Published by P. M. Balasubramaniam1 and S. U. Prabha2*
1Department of Electrical and Electronics Engineering, Sri Shakthi Institute of Engineering and Technology, Coimbatore, India
2Department of Electrical and Electronics Engineering, Sri Ramakrishna Engineering College, Coimbatore, India *Corresponding author. E-mail: email@example.com
Classically, the aim of the electric power system is to generate electrical energy and to deliver this energy to the end-user equipment at an acceptable voltage. As nonlinear loads draw harmonic and reactive power components of current from ac mains, the quality of power deteriorates. This paper presents a review of the main power quality (PQ) problems with their associated causes and solutions with codes and standards. This paper concludes with some solutions to mitigate the Power Quality problems are presented.
Key words: IEEE 519, Total Harmonic Distortion, Point of Common Coupling, Total Demand Distortion
Power quality (PQ) related issues are of most concern nowadays. The widespread use of electronic equipment, such as information technology equipment, power electronics such as adjustable speed drives (ASD), programmable logic controllers (PLC), energy-efficient lighting, led to a complete change of electric loads nature [1-3]. These loads are simultaneously the major causers and the major victims of power quality problems. Due to their non-linearity, all these loads cause disturbances in the voltage waveform.
Although many efforts have been taken by utilities, some consumers require a level of power quality higher than the level provided by modern electric networks . This implies that some measures must be taken in order to achieve higher levels of power quality in Figure 1.
This paper provides an overview of major power quality issues, solutions and related standards based on an extensive number of publications. The referred publications are mainly extracted from IEEE transactions, IEEE Magazines, IEEE Proceedings, IEE/IET (Institution of Electrical Engineers, currently Institution of Engineering and Technology) Proceedings as well as a few, yet very important Conferences and patent documents on power quality and power quality improvement modules.
The review will be particularly useful for: 1) power system designers and researchers engaged in design, optimization, and quality-enhancement activities in today’s competitive environment; 2) practising engineers who would like to enrich their educational background about the system interaction aspects of power system for any application; and 3) undergraduate and postgraduate students who wish to integrate power quality issues and solutions with modern computing practices.
The paper starts with a short introduction to power quality issues (see section 2). In section 3, the solutions reviewed from various sources for power quality are given. A detailed literature review on codes and standards related to power quality are illustrated in sections 4. Finally, section 5 presents the summary and conclusions.
2. Types of Power Quality Issues
The most common types of power quality problems are presented in Table 1.
Table 1. Most common power quality issues [5-15]
Table 1. Continued
3. Solutions for Power Quality Issues
The mitigation of PQ problems may take place at different levels: transmission, distribution and the end-use equipment. Several measures can be taken at these levels are they are described in the following sections.
3.1 Grid Adequacy
Many PQ problems have origin in the transmission or distribution grid. Thus, a proper transmission and distribution grid, with adequate planning and maintenance, is essential to minimize the occurrence of PQ problems.
3.2 Unified Power Quality Conditioner (UPQC)
UPQC allows the alleviation of voltage and current disturbances that could affect sensitive electrical loads while compensating the load reactive power [16-19]. UPQC consists of combined series and shunt active power filters . The main function of UPQC includes [21-25]:
(i) Reactive power compensation.
(ii) Voltage regulation.
(iii) Compensation for voltage sags and swells.
(iv) Unbalance compensation for current and voltage (for 3-phase systems).
(v) Neutral current compensation (for 3-phase 4-wire systems).
3.3 Distributed Resources – Energy Storage Systems
Interest in the use of distributed energy resources (DER) has increased substantially over the last few years because of their potential to provide increased reliability. These resources include distributed generation and energy storage systems. Energy storage systems, also known as restoring technologies, are used to provide the electric loads with ride-through capability in poor PQ environment. Recent technological advances in power electronics and storage technologies are turning the restoring technologies one of the premium solutions to mitigate PQ problems .
The first energy storage technology used in the field of PQ, yet the most used today, is electrochemical battery. Although new technologies, such as flywheels, Super-capacitors and Superconducting Magnetic Energy Storage (SMES) present many advantages, electrochemical batteries still rule due to their low price and mature technology .
A flywheel is an electromechanical device that couples a rotating electric machine (motor/generator) with a rotating mass to store energy for short durations. The motor/generator draws power provided by the grid to keep the rotor of the flywheel spinning. During a power disturbance, the kinetic energy stored in the rotor is transformed to DC electric energy by the generator, and the energy is delivered at a constant frequency and voltage through an inverter and a control system.
