Published by Electrotek Concepts, Inc., PQSoft Case Study: Customer Data Analysis, Document ID: PQS1102, Date: March 15, 2011.
Abstract: Monitoring is often used to characterize power quality levels at various locations on utility and customer power systems. Field measurements provide a convenient means to characterize power quality problems. This case study summarizes a commercial customer power quality measurement data evaluation.
CUSTOMER DATA ANALYSIS CASE STUDY
A commercial customer power quality measurement data analysis case study was completed for the system shown in Figure 1. The utility substation included a 10 MVA, 161 kV/12.47 kV step-down transformer and two 12.47 kV distribution feeders that supplied a mix of residential and commercial customers. One of the feeders had a switched 300 kVAr capacitor bank that was being used for power factor correction and voltage control. The monitoring location is identified as Commercial Customer #1. The customer, which was supplied from 150 kVA transformers with 120/208 V and 480 V secondary buses, was a small office building.
The twelve-month monitoring period was from January 1, 2003 thru December 31, 2003. The power quality instrument used to complete the power quality measurements was the Dranetz-BMI 8010 PQNode™. The instrument samples voltage at 256 points-per-cycle, current at 128 point-per-cycle, and follows the IEC 61000-4-3 method for characterizing harmonic measurement data. The sampling rate also allows characterization of low-to-medium frequency oscillatory transients. The measurement and statistical analysis was completed using the PQView® program.
Figure 2 shows the rms voltage histogram for the twelve-month monitoring period. Statistical analysis of the 37,463 individual steady-state measurements yielded a minimum voltage of 264.1 V, an average voltage of 294.4 V, and a maximum voltage of 306.3 V. In addition, the CP95 value was 299.7 V (108% of nominal). CP95 refers to the cumulative probability, 95th percentile of a value.
Figure 3 shows the measured customer voltage distortion (VTHD) trend during the twelve-month monitoring period. The minimum harmonic distortion was 0.79%, the average distortion was 2.56%, and the maximum distortion 21.18%. The CP95 value was 3.83%. The measured voltage distortion value was below the assumed 5% limit a vast majority of the time.
Figure 1 – Illustration of Oneline Diagram for Commercial Customer Data Evaluation
Figure 2 – Measured Customer Secondary Voltage Histogram
Figure 3 – Measured Customer Secondary Voltage Distortion
Figure 4 shows the statistical summary of total harmonic voltage distortion (VTHD) and number of individual harmonics for the twelve-month monitoring period. The analysis showed that the predominate harmonics for the measured customer secondary bus voltages were the 3rd, 5th, and 7th. The measured values were below the assumed 5% voltage distortion limit.
Figure 4 – Measured Statistical Summary of Voltage Distortion and Harmonics
Voltage sags and momentary interruptions are inevitable on the electric power system. Many of these variations occur during faults on the power system, and since it is impossible to eliminate the occurrence of faults, there will always be voltage variations on customer systems. Other sources of voltage variations include unbalance, induction motor starting, and voltage flicker. Table 1 shows an rms variation event summary listing for several of the sixty rms variation events that occurred during the twelve-month monitoring period. The table shows the date-and-time for each event, as well as the phase-to-neutral voltage magnitude in both volts (kV) and per-unit and the event duration in both seconds and cycles.
Figure 5 shows the corresponding waveform and rms characteristic for one of the voltage sag events measured during the monitoring period (Event #3 in Table 1). The magnitude of the voltage sag was 47.9% and the duration was 7.0 cycles. The voltage sag occurred during a storm. It was caused by a short-duration fault and subsequent fuse clearing on a feeder branch circuit.
Table 1 – Event Listing for Measured RMS Variations
Figure 5 – Measured Customer Secondary Voltage Sag Event
When there are a significant number of events, it is generally not desirable to show the results for each individual measurement. One method for summarizing rms variation event data is to graph the magnitude and duration data on one single scatter plot. This method may also include an equipment tolerance (e.g., ITIC) overlay. Figure 6 shows a summary of the measured rms variation events along with an ITIC overlay. The graph also shows the number of events that are outside the equipment sensitivity characteristic.
Voltage variation indices may be used to assess the service quality for a customer. One commonly used benchmarking value is known as SARFI, which stands for System Average RMS Variation Frequency Index. SARFI represents the average number of specified rms variation measurements that occurred over the assessed period. For example, SARFI70 is a measure of the number of voltage sags that can be expected with a minimum voltage below 70%. Another popular use of SARFI is to define the threshold as a curve. For example, SARFICMEBA would represent the number of rms variation events outside the commonly used CBEMA voltage tolerance envelope. The CBEMA curve was originally developed by the Computer Business Equipment Manufacturers Association. The curve was first published in IEEE Std. 446-1995.
The calculated SARFI values for the twelve-month monitoring period are summarized in Table 2. The SARFI90 value of fifty-six can be determined by counting the number of events with a voltage magnitude below 90%. In addition, the SARFIITIC value of twenty-four that is shown in the table corresponds to the data previously shown in Figure 6.
Table 2 – Summary of RMS Voltage Variation SARFI Values
The causes of the transients measured during the monitoring period included capacitor bank switching, transformer energizing, single-phase faults, switch failure, recloser operations, and current-limiting fuse operations.
Table 3 shows a transient event summary listing for several of the representative transients that were measured during the twelve-month monitoring period. There were several thousand oscillatory transients that were captured. The table shows the date-and-time for each event, as well as the peak phase-to-neutral voltage magnitude in both volts (kVpk) and per-unit and the event duration in both seconds and cycles.
Table 3 – Event Listing for Measured Transient Events
One of the common transient events measured throughout the monitoring period was during energization of the 300 kVAr capacitor bank on the utility distribution feeder. Figure 7 shows a representative measured three-phase customer secondary voltage waveform during uncontrolled energization of the pole-mounted 300 kVAr capacitor bank on feeder #1 (Event #3 in Table 3). The utility capacitor bank was switched on-and-off each day using time clock controls in an attempt to maintain a relatively constant voltage profile. The peak magnitude of the measured transient voltage was 591.1 V (1.51 per-unit) and the principal frequency for the capacitor energizing waveform was approximately 900 Hz. The duration of the transient event was approximately 8.203msec or 0.492 cycles. The capacitor bank was energized using a three-phase oil switch.
Typical voltage magnitude levels for switching distribution capacitor banks range from 1.3 to 1.5 per-unit and typical transient frequencies generally fall in the range from 300 to 1000 Hz. Power quality problems related to utility capacitor bank switching include customer equipment damage or failure, nuisance tripping of adjustable-speed drives or other process equipment, transient voltage surge suppressor failure, and computer network problems.
Utilities switch capacitor banks in-and-out of service routinely to provide voltage support and to improve power factor. One potential disadvantage of capacitor bank switching is the effect that such an operation can have on the topology of the system. Switching capacitor banks into mostly inductive circuits can tune the natural frequency of the circuit closer to harmonic frequencies that might be prevalent on the system. Obviously, this can be a significant problem, possibly resulting in severe voltage and current distortion, increased losses, and overheating of system equipment.
Figure 7 – Measured Customer Transient Voltage during Capacitor Bank Switching
Another relatively common transient event was during a fuse operation on one of the utility distribution feeders. A representative three-phase waveform is shown in Figure 8. The peak magnitude of the measured transient voltage was 593.8 V (1.52 per-unit) and the principal frequency for the transient waveform was approximately 300 Hz.
Figure 8 – Measured Customer Transient Voltage during Fuse Operation
Table 4 shows a summary of relevant terms and indices related to power quality problems on utility and customer power systems.
Table 4 – Power Quality Related Equations and Indices
SUMMARY
This case study summarized a commercial customer power quality measurement data analysis. The case showed that monitoring may be used to characterize power quality levels on customer power systems. The length of the monitoring period, which was twelve-months for this study, is dependent on the nature of the power quality problem. The analysis included trends and statistical summaries of the rms voltage and the harmonic voltage distortion levels.
The results showed that the harmonic voltage distortion levels were below the assumed 5% voltage distortion limit. The results of the analysis also showed that most of the rms variation events were short duration voltage sags. Constant voltage transformers, coil-lock devices, magnetic synthesizers, and a number of power electronic based power conditioners may be used for protection against voltage sag events. Voltage sag protection may be implemented on a single coil or piece of equipment. Correction may also be chosen for large portions of a facility or even for the entire facility.
Mitigation alternatives for reducing harmonic distortion levels include methods for modifying the power system to reduce or eliminate the harmonic resonances that can cause very high current or voltage distortion levels. For example, a passive shunt harmonic filter may be added to the utility or customer system to divert the troublesome harmonic currents off the system and into the filter.
The causes of the transients measured during the monitoring period included capacitor bank switching, single-phase faults, recloser operations, and current-limiting fuse operations. Customer transient mitigation options include power conditioners and TVSSs.
REFERENCES
IEEE Recommended Practice for Monitoring Electric Power Quality,” IEEE Std. 1159-1995, IEEE, October 1995, ISBN: 1-55937-549-3.
IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE Std. 519-1992, IEEE, ISBN: 1-5593-7239-7.
“IEEE Recommended Practice for Emergency & Standby Power Systems for Industrial & Commercial Applications” (IEEE Orange Book, Std. 446-1995), IEEE, ISBN: 1559375981.
“IEEE Guide for Application and Specification of Harmonic Filters,” IEEE Std. 1531-2003, IEEE, ISBN: 0-7381-3718-9.
“IEC Electromagnetic Compatibility Part 4-3: Testing and Measurement Techniques – Radiated, Radio-Frequency, Electromagnetic Field Immunity Test,” IEC 61000-4-3 Consol. Ed. 3.1-2008, International Electrotechnical Commission.
R.C. Dugan, M.F. McGranaghan, S. Santoso, H.W. Beaty, “Electrical Power Systems Quality,” McGraw-Hill Companies, Inc., November 2002, ISBN 0-07-138622-X.
RELATED STANDARDS IEEE Std. 1159, IEEE Std. 519
GLOSSARY AND ACRONYMS ASD: Adjustable-Speed Drive DPF: Displacement Power Factor PF: Power Factor PWM: Pulse Width Modulation THD: Total Harmonic Distortion TPF: True Power Factor
Addressing Evolving Customer Technologies and an Increasingly Complex Power Grid
Published by Electric Power Research Institute (EPRI), Inc. Document ID: 3002022396, March 2022.
Introduction
Over the past 50 years, the concept of power quality (PQ) has evolved from being an unknown and undefined concept to being understood as a fundamental component of grid performance, utility economics, and customer satisfaction.
Until the 1970s, most electrical loads were linear and could ride through most grid voltage and current variations. The emergence of microprocessors, process controls, and other equipment sensitive to voltage and current variations, as well as equipment that caused PQ variations, resulted in an entirely new technology discipline focused on the compatibility between the grid and the devices connected to it. This concept of compatibility is at the heart of PQ issues and research.
While industrial facilities may have the most significant PQ issues, PQ concerns can be important for any customer. For instance, voltage variations caused by large load variations, such as when motors start or when arc furnaces are in use, may cause lights to flicker for customers over a wide area. While light flickers are not as much of a concern these days with LED lights that have their own power supplies, voltage sags occurring over wide areas when there is a fault on the grid can potentially affect a diverse range of equipment and processes. Consumer electronic devices, like televisions, computers, and now electric vehicle chargers, can affect the grid and other customers by causing harmonic distortion.
These types of issues have been the subject of detailed laboratory and field investigations conducted by EPRI’s PQ research program since the 1980s in coordination with other investigators around the world. EPRI’s PQ research has helped raise awareness of this important topic while providing solutions to utilities. As EPRI celebrates its fiftieth anniversary in 2022, it is timely to take a short look at the history of the PQ research program and related activities in the industry as a whole new set of compatibility challenges emerge.
This white paper attempts to accomplish three things:
Provide a timeline and overview of selected historical PQ developments and events.
Offer a starting point for discussions on the topic. In examining this history, lessons learned may be useful when looking for solutions to current and future PQ issues.
Discuss new challenges in the increasingly complex grid environment associated with new technologies and system configurations that are characteristic of the energy transition.
A PQ problem is any power condition manifested in voltage, current, or frequency deviations that results in the failure or maloperation of customer equipment [8]. These problems can include transients, flickers, voltage sags and swells, and harmonic distortion.
The Concept of Compatibility
The concept of “compatibility” is key to virtually all PQ investigations. To achieve compatibility, one must understand the causes of PQ variations (or emissions) and how these can result in voltage variations that can affect other equipment (interaction of emissions with the system impedance characteristics). Equipment immunity levels must be selected to make sure that equipment can operate properly in the presence of “normal” PQ variations. System variations and equipment immunity levels are both probabilistic phenomena, so achieving compatibility is a probabilistic function as well (Figure 1). This concept of compatibility was best described as part of the International Electrotechnical Commission (IEC) 61000 series of standards, first drafted in the 1990s, which addresses the full range of PQ issues from emissions to immunity, from to measurements to testing. Much research has gone into trying to gain an understanding of the relationship between system variations and equipment immunity levels, the economics of PQ impacts, and solutions to problems when they arise.
Figure 1. The concept of compatibility levels as defined in IEC Standard 61000-3-2
Providing the Foundation
Research into PQ events starts with understanding the phenomena associated with PQ variations and impacts. A few of the industry leaders and the publications that resulted from their work are referenced here as foundational in creating a basic understanding
Francois Martzloff—National Institute of Standards and Technology
Early PQ problems often involved transient voltages from lightning and system switching affecting equipment, especially as electronic equipment started to become more common. Transient voltage measurements, transient protection, and grounding practices were implemented to avoid failures due to transient voltages. Francois Martzloff (Figure 2) at the National Institute of Standards and Technology (NIST) was a leader in characterizing these transient voltage concerns, determining how to measure them, and protecting equipment from them. In 1991, Martzolff pulled together state-of-the-art references on surge protection as part of NISTIR 4657 [1].
