charge voltage may be reduced only a few percent, the greatest benefit of the scout scheme may be that it greatly reduces the rate of rise of surge voltages entering the cable. These steep-fronted surges reflect off the open point and frequently cause failures at the first or second pad- mount transformer from the end. Because of lead lengths, arresters are not always effective against such steep impulses. The scout scheme practically eliminates these from the cable. Many distribution feeders in densely populated areas will have scout schemes by default. There are sufficient numbers of transformers that there are already arresters on either side of the riser pole. 4.6 Managing Ferroresonance Ferroresonance in a distribution system occurs mainly when a lightly loaded, three-phase transformer becomes isolated on a cable with one or two open phases. This can happen both accidentally and intention- ally. Strategies for dealing with ferroresonance include ■ Preventing the open-phase condition ■ Damping the resonance with load ■ Limiting the overvoltages ■ Limiting cable lengths ■ Alternative cable-switching procedures Most ferroresonance is a result of blown fuses in one or two of the phases in response to faults, or some type of single-pole switching in the primary circuit. A logical effective measure to guard against fer- roresonance would be to use three-phase switching devices. For exam- ple, a three-phase recloser or sectionalizer could be used at the riser pole instead of fused cutouts. The main drawback is cost. Utilities could not afford to do this at every riser pole, but this could be done in spe- cial cases where there are particularly sensitive end users and frequent fuse blowings. Another strategy on troublesome cable drops is to simply replace the fused cutouts with solid blades. This forces the upline recloser or breaker to operate to clear faults on the cable. Of course, this subjects many other utility customers to sustained interruptions when they would have normally seen only a brief voltage sag. However, it is an inexpensive way to handle the problem until a more permanent solu- tion is implemented. Manual, single-phase cable switching by pulling cutouts or cable elbows is also a major source of ferroresonance. This is a particular problem during new construction when there is a lot of activity and the Transient Overvoltages 157 Transient Overvoltages 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. transformers are not yet loaded. Some utilities have reported that line crews carry a “light board” or some other type of resistive load bank in their trucks for use in cable-switching activity when the transformers have no other load attached. One must be particularly careful when switching delta-connected transformers; such transformers should be protected because voltages may get extremely high. The common grounded wye-wye pad-mounted transformer may not be damaged internally if the exposure time is brief, although it may make consid- erable noise. When switching manually, the goal should be to open or close all three phases as promptly as possible. Ferroresonance can generally be damped out by a relatively small amount of resistive load, although there are exceptions. For the typ- ical case with one phase open, a resistive load of 1 to 4 percent of the transformer capacity can greatly reduce the effects of ferroreso- nance. The amount of load required is dependent on the length of cable and the design of the transformers. Also, the two-phase open case is sometimes more difficult to dampen with load. Figure 4.38 shows the effect of loading on ferroresonance overvoltages for a transformer connected to approximately 1.0 mi (1.61 km) of cable with one phase open. This was a particularly difficult case that dam- aged end-user equipment. Note the different characteristics of the phases. The transformer was of a five-legged core design, and the middle phase presents a condition that is more difficult to control 158 Chapter Four 0 5 10 15 20 25 30 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 A B C Ferroresonant Voltage (per Unit) Resistive Load @ 480 V BUS (% Transformer Capacity) Figure 4.38 Example illustrating the impact of loading on ferroresonance. Transient Overvoltages 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. with loading. Five percent resistive load reduces the overvoltage from approximately 2.8 to 2 pu. The transformer would have to be loaded approximately 20 to 25 percent of resistive equivalent load to limit ferroresonance overvoltages to 125 percent, the commonly accepted threshold. Since such a large load is required, a three-phase recloser was