Traditional flywheel rotors are usually constructed of steel and are limited to a spin rate of a few thousand revolutions per minute (RPM). Advanced flywheels constructed from carbon fibre materials and magnetic bearings can spin in vacuum at speeds up to 40,000 to 60,000 RPM. The stored energy is proportional to the moment of inertia and to the square of the rotational speed. High speed flywheels can store much more energy than the conventional flywheels.
The flywheel provides power during a period between the loss of utility supplied power and either the return of utility power or the start of a back-up power system (i.e., diesel generator). Flywheels typically provide 1-100 seconds of ride-through time, and back-up generators are able to get online within 5-20 seconds.
Super-capacitors (also known as ultra-capacitors) are dc energy sources and must be interfaced to the electric grid with a static power conditioner, providing energy output at the grid frequency. A super-capacitor provides power during short duration interruptions or voltage sags. Medium size super-capacitors are commercially available to implement ride through capability in small electronic equipment, but large super-capacitors are still in development, but may soon become a viable component of the energy storage field [28,29].
Capacitance is very large because the distance between the plates is very small (several angstroms), and because the area of conductor surface (for instance of the activated carbon) reaches 1500-2000 m2/g (16000-21500 ft2/g). Thus, the energy stored by such capacitors may reach 50-60 J/g [30-38].
3.6 Superconducting Magnetic Energy Storage (SMES)
A magnetic field is created by circulating a DC current in a closed coil of superconducting wire. The path of the coil circulating current can be opened with a solid-state switch, which is modulated on and off. Due to the high inductance of the coil, when the switch is off (open), the magnetic coil behaves as a current source and will force current into the power converter which will charge to some voltage level. Proper modulation of the solid-state switch can hold the voltage within the proper operating range of the inverter, which converts the DC voltage into AC power. Atypical SMES system.
Low temperature SMES cooled by liquid helium is commercially available [39,40]. High temperature SMES cooled by liquid nitrogen is still in the development stage and may become a viable commercial energy storage source in the future due to its potentially lower costs. SMES systems are large and generally used for short durations, such as utility switching events.
3.7 Enhanced Interface Devices
Using proper interface devices, one can isolate the loads from disturbances deriving from the grid. The common interface devices are dynamic voltage restorer (DVR), transient voltage surge suppressors (TVSS), constant voltage transformers (CVT), Noise Filters, Active Power Filters, Isolation Transformers, Static VAR Compensators (SVC), Harmonic Filters and reactor for power quality improvement.
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 trough a voltage converter.
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. TVSSs 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.
Constant voltage transformers (CVT) were one of the first PQ solutions used to mitigate the effects of voltage sags and transients. To maintain the voltage constant, they use two principles that are normally avoided: resonance and core saturation. Atypical constant voltage transformer. When the resonance occurs, the current will increase to a point that causes the saturation of the magnetic core of the transformer. If the magnetic core is saturated, then the magnetic flux will remain roughly constant and the transformer will produce an approximately constant voltage output. If not properly used, a CVT will originate more PQ problems than the ones mitigated. It can produce transients, harmonics (voltage wave clipped on the top and sides) and it is inefficient (about 80% at full load). Its application is becoming uncommon due to technological advances in other areas.
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. They should be used when noise with frequency in the kilo Hertz range is considerable.
Static VAR compensators (SVR) 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. The main application of SVR is the voltage regulation in high voltage and the elimination of flicker caused by large loads (such as induction furnaces).
Harmonic filters are used to reduce undesirable harmonics. They can be divided in two groups: passive filters and active filters [41-43]. Passive filters consist in a low impedance path to the frequencies of the harmonics to be attenuated using passive components (inductors, capacitors and resistors). Several passive filters connected in parallel may be necessary to eliminate several harmonic components . If the system varies (change of harmonic components), passive filters may become ineffective and cause resonance.
Active filters analyse the current consumed by the load and create a current that cancel the harmonic current generated by the loads. Active filters were expensive in the past, but they are now becoming cost effective compensating for unknown or changing harmonics .
Reactor for power quality improvement is a patented technology [46-49]. The device is installed on a power line, for removing noise that flows into the power line, characterized by: a reactor having a first coil and a second coil wound on a core disposed between the input end and the output ends of the power line, wherein in the reactor the first end of the first coil is connected to the input end of a first power line, the second end of the first coil is connected to the output end of the first power line, and the first end of the second coil is connected to the output end of the second power line, and the second end of the second coil is connected to the input end of the second power line.
