be effective, economical and easily feasible. In general the principle ALARA should be considered in this case because this principle takes into account value of protected object and value of devices which protect this object.

For the minimalization of risk represented by the incision and fusion of reinforced and steel pylon in transmission system it is possible to apply measures which physically prevent contact with single pylon or make activity of attacker more uncomfortable and time prolonged. Among the suitable security measures we can classify:

• Usage of hardened steel for lower part of pylon construction.
• Concreting of pylon foundations up to height 2 meters above the ground level.
• The definition of perimeter around the pylon by the fence or barbed wire.

Proposed security measures are suitable to be realized mainly in terms of reinforced pylons which are included in grid of cantilevered pylons. Distance between cantilevered pylons of Donau type can be in suitable terrain 500 meters. For the type of attack with bomb usage can be applied similar security measures like in case of the incision and fusion the pylon but in case of bomb attack it depends on the level of energy which will be released within the explosion.

Table 3. Classification of each coefficient

Table 4.

Conclusions

This article describes components which are included in the electricity system and components used for the transmission and distribution of electric energy which is connected to the network. This network can be understood as vertices connected together with edges. In this case was used the graph spectral partitioning method to select the most important power lines. After this selection we are able to consider components included in calculated territory with conventional risk analysis. Applied risk analysis was modified by author from FMEA analysis to have more suitable analysis for the illegal acts. Two new coefficients were applied to this modified analysis – appeal and accessibility. Due to this modification this FMEAIA analysis is more suitable for illegal acts consideration. The result from the analysis is that the most risky devices in terms of illegal acts are mainly devices for transmission of electric energy which have much better accessibility then the devices for electricity production. Due to this conclusion, it is necessary to select the most important devices in specific terrain. Security measures to reduce the risk of attack to an acceptable level must be applied to these devices.

REFERENCES

[1] Internet encyclopedia of energy. Elektrizační soustavy [online].[cit. 2014-03-20]. Available from WWW
[2] Internet source Process Quality Management. FMEA – Failure Mode and Effect Analysis [online]. [cit.2014-03-20]. Available from WWW <URL: http://www.pqm.cz/NVCSS/fmeacs.html>
[3] SLININGER, B.: Fiedler´s Theory of Spectral Graph Partitioning, University of California, March 2013. Available from pdf format on WWW <URL:http://www.cs.ucdavis.edu/~bai/ECS231/returnsfinal/Slininger.pdf>
[5] OCHODKOVÁ, E.: Graph algorithms, Technical university of Ostrava, faculty of electrotechnics and informatics, Ostrava,

Authors: Dalibor VALEK, Radomir SCUREK , Technical university of Ostrava, Faculty of safety engineering, Lumírova 13/630, Ostrava – Výškovice, 70030 , dalikk@email.cz, radomir.scurek@vsb.cz

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 90 NR 11/2014. doi:10.12915/pe.2014.11.41

Overvoltage of Atmospheric Origin

Published by Electrical Installation Wiki, Chapter J. Overvoltage protection – Overvoltage of atmospheric origin

Overvoltage definitions

Overvoltage (in a system) any voltage between one phase conductor and earth or between phase conductors having a peak value exceeding the corresponding peak of the highest voltage for equipment.

Definition from the International Electrotechnical Vocabulary (IEV 604-03-09); available on [1]

Various types of overvoltage

An overvoltage is a voltage pulse or wave which is superimposed on the rated voltage of the network (see Fig. J1).

This type of overvoltage is characterized by (see Fig. J2):

• the rise time tf (in μs);
• the gradient S (in kV/μs).

An overvoltage disturbs equipment and produces electromagnetic radiation. Moreover, the duration of the overvoltage (T) causes an energy peak in the electric circuits which could destroy equipment.

**Fig. J2** – Main characteristics of an overvoltage

Four types of overvoltage can disturb electrical installations and loads:

• Switching surges: high-frequency overvoltages or burst disturbance (see Fig. J1) caused by a change in the steady state in an electrical network (during operation of switchgear).

• Power-frequency overvoltages: overvoltages of the same frequency as the network (50, 60 or 400 Hz) caused by a permanent change of state in the network (following a fault: insulation fault, breakdown of neutral conductor, etc.).

• Overvoltages caused by electrostatic discharge: very short overvoltages (a few nanoseconds) of very high frequency caused by the discharge of accumulated electric charges (for example, a person walking on a carpet with insulating soles is electrically charged with a voltage of several kilovolts).

• Overvoltages of atmospheric origin.

Overvoltage characteristics of atmospheric origin

Lightning strokes in a few figures: Lightning flashes produce an extremely large quantity of pulsed electrical energy (see Figure J4)

• of several thousand amperes (and several thousand volts),
• of high frequency (approximately 1 megahertz),
• of short duration (from a microsecond to a millisecond).

Between 2000 and 5000 storms are constantly undergoing formation throughout the world. These storms are accompanied by lightning strokes which represent a serious hazard for persons and equipment. Lightning flashes hit the ground at an average of 30 to 100 strokes per second, i.e. 3 billion lightning strokes each year.

The table in Figure J3 shows some lightning strike values with their related probability. As can be seen, 50% of lightning strokes have a current exceeding 35 kA and 5% a current exceeding 100 kA. The energy conveyed by the lightning stroke is therefore very high.

**Fig. J3** – Examples of lightning discharge values given by the IEC 62305-1 standard (2010 – Table A.3)

**Fig. J4** – Example of lightning current

Lightning also causes a large number of fires, mostly in agricultural areas (destroying houses or making them unfit for use). High-rise buildings are especially prone to lightning strokes.

Effects on electrical installations

Lightning damages electrical and electronic systems in particular: transformers, electricity meters and electrical appliances on both residential and industrial premises.

The cost of repairing the damage caused by lightning is very high. But it is very hard to assess the consequences of:

• disturbances caused to computers and telecommunication networks;
• faults generated in the running of programmable logic controller programs and control systems.

Moreover, the cost of operating losses may be far higher than the value of the equipment destroyed.

Lightning stroke impacts

Lightning is a high-frequency electrical phenomenon which causes overvoltages on all conductive items, especially on electrical cabling and equipment.

Lightning strokes can affect the electrical (and/or electronic) systems of a building in two ways:

• by direct impact of the lightning stroke on the building (see Fig. J5 a);
• by indirect impact of the lightning stroke on the building:

– A lightning stroke can fall on an overhead electric power line supplying a building (see Fig. J5 b). The overcurrent and overvoltage can spread several kilometres from the point of impact.

– A lightning stroke can fall near an electric power line (see Fig. J5 c). It is the electromagnetic radiation of the lightning current that produces a high current and an overvoltage on the electric power supply network. In the latter two cases, the hazardous currents and voltages are transmitted by the power supply network.

– A lightning stroke can fall near a building (see Fig. J5 d). The earth potential around the point of impact rises dangerously.

**Fig. J5** – Various types of lightning impact

In all cases, the consequences for electrical installations and loads can be dramatic.

**Fig. J6** – Consequence of a lightning stroke impact

The various modes of propagation

Common mode

Common-mode overvoltages appear between live conductors and earth: phase-to-earth or neutral-to-earth (see Fig. J7 ). They are dangerous especially for appliances whose frame is connected to earth due to risks of dielectric breakdown.

Differential mode

Differential-mode overvoltages appear between live conductors:

phase-to-phase or phase-to-neutral (see Fig. J8). They are especially dangerous for electronic equipment, sensitive hardware such as computer systems, etc.

Characterization of the lightning wave

Analysis of the phenomena allows definition of the types of lightning current and voltage waves.

• 2 types of current wave are considered by the IEC standards:
– 10/350 µs wave: to characterize the current waves from a direct lightning stroke (see Fig. J9);

• 8/20 µs wave: to characterize the current waves from an indirect lightning stroke (see Fig. J10).

These two types of lightning current wave are used to define tests on SPDs (IEC standard 61643-11) and equipment immunity to lightning currents.

The peak value of the current wave characterizes the intensity of the lightning stroke.
• The overvoltages created by lightning strokes are characterized by a 1.2/50 µs voltage wave (see Fig. J11).

This type of voltage wave is used to verify equipment’s withstand to overvoltages of atmospheric origin (impulse voltage as per IEC 61000-4-5).

Source URL: https://www.electrical-installation.org/enwiki/Overvoltage_of_atmospheric_origin

The Basics of Grounding Electrical Systems

Published by Lorenzo Mari, EE Power – Technical Articles: The Basics of Grounding Electrical Systems, April 10, 2020.

This article breaks down the complexities found in the fundamental field of grounding for the correct, faultless operation of electrical systems.

Grounding, or earthing, is a fundamental topic for the correct operation of electrical systems and devices. However, few people understand this matter or the reason it is used.

Grounding is a huge topic full of standards, practical rules, misconceptions, surprises, and some magic. The rules for grounding are quite difficult, and at times appear unclear.

This introductory article discusses the basic principles of grounding, provides an overview of the main grounding applications, and lays the basis for examining these applications from first to last.

What is Grounding?

In analyses of electrical installations, you will frequently see the terms ground, grounded and grounding. There are several formal definitions of these terms in different standards and codes. However, as its name suggests, grounding is a connection of the electrical system, electrical devices, and metal enclosures to the ground. It is also known as earthing, i.e., connection to the earth.

Even though non-grounded electrical systems do exist — either because they are excepted from grounding by codes or by operational reasons — most arrays are grounded in one way or another.

Is the Ground a Conductor of Electricity?

Although not the best, yes, the ground is an electrical conductor. It is used to carry fault currents, signals, and radio waves.

Groundwave propagation is particularly important on the low- and medium-frequency portion of the radio spectrum. There are underground low-frequency radio antennas that were developed during the early days of the 20th century. This electrical property becomes visible when lightning travels to and from the earth.

Ground connection. *Image courtesy of Pixabay.*

It is also important to know that sometimes the earth, as a conductor, is assumed to have a potential of zero and is taken as reference in many voltage measurements.

Power system grounding is very important since most faults involve ground. Then, it has a basic role in the protection of its components as well as safety for the operator. There are a variety of grounding techniques utilized for mooring an electrical system to the ground. Let’s look at each type next.

System Grounding

System grounding refers to the limit of the defined values the voltage has to the ground in every part of the electrical system. It connects the current-carrying point of the electrical system to the ground, i.e., the neutral of transformers and rotating equipment as well as lines.

