Published by Information Technology Industry Council (ITI), 1250 Eye Street NW Suite 200, Washington DC 20005, 202-737-8888, http://www.itic.org
The ITI (CBEMA) Curve, included within this Application Note, is published by Technical Committee 3 (TC3) of the Information Technology Industry Council (ITI, formerly known as the Computer & Business Equipment Manufacturers Association). It is available at http://.www.itic.org/technical/iticurv.pdf.
1) SCOPE The ITI (CBEMA) Curve and this Application Note describe an AC input voltage envelope which typically can be tolerated (no interruption in function) by most Information Technology Equipment (ITE). The Curve and this Application Note comprise a single document and are not to be considered separately from each other. They are not intended to serve as a design specification for products or AC distribution systems. The Curve and this Application Note describe both steady-state and transitory conditions.
2) APPLICABILITY The Curve and this Application Note are applicable to 120V nominal voltages obtained from 120V, 208Y/120V, and 120/240V 60Hz systems. Other nominal voltages and frequencies are not specifically considered and it is the responsibility of the user to determine the applicability of these documents for such conditions.
3) DISCUSSION This section provides a brief description of the individual conditions which are considered in the Curve. For all conditions, the term “nominal voltage” implies an ideal condition of 120V RMS, 60Hz. Seven types of events are described in this composite envelope. Each event is briefly described in the following sections, with two similar line voltage sags being described under a single heading. Two regions outside the envelope are also noted. All conditions are assumed to be mutually exclusive at any point in time, and with the exception of steady-state tolerances, are assumed to commence from the nominal voltage. The timing between transients is assumed to be such that the ITE returns to equilibrium (electrical, mechanical, and thermal) prior to commencement of the next transient.
3.1) Steady-State Tolerances The steady-state range describes an RMS voltage which is either very slowly varying or is constant. The subject range is +/- 10% from the nominal voltage. Any voltages in this range may be present for an indefinite period, and are a function of normal loadings and losses in the distribution system.
3.2) Line Voltage Swell This region describes a voltage swell having an RMS amplitude of up to 120% of the RMS nominal voltage, with a duration of up to 0.5 seconds. This transient may occur when large loads are removed from the system or when voltage is supplied from sources other than the electric utility.
3.3) Low-Frequency Decaying Ringwave This region describes a decaying ringwave transient which typically results from the connection of powerfactor-correction capacitors to an AC distribution system. The frequency of this transient may range from 200Hz to 5KHz, depending upon the resonant frequency of the AC distribution system. The magnitude of the transient is expressed as a percentage of the peak 60Hz nominal voltage (not the RMS value). The transient is assumed to be completely decayed by the end of the half-cycle in which it occurs. The transient is assumed to occur near the peak of the nominal voltage waveform. The amplitude of the transient varies from 140% for 200Hz ringwaves to 200% for 5KHz ringwaves, with a linear increase in amplitude with increasing frequency. Refer to Figure 1 for an example of a typical waveform.
Figure 1 – Typical Low Frequency Decaying Ringwave
3.4) High-Frequency Impulse and Ringwave This region describes the transients which typically occur as a result of lightning strikes. Wave shapes applicable to this transient and general test conditions are described in ANSI/IEEE C62.41-1991. This region of the curve deals with both amplitude and duration (energy), rather than RMS amplitude. The intent is to provide an 80 Joule minimum transient immunity.
3.5) Voltage Sags Two different RMS voltage sags are described. Generally, these transients result from application of heavy loads, as well as fault conditions, at various points in the AC distribution system. Sags to 80% of nominal (maximum deviation of 20%) are assumed to have a typical duration of up to 10 seconds, and sags to 70% of nominal (maximum deviation of 30%) are assumed to have a duration of up to 0.5 seconds.
3.6) Dropout A voltage dropout includes both severe RMS voltage sags and complete interruptions of the applied voltage, followed by immediate re-application of the nominal voltage. The interruption may last up to 20 milliseconds. This transient typically results from the occurrence and subsequent clearing of faults in the AC distribution system.
3.7) No Damage Region Events in this region include sags and dropouts which are more severe than those specified in the preceding paragraphs, and continuously applied voltages which are less than the lower limit of the steady-state tolerance range. The normal functional state of the ITE is not typically expected during these conditions, but no damage to the ITE should result.
3.8) Prohibited Region This region includes any surge or swell which exceeds the upper limit of the envelope. If ITE is subjected to such conditions, damage to the ITE may result.
Published by Electrotek Concepts, Inc., PQSoft Case Study: Flickering Lights Wiring and Grounding Case Study, Document ID: PQS0403, March 30, 2004.
Abstract: This case study concerns a residential electrical system. The homeowners were experiencing light flicker when loads were energized and de-energized in their home.
INTRODUCTION
Residential electric systems are served from single-phase transformers employing a spilt secondary winding, often referred to as a single-phase 3-wire system. This type of transformer is used to deliver both 120 volt and 240-volt single-phase power to the residential loads. The primary of the transformer is often served from a 12kV to 15kV distribution system by the local utility. Figure 1 illustrates the concept of a split phase system.
Figure 1 – Residential Split-Phase Electric Service
When this type of service is operating properly, 120 volts can be measured from either leg to the neutral conductor. Due to the polarity of the secondary windings in the transformer, the polarity of each 120-volt leg is opposite the other, thus allowing a total of 240 volts between each leg as illustrated. The proper operation of this type of system is dependent on the physical connection of the neutral conductor or center tap of the secondary winding. If the neutral connection is removed, 240 volts will remain across the two legs, but the line to neutral voltage for either phase can be shifted, causing either a low or high voltage from line to neutral.
Most loads—lighting, televisions, microwaves, home electronics, etc.—in a residential dwelling are operated from 120 volts. However, there are a few major loads that incorporate the use of the 240 volts available. These loads include electric water heaters, electric stoves and ovens, heat pumps, etc.
THE PROBLEM
In this case, there were problems in the residence that caused the homeowner to question the integrity of the power system serving his home. On occasion, the lights would flicker erratically when the washing machine and dryer were operating at the same time. When large single-phase loads were operated, low power incandescent light bulb intensity would flicker.
Measurements were performed at several 120-volt outlets throughout the house. When the microwave was operated, the voltage at several of the 120-volt outlets would increase from 120 volts nominal to 128 volts. The voltage would return to normal after the microwave was turned off. The voltage would also increase when a 1500-Watt space heater was operated. It was determined that the voltage would decrease to approximately 112 volts on the leg from which the large load was served. After the measurements confirmed suspicions of high and low voltages during heavy load operation, finding the source of the problem was the next task at hand.
The hunt began at the service entrance to the house. A visual inspection was made of the meter base & socket after the meter was removed by the local utility. It was discovered that one of the neutral connectors was loose. While attempting to re-tighten this connector, the connector fell off of the meter socket into the bottom of the meter base (see Figure 2). Could this be the cause of the flickering voltage? Let’s examine the effects of the loose neutral connection.
Figure 2 – Photograph of Meter Base
Figure 3 and Figure 4 will be referred to several times during this discussion. Under normal conditions with a solid neutral connection (Figure 3), load current flows through each leg and is returned to the source through the neutral conductor. There is very little impedance in either the hot or the neutral conductor; therefore, no appreciable voltage drop exists.
When the neutral is loose or missing, a significant voltage can develop across the neutral connection in the meter base, as illustrated in Figure 4. When a large load is connected across Leg 1 to N and the other leg is lightly loaded (i.e. Leg 1 to N is approximately 10 times the load on Leg 2 to N), the current flowing through the neutral will develop a voltage across the loose connection. This voltage is in phase with the voltage from Leg 1 to N’ (see Figure 4) and the total voltage from Leg 1 to N will be 120 volts. However, the voltage supplied to any loads connected from Leg 2 to N’ will rise to 128 volts, as illustrated in Figure 4. The total voltage across the Leg 1 and Leg 2 must remain constant at 240 volts. It should be noted that the voltage from Leg 2 to N will be 120 volts since the voltage across the loose connection is 180 degrees out of phase with the Leg 2 to N’ voltage.
Therefore, with the missing neutral connection, the voltage from Leg 2 to N’ would rise causing the light flicker. This explains the rise in voltage when a large load was energized on the system.
Figure 3 – Diagram Illustrating the Effects of Solid Neutral Connection
Figure 4 – Diagram Illustrating the Effects of Loose Neutral Connection
THE SOLUTION
The solution in this case was simple – replace the failed connector.
SUMMARY
Over time the neutral connector had become loose. This loose connection caused heating, which in turn caused the threads on the connector to become worn, and the connector failed. After replacing the connector in the meter base, the flickering light phenomena disappeared.
On systems of this type, if a voltage rise occurs when loads are energized; it is a good indication that the neutral connection may be loose or missing.
RELATED STANDARDS IEEE Standard 1100-1999 – Power and Grounding Electronic Equipment IEEE Standard 142-1991 – Grounding of industrial and Commercial Power Systems The National Electric Code (NEC) – Current Revision
GLOSSARY AND ACRONYMS Neutral: A system or circuit conductor that is intentionally grounded. Also known as the “grounded” conductor. KV: One kV is equal to one thousand volts.
Published by Vedant S. Samudre, Department of electrical, electronics & power, Jawaharlal, Nehru Engineering College, Aurangabad, Maharashtra, India Published in International Journal of Engineering Sciences & Research Technology (IJESRT) Website: www.ijesrt.com Samudre* et al., 5(12): December, 2016 DOI: 10.5281/zenodo.192584
ABSTRACT
Power quality is one of the major concerns and emerging issues in the present era. With increasing quantities of non-linear loads being added to electrical systems, it has become necessary to investigate the power quality issues as all electrical devices are prone to failure when exposed to one or more power quality problems. This paper highlights power quality problems, effect of power quality problems in different apparatuses and methods for its correction. This paper will be very much helpful for engineers, technicians, designers, researchers and system operators as it is necessary for them to become familiar with power quality issues.
KEYWORDS: Power quality (PQ), Standards, Problems.
INTRODUCTION
Power quality problem in the power system has gained importance since the late 1980s. The interest in Power Quality (PQ) is related to all three parties concerned with the power i.e. utility companies, equipment manufacturers and electric power consumers. Problems affecting the electricity supply that were once considered tolerable by the electric utilities and users are now often taken as a problem to the users of everyday equipment. Understanding power quality can be confusing at best. There have been numerous articles and books concerning power quality [1]. There are two terms known in power systems about the quality of power: Good power quality and poor power quality. Good power quality can be used to describe a power supply that is always available, always within the voltage and frequency tolerances and has a pure noise-free sinusoidal wave shape to all equipment, because most equipment was designed on that basis [4]. Unfortunately, most of the equipment that is manufactured also distorts the voltage [3] on the distribution system, leading to what is known as poor power quality. And thus affecting other equipment that was designed with the expectation of consistent undistorted voltage, and are thus sensitive [2] to power disturbances resulting in reduced performance and will cause equipment miss operation or premature failure. The cost of power quality problems can be very high and include the cost of downtimes, loss of customer confidence and, in some cases, equipment damage.
MATERIALS AND METHODS
Why power quality is important?
Along with technology advance, the organization of the worldwide economy has evolved towards globalization and the profit margins of many activities tend to decrease. The increased sensitivity of the vast majority of processes (industrial, services and even residential) to PQ problems turns the availability of electric power with quality a crucial factor for competitiveness in every activity sector. The most critical areas are the continuous process industry and the information technology services. [5] The performance of electronic devices is directly linked to the power quality level. Quality phenomenon or power quality disturbance can be defined as the deviation of the voltage and the current from its ideal waveform.. Faults at either the transmission or distribution level may cause voltage sag or swell in the entire system or a large part of it. Also, under heavy load conditions, a significant voltage drop may occur in the system. Voltage sag and swell can cause sensitive equipment to fail, shutdown and create a large current unbalance. These effects can incur a lot of expensive from the customer and cause equipment damage. So , in order to provide uninterrupted power to the service sectors as well as others for economic growth and prevent equipment damage with varying voltage level and frequency, undoubtedly power quality improvement is utmost important.
