The Application of Power Quality Monitoring Data for Reliability Centered Maintenancee Technical Report The Application of Power Quality Monitoring Data for Reliability Centered Maintenance
1000563 Final Report, December 2000 EPRI Project Manager S. Bhatt EPRI · 3412 Hillview Avenue, Palo Alto, California 94304 · PO Box 10412, Palo Alto, California 94303 · USA 800.313.3774 · 650.855.2121 · askepri@epri.com · www.epri.com DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES
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This report was prepared by EPRI PEAC Corporation 942 Corridor Park Boulevard Knoxville, TN 37932 Principal Investigators S. Floyd A. Mansoor This report describes research sponsored by EPRI. The report is a corporate document that should be cited in the literature in the following manner: The Application of Power Quality Monitoring Data for Reliability Centered Maintenance, EPRI, Palo Alto, CA: 2000. 1000563. iii REPORT SUMMARY This report addresses some requirements of implementing a reliability-centered maintenance (RCM) based power quality system using an open architecture. It also identifies a knowledge base to develop for processing power quality data for maintenance purposes and provides case studies that show how power quality monitoring data can indicate system anomalies. Finally, the report discusses applying artificial intelligence (AI) in such a system. Background The term power quality refers to a wide variety of electromagnetic phenomena that characterize voltage and current at a given time and at a given location on the power system. Increased application of electronic equipment that can cause electromagnetic disturbances or that can be sensitive to them has heightened interest in power quality in recent years. This has led to deployment of power quality monitoring systems at utilities and customer facilities to continuously record power quality parameters in the power system and to capture events based on user-defined thresholds of voltage or current anomalies. Application of power quality monitoring data has so far been limited to diagnosing power quality problems at customer facilities and within the distribution system. The installed base of power quality monitoring systems has increased significantly over the past ten years. Systems that were initially installed for power quality reasons have shown that they also can provide information to improve system reliability. Power quality is tied closely to the reliability of the power distribution system. The ability to predict component failure in distribution systems by using power quality monitoring systems makes them a useful tool in an RCM program. Objective To provide a roadmap for using power quality monitoring systems as an integral part of an RCM approach. Approach The project team first identified characteristics of existing power quality monitoring systems, including hardware and software characteristics, data analysis, data-presentation techniques, and application of power quality monitoring data in different utilities. The team then evaluated the different approaches for reliability-centered maintenance to identify how power quality monitoring data can be used within an RCM framework. Team members developed case studies that illustrate the application of power quality monitoring data to detect system anomalies. They then reviewed different artificial intelligence (AI) techniques that can use power quality monitoring data to predict incipient failures. The team also developed requirements for a power quality predictive maintenance (PQPM) system based on an open architecture and evaluated an v approach using the EPRI Maintenance Management Workstation (MMW) tool to implement an RCM-based system. Results The extensive data that was collected as part of this power quality monitoring system afforded a unique opportunity to mine these data for use within an RCM program. The most useful types of power quality data are trend data of continuous variables such as voltage unbalance and harmonics, trend data of discrete variables such as the number of transients and voltage sags, and waveform events. All of these data can provide an indication of incipient equipment failure. A maintenance system's architecture should be based on an open platform so that data from different power quality monitors can be readily integrated. A system can be implemented as a stand-alone platform or incorporate available software tools such as MMW into an existing system. The project's case studies illustrate use of either trend data or waveform signature to flag potential system anomalies. The biggest challenge in implementing a PQPM system will be developing the required knowledge base. Algorithms that predict and detect equipment problems can be designed and embedded in a software system once the knowledge base is sufficiently developed. EPRI Perspective By providing examples of how to use power quality data in predictive maintenance systems, EPRI is enabling utilities to better use their power quality data. Methods presented in this report will help utilities form their own maintenance approach. The case studies clearly illustrate that power quality data can be abstracted to higher-level information, which can be used in predictive maintenance. The report will help utilities consider the pluses and minuses of a software-based approach (such as MMW), an open system, and various artificial intelligence approaches that are available for predictive maintenance systems. It is important for utilities, regulators, and end users to understand the technical and economic issues that are related to using power quality information in maintenance systems and to make informed decisions regarding their use. Keywords Power quality Reliability Power quality monitoring Reliability-centered maintenance Predictive maintenance Artificial intelligence vi CONTENTS 1 ELECTROMAGNETIC PHENOMENA AND POWER QUALITY MONITORING.................. 1-1