Figure 2. Francois Martzloff
This early work was sponsored by the key organizations that were already working on the range of PQ issues at the time, including:
Computer and Business Equipment Manufacturers Association (CBEMA)
EPRI
Institute of Electrical and Electronics Engineers (IEEE) Standards Coordinating Committee 22 on Power Quality
IEEE Surge Protective Devices Committee
Harmonics—Roger Dugan, Mack Grady, Erich Gunther, Mark Halpin
Concerns for distortion due to nonfundamental components of the voltage and current and the potential for resonances at these higher frequencies were understood since the beginning of alternating current systems. Charles Steinmetz [2] first described these concerns, which were soon to become known as harmonics. The mathematical foundation for characterizing harmonics goes back to Jean-Baptiste Joseph Fourier [3] and the concept of Fourier analysis to represent a signal by a superposition of its harmonic components. Harmonic concerns became more pronounced as electronic equipment became more prevalent in the 1980s. In this decade, major advances were made in harmonic analysis methods and in standards for evaluating compatibility (such as IEEE 519 and IEC 61000-3-2). Roger Dugan, working on an EPRI project in the late 1970s, developed one of the first harmonic analysis tools, in parallel to work at Purdue by Mack Grady. There have been many advancements to these tools over the years, in particular by leaders like Erich Gunther. Mark Halpin took this work and led its incorporation into standards.
Harmonics are sinusoidal voltage or current waveforms whose frequency is an integer multiple of the system frequency (60 Hz in U.S. systems). Periodically distorted waveforms, which often are results from the use of nonlinear loads, can be mathematically described as the sum of ideal waveforms of the fundamental frequency and its harmonics [27].
Voltage Sags—Tom Key, Math Bollen, Dan Sabin
Short-duration voltage variations, known as voltage sags, became one of the most important PQ concerns as industrial processes became automated and the controls for these processes began to include electronic equipment that could be affected by these short voltage variations. CBEMA did some of the early work in 1977 to define the concept of characterizing voltage sags by their magnitude and duration so that they could be compared with the ride-through characteristics of equipment. The CBEMA curve is discussed later in this white paper, including the coordination with Tom Key and the EPRI Power Electronics Application Center (EPRI-PEAC). However, these concepts were advanced significantly by Math Bollen, who wrote the book on voltage sags [4], and Dan Sabin, who developed analytical tools for analyzing PQ measurements that became part of standard utility PQ investigations.
Measuring Power Quality—Abe Dranetz, Alex McEachern
Characterizing PQ with measurements has been key for all types of PQ variations and understanding their impacts. Francois Martzloff, whose work was mentioned previously, led in characterizing variations. One of the first portable PQ measurement instrument that brought forth understanding of PQ issues was developed by Abe Dranetz [5]. Dranetz measurements soon became synonymous with PQ investigations. Alex McEachern took this concept to a new level, first with the Basic Measuring Instruments PQ monitor for portable measurements and then with the PQNode, which was the first device for permanent PQ monitoring with an overall data management and analysis platform called PQView (development led by Erich Gunther and Dan Sabin for the large PQ benchmarking project performed in the late 1980s and early 1990s).
Electrical Power Systems Quality Book—Mark McGranaghan, Roger Dugan, Surya Santoso
Much of this background and the advancements that followed have been documented in the multiple editions of the book Electrical Power Systems Quality (Figure 3). First published in 1996, with subsequent editions released in 2003 and 2012, this textbook has become the de facto textbook for studying PQ issues [6].
Figure 3. T he Electrical Power Systems Quality textbook
A Harbinger of Things to Come: Ben Franklin Brings Home His Point With the Lightning Rod
Coping with lightning long predates the electric power grid but figures prominently in the history of PQ. One of Benjamin Franklin’s most famous inventions was the lightning rod. In 1749, Franklin described the similarities he observed between electricity and lightning—that is, that both looked like light, appeared in forked arcs, crackled, and were able to kill animals [7]. He became determined to find a method of proving that lightning was in fact electricity.
In 1752, Franklin set out to perform an experiment to test his hypothesis that lightning was indeed electricity. One day as a storm moved in, he used a kite that consisted of a wire and silk handkerchief connected to a hemp string, metal house key, and silk string. The hemp string could grow wet from the rain, which meant it could conduct an electrical charge quickly. Using a Leyden jar, he was able to collect “electric fire”—stored electricity—from the key, thereby providing evidence that lightning was made of electricity.
Even since before the experiment with the kite and key, Franklin advocated for sharp-pointed lightning rods to protect public buildings (Figure 4). Franklin’s invention was seen as an effective deterrent to the scourge of fires from lightning strikes, as fewer damaging lighting strikes hit buildings equipped with lighting rods grounded to the earth. The lightning rod became a valuable tool to mitigate lightning damage and served as a harbinger for future developments in PQ.
Figure 4. Franklin’s lightning rod (Source: Franklin Institute)
Creating a Power Quality Research Program
The connection of more nonlinear loads to the grid in the 1980s and the unique customer issues that emerged prompted growing concern about PQ. Primarily affected were process oriented equipment and microcomputers sensitive to minor changes in the nature of electricity supplied. Initial investigations established a knowledge base, leading to basic definitions of PQ phenomena and uncovering the need for measurement, analysis, and benchmarking capabilities. Utilities began investigating customer PQ issues and educating customers on the basics of PQ. Utilities worked with customers and EPRI to collaboratively identify challenges and possible solutions. Over the years, this knowledge has been organized as part of the EPRI Power Quality Online Resource Center (https://mypq.epri.com/).
A technology transfer pipeline coalesced with utilities, customers, EPRI, and equipment manufacturers working together, and PQ emerged as a business. PQ monitoring advanced, allowing for the first instances of large-scale monitoring and benchmarking, accelerated with monitoring and data analytics developed for the EPRI Distribution Power Quality (DPQ) benchmarking initiatives [8], and resulted in significant accumulations of data used for postmortem analyses. From a business perspective, deregulation reduced the focus on customer equipment causing PQ issues and shifted to ensuring utility service did not cause customers issues. PQ standards continued to develop, covering more phenomena and in greater depth. The IEC emerged as a major force on the standards front. Key technology development included the dynamic voltage regulator and superconducting magnetic energy storage systems that could be the basis of enhanced PQ services demonstrated by American Electric Power [9] and Duke Energy [10].
The EPRI Power Electronics Application Center
The PQ research program at EPRI took off with the creation of the EPRI-PEAC in 1988. This center led equipment testing for compatibility, tested new solutions for PQ problems, developed new tools like the voltage sag generator for evaluating compatibility in customer processes, and helped coordinate conferences and publications that led the industry for many years. Leaders in this center included Tom Key, Arshad Mansoor, and Mark Stephens.
PQ Case Studies: Building the Understanding
One of the keys to creating an awareness and understanding of PQ issues was conducting actual investigations of problems and publishing these as PQ case studies. EPRI has collected more than 100 case studies focused on PQ solutions for utility customers in the Next Generation Online PQ Case Study Library (EPRI, 1002281).
Building a Worldwide Collaboration
As understanding of PQ issues was growing, industry conferences and workshops helped spread the word and build a collaboration that still exists today in this field. Three series of conferences were particularly important:
International Conference on Harmonics in Power Systems, later renamed as the International Conference on Harmonics and Quality of Power—Alex Emmanuel.
Power Quality magazine’s Power Quality Conference and John Mungenast Power Quality Leadership Award.
EPRI Power Quality Interest Group and Power Quality Applications conferences sharing case studies and research results from around the world. These conferences accelerated the adoption of compatibility solutions and tools.
Compatibility Means Economics
As the fundamentals of PQ became understood, it became clear that ultimately, PQ issues are economic issues. For example, if a PQ variation causes the shutdown of a semiconductor chip production process or an automotive production, the consequences can be in the millions of dollars. Evaluating the economics is a combination of forecasting the likelihood of a problem, understanding the impacts, and being able to describe potential solutions. Voltage sags were one of the initial areas where this became especially critical, and Math Bollen and Dan Sabin led in characterizing the problem, as described previously. It is also worthwhile to note the contributions of Larry Conrad in leading the work on IEEE 1346 [11] that focused on the economic evaluation and creating an understanding within the industrial community. The work on the economics of PQ continued in the International Council on Large Electric Systems (CIGRE) and IEC working groups and still continues to this day.
Uninterruptible Power Supply to the Rescue
By the 1930s, electrical power supply had become well established in cities and towns, with almost 90% of people living in urban areas having access to electricity [13]. Given this growing dependence on electricity for daily activities, many electrical engineers and inventors were likely trying to work out solutions to riding through the inevitable dips and interruptions in electrical supply. In 1932, a patent application was filed by John J. Hanley for an “Apparatus for Maintaining an Unfailing and Uninterrupted Supply of Electrical Energy,” a device that would become known as the uninterruptible power supply (UPS). Granted on April 3, 1934, the patent described an apparatus that could be used to change automatically from one source of electrical energy supplying an external circuit to another source of electrical energy with no interruption of electrical flow in the external circuit (Figure 5) [14]. The patent went on to describe how the invention could supply energy temporarily from a battery during the period of time after the main source of electricity had been interrupted and before the circuit had been opened to the new source of electrical energy. Further instructions describe the assembly of the unidirectional current valve needed for the apparatus and the design of an audible warning signal to indicate failure of the original commercial circuit.
The emphasis in the patent application was on maintaining “uninterruptedly and with unfailing certainty” the necessary electrical energy for alarms and signals, such as fire alarms and railway signal systems, “where the safety of property and human life depend upon the unfailing operation of the system.” In addition, mention is made of maintaining the lighting systems for hospital operating rooms and for theaters, the latter of which may have provided the original spark of inspiration for Hanley, given his background.
Whether the safety goals expressed in this patent were fueled by a particular calamity or failure in electrical supply is not known. At the time, according to Hanley’s patent, most fire-alarm systems relied on batteries or other sources of power rather than on a commercial power line. Hanley’s invention would have allowed more systems—not just fire-alarm systems, but railroad-crossing systems, traffic stoplights, burglar alarms, and newly invented smoke detectors—to safely take advantage of utility-supplied power.
Today, the UPS is one of the most ubiquitous PQ mitigation technologies. The largest UPS on record can power an entire city and surrounding communities for about seven minutes. The battery electric storage system in Fairbanks, Alaska, is bigger than a soccer field, weighs 1500 tons, consists of 13,760 liquid electrolyte-filled nickel-cadmium battery cells, and can discharge up to 46 MW [15]. On a much smaller scale, a 900-W UPS with digital display and colored lights is presently being marketed to gamers as a stylish all-in-one device for backup, surge protection, and automatic voltage regulation for personal computers, gaming devices, and peripherals.
There are varying options for protection in cases where voltage sags are the main issue, such as in many industrial processes.
Figure 5. John J. Hanley’s original design for an Apparatus for Maintaining an Unfailing and Uninterrupted Supply of Electrical Energy, a technology that would later become known as the uninterruptible power supply
Solutions to issues of voltage sags and momentary interruptions are key to the economic evaluation. These solutions started with a whole industry offering uninterruptible power supplies (UPS) with battery storage to provide ride-through capability. However, many other innovations, from simple protection of process controllers to new power electronic topologies (see, for example, leadership of Deepak Divan in developing novel solutions [12]), were developed and documented in the case studies shared with the industry.
SEMI F47 Standard Improves Compatibility with Many Process Industries (2000)
The semiconductor industry has been vital to the development of electronic devices. Started over 45 years ago, the Semiconductor Equipment and Materials International (SEMI) International Standards Program releases standards aimed at improving product quality and reliability at a reasonable price and steady supply [16]. SEMI F47, which sets equipment voltage sag immunity for the semiconductor industry, was developed through a coordinated effort among semiconductor manufacturers, equipment suppliers, and electric utilities, including many EPRI member companies. The effort began in EPRI’s System Compatibility Research Project Task 24, “Power Quality in Semiconductor Fabrication,” which, through industry collaboration, examined why semiconductor production equipment is susceptible to voltage sags. Task 24 workshops, research, and testing began in 1997, and a request for the establishment of a PQ standard for semiconductor tools was made following the September 1997 workshop in Tempe, Arizona [17]. In February 2000, SEMI F47 was published, defining the minimum voltage sag levels not to cause maloperations of semiconductor equipment. Initially, equipment manufacturers were hesitant to adopt the standard, but researchers showed how the standard could be met with minimum design impact. As a result of this standard being published and adopted, semiconductor customers saw fewer PQ issues, saving manufacturers millions of dollars annually [17].
EPRI’s Consortium for Electric Infrastructure for a Digital Society Report Documents the Economic Value of Power Quality
By 2000, the concept of monitoring and maintaining PQ had been well established as a need in a digital society. Industrial processes were becoming increasingly digitized, prompting greater sensitivity to disturbances in the power supply. Continuous process manufacturing was particularly susceptible as even a small disturbance could lead to lost product, damage, and difficult cleanup. For example, if production stops in injection molding factories, plastic could harden, leading to costly delays before production could resume. In 2000, EPRI’s Consortium for Electric Infrastructure for a Digital Society (CEIDS) report The Cost of Power Disturbances to Industrial and Digital Economy Companies provided a first-of-its-kind comprehensive analysis of economic impacts of PQ issues in key sectors of the economy [18].
In 2000, CEIDS surveyed roughly two million U.S. industrial and digital economy establishments to estimate the cost of different types of power disturbances and the number and cost of disturbances experienced. The report found that across all business sectors, the U.S. economy was losing between $104 billion and $164 billion a year to outages, and another $15 billion to $24 billion to PQ phenomena [18]. In 2020, the report was updated to reflect the current number of U.S. manufacturing facilities, inflation, changes in electrical supply, and sensitivity of processes. The report also clarified terminology used in the 2000 report. Results indicated that in 2020, the total annual costs to all U.S. business establishments from reliability and PQ phenomena were estimated to be roughly $145 billion to $230 billion [19].
Power Quality as a Service
The concept of improving PQ compatibility as a customer service was developed by some leading utilities in the 1990s. Utilities experimented with offering critical customers (such as electronics manufacturing, plastics plants, automotive plants, and so forth) the option of a higher quality power supply by installing additional technology (such as dynamic voltage restorers that helped prevent voltage sags from affecting downline customers). Of course, these offerings came at a cost that was to be considered in the economic evaluation. The overall concept of a service-based offering to improve compatibility and performance continues to be explored to this day. For instance, microgrids provide the capability for local supply to ride through system outages and other PQ issues.