Power quality standards
Power quality is a worldwide issue and its related standards [6] being used by researchers, designer and practitioner to improve power quality are given below:
IEEE 519
IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Electric Power Systems, established limits on harmonic currents and voltages at the point of common coupling (PCC), or point of metering [7,20]. The limits of IEEE 519 are intended to: 1) Assure that the electric utility can deliver relatively clean power to all of its customers 2) Assure that the electric utility can protect its electrical equipment from overheating, loss of life from excessive harmonic currents, and excessive voltage stress due to excessive harmonic voltage. Each point from IEEE 519 lists the limits for harmonic distortion at the point of common coupling (PCC) or metering point with the utility. The voltage distortion limits are 3% for individual harmonics and 5% THD. All of the harmonic limits in IEEE 519 are based on a customer load mix and location on the power system.
IEEE 519 Standard for Current Harmonics:
General Distribution Systems [120V- 69 kV]: Below current distortion limits are for odd harmonics. Even harmonics are limited to 25% of the odd harmonic limits [7, 9, and 11]. For all power generation equipment, distortion limits are those with ISC/IL<20.ISC is the maximum short circuit current at the point of coupling “PCC”.IL is the maximum fundamental frequency 15-or 30- minutes load current at PC.
General Sub-transmission Systems [69 kV-161 kV]: The current harmonic distortion limits apply to limits of harmonics that loads should draw from the utility at the PCC. Note that the harmonic limits differ based on the ISC/IL rating, where ISC is the maximum short circuit current at the PCC, and I is the maximum demand load current at the PCC.
Table no. 1: Current distortion limit for harmonics.
ISC/IL
h<11
11≤h<17
17≤h≤23
23≤h<25
TDD (%)
<20
4.0
2.0
1.5
0.6
5
20-50
7.0
3.5
2.5
1.0
8
50-100
10
4.5
4.0
1.5
12
100-1000
12
5.5
5.0
2.0
15
>1000
15
7.0
6.0
2.5
20
IEEE Standard For Voltage Harmonics:
The voltage harmonic distortion limits apply to the quality of the power. For instance, for systems of less than 69 kV, IEEE 519 requires limits of 3 percent harmonic distortion for an individual frequency component and 5 percent for total harmonic distortion.
Table no. 2: Voltage distortion limit for harmonics.
Bus Voltage
Individual Vh (%)
THDV (%)
V<69 kV
3.0
5.0
69≤V<161 kV
1.5
2.5
V≥161 kV
1.0
1.5
IEC 61000-3-2 and IEC 61000-3-4 (formerly 1000-3-2 and 1000-3-4) IEC 61000-3-2 (1995-03) It specifies limits for harmonic current emissions applicable to electrical and electronic equipment having an input current up to and including 16 A per phase, and intended to be connected to public low-voltage distribution systems. The tests according to this standard are type tests. [8, 15, 19]
IEC/TS 61000-3-4 (1998-10) It specifies to electrical and electronic equipment with a rated input current exceeding 16 A per phase and intended to be connected to public low-voltage ac distribution systems of the following types:
nominal voltage up to 240 V, single-phase, two or three wires;
nominal voltage up to 600 V, three-phase, three or four wires
nominal frequency 50 Hz or 60 Hz
These recommendations specify the information required to enable a supply authority to assess equipment regarding harmonic disturbance and to decide whether or not the equipment is acceptable for connection with regard to the harmonic distortion aspect. The European standards, IEC 61000-3-2 & 61000-3-4, placing current harmonic limits on equipment, are designed to protect the small consumer’s equipment. The former is restricted to 16 A; the latter extends the range above 16 A.
IEEE Standard 141-1993, Recommended Practice for Electric Power Distribution for Industrial Plants A thorough analysis of basic electrical-system considerations is presented. Guidance is provided in design, construction, and continuity of an overall system to achieve safety of life and preservation of property; reliability; simplicity of operation; voltage regulation in the utilization of equipment within the tolerance limits under all load conditions; care and maintenance; and flexibility to permit development and expansion.
IEEE Standard 142-1991, Recommended Practice for Grounding of Industrial and Commercial Power Systems This standard presents a thorough investigation of the problems of grounding and the methods for solving these problems.[9,12]
IEEE Standard 446-1987, Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications This standard is recommended engineering practices for the selection and application of emergency and standby power systems.[12]
IEEE Standard 493-1997, Recommended Practice for Design of Reliable Industrial and Commercial Power Systems
The fundamentals of reliability analysis as it applies to the planning and design of industrial and commercial electric power distribution systems are presented. Included are basic concepts of reliability analysis by probability methods, fundamentals of power system reliability evaluation, economic evaluation of reliability, cost of power outage data, equipment reliability data, and examples of reliability analysis. Emergency and standby power, electrical preventive maintenance, and evaluating and improving reliability of the existing plant are also addressed.[10,12,13,14]
IEEE Standard 1100-1999, Recommended Practice for Powering and Grounding Sensitive Electronic Equipment
Recommended design, installation, and maintenance practices for electrical power and grounding (including both power-related and signal-related noise control) of sensitive electronic processing equipment used in commercial and industrial applications. [12, 14]
IEEE Standard 1159-1995, Recommended active for Monitoring Electric Power Quality
As its title suggests, this standard covers recommended methods of measuring power-quality events. Many different types of power-quality measurement devices exist and it is important for workers in different areas of power distribution, transmission, and processing to use the same language and measurement techniques. Monitoring of electric power quality of AC power systems, definitions of power quality terminology, impact of poor power quality on utility and customer equipment, and the measurement of electromagnetic phenomena are covered.[11,14,16,17,18]
CLASSIFICATION OF POWER SYSTEM DISTURBANCES
Power quality problems occur due to various types of electrical disturbances. Most of the EPQ disturbances depend on amplitude or frequency or on both frequency and amplitude. Based on the duration of existence of EPQ disturbances, events can divided into short, medium or long type. The disturbances causing power quality degradation arising in a power system and their classification mainly include:
Interruption/under voltage/over voltage
These are very common type disturbances. During power interruption, voltage level of a particular bus goes down to zero. The interruption may occur for short or medium or long period. Under voltage and over voltage are fall and rise of voltage levels of a particular bus with respect to standard bus voltage. Sometimes under and over voltages of little percentage is allowable; but when they cross the limit of desired voltage level, they are treated as disturbances. Such disturbances are increasing the amount of reactive power drawn or deliver by a system, insulation problems and voltage stability.
Voltage/current unbalance
Voltage and current unbalance may occur due to the unbalance in drop in the generating system or transmission system and unbalanced loading. During unbalance, negative sequence components appear.
Transients
Transients may generate in the system itself or may come from the other system. Transients are classified into two categories: dc transient and ac transient.AC transients are further divided into two categories: single cycle and multiple cycles. [21, 22]
Voltage sag
It is a short duration disturbance. [23] During voltage sag, r. m. s. voltage falls to a very low level for short period of time.
Voltage swell
It is a short duration disturbance. During voltage sag, r. m. s. voltage increases to a very high level for short period of time.
Harmonic
Harmonics are the alternating components having frequencies other than fundamental present in voltage and current signals. There are various reasons for harmonics generation like non linearity, excessive use of semiconductor based switching devices, different design constrains, etc. Harmonics have adverse effects on generation, transmission and distribution system as well as on consumer equipment’s also. Harmonics are classified as integer harmonics, sub harmonics and inter harmonics. Integer harmonics have frequencies which are integer multiple of fundamental frequency, sub harmonics have frequencies which are smaller than fundamental frequency and inter harmonics have frequencies which are greater than fundamental frequencies. Among these entire harmonics integer and inter harmonics are very common in power system. Occurrence of sub harmonics is comparatively smaller than others. Sometimes harmonics are classified: time harmonics and spatial (space) harmonics. Obviously their causes of occurrence are different. Harmonics are in general are not welcome and desirable. Harmonics are assessed with respect to fundamental. Monitoring of harmonics with respect to fundamental is important consideration in power system application For this purpose different distortion factor with respect to the fundamental have been introduced.
Flicker
It is undesired variation of system frequency.
Ringing waves
Oscillatory disturbances of decaying magnitude for short period of time is known as ringing wave. It may be called a special type transient. The frequency of a flicker may or may not be same with the system frequency.
Outages
It is special type of interruption where power cut has occurred for not more than 60s.
POWER QUALITY SOLUTIONS
Power conditioning devices
Following devices play a crucial role in improving power quality strategy.
Transient Voltage surge suppressor (TVSS)
It provides the simplest and least expensive way to condition power. These units clamp transient impulses (spikes) to a level that is safe for the electronic load. Transient voltage surge suppressors are used as interface between the power source and sensitive loads, so that the transient voltage is clamped by the TVSS before it reaches the load. TVSS usually contain a component with a nonlinear resistance (a metal oxide varistor or a Zener diode) that limits excessive line voltage and conduct any excess impulse energy to ground. [24]
Filter
Filters are categorized into noise filters, harmonic filters (active and passive) etc. [25] Noise filters are used to avoid unwanted frequency current or voltage signals (noise) from reaching sensitive equipment. This can be accomplished by using a combination of capacitors and inductances that creates a low impedance path to the fundamental frequency and high impedance to higher frequencies, that is, a low-pass filter. Harmonic filters are used to reduce undesirable harmonics. Passive filters consist in a low impedance path to the frequencies of the harmonics to be attenuated using passive components (inductors, capacitors and resistors)
Motor generator set
They are usually used as a backup power source for a facility’s critical systems such as elevators and emergency lighting in case of blackout. However, they do not offer protection against utility power problems such as over voltages and frequency fluctuations. Motor generators are consists of an electric motor driving a generator with coupling through a mechanical shaft. This solution provides complete decoupling from incoming disturbances such as voltage transients, surges and sags.
Isolation transformer
Isolation transformers [26] are used to isolate sensitive loads from transients and noise deriving from the mains. The particularity of isolation transformers is a grounded shield made of nonmagnetic foil located between the primary and the secondary. Any noise or transient that come from the source in transmitted through the capacitance between the primary and the shield and on to the ground and does not reach the load. Isolation transformers reduce normal and common mode noises, however, they do not compensate for voltage fluctuations and power outages. [26]
Fig 1: Noise attenuation by isolation transformer
Voltage regulators
Voltage regulators are normally installed where the input voltage fluctuates, but total loss of power is uncommon. There are three basic types of regulators:
Tap changers: Designed to adjust for varying input voltage by automatically transferring taps on a power transformer.
Buck boost: Utilize similar technology to the tap changers except transformer is not isolated.
Constant voltage transformer (CVT): It is completely static regulator that maintains nearly constant output voltage during large variation in input voltage.
Uninterrupted power supply (UPS)
UPS systems provide protection in the case of a complete power interruption (blackout). They should be applied where “down time” resulting from any loss of power is unacceptable. UPS are designed to provide continuous power to the load in the event of momentary interruptions. They also provide varying degrees of protection from surges, sags, noise or brownouts depending on the technology used. [24]
There are three major UPS topologies each providing different levels of protection:
Offline UPS: Low cost solution for small, less critical, stand-alone application such as PLC, personal computers and peripherals. Advantages of offline UPS are high efficiency, low cost and high reliability.
Line interactive UPS: These UPS provides highly effective power conditioning plus battery backup. Advantages are good volume regulation and high efficiency. Disadvantages are noticeable transfer time and difficulty in comparing competing units.
True Online UPS: It provides the highest level of power protection, conditioning and power availability. Advantages includes the elimination of any transfer time and superior protection from voltage fluctuation.
Fig 2: Offline UPS system
Fig 3: Online UPS system
Dynamic Voltage Regulator (DVR)
A dynamic voltage restorer (DVR) acts like a voltage source connected in series with the load. The output voltage of the DVR is kept approximately constant voltage at the load terminals by using a step-up transformer and/or stored energy to inject active and reactive power in the output supply through a voltage converter. [27]
Fig 4: Dynamic Voltage Regulator.