Introduction ........................................................................................................................ 1-1 Power Quality Measurements............................................................................................. 1-1 Transients...................................................................................................................... 1-2 Impulsive Transients ................................................................................................. 1-2 Oscillatory Transients................................................................................................ 1-3 Short-Duration Variations............................................................................................... 1-4 Interruption................................................................................................................ 1-4 Sags.......................................................................................................................... 1-5 Swells ....................................................................................................................... 1-7 Long-Duration Variations ............................................................................................... 1-7 Overvoltage............................................................................................................... 1-8 Undervoltage............................................................................................................. 1-8 Sustained Interruption ............................................................................................... 1-8 Voltage Unbalance ........................................................................................................ 1-8 Waveform Distortion ...................................................................................................... 1-9 DC Offset ................................................................................................................ 1-10 Harmonics............................................................................................................... 1-10 Interharmonics ........................................................................................................ 1-10 Notching.................................................................................................................. 1-10 Noise....................................................................................................................... 1-11 Voltage Fluctuations .................................................................................................... 1-11 Power-Frequency Variations........................................................................................ 1-12 Power Quality Monitoring Equipment................................................................................ 1-12 AC Voltage Measurements .......................................................................................... 1-13 AC Current Measurements .......................................................................................... 1-13 Monitoring Instruments ................................................................................................ 1-14 vii Oscilloscopes.......................................................................................................... 1-14 Disturbance Monitors .............................................................................................. 1-14 Application of Power Quality Monitoring at Utilities ........................................................... 1-15 Background ................................................................................................................. 1-15 Role of Power Quality Monitoring in Maintenance........................................................ 1-16 References....................................................................................................................... 1-17 2 POWER QUALITY DATA IN RELIABILITY-CENTERED MAINTENANCE ......................... 2-1
Introduction ........................................................................................................................ 2-1 Maintenance Categories..................................................................................................... 2-1 Reactive Maintenance ................................................................................................... 2-2 Preventive Maintenance ................................................................................................ 2-2 Predictive Maintenance.................................................................................................. 2-3 Proactive Maintenance .................................................................................................. 2-4 Reliability Centered Maintenance ....................................................................................... 2-4 How Power Quality Data Can Help..................................................................................... 2-6 Reactive......................................................................................................................... 2-6 Preventive...................................................................................................................... 2-6 Predictive....................................................................................................................... 2-6 Proactive........................................................................................................................ 2-6 Conclusion ......................................................................................................................... 2-6 References......................................................................................................................... 2-7 3 CASE STUDIES FOR PREVENTIVE MAINTENANCE SYSTEMS ...................................... 3-1
Need for Knowledge Base .................................................................................................. 3-1 Case Study 1: Gating Errors in Static Source Transfer Switch Causes High Even Harmonics .......................................................................................................................... 3-2 The Situation ................................................................................................................. 3-2 System Description........................................................................................................ 3-3 Power Quality Monitoring Data ...................................................................................... 3-3 Application of PQPM...................................................................................................... 3-5 Configuration ................................................................................................................. 3-6 Case Study 2: Waveform Signature Indicates Faulty Switch............................................... 3-6 The Situation ................................................................................................................. 3-6 System Description........................................................................................................ 3-6 viii Power Quality Monitoring Data ...................................................................................... 