Power Quality Standards Define Compatibility
Standards have been essential in defining the issues and supporting the evaluation of compatibility. Standards activities in IEEE and IEC continue to advance the understanding of PQ issues and provide a forum for discussion of new issues and new approaches for dealing with these issues. Figure 6 illustrates how different standards are needed for definitions, system performance, equipment compatibility, measurements, and assessment methods. With the changing nature of equipment and the characteristics of the power system, this will be an ongoing effort.
Figure 6. Flow of PQ standards development
The 1990s: A Key Decade for Power Quality Standards
As attention to and knowledge of PQ issues grew, the need for a common understanding of different phenomena and solutions among engineers, equipment manufacturers, and research groups became evident. Through engagement with all of these parties, standards were developed to improve equipment performance and reduce PQ issues on the grid. The 1990s saw the publication of some seminal standards by groups including IEEE, IEC, and SEMI.
Initially released in 1992 and subsequently updated in 2014, IEEE 519—IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems gives a recommended practice establishing goals for the design of electrical systems that include both linear and nonlinear loads [20]. The 1992 version served as both an educational tutorial and a standard, defining the requirements and responsibilities of the utilities that supply power and those of the end users. The standard set limits on voltage and current distortion at point of common coupling and gave specific limits on total harmonic distortion and total demand distortion. It held that the customer is responsible for limiting the amount of harmonic current injected into the grid and the utility is responsible for avoiding resonance conditions. While many people contributed to the development of IEEE 519 and the application guide for this standard, Mark Halpin deserves a lot of the credit for continuing to advance this important effort.
Standards in the IEC 61000 family introduced the concept of electromagnetic compatibility, dictating how electrical equipment and systems can function acceptably in their electromagnetic environment. The first standard in this family was released in 1992 and defined basic terms [21]. Following standards set limits for harmonic emissions, bounds for voltage fluctuations, and guidelines for testing and measurement techniques [22, 23]. In 2003, IEC 61000-4-30 was first published, defining methods for measurement and interpretation of PQ data [23]. Prior to this standard being released, engineers could collect identical data and arrive at multiple different interpretations. Updated in 2008 and 2015, IEC 61000-4-30 resolved this issue by standardizing data analysis. Early leaders like Alain Robert led these development efforts with tremendous contributions from Robert Koch, Emmanuel DeJaeger, and many others.
Today, standards continue to evolve and be updated as loads and generation sources change. As more nonlinear loads are connected to the system and more inverter-based resources are introduced, standards can play a key role in reducing PQ issues on the grid.
Power Quality Characterization, Monitoring, and Benchmarking
Understanding system performance and specific PQ variations is important to finding PQ solutions. Variations may be characterized by abnormalities in voltage, current, frequency, and duration of event. PQ monitoring, or the process of gathering, analyzing, and interpreting measurement data, is essential for identifying and characterizing variations. Advancements in PQ monitoring devices made interpretation easier, streamlined data collection processes, and enabled the development of software to manage large quantities of PQ data.
The CBEMA and Information Technology Industry Council Power Acceptability Curves
The key to the compatibility problem for voltage sags and interruptions is characterizing the performance from the supply system in a way that can be compared with the ride through characteristics of equipment. The famous “CBEMA curve” was one of the first attempts to document and enable compatibility between common high-tech end-use devices and the electric grid. In 1977, the CEBMA ESC-3 working group was asked to provide input on an energy performance profile for computer equipment that was being proposed for publication in IEEE Standard 446. This standard focused on industrial and commercial users’ needs for the selection and application of emergency and standby power systems. After making minor modifications, the working group approved this power acceptability curve. What became known as the “CBEMA curve” (see Figure 6) was derived from experimental and industry-provided data from mainframe computers. The CBEMA curve attempted to describe the tolerance of single-phase computer business equipment to the magnitude and duration of voltage variations on the power system [24]. This was the first attempt to develop a simple compatibility guideline for sensitivity and robustness of end-use equipment to common voltage sags and became one of the most frequently used power acceptability curves.
CBEMA Curve Based on Mainframe Computers (1977)
Despite the focus on mainframe computers and 120-V, single-phase systems, the CBEMA curve (Figure 7) was considered widely useful. Data on voltage variations gathered from PQ testing equipment located near sources of suspected disturbances or placed to measure the operation of a distribution system could be analyzed in combination with the CBEMA curve. Balanced voltage-sag events in three-phase systems could be treated as a single-phase equivalent and the curve applied directly; however, unbalanced voltage sags made the application of the CBEMA curve to three-phase systems more complicated. Nevertheless, the CBEMA curve would eventually be used in the design of system reliability for electronic equipment and the design of sensitive equipment on the power system, and it would also be used as a common format for reporting on PQ variation data.
Figure 7. The CBEMA curve. Voltage magnitude is indicated on the vertical axis, while the horizontal axis represents the duration of the PQ event. Points below the envelope are presumed to cause the load to drop out due to lack of energy. Points above the envelope are presumed to cause malfunctions like insulation failure, overvoltage trip, and overexcitation.
Information Technology Industry Council Curve Based on Tests of Computer Power Supplies (1996)
In 1994, CBEMA was renamed the Information Technology Industry Council (ITIC), and a new curve evolved in 1996 that became known as the “ITIC curve.” A working group and several sponsors revised the original curve based on results from tests that were conducted on a representative sample of eight personal-computer power supplies supplied by eight different manufacturers [25].
Unlike the original CBEMA curve, which was very “smooth,” the new ITIC curve has discrete steps (see Figure 8), making it easier to program in PQ meters and spreadsheet platforms. Uses for this curve have expanded to include defining specification criteria for electronic equipment and using it as a basis for PQ performance contracts between utilities and large industrial customers [26].
Figure 8. The ITIC curve. The prohibited region (above the top blue line) indicates overvoltage conditions where the equipment may not operate normally for any duration and may suffer damage if allowed to remain at that voltage. The region in green, between the blue lines, represents voltages and durations for which the equipment should operate normally. Voltages within the “no damage region” at the bottom of the graph may not be high enough to allow equipment to operate normally for any duration and may not cause any damage to equipment but may cause shutdowns.
Power Quality Monitoring Equipment
One of the first PQ monitoring devices was the General Electric lightning strike recorder [27]. Developed in the 1920s, this device recorded lightning strike date and time with marks on strip-chart paper [28]. Devices producing more quantitative results did not emerge until the 1960s, and it was not until the mid-1970s that the first well-recognized PQ monitor was developed. This monitoring device, the Dranetz Series 606 created by Abe Dranetz, was the first microprocessor-based device of its kind. It measured voltage only and printed its text-based output describing disturbances by event type and voltage magnitude on paper tape [28]. In the 1980s, Alex McEachern saw an opportunity to improve the world of PQ monitoring and created a new generation of monitoring devices with graphical displays, digital memory, and improved triggering approaches. In the 1990s, in part due to the scope of the EPRI DPQ Project and the research that went into both the hardware and the software to support permanent PQ monitoring, a third generation of monitors was coupled with software systems to collect and manage data as part of a complete PQ monitoring system [28]. Today, PQ equipment is coupled with software that analyzes data and generates usable information. In many cases, the monitors themselves perform mathematical calculations, including the fast Fourier transform to calculate harmonics. The focus is turning towards automated PQ monitors that may enable proactive grid controls [29].
Widespread Monitoring and Benchmarking Establishes Power Quality Baselines
Complaints of PQ issues increased significantly with the increased use of electronic controls and automated, continuous manufacturing, along with the use of more sensitive equipment in industrial facilities. Individual solutions, such as outfitting customer facilities with UPS devices, were not cost-effective compared to a systemwide approach to solving PQ issues. The concept of “premium power quality” emerged as a service that utilities could provide to stay competitive and retain customers. Before offering premium PQ, utilities needed to understand their baseline levels of PQ, prompting widescale PQ monitoring and benchmarking projects around the globe.
In fall 1989, EPRI launched the first iteration of the Distribution Power Quality (DPQ-I) project [8]. The chief goal of the project was to provide baseline statistics regarding quantities that fall under the general category of distribution PQ, including the frequency and duration of PQ events. Methods included collecting, analyzing, and reporting of distribution PQ data at the national level with a degree of statistical importance. Monitors were placed at 300 locations on 100 distribution feeders, resulting in 27 months of monitoring and more than 30 gigabytes of data stored in the DPQ Database, making DPQ-I the most extensive distribution system PQ study ever conducted. Figure 9 shows an 8010 PQNode device, which was used for PQ monitoring during DPQ-I. Following DPQ-I was DPQ-II in 2001 and 2002, which characterized short term variations based on data from 480 monitors, including data collected during DPQ-I [30]. In 2014, results from the Transmission Power Quality-Distribution Power Quality (TPQ-DPQ-III) project were reported, expanding efforts from the previous DPQ projects by monitoring PQ characteristics in both distribution and transmission systems [31]. In 1997, EPRI’s Reliability Benchmarking Methodology provided methods and defined indices so that service quality could be quantified from the data collected in surveys such as DPQ [32].
Figure 9. Photograph of a BMI 8010 PQNode within a NEMA 4 Enclosure. This instrument was developed at the initial phase of the EPRI DPQ project.
PQ monitoring began to be ubiquitous around the world to ensure acceptable levels of quality for consumers. Along with the EPRI DPQ projects, different utilities and research groups conducted large-scale surveys and monitoring of PQ levels in multiple countries. Examples include:
In 1991, the Canadian Electricity Association began a three-year survey, resulting in 550 customer sites monitored for one month each [33].
In the mid-1990s, Electricité de France’s Qualimat project involved monitoring PQ at every medium-voltage substation, with the goal of ensuring a specified level of PQ nationwide.
Similarly, by the same time, East Midlands Electricity in Nottingham, England, was monitoring PQ within its territory [34].
In 2001, the Council of European Energy Regulators released its first Benchmarking Report on Quality of Electricity Supply, with following editions released in 2003, 2005, 2008, 2011, and 2016, all of which addressed continuity of supply, voltage quality, and commercial quality [35].
Power Quality Analysis and the Role of PQView
While there was significant advancement in monitoring devices and software in the 1980s, there remained a question of how to extract meaning from the data collected. This problem was compounded when large-scale monitoring efforts began, prompting the need for improved methods of collection, analysis, and reporting of massive amounts of data. Continuous monitoring of system performance was also growing as a proactive measure against PQ issues, increasing the need for analysis that could actively characterize phenomena. Additionally, different types of PQ variations required different types of analysis to characterize system performance. To address these needs, software was developed to analyze and characterize data.
One example of a PQ statistical analysis program is PQView, the software system designed to manage and analyze the data collected and stored during DPQ-I [8]. Electrotek Concepts and EPRI began developing the software in 1989, and version 1.0 was released in 1994 [36]. Key functionalities of this software included the ability to characterize data automatically from a database and use analysis tools to generate summary statistical reports.
Since its initial development, PQView has gone through multiple updates to advance its abilities and functionality. Version 1.0 could connect to one meter and output a static report. Following versions could connect to multiple monitors of different types and evolved to be an investigative tool. Besides data analytics for benchmarking, tools like PQView could be used for diagnostics, operations, asset management, and planning. A great example was the first implementation of fault location based on PQ waveforms at Con Edison [37]. This tool reduced time needed to locate and repair faults, and it demonstrated that PQ monitoring could be more than just reacting to customer impacts but rather could be a proactive tool used throughout grid operational services.
PQView has now grown into enterprise-class analysis software. There has also been emergence of open source analysis platforms, such as PQDashboard, an online tool for systemwide PQ data [38].
Power Quality Monitoring Into the Future
The new millennium brought an increase in the number of advanced PQ monitors on the market and the recognition of the need for better handling of PQ and other data streams. Most importantly, PQ is becoming part of standard power system monitoring. With equipment like advanced meters, sensors throughout the grid, phasor measurement units, and monitoring within individual equipment, a vast amount of data can be collected. A key challenge today has moved to data management and analytics that will turn these data into actionable information to improve grid and equipment performance.
The Impacts of Distributed Resources
Managing the power grid hit an inflection point as distributed energy resources (DER) emerged rapidly, creating a complex environment with thousands of decentralized generation sources, many of which are inverter based. Between 2005 and 2018, annual global solar photovoltaic (PV) generation grew from 3.7 terawatt hours (TWh) to 554.4 TWh [39]. While these resources play a major role in efforts toward decarbonization, they introduce PQ concerns as they are noninertial and inverter based, and they can inject significant harmonics into the grid. Research is ongoing to determine how the grid and end-use equipment are affected by DER-induced PQ issues.
IEEE Standard 1547 provides for the interconnection and interoperability of DER with electric power systems. First released in 2003 and updated in 2018, Standard 1547 provides requirements for the performance, operation, testing, safety, and maintenance of the interconnection between DER and the power system [40]. If DER may be causing PQ issues (including harmonics), this standard advises what actions to take and formalizes PQ considerations for the grid.
New Power Quality Issues
There are always new issues to study in the world of PQ compatibility. Some of the most important areas of investigation today include:
Voltage regulation with increased penetration of distributed resources
Hosting capacity—the ability of the power system to function properly with more and more devices that may affect quality or reliability
PQ research has been driven by digitization in equipment and processes, customer needs and economic impacts, standards development, and a changing power grid environment.
Continued increased use of computers, inverters, and microprocessors potentially will introduce new challenges and influence research pathways. Integrating DER and increased system complexity will remain a core PQ challenge unique to each local system as the grid continues to evolve. Ongoing integration of solar PV, electric vehicle chargers, and other inverter-connected devices may introduce entirely new PQ issues as electrification efforts increasingly permeate all economic sectors. With possibly millions of these devices operating throughout the grid, it is important to consider systemwide PQ for the reliable operation of these devices and to minimize PQ impacts on the grid.
Updated and new standards, such as IEEE 1547 and IEEE P2800, provide guidelines on how DER and inverter-based resources may minimize impact to PQ on the grid, including those resources interconnecting with transmission systems. Instead of reacting to PQ issues, a systemwide, data-driven approach may aid in proactively identifying PQ issues that could lead to major incidents. Artificial intelligence and machine learning using large amounts of data could bolster analysis programs to generate localized solutions. UPS and battery storage also may have value in the future energy system, not only as storage devices to balance dips in generation but also for voltage regulation and PQ in general, a potential incentive for larger industrial users to install their own on-site batteries
Utilities, manufacturers, vendors, researchers, and customers are collaborating to find mutual understanding and solutions beneficial to all stakeholders. EPRI’s Program 1: Power Quality is helping to continue these efforts, with research ongoing to anticipate and address PQ issues in the changing grid environment.