Unified Power Quality Conditioner (UPQC)
The UPQC employs two voltage source inverters (VSI) that is connected to a dc energy storage capacitor .A UPQC, combines the operations of a Distribution Static Compensator (DSTATCOM) and Dynamic Voltage Regulator (DVR) together. This combination allows a simultaneous compensation of the load currents and the supply voltages, so that compensated current drawn from the network and the compensated supply voltage delivered to the load are sinusoidal and balanced. [28]
Fig 5: Basic structure of UPQC
Thyristor based switch
The static switch is a versatile device for switching a new element into the circuit when voltage support is needed. To correct quickly for voltage spikes, sags, or interruptions, the static switch can be used to switch in capacitor, filter, alternate power line, energy storage system etc. It protects against 85% of the interruptions and voltage sags. [28]
Static VAR compensator (SVC)
Static VAR compensators (SVC) use a combination of capacitors and reactors to regulate the voltage quickly. Solid-state switches control the insertion of the capacitors and reactors at the right magnitude to prevent the voltage from fluctuating. It is normally applied to transmission networks to counter voltage dips/surges during faults. [24]
Fig 6: Static VAR Compensator (SVC)
CONCLUSION
An extensive review of work done power quality issues has been presented to provide a clear perspective on various aspects of the power quality to the researchers and engineers working in this field. To overcome the negative impact of poor power quality on equipment and businesses, suitable power quality equipment can be invested.
REFERENCES
[1] “Details of equipment sensitivity,” http://www.powerquality.com/pqpark/pqpk1052.hm [2] A. Rash, “Power quality and harmonics in the supply network: a look at common practices and standards,” in Proc. On MELECON’ 98, Vol.2, pp.1219-1223, May1998 [3] R.C. Sermon, “An overview of power quality standards and guidelines from the end-user’s point-of-view,” in Proc. Rural Electric Power Conf., pp. 1-15, May 2005 [4] IEC 61000-4-30, “Testing and measurement techniques – Power quality measurement methods,” pp. 19, 78, 81, 2003 [5] Ferracci, P., “Power Quality”, Schneider Electric Cahier Technique no. 199, September 2000. [6] S. Khalid et al “Power quality issues, problems, standards & their effects in industry with corrective means” International Journal of Advances in Engineering & Technology(IJAET),vol-1,issue 2, pp 1-11,May 2011. [7] IEEE, “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems,” IEEE Std. 519-1992, revision of IEEE Std. 519-1981 [8] IEC, Electromagnetic Compatibility, Part 3: Limits- Sect.2: Limits for Harmonic Current Emission,” IEC 1000-3-2, 1st ed., 1995 [9] V. K. Dhar, “Conducted EMI Analysis—A Case Study,” Proceedings of the International Conference on Electromagnetic Interference and Compatibility ‘99, December 6–8, 1999, pp. 181–186. [10] IEEE, “IEEE Guide for Service to Equipment Sensitive to Momentary Voltage Disturbances,” IEEE Std. 1250–1995. [11] IEEE, “IEEE Recommended Practice for Evaluating Electric Power System Compatibility with Electronic Process Equipment,” IEEE Std. 1346-1998. [12] IEEE Std 446-1987, “IEEE Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications,” [13] IEEE Std 1250-1995, “IEEE Guide for Service to Equipment Sensitive to Momentary Voltage Disturbances,” Art 5.1.1, Computers. [14] IEEE 100, The Authoritative Dictionary of IEEE Standard Terms, seventh edition, 2000, p. 234. [15] Mohan, Underland and Robbins, Power Electronics, John Wiley and Sons, 1995. [16] R. C. Dugan, M. F. McGranaghan, S. Santosa, and H. W. Beaty, Electrical Power Systems Quality, 2nd edition, McGraw-Hill, 2002. [17] Blajszczak, G. Antos, P, “Power Quality Park – Idea and feasibility study,” Proc. Of Electric Power Quality and Supply Reliability Conference(PQ), 16-18 June, pp 17 – 22, 2010. [18] S.Khalid, B.Dwivedi, “A Review of State of Art Techniques in Active Power Filters and Reactive Power Compensation,” National Journal of Technology, No 1, Vol. 3, pp.10-18, Mar. 2007. [19] Alexander Kusko, Marc T. Thompson, “Power Quality in Electrical Systems, McGraw-Hill, New York, 2007. [20] J. G. Boudrias, “Harmonic Mitigation, Power Factor Connection, and Energy Saving with Proper Transformers and Phase Shifting Techniques,” Proc. Of Power Quality Conference, ‘04, Chicago, IL [21] Bollen, M.H.J., Styvaktakis, E., Yu-HuaGu, I.: Categorization and analysis of power system transients. IEEE Trans. Power Deliv. 20(3), 2298–2306 (2005) [22] Herath, C., Gosbell,V., Perera, S.:A transient index for reporting power quality (PQ) surveys. Proceedings CIRED 2003, pp. 2.61-1–2.61-5. Bercelona, Spain (2003) [23] Djokic, S.Z., Desmet, J., Vanalme, G., Milanovic, J.V., Stockman, K.: Sensitivity of personal computer to voltage sags and short interruption. IEEE Trans. Power Deliv. 20(1), 375–383 (2005) [24] Marty Martin, “Common power quality problems and best practice solutions,” Shangri la Kuala Lumpur, Malaysia 14. 1997 [25] Singh, B., AL Haddad K., Chandra, A., “A review of active filters for power quality improvement,” IEEE Trans. Ind. Electron., Vol. 46, pp 960–970, 1999 [26] Arrillaga, J., Watson N.R., Chen, S., Power system quality assessment, John Wiley and Sons, 2000. [27] Sabin D.D., Sundaram, A., “Quality enhances reliability”. IEEE Spectrum, Feb. 1996. 34 41. [28] Anurag Agarwal, Sanjiv Kumar, Sajid Ali, “A Research Review of Power Quality Problems in Electrical Power System”. MIT International Journal of Electrical and Instrumentation Engineering, Vol. 2(2), pp. 88-93, 2012.
Published by Mark Stephens, PE, Manager, Industrial Studies, Electric Power Research Institute (EPRI), 942 Corridor Park Blvd, Knoxville, Tennessee 37932, Phone 865.218.8022, Email: mstephens@epri.com
Published by Electrotek Concepts, Inc., PQSoft Case Study: General Reference – Performing Power Quality Audits, Document ID: PQS0306, Date: February 4, 2003.
Abstract: Power quality investigations often require monitoring to identify the exact problem and then to verify the solutions that are implemented. Before embarking on extensive monitoring programs, it is important to develop an understanding of the customer facility, equipment being affected, wiring and grounding practices, and operating considerations. Often, power quality problems can be solved without extensive monitoring by asking the right questions when talking to the customer and performing an initial power quality audit.
This document provides a guide for performing a power quality audit.
PERFORMING POWER QUALITY AUDITS
Power quality investigations often require monitoring to identify the exact problem and then to verify the solutions that are implemented. Before embarking on extensive monitoring programs, it is important to develop an understanding of the customer facility, equipment being affected, wiring and grounding practices, and operating considerations. Often, power quality problems can be solved without extensive monitoring by asking the right questions when talking to the customer and performing an initial site survey. Audit procedures generally include the following steps:
Obtaining customer input
Defining objectives (scope) of the work
Collecting data and conducting an equipment inventory
Wiring and grounding inspections
Determining monitoring locations
Selecting appropriate monitoring equipment
Preparing an audit report
Utilities are generally blamed for power quality problems. They are not necessarily responsible. There are four sources for most customer-encountered problems:
Natural phenomena (e.g., inclement weather)
Normal utility operations (e.g., automatic protection system operations)
Neighboring customers (e.g., welding equipment adjacent to an office)
Customer’s own equipment and facilities.
While most problems have nothing to do with the utility, customers often blame the utility for causing or contributing to the problem. In fact, eighty percent of all power quality related problems in commercial and industrial facilities are caused on the customer side of the meter. In residential facilities, eighty percent of the problems are due to weather and weather-related actions.
The first step is to understand how customers perceive power quality problems. Customers rarely see or understand these problems. They see symptoms of them and the resulting difficulties in their businesses and homes.
Some of the more common symptoms include:
− Equipment damage − Blinking digital displays − Data or information loss / software glitches − Loss of instructional programming and controller timing − An abnormal number of service calls on sensitive equipment − Disk drive problems / computers re-booting − Static shock
Performing a Site Survey
Data Collection Process
The initial site survey should be designed to obtain as much information as possible about the customer facility and the problems being experienced. Specific information that should be obtained at this stage includes:
Customer information: a. company and contact name b. address and phone/fax/e-mail
Nature of the problems: a. data or information loss b. nuisance trips of motor drives or other power-electronic devices c. electronic component failures d. control system malfunctions e. equipment damage
Impact on operations: a. stop or slow production b. lost production or sales c. reduced product quality d. scrap / restart
Characteristics of the sensitive equipment experiencing problems: a. equipment design information b. equipment ride-through characteristics c. equipment application guides or installation/user manuals
Frequency and timing: a. time of day, day of week/month, dates of occurrence b. repetitive (e.g., same time every day)
Coincident problems or known operations that occur at the same time: a. motor starting or slowing down b. capacitor switching c. lights blink on and off momentarily d. weather conditions
Possible sources of power quality variations within the facility: a. motor starting b. capacitor switching c. power electronic equipment operation (e.g., ASDs, PCs, electronic ballast fluorescent lights) d. arcing equipment (e.g., magnetic ballast fluorescent lights, arc furnaces, etc.) e. copy machines, HVAC
Power conditioning equipment being used: a. surge suppressors (e.g., TVSS, arresters, etc.) b. motor-generator sets c. ferroresonant transformers (also know as CVTs) d. UPS systems e. isolation transformers / chokes
Electrical system data: a. oneline or facility wiring diagrams b. transformer information (e.g., size and impedance) c. load information d. capacitor information (e.g., size, connection, and placement) e. feeder/cable data
Data Recording Process
Once this basic data is obtained through discussions with the customer, a site survey should be performed to verify the oneline diagrams, electrical system data, wiring and grounding integrity, load levels, and basic power quality characteristics. Data forms that can be used for this initial verification of the power distribution system are provided in Figure 1 through Figure 4.
Figure 1 – Form for Recording Supply Transformer Data
Figure 2 – Form for Recording Feeder Circuit Data
Figure 3 – Form for Recording Branch Circuit Data
Figure 4 – Form for Recording Individual Load Data
Performing Wiring and Grounding Inspections
Wiring and grounding problems are responsible for many power quality variations within customer facilities. Some electric utility engineers have estimated that 80% of all the power quality problems reported by customers are found to be due to their own wiring and grounding problems. While end-users may have a different opinion, it is commonplace for many power quality problems to be resolved by simply tightening a loose connection, removing an unnecessary ground connection, bonding ground conductors, or replacing a corroded conductor. Therefore, the first step in most power quality investigations is to evaluate the wiring and grounding practices of the facility.
Wiring and grounding problems are identified by physical inspections of wiring, connections etc.; infrared scans to identify heating that may be caused by overloaded conductors or bad connections; and measurements to characterize circuit loading and identify grounding problems. Steps for a wiring and grounding inspections include:
1. Check rms voltage levels.
2. Check for extra neutral-ground bonds. There should be only one neutral-to-ground bond per separately derived system. This is a common problem that causes load currents to flow in the building ground system, creating the potential for serious interference problems. This can be checked by measuring the current in the green wire grounds at the service entrance or at the source of the separately derived system. These currents should be very close to zero. If any current is flowing in the ground, the source of the current should be found and corrected.
3. Check for overloaded neutral conductors. In three-phase, four wire systems supplying single-phase electronic loads, the neutral currents can be as high as 173% of the rms phase current. This can cause overloading of the neutral conductor because the code does not require the neutral conductor to be rated for currents higher than the phase conductor. The neutral currents should be measured with a true rms meter and checked against the ampacity of the neutral conductors. This problem can be corrected by filtering the harmonics of the electronic loads, using a zig-zag transformer, reducing the load, or increasing the neutral conductor capacity.