Neutral Grounding

The art and science of neutral grounding are of paramount importance in this analysis. A choice of methods for grounding the neutral in transformers and rotating equipment has emerged to control the fault rate and transient disturbances, improving the continuity of service. The main types of neutral grounding are:

• Ungrounded: Grounding is not done on purpose, but the system is grounded because of its natural capacitance to the ground
• Through impedance
* Resistance — high-resistance, low resistance
* Reactance — high-reactance, resonant (high-reactance, as well), low reactance
• Solid (effective)

Most neutral grounding is solid. In this method the neutral is kept at ground potential, with the following advantages:

• Limits the voltage that will be applied to the equipment insulation. Recall that the materials used in the insulation have to be able to withstand the applied voltage;
• Limits system voltage to ground or equipment enclosures, under normal and fault conditions, increasing personnel safety;
• Minimizes potential transient overvoltages;
• Provides for a source of ground-fault current relaying, allowing fast fault clearing.

Other Grounding Methods

Other grounding methods are sometimes employed in systems 600V and below.

• Line grounding
* Zig-zag grounding transformer
* Corner-of-the-delta
• Mid-phase grounding

Equipment and Safety Grounding

People must be safeguarded because a small quantity of current circulating through the body may cause big damage or death.

Equipment grounding connects all non-current-carrying metal parts of the wiring system or apparatus to the ground. Examples include the cabinet of the service equipment, the frames of transformers and motors, the metal conduit and boxes, the metal shield of shielded cables, poles, towers, and more.

Equipment grounding limits voltage between non-current-carrying parts and between these parts and earth to a safe value, boosting protection. It also enables fast fault clearing.

Furthermore, to protect people and animals in the vicinity, power plants and substations are built on grounding mats. This practice minimizes electric shock potentials.

Equipment grounding. *Image courtesy of Pixabay.*

Bonding Equipment for Meeting Safety Standards

Bonding consists of the interconnection of all non-current-carrying metal parts of the installation to assure electrical continuity and conductivity. In this way, metal pieces are at a common and minimum potential above the ground. Codes require bonding in grounded and ungrounded arrays.

This interconnection behaves as a low impedance path that conducts ground-fault current safely and helps the swift operation of overcurrent protective devices on a grounded system as well as the operation of ground fault detectors on high-impedance grounded and on ungrounded systems.

Codes also deal with the bonding of metal building parts (non-electrical) that may be energized accidentally.

Protecting Against Static Electricity With Static Grounding

The purpose of controlling static charges is to protect people and property.

The friction between two surfaces of insulating materials can cause electrons to be transferred from one surface to the other, creating a potential difference of thousands of volts. This potential difference can cause static sparks, which are a source of fires and explosions.

Electronic components and equipment are incapable of withstanding the instantaneous power produced by static. There are several methods of safeguarding from the hazards of static electricity, grounding and bonding are two of them.

Static grounding provides a low-resistance ground connection, mitigating the static electricity generation. This practice prevents sparking between the bodies.

Hazardous locations are particularly important to grounding because they may have flammable or ignitable materials and sparks caused by static could ignite the atmosphere.

Electrostatic induction may also be the origin of transient conditions that triggers unintentional events in adjacent circuits, producing false relay operations, tripping of circuit breakers, or false signals in control circuits, to name a few.

Lightning Protection Grounding

Lightning protection plays a key role in the design and operation of electric power systems. In areas with frequent storms, lightning is the most common cause of outages and damage.

A lightning protection system intercepts or diverts lightning and provides a certain path for conducting the surges safely to the ground by adequate down conductors to grounding electrodes. Thus, it helps prevent disastrous events like fires, injuries, and deaths.

Lightning protection plays a key role in the design and operation of electric power systems. *Image courtesy of Pixabay.*

In addition to electric power systems, tall structures like smokestacks, tanks, towers, and buildings may require lightning protection systems, although not all objects or structures at a given site will need them. Again, hazardous locations are important because lightning produces sparks and the risk of fires and explosions is high.

Keep in mind that it is impossible to protect 100% of a structure from direct impacts, except by completely encapsulating it with metal.

Regarding transmission systems, a well-devised ground wire system can substantially reduce the outage rate because it will shield the phase conductors by receiving the direct impact of lightning strikes.

Protecting Against Lightning-Induced Overvoltages

Transient overvoltages are daily events in electric power systems. Switching is their main initiator, but switching surges are relatively easy to handle. However, lightning surges are the most severe and difficult to manage. They may increase the system voltage to many times the rated voltage. If the equipment in the power system is not protected against lightning surges, considerable damage will occur.

Over-running ground wires, besides shielding against direct lightning strokes, lessens the effects of induced surges.

Likewise, surge arresters are connected in shunt across the pieces of electrical equipment to divert transients to the ground.

Grounding Techniques for Protecting Electronic Equipment

Computers, communication systems, instrumentation, and control equipment require proper grounding for correct operation. More often than not, the safety grounding of equipment is the same for electronic equipment as it is for any other kind of apparatus.

Control room. *Image courtesy of Unsplash.*

Occasionally special grounding techniques, different from conventional safe grounding practices, are applied to electronic equipment, but care must be taken to avoid these techniques leading to unsafe practices.

Some electrical distribution systems for electronic equipment have been installed mistakenly in an intent to minimize the amount of electrical noise seen in the grounding system. But these installations do not comply with the rules of the National Electrical Code (NEC), jeopardizing personnel safety.

Protection of data circuits from disturbances or damage does not always involve grounding, although good grounding makes this protection easier.

A Review of Grounding Techniques and Uses

One of the most important but least understood considerations in the design of electrical systems is grounding.

Grounding consists of a low impedance connection to the earth. The ground is a poor conductor but good enough for this purpose.

Grounding has a key role in the correct operation of the electrical systems, either power or electronics, as well as protecting people.

• System grounding helps detect and clear ground faults.
• Equipment grounding provides a return path for ground-fault current.
• Bonding keeps electrical continuity and conductivity.
• Static grounding prevents the build-up of static electricity reducing the chance of fires or explosions where hazardous materials are handled.
• Lightning protection grounding helps protect structures and equipment from direct strikes.
• Overhead ground wires and surge arresters, connected to ground, can limit dangerous system overvoltages to safe values.

Fundamentally, grounding an electronic system is the same as grounding any electrical system. However, care must be taken to prevent special grounding techniques from generating hazardous conditions.

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/the-basics-of-grounding-electrical-systems/

Stator Earth Fault in 200MW Generator – A Case Study

Published by Mr. V.Veerapandi, Sr.Manager(ser)/SAS, Email:vpandi@bhelpswr.co.in

Introduction

One of 200 MW old machine was experienced stator earth fault and the unit was tripped when the load was around 154 MW. The stator earth fault was attended on emergency basis and the unit was brought back with minimum possible time. This article covers observations, analysis of the problem and also suggests remedial measures to avoid recurrence of such a problem.

Problem Faced

The 200MW TG set was brought back to grid after major overhauling of LP Turbine for a month and no work was carried out in Generator during this overhaul. Before synchronization of machine electrical test was carried out on generator and during the test, winding temperature marginal rise was observed in slot no.52. After confirmation of healthiness of stator winding during electrical test, the machine was synchronized and loaded to 154 MW. The set was tripped on stator earth fault after eight hours of service.

Observations made

During the fact finding and thorough inspection, it has been observed that the following Relays were operated

• 64G1- Earth fault protection.
• 64G2- Standby earth fault protection.

Problem Analysis

To find out the cause for the stator earth fault, the following activities were carried out .

IR value checked in Generator Bus duct + GT Primary + UAT Primary after isolating
of Generator from phase bushing. Value observed as 200 MΩ.
Generator phase to phase PT voltage measured at Primary and the value observed
as RY – 2.024 V, YB – 2.08 V, BR – 2.05 V.
Generator all three phase winding together Insulation Resistance value taken with
isolation of NGT and the value observed as – 770 KΩ (with 1 KV megger).
Generator individual phase winding IR value taken with isolation of NGT and the
value observed as R- 486 KΩ, Y- 28.7 MΩ,B- 46.5 MΩ
Generator individual Phase winding IR value recorded after cleaning of all bushing
and the value observed as R- 938 KΩ,Y- 28.28 MΩ,B- 46.5 MΩ
R-phase bushing was inspected visually and no abnormality was found and heating
of phase bushing from bus duct side done for 24 hrs and the megger value observed
as R-1.19 MΩ,Y-40.4 MΩ,B- 117 MΩ.
IR value of R-phase found improvement after cleaning & heating but the value
observed less than Y & B phase. Hence the existing R-phase bushing was replaced
by new one and the megger value observed as R-2.78 GΩ, Y-68.5 MΩ, B-79.7 MΩ

The above checks reveal that stator windings are in healthy condition and the following Electrical tests were carried out further confirmation.

• Generator DC high voltage applied for about 1 minute- R-12.7 KV,Y-12.6 KV,B- 12.6 KV and found ok.
• Tan delta test carried out at 10 KV and the results observed was in line with previous test values.
• DC hi pot test carried out at 16 KV for 1 minute and found ok.

All above electrical test results were found satisfactory and no significant fault was identified for the stator earth fault occurrence. Hence the following components & systems were visually checked for any abnormality.

Inspection of stator water system

Thorough visual inspection was made on stator core bar, stator water headers and all inlet and outlet Teflon hoses. Stator core bar and stator water headers are found in good condition. During the inspection of Teflon hose, a black spots was noticed in one of Teflon Tube connecting to bar no.52 Top and bar no.16 Bottom.

For further investigation, the particular Teflon hose was removed from the position and through inspection was made on Teflon hose. During inspection it was observed that the stator water inlet passageway of bar no16 bottom bar was completely blocked by small foreign material (rubber piece) as shown in Figure-1. The figure no.2 illustrates heavy carbon deposition inside the hose.

Figure 2 – Carbonization in coil no.16 B of stator water Teflon hose

Root cause analysis for ingress of foreign materials into the stator water flow path:

• Stator water for cooling the stator winding, phase connectors and bushing is circulated in a closed circuit (see figure 3&4).

• The stator water system comprises the Expansion Tank, Stator water pumps, stator water coolers, stator water filters and Magnetic filter.

• The operating pump draws the water from expansion tank and feed the water to winding through water coolers, stator water filters and magnetic filter and returns back to the expansion tank.

• Makeup water is supplied for stator water is from DM makeup water and this makeup water line pipe inner surface is coated with rubber lining. If any deterioration in the rubber lining will be collected at stator water filters which are installed in the system. Hence there is no chance to enter foreign materials during operation.

Water path of stator winding and Terminals

Possibility of entering foreign material into the stator water system

During Generator inspection, Stator winding hydraulic test is one of the main test to check the healthiness of stator winding & Teflon hose connections. During overhauling the complete water path is subjected to rigorous hydraulic test. For performing hydraulic test, stator water circuit is to be isolated from stator winding by removing the spool pieces in the circuit as shown in Figure.5

In normal operating condition these spool piece joints are to be made with Teflon/ PVC gasket for long run operation and sturdy life. No rubber gaskets are to be used in the stator water circuit and during hydraulic test also.