used to switch the cable. On many utility systems, arresters are not applied on every pad- mounted distribution transformer due to costs. However, surge arresters can be an effective tool for suppressing the effects of ferrores- onance. This is particularly true for transformers with ungrounded pri- mary connections where the voltages can easily reach 3 to 4 pu if unchecked. Primary arresters will generally limit the voltages to 1.7 to 2.0 pu. There is some risk that arresters will fail if subjected to fer- roresonance voltages for a long time. In fact, secondary arresters with protective levels lower than the primary-side arresters are frequent casualties of ferroresonance. Utility arresters are more robust, and there often is relatively little energy involved. However, if line crews encounter a transformer with arresters in ferroresonance, they should always deenergize the unit and allow the arresters to cool. An over- heated arrester could fail violently if suddenly reconnected to a source with significant short-circuit capacity. Ferroresonance occurs when the cable capacitance reaches a critical value sufficient to resonate with the transformer inductance (see Fig. 4.11). Therefore, one strategy to minimize the risk of frequent ferroreso- nance problems is to limit the length of cable runs. This is difficult to do for transformers with delta primary connections because with the high magnetizing reactance of modern transformers, ferroresonance can occur for cable runs of less than 100 ft. The grounded wye-wye connec- tion will generally tolerate a few hundred feet of cable without exceeding 125 percent voltage during single-phasing situations. The allowable length of cable is also dependent on the voltage level with the general trend being that the higher the system voltage, the shorter the cable. However, modern trends in transformer designs with lower losses and exciting currents are making it more difficult to completely avoid fer- roresonance at all primary distribution voltage levels. The location of switching when energizing or deenergizing a trans- former can play a critical role in reducing the likelihood of ferroreso- nance. Consider the two cable-transformer switching sequences in Fig. 4.39. Figure 4.39a depicts switching at the transformer terminals after the underground cable is energized, i.e., switch L is closed first, fol- lowed by switch R. Ferroresonance is less likely to occur since the equivalent capacitance seen from an open phase after each phase of switch R closes is the transformer’s internal capacitance and does not involve the cable capacitance. Figure 4.39b depicts energization of the Transient Overvoltages 159 Transient Overvoltages 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. transformer remotely from another point in the cable system. The equivalent capacitance seen from switch L is the cable capacitance, and the likelihood of ferroresonance is much greater. Thus, one of the com- mon rules to prevent ferroresonance during cable switching is to switch the transformer by pulling the elbows at the primary terminals. There is little internal capacitance, and the losses of the transformers are usually sufficient to prevent resonance with this small capacitance. This is still a good general rule, although the reader should be aware that some modern transformers violate this rule. Low-loss transform- ers, particularly those built with an amorphous metal core, are prone to ferroresonance with their internal capacitances. 4.7 Switching Transient Problems with Loads This section describes some transient problems related to loads and load switching. 160 Chapter Four (a) underground cable Switch L Switch R (b) underground cable Switch L Switch R Figure 4.39 Switching at the transformer terminals (a) reduces the risk of iso- lating the transformer on sufficient capacitance to cause ferroresonance as opposed to (b) switching at some other location upline. Transient Overvoltages 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. 4.7.1 Nuisance tripping of ASDs Most adjustable-speed drives typically use a voltage source inverter (VSI) design with a capacitor in the dc link. The controls are sensitive to dc overvoltages and may trip the drive at a level as low as 117 per- cent. Since transient voltages due to utility capacitor switching typi- cally exceed 130 percent, the probability of nuisance tripping of the drive is high. One set of typical waveforms for this phenomenon is shown in Fig. 4.40. The most effective way to eliminate nuisance tripping of small drives is to isolate them from the power system with ac line chokes. The addi- tional series inductance of the choke will reduce the transient voltage magnitude that appears at the input to the adjustable-speed drive. Determining the precise inductor size required for a particular appli- cation (based on utility capacitor size, transformer size, etc.) requires a fairly detailed transient simulation. A series choke size of 3 percent based on the drive kVA rating is usually sufficient. 