4. Checking grounding electrode system. The grounding electrode system consists of all the grounded elements of the building that are bonded together to form a grounding system. This can include ground rods, metal water pipe, building steel, concrete-encased electrodes, a ground ring, etc. All of these things should be bonded together to form the best equipotential reference for equipment in the building as possible. It is not advisable to have separate, isolated ground rods for individual equipment in the facility. If a separate ground rod is driven for equipment, it should be bonded with the overall building grounding electrode system. Guidelines for the grounding electrode conductor are provided in Table 1.
5. Check isolated ground receptacle wiring. Isolated ground receptacles are a good way to provide a separate, clean ground for sensitive equipment. These receptacles require a separate ground wire in addition to the safety ground. The isolated ground is insulated from the case of the receptacle and should go back to the ground of the separately derived system, where it is tied in to the building grounding electrode system.
Table 1 – Grounding Electrode Conductor for AC Systems
Grounding Electrode Conductor for AC Systems
6. Check overall circuit layouts. Are sensitive equipment loads on separate circuits from disturbing loads? Loads that are switched or that have power electronic components can create transient disturbances that can impact the operation of some sensitive equipment. Loads like switched motors, copiers, laser printers, elevators, etc. should be on separate circuits from sensitive equipment. The separate circuits provide isolation for high frequency transients and a clean ground reference for the sensitive loads.
7. Check for use of separately derived systems. Separately derived systems permit the bonding of the ground and neutral. In circuits with significant neutral currents (e.g., single-phase electronic loads), a significant neutral-to-ground voltage will build up if there is a significant length between the loads and the supplying transformer. Using an isolation transformer close to the loads minimizes the neutral-to-ground voltage and provides isolation for transient overvoltages.
8. Check for ground loops. Ground loops are probably the most common cause of interference in network systems and the most common problems with multi-port devices in general. Multi-port devices have more than one type of interface. For instance, a television has a power input and a cable input; a computer has a power input and a phone input for the modem and a network input for a LAN. All of these ports require a ground reference. This multiple ground reference scenario creates the potential for serious ground loop problems. Ground loop problems are best avoided by making sure all equipment that is tied together through other ports (e.g., on a LAN) has the same ground reference. This means that all the equipment is part of the same separately derived system.
9. Apply protection to data/communication lines if there is ground loop potential. Sometimes, the ground loop problem described above cannot be avoided. In these cases, protection for data circuits, communication circuits, etc. should be applied. Optical coupling provides the most isolation and prevents the ground loops completely. Where ground loop problems exist, data lines should be protected with baluns, ferrite cores, or data line surge protectors.
A data form that can be used for recording the power distribution and grounding information is provided in Figure 5.
Figure 5 – Form for Recording Power Distribution and Grounding Data
Problems with Conductors and Connectors
The first things to look for when inspecting the service entrance, panel boards, and equipment wiring during a site survey are problems with conductors or connections. A bad connection (faulty, loose, or resistive connection) will result in heating, possible arcing, and burning of insulation. Table 2 summarizes some of the wiring problems that can be uncovered during a site survey.
Table 2 – Problems with Conductors and Connectors
Problems with Conductors and Connectors
Missing Safety Ground
If the safety ground is missing, a fault in the equipment from the phase conductor to the enclosure results in line potential on the exposed surfaces of the equipment. No breakers will trip and a hazardous situation results.
Multiple Neutral to Ground Connections
Unless there is a separately derived system, the only neutral to ground bond should be at the service entrance. The neutral and ground should be kept separate at all panel boards and junction boxes. Double neutral-to-ground bonds result in parallel paths for the load return current where one of the paths becomes the ground circuit. This can cause misoperation of protective devices. In addition, during a fault condition, the fault current will split between the ground and the neutral that could prevent proper operation of protective devices (a serious safety concern). This is a direct violation of the NEC.
Ungrounded Equipment
Isolated grounds are sometimes used due to the perceived notion of obtaining a clean ground. Procedures which involve an illegal insulating bushing in the power source conduit and replacing the prescribed equipment grounding conductor with one to an Isolated Dedicated Computer Ground are dangerous, violate code, and are unlikely to solve noise problems.
Additional Ground Rods
Ground rods for a facility should be part of a grounding system, connected where all the building grounding electrodes are bonded together. Multiple ground rods can be bused together at the service entrance to reduce the overall ground resistance. Isolated grounds can be used for sensitive equipment, as described previously. However, these should not include isolated ground rods to establish a new ground reference for the equipment. The most important problem with additional ground rods is that they create additional paths for lightning stroke currents to flow. With the ground rod at the service entrance, any lightning stroke current reaching the facility goes to ground at the service entrance and the ground potential of the whole facility rises together. With additional ground rods, a portion of the lightning stroke current will flow on the building wiring to reach the additional ground rods. This creates a possible transient voltage problem for equipment and a possible overload problem for the conductors.
Ground Loops
Ground loops are one of the most important grounding problems in many commercial and industrial environments that include data processing and communication equipment. If two devices are grounded via different paths and a communication cable between the devices provides another ground connection between them, a ground loop results. Slightly different potentials in the two power system grounds can cause circulating currents in this ground loop. Because the communication signal levels can be quite low (e.g., five volts), very low magnitudes of circulating current can cause serious noise problems. The best solution to this problem is to use optical couplers in the communication lines, thereby eliminating the ground loop.
Insufficient Neutral Conductor
Switched-mode power supplies and fluorescent lighting with electronic ballasts are becoming increasingly prevalent in commercial facilities. The high harmonic currents produced by these loads can have a very important impact on the required neutral conductor rating for the supply circuits. The most important harmonic component in these load currents is the third. Third harmonic currents in a balanced system appear in the zero sequence circuit. This means that third harmonic currents from three single phase loads will add in the neutral, rather than cancel as is the case for the 60 Hz current. In typical commercial buildings with a diversity of switch-mode power supply loads, the neutral current is typically in the range 140%-170% of the fundamental frequency phase current magnitude. CBEMA has recognized this concern and has prepared a brief to alert the industry to problems caused by harmonics from computer power supplies.
Preparing an Audit Report
The result of a power quality audit is often a formal written report to a customer. The report may be as simple as a one or two page summary letter, or as detailed as a multi-section report. A standardized format for writing audit reports is recommended. In addition, since power quality is very technical and often confusing for a non-technical person, the report should be presented in easily understood language and organization. A suggested outline for an audit report includes the following sections:
− Executive Summary • Description of the Problem • Objectives of the Investigation • Important Conclusions and Recommendations
− System Description • Overview of the Utility Supply System • Overview of the Customer System
− Engineering Analysis Summary • Power Quality Concerns and Related Symptoms Evaluated in the Report • Summary of Computer Simulation Results (if applicable) • Mitigation Alternatives • Economic Analysis (cost/benefit)
− Monitoring Results Summary • Monitoring Period • System Voltage Performance • Summary Harmonic Distortion Levels • Summary of Voltage Sags and Interruptions • Summary of Transient Overvoltages • Discussion of Major Events
− Appendices • Glossary of Terms
The Executive Summary section should acknowledge that the utility or company is interested in helping the customer use electricity without problem, and is pleased to assist the customer in that pursuit. This section should provide a brief history of the events leading up to the audit work, and a description of all work that was done. Include any information that helps the reader understand the purpose and use of the audit report.
The Important Conclusions and Recommendations subsection may be organized by each conclusion drawn from the engineering and monitoring effort. In this structure, the rationale for each conclusion is stated and a specific recommendation is made. Each conclusion should state the cause of the problem, and its relative impact and importance to the customer. Recommendations should be described completely and leave no ambiguity about what actions should be undertaken.
The System Description section should include an overview of both the utility and customer systems, including all power system data collected during the investigation. Oneline or facility wiring diagrams should be included where appropriate.
The Engineering Analysis Summary section should present exactly what was found in each step of the diagnostic process. For example, what types of power quality problems or disturbances were found, and where. Include specific information to help the reader understand what was done. Photos and references to specific points on wiring diagrams or facility layouts are helpful. This section is also a good place to discuss the estimates made by facility personnel of the actual or estimated cost associated with each identified problem. This information is valuable in establishing a cost/benefit analysis for the customer and demonstrating the value of problem mitigation. This is particularly true if the solution to a specific problem involves the outlay of capital funds.
The Monitoring Results Summary section should include summary results for the relevant steady-state and disturbance quantities.
The Appendices section(s) should include supplemental information such as a glossary of terms and detailed monitoring results.
REFERENCES
IEEE Standard 1100. IEEE Recommended Practice for Powering and Grounding Sensitive Equipment (The Emerald Book). IEEE Standard 1159. IEEE Recommended Practice on Monitoring Electric Power Quality. ANSI/NFPA 70-1993, National Electrical Code. Power Quality Considerations for Adjustable Speed Drives, EPRI Publication CU.3036.4.91, Electric Power Research Institute, 1991.
RELATED STANDARDS IEEE Standard 1159 IEEE Standard 1346 IEEE Standard 1250 IEEE Standard 1036 IEEE Standard 519
GLOSSARY AND ACRONYMS ASD: Adjustable-Speed Drive CVT: Constant Voltage Transformer GPR: Ground Potential Rise IEEE: Institute of Electrical and Electronics Engineers MOV: Metal Oxide Varistor PWM: Pulse Width Modulation TVSS: Transient Voltage Surge Suppressors UPS: Uninterruptible Power Supply
Published by Pablo Diaz, P.Eng, Grounding Systems Technologies (GST), The Electricity Forum, Power Quality and Ground Articles.
What is Electrical Grounding?
Electrical Grounding is defined as the process of transferring the immediate discharge of electricity directly to the Earth with the help of a low-resistance wire.
Electrical grounding was invented as a safety system to help prevent people and equipment from accidentally coming in contact with electrical hazards. In a short circuit situation, electrical current wants to find the shortest distance to the earth that has the least possible resistance. A system has to be designed to allow a controlled path to ground that is safe. If not, then the uncontrolled path might be a person’s body or equipment. This can be injurious to people and damaging to equipment. Therefore, electrical grounding is accomplished by connecting the non-current-carrying part of electrical equipment or the neutral part of the supply system to the ground.
The National Electrical Code and Canadian Electrical Code provide rules to govern the proper installation of ground wires in an electrical system to avoid potentially dangerous electric shock accidents. The ground connection provides a path for electric current to flow, for instance from a circuit breaker or electrical box through an electrical connection to a cold water pipe that travels into the earth, and that assists the dissipation of energy into the actual ground.
These are the Top 10 Electrical Grounding questions people ask.
1. How do you Ground Electrical Boxes?
Plastic electrical boxes have their pluses and minuses. Because they are plastic, there is no need to attach a bonding conductor to it. Since it is made of a non-conductive material, switches and outlets cannot short out if they touch the side of the box. The only requirement is that the system is grounded.
When using plastic electrical boxes there is no need to attach a “Bonding Conductor”, because they are plastic, which means they are made with “No Conductive Materials.
You only need to bond those metallic boxes or “non-current carrying” metallic enclosures” which can carry electrical currents only during a “Ground Fault” or exposed to energized parts of the electrical system.
It is needed to attach a ground wire only to metallic boxes or boxes made of conductive plastics that conduct electrical current.
If the box is made from a material that doesnot conduct electricity, such as polyethylene plastic or glass, bonding or grounding is not necessary.
2. What is an electrical ground loop?
Improper grounding can create a lethal hazard. Correct grounding is essential for correct operation and safety of electrical equipment. Grounding can solve many problems, but it can also create new ones. One of the most common problems is called “ground loop”.
Generally, a ground loop connection is created when an electrical system is connected to ground at several points. When two or more pieces of equipment are connected to a common ground through different paths, a “ground loop occurs”. Currents flow through these multiple paths and develop voltages which can cause damage and noise in audio or video systems.
These problems occur because there is a conflict between safety and noise reduction.
To prevent ground loops, all signal grounds need to have a common point and when two grounding points can not be avoided, one side must isolate the signal and ground from the other.