References
F. Martzloff, “Proceedings: Open Forum on Surge Protection Application,” National Institutes of Standards and Technology, 1991.
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J. J. Hanley, “Apparatus for Maintaining an Unfailing and Uninterrupted Supply of Electrical Energy.” United States Patent Office, Patent 1953602, 1934.
“Battery Energy Storage System for Golden Valley Electric Association,” Energize, November 2004, pp. 27–30.
Impact of SEMI F47 on Utilities and Their Customers. EPRI, Palo Alto, CA, 2004. 1002284.
The Cost of Power Disturbances to Industrial and Digital Economy Companies June 2001 ID: 1006274. EPRI, Palo Alto, CA, 3002000476.
Understanding the Cost to U.S. Business from Unmitigated Reliability and Power Quality Events. EPRI, Palo Alto, CA, 2020. 3002019395.
Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE Standard 519-1992.
1992 Electromagnetic Compatibility (EMC)—Part 1: General—Section 1: Application and Interpretation of Fundamental Definitions and Terms, IEC TR 61000-1-1.
1995 Electromagnetic Compatibility (EMC)—Part 3: Limits— Section 2: Limits for Harmonic Current Emissions (Equipment Input Current <= 16 per Phase), IEC 61000-3-2.
Standards for Characterizing Voltage Sag Performance. EPRI, Palo Alto, CA, 2015. 3002005885.
A. McEachern, “World-wide Power Quality—20-Year Cycles of Development, 1965 to 2025,” presented at China International Conference on Electricity Distribution, 2006.
R. Dugan, M. McGranaghan, S. Santoso, and H. W. Beaty. Electrical Power Systems Quality. McGraw-Hill Education, 2002.
PQ TechWatch: Power Quality Monitoring for Micro-Grid and Comparable DER Installations. EPRI, Palo Alto, CA, 2019. 3002016377.
Voltage Sags, Swells, and Interruptions Characterized in DPQ Phase II Project. EPRI, Palo Alto, CA, 2005. 1010927.
Transmission—Distribution Power Quality Report (TPQ-DPQ III). EPRI, Palo Alto, CA, 2014. 3002003995.
Reliability Benchmarking Methodology. EPRI, Palo Alto, CA, TR-107938.
M. B. Hughes and J. Chan, “Canadian National Power Quality Survey Results.” Proceedings of 1996 Transmission and Distribution Conference and Exposition, 1996.
D. D. Sabin and A. Sundaram, “Quality Enhances Reliability [Power Supplies],” IEEE Spectrum, Vol. 33, No. 2, pp. 34–41, February 1996.
Council of European Energy Regulators (CEER). 6th CEER Benchmarking Report on the Quality of Electricity and Gas Supply. CEER, Brussels, Belgium, 2016.
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About EPRI
Founded in 1972, EPRI is the world’s preeminent independent, non-profit energy research and development organization, with offices around the world. EPRI’s trusted experts collaborate with more than 450 companies in 45 countries, driving innovation to ensure the public has clean, safe, reliable, affordable, and equitable access to electricity across the globe. Together, we are shaping the future of energy.
Published by Electrotek Concepts, Inc., PQSoft Case Study: General Reference – Approach to Systems Monitoring, Document ID: PQS0310, Date: April 16, 2003.
Abstract: Power quality monitoring is an essential service many utilities perform for their industrial and other key commercial customers. Because of the technology and software now available, this monitoring is highly effective. Not only can a monitoring system provide information about the quality of the power and the causes of power system disturbances, but also it can identify conditions throughout the system before they cause problems. Power quality problems are not necessarily limited to the utility power system. Many surveys have shown that the majority of problems are localized within customer facilities. Given this fact, monitoring provides a key opportunity for a utility to protect its reputation and improve its relationship with customers.
This document provides an overview of the approach to systems monitoring.
APPROACH TO SYSTEMS MONITORING
Power quality monitoring is an essential service many utilities perform for their industrial and other key commercial customers. Because of the technology and software now available, this monitoring is highly effective. Not only can a monitoring system provide information about the quality of the power and the causes of power system disturbances, but also it can identify conditions throughout the system before they cause problems. Power quality problems are not necessarily limited to the utility power system. Many surveys have shown that the majority of problems are localized within customer facilities. Given this fact, monitoring provides a key opportunity for a utility to protect its reputation and improve its relationship with customers.
Monitoring system functions include extensive data processing capabilities, easily understood reporting, and universal sharing of information. These systems may be tailored to specific customer needs. The scope can vary from a few monitoring devices to several hundred. Information may be made available through the Internet or in-house via a company network. Reports can be customized to produce pertinent, vital information in a consistent format. Utility and/or customer requirements are the only limiting factor for a system’s flexibility and scope.
An essential requirement for any monitoring system is easy access and analysis of a large volume of data. This involves the ability to generate automated reports and the ability to distribute the data to utility personnel or customers.
Components of a Monitoring System
Power quality monitoring systems are structured using basic hardware components. There is no limit to a system’s size. Essential components (illustrated in Figure 1) for the monitoring system include
Power disturbance monitors
Mass disk storage for data
Computer workstations
Computer for downloads
Web or company Intranet server
Analysis software
Figure 1 – Example of System Monitoring Concept
Objectives for a Power Quality Monitoring Project
The objectives for a monitoring program determine the choice of measurement equipment and triggering thresholds, the methods for collecting data, the data storage and analysis requirements, and the overall level of effort required. Several general classifications for monitoring objectives include:
Monitoring to characterize system performance. This is the most general requirement. A power producer may find this objective important if it has the need to understand its system performance and then be able to match that system performance with the needs of customers. System characterization is a proactive approach to power quality monitoring. By understanding the normal power quality performance of a system, a provider can quickly identify problems and can offer information to its customers to help them match their sensitive equipment’s characteristics with realistic power quality characteristics.
Monitoring to characterize specific problems. Many power quality service departments or plant managers solve problems by performing short-term monitoring at specific customers or at difficult loads. This is a reactive mode of power quality monitoring, but it frequently identifies the cause of equipment incompatibility that is the first step to a solution.
Monitoring as part of an enhanced power quality service. Many power producers are currently considering additional services to offer customers. One of these services would be to offer differentiated levels of power quality to match the needs of specific customers. A provider and customer can together achieve this goal by modifying the power system or by installing equipment within the customer’s premises. In either case, monitoring becomes essential to establish the benchmarks for the differentiated service and to verify that the utility achieves contracted levels of power quality.
Power quality encompasses a wide variety of conditions on the power system. Important disturbances can vary in duration from very high frequency impulses caused by a lightning stroke, to long-term overvoltages caused by a regulator tap switching problem. The range of conditions that a power quality instrument must characterize creates problems both in terms of the monitoring equipment complexity and in the data collection requirements.
The methods of characterizing are important for the monitoring requirements. For instance, characterizing most transients requires high frequency sampling of the actual waveform. Characterization of voltage sags involves a plot of the rms voltage versus time. Outages can be defined just by a duration. Monitoring to characterize harmonic distortion levels and normal voltage variations requires steady-state sampling with trending of the results over time.
It may be prohibitively expensive to monitor all the different types of power quality variations at each location. The priorities for monitoring should be determined up front based on the objectives of the effort. Projects to benchmark system performance should involve a reasonably complete monitoring effort. Projects designed to evaluate compliance with IEEE Standard 519 for harmonic distortion levels may only require steady-state monitoring of harmonic levels. Other projects focused on specific industrial problems may only require monitoring of rms variations, such as voltage sags or momentary interruptions.
Monitoring Equipment
There are many different types of monitoring equipment that form part of a power quality monitoring project. Four basic categories of equipment are often utilized:
1. Digital Fault Recorders (DFR). These may already be in place at many substations. DFR manufacturers do not design the devices specifically for power quality monitoring. However, a DFR will typically trigger on fault events and record the voltage and current waveforms that characterize the event. This makes them valuable for characterizing rms disturbances, such as voltage sags, during power system faults. DFRs also offer periodic waveform capture for calculating harmonic distortion levels.
2.Voltage Recorders. Power providers use a variety of voltage recorders to monitor steady-state voltage variations on distribution systems. These devices are becoming increasingly sophisticated and fully capable of characterizing momentary voltage sags and even harmonic distortion levels. Typically, the voltage recorder provides a trend that gives the maximum, minimum, and average voltage within specified sampling window (for example, 2 seconds). With this type of sampling, the recorder can characterize a voltage sag magnitude adequately. However, it will not provide the duration with a resolution less than two seconds.
3.In-Plant Power Monitors. It is now common for monitoring systems in industrial facilities to have some power quality capabilities. These monitors, particularly those located at the service entrance, can be used as part of a utility monitoring program. Capabilities usually include waveshape capture for evaluation of harmonic distortion levels, voltage profiles for steady-state rms variations, and triggered waveshape captures for voltage sag conditions. It is not common for these instruments to have transient monitoring capabilities.
4.Special-Purpose Power Quality Monitors. The monitoring instrument developed for the EPRI Distribution Power Quality (DPQ) Project was specifically designed to measure the full range of power quality variations. This instrument features monitoring of three-phases and current plus neutral. A 14-bit A/D board provides a sampling rate of 256 points per cycle for voltage, and 128 points per cycle for current. The high sampling rate allowed detection of voltage harmonics as high as the 100th in order and current harmonics as high as the 50th. Most power quality instruments can record both triggered and sampled data. Triggering should be based upon rms thresholds for rms variations and on waveshape for transient variations. Simultaneous voltage and current monitoring with triggering of all channels during a disturbance is an important capability for these instruments. Power quality monitors have proved suitable for substation, feeder locations, and customer service entrance locations.
Analysis of Measurement Data
Analysis of power quality measurement data is an important component of the monitoring project. This section presents some basic methods of summarizing power quality phenomena, including:
probability distributions
correlations
time trends
Distributions
Distributions can be illustrated as histograms of event incidence (frequency, relative frequency or cumulative frequency) versus time interval, event characteristic interval, or site descriptor interval. For example, Figure 2 shows a distribution of steady-state rms voltage measurements. Each bar indicates frequency – the number of samples that possess the characteristic value indicated at the base of the bar. The line plots cumulative frequency, the percent of all samples with the characteristic value less than or equal to the value indicated on the x-axis.
Figure 2 – RMS Voltage Histogram
Correlations
A correlation can be visualized graphically as a scatter plot of an event characteristic value or event incidence versus an event characteristic value or a site descriptor value. The most common correlation used for power quality data is the magnitude duration plot which shows voltage variations over a given time period. Events are plotted on a grid with the horizontal axis representing event duration, and the vertical axis representing the maximum (or minimum) rms level recorded during the event. The curves superimposed on the magnitude duration plot in Figure 3 indicate typical computer equipment voltage tolerance, as defined in ANSI/IEEE Standard 446.
Figure 3 – Magnitude Duration Plot with CBEMA Overlay
Time Trends
A time trend is a plot of an event characteristic versus time. For example, Figure 4 shows current TDD (total demand distortion) variation over several months at a single monitoring site.
Figure 4 – Current Total Demand Distortion Trend
Data Analysis Tools
Most of the graphs illustrated on the previous pages were developed with computer analysis tools, such as spreadsheets, statistical programs, databases, and spreadsheets. A software package for statistical analysis of power quality measurement data was developed under the EPRI DPQ Project. The capabilities of this program, called PQView®, are summarized in Table 1.
Table 1 – PQView Data Analysis Functions
PQView® is a database software application developed by Electrotek Concepts, Inc. that is designed to store and analyze large quantities of power quality-related disturbance and steady-state measurement data. Featuring data management tools that can quickly characterize this data, PQView includes statistical analysis and plotting tools that can provide single- or multiple-site analyses for power systems.
PQView enables users to organize data from a variety of instruments, such as power quality monitors, voltage recorders, in-plant monitors, and fault recorders. It also stores site characteristics and event information. This information is valuable not only in establishing a disturbance’s source, but also the customer equipment sensitivity to power system problems.
PQView brings all this information together in one relational database and provides the means to automate both the loading of new data and the generation of monitoring reports. PQView comes with a base set of reports that provide information on raw measurements, detailed statistical analysis, and executive summaries. These reports enable the user to reach all of the audiences interested in the results and allow reports to be customized as needed.
Using technology developed under EPRI sponsorship, PQView combines powerful features in a user-friendly interface. It utilizes Microsoft® Access as its foundation, providing a database engine, development tools, support for database editing and security, and integration with other Microsoft Windows® applications such as word processing and spreadsheet programs. PQView consists of two major components, the Power Quality Data Manager (PQDM), which creates, loads, and edits power quality databases; and the Power Quality Data Analyzer (PQDA), which generates reports and analyzes the data.
REFERENCES
IEEE Standard 1159. IEEE Recommended Practice on Monitoring Electric Power Quality. Measuring Voltage and Current Harmonics in Distribution Systems, M. F. McGranaghan, J. H. Shaw, R. E. Owen, IEEE Paper 81WM126-2, November 1981. A Guide to Monitoring Power Quality, EPRI TR-103208, Project 3098-01, Electric Power Research Institute, April 1994.
RELATED STANDARDS IEEE Standard 1159 IEEE Standard 1346 IEEE Standard 1250 IEEE Standard 519
GLOSSARY AND ACRONYMS DFT: Digital Fault Recorders IEEE: Institute of Electrical and Electronics Engineers PQDA: Power Quality Data Analyzer PQDM: Power Quality Data Manager TDD: Total Demand Distortion UPS: Uninterruptible Power Supply
Published by Electrotek Concepts, Inc., PQSoft Case Study: Monitoring Objectives and Screening Procedures – Distribution Feeder Capacitor Bank Application, Document ID: PQS0708, Date: January 1, 2007.
Abstract: Power quality problems encompass a wide range of disturbances and conditions on utility and customer power systems. They include everything from very fast transient overvoltages to long duration outages. Power quality problems also include steady-state phenomena such as harmonic distortion, and intermittent phenomena, such as voltage flicker. This wide variety of conditions that make up power quality makes the development of standard measurement procedures and equipment very difficult. This case study introduces the subject of monitoring objectives and screening procedures and provided a distribution feeder capacitor bank application monitoring example.