3-7 Application of PQPM...................................................................................................... 3-8 Configuration ................................................................................................................. 3-8 Final Considerations ...................................................................................................... 3-9 Case Study 3: Waveform Signature Indicates Incipient Cable Failure ................................ 3-9 The Situation ................................................................................................................. 3-9 System Description........................................................................................................ 3-9 Power Quality Monitoring Data .................................................................................... 3-10 Application of PQPM.................................................................................................... 3-11 Configuration ............................................................................................................... 3-11 Final Considerations .................................................................................................... 3-12 Case Study 4: Voltage Unbalance Trend Indicates Regulator Problem ............................ 3-12 The Situation ............................................................................................................... 3-12 System Description...................................................................................................... 3-12 Power Quality Monitoring Data .................................................................................... 3-13 Application of PQPM.................................................................................................... 3-15 Configuration ............................................................................................................... 3-15 Final Considerations .................................................................................................... 3-16 Case Study 5: Waveform Indicates Interaction between Utility and Static Series Compensation Device ...................................................................................................... 3-16 The Situation ............................................................................................................... 3-16 System Description...................................................................................................... 3-16 Power Quality Monitoring Data .................................................................................... 3-17 Application of PQPM.................................................................................................... 3-18 Configuration ............................................................................................................... 3-19 Final Considerations .................................................................................................... 3-20 Combining Power Quality Data with Predictive Maintenance ....................................... 3-20 4 PQPM SYSTEM APPLICATION FOR CIRCUIT BREAKERS AND RECLOSERS.............. 4-1
Circuit Breaker Introduction ................................................................................................ 4-1 Application of Power Quality Monitors for On-Line Monitoring of Breaker Condition ........... 4-3 Maintaining and Testing Circuit Breakers ........................................................................... 4-5 Different Maintenance Strategies ................................................................................... 4-6 Testing Circuit Breakers................................................................................................. 4-6 Trip and Close-Coil Currents ..................................................................................... 4-7 ix Auxiliary (A/B) Contact Timing................................................................................... 4-7 Travel Motion ............................................................................................................ 4-7 Load Current/Main Contact Timing............................................................................ 4-8 DC-Voltage Supply.................................................................................................... 4-8 Vibration.................................................................................................................... 4-8 Application of Power Quality Monitoring System for Breaker Maintenance ......................... 4-8 Limitation of Power Quality Monitoring System for Breaker Maintenance....................... 4-9 Circuit Breaker Main Contact Time Variation ................................................................. 4-9 Circuit Breaker Protection for Failure to Close ............................................................. 4-10 Protection for Current Through an Open Breaker......................................................... 4-10 Maintenance Scheduling Based on Breaker Operation ................................................ 4-10 Circuit Breaker Emergency Load-Current-Carrying Capability ..................................... 4-12 Conditions for Emergency Load-Current-Carrying Capability .................................. 4-12 Four-Hour and Eight-Hour Emergency Load-Current-Carrying Capability Factors.................................................................................................................... 4-12 Emergency Operation at an Ambient Temperature Other Than 40°C...................... 4-13 Algorithms for PQPM Flag Generation .................................................................... 4-14 Reclosers ......................................................................................................................... 4-14 Existing Maintenance Practices for Reclosers.............................................................. 4-15 Application of Duty Factor for Determining Maintenance Basis .................................... 4-15 Application of a PQPM System for Reclosers .............................................................. 4-16 References....................................................................................................................... 4-18 5 OPEN ARCHITECTURE FOR A POWER QUALITY PREDICTIVE MAINTENANCE SYSTEM ................................................................................................................................. 5-1