3. What does “in open ground” mean in electrical terms?
An open ground is present when a three-prong receptacle does not have a “Bonding Conductor”.
These open grounds are common in houses built prior to the adoption of the 1962 electrical code. In these older homes the original wiring does not have the safety ground called the “bonding conductor”. In some instances, these houses have ungrounded outlets distinguished by their two-hole/slot rather than the tree holes slots.
The best fix available, rather than rewire the entire house is to utilize an ‘Ground Fault Circuit Interrupter” (GFCI), This device takes place of the standard outlet and replaces it with a special GFCI protected outlet. These are commonly used in the kitchen and bathrooms. They are known for their black and red buttons.
4. What happens if an electrical outlet is not grounded?
Ungrounded outlets increase the chance of: Electricalfire. Without the ground present, errors that occur with the outlet may cause arcing, sparking and electrical charge that can create fire
Grounding of electrical receptacles (which some laypeople refer to as outlets) is an important safety feature that has been required in new construction since 1962, as it minimizes the risk of electric shock and protects electrical equipment from damage.
An ungrounded outlet is it dangerous in that it dramatically increases personal and property risk and increase the chance of Electrical fire, health hazards (risk of shock) and property loss. It is important to note that it takes only 80 milliamps to cause death by electrocution.
5. What is a common ground electrically?
Electrical ground or earthis the reference point in an electrical circuit from which voltages are measured.
A neutral wire is the common current carrying wire in a multiwire electrical circuit. It is generally the wire to which all current carrying conductors are voltage referenced.
It may or may not be grounded, depending on the circuit requirements.
It is not the same as the ground wire in an electrical circuit, because a ground wire does not normally carry any current except in abnormal circumstances such as a ground fault.
But in a network set up (two hot’s sharing a neutral) the neutral is common. Terminology can cause great confusion in electrical work, be sure exactly which way the terms are being used or you could be in trouble. It is essential to use the terminology of the CEC.
6. What is the purpose of electrical grounding?
The term grounding is commonly used in the electrical industry to mean both “equipment grounding” and “system grounding”.
Equipment grounding means the connection of earth ground to non-current carrying conductive materials such as conduit, cable trays, junction boxes, enclosures and motor frames.
System grounding means the connection of earth ground to the neutral point of current carrying conductors such as the neutral point of a circuit, a transformer, rotating machinery, etc.
System grounding, or the intentional connection of a phase or neutral conductor to ground, is for the purpose of limiting the voltage to ground, facilitate the over-current device operation, stabilize the voltage-to-ground during normal operations and to drain leakage currents.
7. What does it mean by electrical grounding?
Electrical grounding is a direct connection to the earth.
Electrical grounding is important because it provides a reference voltage (called zero potential or ground potential) against which all other voltages in a system are established and measured. An effective electrical ground connection also minimizes the susceptibility of equipment to interference, reduces the risk of equipment damage due to lightning and eliminates electrostatic buildup that can damage system components.
When a point is connected to a good ground, that point tends to stay at a constant voltage, regardless of what happens elsewhere in the circuit or system. The earth, which forms the ultimate ground, has the ability to absorb or dissipate an unlimited amount of electrical charge.
8. How do I ground an electric fence?
An effective grounding system consists of three 6 to 8 foot grounding rods, ground rod clamps, and 20 KV insulated hook up wire.
The first grounding rod must be driven into the soil within 20 feet of the fence energizer. Additional grounding rods should be spaced 10 feet apart from the previous rod.
Using a ground rod clamp at each rod, connect the 20 KV insulated hook up wire to each rod in a linear fashion. Remember to strip the end of the wire so the metal is exposed when clamping it to the rod in order to establish a connection.
Once all three rods are connected one to another, the wire can be hooked up to the fence energizer. It must go to the ground terminalon the charger. Do not make the mistake of connecting it to the fence wire or fence terminal
9. How do I ground an electric motor?
Electrical motor windings are normally insulated from all mechanical parts of the motor. However, if the insulation system should fail, then the motor frame could become energized at line voltage. Any person contacting a grounded surface and the energized motor frame simultaneously could be severely injured or killed. Grounding the motor frame prevents this possibility.
The most common method is to use a grounding connector. This connector is placed into the motor’s frame. The other common method is to weld or braze a copper or steel ground pad to the motor frame. The ground should always bolt directly to the main part of the motor frame.
10. How to ground the main electrical panel?
The NEC/Canadian Electrical Code gives for ensuring electrical continuity at Main Electrical panel or service equipment. It requires bonding of all metallic electrical components and a Neutral-to-Ground connection
It requires Bonding jumpers to bond metal parts to the service neutral conductor. This requires a main bonding jumper because the service neutral conductor provides the effective ground-fault current path to the power supply.
It will require threaded fittings, to terminate metal raceways to metal enclosures by threaded hubs on enclosures. If using thread less fittings it must terminate metal raceways to metal enclosures if made tight.
It can also use. bonding-type locknuts, bushings, wedges, or bushings with bonding jumpers.
When a metal raceway containing service conductors terminates to an enclosure without a ringed knockout, you can use a bonding-type locknut. Bonding one end of a service raceway to the service neutral provides the low-impedance fault current path to the source.
Published by Keaton A. Wheeler, Anthony W. Bowers, Charlie H. Wong, Jonathan Y. Palmer and Xue Wang, Primary Engineering and Construction, Saskatoon/Kelowna, Canada {kwheeler, abowers, cwong, jpalmer & swang}@primaryeng.com Published in: 2018 IEEE Electrical Power and Energy Conference (EPEC) Date of Conference: 10-11 Oct. 2018 DOI: 10.1109/EPEC.2018.8598358
Abstract— This paper conducts a load and power quality analysis on a 3-feeder distribution system with a cryptocurrency mining system connected. The study includes power factor and harmonic distortion analysis of actual measured 15-minute data over three days in conjunction with measurements of startup and steady state waveforms to determine the load profile of a cryptocurrency mine when connecting to the electrical grid. In addition, comments are made on the effect of the substation transformer. In the context of this paper, multiple load levels are investigated through a thorough analysis of a sample test system. The effects of the cyptocurrency mine are then quantified through comparison to standards indicated in IEEE Std. 519.
Index Terms — Cryptocurrency mine, power quality, load analysis.
I. INTRODUCTION
Modern power systems are aiming to deliver economic and reliable power to various customers connecting to the grid. The load characteristics associated to these customers can affect utility networks through their interconnection. Power quality (PQ) is one of the most important indicators of safety and efficiency required to be considered when connecting independent facilities to the electrical grid. Most utility company has a specified guideline for interconnection systems, including factors such as total harmonic distortion (THD), individual harmonic currents and voltages and power factors [1]-[2]. In cases where the consumer’s load does not meet these criteria, mitigation factors are required to be employed to meet all the interconnection requirements. These interconnection requirements exist to prevent unnecessary burden on the utility grid and prevent detrimental operation on other energy users.
When connecting a customer-owned facility to the electrical grid, two key factors that are of interest to the utility are power demand and power quality. Power demand is of interest as loads absorb real and reactive power from the network which can result in changes in the existing operational behaviors of the grid when significant energy demands are present. A load analysis may be mandated by the utility when there is limited information available on a certain type of load. Based on the reactive power characteristic of the load, the utility may require power factor correction via methods such as capacitive power compensation to mitigate detrimental effects or reduce burden on the network [1]-[2].
The level of harmonics is another parameter which utilities will consider when a new load is being connected to the network. When new loads are nonlinear in nature they can affect the sinusoidal characteristics of existing current and voltage waveforms thereby introducing harmonics into the network, affecting the operation of electrical equipment [3]-[4]. Different utility companies tend to develop local standards for interconnecting customers. IEEE Std. 519 [3] has been employed by many utilities to quantify limits that are required to maintain power quality while also offering determination methods. Therefore, in this paper, IEEE Std. 519 is used to quantify the power quality of a system connecting with cryptocurrency mine.
Cryptocurrencies are defined as peer-to-peer digital exchange systems which then generate and distribute currency units [5]. Transactions are verified through a process known as “mining”. The concept of cyptocurrency mining can be explained as a process where digital currency is verified (confirms the transaction amount and that the seller originally owned the currency) and added to the blockchain (an electronic ledger) [6]. This verification process is completed using specialized computing devices to solve computationally difficult mathematical problems to be able to create a new block on the chain. The resulting creation of the block rewards the creator with newly created cryptocurrency units and transaction fees [6]. Due to the fast-growing market for cryptocurrencies [7], facilities dedicated to the mining process are beginning to be constructed and connected to the electrical grid. These facilities connect thousands of individual processing units together with the same purpose of creating blocks in the chain, resulting in significant energy demands on the electrical grid [8].
As evident in the literature, an understanding of load electrical characteristics in the context of energy demand and power quality is essential to dictating whether additional infrastructure requirements will need to be employed by the consumer. Through a literature survey, it is apparent that no power quality or loading analysis has been reported on cryptocurrency mines despite their growing prevalence in grid energy demands. As a result, in this paper a cryptocurrency mine is analyzed to determine loading profiles and power quality using measured data. Relevant parameters are calculated and compared to interconnection standards to offer insights into how these mines will affect the electrical grid.
II. THE APPROACH
A. System Under Study
The system utilized in the studies of this paper is depicted in Fig. 3. The system consists of three distribution feeders which are constructed with four switching cubicles while servicing three unit substation transformers per switching cubicle. This gives a total of 36 unit substation transformers (12 per feeder) and 12 switching cubicles (4 per feeder). All feeders and unit transformers are assumed to be the same. The point of common coupling (PCC) is the location where the transmission network connects to the substation transformer. The system nominal frequency is 60 Hz.
Fig. 1. Single line diagram of the cryptocurrency mining facility.
Cryptocurrency mining systems include many individual processing units which are connected to perform a transaction verification purpose. At the time of this analysis, the AntMiner s9 was one of the most efficient hardware’s available on the market for the purposes of cryptocurrency mining [9]. Each AntMiner can be considered to be the smallest power consuming component in one cryptocurrency mining system. A simplified mining system setup is shown in Fig. 2. The loading characteristics would be determined by the accumulated loading profile of individual AntMiner units. For cooling purposes, induction motor-based fans are installed in the facility housing the processing units. These fan loads can be assumed to be 5 MVA operating at a lagging power factor of 0.8. It is assumed that the building fan load is distributed evenly among all the unit substation transformers and all AntMiner units are distributed evenly amongst the three phases.
As shown in Fig. 1, The distribution feeder consists of 500 meters of Hawk 477 kcmil ACSR conductor. The short circuit current of the network is 2.95 kA at the PCC on the high voltage side of the transformer. Conductor data is available in the Appendix.
B. Power Quality and Load Analysis Test Setup
In order to determine how the processor loads behave electrically, an ION 7650 PQ meter capable of sampling at 1028 samples/cycle was connected to ten AntMiner s9 processing units with the default firmware framework employed. It should be noted that the ten AntMiner units were connected in parallel with one another.
Fig. 2. Power quality measurement test setup.
The meter was wired in a single-phase mode as outlined in Fig. 3. It should be noted that the neutral current transformer (CT) was not used as the loads were connected directly across the phase conductors resulting in no neutral current flow [10].
Fig. 3. Wiring diagram of the power quality meter [10].
Utilizing the test setup depicted in Figs. 2 and 3, 15-minute data and waveform captures were logged for three full days while the 10 processing units were running. This data included information on real time energy demand and power quality.
III. POWER DEMAND ANALYSIS FOR A CRYPTOCURRENCY MINE
To determine the load demand and power factor associated with the processing units, three days of 15-minute data was accumulated while the PQ meter was connected to 10 AntMiner s9 units.
Due to the accumulated data, it can be observed that in 99.3% of all logged values, the power factor was between 0.994 and 0.995 leading. It should be noted that the power factor was observed at lower levels only during processing unit startup conditions. Due to this observation, when conducting the remaining studies in this paper, the processing units were assumed to have a power factor of 0.994 leading.
