island. Therefore, some means of direct transfer trip is generally required to ensure that the generator disconnects from the system when certain utility breakers operate. A more normal connection of DG is to use power and power factor control. This minimizes the risk of islanding. Although the DG no longer attempts to regulate the voltage, it is still useful for voltage reg- ulation purposes during constrained loading conditions by displacing some active and reactive power. Alternatively, customer-owned DG may be exploited simply by operating off-grid and supporting part or all of the customer’s load off-line. This avoids interconnection issues and provides some assistance to voltage regulation by reducing the load. The controls of distributed sources must be carefully coordinated with existing line regulators and substation LTCs. Reverse power flow can sometimes fool voltage regulators into moving the tap changer in the wrong direction. Also, it is possible for the generator to cause regu- lators to change taps constantly, causing early failure of the tap-chang- ing mechanism. Fortunately, some regulator manufacturers have anticipated these problems and now provide sophisticated microcom- puter-based regulator controls that are able to compensate. To exploit dispersed sources for voltage regulation, one is limited in options to the types of devices with steady, controllable outputs such as reciprocating engines, combustion turbines, fuel cells, and battery stor- age. Randomly varying sources such as wind turbines and photo- voltaics are unsatisfactory for this role and often must be placed on a relatively stiff part of the system or have special regulation to avoid voltage regulation difficulties. DG used for voltage regulation must also be large enough to accomplish the task. Not all technologies are suitable for regulating voltage. They must be capable of producing a controlled amount of reactive power. Manufacturers of devices requiring inverters for interconnection some- times program the inverter controls to operate only at unity power factor while grid-connected. Simple induction generators consume reactive power like an induction motor, which can cause low voltage. 7.7 Flicker* Although voltage flicker is not technically a long-term voltage varia- tion, it is included in this chapter because the root cause of problems is the same: The system is too weak to support the load. Also, some of the solutions are the same as for the slow-changing voltage regulation problems. The voltage variations resulting from flicker are often within the normal service voltage range, but the changes are sufficiently rapid to be irritating to certain end users. 316 Chapter Seven *This section was contributed by Jeff W. Smith. Long-Duration Voltage Variations Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Flicker is a relatively old subject that has gained considerable attention recently due to the increased awareness of issues concern- ing power quality. Power engineers first dealt with flicker in the 1880s when the decision of using ac over dc was of concern. 2 Low-fre- quency ac voltage resulted in a “flickering” of the lights. To avoid this problem, a higher 60-Hz frequency was chosen as the standard in North America. The term flicker is sometimes considered synonymous with voltage fluctuations, voltage flicker, light flicker, or lamp flicker. The phenom- enon being referred to can be defined as a fluctuation in system voltage that can result in observable changes (flickering) in light output. Because flicker is mostly a problem when the human eye observes it, it is considered to be a problem of perception. In the early 1900s, many studies were done on humans to deter- mine observable and objectionable levels of flicker. Many curves, such as the one shown in Fig. 7.14, were developed by various companies to determine the severity of flicker. The flicker curve shown in Fig. 7.14 was developed by C. P. Xenis and W. Perine in 1937 and was based upon data obtained from 21 groups of observers. In order to account for the nature of flicker, the observers were exposed to vari- ous waveshape voltage variations, levels of illumination, and types of lighting. 