4.7.2 Transients from load switching Deenergizing inductive circuits with air-gap switches, such as relays and contactors, can generate bursts of high-frequency impulses. Figure 4.41 shows an example. ANSI/IEEE C62.41-1991, Recommended Practice for Surge Voltages in Low-Voltage AC Power Circuits, cites a representative 15-ms burst composed of impulses having 5-ns rise Transient Overvoltages 161 480-V Bus Voltage (phase-to-phase) 33.3 50.0 66.7 83.3 100.0 116.7 –1500 –1000 –500 0 500 1000 1500 Voltage (V) Time (ms) Figure 4.40 Effect of capacitor switching on adjustable-speed-drive ac current and dc voltage. Transient Overvoltages 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. times and 50-ns durations. There is very little energy in these types of transient due to the short duration, but they can interfere with the operation of electronic loads. Such electrical fast transient (EFT) activity, producing spikes up to 1 kV, is frequently due to cycling motors, such as air conditioners and ele- vators. Transients as high as 3 kV can be caused by operation of arc welders and motor starters. The duration of each impulse is short compared to the travel time of building wiring, thus the propagation of these impulses through the 162 Chapter Four ac Drive Current during Capacitor Switching 33.3 50.0 66.7 83.3 100.0 116.7 –300 –200 –100 0 100 200 300 Time (ms) Current (A) dc Link Voltage during Capacitor Switching 33.3 50.0 66.7 83.3 100.0 116.7 500 550 600 650 700 750 Time (ms) Current (A) Figure 4.40 (Continued) Transient Overvoltages 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. wiring can be analyzed with traveling wave theory. The impulses atten- uate very quickly as they propagate through a building. Therefore, in most cases, the only protection needed is electrical separation. Physical separation is also required because the high rate of rise allows these transients to couple into nearby sensitive equipment. EFT suppression may be required with extremely sensitive equip- ment in close proximity to a disturbing load, such as a computer room. High-frequency filters and isolation transformers can be used to pro- tect against conduction of EFTs on power cables. Shielding is required to prevent coupling into equipment and data lines. 4.7.3 Transformer energizing Energizing a transformer produces inrush currents that are rich in harmonic components for a period lasting up to 1 s. If the system has a parallel resonance near one of the harmonic frequencies, a dynamic overvoltage condition results that can cause failure of arresters and problems with sensitive equipment. This problem can occur when large transformers are energized simultaneously with large power factor cor- rection capacitor banks in industrial facilities. The equivalent circuit is shown in Fig. 4.42. A dynamic overvoltage waveform caused by a third- harmonic resonance in the circuit is shown in Fig. 4.43. After the expected initial transient, the voltage again swells to nearly 150 per- cent for many cycles until the losses and load damp out the oscillations. This can place severe stress on some arresters and has been known to significantly shorten the life of capacitors. Transient Overvoltages 163 400V 0V –400V 200 V/div VERTICAL 102.4 ␮s/div HORIZ 1000V 0V –1000V 500.0 V/div VERTICAL 5.0 ms/div HORIZ Figure 4.41 Fast transients caused by deenergizing an inductive load. Transient Overvoltages 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. This form of dynamic overvoltage problem can often be eliminated simply by not energizing the capacitor and transformer together. One plant solved the problem by energizing the transformer first and not energizing the capacitor until load was about to be connected to the transformer. 