3.8 Make End-use Devices Less Sensitive
Designing the equipment to be less sensitive to disturbances is usually the most cost effective measure to prevent PQ problems. Some manufacturers of end-use equipment are now recognising this problem, but the competitive market means that manufacturers should reduce costs and only respond to customers’ requirements. The exception is the ASD market, where manufacturers are actively promoting products with enhanced ride-through capabilities.
Some of the energy meters have the capability for power quality detection, monitoring, reporting, recording and communication in a revenue accuracy electrical power meter is disclosed . Transient events are detected by monitoring the Wave shape of the electrical power and comparing deviations to a known threshold. Sags and Swells are detected by computing root mean square value over a rolling Window and comparing the computed value with a known threshold. Harmonic frequencies and symmetrical components are quantified by a known algorithm and compared with a known threshold . Adding a capacitor with a larger capacity to power supplies, using cables with larger neutral conductors, derating transformers and adjusting under voltage relays, are measures that could be taken by manufacturers to reduce the sensitivity of equipment to PQ problems.
4. Codes and Standards
Some measures have been taken to regulate the minimum PQ level that utilities have to provide to consumers and the immunity level that equipment should have to operate properly when the power supplied is within the standards. Standardization organizations like IEC, CENELEC, and IEEE have developed a set of standards with the same purposes. In Europe, the most relevant standards in PQ are the EN 50160 (by CENELEC) and IEC 61000. IEEE power quality standards do not have such a structured and comprehensive set as compared to IEC . Nonetheless, the IEEE standards give more practical and some theoretical background on the phenomena, which makes it a very useful reference. Some of the IEEE power quality standards are described in the ensuing sections.
4.1 IEEE 519
Power system problems that were associated with harmonics began to be of general concern in the 1970s, when two independent developments took place. The first was the oil embargo, which led to price increases in electricity and the move to save energy. Industrial consumers and utilities began to apply power factor improvement capacitors. The move to power factor improvement resulted in a significant increase in the number of capacitors connected to power systems. American standards regarding harmonics have been laid out by the IEEE in the 519 Standard: IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems. There is a combined effect of all nonlinear loads on utility systems that have a limited capability to absorb harmonic current. Further, utilities are charged with the responsibility to provide a high quality supply in terms of voltage level and waveform. IEEE 519 recognizes not only the absolute level of harmonics produced by an individual source but also their size relative to the supply network. It should be noted that IEEE 519 is limited to being a collection of Recommended Practices that serve as a guide to both suppliers and consumers of electrical energy. Where problems exist, because of excessive harmonic current injection or excessive voltage distortion, it is incumbent upon supplier and consumer to resolve the issues within a mutually acceptable framework .
4.2 IEEE 519 Standard for Harmonic Voltage Limits
According to IEEE 519 Table 2 shows that, harmonic voltage distortion on power system 69 kV and below is limited to 5% Total Harmonic Distortion with each individual harmonic limited 3% .
Table 2. Harmonic voltage distortion limits
4.3 IEEE 519 Standard for Harmonic Current Limits
General distribution systems [GDS 120V-69,000 V]: Current distortion limits are for odd harmonics. Even harmonics are limited to 25% of the odd Harmonic limits. 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 PCC. TDD is the total demand distortion (= THD normalized by IL are shown in Table 3).
Table 3. Harmonic current distortion limits
General sub-transmission systems [GSTS 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.
ISC is the available short circuit current at the point of common coupling. The ISC is determined by the size, impedance, and voltage of the service feeding the PCC. IL is the maximum demand load current (fundamental frequency component) measured at the PCC are shown in Table 4. It is suggested that existing facilities measure this over a period of time and average it. Those creating new designs should calculate the IL using anticipated peak operation of the facility. The point of common coupling with the consumer/utility interface is the closest point on the utility side of the customer service where another utility service customer is or could be supplied. The ownership of any apparatus such as a transformer that the utility might provide in the customers system is immaterial to the definition of PCC. This definition has been approved by IEEE working group.
Table 4. Maximum harmonic current distortion level
4.4 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. There is a separate chapter for grounding sensitive equipment.
4.5 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. It provides facility designers, operators and owners with guidelines for assuring uninterrupted power, virtually free of frequency excursions and voltage dips, surges, and transients.
4.6 IEEE Standard 1100-1999, Recommended Practice for Powering and Grounding Sensitive Electronic Equipment
Recommended design is 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.
4.7 IEEE Standard 1346-1998 Recommended Practice for Evaluating Electric Power System Compatibility with Electronic Process Equipment
A standard methodology for the technical and financial analysis of voltage sag compatibility between process equipment and electric power systems is recommended. The methodology presented is intended to be used as a planning tool to quantify the voltage sag environment and process sensitivity.