In this case by oversight, over size rubber gasket might have been used during hydraulic test and it might leads to scratch/cut due to sharp collar in the spool piece mating flanges (see figure no.5). The damage /scratch pieces may travelled through stator water header, stator water Teflon hose and stuck up in the winding inlet.

Figure 5 – Generator stator water spool piece arrangement

Corrective action done

• Teflon tubes of slot no 52 top, 16 bottom and 14 top were replaced by new ones.
• Reverse water flushing done for thorough cleaning of stator winding.
• Hot stator water flushing done by installing heaters in stator water line.
• Flow measurement done from inlet of 52T and outlet of 16B and ensured proper flow in both the bar.

Performance of the Machine after corrective action

• Insulation Resistance measurement done with stator water in circuit and value observed as R phase-1.06 MΩ, Y phase-1.04 MΩ, B phase-1.08 MΩ
• Tan Delta test at 10 KV AC of stator winding done with stator water in circuit and found ok.
• Hydro Test of stator winding done at 6Kg/cm² pressure for 12 Hrs and found ok.
• OCC & SCC test on generator carried out during rolling and found ok.

Suggestion / Recommendations

1) Stator water chemistry is to be monitored thoroughly and conductivity to be maintained as per design.
2) Vacuum in stator water damper tank may be checked for any leakage of dust from outside to inside during running of vacuum pump.
3) Stator winding temperature may be monitored continuously.
4) During overhaul Generator Hot water Flushing / Reverse flushing may be carried out and stator water filter may be checked for debris collected after flushing. (Procedure is available in Generator Service Manual).

Conclusion

During first rolling slot number 52T temperature was raised due to blockage of water flow path at Bar no.16B. The carbon deposition observed inside the Teflon hose no.16B might be the main cause for flash over and earth fault operation in stator.

Transmission System Harmonics Study

Published by Electrotek Concepts, Inc., PQSoft Case Study: Transmission System Harmonics Study, Document ID: PQS0501, Date: March 31, 2005.

Abstract: A utility presently operates two 16.8 MVAr capacitor banks in a 161/69kV substation. Supplied by this substation are residential, commercial, and industrial loads, including a steel manufacturing facility. For increased voltage support, the utility is investigating the installation of three additional 16.8 MVAr, 69kV capacitors to be installed within the substation. Concerns have been raised with regards to the change in resonance as a result of these additional capacitors. The proposed capacitors could result in resonance conditions that could be excited by the locally operated arc furnace, thus possibly producing high levels of voltage distortion at the substation. This case study presents some of the findings associated with a resonance study that included power quality monitoring and harmonic distortion simulations.

INTRODUCTION

The scope of this study is to investigate the impact a capacitor bank has on the harmonic impedance at a utility owned, 69kV substation. The utility presently operates two 16.8 MVAr, 69kV capacitor banks at the substation. Upgrade plans include the installation of three additional 16.8 MVAr banks. For regulation of the 69kV bus voltage, automatic capacitor controls will provide staging of capacitors in 16.8 MVAr increments, from 0 to 84 MVAr. Due to a locally operated arc furnace, there are concerns that the additional capacitance could result in a shift in resonance; thus possibly resulting in overvoltage conditions due to excitation from the arc furnace. Of particular interest is the impact on harmonic distortion levels that result due to the interaction (resonance) between the capacitors and system impedance.

The scope of this harmonic evaluation study includes:

− Frequency Response Evaluation
− Field Measurements
− Harmonic Distortion Evaluation

SYSTEM MODEL DESCRIPTION AND DEVELOPMENT

A one-line diagram of the substation and surrounding transmission system is shown in Figure 1 and Figure 2.

Figure 1 – Oneline Diagram of Utility Substation

Figure 2 – Oneline Diagram of Surrounding 69/138/161kV Transmission System

The system model was developed using Electrotek’s SuperHarm^® program. A three-phase model was used to simplify the analysis of transformer connections on harmonic cancellation. All transmission lines were modeled using a three-phase PI model, thus taking into consideration charging capacitance. Linear load was modeled to provide a realistic amount of system damping. Without these loads, the simulation results would be too conservative, especially at or near system resonance. Transformers were modeled using the SuperHarm three-phase transformer model. Transformer test report data was used when provided, and a default X/R ratio of 20 was used when resistances were not available. A typical capacitance of 16nF/mile was used to represent the charging MVAr of the overhead lines.

Analysis of Resonance Concerns for Harmonics

Frequency scans were performed at the substation to evaluate the affect of different capacitor configurations on system resonance. The scans demonstrate the expected harmonic voltage at the 69kV level per amp of harmonic current injected into the system. A summary of some of these cases is described in this section.

Figure 3 illustrates the affect of staging the substation capacitors (C7) from 16.8 MVAr to 84 MVAr on system resonance. For Case 15 (16.8 MVAr capacitor), a significant amount of resonance occurs between the 7^th and 9^th harmonic. As additional capacitance is added, this resonance is slightly decreased in magnitude. Unfortunately, this resonance is shifted to lower harmonic values (5^th harmonic for Case 17 and 3^rd harmonic for Case 19).

Figure 4 demonstrates the result of 3 levels of capacitance at the substation (C7 – 16.8, 50.4, and 84 MVAr) and capacitor banks C1, C2, C3, C4, and C5. Compared to Figure 3, the overall affect on resonance is a slightly decreased magnitude with a shift to higher harmonics.

The cases shown in Figure 5 are similar to that of Figure 3 with the exception of the added 24.4 MVAr capacitor C6. As illustrated in Figure 5, the interaction of the substation capacitors and C6 have a strong impact on the overall system resonance at the substation. Increased resonance occurs at the 9^th harmonic (Case 24) and 10^th (Case 23). High levels of resonance also occur between the 3^rd and 6^th harmonics for all five cases.

Figure 6 illustrates the affect of total capacitance (all capacitors switched in at the 13.2 kV level and C6) with the substation capacitors varied in 16.8 MVAr stages. The affect of additional capacitors at the 13.2 kV level have an overall positive effect on system resonance by reducing the magnitude of the lower order harmonic resonance considerably (compared to Figure 3 and Figure 5). High resonance conditions, however, do occur at the 11^th harmonic (Case 28) and the 9^th harmonic (Case 29).

Field Measurements – Power Quality Monitoring

A Dranetz-BMI PQNode 8020 was installed at the substation and power quality data was collected for a period of 30 days. The PQNode monitored three-phase voltage (line-to-line) and current on the 69kV bus supplying the feeder to the arc furnace.

This section will summarize the data collected.

During the 30 day monitoring period, the arc furnace was not fully operational, thus limiting the amount of usable data for statistically characterizing the harmonic content on the feeder. During the first week of monitoring, arc furnace was not operating 100% of the time, and unfortunately, it was switched completely offline for approximately two weeks for routine maintenance, thus limiting the amount of usable data to roughly two weeks.

Figure 7 and Figure 8 illustrate the RMS line current and line-to-line voltage measured on the arc furnace feeder for the 30 day period. The various operating conditions in which the furnace undergoes are most evident in the current measurements.

Figure 7 – Measured A-Phase RMS Line Current (7/15 – 8/14)

Figure 9 and Figure 10 illustrate the Phase A current THD from 8/8/99 to 8/14/99. Using the Electrotek/EPRI PQView^® software, a statistical analysis of the measured data was easily performed with the results shown in Figure 10. These calculations were also performed for both B and C phases.

Figure 10 – Histogram of Phase A Current THD

Using the data shown in Figure 10, PQView was used to calculate the average, CP05, and CP95 harmonic current values over this period. The statistical summary for the TDD and harmonics are shown in Figure 11.

Figure 11 – Average, CP05, and CP95 Harmonic Current Measured 8/8 – 8/14
*Note: Base Current CP99 – 240A*

Using the data in Figure 11, a representative current spectrum, shown in Figure 12, was chosen from the 12-cycle samples taken. The fundamental (and corresponding current spectrum) was scaled up from 150A to 240A, a factor of 1.6, to simulate CP99 conditions. By doing so, this creates a considerably conservative approach. This scaled current spectrum was then used to represent the nonlinear characteristics of the arc furnace load.

Figure 12 – Snapshot of Current Spectrum for Distortion Simulations

Harmonic Distortion Simulations

Using the system model developed for the resonance simulations, the scaled harmonic spectrum in Figure 12 was modeled in SuperHarm as a non-linear load connected directly to the utility substation 69kV bus. A conservative approach was taken in modeling the transmission system by removing the parallel resistance used with each source equivalent. This parallel resistance provides the damping necessary to estimate “actual” system conditions. By removing this resistance, “worse than actual” system conditions can be simulated, thus providing extremely conservative results. Note: Because this source damping is not included, a direct comparison with measured THD values cannot be performed.

Based upon the frequency scan results found in Figure 3 through Figure 6, various worst case system configurations were considered for calculating the expected THD as the additional 50.4 MVAr capacitors are added at the utility substation. These cases are summarized in Table 1. For each of the below cases, the amount of capacitance at the substation is increased from the presently operating 33.6 MVAr to 84 MVAr in increments of 16.8 MVAr.

The capacitor configurations considered in Cases 1 through 8 are not considered realistic scenarios. However, to simulate the absolute worst case conditions, they were included.

Table 1 – Summary of Harmonic Distortion Simulations Performed

Figure 13 – Cases 1 – 4: Voltage THD Simulation Results

As seen in Figure 13, with only capacitors operational at the substation, the maximum THD value calculated is found in Case 2, with less than 1.8% THD.

Figure 14 – Cases 5 – 8: Voltage THD Simulation Results

With the C6 24.4 MVAr capacitor online (Figure 14), the maximum THD values are found in Case 5, which represents the existing substation capacitor configuration. The maximum THD value reaches 1.6%, with the 5^th harmonic topping at approximately 1.5%. As capacitance is increased at the substation, it is shown that the THD values are actually reduced in magnitude.

Figure 15 – Cases 9 – 12: Voltage THD Simulation Results

Figure 15 represents typical system capacitor configurations with the addition of C1, C2, C3, C4, C5, and C6 capacitors in service. The additional capacitance reduces the THD to approximately less than 0.5% for all cases. This reduction in THD is due to the reduced driving point impedance as the additional capacitors are switched into service (see Figure 5 and Figure 6).

Similar results are found with the C6 24.4 MVAr capacitor taken out of the circuit (Figure 16). The maximum THD value reaches approximately 0.5%.

Figure 16 – Cases 13 – 16: Voltage THD Simulation Results

SUMMARY

The purpose of this study was to investigate the possible resonance conditions that would occur with the introduction of 3 additional 16.8 MVAr capacitors at a utility-owned substation and to determine if any harmonic distortion limits would be exceeded due to excitation from a locally operated arc furnace. A complete three-phase model of the substation and surrounding transmission system was modeled in SuperHarm and the frequency response characteristics were examined for various system capacitor configurations.