4.8 Computer Tools for Transients Analysis The most widely used computer programs for transients analysis of power systems are the Electromagnetic Transients Program, com- monly known as EMTP, and its derivatives such as the Alternate Transients Program (ATP). EMTP was originally developed by Hermann W. Dommel at the Bonneville Power Administration (BPA) in the late 1960s 15 and has been continuously upgraded since. One of the reasons this program is popular is its low cost due to some versions being in the public domain. Some of the simulations presented in this 164 Chapter Four Figure 4.42 Energizing a capacitor and transformer simultaneously can lead to dynamic overvoltages. –2 –1 0 1 2 0 200 400 600 800 Phase A Time (ms) Voltage (V pu) Figure 4.43 Dynamic overvoltages during transformer energizing. Transient Overvoltages 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. book have been performed with a commercial analysis tool known as PSCAD/EMTDC, a program developed by the Manitoba HVDC Research Center. This program features a very sophisticated graphical user interface that enables the user to be very productive in this diffi- cult analysis. Some power system analysts use computer programs developed more for the analysis of electronic circuits, such as the well- known SPICE program 16 and its derivatives. Although the programs just discussed continue to be used exten- sively, there are now many other capable programs available. We will not attempt to list each one because there are so many and, also, at the present rate of software development, any such list would soon be out- dated. The reader is referred to the Internet since all vendors of this type of software maintain websites. Nearly all the tools for power systems solve the problem in the time domain, re-creating the waveform point by point. A few programs solve in the frequency domain and use the Fourier transform to convert to the time domain. Unfortunately, this essentially restricts the address- able problems to linear circuits. Time-domain solution is required to model nonlinear elements such as surge arresters and transformer magnetizing characteristics. The penalty for this extra capability is longer solution times, which with modern computers becomes less of a problem each day. It takes considerably more modeling expertise to perform electro- magnetic transients studies than to perform more common power sys- tem analyses such as of the power flow or of a short circuit. Therefore, this task is usually relegated to a few specialists within the utility orga- nization or to consultants. While transients programs for electronic circuit analysis may formu- late the problem in any number of ways, power systems analysts almost uniformly favor some type of nodal admittance formulation. For one thing, the system admittance matrix is sparse allowing the use of very fast and efficient sparsity techniques for solving large problems. Also, the nodal admittance formulation reflects how most power engi- neers view the power system, with series and shunt elements con- nected to buses where the voltage is measured with respect to a single reference. To obtain conductances for elements described by differential equa- tions, transients programs discretize the equations with an appropri- ate numerical integration formula. The simple trapezoidal rule method appears to be the most commonly used, but there are also a variety of Runge-Kutta and other formulations used. Nonlinearities are handled by iterative solution methods. Some programs include the nonlineari- ties in the general formulation, while others, such as those that follow Transient Overvoltages 165 Transient Overvoltages 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 EMTP methodology, separate the linear and nonlinear portions of the circuit to achieve faster solutions. This impairs the ability of the program to solve some classes of nonlinear problems but is not usually a significant constraint for most power system problems. 4.9 References 1. Electrical Transmission and Distribution Reference Book, 4th ed., Westinghouse Electric Corporation, East Pittsburgh, Pa., 1964. 2. Electrical Distribution-System Protection, 3d ed., Cooper Power Systems, Franksville, Wis., 1990. 3. K. Berger, R. B. Anderson, H. Kroninger, “Parameters of Lightning Flashes, “ Electra, No. 41, July 1975, pp. 23–27. 4. R. Morrison, W. H. Lewis, Grounding and Shielding in Facilities, John Wiley & Sons, New York, 1990. 5. G. L. Goedde, L. J. Kojovic, M. B. Marz, J. J. Woodworth, “Series-Graded Gapped Arrester Provides Reliable Overvoltage Protection in Distribution Systems,” Conference Record, 2001 IEEE Power Engineering Society Winter Meeting, Vol. 3, 2001, pp. 1122–1127. 6. Randall A. Stansberry, “Protecting Distribution Circuits: Overhead Shield Wire ver- sus Lightning Surge Arresters,” Transmission & Distribution, April 1991, pp. 56ff. 7. IEEE Transformers Committee, “Secondary (Low-Side) Surges in Distribution Transformers,” Proceedings of the 1991 IEEE PES Transmission and Distribution Conference, Dallas, September 1991, pp. 998–1008. 