3 Long-Duration Voltage Variations 317 1.0 2.0 3.0 4.0 5.0 6.0 7.0 0.1 1.0 10.0 100.0 Frequency of Flicker in Seconds Voltage Change (in Volts) on 120-V System T h r e s h o l d o f P e r c e p t i o n T h r e s h o l d o f O b j e c t i o n Figure 7.14 General flicker curve. Long-Duration Voltage Variations Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Flicker can be separated into two types: cyclic and noncyclic. Cyclic flicker is a result of periodic voltage fluctuations on the system, while noncyclic is a result of occasional voltage fluctuations. An example of sinusoidal-cyclic flicker is shown in Fig. 7.15. This type of flicker is simply amplitude modulation where the main signal (60 Hz for North America) is the carrier signal and flicker is the modu- lating signal. Flicker signals are usually specified as a percentage of the normal operating voltage. By using a percentage, the flicker signal is independent of peak, peak-to-peak, rms, line-to-neutral, etc. Typically, percent voltage modulation is expressed by Percent voltage modulation ϭϫ100% where V max ϭ maximum value of modulated signal V min ϭ minimum value of modulated signal V 0 ϭ average value of normal operating voltage The usual method for expressing flicker is similar to that of percent voltage modulation. It is usually expressed as a percent of the total change in voltage with respect to the average voltage (⌬V/V) over a cer- tain period of time. V max Ϫ V min ᎏᎏ V 0 318 Chapter Seven –200 –150 –100 –50 0 50 100 150 200 0.000 0.058 0.117 0.175 0.233 0.292 0.350 0.408 0.467 0.525 0.583 0.642 0.700 0.758 0.817 0.875 0.933 Time (s) Voltage (V) Figure 7.15 Example flicker waveform. Long-Duration Voltage Variations Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. The frequency content of flicker is extremely important in determin- ing whether or not flicker levels are observable (or objectionable). Describing the frequency content of the flicker signal in terms of mod- ulation would mean that the flicker frequency is essentially the fre- quency of the modulating signal. The typical frequency range of observable flicker is from 0.5 to 30.0 Hz, with observable magnitudes starting at less than 1.0 percent. As shown in Fig. 7.14, the human eye is more sensitive to luminance fluctuations in the 5- to 10-Hz range. As the frequency of flicker increases or decreases away from this range, the human eye generally becomes more tolerable of fluctuations. One issue that was not considered in the development of the tradi- tional flicker curve is that of multiple flicker signals. Generally, most flicker-producing loads contain multiple flicker signals (of varying magnitudes and frequencies), thus making it very difficult to accu- rately quantify flicker using flicker curves. 7.7.1 Sources of flicker Typically, flicker occurs on systems that are weak relative to the amount of power required by the load, resulting in a low short-circuit ratio. This, in combination with considerable variations in current over a short period of time, results in flicker. As the load increases, the cur- rent in the line increases, thus increasing the voltage drop across the line. This phenomenon results in a sudden reduction in bus voltage. Depending upon the change in magnitude of voltage and frequency of occurrence, this could result in observable amounts of flicker. If a light- ing load were connected to the system in relatively close proximity to the fluctuating load, observers could see this as a dimming of the lights. A common situation, which could result in flicker, would be a large industrial plant located at the end of a weak distribution feeder. Whether the resulting voltage fluctuations cause observable or objec- tionable flicker is dependent upon the following parameters: ■ Size (VA) of potential flicker-producing source ■ System impedance (stiffness of utility) ■ Frequency of resulting voltage fluctuations A common load that can often cause flicker is an electric arc furnace (EAF). EAFs are nonlinear, time-varying loads that often cause large voltage fluctuations and harmonic distortion. Most of the large current fluctuations occur at the beginning of the melting cycle. During this period, pieces of scrap steel can actually bridge the gap between the elec- trodes, resulting in a highly reactive short circuit on the secondary side Long-Duration Voltage Variations 319 Long-Duration Voltage Variations Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. of the furnace transformer. This meltdown period can generally result in flicker in the 1.0- to 10.0-Hz range. Once the melting cycle is over and the refining period is reached, stable arcs can usually be held on the elec- trodes resulting in a steady, three-phase load with high power factor. 4 Large induction machines undergoing start-up or widely varying load torque changes are also known to produce voltage fluctuations on systems. As a motor is started up, most of the power drawn by the motor is reactive (see Fig. 7.16). This results in a large voltage drop across distribution lines. The most severe case would be when a motor is started across the line. This type of start-up can result in current drawn by the motor up to multiples of the full load current. An example illustrating the impact motor starting and torque changes can have on system voltage is shown in Fig. 7.17. In this case, a large industrial plant is located at the end of a weak distribution feeder. Within the plant are four relatively large induction machines that are frequently restarted and undergo relatively large load torque variations. 