A wide range of capacitor combinations was simulated using SuperHarm. As shown in Figure 3 through Figure 6, the introduction of the additional 3 capacitor banks at the substation reduces the magnitude of the present harmonic resonance. However, as a result of the added MVAr introduced into the system, the resonance frequency is also reduced to lower order harmonic values. This shifting of harmonic resonance has resulted in peaks occurring around the 3^rd, 5^th, and 7^th harmonics.

With the locally connected arc furnace, it was quite possible for the furnace to excite this lower-order resonance, thus resulting in extreme overvoltages occurring at the substation. Due to the shifting of resonance to the 3^rd, 5^th, and 7^th harmonics, it was determined that additional investigation would be required before conclusions with regards to possibly harmonic distortion problems could be made.

The second portion of this study involved monitoring the 69kV bus for possible harmonic currents that could excite the resonant conditions as a result of the additional capacitor bank installation. To simulate the affects of the arc furnace, the measured data was then incorporated into the three-phase model developed previously.

The harmonic distortion simulations proved that the amount of harmonic current being drawn by the arc furnace was not enough to cause problematic voltage distortion levels. The maximum voltage distortion levels were calculated at approximately 1.8%, well below the 5% THD limit specified in IEEE Std. 519-1992. Therefore, the additional three capacitor banks will not cause excessive voltage distortion at the utility substation and surrounding transmission system.

REFERENCES

1. IEEE Std. 519-1992, “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.”

Case Study on Fault Analysis Using PMU

Published by Prithwish Mukhopadhyay, General Manager, prithwish.mukhopadhyay@posoco.in,
Rajkumar Anumasula, Senior Engineer, rajkumar@posoco.in,
Abhimanyu Gartia, Deputy General Manager, agartia@posoco.in,
Chandan Kumar, Engineer, chandan.wrldc@posoco.in,
Pushpa Seshadri, Chief Manager, pushpa_seshadri@posoco.in,
Sunil Patil, Engineer, skpatil@posoco.in , WRLDC, Mumbai, India

Abstract — Synchrophasors are used extensively in Indian Grid to detect and analyze faults occurring in the system. This paper majorly discusses the case studies of power system faults that occurred in the Indian grid and its identification and classification using data obtained from Synchrophasors. The types of faults identified include symmetrical and unsymmetrical faults in power system. Further in case of single phase to ground faults, the successful, unsuccessful and non -operation of autoreclosure has been verified. Data from Phasor measurement units (PMUs) helped in identifying the fault recovery time and operation of autoreclosure and provided a strong tool for monitoring the status of the protection system in the grid. A comparative analysis of the disturbance recorder files and the synchrophasor data has also been discussed in this paper. The comparative analysis is used for validation of the DR and Synchrophasor data. This paper has led to the development of disturbance analysis tool for fault analysis using both DR and Synchrophasor data which will ease the system operator decision making ability while taking decision during a contingency in the system.

Index Terms—Synchrophasor, Faults, Autoreclosure, Indian grid, Disturbance recorder.

I. INTRODUCTION

Power system operation in India is complex due to disparity in geographical distribution of resources and loads, network complexity with rapid changes in network configuration and increasing combination of UHV, EHV, HVDC lines and FACTS devices in the system. Synchrophasor measurement units also known as “Phasor measurement units (PMUs) have become a very effective tool for system monitoring and analysis.

The Indian Grid is a large synchronous grid which constitutes of Eastern, Northern, North-Eastern, Southern and Western grids. Synchrophasor measurement units have been deployed in each regional grid under the pilot projects for monitoring and development of analytics which will help in system operation [1-2]. This has proved to be a boon for understanding the behavior of Indian power system. The high resolution data obtained from PMUs (25 samples/sec) with accurate time stamping provides a continuous snapshot of the system. Among the various applications explored from this technology, fault analysis is used extensively for understanding the sequence of events and also for strengthening protection schemes. It has helped in post facto analysis and real time detection of faults occurring in the grid.

In the Western regional grid, ,PMUs are placed strategically covering all types of buses like generator buses, intermediate pooling stations, load centers, HVDC substations etc. [2]. The potential transformer (PT) of the Bus and current transformer (CT) of the feeders have been used as analog inputs to the PMUs. Prior to Synchrophasor technology implementation in India, the identification, detection and classification of fault was confirmed from the Disturbance recorder (DR) files, Sequence of events (SOE) from the Relays and Sub-station SCADA. Obtaining these details from various substations resulted in delayed analysis and information about the power system phenomenon. With the PMU data available at control centers, the fault can be detected identified and classified in real time by observing the trends in various parameters of the system. This has saved a lot of time for analysis as the nature and type of fault can be confirmed at the control center itself although the DR and SOE are still used for its confirmation.

This paper is focused on the fault analysis of EHV transmission lines and bus bar, their characterization using synchrophasor and their validation using disturbance recorder data. Section II illustrates the single phase fault in the Indian power system followed by the Sections III and IV where phase to phase and three phase fault have been discussed. Section V discusses the validation of observations from Synchrophasor data with DR for power system. Section VI concludes the paper.

II. SINGLE PHASE TO GROUND FAULT

Single phase to ground fault is the most common type of fault in power system. Such faults are very common in transmission lines and can be either temporary/transient or permanent in nature. The ground path provided by trees, fogs etc. are temporary in nature and these ground paths gets cleared immediately. The permanent nature fault appears in case the conductor breaks down (in case of transmission line), current or potential transformer bushing burst (in sub-station) or by some other permanent grounding path. To avoid permanent isolation of an element in case of a transient fault, auto-reclosure (A/R) for single phase faults is provided for 400 kV and higher level EHV transmission lines On the basis of that, four cases of phase to ground fault are discussed here which are:

1. Single phase to earth fault on transmission line with successful A/R
2. Single phase to earth fault on transmission line with unsuccessful A/R
3. Single phase to earth fault on transmission line with no A/R.
4. Single phase to earth fault on Bus bar.

The first case of single phase to earth fault with successful auto-reclosure is shown in figure 1 and figure 2 where a fault on B phase to earth occurred on 400 kV Jabalpur-Vindhyachal circuit 4. It can be observed that when a single phase fault occur on transmission line the voltage of faulty phase dips to a lower value while remaining phases will observe a smaller dip due to unbalance in the system as the fault is being fed by the bus whose voltage has been as shown in figure 1.

Figure 1. Voltage of 400 kv Jabalpur bus from Synchrophasor data during single phase to ground fault on 400 kV Vindhyachal-Jabalpur circuit 4.

Figure 2. Current of Vindhyachal-Jabalpur circuit 4 observed from Jabalpur end from Synchrophasor data

Figure 2 shows how the fault current is increased in the faulty phase of the circuit indicating fault feeding from the affected phase. The line protection operated in zone 1 from both end and the faulty phase got isolated from both end resulting in recovery of voltage in faulty phase while current in the phase became zero. This has resulted in A/R initiation for the faulty phase and after 1 second, that phase got reclosed and further no fault was sensed resulting in successful auto reclosure. The line came back in service and the system again came to a balanced state. The voltage channel in PMU is obtained from the Bus PT so it will not become zero but due to the fault, the voltage of bus will get unbalanced and will observe a large dip in faulty phase.

Second case is the single phase fault with unsuccessful auto re-closure of the line. In cases where the fault is not of transient nature, permanent tripping of the line is required. One such case was observed in 400 kV Solapur Parli circuit 1. Figure 3 shows the voltage of 400 kV Solapur Bus during the fault while figure 4 gives the current observed in the faulty circuit from Solapur end.

Figure 3. Voltage of 400 kV Solapur bus during single phase to ground fault on 400 kV Solapur Parli 1 circuitfrom Synchrophasor data

Figure 4. Current of 400 kV Solapur parli citcuit 1 observed from Solapur end from Synchrophasor data

B phase to earth fault occurred in the 400 kV Solapur Parli circuit 1 due to the heavy rainfall in the area. As observed from the voltage of 400 kV Solapur in figure 3, the voltage of faulty phase dipped to a very low value due to feeding of the fault through the bus. The fault current as shown in figure 4 increased and with carrier aided zone 1 protection, the line tripped in the faulty phase from both ends. A/R was initiated after one second. The faulty phase of the circuit got reclosed but due to permanent nature of fault, all the three phases tripped thus isolating the faulty line.

There are cases in which the Auto-reclosure did not occur on the line in case of single phase to ground fault. This is mainly due to issue with the carrier communication or relay. Such cases are very much essential to be analyzed and informed to the concerned utility to rectify it at the earliest. One such case has been shown in figure 5 and 6 where the auto reclosure activity was not observed during single phase to earth fault on 400 kV Korba-Bhatapara.

Figure 5. Voltage of 400 kV Korba bus during single phase to ground fault on 400 kV Korba Bhatapar circuit from Synchrophasor data

On 400 kV Korba Bhatapra circuit, Y phase to earth fault occurred on the circuit. As observed from the figure 5 , the faulty phase voltage dipped at Korba and the fault got cleared with operation of line protection [2]. While figure 6 shows the current of the feeder from Korba end which confirms that the all the three phases of the line tripped rather than single phase with A/R initiation. This was informed to the concern utility and rectified.

Figure 6. Current of 400 kV Korba Bhatapara citcuit observed from Korba end from Synchrophasor data

The last case in the section of single phase to ground fault is associated with bus fault, which in general occur due to problem with CT/PT bursting. One such case of bus fault at 220 kV level was observed by the synchrophasor unit installed at 400 kV nearby Bus is shown in figure 7.

Figure 7. Voltage of 400 kV Boisar bus during single phase to ground fault on 220 kv Padghe Bus II from Synchrophasor data

From Figure 7, it can be observed that Y phase to ground fault occurred at Padghe Sub-station on 220 kV Bus II due to bursting of Y phase Bus PT. This resulted in fault feeding by the nearby buses resulting in voltage dip as observed in figure 7. The fault got cleared in 160 ms with the operation of 220 kV Bus bar protection of Bus II.

With this it can be understood that how easily single phase fault can be analyzed at control centre without any DR/EL from the sub-station with the help of synchrophasor data.

III. PHASE TO PHASE FAULT

The second category of fault in the power system is phase to phase fault. In such cases, the fault may involve a ground path or may not. Such types of faults are rare in nature and generally found in transmission lines.

The case as shown in figure 8 and 9 is for the R-Y Phase fault on 400 kV Dehgam-Gandhar circuit 2. It can be observed from figure 8 that the phase voltages of the faulty phases have dipped to a lower value till the fault got cleared with the tripping of line. The current observed from Dehgam for this line show that the fault current in the faulty phases has increased drastically and it got cleared with tripping of the line from both end on zone 2 protection.