In the context of power demand; it can be determined that the peak power consumption was 1.595 kVA. As can be observed in the load distribution depicted in Fig. 4, the load demand of the processing units is between 1.55 and 1.6 kVA in 66.2% of logged values. In addition, it is above 1.5 kVA in 98.26% of all cases. Again, it should be noted that the power demand was lower only during startup conditions. As a result, the power consumption was taken as 1.6 kVA per processing unit for all studies conducted in this paper.
Occurrence of load demand intervals
Fig. 4. Load distribution for the network under study.
A. Power Factor Analysis
To determine the expected power factor of the system under study, four different load levels were selected with a constant fan load to demonstrate how the power factor changes according to mine size. The load levels used in this case study can be expressed in Table I.
TABLE I: PROCESSING UNITS WITH CORRESPONDING LOAD LEVELS
# of Units
Load Level (MVA)/unit sub
Power factor (lead)
1302
2.187
0.994
1128
1.908
0.994
954
1.630
0.994
781
1.354
0.994
In addition to the processing units, a fan load of 0.139 MVA with a lagging power factor of 0.8 is assumed to be present on each unit substation transformer.
As depicted in Section II, there are a total of 36 unit substation transformers that are connected in parallel in the network. By superposition the total power demand of the system at the PCC can be observed as per Table II.
TABLE II: SYSTEM POWER DEMAND IN TERMS OF POWER FACTOR AND LOAD LEVEL
Total Load Level (MVA)
Power factor
Leading/Lagging?
78.72
0.9978
leading
68.71
0.9982
leading
58.70
0.9987
leading
48.75
0.9992
leading
Due to scaling of the power demand by the number of unit substation transformers in the network, the total power demand of the system can be determined.
TABLE III: SYSTEM POWER DEMAND
System Load Demand (MVA)
System Power Demand (MW)
System Reactive Power Demand (MVAR) (capacitive)
78.72
78.55
5.20
68.71
68.58
4.11
58.70
58.62
3.01
48.75
48.72
1.92
As can be observed in Table III, the load from the system is expected to be capacitive in nature. This result is significant in that it has the potential to prevent utilities from requiring private facilities owners from adding capacitive compensation to attain power factor correction. In addition, observation of Table II makes it apparent that the power factor only changes marginally when varying the load demand of the system while keeping fan loads constant. This highlights that the system’s power factor is influenced by the level of fan load utilized in the network. As a result, it can be inferred that most cryptocurrency mining facilities would not be required to employ methods such as capacitive compensation in the context of power factor correction.
An additional key consideration from this finding is there is an indication, due to its capacitive nature, that the processing unit loads may be prone to harmonic resonant conditions when combining the system impedance along with the overall network impedance. Although outside of the scope of this paper, this is significant as the processing units may be subject to significant resonant conditions and thus causing a detrimental effect on their operation [11]-[12].
IV. POWER QUALITY ANALYSIS OF A CRYPTOCURRENCY MINE
To ascertain the power quality effects of the processing units utilized in the system under study, the voltage and current waveforms of ten of the AntMiner s9’s were captured in real time during startup and steady state. The startup waveform is depicted in Fig. 5.
Fig. 5. Startup voltage and current for ten AntMiner s9 units.
As can be observed in Fig.5, the startup voltage appears to not suffer from distortion. This is to be expected as the processing units were connected directly to a power supply rather than through a distribution conductor which would be present in the actual system. The current waveform suffers from distortion, this can be attributed to the processing unit’s power supply circuitry. As per the measured data, this waveform begins to appear like the steady state within a 15-minute timeframe.
The steady state current and voltage waveforms of the ten AntMiner s9 units is depicted in Fig. 6
Fig. 6. Steady state voltage and current for ten AntMiner s9 units.
As can be seen in Fig. 6, it is apparent that there is a notching phenomenon occurring at the zero crossing of the current waveform. This indicates that the current waveform is subject to harmonic distortion. Taking the fast Fourier transform (FFT) of the current signal at a sampling frequency of 256 samples/cycle, the current harmonic spectrum demonstrated in Fig. 7 is obtained.
It should be noted that Fig. 7 only displays up to the 18th harmonic however harmonics were factored into calculations in this paper up to the order of 34. As can be observed in Fig. 7, the individual harmonics with the greatest magnitude are harmonics 3, 5, 7 and 9 with percentages of 7.97, 4.23, 2.17 and 1.78 percent respectively.
Fig. 7. Current harmonic spectrum for the AntMiner s9 units in the system under study.
In order to determine the THD from the individual current harmonics, (1) is utilized as defined by references [3]-[4]:
Where THDI is the total harmonic distortion of the current, Isys is the system nonlinear load current in Amps, Iload is the total system load current in Amps and h is the harmonic number.
As indicated in Fig. 1, the substation transformer in the system under study has a delta to wye grounded configuration. Therefore, the triplen harmonics will circulate in the delta winding of the transformer and thus will not be observed in the transmission system [4]. The THD levels experienced by the system for both the voltage and current will be lower than when there is no delta winding in the configuration. For the purposes of comparison, the THD levels are calculated to account for triplen filtering but are also given for the topology where triplen filtering does not occur.
Utilizing (1) in conjunction with the harmonic distortions indicated in Fig. 7 with the network loading profiles yields a current THD at the PCC with the following values:
TABLE IV: PCC CURRENT THD FOR THE NETWORK UNDER STUDY
Load Level (MVA)
78.72
68.71
58.70
48.75
Current THD (%) (no triplen filtering)
9.045
8.958
8.842
8.681
Current THD (%) (with triplen filtering)
4.732
4.686
4.626
4.541
With the knowledge of the current harmonic levels, it is possible to determine the voltage harmonics on the network from the given measured PQ data. Utilizing calculation methods outlined in reference [4] whereby the impedance is determined at each harmonic level and multiplied by the harmonic current, the individual voltage harmonics can be determined for the system under study:
TABLE V: PCC INDIVIDUAL VOLTAGE HARMONICS FOR THE NETWORK UNDER STUDY
Harmonic Number
Load Level (MVA)
Load Level (MVA)
Load Level (MVA)
Load Level (MVA)
78.72
68.71
58.70
48.75
3
2.32%
2.01%
1.70%
1.39%
5
2.05%
1.78%
1.50%
1.23%
7
1.47%
1.27%
1.08%
0.88%
8
1.55%
1.35%
1.14%
0.93%
It should be noted that the Table V only indicates the worst of the voltage harmonics. With the knowledge of the voltage harmonic levels, the THD can be determined using (2).
Where THDV is the total harmonic distortion of the voltage and Vsysis the system voltage.
TABLE VI: PCC VOLTAGE THD FOR THE NETWORK UNDER STUDY
Load Level (MVA)
78.72
68.71
58.70
48.75
Voltage THD (%) (no triplen filtering)
4.623
4.005
3.387
2.773
Voltage THD (%) (with triplen filtering)
3.514
3.045
2.575
2.108
As can be observed in Tables IV and VI, the THD for both the voltage and current is significantly changed when accounting for triplen filtering due to the connection topology of the transformer.
A factor that would need to be considered during the design stage of any cryptocurrency mine project would be heating of the transformer due to harmonics. This heating factor can be assessed by using the k-factor which is defined in (3) [13]:
The k-factor weights the harmonic load currents in accordance with their effects on transformer heating. The k-factor will be 1.0 when a purely linear load is present. This is of interest when creating specifications for transformers, particularly in the context of substations. A k-factor rating greater than the calculated value from the load for a transformer will mitigate degradation associated to heating effects of harmonics. Utilizing the data obtained in conjunction with (3), the k-factor calculated for the network given in Fig. 1 can be determined as:
𝑘−𝑓𝑎𝑐𝑡𝑜𝑟 = 1.218
In the case of the system under study indicated in Fig. 1, the ratio of the three-phase short circuit current to the load current is indicated in Table VII:
TABLE VII: SHORT CIRCUIT OVER LOAD CURRENT RATIO OF THE SYSTEM
Load Level (MVA)
78.72
68.71
58.70
48.75
Isc/Iload
10.28
11.76
13.72
16.46
As can be observed in Table VII, the short circuit over load current ratio is below 20 in all calculated cases.
IEEE Std. 519 quantifies a measure for power quality when loads and generation sources connect to the utility network. As per Table VII, the short circuit over load ratio is always less than 20. As such the limits expressed in Table VIII can be applied from IEEE Std. 519. It should be noted that the individual harmonic values in Table VIII should be 25 percent of the specified values for even harmonics. As per IEEE Std. 519 the voltage harmonics should be limited to a maximum of 1 percent for individual values and 1.5 percent for voltage THD for system voltages above 138 kV.
TABLE VIII: CURRENT HARMONIC LIMITS
Harmonic Number
Less than 11
11 to 17
17 to 23
23 to 35
Harmonic current limit (%)
4.0
2.0
1.5
0.6
Current THD limit (%)
5.0
5.0
5.0
5.0
Observation of Fig. 7 and Table IV in conjunction with harmonic requirements outlined in IEEE Std. 519 [3] give the following observations:
The THD of the current is below the prescribed maximum of 5% when triplen filtering occurs.
The THD of the current is above the prescribed maximum of 5% when no triplen filtering takes place.
The third (7.97%) and fifth harmonic (4.23%) current is above the prescribed 4% maximum. If the system under study were to connect to a utility that requires the consumer to meet IEEE Std. 519 then the third and fifth harmonic would be required to be filtered. In the case of the third harmonic, installing a transformer at the substation with a delta winding would be adequate to filter the triplen harmonic.
Through observation of results in Tables V and VI in conjunction with voltage harmonic requirements outlined in IEEE Std. 519 [3] the following conclusions can be drawn for a system voltage of 161 kV:
The THD is above the prescribed maximum of 1.5% when triplen filtering does or does not occur.
Several individual voltage harmonics are above the prescribed minimum of 1%.
Through observation of results obtained in conjunction with IEEE Std. 519, it is apparent that the connection topology of the transformer is required when determining the harmonic effects employed on the system. Furthermore, in the case of the AntMiner s9 processing units, when a delta winding is not present in the substation transformer the current harmonics propagating back into the grid can be as high as 9%, nearly double that of the recommended levels outlined in the literature. Furthermore, it can be observed that the harmonic voltages are consistently higher than the recommendations outlined in IEEE Std. 519.
A potential solution to the excessive harmonics would be to use a higher quality power supply that is capable of both power factor correction and harmonic filtering although this is outside the scope of investigations in this paper.
Although IEEE Std. 519 indicates its own recommended harmonic distortion maximums, individual utilities may indicate their own values. In this case it is imperative that these levels be understood and communicated to stakeholders when attempting to connect a cryptocurrency mine to the network.
V. CONCLUSION
This paper conducts a load and power quality analysis on a cryptocurrency mine using AntMiner s9 processing units. Startup and steady state voltage and current waveforms are presented in conjunction with three days of 15-minute data to determine the power demand and harmonic distortion of the processing units.
Results indicate that the expected power factor of the processing units is in excess of 0.99 with a leading characteristic. This is significant in that it may mean that consumers wishing to connect a cryptocurrency mine to the utility network may not be mandated to install power factor compensation equipment such as capacitor banks. This, from an economic system operation and feasibility standpoint, can be critical as it can dictate the expectations of the utility on the customer.
Furthermore, it can be observed that the cyptocurrency processing units have a nonlinear characteristic and are subject to harmonic distortion. Results indicate that triplen filtering via delta windings in the transformer should be present to prevent high THD levels when significant load demand is present. In addition, it can be observed that despite triplen filtering, individual harmonics may exceed standard prescribed maximums. As a result, it can be determined that a PQ and load analysis, such as the one conducted in this paper, should be mandated by utilities to ensure that power quality conditions are met by facility owners wishing to connect to the grid.
Considering the rapid rise of occurrence of cryptocurrency mines in utility grids, the results and considerations discussed in this paper offer significant practical value in the context of network expansion planning.
ACKNOWLEDGMENT
The authors would like to acknowledge and thank DMG Blockchain Solutions Inc. for providing the AntMiner s9 units and supporting the research conducted in these studies.