MONITORING OBJECTIVES AND SCREENING PROCEDURES
Power quality monitoring is used to characterize harmonics and transients (and other variations) at various locations on utility and customer power systems over a period. The length of the monitoring period is dependent on the nature of the power quality problem. For example, capacitor bank switching transients may be collected in several days, while harmonic distortion levels may need to be monitored for weeks, months, or even years to show the influence of load and seasonal variations. The current trend for power quality monitoring is fixed instruments that continuously monitor the power system.
The objectives of a monitoring program determine the choice of measurement equipment, method of collecting data, disturbance thresholds, data analysis requirements, and the overall effort required. Monitoring objectives generally fall into one of the following categories:
− Diagnostic: Monitoring to characterize power quality problems that are affecting an existing customer, or that may affect a new facility.
− Evaluative: Identify critical design, construction, and environmental parameters that affect power quality; appraise measures to improve power quality; or refine power quality modeling techniques.
− Predictive: Estimate existing levels of power quality on the system.
Generally, it is difficult to design a power quality monitoring program that will satisfy all of these goals. This is due to the site selection criteria conflict. Prediction of system-wide power quality is based on a random selection of monitoring locations. Data from randomly selected sites may be used to diagnose some power quality problems, but diagnostic monitoring is clearly more effective when monitoring locations are targeted – using customer complaints, equipment failure reports, etc. Conversely, data from targeted sites, even if those sites are large in number and well dispersed throughout the system, cannot legitimately be used to extrapolate system-wide power quality levels. A statistical monitoring program may produce, as a by-product, data that is useful for an evaluative effort. However, as with diagnostic monitoring, evaluative monitoring is most effective when situation-specific monitoring sites are targeted.
The objectives of a monitoring program also influence equipment requirements because at present, no instrument is completely satisfactory for performing all types of monitoring. Diagnostic/evaluative monitoring can usually be conducted with portable single-purpose instruments, while predictive monitoring requires permanently installed instruments that can measure the whole range of power quality phenomena.
Screening procedures for characterizing overall power quality levels for a particular utility or customer system include:
− Identify the power quality phenomena of concern. Examples of these include capacitor bank switching voltage and current transients, harmonic distortion, voltage sags and interruptions, etc. The types of disturbances that will be monitored influence the selection of monitoring equipment, transducers, and monitoring locations.
− Identify the system parameters that may affect the disturbance magnitudes and frequencies. Examples of utility system parameters include radial or network feeders, overhead vs. underground, transformer connections, and power factor correction. Examples of customer system parameters include load characteristics, wiring, and grounding practices, protection practices, and power factor correction. Additional factors may include lightning levels, trees or animals, maintenance practices, or construction activity.
− Characterize power quality variations as a function of important system parameters. This requirement will influence the overall monitoring plan and data collection requirements.
− Compare power quality variations on the system with results from published standards or national monitoring efforts. This will provide a benchmark for evaluating the level of concern associated with each type of disturbance.
Screening procedures for a diagnostic evaluation to determine a particular power quality problem for a utility or customer facility include:
− Characterize the problem – which equipment is mis-operating or failing.
− Correlate problems with changes in the system, such as motor starting, capacitor bank switching, operation of nonlinear loads, etc.
− Characterize the magnitudes, durations, and frequencies of the disturbances causing the problem. This will usually require monitoring at different voltage levels – distribution system, customer service entrance, and at the affected equipment.
− Identify possible solutions. Characterizing problems and causes will lead to a range of possible solutions.
FEEDER CAPACITOR BANK APPLICATION
A power quality monitoring example specific to a distribution capacitor bank application is shown in the following four figures. Figure 1 shows a 13.8kV distribution feeder current and Figure 2 shows the bus voltage before and after energization of a pole-mounted 900 kVAr capacitor bank. The resulting transient overvoltage is approximately 1.3 per-unit (130%) and the steady-state voltage rise is approximately 1.2%.
Insertion of the capacitor bank creates a harmonic resonance condition on the feeder, thereby increasing the harmonic current distortion recorded by the power quality monitor. The harmonic current distortion before the capacitor bank is energized is approximately 1.8% and the distortion after energization is approximately 11.8%. A spectral analysis of the harmonic current waveform after energization of the capacitor bank shows that the highest harmonic is the 9th.
Figure 3 shows the rms voltage trend for the same feeder for a two day period that includes the energization of the feeder capacitor bank. The trend shows the corresponding steady-state voltage rise when the capacitor bank is energized.
Figure 4 shows the voltage distortion trend and Figure 5 shows the current distortion trend for the same feeder for the same two day period that includes the energization of the feeder capacitor bank. The current distortion before the capacitor bank is energized is approximately 1.8% and the distortion after energization is approximately 11.8%.
Figure 1 – Measured Distribution Feeder Current during Capacitor Bank Energization
Figure 2 – Measured Distribution Feeder Voltage during Capacitor Bank Energization
Figure 3 – Measured Distribution Feeder Voltage Profile
Figure 4 – Measured Distribution Feeder Voltage Distortion Trend
Figure 5 – Measured Distribution Feeder Current Distortion Trend
SUMMARY
Power quality problems encompass a wide range of disturbances and conditions on utility and customer power systems. They include everything from very fast transient overvoltages (microsecond) to long duration outages (hours or days). Power quality problems also include steady-state phenomena such as harmonic distortion, and intermittent phenomena, such as voltage flicker. This wide variety of conditions that make up power quality makes the development of standard measurement procedures and equipment very difficult. This case study introduces the subject of monitoring objectives and screening procedures and provided a distribution feeder capacitor bank application monitoring example.
REFERENCES
IEEE Standard 1159. IEEE Recommended Practice on Monitoring Electric Power Quality. Measuring Voltage and Current Harmonics in Distribution Systems, M. F. McGranaghan, J. H. Shaw, R. E. Owen, IEEE Paper 81WM126-2, November 1981. A Guide to Monitoring Power Quality, EPRI TR-103208, Project 3098-01, Electric Power Research Institute, April 1994.
RELATED STANDARDS IEEE Standard 1159 IEEE Standard 1346 IEEE Standard 1250 IEEE Standard 519
GLOSSARY AND ACRONYMS DFT: Digital Fault Recorders IEEE: Institute of Electrical and Electronics Engineers PQDA: Power Quality Data Analyzer PQDM: Power Quality Data Manager TDD: Total Demand Distortion UPS: Uninterruptible Power Supply
Published by Electrotek Concepts, Inc., PQSoft Case Study: Transformer Energizing and Dynamic Overvoltages, Document ID: PQS0709, Date: October 15, 2007.
Abstract: Energizing power transformers results in inrush currents that are rich in harmonic components. The inrush current interacts with the system impedance vs. frequency characteristics to create a voltage waveform that can have significant harmonic components for the duration that the inrush current is present.
Dynamic overvoltages are long-term resonant overvoltages lasting many cycles that can cause damage to capacitor units and other adjacent equipment, such as transformers and surge arresters.
This case study presents a transformer energizing and dynamic overvoltage evaluation for a 34.5kV distribution system.
INTRODUCTION
A transformer energizing and dynamic overvoltage evaluation was completed for the system shown in Figure 1.
Figure 1 – Oneline Diagram for Transformer Energizing and Dynamic Overvoltage
BACKGROUND
Energizing saturable devices (devices with magnetic cores), such as power transformers, results in inrush currents that are rich in harmonic components. The inrush current interacts with the system impedance vs. frequency characteristics to create a voltage waveform that can have significant harmonic components for the duration that the inrush current is present. Transformer inrush current typically decays over a period on the order of one second.
This phenomenon combines concerns for harmonic current distortion and transient voltages. The harmonics of concern are low order (dominated by the 2nd through the 5th harmonics). If the circuit has a high impedance resonance near one of these frequencies, a dynamic overvoltage condition results that can cause failure of arresters and problems with sensitive equipment.
This problem is typically limited to cases of energizing large transformers with large power factor correction capacitor banks (e.g., arc furnace installations or other large industrial facilities). The solution to problems with dynamic overvoltages is to assure that the conditions causing the system resonance are not present when the transformer is energized. This could mean making sure a capacitor bank is out of service whenever a large transformer is energized.
Figure 2 shows an example measured current waveform on a 12.5kV feeder during a transformer energizing operation.
Figure 2 – Example Transformer Inrush Current during Circuit Restoration
SIMULATION RESULTS
The accuracy of the system model was verified using three-phase and single-line-to-ground fault currents and other steady-state quantities, such as transformer, load, and capacitor bank rated currents and voltage rise.
The initial case (Case 1a) involved energizing the 18 MVA transformer using the high-side circuit breaker T1 with no secondary load or the 3.6 MVAr capacitor bank in-service on the 34.5kV bus. Figure 3 shows the worst-case simulated primary transformer current (Phase A) during energization of the unloaded transformer. The peak inrush current is nearly 175 amps. For reference, the full-load current for the transformer is approximately 45 amps. Figure 4 shows the corresponding secondary 34.5kV bus voltage waveform, which contains virtually no distortion.
Figure 3 – Transformer Energizing Primary Inrush Current
Figure 4 – Transformer Secondary Voltage during Transformer Energizing
Transformer inrush current is rich in 2nd, 3rd, 4th, and 5th harmonics. The exact characteristics of the inrush current are dependent on transformer parameters (e.g., saturation curve) and the initial condition of the residual transformer flux.
The second case (Case 1b) involved energizing the 18 MVA transformer using the high-side circuit breaker T1 with no secondary load and with the 3.6 MVAr capacitor bank in-service on the 34.5kV bus.
The resonant frequency for the 18 MVA transformer and the 3.6 MVAr capacitor bank may be approximated using the following expression:
where: hr = parallel resonant frequency (x fundamental) MVA3φ = three-phase short circuit capacity (MVA = √3 * 34.5 kV * 3.8 kA ≈ 227 MVA) MVAr3φ = three-phase capacitor bank rating (MVAr)
Figure 5 shows the worst-case simulated primary transformer current (Phase A) during energization of the unloaded transformer with the 3.6 MVAr capacitor bank in-service on the 34.5kV bus. Figure 6 shows the corresponding worst-case secondary 34.5kV bus voltage (Phase C).
As can be observed from the simulation results, the current waveform is much more distorted than the initial case that did not have the 3.6 MVAr capacitor bank in-service. In addition, the voltage waveform shows a resonance condition and the resulting dynamic overvoltage. Dynamic overvoltages are defined as long-term resonant overvoltages lasting many cycles that can cause damage to capacitor units and other adjacent equipment, such as transformers and surge arresters. Figure 7 shows the results of a Fourier analysis of the voltage waveform. The highest harmonic voltage component is at 480 Hz, which corresponds to the previous resonance calculation.
Figure 5 – Transformer Inrush Current with Secondary Capacitor Bank In-Service
Figure 6 – Transformer Secondary Voltage with Capacitor Bank In-Service
Figure 7 – Transformer Secondary Voltage Harmonic Spectrum
The final case (Case 1c) shows the effect of adding a secondary load on the 34.5kV bus, while still energizing the 18 MVA transformer using the high-side circuit breaker T1 with the 3.6 MVAr capacitor bank in-service. Figure 8 shows the worst-case simulated primary transformer current (Phase A) during energization of the loaded transformer with the 3.6 MVAr capacitor bank in-service on the 34.5kV bus. Figure 9 shows the corresponding worst-case secondary 34.5kV bus voltage (Phase C). As can be observed from the simulation results, the current and voltage waveforms are much less distorted with 5,000 kVA of resistive load included on the 34.5kV bus.
Figure 8 – Transformer Inrush Current with Secondary Load In-Service
Figure 9 – Transformer Secondary Voltage with Secondary Load In-Service
CONCLUSIONS
Observations and conclusions for this case study include:
− Transformer inrush currents contain harmonic currents that may produce dynamic overvoltages if the transformer is energized with a capacitor bank on the secondary bus.
− Solutions to this problem include energizing the capacitor bank separately from the transformer (this prevents the inrush current from exciting the resonant circuit) and energizing the transformer/capacitor combination with enough secondary load to sufficiently damp the transient overvoltage. The combination of switching an unloaded transformer and capacitor bank is the most susceptible to dynamic overvoltages.
RELATED STANDARDS IEEE Std. 1036
GLOSSARY AND ACRONYMS MOV: Metal Oxide Varistor Arrester MSSPL: Maximum Switching Surge Protective Level SiC: Silicon Carbide Arrester
Published by Electrotek Concepts, Inc., PQSoft Case Study: ModelingFerroresonance in an Underground Distribution System, Document ID: PQS0610, Date: July 1, 2006.
Abstract: The objective of this case study is to provide an overview of ferroresonance phenomena, its modeling aspects, and practical experience in recognizing, avoiding, and solving the problem. In particular, it will present symptoms of ferroresonance. An actual example simulation analysis involving an underground cable circuit with blown fuses is presented along with solutions to avoid ferroresonance.
INTRODUCTION
Ferroresonance is a general term applied to a wide variety of resonance interactions involving capacitors and saturable iron-core inductor. During the resonance the capacitive and inductive reactances are equal with opposite values, thus the current is only limited by the system resistance resulting in unusually high voltages and/or currents. Ferroresonance in transformers are more common than any other power equipment since their cores are made of saturable ferrous materials.
Ferroresonant overvoltages on distribution systems were observed early in the history of power systems (i.e., early 1900s). In this case study, the theory of ferroresonance is briefly presented. More theoretical description can be found in documents listed in the reference section. Symptoms of ferroresonance and personal accounts of engineers who witnessed the phenomena are provided. Ferroresonant modeling and a case study of an actual ferroresonance problem with its corresponding solutions is also included in the case.
PRINCIPLES OF FERRORESONANCE
There are various ways to understand ferroresonance. One method is to begin with a review of a simple RLC circuit. Figure 1 shows a voltage source with an arbitrary frequency, such as 50 Hz or 60 Hz.