Figure 8. Voltage of 400 kV Dehgam bus during Phase to Phase fault on 400 kV Dehgam-Gandhar circuit 2 from Synchrophasor data

Figure 9. Current of 400 kV Dehgam- Gandhar citcuit 2 observed from Dehgam end from Synchrophasor data

It can be observed that how easily the phase to phase fault can be detected with the synchrophasor data. This has reduced the time taken for analysis of any complex fault in power system.

IV. THREE PHASE FAULT

One of the most severe and rare occurring fault in the power system is three phase fault. This fault may or may not involve ground. The severity is very high and may result in commutation failure of nearby HVDC and stalling of AC motor in the nearby area.

Two cases are considered here for example, one at a bus and other on the transmission line. Figure 10 shows the voltage of nearby bus during a three phase fault on 220 kV Bus 1 at Padghe. Voltage has dipped in all the three phases as the fault was being fed by the nearby buses. Figure 11 shows the current in the transmission line connecting the two sub-stations, which has increased as the Boisar Bus is feeding the fault at padghe. The fault got cleared after the operation of bus bar protection.

Figure 10. Voltage of 400 kV Boisar bus during three phase fault on 220 kV Dehgam-Gandhar circuit 2 from Synchrophasor data.

Figure 11. Current of 400 kV Boisar-Padghe citcuit duing the three phase bus fault on 220 kV Padghe Bus 1 from Synchrophasor data

The second case is of the three phase fault on a transmission line. Figure12 and 13 shows the synchorphasor data for the three phase fault on 400 kV Raipur Wardha circuit 1. From figure 13 it can be seen that during three phase fault on 400 kV Raipur wardha 1 circuit , the voltage of the Raipur bus I all the three phases dipped to a lower value and recoverd with tripping of circuit on zone 1 line protection.

Figure 12. Voltage of 400 kV Raipur bus during three phase fault on 400 kv Raipur –Wardha circuit 1 from Synchrophasor data.

Figure 13. Current of 400 kV Raipur Bhadrawati circuit 1 observed from Raipur end from Synchrophasor data

With this all the three types of fault has been discussed with the help of synchrophasor data. This has shown the importance of synchrophasor measurement units in fault analysis and event detection at control centers. In the next section the validation of synchrophasor data with the disturbance recorder file is discussed for determining up to what extent it can be used and what are their limitations.

V. VALIDATION OF SYNCHROPHASOR DATA FOR FAULT ANALYSIS AND ITS LIMITATION

Synchrophasor data has come out as a very effective tool for analyzing faults in power system in real time. But the synchorphasor data has its own limitation compared to the disturbance recorder file which has higher sampling rate (kHz). Figure 14, 15, 16 shows the DR for the first three events discussed in section II in case of single phase to earth fault with successful A/R, unsuccessful A/R and no A/R respectively using SIGRA [3]. The synchorphasor are the fundamental frequency component calculated using Discrete Fourier Transform (DFT). So higher harmonic are being neglected which may result in different fault current observed from synchrophasor data and actual data from fault recorder.

Figure 14. Current from the DR of 400 kV Jabalpur -Vindhyachal circuit 4 from Jabalpur end for the single phase fault (successful A/R).

Figure 15. Current from the DR of 400 kV Solapur -Parli circuit 1 from Solapur end for the single phase fault (unsuccessful A/R).

Figure 16. Current from the DR of 400 kV Korba Bhatapara circuit 1 from Korba end for the single phase fault (No A/R)

The validation analysis of the three cases is shown in table 1. Here major two parameters i.e. fault current value and fault clearing time has been tabulated for all the three cases.

Table I. Validation of DR and Synchrophasor Measurement During Fault

It can be observed that fault clearing time from DR and PMU has slight difference which is attributed to mainly three factors:

1. Reporting rate of PMU (here at every 40 ms)
2. DFT Window length (generally varies from two cycles to 10 cycles).
3. Time stamping of phasor (time stamping at the starting/middle/end of DFT calculation window) [4].

This in general result in a ±40 ms error in the time for fault recovery time [2]. This can be improved with increasing the reporting rate of the PMU.

Also, when the fault current is analyzed it can be observed that there is a difference between the maximum fault current from synchrophasor measurement and disturbance recorder, both of which are taken from the same CT of the feeder. This can be due to various reasons and out of these major two are:

1. PMU uses measurement core while the DR uses the protection core of the CT which is having higher accuracy. (Although PMU can be made to use protection core of the CT also, but in India measurement core of the CT is used)

2. The second reason is the contribution of higher harmonics during faults.

During faults, the fault current is having DC, Fundamental frequency component and higher harmonic component. DR measured currents are having all these components. While synchrophasor is giving only the fundamental frequency component resulting in lower current values. Figure 17 shows the various components in the faulty phase current for the three cases discussed for validation in this section. It can be observed that the Current associated with fundamental component of the frequency are very close to what being observed by the synchrophasor units.

Figure 17. Various compoent of Current when maxium fault current appear in the three cases 1. Successfulr A/R 2. Unsuccesdful A/R and 3. No A/R.

Thus, it can be inferred that, synchrophasor data has given a very good insight into the various aspect of fault observed in the power system. Synchrophasor data can be used for event detection engine at control centers in real time by setting various thresholds to detect the individual types of faults. It also helps in quicker detection of A/R in case it has been enabled during single/three phase fault in zone 1 for transmission line. The extent of fault current signifies the severity of fault to the real time operator in taking decision for charging of the faulty line. Synchrophasor data in combination with field records will make fault analysis easier.

VI. CONCLUSION

This paper has discussed the various types of faults as observed from the synchrophasor measurements. This has helped in finding the various characteristic of different types of faults in power system. The paper has led the basis of disturbance analysis tool development for analysis of the fault in offline mode with the helped of Synchphasor and DR data. Also has set path to the course of event detection engine in real time using synchrophasor data stream. Such development will help real time system operator to access the severity of power system faults and take corrective action to reduce its after effects.

ACKNOWLEDGMENT: The authors acknowledge the guidance and support given by managements of POSOCO as well as PGCIL and for permitting the publication of this paper. The authors are sincerely thankful to Shri. P.Pentayya, Ex General Manager, WRLDC for his guidance during Synchrophasor Pilot project implementation in western region. The authors are also thankful to WRLDC personnel for their support. The views expressed in this paper are of authors and not necessarily that of the organizations they represent.

REFERENCES

[1] “Synchrophasors Initiative in India,” POSOCO, New Delhi, Tech.Rep. July 2012
[2] “Synchrophasors Initiative in India,” POSOCO, New Delhi, Tech.Rep. December 2013
[3] “User Manual –Fault Record Analysis SIGRA 4 ”, Siemens
[4] IEEE Standard for Synchrophasor Measurements for Power Systems, IEEE Std C37.118.1^TM-2011,Dec.2011

Source & Publisher Item Identifier: 978-1-4799-5141-3/14/$31.00 @ 2014 IEEE

AC Equipment Grounding: Creating a Safe Fault Current Path to Ground

Published by Lorenzo Mari, EE Power – Technical Articles: AC Equipment Grounding: Creating a Safe Fault Current Path to Ground, August 07, 2020.

Learn about the merits of the equipment grounding to protect people in faulted AC power systems.

Electrical equipment casings should be at earth potential under normal conditions. When a fault occurs in an ungrounded casing, it will remain energized, creating a threat to people’s safety. If you are grounded and touch that housing, a dangerous current will flow through your body.

The magnitude of this current may not be enough to activate the circuit protection, but it could be enough to kill someone. Equipment grounding provides a secure, low impedance path to ground, allowing a sufficient amount of current to flow to trip a breaker or blow a fuse, removing the hazard.

In the last article regarding AC equipment grounding, we analyzed the dangers of electric shocks. This article will focus on proper safety measures as noted by the National Electrical Code (NEC).

Equipment Grounding Regulations and Language

The basis of this article is the National Electrical Code (The NEC). The NEC is a set of rules and regulations for the safe installation of electrical equipment and wiring used in the United States and other countries. The methods discussed in this article are mainly for low voltage power, heat, and light.

One of the reasons for the misunderstandings that appear when handling the subject of equipment grounding is the use of inappropriate terms. To avoid this situation, everyone should speak the same language, and the recommendation is to use the terms defined in the NEC, Articles 100 – Definitions – and 250 – Grounding.

The use of unofficial or ambiguous terminology can cause confusion and increases the chance of making mistakes in grounding installations.

Three essential terms to be aware of are “grounded conductor,” “equipment grounding conductor,” and “grounding electrode conductor.”

The NEC is a complex set of standards. This article will give a general overview. Readers should still consult the NEC before working with AC equipment grounding techniques.

Equipment Grounding Basics

The term equipment grounding refers to the grounding of parts of the electric system that do not regularly conduct electricity, like service equipment, the frames of appliances, metal conduits, and metal shields of shielded cable. The primary purpose of equipment grounding is safety.

Grounding keeps all equipment at the same potential, decreasing the possibility of harming a person in contact with two metal parts that could have different potentials when electrical failures occur. This permanent joining of metallic parts to form an electrically conductive path is known as bonding.

When the electrical systems fail, people’s lives may depend on a proper grounding installation. Equipment grounding provides a dependable, low impedance path to the fault current for the correct operation of protective devices.

Equipment grounding and system grounding come together only at the power source, such as the service equipment.

Basic Concepts of Grounding for Safety

The NEC defines service as the conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served. But the conductors and equipment are feeders, and not a service, when other than the serving utility supplies the electric energy.

In this article, we’ll use the terms service, service equipment, and service entrance. The service equipment may comprise one or more main fused switches or circuit breakers.

Figure 1 shows a 3-wire, single-phase overhead service to a building. It starts at the service point on the load side of the transformer. The transformer usually belongs to the utility company, but may also be a customer’s property.

A load is any piece of equipment connected to the wiring that consumes power, like a lamp, a motor, a fridge, or a microwave oven.

Figure 1. Single-phase, 3-wire service. Conduit grounded incidentally.

The power comes into the premises over three conductors, the service-entrance conductors. The conductors marked a and b are ungrounded and called phase conductors or hot wires.

The conductor marked N, is the neutral or grounded conductor, grounded at the service equipment and the transformer. The voltage between a or b and the neutral N is 120 V; the voltage between a and b is 240 V.

A person touching a and b will receive a 240 V shock. Touching a or b and N will cause a 120 V shock. Note that N is grounded, and the person will also receive a 120 V shock by touching a or b while standing on the ground.