APPENDIX A TABLE A.I CONDUCTOR DATA
Conductor
R1 (Ω/km)
L1 (Ω/km)
Hawk 477 kcmil ACSR
0.120
0.267
REFERENCES
[1] FortisBC, “Facility Connection Requirements,” September 2016. [Online]. Available: https://www.fortisbc.com/Electricity/ CustomerService/TransmissionServices/Documents/FortisBC_Facility_Connection_Requirements.pdf. [Accessed February 2018]. [2] SaskPower, “Electric Service Requirements,” November 2017. [Online]. Available: http://www.saskpower.com/wp-content/uploads/Guide-ElectricalInspections-ServiceRequirements.pdf. [Accessed: February 2018]. [3] IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems, IEEE Std. 519, 2014. [4] M.A. Masoum and E.F. Fuchs, Power Quality in Power Systems and Electrical Machines, 2nd ed., London: Elsevier Inc., 2015. [5] U. Mukhopadhyay, A. Skjellum, O. Hambolu, J. Oakley, L. Yu and R. Brooks, “A Brief Survey of Cryptocurrency Systems,” in IEEE Conference on Privacy, Security and Trust, pp. 745-752, Auckland, 2017. [6] I. Eyal, “The Miner’s Dilemma,” in IEEE Symposium on Security and Privacy, San Jose, 2015. [7] R.C. Phillips and D. Gorse, ” Predicting Cryptocurrency Price Bubbles Using Social Media Data and Epidemic Modelling,” in IEEE Symposium on Computational Intelligence, Honolulu, 2017. [8] Digiconomist, Bitcoin Energy Consumption Index. 2018, [Online]. Available: https://digiconomist.net/bitcoin-energy-consumption. [Accessed: March 2018]. [9] N. Drake. (2018, January). “Best ASIC devices for Bitcoin mining in 2018,” TechRadar. [Online]. Available: https://www.techradar.com/ news/best-asic-devices-for-bitcoin-mining-in-2018. [Accessed: March 2018]. [10] Schneider Electric, “PowerLogic ION7550/ION7650 Energy and power quality meter User Guide,” 2009. [Online]. Available: http://www2.schneider-electric.com/resources/sites/ SCHNEIDER_ELECTRIC/content/live/FAQS/31000/FA31689/es_ES/ION7650%20Third%20Party%20protocols%20(70002-0248-06).pdf. [Accessed: January 2018]. [11] S.S. Rangarajan, E.R. Collins and J.C. Fox, “Harmonic Resonance Repercussions of PV and Associated Distributed Generators on Distribution Systems,” in IEEE North American Power Symposium, Morgantown, 2017. [12] N.D. Rao, S.I. Sporea and A.Sawma, “Analysis of Resonance Problems and Harmonic Filter Design in Power Factor Correction Capacitor Applications,” in IEEE Canadian Conference on Electrical and Computer Engineering, Waterloo, 1998. [13] IEEE Recommended Practice for Establishing Liquid-Filled and Dry-Type Power and Distribution Transformer Capability When Supplying Nonsinusoidal Load Currents, IEEE Std. C57.110, 2008.
Published by Electrotek Concepts, Inc., PQSoft Case Study: General Reference – Power Quality Assessment Procedure, Document ID: PQS0608, Date: July 1, 2006.
Abstract: Power quality is a frequently used term that means different things to different people. Common power quality problems include all of the issues that arise from the incompatibility between a utility’s power and the customer’s energy-using equipment that result in impaired operation. These include transients, sags and swells, harmonics, and short- and long-term voltage variations and outages. Also included under this broad area are issues of power reliability.
This case describes a general procedure for performing on-site case studies of power quality concerns.
POWER QUALITY ASSESSMENT PROCEDURE
This case describes a general procedure for performing on-site case studies of power quality concerns. The power quality assessment procedure is based on the variety of different power quality concerns that can exist and focuses on a combination of monitoring and analysis to characterize these concerns. Once the power quality concerns are characterized, the analysis procedures developed can be used to evaluate possible solutions to the power quality problems. These solutions must then be evaluated from both a technical and an economic perspective. There are a number of important areas that must be addressed in the power quality assessment procedure. These include:
Data collection requirements
Important power quality concerns as a function of the type of customer
Equipment sensitivity
Important parameters of the power quality concern
Roles of measurements and simulations in evaluating the concern
Implementation of possible solutions to solve the problem
Technical considerations
Economics
Politics
The procedure presented here, along with the information included in the previous sections, provides the framework for performing case studies in a variety of customer categories. It is worthwhile to group customers in categories that involve the application of similar equipment and have similar electrical system designs. With this in mind, the following list of customer categories is provided as an example of possible customer segments:
Office Buildings
Hospitals
Semiconductor Manufacturers
Data Processing/Banking
Telecommunications
Computer Manufacturers
Point of Sale Retail Operations
Food Processing
Pulp and Paper Mills
Refinery and Chemicals
Automotive Plants
Printing/Publishing
Steel Mills
Rubber/Plastics Plants
Besides classifying the case studies by the type of customer involved, it is also possible to categorize the studies by the type of sensitive equipment involved or the specific type of power quality variation of concern. The results of multiple case studies can be combined to develop descriptions of general power quality concerns for various classes of customer equipment. Some of the most important equipment categories to consider include:
Adjustable speed drives – sensitivity to transient voltages
Electronic controls, ASDs, robotics, and PLCs – sensitivity to voltage sags
Switch-mode power supplies – harmonic current generation and neutral current concerns
Fluorescent lighting (especially with electronic ballasts) – harmonic generation
Power factor correction capacitors – switching transients and magnification
Power factor correction capacitors – harmonic distortion concerns (resonance)
Motor contactors – sensitivity to voltage sags
Power conditioning equipment selection – matching to requirements of protected equipment
Data processing equipment – UPS system specification
Electronic equipment – sensitivity to high frequency transients
Transformers – harmonic heating
Motors – voltage imbalances and harmonic heating
The power quality assessment procedure provides a general framework that contains all the possible elements that may be needed in a power quality case study. Each case study will have unique requirements, depending on the type of customer, equipment sensitivity, and other factors. These unique requirements will influence the level of effort needed for each step or may even permit bypassing a particular step of the overall procedure. The following sections summarize the general steps involved in the procedure.
Identify Power Quality Concerns
The specific power quality concerns that need to be evaluated will be different from customer to customer. A review of the types of equipment used by the customer, process requirements, and economic impacts of problems will lead to a list of concerns that need to be studied. The concerns can include possible problems with both the utility distribution system and the customer facilities. Possible power quality problem categories to be evaluated include the following:
Voltage transients caused by circuit switching and load switching within the customer facility.
Harmonic distortion from adjustable speed drives or other nonlinear loads.
Transformer heating caused by harmonic current levels.
Transient magnification at low voltage capacitor banks.
Sensitivity of adjustable speed drives and control systems to utility capacitor switching transients.
Transients and notching associated with power electronics equipment operation.
Neutral conductor overloading due to harmonic producing loads in commercial installations.
Voltage flicker from arc furnace loads and arc welding loads.
Voltage sags due to faults on parallel circuits on the same distribution system or faults on the transmission system.
Momentary interruptions at industrial and commercial installations due to recloser operations on feeder circuit breakers.
Coupled voltages at customer facilities due to lightning transients on the primary distribution systems.
Identification of the particular concerns involved for an installation provides a focus for the study. Development of a model for analysis of the problem is dependent on the frequency range of the power quality variations that need to be studied. The model can be for computer simulations, hand calculations, or application of simple rules. Analysis of voltage sags often requires modeling all the way up to the utility transmission system. Analysis of high frequency transients might only require a model for a local part of the customer facility.
Monitoring requirements are also based on the particular concern involved. If harmonic distortion is a concern, monitoring of steady state conditions with a harmonic analyzer is required. Analysis of disturbances requires a disturbance monitor. The duration of monitoring depends on how often the problems occur. Some problems with voltage sags or momentary interruptions might only occur a few times per year due to faults on the transmission system while problems caused by capacitor switching might occur every day. Other voltage variations of interest probably fall somewhere between these extremes.
Collect Data/Develop Initial Models
A representation of the customer system and important parts of the utility system should be developed for preliminary analysis. This model can be used for preliminary simulations or analysis to predict power quality problems and evaluate possible solutions to problems. In cooperation with the customer, the data for the model is collected and compiled into a database for convenient reference during the analysis. Important information includes:
Load characteristics − Motors − Power electronics − Process controls − Computers − Adjustable speed drives − Lighting
Power conditioning equipment − Surge suppressors (arresters, varistors, etc.) − Isolation transformers − Constant voltage transformers − Voltage regulators − Power conditioners − UPS systems − Harmonic filters
Distribution system characteristics − Primary Voltage − Underground/Overhead − Protection practices, switching procedures − Capacitor applications (locations, sizes, switching)
Perform Simulations
Simulations provide a convenient means to characterize power quality problems, predict disturbance characteristics, and evaluate possible solutions to problems. They should be performed in conjunction with monitoring efforts and measurements for verification of models and identification of important power quality concerns. The models required for the simulations depend on the system characteristics and the power quality variations being evaluated. The simulations fall into three major categories:
1.Transients. Transient simulations can be performed with the Electromagnetic Transients Program (EMTP). This is a valuable tool for analysis of circuit switching operations, capacitor switching, lightning transients, and transients associated with power electronic equipment operation.
2.Harmonics. Harmonic PLCs is usually performed using steady state analysis techniques at the individual harmonic frequencies. The SuperHarm program can be used for these simulations. Harmonic producing loads can be modeled as harmonic current sources and the simulations used to predict harmonic voltages and currents throughout the customer and utility systems.
Overloading of neutral conductors, transformer heating considerations, resonances caused by capacitor applications, and harmonic currents injected onto the utility system can be evaluated in the simulations.
3.60 Hz Voltage Variations. Variations in the fundamental frequency voltage can be evaluated with conventional analysis tools. Load flow programs give system voltages as a function of load levels on the system. Fault programs can calculate system voltage profiles during fault conditions for analysis of voltage sag concerns.
The models being developed during the case studies in the second phase of this project will serve as templates for future case studies by utility personnel. The models will be verified and refined using the results of monitoring at customer sites.
Develop Monitoring Program
The utility and customer systems being evaluated should be monitored to characterize the power quality variations and to verify the analytical models developed for simulations. The measurement program should be designed based on initial simulation results and on the particular sensitive loads existing at the customer facilities. Monitoring will typically be performed at the customer service entrance and close to particular sensitive loads in order to characterize disturbances coming from the utility system and disturbances that are localized at the sensitive loads. A measurement program plan should be developed which specifies:
quantities to monitor
monitoring durations
threshold levels which will trigger recording of disturbances
waveform sampling and data storage requirements
analysis procedures and data presentation formats
Available monitoring instruments should be evaluated for the measurements required. The problem of obtaining adequate representation of both harmonic and transient conditions must be addressed in particular if both of these concerns exist at a facility.
A customer site survey should be part of the measurement program design. The site survey should characterize the wiring and distribution system integrity and provide basic information about circuit and equipment loading. The data collection forms provided in Chapter 8 can be used to assist with this data collection effort. The site survey should also include discussions with facility personnel regarding characteristics of equipment problems and known customer system conditions at the time power quality variations have occurred.
The actual monitoring effort requires close cooperation between the customer and utility personnel. Monitoring sites and instrumentation should be selected based on the particular concerns being characterized. The duration of monitoring will depend on the parameters that can affect the power quality concerns. It is likely that the customer will need to be responsible for making sure the monitor is operating properly on a day-to-day basis. The monitoring results should be compiled and analyzed for verification of analytical models and to provide a concise description of the possible concerns.
The customer should participate in the monitoring effort by keeping a log of equipment problems during the monitoring period. It is very important to correlate actual equipment problems with power quality variations and with operations on the customer system or the utility system.