Figure 1 – Simple RLC Circuit for Explaining Ferroresonance
The inductive (XL) and capacitive (XC) reactances are assumed to be constant or linear. Furthermore, it is assumed that the resistance (R) is much smaller than |XL| and |XC|. The magnitude of current flowing in the circuit is approximately:
I = V / R + XL – XC ≅ V / XL – XC
Let us vary XC and hold R and XL at a constant value. When XC = 0, the current flowing in the circuit is I=V/ XL, and when XC is very large, the current becomes negligible. In between these two extremes, |XC|=|XL|. The current becomes very large limited only by R, i.e., I=V/ R. The large current can produce considerable overvoltage. Figure 2 illustrates the magnitude of current under various XC values. The possibility of XC exactly matching XL is remote since both values are linear or constant. However, if the value of XL varies, such as in an iron core transformer, the possibility of XC equaling XL increases considerably.
Figure 2 – Current in the Simple Series RLC Circuit with Various XC Values
Alternatively, the solution to the above circuit can be written as follows:
VL = jXLI = V – (-jXC)I or, v = XLI, and v = V – XCI
where V is an arbitrary voltage.
Figure 3 – Graphical Solution of Linear LC Circuit
The intersection between the inductive reactance XL line and the capacitive reactance XC line yields the current in the circuit and the voltage across the inductor, VL. The above solution is depicted in Figure 3. At resonance, these two lines become parallel, yielding solutions of infinite voltage and current (assuming lossless element).
When XL is no longer linear, such as a saturable inductor, the XL reactance can no longer be represented with a straight line. The graphical solution is now as shown in Figure 4.
Figure 4 – Graphical Solution of a Nonlinear LC Circuit
It is obvious that there may be as many as three intersections of the capacitive reactance line with the inductive reactance curve. Intersection 2 is an unstable operating point and the solution will not remain there in steady-state. However, it may pass through this point during a transient. Intersections 1 and 3 are stable and will exist in steady-state. Obviously, if the values become an intersection 3 solution, there will be both high voltages and high currents. For small capacitances, the XC line is very steep, usually resulting in only one intersection in the third quadrant. The capacitive reactance is larger than the inductive reactance, resulting in a leading current and higher than normal voltages across the capacitor. The voltage across the capacitor is the length of the line from the system voltage intersection to the intersection with the inductor curve. As the capacitance increases, there can be multiple intersections as shown. The natural tendency then is to achieve a solution at intersection 1, which is an inductive solution with lagging current and little voltage across the capacitor. Note that the voltage across the capacitor will be the line-to-ground voltage on the cable in a typical power system ferroresonance case.
If there is a slight increase in the voltage, the capacitor line would shift upward, eliminating the solution at intersection 1. The solution would then try to jump to the third quadrant. Of course, the resulting current might be so large that the voltage drops again and the solution point jumps between 1 and 3. Indeed, phenomena like this are observed during instances of ferroresonance. The voltage and current appear to vary randomly and unpredictably.
In the usual power system case, ferroresonance occurs when a transformer becomes isolated on a cable section in such a manner that the cable capacitance appears to be in series with the magnetizing characteristic of the transformer. For short lengths of cable, the capacitance is very small and there is one solution in the third quadrant at relatively low voltage levels. As the capacitance increases the solution point creeps up the saturation curve in the third quadrant until the voltage across the capacitor is well above normal. These operating points may be relatively stable, depending on the nature of the transient events that precipitated the ferroresonance.
SYMPTOMS OF FERRORESONANCE
There are several modes of ferroresonance with varying physical and electrical characteristics. Some have very high voltages and currents while others have voltages close to normal. In this section symptoms of ferroresonance are presented.
Audible Noise
One thing common to all types of ferroresonance is that the steel core is driven into saturation, often deeply and randomly (otherwise, it is conventional resonance and not considered ferroresonance). As the core goes into a high flux density, it will make an audible noise due to the magnetostriction of the steel and to the actual movement of the core laminations. In ferroresonance, this noise is often likened to shaking a bucket of bolts, whining, or to a chorus of a thousand hammers pounding on the transformer from within. In any case, the sound is distinctively different and louder than the normal hum of a transformer.
Overheating
Another reported symptom of the high magnetic field is due to stray flux heating in parts of the transformer where magnetic flux is not expected. Since the core is saturated repeatedly, the magnetic flux will find its way into the tank wall and other metallic parts. One possible side effect is the charring or bubbling of paint on the top of the tank. This is not necessarily an indication that the unit is damaged, but damage can occur in this situation if the ferroresonance has persisted sufficiently long to cause overheating of some of the larger internal connections. This may in turn damage insulation structures beyond repair.
Arrester and Surge Protector Failure
The arrester failures are related to heating of the arrester block. One common failure scenario is for line personnel to discover an open fused cutout and to simply replace the fuse. Meanwhile, the arrester on that phase has become very hot and goes into thermal runaway upon restoration of full power to that phase. Failures are often catastrophic with parts being expelled from the arrester housing. Under-oil arresters are less susceptible to this problem because they are able to dissipate the heat due to the ferroresonance current more rapidly.
Flicker
Customers are frequently subjected to a wavering voltage magnitude. Light bulbs will flicker between very bright and dim. Some electronic appliances are reportedly very susceptible to the voltages that result from some types of ferroresonance, but we have no knowledge of the alleged failure mode. Perhaps, it is simply MOV failure in the power front end. These frequently fail catastrophically, going into thermal runaway and then burning open with considerable arcing display. This may do nothing more than pop a breaker, but surge protection is lost for any subsequent surge that might damage the appliance.
Cable Switching
The transformers themselves can usually withstand the overvoltages without failing. Of course, they would not be expected to endure this stress repeatedly because the forces often shake things loose inside and abrade insulation structures. The cable is also in little danger unless its insulation stress had been reduced by aging or physical damage. Of course, operating a solid dielectric system above its normal stress level for an extended period can be expected to create some shortage of life.
It may be difficult to clear arcs when pulling cable elbows if ferroresonance is in progress. The currents may be much higher than expected and the peak voltages may be high enough to cause reignition of the arc.
Some utilities will not perform cable switching involving three-phase padmount transformers without first verifying that there is substantial load on the transformers. One of the common solutions to ferroresonance during cable switching is to always pull the elbows and energize the unit at the primary terminals. This will normally work because there is no external cable capacitance to cause ferroresonance. There is little internal capacitance, and the losses of the transformers are usually sufficient to prevent resonance with this small capacitance. Unfortunately, modern transformers are changing the old rules of thumb. The newer low-loss transformers, particularly, those with amorphous metal core, are prone to ferroresonance.
TRANSFORMER MODELING
Ferroresonance can be mysterious subject. Probably the main reason is that the analysis requires sophisticated nonlinear circuit analysis techniques. The results are sometimes unpredictable and certainly difficult to visualize (unlike linear circuit phenomena). Another issue that complicates the analysis of ferroresonance is that there are several different types of three-phase transformers such as three single-phase transformers connected as a three-phase transformer, three-legged core transformers, three-phase shell-type transformers, four-legged core, and five-legged cores. The conventional T model of a two-winding transformer, and the five-legged core transformer model will be summarized in this section.
For single-phase transformers, three-phase shell form transformers, and three-phase triplexed transformers (three single-phase units stacked in one can), the conventional T model will suffice because there is no coupling between the magnetic circuits. Figure 5 shows the T model for a two winding transformer, which will suffice for standard switching surge and ferroresonance studies. For higher frequencies, it would be necessary to model the capacitances and inner winding construction.
Figure 5 – Standard T Model of a Two-Winding Transformer
The terminals of this model can be connected to represent any two-winding transformer with magnetically independent phases. The saturable inductance data are readily available from the manufacturer’s test data. Note that manufacturers supply the rms v-i curve. This must be converted to a peak flux-current curve before it can be used in EMTP or other transients programs. This conversion is a bit tricky because the current waveforms are not sinusoidal. Therefore, one cannot simply multiply the current values on the rms curve by 1.414 to determine the correct peak value. The usual procedure is to use a computer program that reconstructs the peak saturation curve by iterative solution. The first point can be established by multiplying by 1.414. Then a guess is made at the next point and the waveform is reconstructed. The guess is adjusted until the rms of the reconstructed waveform matches that supplied by the manufacturer.
The five-legged core transformer design [3] is illustrated in Figure 6. The design typically consists of four individual cores tied together to create the five-legged core transformer. The inner three legs carry the phase windings with flux paths as indicated. The equivalent circuit can be derived from the flux path direction and is shown in Figure 7.
Figure 6 – Five-Legged Transformer Design and its Flux Paths
Figure 7 – Equivalent Circuit for a Five-Legged Transformer
CASE STUDY
In this section, an actual case study is presented. A ferroresonance condition developed on an approximately 5,000-foot underground cable feed to a medical facility. When one of the riser pole fuses blew, severe voltage fluctuations occurred at the load. As a temporary solution the utility replaced the fuses with a three-phase recloser and wanted to see under what conditions the three-phase recloser might be removed and the fuses reinstalled. Therefore, the purpose of this case study was to determine under what conditions the ferroresonance at the underground distribution network could be avoided (and whether fuses might be reinstalled instead of keeping the three-phase recloser).
The ferroresonance condition apparently did not cause damages to the two 500 kVA transformers nor the customer loads at the medical facility. However, it was reported that a sudden overvoltage did occur and lights flickered between bright and dim.
A simplified one-line diagram to study the ferroresonance problem is shown in Figure 8. The simulation model was developed using the EMTP program.
Figure 8 – Oneline Diagram fro the Underground Cable-Fed Run
The lengths of the cable from the first pole to the first switch (S1), and from the first switch (S1) to the second switch (S2) were approximately 1,900 feet and 2,150 feet long. The cable size was 600 MCM with the following characteristics:
Insulation: 0.1406 outside diameter in feet Jacket: 0.1412 outside diameter in feet Neutral: 0.1409 outside diameter in feet
A line constant program was used to compute the positive and zero-sequence impedances of the cable, yielding the following results:
The lengths of the underground cable from the second switch (S2) to the first transformer (JL61), and from the first transformer (JL61) to the second transformer (KL30) were 420 and 450 feet long, respectively. The type of the cable was 1/0 with the following characteristics:
Insulation: 0.0629 outside diameter in feet Jacket: 0.0688 outside diameter in feet Neutral: 0.0794 outside diameter in feet
The computed positive and zero-sequence impedances were:
The two 500 kVA transformers were modeled according to the five-legged core transformer design. In order to investigate overvoltage due to ferroresonance, one phase of the cable was intentionally opened to simulate circumstances leading to ferroresonance (e.g., fuse blows, cable connector or splice opening, etc.). In the simulation, phase B at the first pole was open-circuited, while switches S1 and S2 shown in Figure 8 were closed at all times. Resistive loads at the secondary winding of transformers JL61 and KL30 were increased from zero to 30% of the transformer capacities, i.e., from 0 to 150 kW.
Figure 9 shows voltage waveforms at the secondary winding of transformer JL61 when both JL61 and KL30 transformers are unloaded. Since the voltage at the secondary of transformer KL30 is nearly identical to that of JL61, the voltage waveforms are not shown. Industry analysts have historically assumed that when the voltage exceeds 1.25 per-unit, the system is said to be “in ferroresonance”. Figure 9 clearly illustrates that the system is in ferroresonance condition since phase B exhibits sustained overvoltage approaches 3.0 per-unit.
Figure 9 – Voltage Waveforms at Secondary Winding of Transformer JL61
Figure 10 (top left) shows the voltage waveforms at the secondary winding of JL61 transformer when both JL61 and KL30 transformers are loaded with resistive load equivalent to 5% of transformer capacities. In other words, JL61 and KL30 transformers are loaded with 25 kW loads. The overvoltage at phase B is now approximately 2 per-unit, much less compare to when both transformers are unloaded.
In the similar fashion, loads at both transformers are added successively, i.e., 10, 15, 20, 25, and 30% of the transformer capacities. As loads increase the overvoltage drops quickly. Figure 10 shows the voltage waveforms at the secondary winding of transformer JL61 when both transformers are loaded with 5% (top left), 15% (top right), 20% (bottom left), and 30% (bottom right) of their respective capacities.
With 15% of load, the system remains in ferroresonance condition since it exhibits sustained overvoltage of 1.5 per-unit. The ferroresonance condition is practically eliminated when both transformers are loaded with 20% of resistive load. The overvoltage magnitude is about 1.4 per-unit at when phase B is open, however this overvoltage is not sustained and quickly decays to a low voltage. With 30% of load, the system is not in ferroresonance either. Twenty percent of resistive load is sufficient to avoid the ferroresonance condition.
Figure 10 – Voltage Waveforms at the Secondary Winding of Transformer JL61 (with (a) 5%, (b) 15%, (c) 20%, and (d) 30% of their Respective Capacities)
Figure 11 shows the summary of peak overvoltages when both transformers are loaded from 0% to 30% of their capacities. The overvoltage on phase B drops quickly as both transformers become more loaded. From the analysis presented in this section, it can be concluded that both transformers should be loaded with a minimum of 100 kW resistive load or loads equivalent to 20% of transformer capacity to avoid the ferroresonance condition. The rapid drop in ferroresonant voltage magnitude is due in large part to the introduction of the resistive load.
Based on the study, the ferroresonance condition can be avoided by having both transformers loaded with at least 20 percent of their respective capacities. In other words, each transformer must have 100 kW (resistive) at its secondary winding. When one phase is open-circuit, there will be a momentary overvoltage as high as 1.4 per-unit, however it quickly decays to a low voltage. There will be no sustained ferroresonance overvoltage. If this minimum loading can be guaranteed, it is safe to replace a three-phase recloser with three fuses.
In the event that the loading cannot be achieved, it is advised to use the three-phase switchgear to avoid the ferroresonance condition. The minimum load of 20% to avoid ferroresonance is much higher than the usual minimum load of 5%. The higher minimum is primarily due to the length of the cable involved, which is approximately 1 mile long.
Figure 11 – Change in Peak Transient Overvoltage vs. % Resistive Load on a 500kVA Transformer
SUMMARY
A fundamental description of ferroresonance has been presented in this case study. In particular, analysis of the ferroresonance condition based on a simple graphical approach is presented. Several transformer models are also included for reference. Finally, a representative case study showing ferroresonance in an underground cable circuit is included. Minimum load levels for mitigating ferroresonance are evaluated.