Background ........................................................................................................................ 5-1 Introduction ........................................................................................................................ 5-1 Architecture ........................................................................................................................ 5-2 Data-Collection Layer .................................................................................................... 5-3 Database Layer ............................................................................................................. 5-5 Analysis Modules........................................................................................................... 5-5 Structure of an Analysis Module................................................................................ 5-5 Analysis Module Objects ........................................................................................... 5-6 Event Object......................................................................................................... 5-7 Trend Object......................................................................................................... 5-7 x Signature Object................................................................................................... 5-7 Analysis-Method Object........................................................................................ 5-8 Response Object .................................................................................................. 5-8 Learned-Performance Module ....................................................................................... 5-8 Helper Applications........................................................................................................ 5-8 Internet Capabilities ....................................................................................................... 5-9 6 IMPLEMENTATION OF A PQPM SYSTEM USING MAINTENANCE MANAGEMENT WORKSTATION..................................................................................................................... 6-1
Introduction ........................................................................................................................ 6-1 Generic MMW Implementation ........................................................................................... 6-3 Groundwork ................................................................................................................... 6-4 Development of Site-Requirement Specifications .......................................................... 6-4 Implementation .............................................................................................................. 6-5 Physical Installation................................................................................................... 6-5 Business Application Development ........................................................................... 6-5 Acceptance Testing ....................................................................................................... 6-8 Training and Customer Support ..................................................................................... 6-8 An Example Use of MMW to Improve Power Quality .......................................................... 6-8 The Objective and Approach.......................................................................................... 6-8 Confirmation of Prerequisites......................................................................................... 6-9 Establishment of Indicators .......................................................................................... 6-10 Notification Setup......................................................................................................... 6-12 Summary.......................................................................................................................... 6-13 7 ARTIFICIAL INTELLIGENCE TECHNIQUES IN A POWER QUALITY PREDICTIVE MAINTENANCE SYSTEM ...................................................................................................... 7-1
Problem Introduction .......................................................................................................... 7-1 Artificial Intelligence Approach............................................................................................ 7-3 Case-Based Reasoning ................................................................................................. 7-3 Artificial Neural Networks............................................................................................... 7-3 Expert Systems.............................................................................................................. 7-3 Fuzzy Logic ................................................................................................................... 7-3 Genetic Algorithms and Fuzzy Logic Hybrid Approach................................................... 7-3 Design Requirements of the PQPM-AI System................................................................... 7-4 Case-Based Reasoning...................................................................................................... 7-5 xi Description..................................................................................................................... 7-5 Interface to the PQPM System....................................................................................... 7-7 Example of Usage ­ Current-Limiting Fuse Operation ................................................... 7-8 Advantages.................................................................................................................... 7-9 Disadvantages ............................................................................................................... 7-9 Artificial Neural Networks ................................................................................................... 7-9 Description..................................................................................................................... 7-9 Interface to the PQPM System..................................................................................... 7-11 Example of Usage ­ Back-to-Back Capacitor Switching .............................................. 7-11 Advantages.................................................................................................................. 7-12 Expert Systems ................................................................................................................ 7-13 Description................................................................................................................... 7-13 Interface to the PQPM System..................................................................................... 