Evaluate Measurements Results
The measurement results are analyzed along with the results of simulations to correlate customer problems with the utility system power quality levels. The initial measurements and the site survey are used to identify the phenomena involved and the important parameters. The subsequent measurement results are used to verify the model and characterize the actual power quality variations. Using this information, the model can then be used for more detailed simulations of possible solutions to the power quality problem. The simulations provide the means to evaluate a range of possible solutions from a technical point of view.
Once the range of technical solutions is identified, economic analyses need to be performed to evaluate the possible alternatives for solving customer power quality problems. These alternatives will generally include the following options: − Power conditioning and/or filtering at the sensitive loads.
Central power conditioning and/or filtering at the customer service entrance.
Changing operating procedures or system design on the utility distribution system.
Modification to the design of loads to make them less sensitive to power quality variation
The requirements for each of these options will be developed from the simulation effort and the analysis of measurement results. Power conditioning in this case includes surge suppression, voltage regulation, and possibly backup for momentary interruptions. Harmonic filtering to solve harmonic problems can be applied either at individual loads or at the main service for a facility. Customer system design modifications, such as changing power factor correction procedures and equipment, can have an important impact on power quality variations. If particular loads are much more sensitive that other loads in the facility, either power conditioning at the particular load or design changes to the load equipment should be considered.
Momentary interruptions and voltage sags deserve careful consideration. Distribution system modifications could include implementation of switching procedures to minimize transients associated with capacitor switching events or addition of current limiting devices to minimize the voltage sags that occur during faults on parallel feeders. The impact of protection practices on power quality levels experienced by customers should be evaluated carefully using both the analytical and measurement results. Conclusions will be developed from this effort regarding optimum locations for power quality improvement and the impact of distribution system design practices on customer power quality levels.
SUMMARY
The procedure described in the previous sections is summarized by the block diagram in Figure 1.
Figure 1 – Assessment Procedure for Power Quality Evaluations
REFERENCES
IEEE Standard 100. Terms and Definitions IEEE Standard 1100. IEEE Recommended Practice for Powering and Grounding Sensitive Equipment (The Emerald Book). IEEE Standard 1159. IEEE Recommended Practice on Monitoring Electric Power Quality.
GLOSSARY AND ACRONYMS ASD: Adjustable-Speed Drive CVT: Constant Voltage Transformer GPR: Ground Potential Rise IEEE: Institute of Electrical and Electronics Engineers MOV: Metal Oxide Varistor PWM: Pulse Width Modulation TVSS: Transient Voltage Surge Suppressors UPS: Uninterruptible Power Supply VCR: Video Cassette Recorder
Published by Mirus International Inc., [2010-01-08] MIRUS-FAQ001-B2, FAQ’s Harmonic Mitigating Transformers, 31 Sun Pac Blvd., Brampton, Ontario, Canada. L6S 5P6.
Some manufacturers are promoting the use of 3rd harmonic (180 Hz) blocking filters for the treatment of high neutral currents caused by non-linear loads such as personal computers. These devices are parallel L-C filters tuned to 180 Hz and are connected in the neutral of 4-wire systems between the transformer secondary and the neutral-to-ground connection. Their high impedance to the flow of 3rd harmonic current forces all connected equipment to draw current that does not contain the 3rd harmonic. Although their use will result in a significant reduction in 3rd harmonic current, it is achieved at the risk of rather severe consequences.
Figure 1: Typical installation of 3rd Harmonic Blocking Filter
Some reasons for concern are as follows:
1. The installation raises questions with respect to NEC 2002 compliance. NEC 250.30(A)(2)(a) states that “a grounding electrode conductor for a single separately derived system … shall be used to connect the grounded conductor of the derived system to the grounding electrode…” In addition, “the grounding electrode conductor shall be installed in one continuous length without a splice or joint…” [italics added. See NEC 250.64(C)]. If a simple splice connection is not allowed, then certainly the L-C circuit of the 3rd harmonic blocking filter should not be allowed either. Also, the installation results in an impedance grounded wye system rather than a solidly grounded system.
The only reference in NEC that allows for the introduction of an impedance between the neutral and the grounding electrode is found in Section 250.36, High-Impedance Grounded Neutral Systems. However, these systems are permitted only at 480V and higher and only if they do not serve line-to-neutral loads. They also require the use of ground fault detectors. None of these requirements is met in the normal application of the 3rd harmonic blocking filter where the loads are primarily 120V, phase-to-neutral connected computer or other power electronic equipment.
2. Although tuned to 180 Hz, the L-C circuit will introduce some impedance at 60 Hz as well. The consequences are:
a. Line-neutral short circuit current will be reduced which will limit a circuit breakers ability to clear a line-neutral fault. This can be very dangerous because an uninterrupted fault (commonly referred to as an arcing fault) will often result in an electrical fire.
b. The neutral point at the transformers wye secondary can shift. This can result in 120V line-neutral voltages that rise and fall unpredictably as the load balance between the phases varies.
3. High impedance to the flow of 3rd harmonic current will produce voltage distortion in the form of flat-topping – a dramatic reduction in peak to peak voltage. This will:
a. Significantly reduce the ride-through capability of switch-mode power supplies (SMPS) since the DC smoothing capacitors will not be allowed to fully charge.
b. Reduce the SMPS DC bus voltage, thereby increasing the current demand and the associated I2R losses. Component reliability will be reduced due to higher operating temperatures.
c. Often cause 1-ph UPS systems to switch to battery back-up.
d. Force connected equipment to operate without 3rd harmonic current – an operating mode for which they have not been intended or tested.
At first, when loading is light, problems may not be extremely obvious. However, as the load increases, voltage distortion and flat-topping will also increase until problems do arise. Figure 2 shows the voltage waveform of a 3rd Harmonic Blocking Filter installation at a financial institution. Although neutral current was indeed reduced, it was achieved at the expense of a tremendous increase in voltage distortion. At 30%, the voltage distortion was 6 times the maximum limit of 5% recommended by IEEE std 519. In addition, the crest factor of 1.19 was 19% below the normal sinusoidal crest factor of 1.414. (For an explanation of the effect of voltage flat-topping on connected equipment, blog)
Figure 2: Voltage Flat-topping caused by 3rd Harmonic Blocking Filter
4. The 180 Hz L-C blocking filter requires the use of capacitors and it is well known that capacitors are less reliable than inductors and transformers. Failure of the capacitor or its protection could result in a very high impedance ground at the neutral over the full frequency range. This would have a dramatic effect on 60 Hz unbalance and fault currents.
5. At frequencies above the resonant point (180 Hz), the parallel L-C circuit becomes capacitive which could result in a resonant condition at some higher harmonic frequency.
A much better strategy for 3rd harmonic current treatment is the use of a parallel connected low zero sequence impedance filter such as the MIRUS Neutral Current EliminatorTM (NCETM). This device provides a lower impedance, alternate path for the flow of 3rd harmonic and other zero sequence currents, thereby off-loading the neutral conductor and upstream transformer. In addition, voltage distortion is decreased because the harmonic currents no longer pass through the transformer and cable impedance. For more information on the zero sequence filter, blog.
References: 1. A. Hoevenaars, 3rd Harmonic Blocking Filters – Is the Cure Worse than the Disease, IAEI News, Sept/Oct 2002, pp. 68 – 74
Harmonics and Harmonic Mitigating Transformers (HMT’s) Questions and Answers
This document has been written to provide answers to the more frequently asked questions we have received regarding harmonics and the Harmonic Mitigating Transformer technology used to address them. This information will be of interest to both those experienced in harmonic mitigation techniques and those new to the problem of harmonics. For additional information visit our Website at www.mirusinternational.com.
Published by Marcus Lu, Visual Capitalist, Energy Archive, April 20, 2021 Website: visualcapitalist.com
Visualizing the Power Consumption of Bitcoin Mining
Cryptocurrencies have been some of the most talked-about assets in recent months, with bitcoin and ether prices reaching record highs. These gains were driven by a flurry of announcements, including increased adoption by businesses and institutions.
Lesser known, however, is just how much electricity is required to power the Bitcoin network. To put this into perspective, we’ve used data from the University of Cambridge’s Bitcoin Electricity Consumption Index (CBECI) to compare Bitcoin’s power consumption with a variety of countries and companies.
When people mine bitcoins, what they’re really doing is updating the ledger of Bitcoin transactions, also known as the blockchain. This requires them to solve numerical puzzles which have a 64-digit hexadecimal solution known as a hash.
Miners may be rewarded with bitcoins, but only if they arrive at the solution before others. It is for this reason that Bitcoin mining facilities—warehouses filled with computers—have been popping up around the world.
These facilities enable miners to scale up their hashrate, also known as the number of hashes produced each second. A higher hashrate requires greater amounts of electricity, and in some cases can even overload local infrastructure.
Putting Bitcoin’s Power Consumption Into Perspective
On March 18, 2021, the annual power consumption of the Bitcoin network was estimated to be 129 terawatt-hours (TWh). Here’s how this number compares to a selection of countries, companies, and more.
Name
Population
Annual Electricity Consumption (TWh)
China
1,443M
6,543
America
330.2M
3,989
All of the world’s data centers
–
205
State of New York
19.3M
161
Bitcoin network
–
129
Norway
5.4M
124
Bangladesh
165.7M
70
Google
–
12
Facebook
–
5
Walt Disney World Resort (Florida)
–
1
Note: A terawatt hour (TWh) is a measure of electricity that represents 1 trillion watts sustained for one hour. Source: Cambridge Centre for Alternative Finance, Science Mag, New York ISO, Forbes, Facebook, Reedy Creek Improvement District, Worldometer
If Bitcoin were a country, it would rank 29th out of a theoretical 196, narrowly exceeding Norway’s consumption of 124 TWh. When compared to larger countries like the U.S. (3,989 TWh) and China (6,543 TWh), the cryptocurrency’s energy consumption is relatively light.
For further comparison, the Bitcoin network consumes 1,708% more electricity than Google, but 39% less than all of the world’s data centers—together, these represent over 2 trillion gigabytes of storage.
Where Does This Energy Come From?
In a 2020 report by the University of Cambridge, researchers found that 76% of cryptominers rely on some degree of renewable energy to power their operations. There’s still room for improvement, though, as renewables account for just 39% of cryptomining’s total energy consumption.
Here’s how the share of cryptominers that use each energy type vary across four global regions.
Energy Source
Asia-Pacific
Europe
Latin America and the Caribbean
North America
Hydroelectric
65%
60%
67%
61%
Natural gas
38%
33%
17%
44%
Coal
65%
2%
0%
28%
Wind
23%
7%
0%
22%
Oil
12%
7%
33%
22%
Nuclear
12%
7%
0%
22%
Solar
12%
13%
17%
17%
Geothermal
8%
0%
0%
6%
Source: University of Cambridge Editor’s note: Numbers in each column are not meant to add to 100%
Hydroelectric energy is the most common source globally, and it gets used by at least 60% of cryptominers across all four regions. Other types of clean energy such as wind and solar appear to be less popular.
Coal energy plays a significant role in the Asia-Pacific region, and was the only source to match hydroelectricity in terms of usage. This can be largely attributed to China, which is currently the world’s largest consumer of coal.
Researchers from the University of Cambridge noted that they weren’t surprised by these findings, as the Chinese government’s strategy to ensure energy self-sufficiency has led to an oversupply of both hydroelectric and coal power plants.
Towards a Greener Crypto Future
As cryptocurrencies move further into the mainstream, it’s likely that governments and other regulators will turn their attention to the industry’s carbon footprint. This isn’t necessarily a bad thing, however.
Mike Colyer, CEO of Foundry, a blockchain financing provider, believes that cryptomining can support the global transition to renewable energy. More specifically, he believes that clustering cryptomining facilities near renewable energy projects can mitigate a common issue: an oversupply of electricity.
“It allows for a faster payback on solar projects or wind projects… because they would [otherwise] produce too much energy for the grid in that area”
– MIKE COLYER, CEO, FOUNDRY
This type of thinking appears to be taking hold in China as well. In April 2020, Ya’an, a city located in China’s Sichuan province, issued a public guidance encouraging blockchain firms to take advantage of its excess hydroelectricity.
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