REFERENCES
[1] R. Rudenberg, Transient Performance of Electric Power Systems, New York, NY, McGraw-Hill Company, 1950. [2] C. Hayashi, Nonlinear Oscillations in Physical Systems, New York, NY, McGraw-Hill Company, 1964. [3] D. L. Stuehm, B. A. Mork, D. D. Mairs, “Five-legged core transformer equivalent circuit”, IEEE Transactions on Power Delivery, Vol 4, No. 3, July 1989, pp. 1786. [4] Slow Transient Task Force of the IEEE Working Group on Modeling and Analysis of System Transients Using Digital Programs, “Modeling and analysis guidlines for slow transients – Part III: The study of ferroresonance,” IEEE Trans. on Power Delivery, vol. 15, No. 1., Jan. 2000, pp. 255 – 265.
RELATED STANDARDS IEEE Std. C57.105-1978
GLOSSARY AND ACRONYMS MOV: Metal Oxide Varistor Arrester MSSPL: Maximum Switching Surge Protective Level SiC: Silicon Carbide Arrester
Published by Electrotek Concepts, Inc., PQSoft Case Study: General Reference – Ferroresonance, Document ID: PQS0607, Date: July 1, 2006.
Abstract: The term ferroresonance refers to a special kind of resonance that involves capacitor and iron-core inductance. The most common condition in which it causes disturbances is when the magnetizing impedance of a transformer is placed in series with a system capacitor. There are several modes of ferroresonance with varying physical and electrical characteristics. Some have very high voltages and currents, while others have voltages close to normal. There may or may not be equipment failures or other evidence of ferroresonance in the electrical equipment. In many cases it may be may be difficult to tell if ferroresonance has occurred, unless there are witnesses or power quality monitoring instruments installed.
This case presents an overview of ferroresonance and a number of example measured and simulated waveforms.
OVERVIEW OF FERRORESONANCE
Ferroresonance is a term generally applied to a wide variety of interactions between capacitors and iron-core inductors that result in unusual voltage and/or currents. In linear circuits, resonance occurs when the capacitive reactance equals the inductive reactance at the frequency at which the circuit is excited. Iron-core inductors have a nonlinear characteristic and therefore a range of inductance values. This relationship may lead to a number of operating conditions where the inductive reactance does not equal the capacitive reactance, but very high and damaging overvoltages occur.
In a typical power system, ferroresonance occurs when a transformer becomes isolated on a cable section in such a manner that the cable capacitance appears to be in series with the magnetizing characteristic of the transformer. An unbalanced switching operation is required to initiate the condition.
Several of the more common causes include:
single-phase cutouts
fuse blowing or opening (transformer or line fuse) (or a lineman pulls an elbow connector)
single-phase reclosers
cable connector of splice opening
manual cable switching to reconfigure a cable circuit during an emergency condition
open conductor fault in overhead line feeding cable
three-phase switch with large pole closing span
Two additional conditions must be satisfied for ferroresonance to occur:
The length of cable between the transformer and open conductor location must have sufficient capacitance to produce excessive ferroresonant voltages
The losses in the circuit and the resistive load on the transformer must be low.
These conditions may be met at a variety of times. The low resistive load requirement is often satisfied on new construction projects, when there may be no load on the transformer for a period of time. As the load increases, ferroresonance becomes much less likely (unless the customer separates from the utility during emergency conditions). Figure 1 illustrates one possible switching condition that can lead to ferroresonance. The case, which was investigated using computer simulations, involves a stuck-pole in the feeder switch. A number of different scenarios may lead to this condition, however, for the purposes of simulation, it is simply modeled as two phases open and one closed.
There are several modes of ferroresonance with varying physical and electrical characteristics. Some have very high voltages and currents, while others have voltages close to normal, as illustrated in Figure 2. There may or may not be equipment failures of other evidence of ferroresonance in the electrical equipment. In many cases it may be may be difficult to tell if ferroresonance has occurred, unless there are witnesses or power quality monitoring instruments installed.
Figure 2 – Example Measured Distribution Feeder Voltage during Ferroresonance
One thing common to all types of ferroresonance is that the steel core is driven into saturation, often deeply and randomly (otherwise, it is conventional resonance). As the core goes into a high flux density, it will make an audible noise due to the magnetostriction of the steel and movement of the core laminations. The sound produced is distinctly different and louder than the normal hum of a transformer. Another reported symptom of the high magnetic field is charring or bubbling of the paint on the top of the transformer tank. This is due to stray flux heating in parts of the transformer where magnetic flux is not expected. Since the core is saturated repeatedly, the magnetic flux will find its way into the tank wall and other metallic parts.
If high voltages accompany the ferroresonance, there could be electrical damage to both the primary and secondary circuits. Surge arresters commonly fail during this condition. Arrester failures are related to the heating of the arrester block, and at times, the failures can be catastrophic, with parts being expelled from the arrester housing.
Ferroresonance cannot always be entirely avoided; however, steps can be taken to reduce the probability of occurrence. These include locating fuses or disconnects near the transformer (to minimize capacitance), and using three-phase switches. However, neither of these remedies will provide protection for the broken conductor case. Another common solution involves using grounded-wye / grounded-wye transformers. When each phase is magnetically independent, this connection prevents ferroresonance. However, the common five-legged core design of three-phase padmount transformers is still susceptible to ferroresonance because the phases are magnetically coupled.
FERRORESONANCE WAVEFORM EXAMPLES
Figure 3 illustrates an example distribution voltage during a ferroresonance event. Phases A-B and C-A are shown. These waveforms were recorded with a Dranetz-BMI 8010 PQNode.
Figure 3 – Example Measured Ferroresonance Voltage Waveform
Figure 4 shows an example simulated distribution system ferroresonance event. This voltage waveform was produced using the Electromagnetic Transients Program (EMTP).
Figure 4 – Example Simulated Ferroresonance Voltage Waveform
Figure 5 shows an example simulated distribution system ferroresonance event. This voltage waveform was produced using the FerroView program.
Figure 5 – Example Simulated Ferroresonance Voltage Waveform
Figure 6 shows an example simulated distribution system ferroresonance event. This voltage waveform was produced using the PSCAD program.
Figure 6 – Example Simulated Ferroresonance Voltage Waveform
Figure 7 shows an example simulated distribution system ferroresonance event. A resistive load is added to the circuit at 0.3 seconds. This voltage waveform was produced using the PSCAD program
Figure 7 – Example Simulated Ferroresonance Voltage Waveform
Figure 8 shows an example distribution system voltage during a ferroresonance event. These voltage waveforms were recorded with a Dranetz-BMI 8010 PQNode.
Figure 8 – Example Measured Ferroresonance Voltage Waveform
Figure 9 shows an example distribution system feeder current during a ferroresonance event. These current waveforms were recorded with a Dranetz-BMI 8010 PQNode.
Figure 9 – Example Measured Ferroresonance Current Waveform
SUMMARY
Ferroresonance is a term generally applied to a wide variety of interactions between capacitors and iron-core inductors that result in unusual voltage and/or currents. These interactions may lead to a number of operating conditions where high and damaging overvoltages occur. Ferroresonance is different than resonance in linear system elements. In linear systems, resonance results in high sinusoidal voltages and currents of the resonant frequency. Ferroresonance can also result in high voltages and currents, but the resulting waveforms are usually irregular and chaotic in shape, as illustrated with the example waveforms in the case study.
REFERENCES
Tennessee Valley Public Power Association, Inc., Power Quality Manual, Final Report, Project PQ 2, 2002.
RELATED STANDARDS IEEE Std. C57.105-1978
GLOSSARY AND ACRONYMS MOV: Metal Oxide Varistor Arrester MSSPL: Maximum Switching Surge Protective Level SiC: Silicon Carbide Arrester
Published by Siemens Canada, Siemens Limited Power Product Catalogue, Canadian Edition 2019. Section 18 – Technical, Contents: Harmonics / K-factor Ratings (18-16, 18-17)
Non-Linear Loads
When a sinusoidal voltage is applied to a linear load, the resultant current waveform takes on the shape of a sine wave as well. Typical linear loads are resistive heating and induction motors. In contrast, a non-linear load either:
Draws current during only part of the cycle and acts as an open circuit for the balance of the cycle,
or
Changes the impedance during the cycle, hence the resultant waveform is distorted and no longer conforms to a pure sine wave shape
In recent years, the use of electronic equipment has mushroomed in both offices and industrial plants. These electronic devices are powered by switching power supplies or some type of rectifier circuit. Examples of these devices used in offices are: computers, fax machines, copiers, printers, cash registers, UPS systems, and solid-state ballasts. In industrial plants, one will find other electronic devices such as variable speed drives, HID lighting, solid-state starters and solid-state instruments. They all contribute to the distortion of the current waveform and the generation of harmonics. As the use of electronic equipment increases and it makes up a larger portion of the electrical load, many concerns are raised about its impact on the electrical power supply system.
Harmonics
As defined by ANSI / IEEE Std. 519-1992, harmonic components are represented by a periodic wave or quantity having a frequency that is an integral multiple of the fundamental frequency. Harmonics are voltages or currents at frequencies that are integer multiples of the fundamental (60 Hz) frequency: 120 Hz, 180 Hz, 240 Hz, 300 Hz, etc. Harmonics are designated by their harmonic number, or multiple of the fundamental frequency. Thus, a harmonic with a frequency of 180 Hz (three times the 60 Hz fundamental frequency) is called the 3rd harmonic. Harmonics superimpose themselves on the fundamental waveform, distorting it and changing its magnitude. For instance, when a sine wave voltage source is applied to a non-linear load connected from a phase-leg to neutral on a 3-phase, 4-wire branch circuit, the load itself will draw a current wave made up of the 60 Hz fundamental frequency of the voltage source, plus 3rd and higher order odd harmonic (multiples of the 60 Hz fundamental frequency), which are all generated by the non-linear load. Total Harmonic Distortion (THD) is calculated as the square root of the sum of the squares of all harmonics divided by the normal 60 Hz value.
This yields an RMS value of distortion as a percentage of the fundamental 60 Hz waveform.
Therefore, it is the percentage amount of odd harmonics (3rd, 5th, 7th ,…, 25th,…) present in the load which can affect the transformer, and this condition is called a “Non-Linear Load” or “Non-Sinusoidal Load”. To determine what amount of harmonic content is present, a K-Factor calculation is made instead of using the THD formula. The total amount of harmonics will determine the percentage of non-linear load, which can be specified with the appropriate K-Factor rating.
Figure 30 — Effect of Harmonics on Current Waveform
Typical Symptoms of Harmonic Problems
Distribution / lighting transformers overheating even when measured load current is within transformer rating
Neutral cable / bus overheating even with balanced load
Fuses blowing and circuit breakers tripping at currents within rating
Effect Of Harmonics On Transformers
Non-sinusoidal current generates extra losses and heating of transformer coils thus reducing efficiency and shortening the life expectancy of the transformer. Coil losses increase with the higher harmonic frequencies due to higher eddy current loss in the conductors. Furthermore, on a balanced linear power system, the phase currents are 120 degrees out of phase and offset one another in the neutral conductor. But with the “Triplen” harmonics (multiple of 3) the phase currents are in phase and they are additive in this neutral conductor. This may cause installations with non-linear loads to double either the size or number of neutral conductors.
Measurement of Harmonics
Table A.3 K-Factor Ratings
Sizing Transformers for Non-Linear Loads
ANSI / IEEE C57.110-2008 has a procedure for de-rating standard distribution transformers for non-linear loading. However this is not the only approach. A transformer with the appropriate K-Factor specifically designed for non-linear loads can be specified.
K-Factors
K-Factor is a ratio between the additional losses due to harmonics and the eddy losses at 60 Hz. It is used to specify transformers for non-linear loads. Note that K-Factor transformers do not eliminate harmonic distortion; they withstand the non-linear load condition without overheating.
Calculating K-Factor Loads
List the kVA value for each load category to be supplied. Next, assign a K-factor designation that corresponds to the relative level of harmonics drawn by each type of load. Refer to Table A.4
Multiply the kVA of each load or load category times the Index of Load K-rating (ILK) that corresponds to the assigned K-factor rating. This result is an indexed kVA-ILK value. KVA x ILK = kVA-ILK.
Tabulate the total connected load kVA for all load categories to be supplied.
Next, add-up the kVA-ILK values for all loads or load categories to be supplied by the transformer.
Divide the grand total kVA-ILK value by the total kVA load to be supplied. This will give an average ILK for that combination of loads. Total kVA-ILK/ Total kVA = average ILK.
From Table A.4 find the K-factor rating whose ILK is equal to or greater than the calculated ILK
Measurement of Harmonics Table A.3 K-Factor Ratings
For existing installations, the extent of the harmonics can be measured with appropriate instruments commonly referred to as “Power Harmonic Analyzers”. This service is offered by many consulting service organizations. For new construction, such information may not be obtainable. For such situations, it is best to assume the worse case condition based on experience with the type and mix of loads.
Published by Electrotek Concepts, Inc., PQSoft Case Study: Ferroresonance in an Underground Distribution System, Document ID: PQS0710, Date: October 15, 2007.
Abstract: The term ferroresonance refers to a special kind of resonance that involves capacitor and iron-core inductance. The most common condition in which it causes disturbances is when the magnetizing impedance of a transformer is placed in series with a system capacitor. There are several modes of ferroresonance with varying physical and electrical characteristics. Some have very high voltages and currents, while others have voltages close to normal. There may or may not be equipment failures or other evidence of ferroresonance in the electrical equipment. In many cases it may be may be difficult to tell if ferroresonance has occurred, unless there are witnesses or power quality monitoring instruments installed.
This case study presents a ferroresonance evaluation for a 34.5kV underground distribution system.
INTRODUCTION
An underground distribution system ferroresonance evaluation was completed for the system shown in Figure 1.
Figure 1 – Oneline Diagram for Underground Distribution Ferroresonance
BACKGROUND
Ferroresonance is a term generally applied to a wide variety of interactions between capacitors and iron-core inductors that result in unusual voltages and/or currents. In linear circuits, resonance occurs when the capacitive reactance equals the inductive reactance at the frequency at which the circuit is excited. Iron-core inductors have a nonlinear characteristic and therefore a range of inductance values. This relationship may lead to a number of operating conditions where the inductive reactance does not equal the capacitive reactance, but very high and damaging overvoltages occur.