7-13 Example of Usage ­ Voltage Unbalance Control Chart................................................ 7-15 Advantages.................................................................................................................. 7-16 Disadvantages ............................................................................................................. 7-17 Fuzzy Logic ...................................................................................................................... 7-17 Description................................................................................................................... 7-17 Interface to the PQPM System..................................................................................... 7-19 Example of Usage ­ Six-Pulse Converter Operation.................................................... 7-19 Advantages.................................................................................................................. 7-20 Disadvantages ............................................................................................................. 7-20 Genetic Algorithms and Fuzzy Logic ­ A Hybrid Approach ............................................... 7-20 Description................................................................................................................... 7-20 Interface to the PQPM System..................................................................................... 7-21 Independent Variable Selection............................................................................... 7-24 Cluster Analysis ...................................................................................................... 7-25 Genetic Algorithms.................................................................................................. 7-26 Example of Usage ­ Predicting Energy Consumption .................................................. 7-27 Advantages.................................................................................................................. 7-31 Disadvantages ............................................................................................................. 7-31 References....................................................................................................................... 7-32 8 ROADMAP FOR FUTURE WORK ...................................................................................... 8-1
Knowledgebase Creation ................................................................................................... 8-1 xii Failure Testing ............................................................................................................... 8-1 Existing Field Data......................................................................................................... 8-1 Manual Operation of Equipment .................................................................................... 8-1 Simulation...................................................................................................................... 8-2 Review of Existing Literature.......................................................................................... 8-2 Algorithm Development and System Implementation.......................................................... 8-2 A HISTORICAL OVERVIEW OF ARTIFICIAL INTELLIGENCE .............................................A-1 xiii LIST OF FIGURES
Figure 1-1 Impulsive Current Transient Caused by a Lightning Strike ..................................... 1-3 Figure 1-2 Oscillatory Transient Induced by Capacitor Switching ............................................ 1-4 Figure 1-3 Momentary Interruption Caused by a Fault ............................................................ 1-5 Figure 1-4 A Sag Caused by an SLG Fault ............................................................................. 1-6 Figure 1-5 A Sag Caused by the Starting of a Large Motor ..................................................... 1-6 Figure 1-6 A Swell Caused by an SLG Fault ........................................................................... 1-7 Figure 1-7 Trend Chart of Voltage Unbalance ......................................................................... 1-9 Figure 1-8 Example of Voltage Notching ............................................................................... 1-11 Figure 1-9 Example of Voltage Fluctuations .......................................................................... 1-12 Figure 2-1 Distribution of Maintenance Techniques................................................................. 2-1 Figure 2-2 Modes of Failure and the Percent of Equipment Exhibiting Such Failure Modes [3] ........................................................................................................................ 2-3 Figure 2-3 Categories of Maintenance Approaches [3] ............................................................ 2-5 Figure 3-1 Diagram of SSTS and Monitoring System in a Utility Distribution System .............. 3-3 Figure 3-2 High Even Harmonic Distortion in Voltage and Current .......................................... 3-4 Figure 3-3 Normal Voltage and Current Waveforms at the SSTS Output ................................ 3-4 Figure 3-4 Daily Average Even Harmonic Current at the Output of the SSTS ......................... 3-5 Figure 3-5 One-Line Diagram of the 4.8-kV Distribution System ............................................. 3-7 Figure 3-6 Voltage and Current Waveforms during Initiation of Ferroresonant Conditions....... 3-8 Figure 3-7 One-Line Diagram of a 13-kV Distribution System ............................................... 3-10 Figure 3-8 Voltage Waveform Captured Before the Circuit Breaker Failed............................ 3-11 Figure 3-9 Current Waveform on Phase C during the Fault (Note that the clipping at the peaks of the waveform was caused by CT saturation)................................................... 3-11 Figure 3-10 Diagram of the 4.8-kV Distribution System......................................................... 3-13 Figure 3-11 Voltage Unbalance Trend Showing High Levels of Unbalance around April 11 .................................................................................................................................. 3-14 Figure 3-12 Zoomed View of the High Voltage Unbalance .................................................... 3-14 Figure 3-13 Example of a Control Chart ................................................................................ 3-15 Figure 3-14 Diagram of the Medium-Voltage Distribution System, Including Capacitor Banks ............................................................................................................................ 