5 Although starting large induction machines across the line is gener- ally not a recommended practice, it does occur. To reduce flicker, large motors are brought up to speed using various soft-start techniques such as reduced-voltage starters or variable-speed drives. In certain circumstances, superimposed interharmonics in the sup- ply voltage can lead to oscillating luminous flux and cause flicker. Voltage interharmonics are components in the harmonic spectrum that are noninteger multiples of the fundamental frequency. This phenom- enon can be observed with incandescent lamps as well as with fluores- cent lamps. Sources of interharmonics include static frequency converters, cycloconverters, subsynchronous converter cascades, induction furnaces, and arc furnaces. 6 320 Chapter Seven 1.0 0.9 0.8 0.7 0.6 0.5 Slip 0.4 0.3 0.2 0.1 0.0 Active Power Reactive Power Q P Figure 7.16 Active and reactive power during induction machine start-up. Long-Duration Voltage Variations Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 7.7.2 Mitigation techniques Many options are available to alleviate flicker problems. Mitigation alternatives include static capacitors, power electronic-based switch- ing devices, and increasing system capacity. The particular method chosen is based upon many factors such as the type of load causing the flicker, the capacity of the system supplying the load, and cost of miti- gation technique. Flicker is usually the result of a varying load that is large relative to the system short-circuit capacity. One obvious way to remove flicker from the system would be to increase the system capacity sufficiently to decrease the relative impact of the flicker-producing load. Upgrading the system could include any of the following: reconductoring, replac- ing existing transformers with higher kVA ratings, or increasing the operating voltage. Motor modifications are also an available option to reduce the amount of flicker produced during motor starting and load varia- tions. The motor can be rewound (changing the motor class) such that the speed-torque curves are modified. Unfortunately, in some cases this could result in a lower running efficiency. Flywheel energy systems can also reduce the amount of current drawn by motors by delivering the mechanical energy required to compensate for load torque variations. Recently, series reactors have been found to reduce the amount of flicker experienced on a system caused by EAFs. Series reactors help sta- bilize the arc, thus reducing the current variations during the beginning of melting periods. By adding the series reactor, the sudden increase in current is reduced due the increase in circuit reactance. Series reactors Long-Duration Voltage Variations 321 Motor Starting and Load Torque Variations 40 60 80 100 120 140 299000 302000 305000 308000 311000 314000 Time (ms) Figure 7.17 Voltage fluctuations caused by induction machine operation. Long-Duration Voltage Variations Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. also have the benefit of reducing the supply-side harmonic levels. 7 The design of the reactor must be coordinated with power requirements. Series capacitors can also be used to reduce the effect of flicker on an existing system. In general, series capacitors are placed in series with the transmission line supplying the load. The benefit of series capacitors is that the reaction time for the correction to load fluctuations is instan- taneous in nature. The downside to series capacitors is that compensa- tion is only available beyond the capacitor. Bus voltages between the supply and the capacitor are uncompensated. Also, series capacitors have operational difficulties that require careful engineering. Fixed shunt-connected capacitor banks are used for long-term volt- age support or power factor correction. A misconception is that shunt capacitors can be used to reduce flicker. The starting voltage sag is reduced, but the percent change in voltage (⌬V/V) is not reduced, and in some cases can actually be increased. A rather inexpensive method for reducing the flicker effects of motor starting would be to simply install a step-starter for the motor, which would reduce the amount of starting current during motor start-up. With the advances in solid-state technology, the size, weight, and cost of adjustable-speed drives have decreased, thus allowing the use of such devices to be more feasible in reducing the flicker effects caused by flicker-producing loads. Static var compensators (SVCs) are very flexible and have many roles in power systems. SVCs can be used for power factor correction, flicker