4.8 IEEE Standards Related to Voltage Sag and Reliability
The distribution voltage quality standard i.e. IEEE Standard P1564 gives the recommended indices and procedures for characterizing voltage sag performance and comparing performance across different systems. A new IEC Standard 61000-2-8 titled “Environment – Voltage Dips and Short Interruptions” has come recently. This standard warrants considerable discussion within the IEEE to avoid conflicting methods of characterizing system performance in different parts of the world.
4.9 IEEE Standards Related to Flicker
Developments in voltage flicker standards demonstrate how the industry can successfully coordinate IEEE and IEC activities. IEC Standard 61000-4-15 defines the measurement procedure and monitor requirements for characterizing flicker. The IEEE flicker task force working on Standard P1453 is set to adopt the IEC standard as its own.
4.10 Standards Related to Custom Power
IEEE Standard P1409 is currently developing an application guide for custom power technologies to provide enhanced power quality on the distribution system. This is an important area for many utilities that may want to offer enhanced power quality services.
4.11 Standards Related to Distributed Generation
The new IEEE Standard P1547 provides guidelines for interconnecting distributed generation with the power system.
4.12 420-2013 – IEEE Standard for the Design and Qualification of Class 1E Control Boards, Panels and Racks Used in Nuclear Power Generating Stations
This standard specifies the design requirements for new and/or modified Class 1E control boards, panels, and racks and establishes the methods to verify that these requirements have been satisfied. Methods for meeting the separation criteria contained in IEEE Std 384 are addressed. Qualification is also included to address the overall requirements of IEEE Std 323 and recommendations of IEEE Std 344.
4.13 IEEE Standard 384-2008 – IEEE Standard Criteria for Independence of Class 1E Equipment and Circuits
The independence requirements of the circuits and equipment comprising or associated with Class 1E systems are described. Criteria for the independence that can be achieved by physical separation and electrical isolation of circuits and equipment that are redundant are set forth. The determination of what is to be considered redundant is not addressed.
4.14 IEEE Standard C57.18.10-1998 – IEEE Standard Practices and Requirements for Semiconductor Power Rectifier Transformers
Practices and requirements for semiconductor power rectifier transformers for dedicated loads rated single-phase 300 kW and above and three-phase 500 kW and above are included. Static precipitators, high-voltage converters for DC power transmission, and other nonlinear loads are excluded. Service conditions, both usual and unusual, are specified, or other standards are referenced as appropriate. Routine tests are specified. An informative annex provides several examples of load loss calculations for transformers when subjected to non-sinusoidal currents, based on calculations provided in the standard.
4.15 IEEE Standard C57.21-1990 -IEEE Standard Requirements, Terminology and Test Code for Shunt Reactors Rated Over 500 kVA
All oil-immersed or dry-type, single-phase or three phase, outdoor or indoor shunt reactors rated over 500 kVA are covered. Terminology and general requirements are stated, and the basis for rating shunt reactors is set forth. Routine, design, and other tests are described, and methods for performing them are given. Losses and impedance, temperature rise, dielectric tests, and insulation levels are covered. Construction requirements for oil immersed reactors and construction and installation requirements for dry-type reactors are presented.
5. Summary and Conclusions
This paper gives a comprehensive review by critical analyzing about power quality problems, issues, related international standards, and the solutions. The correct solutions are also discussed which can be remedy for power quality problems generated in different phenomena. Coordination with existing industry practices and international harmonic standards is also considered in this paper. To overcome the negative impact of poor power quality on equipment and businesses, suitable power quality equipment can be invested. Identifying the right solution remains the first step. Many power quality problems are easily identified once a good description of the problems is obtained. Unfortunately, the tensions caused by power problems often result in vague or overly dramatic descriptions of the problem.
A power quality audit can help determine the causes of your problems and provide a well designed plan to correct them. The power quality audit checks the facility’s wiring and grounding to ensure that it is adequate for your applications and up to code. The auditor normally will check the quality of the ac voltage itself, and consider the impact of the utility’s power system. Many businesses and organizations rely on computer systems and other electrical equipment to carry out the mission critical functions, but they aren’t safeguarding against the dangers of an unreliable power supply. It is time utilities as well as businesses engage in more proactive approach to power quality treats by engaging in power quality analysis.
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Manuscript Received: Sep. 24, 2013
Accepted: Jul. 23, 2015
Source & Publisher Item Identifier: Journal of Applied Science and Engineering, Vol. 18, No. 4, pp. 371-380 (2015), DOI: 10.6180/jase.2015.18.4.08