The utility company grounds the neutral at the transformer for three reasons:

• Connect the neutral to 0 V, ensuring that VaN = VbN = 120 V
• Protect the load side from high voltages resulting from a primary-to-secondary short, a higher voltage line falling on the service-entrance conductors or a nearby lightning strike
• Make sure that the neutral is grounded even if the connection at the service equipment is missing

Figure 1 also shows phase a supplying a single-phase, 2-wire, 120 V load. Here, conductor N is not a neutral but just a grounded conductor. The reason is that a neutral carries the unbalanced current of phases a and b, and in this circuit, the conductor takes all phase a (or load) current.

The conduit connection to the ground is accidental or incidental, meaning that the grounding is not by design. Incidental grounding happens when attaching the conduit to a support connected to the earth. So there is very high resistance in the ground circuit between the conduit and the neutral ground connection.

If the phase conductor insulation fails (ground fault), the fault current will flow through a high resistance circuit. It will not be of the magnitude necessary to operate the circuit protection. Then the conduit will be energized at 120 V, which will pose a risky condition for people who can touch it.

The conduit shown in Figure 1 represents any ungrounded metallic frame of electrical equipment.

Figure 2 shows an ungrounded equipment frame under ground fault conditions. The protection devices do not clear the fault, allowing the structure to sustain 120V to ground. A person’s body in contact with a grounded element touches the frame and becomes part of the fault circuit. The dotted line shows the path the fault current follows through the body, a condition that will probably cause a sharp shock and perhaps death.

Figure 2. Shock hazard from an ungrounded equipment frame.

Figure 3 shows the same ground fault but this time with a grounded equipment frame. Here, a ground-fault current of sufficient magnitude flows to operate the circuit protection adequately, and the structure, at ground potential, does not pose a shock risk.

Figure 3. Ground-fault current path with the equipment frame grounded

Do not connect the equipment frame only to the physical ground, but in the way indicated by Article 250 of the NEC as described in the following paragraphs. The NEC prohibits using the earth as an effective fault-current path.

The Grounded Conductor

A grounded conductor is a system or circuit conductor grounded intentionally. A fundamental requirement to bear in mind is that a grounded conductor is never interrupted by a circuit breaker, switch, fuse, or another device unless this device opens the grounded and ungrounded conductors simultaneously.

The grounded conductor runs from the source to the load without switches or overcurrent protection, although it may be spliced or connected to terminals.

Once again, the grounded conductor, whether it is a neutral or not, is grounded twice. Once to the grounding electrode system at the service equipment, and once to the transformer serving the property.

No harm follows if someone touches an exposed grounded conductor or a component of the grounding electrode system, as described in the following paragraphs. Every time you touch a faucet or a water pipe, you are in contact with the grounded conductor.

The Equipment Grounding Conductor

Figure 4 shows a protective grounding conductor connected from the equipment frame to the system grounded conductor. This conductor is NEC’s equipment grounding conductor (EGC). It connects non–current-carrying metal parts of equipment together and to the system grounded conductor or the grounding electrode conductor, or both.

Figure 4. Ground-fault current path through EGC

The EGC does not carry current during regular operation, but it does when there are damages or defects in the wiring system or the connected pieces of equipment. The dotted line shows the path of the fault current. Since this path has a low resistance, the fault current will have the required magnitude to operate the protective devices.

The NEC recognizes that the EGC also performs bonding.

When using metal conduit or cable with a metal shield, the conduit or shield serves as the equipment grounding conductor. Figure 5 shows a metallic conduit fulfilling this task.

Although the metallic conduit provides an electrical path, loose connections could interrupt this grounding conductor. The recommended practice is to run an additional wire in each conduit to offer a continuous and reliable second route. By doing this, the fault current divides between the conduit and the additional wire.

Adding a wire does not decrease the need for electrical continuity between the housings and the conduits. Either way, the current will flow through the metal components and, if there are loose connections with high resistance, arcs and heating can occur with the risk of fire.

Figure 6 shows the EGC connections, in a 3-wire, single-phase service. The EGC goes from the equipment frame to the grounding terminal bar in the service equipment, bonded to its enclosure.

Figure 6. EGC connections, in a 3-wire, single-phase service

The Main Bonding Jumper

A bonding jumper connects two or more portions of the equipment grounding conductor throughout the electrical system, to ensure a ground-fault current path. But, the main bonding jumper, one of the most critical elements in the grounding system, connects the equipment grounding conductor, the service grounded conductor, and the grounding electrode conductor at the service, closing the fault circuit. It carries all the fault current from the equipment grounding arrangement returning to the source.

The Grounding Electrode System

The grounding electrode system is the physical connection to the earth. It bonds together the multiple electrodes that the building may have; using multiple grounding electrodes is more effective than depending on a single grounding electrode.

The electrodes permitted by the NEC for grounding are metal underground water pipe, metal in-ground support structures, concrete-encased electrodes, ground rings, rod and pipe electrodes, and plate electrodes.

The Grounding Electrode Conductor

The grounding electrode conductor connects the grounded conductor, the equipment grounding conductor, or both, to a point on the grounding electrode system or a single grounding electrode.

Figures 4, 5, and 6 show the relative location of the main bonding jumper, grounding electrode system, and grounding electrode conductor.

Effects of Several Grounding Connections

Except in some special applications, the equipment grounding conductor may be grounded at several points. But the neutral or grounded conductor must be grounded only at the service.

You may ask, why insulate the grounded conductor if it is grounded and there is no risk in touching it? The grounded conductor carries current, and without the insulation, it will contact the piping and other metallic structures.

The load current, supplied by the hot wire, will return to the source through the grounded conductor and all the grounding system, including the equipment grounding conductors, building steel, metal raceways, and water pipes.

These circulating currents will energize metallic frames and generate potential differences between the neutral of electronic equipment and ground.

The term used by the NEC is objectionable current. The NEC requires installing and arranging the grounding system in such a way that it will prevent objectionable current.

Figure 7 shows the flow of objectionable current when the grounded conductor has various grounding connections.

Figure 7. Objectionable current flowing through the equipment grounding conductor

If you suffer a shock when touching the fridge or a metal frame at home, the most likely cause is a neutral or grounded conductor connected to ground in more than one location. If you are wet, the shock can be much larger than expected.

Identification for Grounded and Equipment Grounding Conductors

The identification of grounded and equipment grounding conductors is critical, and the NEC gives methodical rules to accomplish this.

When using color codes, the general guidelines are:

Grounded conductors – A continuous white or grey outer finish or three continuous white or grey stripes along the conductor’s entire length on other than green insulation.

Equipment grounding conductors – For individually covered or insulated equipment grounding conductors, a continuous green outer finish, or green with one or more yellow stripes. The hot and grounded wires cannot use these colors. The EGC can also be bare.

There are no explicit color rules for hot wires. They may be any color excluding those mentioned above. Common choices are black, red, and blue.

Wiring Low Voltage AC Grounding Receptacles

It is essential to wire the grounding receptacles correctly to protect the users from shocks. There is no rationale for doing it wrong because this is an easy job.

Figure 8 shows the correct wiring of a conventional grounded receptacle. The installation follows the rule black to brass, white to light, green to green.

Figure 8. Correct wiring of a grounding receptacle

Although not mandatory, the typical installation practice is placing the opening for the grounding prong of the plug at the top. In this way, if a plug is inserted partially in the receptacle, exposing part of the prongs, and a metal object falls into the space between plug and receptacle, the grounding prong will block the object’s route to the hot prong, avoiding a ground fault.

Ground Fault Circuit Interrupter (GFCI)

Sometimes a standard circuit breaker or fuse cannot protect people against ground faults because the fault current is too low for them to operate. However, it could flow through a person in contact with the faulty equipment and a grounded surface.

A GFCI is an electronic device that effectively protects people by quickly interrupting the current to a circuit or a portion of it under ground-fault conditions. It is very sensitive and reacts to small fault currents, but there is no protection for phase-to-neutral and phase-to-phase faults.

The GFCI monitors the current to and from the load. If the wiring, a tool, or appliance allows some current to leak to ground, the GFCI will detect the difference in current in the two wires. When the fault current exceeds the trip level, it will disconnect the circuit in as little as 1/40 s.

The GFCI is an excellent supplementary safety precaution, especially when using electrical equipment in wet places like outdoors, bathrooms, and kitchens. The NEC requires the use of GFCI in specific locations in a dwelling and other units where the likelihood and severity of shocks are high. The GFCI will not prevent a person from receiving a shock, but it will open a circuit quickly enough to prevent injuries.

Several types of GFCIs are available, and they do not require an equipment grounding conductor.

A Review of Safe Grounding Techniques

The National Electrical Code (The NEC) establishes the minimum standards for wiring design and installation practice in the United States and other countries. These rules protect people from fire and other hazards.

In addition to the safety rules, the NEC provides technical terminology that, when used by everyone, reduces the misunderstandings that exist regarding the topic of grounding.

Equipment grounding connects to ground the metallic components of the electric system that do not carry electricity under normal conditions.

The grounded conductor connects to ground at the service only. It goes from the source to the load without any interruption or overcurrent protection.

The equipment grounding conductor, bonded to the grounded conductor, provides a safe and secure low impedance path for the ground-fault current, expediting the operation of overcurrent devices. It can be connected to the ground in several places.

The main bonding jumper is a crucial element that carries all the fault current from the equipment grounding arrangement back to the power supply.

The grounding electrode system provides the connection of the electrical system to the earth.

The grounding electrode conductor is the only connection to the grounding electrode system.

The NEC does not allow the flow of current through the grounding system – called objectionable currents – other than temporary ones that appear under ground-fault conditions.

The color of the insulation identifies the category of the conductor.

The use of a GFCI is convenient and required by the NEC in places with an increased chance of shocks, like wet environments.

For more information about AC electrical grounding, read other EE Power articles from Lorenzo Mari.

Source URL: https://eepower.com/technical-articles/ac-equipment-grounding-providing-a-safe-path-to-ground/

Analysis of Long-Time Efficiency of Photovoltaic Installation

Published by Jacek DĄBROWSKI, Ewa KRAC, Krzysztof GÓRECKI
Katedra Elektroniki Morskiej, Akademia Morska w Gdyni

Abstract. In the paper the results of long-time investigations of the photovoltaic installation situated in Gdynia Maritime University are presented. A short description of the considered installation and the weather station making possible continuous monitoring of weather parameters are included. The selected results of measurements of exploitive parameters of the considered installation obtained in the selected days and the corresponding to them results of measurements of power density of solar radiation are shown.

Streszczenie. W pracy przedstawiono wyniki długookresowych badań instalacji fotowoltaicznej zlokalizowanej w Akademii Morskiej w Gdyni. Zaprezentowano krótki opis rozważanej instalacji fotowoltaicznej i stacji pogodowej umożliwiającej ciągłe monitorowanie stanu pogody w miejscu zamontowania instalacji fotowoltaicznej. Pokazano wybrane wyniki pomiarów parametrów eksploatacyjnych rozważanej instalacji uzyskane w wybranych dniach oraz odpowiadające im wyniki pomiarów gęstości mocy promieniowania słonecznego. (Analiza długookresowej wydajności instalacji fotowoltaicznej).