3-17 Figure 3-15 Voltage Captured on the Utility Side before and during the Event ...................... 3-17 Figure 3-16 Voltage Captured on the Load Side before and during the Event ....................... 3-18 xv Figure 3-17 Voltage Captured on the Utility Side during the Clearing of the Event ................ 3-18 Figure 3-18 Voltage Captured on the Load Side during the Clearing of the Event................. 3-18 Figure 3-19 Increase in Peak Voltage Caused by the Interaction between the SSC Device and the Utility System ........................................................................................ 3-19 Figure 3-20 Increase in the Sixth-Harmonic Voltage Distortion Caused by the Interaction between the SSC Device and the Utility System............................................................ 3-19 Figure 4-1 Performance of a 5A and a 45A Power Quality Monitor Clamp-On CT................. 4-11 Figure 4-2 Recloser Location on Distribution Feeders........................................................... 4-17 Figure 5-1 System Architecture ............................................................................................... 5-3 Figure 5-2 Data Collection Layer............................................................................................. 5-4 Figure 5-3 Database Layer...................................................................................................... 5-5 Figure 5-4 Analysis Module Objects........................................................................................ 5-7 Figure 5-5 Internet Capability .................................................................................................. 5-9 Figure 6-1 MMW Integrates Disparate Sources of Information ................................................ 6-2 Figure 6-2 Example of a High-Level Status Panel ................................................................... 6-3 Figure 6-3 Accumulation of Breaker Trips ............................................................................... 6-7 Figure 6-4 Outage Duration and Categorization by Feeder ..................................................... 6-7 Figure 6-5 Example of Detailed Analysis Results .................................................................. 6-11 Figure 7-1 System Architecture of a PQPM System................................................................ 7-4 Figure 7-2 The CBR Cycle ...................................................................................................... 7-7 Figure 7-3 Example Waveform Showing Operation of Current-Limiting Fuse .......................... 7-8 Figure 7-4 Frequently Used ANN Topologies ........................................................................ 7-10 Figure 7-5 ANN Topology for Back-to-Back Capacitor-Switching Waveforms ....................... 7-12 Figure 7-6 Rule-Based Expert System .................................................................................. 7-14 Figure 7-7 Example of Expert System and Knowledge Bases Tied into PQPM System [5] ... 7-14 Figure 7-8 Example of Operation of One Rule Base [5]......................................................... 7-15 Figure 7-9 Example Control Chart......................................................................................... 7-16 Figure 7-10 Example of Fuzzy Inference System .................................................................. 7-18 Figure 7-11 Example of Fuzzy Inference System Interconnected with Expert System........... 7-19 Figure 7-12 Typical Waveform (One Phase) of Voltage Notching Event Due to a SixPulse Converter Operation [5] ....................................................................................... 7-20 Figure 7-13 Block Diagram of GA-based Learning System ................................................... 7-23 Figure 7-14 Block Diagram of the Fuzzy Expert System ....................................................... 7-24 Figure 7-15 Example of a Genetic Algorithm Minimum Point Search .................................... 7-26 Figure 7-16 Fuzzy Associative Memory for Electrical Energy Consumption .......................... 7-28 Figure 7-17 Fuzzy Membership Function for Time ................................................................ 7-29 Figure 7-18 Block Diagram of the Fuzzy Logic Module ......................................................... 7-30 xvi LIST OF TABLES
Table 4-1 Emergency Load Current-Carrying Capability Factors (Ie/Ir) (Based on an Ambient Temperature of 40°C)...................................................................................... 4-13 xvii 1
ELECTROMAGNETIC PHENOMENA AND POWER QUALITY MONITORING Introduction
To understand how power quality data can be used in Reliability Centered Maintenance (RCM) systems, one must understand the electromagnetic phenomena that cause power quality problems and the power quality monitoring systems that are used to gather data on such phenomena. Electricity providers and their customers desire good power. For customers, the economic impact of power disturbances can reach into the millions of dollars in product losses. For utilities, customer dissatisfaction and loss of loads and revenues can result from system disturbances. The increased use of sensitive electronic equipment in recent years also has heightened the need for quality power. Such equipment can cause and are sensitive to electromagnetic disturbances. Power quality monitoring is used to help understand these problems to properly apply mitigation solutions. Utilities can measure power disturbances and interpret the resulting data to manage power quality issues. Power Quality Measurements
Diverse electromagnetic phenomena can adversely affect the voltage and current of a power system. Characterizing power disturbances that are known to cause problems is important for several reasons. For example, it provides a common understanding and nomenclature for power disturbances that can be used by power quality professionals, and designers of monitoring systems can focus on producing a product that is better able to detect them. There are different ways to categorize power disturbances. From a maintenance and repair perspective, power quality problems are solved differently depending on the particular power variations of concern, so grouping the problems by their solution is helpful. Categorization of disturbances for measurement and analysis is also important. The following categories are used to describe electromagnetic phenomena: · · · · · Transients Short-duration variations Long-duration variations Voltage unbalance Waveform distortion 1-1 Electromagnetic Phenomena and Power Quality Monitoring · · Voltage fluctuations Power-frequency variations Each type of electromagnetic phenomena listed above is described below in more detail. Transients