reduction, and steady-state voltage control, and also have the benefit of being able to filter out undesirable frequencies from the sys- tem. SVCs typically consist of a TCR in parallel with fixed capacitors (Fig. 7.18). The fixed capacitors are usually connected in ungrounded wye with a series inductor to implement a filter. The reactive power that the inductor delivers in the filter is small relative to the rating of the filter (approximately 1 to 2 percent). There are often multiple filter stages tuned to different harmonics. The controls in the TCR allow con- tinuous variations in the amount of reactive power delivered to the sys- tem, thus increasing the reactive power during heavy loading periods and reducing the reactive power during light loading. SVCs can be very effective in controlling voltage fluctuations at rapidly varying loads. Unfortunately, the price for such flexibility is high. Nevertheless, they are often the only cost-effective solution for many loads located in remote areas where the power system is weak. Much of the cost is in the power electronics on the TCR. Sometimes this can be reduced by using a number of capacitor steps. The TCR then need only be large enough to cover the reactive power gap between the capacitor stages. 322 Chapter Seven Long-Duration Voltage Variations Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Thyristor-switched capacitors (TSCs) can also be used to supply reac- tive power to the power system in a very short amount of time, thus being helpful in reducing the effects of quick load fluctuations. TSCs usually consist of two to five shunt capacitor banks connected in series with diodes and thyristors connected back to back. The capacitor sizes are usually equal to each other or are set at multiples of each other, allowing for smoother transitions and increased flexibility in reactive power control. Switching the capacitors in or out of the system in dis- crete steps controls the amount of reactive power delivered to the sys- tem by the TSC. This action is unlike that of the SVC, where the Long-Duration Voltage Variations 323 Fixed Capacitors and Tuning Reactors TCR Fixed Capacitors (Single-Phase) Tuning Reactors 5th Harmonic 7th Harmonic 11th Harmonic 13th Harmonic High-Pass Filter Figure 7.18 Typical SVC configuration. Long-Duration Voltage Variations Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. capacitors are static and the reactors are used to control the reactive power. An example diagram of a TSC is shown in Fig. 7.19. The control of the TSC is usually based on line voltage magnitude, line current magnitude, or reactive power flow in the line. The control circuits can be used for all three phases or each phase separately. The individual phase control offers improved compensation when unbal- anced loads are producing flicker. 7.7.3 Quantifying flicker Flicker has been a power quality problem even before the term power quality was established. However, it has taken many years to develop an adequate means of quantifying flicker levels. Chapter 11 provides an in-depth look at power quality monitoring, with a section that describes modern techniques for measuring and quantifying flicker. 7.8 References 1. L. Morgan, S. Ihara, “Distribution Feeder Modification to Service Both Sensitive Loads and Large Drives,” 1991 IEEE PES Transmission and Distribution Conference Record, Dallas, September 1991, pp. 686–690. 2. E. L. Owen, “Power Disturbance and Power Quality—Light Flicker Voltage Requirements,” Conference Record, IEEE IAS Annual Meeting, Denver, October 1994, pp. 2303–2309. 3. C. P. Xenis, W. Perine, “Slide Rule Yields Lamp Flicker Data.” Electrical World, Oct. 23, 1937, p. 53. 4. S. B. Griscom, “Lamp Flicker on Power Systems,” Chap. 22, Electrical Transmission and Distribution Reference Book, 4th ed., Westinghouse Elec. Corp., East Pittsburgh, Pa., 1950. 5. S. M. Halpin, J. W. Smith, C. A. Litton, “Designing Industrial Systems with a Weak Utility Supply,” IEEE Industry Applications Magazine, March/April 2001, pp. 63–70. 6. Interharmonics in Power Systems, IEEE Interharmonic Task Force, Cigre 36.05/CIRED 2 CC02, Voltage Quality Working Group. 7. S. R. Mendis, M. T. Bishop, T. R. Day, D. M. Boyd, “Evaluation of Supplementary Series Reactors to Optimize Electric Arc Furnace Operations,” Conference Record, IEEE IAS Annual Meeting, Orlando, Fla., October 1995, pp. 2154–2161. 324 Chapter Seven Figure 7.19 Typical TSC configuration. Long-Duration Voltage Variations Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 7.9 Bibliography IEEE Standard 141-1993: Recommended Practice for Power Distribution in Industrial Plants, IEEE, 1993. IEEE Standard 519-1992: Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE, 1993. IEC 61000-4-15, Electromagnetic Compatibility (EMC). Part 4: Testing and Measuring Techniques. Section 15: Flickermeter—Functional and Design Specifications. Long-Duration Voltage Variations 325 Long-Duration Voltage Variations Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. [...]