Słowa kluczowe: Systemy fotowoltaiczne, parametry pogodowe, pomiary.
Keywords: Photovoltaic systems, weather parameters, measurements.

Introduction

Photovoltaic installations are used more and more often in households, in objects of public utility and in great solar power stations [1, 2, 3]. A drawback of these installations is variability of their efficiency at different seasons of the year and at different times of the day [1]. The efficiency of such installations strongly depends on weather conditions, characterized, among other things by power density of solar radiation and temperature [3, 4, 5].

A basic component of the considered photovoltaic system are photovoltaic panels. The value of electrical power produced in such panels depends also on the direction of the wind and its speeds, because these parameters influence efficiency of convection of heat generated in the panel as a result of solar infrared radiation and self-heating phenomena [5, 6].

On the other hand, the considered weather parameters are subject to seasonal and days fluctuations and they strongly depend on localisation of the considered photovoltaic installation.

From the literature [7,8,9] it is known that energy produced by photovoltaic panels, being basic components of the considered installation, is proportional to power density of solar radiation, and it also strongly depends on temperature.

In the paper the set-up to investigate the influence of weather parameters on exploitive parameters of the photovoltaic installation situated in Gdynia Maritime University and the selected results of measurements of this installation are shown.

The Weather Station

To register weather parameters the weather station designed by the Authors and installed on the roof of the building of Gdynia Maritime University was used. This station, shown in Fig.1, consists of 3 sensors measuring the ambient temperature, moisture, directions and speeds of the wind and power density of radiation, it also has the module LB-480 used to stockpile the measuring results. This data can be watched and analysed online from any place on the Earth thanks to the interface Ethernet with which the module LB-480 is equipped.

The module LB-480 assures the data access and settings with the use of the protocol http. The default home page contains the table with the running results of measurements. The page is refreshed automatically every second and contains the current results of measurements [10].

Fig. 1 Sensors of the weather station situated on the mast of the building of Gdynia Maritime University (A -thermo-hydrometer, B – the wind-gauge, C – the sensor of power density of the solar radiation)

In the paper [11] the influence of weather conditions on the operation of the selected photovoltaic panels is analysed. The weather station making possible measurements of values of the selected weather parameters on the ground of Gdynia Maritime University is described. The measured waveforms of power density of solar radiation and air temperature in the selected days are presented. The electrothermal transient analysis of the polycrystalline photovoltaic panel operating with the linear resistance load was conducted. The obtained results of calculations were presented and discussed. The problem of the influence of weather parameters on cooling conditions of the photovoltaic panels was pointed out.

Photovoltaic installation

The photovoltaic system installed in Gdynia Maritime University is characterized by the installed top-power of silicon photovoltaic panels equal to 10.4 kWp. This system, shown in Fig. 2, is made according to the topology of the system SMA FLEXIBLE STORAGE SYSTEM based on the devices by SMA company including two network inverters of the type Sunny Boy 5000TL-21 operating in the two-phase-configuration and the insular inverter of the type Sunny Island 6.0H. In the considered system forty panels are used, each of the top-power equal to 260 kWp, including 20 monocrystalline panels – Axitec AC-260M/156-60S and 20 poly-crystalline panels – Axitec AC-260P/156-60S. The mounting construction of the panels makes possible manual regulation of the depression angle of these panels, which can be 20°, 35°, 50° or 60°.

The photovoltaic system provides electrical energy to the local energy-network installed in the laboratory of optoelectronics, photovoltaics and optical fibre technique.

Fig. 2. View of the photovoltaic installation in Gdynia Maritime University

The insular inverter makes possible the transformation of electrical energy of the direct current, aggregated in the battery of four gel batteries connected serially, characterised by the nominal voltage amounting to 12 V and the capacity of 220 Ah, into energy of the alternating current. The production of additional energy across the network inverter follows in the case of the too weak sun exposure (eg. large cloudiness, nighttime) and in the case of damage of the university energy-network. The communication between each device is realised across the local Internet network. Parameters of the system are sent to the Internet service Sunny Portal across the device of the name Sunny Home Manager. This service allows remote management with settings of the photovoltaic system and its monitoring.

Investigations results

With the use of the weather station presented in Section 2, weather parameters in the place of location of the photovoltaic installation described in Section 3 were measured and recorded. The measurements of weather parameters and exploitive parameters of the considered installation have been performed since October 2015.

As an example, in Fig. 3 the measured courses of changes of power density of solar radiation during twenty-four hours are shown, and in Fig. 4 – the corresponding to them courses of power produced by the considered installation. For the presentation the Authors selected the arbitrarily data from the following days – 27^th October 2015, 27^th November, 24^th December 2015, 29^th January 2016, 27^th February 2016, 2^nd April 2016, 26^th April 2016, 25^th May 2016, 30^th June 2016, 29^th July 2016 and 25^thAugust 2016.

As it is visible, the considered waveforms obtained for all the considered days have similar courses, and over the considered period of time the maximum power density of solar radiation in winter and in summer changes even four times. As a result of changing times of the sunrise and the sunset, as well as changes of the height of the sun over the horizon at noon, there are visible changes of the maximum value of pV and time, in which the values of pV are much more than zero.

In the considered curves obtained in the selected days many local minima and maxima are observed. These local extrema are a result of changes in cloudiness during the considered days. These changes cause fast changes in the value of power density of solar radiation. For example in the curve measured on 29^th July 2016 it is visible that during 10 minutes only the value of pV decreases from 1100 W/m² to 300 W/m². On the other hand, for a sunny day the curve pV(time) has the shape near the Gaussian curve.

Fig. 3. Measured time distribution of power density of solar radiation

As it is visible in Fig. 4, the output power of the photovoltaic system visibly changes every day and during the year. It is worth noticing that the maximum output power of the photovoltaic system changes from about 5 kW in December to about 9 kW in June.

Fig. 4. Measured time distribution of the output power of the photovoltaic system

In the observed curves it is visible that the energy produced by the considered system depends on power density of solar radiation pV. Particularly, changes in values of pV (observed in Fig. 3) cause changes in the values of the output power of the considered system. The maximum value of the output power equal to the nominal value of this parameter was obtained in the period between April and October.

The information about time at which the power generation starts and stops in the considered days is presented in Fig.5.

Fig. 5. Time of starting and stopping energy generation by the photovoltaic system in the selected days

It is visible that this time is very short between November and January and its value is about only 7 hours, whereas in summer this time is equal to about 16 hours.

From the point of view of the user of a photovoltaic system information about electrical energy produced by this system is very important. The value of this energy depends on solar energy. In Fig.6 the values of solar energy density in the selected days are shown, whereas in Fig. 7 – the values of electrical energy produced by the considered system in the same days are presented.

Fig. 6. Solar energy density in the selected days

Fig. 7. Electrical energy produced by the photovoltaic system in the selected days

As it is visible, shapes of the curves shown in Fig. 4 and 5 are similar. The minimum value of the produced energy is observed in December and it is equal to 25% only of the value of this energy produced in May. The observed changes in the value of the produced electrical energy is twice smaller than changes in the value of density of solar energy. The value of electrical energy produced each day in the period from the beginning of April to the end of August is nearly the same and it is equal to about 60 kWh.

Conclusions

In the paper the investigations results of the photovoltaic installation and the weather station situated in Gdynia Maritime University are presented. The long-time investigations confirmed that the output power of the considered installation was not proportional to power density of solar radiation. It is also visible that changes of power density of solar radiation due, among other things to variable cloudiness get averaged. The energy produced by the considered installation changes even four times during the period from the end of December to the beginning of April and the time, in which energy is produced during this period, increases twice. During 5 months the value of the produced each day electrical energy is equal to its maximum.

REFERENCES

[1] D. Mulvaney, Solar’s Green Dilemma, IEEE Spectrum, No. 9, 2014, pp. 26-29
[2] J. Singh, “Semiconductor devices: basic principles”, John Wiley&Sons, 2000
[3] E. Klugmann-Radziemska, „Fotowoltaika w teorii i praktyce”, Wydawnictwo BTC, Legionowo, 2010.
[4] L. Castaner, S. Silvestre, „Modelling photovoltaic systems using Pspice”, John Wiley&Sons, 2002.
[5] M.H. Rashid, “Power Electronic Handbook”, Academic Press, Elsevier, 2007.
[6] E. Krac, K. Górecki: Modelling characteristics of photovoltaic panels with thermal phenomena taken into account. IOP Conference Series: Materials Science and Engineering, Vol. 104, 2016, 39^th International Microelectronics and Packaging IMAPS Poland 2015 Conference, 012013, pp. 1-7, doi:10.1088/1757-899X/104/1/012013.
[7] W. Marańda, M. Piotrowicz: “Extraction of Thermal Model Parameters for Field-Installed Photovoltaic Module”, PROC. 27^th International Conference on Microelectronics (MIEL 2010), NIŠ, Serbia, 16-19 MAY, 2010.
[8] K. Górecki, J. Zarębski: Modeling the influence of selected factors on thermal resistance of semiconductor devices. IEEE Transactions on Components, Packaging and Manufacturing Technology, Vol. 4, No. 3, 2014, pp. 421-428
[9] F.F. Oettinger, D.L. Blackburn: Semiconductor measurement technology: thermal resistance measurements U. S. Department Commerce NIST/SP-400/86, 1990
[10] T Markvart, L Castañer Practical handbook of photovoltaics : fundamentals and applications; Oxford: Elsevier Advanced Technology, 2003
[11] http: //mk.label.pl/lb480-user-manual/pl/lb480-usermanual.pl.pdf
[12] E. Krac, K. Górecki: The influence of selected weather parameters on characteristics of photovoltaic panels. Elektronika, No. 5, 2016, pp. 40-43

Autorzy: dr inż. Jacek Dąbrowski, mgr inż. Ewa Krac, prof. dr hab. inż. Krzysztof Górecki, Akademia Morska w Gdyni, Katedra Elektroniki Morskiej, ul. Morska 81-87, 81-225 Gdynia, E-mails: j.dabrowski@we.am.gdynia.pl, e.krac@we.am.gdynia.pl, k.gorecki@we.am.gdynia.pl

Source & Publisher Item Identifier: PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 93 NR 2/2017. doi:10.15199/48.2017.02.44

An Introduction to Microgrid Energy Management Systems

Published by Anushree Ramanath, EE Power – Technical Articles: An Introduction to Microgrid Energy Management Systems, July 05, 2021.