A transient is a brief variation of the voltage and/or current in an electric circuit. Transients can be classified as either impulsive or oscillatory. Impulsive Transients Lightning strikes are a common cause of impulsive transients, which are characterized by a rapid rise time followed by a longer decay time. As such, they are usually described by their rise and decay times. For example, a 1.1/65-µs 1500-V impulsive transient rises to 1500 volts in 1.1 µs before the voltage decays to 50% of its peak value 65 µs later. An impulsive transient is unidirectional in polarity, either positive or negative. Such disturbances can induce transformer and arrester failures and damage customer equipment. The shape of impulsive transients can be changed quickly by circuit components and may have significantly different characteristics when viewed from different parts of the power system because of the high frequencies involved. They are generally not conducted far from where they enter the power system, but can be conducted for some distance along utility lines. Impulsive transients can excite the natural frequency of power system circuits and produce oscillatory transients. Figure 1-1 shows an example of an impulsive transient caused by a lightning strike. 1-2 Electromagnetic Phenomena and Power Quality Monitoring Figure 1-1 Impulsive Current Transient Caused by a Lightning Strike Oscillatory Transients Unlike impulsive transients, oscillatory transients are characterized by sudden changes in the instantaneous value of voltage and/or current that alternate polarity quickly. They can be further categorized into the frequency subclasses of low, medium, and high spectral content. It is also possible to categorize transients (and other disturbances) according to their mode. Basically, a transient in a three-phase system with a separate neutral conductor can be either common-mode or normal-mode. Common-mode transients may be measured between currentcarrying conductors and ground. Normal-mode transients occur between current-carrying conductors. Capacitor switching is a common cause of low-frequency (< 5 kHz) oscillatory voltage transients, which typically have a primary frequency between 300 and 900 Hz. Peak magnitudes are generally between 1.3 and 1.5 pu and last between ½ and 3 cycles. They can result in the tripping of sensitive equipment such as adjustable-speed drives (ASDs). Utility capacitorswitching transients may react with customer low-voltage power-factor-correction capacitors, which results in magnification of the capacitor-switching transient. Figure 1-2 shows an example of an oscillatory transient caused by capacitor switching. 1-3 Electromagnetic Phenomena and Power Quality Monitoring Figure 1-2 Oscillatory Transient Induced by Capacitor Switching Traveling waves resulting from lightning are an example of a medium-frequency (5 to 500 kHz) oscillatory transient. Local ferroresonance, switching on secondary systems, and lightning-induced ringing can be causes of high-frequency (> 500 kHz) oscillatory transients. Low-voltage power supplies can fail as a result of the high rise rate inherent in such a disturbance. Short-Duration Variations
Short-duration variations are characterized as variations of the RMS value of nominal voltage or current for a time greater than ½ cycle of the power frequency but less than or equal to one minute. Such variations are typically described in terms of their magnitude and duration. Causes of short-duration variations include faults, energization of large loads that require high starting currents, and intermittent loose connections in power wiring. Depending on the fault location and the system conditions, a fault can cause complete loss of voltage (interruptions), temporary voltage drops (sags), or voltage rises (swells). Interruption A decrease in voltage or current to less than 0.1 pu for less than 1 minute is called an interruption. Duration is the only measure typically used for interruptions because magnitude is always less than 10% of nominal. The duration of a momentary interruption ranges from 0.5 cycles to 3 seconds. The duration of a temporary interruption ranges from 3 seconds to 1 minute. Figure 1-3 shows an example of a momentary interruption where voltage drops for about 2 seconds. 1-4 Electromagnetic Phenomena and Power Quality Monitoring Figure 1-3 Momentary Interruption Caused by a Fault The residual voltage from the back-EMF effect of induction motors can be seen in the waveform plot at the bottom of Figure 1-3. This effect causes the instantaneous voltage not to immediately drop to zero. Sags The terms sag and dip are considered to be synonyms, with sag being the preferred term in the United States power quality community. It is a decrease in the RMS voltage or current to between 0.1 and 0.9 pu at the power frequency for a duration between 0.5 cycles and 1 minute. The term sag can be used in a manner that causes confusion. For example, if someone refers to an event as an "80% sag," the statement can be interpreted as "the voltage dropped to 80% of nominal," or it can be interpreted as "the voltage dropped to 20% (100%-80%) of nominal." Ambiguity such as this is avoided when the recommended terminology is used. For example, "a sag to 80%" means that the line voltage is reduced by 20% down to 80% of its nominal value. Figure 1-4 shows a voltage sag that is attributable to a single line-to-ground (SLG) fault. 1-5 Electromagnetic Phenomena and Power Quality Monitoring Figure 1-4 A Sag Caused by an SLG Fault Sags are typically associated with system faults but can also be caused by the energization of heavy loads or the starting of large motors. For example, an induction motor can draw up to ten times its full-load current when starting, which causes a voltage drop across the impedance of the system. The resulting voltage sag can be significant if the current magnitude is large relative to the available system fault current. Figure 1-5 is an example showing the effect that the starting of a large motor has on RMS voltage variation. Figure 1-5 A Sag Caused by the Starting of a Large Motor 1-6 Electromagnetic Phenomena and Power Quality Monitoring Swells The term momentary overvoltage is sometimes used as a synonym for the term swell. A swell is caused when the RMS voltage or current increases to between 1.1 and 1.8 pu at the power frequency for a duration between 0.5 cycles and 1 minute. Characterization of a swell is given by the magnitude of its RMS value and its duration. Single line-to-ground faults are a cause swells. For example, an SLG fault on one phase results in a temporary voltage rise on the other phases. Oth