... SARFI10 SARFICBEMA SARFIITIC SARFISEMI 0.000 11.8 87 43.9 87 56.308 135.185 2 07. 644 0.000 5.594 22.813 28 .72 9 66.260 103.405 0.000 0.000 12.126 18.422 51.000 70 .535 0.000 0.000 5.165 8.926 27. 0 37 56.311 0.000 0.000 1.525 3.694 13.519 35.689 0.000 5.316 25.465 33.293 71 .413 149.488 0.000 2 .79 1 18 .76 5 25.390 51.500 140 .76 8 0.000 2.362 13.619 18.535 38.238 140 .76 8 *Submitted for IEEE Standard P1564.8 †CP05,... solving power quality problems is that disturbances in the electric power system are not restricted by legal boundaries Power suppliers, power consumers, and equipment suppliers must work together to solve many problems Before they can do that, they must understand the electrical environment in which enduse equipment operates This is necessary to reduce the long-term economic impact of inevitable power. .. level Phase A Voltage RMS Variation % Volts 140 120 100 80 60 40 20 0 0 February 20, 1994 at 12:52:52 Local Trigger Duration 0.633 s Min 0.166 Ave 75 .50 Max 138.8 Ref Cycle 4 376 0 0.25 0.5 0 .75 25 50 75 % Volts 1 1.25 Time (s) 1.5 1 .75 2 100 125 Time (ms) 150 175 200 150 100 50 0 –50 –100 –150 0 Figure 8.1 Multicomponent, nonrectangular rms variation Downloaded from Digital Engineering Library @ McGraw-Hill... subject to the Terms of Use as given at the website Source: Electrical Power Systems Quality Chapter 8 Power Quality Benchmarking EPRI has been studying power quality (PQ) problems and solutions for over 15 years This chapter presents many new and innovative approaches to PQ monitoring, analysis, and planning that have been developed since the First Edition of this book The authors have been intimately involved... 80 70 50 10 0.0 0.0 0.5 27. 5 13.6 7. 3 4.8 4.3 0.0 0.0 0.5 22 .7 8.8 2.5 0.5 Undefined 0.0 0.0 0.0 4.3 4.3 4.3 3.8 3.8 0.0 0.0 0.0 0.5 0.5 0.5 0.5 0.5 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Power Quality Benchmarking Power. .. Companies All rights reserved Any use is subject to the Terms of Use as given at the website Power Quality Benchmarking Power Quality Benchmarking 341 100% 80% 70 % 60% 50% 40% 30% 20% CP95 = 3. 17% Cumulative Frequency 90% 4.0% 3.6% 3.2% 2.8% 2.4% 2.0% 1.6% 1.2% 0.8% 0.4% 0.0% 10% 0% VTHD 100% 90% 80% 70 % 60% 50% 7 Count of Sites 6 5 4 3 40% 30% 20% 2 1 10% 0% 5.2% 4.8% 4.4% 4.0% 3.6% 3.2% 2.8% 2.4% 2.0%... 1.43 and 1 .71 percent The mean of the distribution is the same as that of the SATHD index 18% 100% Mean (SATHD): 1. 57% Standard Deviation: 0. 071 4% 95% Confidence Interval: 1.43% to 1 .71 % Frequency of Sites 14% 12% 80% 60% 10% 8% 40% 6% 4% 20% Cumulative Frequency 16% 2% 0% 6.6 6.0 5.4 4.8 4.2 3.6 3.0 2.4 1.8 1.2 0.6 0.0 0% VTHD (%) Figure 8.8 Histogram of average value for voltage THD at 277 mon- itoring... are deploying power quality monitoring infrastructures that provide the data required for accurate benchmarking of the service quality provided to consumers These are permanent monitoring systems due to the time needed to obtain accurate data and the importance of power quality to the end users where these systems are being installed For most utilities and consumers, the most important power quality... of this nonrectangular event have magnitudes well below 50 percent T10%, however, comprises only the duration of the second component, 200 ms 140 120 Measurement Event #1 % Volts 100 T80% 80 60 T50% 40 20 0 0.000 0.1 67 T10% 0.333 0.500 0.6 67 0.833 1.000 1.1 67 Time (s) 1.333 1.500 1.6 67 Figure 8.2 Illustration of specified voltage characterization of rms variation phase mea- surements Downloaded from... Terms of Use as given at the website Power Quality Benchmarking 348 Chapter Eight power quality variations may be the result of events on the local distributor’s system, there will also be events on the transmission system that will affect large areas Forced outages at power plants can cause spikes in the power market and, perhaps, voltage regulation issues if the power supply becomes constrained The . Seven –200 –150 –100 –50 0 50 100 150 200 0.000 0.058 0.1 17 0. 175 0.233 0.292 0.350 0.408 0.4 67 0.525 0.583 0.642 0 .70 0 0 .75 8 0.8 17 0. 875 0.933 Time (s) Voltage (V) Figure 7. 15 Example flicker waveform. Long-Duration. Volts –150 –100 –50 0 50 100 150 0 0 25 50 75 100 125 150 175 200 0.25 0.5 0 .75 1 1.25 1.5 1 .75 2 Time (s) Time (ms) Duration 0.633 s Min 0.166 Ave 75 .50 Max 138.8 Ref Cycle 4 376 0 Figure 8.1 Multicomponent,. Perine, “Slide Rule Yields Lamp Flicker Data.” Electrical World, Oct. 23, 19 37, p. 53. 4. S. B. Griscom, “Lamp Flicker on Power Systems, ” Chap. 22, Electrical Transmission and Distribution Reference