This article highlights the growth of microgrids and the components of these systems.

With the growing number of industries and businesses, access to reliable and cost-effective power is critical. This leads to demand for small-scale power supply networks to cater to the communities. The microgrid thus formed serves as a connection between the power generation facility and the utility grid [1]. It enables resilience, reliability, energy efficiency, environmental benefits, and economic gains. This promises uninterrupted power, thus prevents outages, and manages energy loads of multiple generation systems along with storage systems. The management aspect of the microgrid is handled through dedicated software and control systems. Read on to learn more about what a microgrid is, how it works, and its pros and cons.

Microgrids are a growing segment of the energy industry and represent a paradigm shift from remote central power plants to more localized distributed generation [2]. The microgrid concept has been around for several years, but it has gained significant traction in recent years as many projects are put into production, turning the concept into reality. If we observe closely, we realize that the early power systems developed by Thomas Edison and Nikola Tesla were all essentially microgrids. They are known to be revolutionary as they act as new drivers supporting the expansion of electrification while reducing the carbon footprint and enabling new technologies to support clean energy endeavors.

The microgrid is a local energy system capable of producing and distributing energy and is composed of different types of assets, also known as distributed energy resources (DERs), as illustrated in Figure 1. It can also be termed as a miniature power grid system that manages DERs, including both renewable and non-renewable sources of energy. It can be connected to the grid-like in healthcare facilities where continuous power is inevitable or be run entirely off the grid as it is self-sustaining in nature, like in remote sites that require tremendous amounts of energy.

Figure 1: Illustration of a microgrid [4]

The process of building a microgrid can be described as that of a Paladin lifecycle [3]. It involves the initial feasibility study of the site, the possible design, and the modeling of it. It is followed by the power study, including model-based studies, forecasting, and optimization. The next stage involves system design and management, including analysis of interconnection, implementation, and real-time simulations. The final step involves real-time control and optimization of all the power components.

Components of a Microgrid

The U.S. Department of Energy (DOE) defines a microgrid as “A group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid and island modes” [5]. A microgrid generally comprises renewable or fossil-fueled generators, loads, energy storage systems, circuit breakers, and control equipment, as illustrated in Figure 2.

The most commonly employed assets to generate power are photovoltaics (PV), wind turbines, and power generators. The other elements critical in terms of the functionality of a microgrid include storage systems, smart controls, and software that facilitates interconnection. All of these components need to work well together to ensure a seamless customer experience while adhering to standard regulatory requirements.

Advantages of Microgrids

The formation of microgrids assures efficient and low-cost clean energy along with reducing grid congestion and peak loads. It helps improve the stability of the grid while enhancing the reliability and resilience of the critical infrastructure. It also helps in reducing the carbon footprint and line losses while promoting the use of renewable energy sources.

Microgrids offer several financial benefits, including tax breaks, discounts and avoid peak pricing burden. It is also diversified and produces energy from multiple sources like wind, solar, and fuel cells, adding several levels of security [3]. They come in handy as they are smaller and can be installed quickly than traditional power plants. Further, microgrids can integrate with the grid along with other smart grid technologies with ease.

Disadvantages of Microgrids

The main disadvantage of a microgrid is the resynchronization with the main grid.

There is also a need for ample storage, which again demands an additional cost, maintenance, and space for installation. There is some resistance from the utilities to implement microgrid technologies. There are also regulatory issues that might be difficult to comply with, and it is difficult for laws to catch up with building technology.

Key references:

• Microgrid – basics, structure, advantages, disadvantages – Electrical – Industrial Automation, PLC Programming, scada & Pid Control System
• Features and Benefits – Microgrids
• Power Analytics, Microgrid Basics, Part 1: The Need for a Feasibility Study.
• The Pros and Cons of Microgrids
• Microgrid Fundamentals | What is and Role Generators Play w/ Video & Graphics!
• Microgrid-graphic – Midwest

Author: Anushree Ramanath is a seasoned engineering professional skilled in system-level design, building hardware, coding, firmware, industry-oriented research, software architecture, modeling, and simulations. She received a Ph.D. in Electrical and Computer Engineering from the University of Minnesota Twin Cities with a focus on power and controls. She loves experiencing different cultures through languages, food, or travel while indulging in a variety of fine arts

Source URL: https://eepower.com/technical-articles/an-introduction-to-microgrid-energy-management-systems/

Solutions to Mitigate Harmonics

Published by Electrical Installation Wiki, Chapter M. Power harmonics management – Solutions to mitigate harmonics

There are three different types of solutions to attenuate harmonics:

• Modifications in the installation
• Special devices in the supply system
• Filtering

Basic solutions to mitigate harmonics

To limit the propagation of harmonics in the distribution network, different solutions are available and should be taken into account particularly when designing a new installation.

Position the non-linear loads upstream in the system

Overall harmonic disturbances increase as the short-circuit power decreases.

All economic considerations aside, it is preferable to connect the non-linear loads as far upstream as possible (see Fig. M24).

**Fig. M24** – Non-linear loads positioned as far upstream as possible (recommended layout)

Group the non-linear loads

When preparing the single-line diagram, the non-linear devices should be separated from the others (see Fig. M25). The two groups of devices should be supplied by different sets of busbars.

**Fig. M25** – Grouping of non-linear loads and connection as far upstream as possible (recommended layout)

Create separate sources

In attempting to limit harmonics, an additional improvement can be obtained by creating a source via a separate transformer as indicated in the Figure M26.

The disadvantage is the increase in the cost of the installation.

**Fig. M26** – Supply of non-linear loads via a separate transformer

Transformers with special connections

Different transformer connections can eliminate certain harmonic orders, as indicated in the examples below:

• A Dyd connection suppresses 5^th and 7^th harmonics (see Fig. M27)
• A Dy connection suppresses the 3^rdharmonic
• A DZ 5 connection suppresses the 5^thharmonic

**Fig. M27** – A Dyd transformer blocks propagation of the 5^th and 7^th harmonics to the upstream network

Install reactors

When variable-speed drives are supplied, it is possible to smooth the current by installing line reactors. By increasing the impedance of the supply circuit, the harmonic current is limited.

Installation of harmonic suppression reactors on capacitor banks increases the impedance of the reactor/capacitor combination for high-order harmonics.

This avoids resonance and protects the capacitors.

Select the suitable system earthing arrangement

TNC system

In the TNC system, a single conductor (PEN) provides protection in the event of an earth fault and the flow of unbalance currents.

Under steady-state conditions, the harmonic currents flow in the PEN. Because of the PEN impedance, this results in slight differences in potential (a few volts) between devices that can cause electronic equipment to malfunction.

The TNC system must therefore be reserved for the supply of power circuits at the head of the installation and must not be used to supply sensitive loads.

TNS system

This system is recommended if harmonics are present.

The neutral conductor and the protection conductor PE are completely separate and the potential throughout the distribution network is therefore more uniform.

Harmonic filtering

In cases where the preventive action presented above is insufficient, it is necessary to equip the installation with filtering systems.

There are three types of filters:

• Passive
• Active
• Hybrid

Passive filters

Typical applications

• Industrial installations with a set of non-linear loads representing more than 500kVA (variable-speed drives, UPSs, rectifiers, etc.)
• Installations requiring power-factor correction
• Installations where voltage distortion must be reduced to avoid disturbing sensitive loads
• Installations where current distortion must be reduced to avoid overloads

Operating principle

An LC circuit, tuned to each harmonic order to be filtered, is installed in parallel with the non-linear load (see Fig. M28). This bypass circuit absorbs the harmonics, thus avoiding their flow in the distribution network.

Generally speaking, the passive filter is tuned to a harmonic order close to the order to be eliminated. Several parallel-connected branches of filters can be used if a significant reduction in the distortion of a number of harmonic orders is required.

**Fig. M28** – Operating principle of a passive filter

Active filters (active harmonic conditioner)

Typical applications

• Commercial installations with a set of non-linear loads representing less than 500kVA (variable-speed drives, UPSs, office equipment, etc.)
• Installations where current distortion must be reduced to avoid overloads.

Operating principle

These systems, comprising power electronics and installed in series or parallel with the non-linear load, compensate the harmonic current or voltage drawn by the load.

Figure M29 shows a parallel-connected active harmonic conditioner (AHC) compensating the harmonic current (I_har = -I_act).

The AHC injects in opposite phase the harmonics drawn by the non-linear load, such that the line current Is remains sinusoidal.

**Fig. M29** – Operating principle of an active filter

Hybrid filters

Typical applications

Industrial installations with a set of non-linear loads representing more than 500kVA (variable-speed drives, UPSs, rectifiers, etc.)

• Installations requiring power-factor correction
• Installations where voltage distortion must be reduced to avoid disturbing sensitive loads
• Installations where current distortion must be reduced to avoid overloads
• Installations where strict limits on harmonic emissions must be met

Operating principle

Passive and active filters are combined in a single system to constitute a hybrid filter (see Fig. M30). This new filtering solution offers the advantages of both types of filters and covers a wide range of power and performance levels.

Selection criteria

Passive filter

It offers both power-factor correction and high current-filtering capacity. Passive filters also reduce the harmonic voltages in installations where the supply voltage is disturbed. If the level of reactive power supplied is high, it is advised to turn off the passive filter at times when the percent load is low.

Preliminary studies for a filter must take into account the possible presence of a power factor correction capacitor bank which may have to be eliminated.

**Fig. M31** – Example of MV passive filter equipment

Active harmonic conditioners

They filter harmonics over a wide range of frequencies and can adapt to any type of load. On the other hand, power ratings are limited.

**Fig. M32** – Active Harmonic Conditioner (AccuSine range)

Hybrid filters

They combine the performance of both active and passive filters.

The method to optimize harmonics mitigation

The best solution, in both technical and financial terms, is based on the results of an in-depth study.

Harmonic audit of MV and LV networks

By calling on an expert, you are guaranteed that the proposed solution will produce effective results (e.g. a guaranteed maximum THD_u).

A harmonic audit is carried out by an engineer specialised in the disturbances affecting electrical distribution networks and equipped with powerful analysis and simulation equipment and software.

The steps in an audit are the following:

• Measurement of disturbances affecting current and phase-to-phase and phase-to-neutral voltages at the supply source, the disturbed outgoing circuits and the non-linear loads

• Computer modelling of the phenomena to obtain a precise explanation of the causes and determine the best solutions

• A complete audit report presenting:

– The current levels of disturbances
– The maximum permissible levels of disturbances (refer to IEC 61000, IEEE 519, etc.)

• A proposal containing solutions with guaranteed levels of performance

• Finally, implementation of the selected solution, using the necessary means and resources.

The entire audit process should be certified ISO 9002.

Source URL: https://www.electrical-installation.org/enwiki/Solutions_to_mitigate_harmonics