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.

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.

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]

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.

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]

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]

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]

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 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:

1. Obtaining customer input
2. Defining objectives (scope) of the work
3. Collecting data and conducting an equipment inventory
4. Wiring and grounding inspections
5. Determining monitoring locations
6. Selecting appropriate monitoring equipment
7. 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:

1. Natural phenomena (e.g., inclement weather)
2. Normal utility operations (e.g., automatic protection system operations)
3. Neighboring customers (e.g., welding equipment adjacent to an office)
4. 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:

1. Customer information:
   a. company and contact name
   b. address and phone/fax/e-mail

2. 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

3. Impact on operations:
   a. stop or slow production
   b. lost production or sales
   c. reduced product quality
   d. scrap / restart

4. Characteristics of the sensitive equipment experiencing problems:
   a. equipment design information
   b. equipment ride-through characteristics
   c. equipment application guides or installation/user manuals

5. Frequency and timing:
   a. time of day, day of week/month, dates of occurrence
   b. repetitive (e.g., same time every day)

6. 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

7. 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

8. 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

9. 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.

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

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.

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

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 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 – harmonic distortion concerns
• 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:

1. Load characteristics
   − Motors
   − Power electronics
   − Process controls
   − Computers
   − Adjustable speed drives
   − Lighting
2. Transformer sizes/ratings
3. Conductor lengths, characteristics
4. Customer capacitor sizes, locations, switching procedures
5. Customer equipment and circuit switching
6. Power conditioning equipment
   − Surge suppressors (arresters, varistors, etc.)
   − Isolation transformers
   − Constant voltage transformers
   − Voltage regulators
   − Power conditioners
   − UPS systems
   − Harmonic filters
7. 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.

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.

RELATED STANDARDS
IEEE Std. 1159
IEEE Std. 1346
IEEE Std. 1250
IEEE Std. 1036
IEEE Std. 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
VCR: Video Cassette Recorder