In a typical power system, ferroresonance occurs when a transformer becomes isolated on a cable section in such a manner that the cable capacitance appears to be in series with the magnetizing characteristic of the transformer. An unbalanced switching operation is required to initiate the condition. Several of the more common causes include:
− Single-phase cutouts − Fuse blowing or opening (transformer/line fuse, or a lineman pulls an elbow connector) − Single-phase reclosers − Cable connector of splice opening − Manual cable switching to reconfigure a cable circuit during an emergency condition − Open conductor fault in overhead line feeding cable − Three-phase switch with large pole closing span
Two additional conditions must be satisfied for ferroresonance to occur:
− The length of cable between the transformer and open conductor location must have sufficient capacitance to produce excessive ferroresonant voltages. − The losses in the circuit and the resistive load on the transformer must be low.
SIMULATION RESULTS
The accuracy of the system model was verified using three-phase and single-line-to-ground fault currents and other steady-state quantities, such as transformer and customer load rated currents.
A ferroresonance condition developed on a roughly 7,000-foot underground cable supplying a medical facility. Severe voltage fluctuations occurred at the customer load when one of the riser pole fuses blew. As a temporary solution, the utility replaced the fuses with a three-phase recloser. A study was completed to determine under what conditions the three-phase recloser might be removed and the fuses reinstalled.
The ferroresonance condition did not damage the two 500 kVA transformers or the customer loads at the medical facility. However, it was reported that a sudden overvoltage did occur and lights flickered between bright and dim.
The lengths of the cable from the first pole to the first switch (S1), and from the first switch (S1) to the second switch (S2) were approximately 2,750 feet and 3,200 feet long. The cable size was 600 MCM with the following characteristics:
Insulation:……………………………….0.1406 outside diameter in feet Jacket:…………………………………….0.1412 outside diameter in feet Neutral:…………………………………..0.1409 outside diameter in feet
A line constant program was used to compute the positive and zero-sequence impedances of the cable, yielding the following results:
Z1 = 0.0231 + j 0.0824 ohms/1000 ft Z0 = 0.1828 + j 0.6854 ohms/1000 ft C1 = C0 = 78.24 ηF/1000 ft
The lengths of the underground cable from the second switch (S2) to the first transformer, and from the first transformer to the second transformer were 420 and 450 feet long, respectively. The type of the cable was 1/0 with the following characteristics:
Insulation:……………………………….0.0629 outside diameter in feet Jacket:…………………………………….0.0688 outside diameter in feet Neutral:…………………………………..0.0794 outside diameter in fee
The computed positive and zero-sequence impedances were:
Z1 = 0.0803 + j 0.0952 ohms/1000 ft Z0 = 0.4061 + j 0.5150 ohms/1000 ft C1 = C0 = 97.34 ηF/1000 ft
The two 500 kVA transformers were modeled using a five-legged core transformer design. In order to investigate overvoltage due to ferroresonance, one phase of the cable was intentionally opened to simulate the circumstances leading to the ferroresonance (e.g., fuse blows, cable connector or splice opening, etc.) event. In the transient simulation, Phase B at the first pole was open-circuited, while the switches S1 and S2 (refer to Figure 1) were closed at all times. Figure 2 shows the three-phase voltage waveform at the secondary winding of one of the 500 kVA transformers with both of the 500 kVA transformers unloaded (Case 2a). Industry analysts have historically assumed that when the voltage exceeds 1.25 per-unit, the system is said to be in ferroresonance. Figure 2 clearly illustrates that the system is in a ferroresonance condition because Phase A exhibits sustained overvoltages greater than 2.0 per-unit.
Figure 2 – Customer Transformer Secondary Voltage with no Secondary Load
A series of cases were completed to determine the relationship between the amount of customer secondary load and the resulting ferroresonant overvoltages. As the load is increased, the transient overvoltage drops very quickly. Figure 3 shows the three-phase voltage waveform at the secondary winding of one of the 500 kVA transformers with both of the 500 kVA transformers having a resistive load equal to 5% (25 kW) of the transformer rating (Case 2b).
Figure 3 – Customer Transformer Secondary Voltage with 5% Secondary Load
The final case investigates the use of a three-phase recloser rather than three single-phase fuses. Figure 4 shows the three-phase voltage waveform at the secondary winding of one of the 500 kVA transformers with both of the 500 kVA transformers unloaded and with a three-phase switch opening on the primary distribution feeder (Case 2c). No ferroresonance occurs during the three-phase switching.
Figure 4 – Customer Transformer Secondary Voltage with a Three-Phase Switch
CONCLUSIONS
Observations and conclusions for this case study include:
− The term ferroresonance refers to a special kind of resonance that involves a capacitance and a variable iron-core inductance. A 34.5kV underground distribution feeder ferroresonance event occurred when a single-phase riser fuse blows.
− Transient computer simulations of the underground distribution feeder circuit indicated that the ferroresonant overvoltages were very dependent on the circuit configuration (e.g., cable length and capacitance, transformer ratings, etc.) and on the rating of the load on the customer secondary circuits.
− Solutions to the ferroresonance problem generally include adding resistive load to the secondary of each customer transformer. For this circuit configuration, a resistive load representing 5% of the transformer rating significantly reduced the secondary transient overvoltages. In addition, the potential for ferroresonance may be eliminated by using three-phase switches in place of single-phase fuses.
RELATED STANDARDS IEEE C57.105-1978, IEEE Std. 1036
GLOSSARY AND ACRONYMS MOV: Metal Oxide Varistor Arrester MSSPL: Maximum Switching Surge Protective Level SiC: Silicon Carbide Arrester
Published by Electrotek Concepts, Inc., PQSoft Case Study: Ferroresonance Analysis – 25kV Single 5-Legged Core Transformer, Document ID: PQS0318, Date: July 18, 2002.
Abstract: Ferroresonance is a concern for medium voltage underground systems. As part of an evaluation of the design of a new 25kV underground system, the potential for ferroresonance was analyzed. Ferroresonance requires a certain length of cable to generate severe overvoltages. With modern transformers, this length is frequently less than 200 ft. Our calculations suggest that the threshold of ferroresonance is about 100 ft for a modern 2500 kVA transformer and 1000 kcmil cable and it is certainly in ferroresonance at 200 ft. These distances are proportionately shorter for smaller transformers. This makes it more difficult to achieve ferroresonance-free installations where single-phase switching or fusing is permitted because the maximum cable length is impractically short. As a general guideline, three-phase switching should be strongly considered for cable length exceeding 100 ft. If the switchgear can be mounted closer to the transformer, fused switching may be used with little fear of serious overvoltages.
PROBLEM STATEMENT
An electric utility was evaluating the proposed design of a new, predominantly underground, 25kV system. One of the concerns of the utility related to the new system was the likelihood of 25kV transformers experiencing ferroresonant conditions. This case study was an effort to characterize the combinations of parameters that would increase the probability of ferroresonance.
SYSTEM MODEL
The complete three-phase schematic of the circuit used to evaluate ferroresonance on a single 5-legged core transformer is shown in Figure 1.
Figure 1: Circuit Model used for Ferroresonance Study.
This model was represented in the EMTP program and executed for a number of cases. The variables were:
The length of the cable. Lengths from 100 ft up to 2000 ft were considered.
Transformer size. The transformer model was scaled from 2500 kVA down to 150 kVA.
The number of phases open: either one or two.
The simulation was started with all three switches open and then either one or two phases were closed within a few milliseconds. This creates a transient flux condition in the transformer that is intended to send it into a high ferroresonance mode if one exists. The transformer was assumed to be a 5-legged core design. The five-legged core transformer is of particular interest because it is likely to be the most common 3-phase transformer applied to the 25kV system. It is also the most difficult to model. The 5-legged core transformer model utilized for the evaluation is a model based on Transient Network Analyzer (TNA) technology that basically assumes that the 5-legged core transformer looks the same from each phase and essentially behaves like a symmetrical 4-legged core. This model is shown in Figure 2 and well represents imbalances associated with the center leg of the transformer. The results presented in this case study are those from simulations using this simplified model.
Figure 2: Simplified Model based on TNA Modeling Methods obtained by inserting a saturable element in the corner of the delta in the “phantom tertiary”.
ANALYSIS
Figure 3 shows a plot of the computed voltages appearing on the cable versus cable length for a variety of transformer sizes. The peak of the transient voltage that appears upon energization of the second phase in phase-by-phase switching is plotted. The inrush transient generates a fair amount of high frequency “hash” as is evident in the representative waveforms in Figure 6 and Figure 7. This high frequency transient results from a combination of the transformer being driven heavily into saturation upon inrush and the low assumed losses in the transformer model. It disappears quickly in the presence of a few additional losses. In practice, it may die out in few seconds (although it would be interesting to conduct live tests for comparison to see if actual transformers really do demonstrate the low losses).
For the purposes of comparison, this transient peak is much higher than the steady state values reported in the literature where the typical peak voltage is 2 to 2.5 per unit and is quite sensitive to the loss model. However, we have chosen to plot this voltage because the resulting curves clearly point to the lengths of cable at which ferroresonance begins to be a problem.
Industry analysts have historically assumed that when this voltage exceeds 125%, the system is said to be “in ferroresonance”. Of course, there is similar activity even when lower voltages appear. At 100 ft of cable, all the transformer sizes meet this criterion when there is no load. The 2500 kVA transformer just meets this criterion at 100 ft and is definitely in ferroresonance by 200 ft. The smaller transformers require proportionately less cable. The 150 kVA transformer is somewhat of an anomaly, indicating the fickleness of ferroresonance.
Figure 3: Plot of Peak Transient Voltage vs. Cable Length upon energization of the second phase, for various transformer sizes; no load.
Figure 4 shows the effect of resistor load on ferroresonance. We took two lengths of cable, 500 ft and 2000 ft, for which the 2500-kVA transformer exhibited significant ferroresonance at no load and plotted the peak voltage appearing on the cable for a range of balanced resistive loads on the transformer. As can be seen, the overvoltage drops quickly with the addition of only 1 or 2% load. Technically, the transformer is still “in ferroresonance” up to about 5% load and the voltage magnitudes are near normal, or less than normal, at 15% load. This is in almost complete agreement with what has been reported in the literature.
Figure 4: Change in Peak Transient Overvoltage vs. Percent Resistive Load
Figure 5 shows a set of three waveforms of the voltage for successively increasing load. The rapid drop in ferroresonant voltage magnitude with the introduction of load is due in large part to the damping of the high frequency portion of the transient. This disappears quickly. The remaining voltage waveform bears a strong resemblance to many of the steady state waveforms reported in the literature.
Figure 6 and Figure 7 show other representative waveforms illustrating the variation of waveforms with transformer size and cable length, respectively.
Figure 5: Effect of Load on the Ferroresonant Voltage Magnitude
Figure 6: Effect of Transformer Size with 100 ft. of Cable.
Figure 7: Effect of Cable Length for a 2500 kVA Transformer.
CONCLUSIONS
From these results and the review of the literature concerning trends in ferroresonance with modern transformer designs, we have concluded that it is likely that ferroresonant activity resulting in voltages exceeding 1.25 per unit will occur during normal phase-by-phase switching events and abnormal failure-related events. Based on what we know about how the system will be designed at this time, it appears that the transformers can become isolated on sufficiently long pieces of cable to cause ferroresonance. The threshold of ferroresonance occurs at approximately 100 ft. of cable.
Grounded wye-wye, 5-legged core transformers are not likely to get into a ferroresonance mode that will result in immediate failure of the transformer or connected cables. In fact, for many cases the voltage will not be high enough to cause utility arresters to operate (although customer arresters may). Therefore, if ferroresonance occurs during a switching operation, no damage would be expected if the operation is completed promptly. The greater danger is with ferroresonance that persists undetected for several minutes or hours. Customer equipment may suffer damage even on brief occurrences.
Although we did not simulate it extensively, many sources indicate that delta-connected transformers can easily achieve voltages exceeding 2.5 per unit. They tend to stay in the higher ferroresonance modes we observed in our simulations of the 5-legged core transformer. This can result is rather rapid failure of equipment and protection should be applied
RECOMMENDATIONS
− Fuse protection should be avoided when the length of cable between the fuse and a ferroresonance-susceptible transformer exceeds about 100 ft. Use three-phase tripping fault interrupters instead. This is similar to the recommendations of other investigators who would suggest critical lengths in the range of 60 to 200 ft for a 2500 kVA transformer.
− Phase-by-phase switching of unloaded transformer when more than 100 ft of cable is involved should be done with the anticipation of ferroresonance.
− Operating procedures should be reviewed in light of the possibility of ferroresonance and revised where necessary.
− Customers with critical loads who may have a desire to dump the utility bus and switch to backup power at the first sign of utility system trouble, should be advised to design their systems to leave significant lighting loads on the utility bus to reduce the chances that the ferroresonance will be damaging.
− Delta-connected transformers should be avoided on the 25-kV system. If present, they should be protected with adequate arresters. If such a transformer is discovered in ferroresonance, it should be de-energized completely for a sufficient time to allow the arresters to cool before re-energization.
REFERENCES
Reinhold Rudenberg, Transient Performance of Electric Power System, MIT Press, May 1970, Chapter 48.
B.A. Mork, D. L. Stuehm, “Application of Nonlinear Dynamics and Chaos to Ferroresonance in Distribution Systems,” IEEE/PES Summer Meeting, Vancouver, 1993, Paper No. 93 SM 415-0 PWRD.
Xusheng Chen, “A Three-phase Multi-legged Transformer Model in ATP using the Directly-formed Inverse Inductance Matrix,” Paper No. 95 SM 421-8 PWRD, IEEE/PES Summer Meeting, Portland, OR, 1995.
D. L. Stuehm, B. A. Mork, D. D. Mairs, “Five-legged Core Transformer Equivalent Circuit,” IEEE Transactions on Power Delivery, Vol 4, No. 3, July 1989.
R. A. Walling, et. al., “Performance of Metal-Oxide Arresters Exposed to Ferroresonance in Padmount Transformers,” IEEE Transactions on Power Delivery, Vol 9., No. 2, April 1994, pp. 788 ff.
RELATED STANDARDS IEEE C57.105-1978
GLOSSARY AND ACRONYMS TNA: Transient Network Analyzer
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