8. C. W. Plummer, et al., “Reduction in Distribution Transformer Failure Rates and Nuisance Outages Using Improved Lightning Protection Concepts,” Proceedings of the 1994 IEEE PES Transmission and Distribution Conference, Chicago, April 1994, pp. 411–416. 9. G. L. Goedde, L. A. Kojovic, J. J. Woodworth, “Surge Arrester Characteristics That Provide Reliable Overvoltage Protection in Distribution and Low-Voltage Systems,” Conference Record, 2000 IEEE Power Engineering Society Summer Meeting, Vol. 4, 2000, pp. 2375–2380. 10. P. Barker, R. Mancao, D. Kvaltine, D. Parrish, “Characteristics of Lightning Surges Measured at Metal Oxide Distribution Arresters,” IEEE Transactions on Power Delivery, October 1993, pp. 301–310. 11. R. H. Hopkinson, “Better Surge Protection Extends URD Cable Life,” IEEE Transactions on Power Apparatus and Systems, Vol. PAS-103, 1984, pp. 2827–2834. 12. G. L. Goedde, R. C Dugan, L. D. Rowe, “Full Scale Lightning Surge Tests of Distribution Transformers and Secondary Systems,” Proceedings of the 1991 IEEE PES Transmission and Distribution Conference, Dallas, September 1991, pp. 691–697. 13. S. S. Kershaw, Jr., “Surge Protection for High Voltage Underground Distribution Circuits,” Conference Record, IEEE Conference on Underground Distribution, Detroit, September 1971, pp. 370–384. 14. M. B. Marz, T. E. Royster, C. M. Wahlgren, “A Utility’s Approach to the Application of Scout Arresters for Overvoltage Protection of Underground Distribution Circuits,” 1994 IEEE Transmission and Distribution Conference Record, Chicago, April 1994, pp. 417–425. 15. H. W. Dommel, “Digital Computer Solution of Electromagnetic Transients in Single and Multiphase Networks,” IEEE Transactions on Power Apparatus and Systems, Vol. PAS-88, April 1969, pp. 388–399. 16. L. W. Nagel, “SPICE2: A Computer Program to Simulate Semiconductor Circuits,” Ph. D. thesis, University of California, Berkeley, Electronics Research Laboratory, No. ERL-M520, May 1975. 166 Chapter Four Transient Overvoltages 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. [...]... simplifications power engineers use for the fundamental frequency analysis do not apply 5 .4. 1 Active, reactive, and apparent power There are three standard quantities associated with power: ■ Apparent power S [voltampere (VA)] The product of the rms voltage and current ■ Active power P [watt (W)] The average rate of delivery of energy ■ Reactive power Q [voltampere-reactive] (var)] The portion of the apparent power. .. power to perform real work (active power) to the power supplied by a utility (apparent power) , i.e., P PF ϭ ᎏ S (5.10) In other words, the power factor ratio measures the percentage of power expended for its intended use Power factor ranges from zero to unity A load with a power factor of 0.9 lagging denotes that the load can effectively expend 90 percent of the apparent power supplied (voltamperes) and... harmonics in power systems When electronic power converters first became commonplace in the late 1970s, many utility engineers became quite concerned about the ability of the power system to accommodate the harmonic distortion Many dire predictions were made about the fate of power systems if these devices were permitted to exist While some of these concerns were probably overstated, the field of power quality... 1 ᎏ Ih ͙2 ෆ 2 1 ϭ ᎏ ͙2 ෆ (5 .4) where Vh and Ih are the amplitude of a waveform at the harmonic component h In the sinusoidal condition, harmonic components of Vh and Ih are all zero, and only V1 and I1 remain Equations (5.3) and (5 .4) simplify to Eq (5.2) The active power P is also commonly referred to as the average power, real power, or true power It represents useful power expended by loads to perform... waveform distortion for many seconds and has been known to excite system resonances 5 .4 Power System Quantities under Nonsinusoidal Conditions Traditional power system quantities such as rms, power (reactive, active, apparent), power factor, and phase sequences are defined for the fundamental frequency context in a pure sinusoidal condition In the presence of harmonic distortion the power system no longer... cosine of the phase angle as in Eq (5.11) The power factor that takes into account the contribution from all active power, including both fundamental and harmonic frequencies, is known as the true power factor The true power factor is simply the ratio of total active power for all frequencies to the apparent power delivered by the utility as shown in Eq (5.10) Power quality monitoring instruments now commonly... Copyright © 20 04 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Fundamentals of Harmonics Fundamentals of Harmonics 177 tribute the traditional sinusoidal components to S, while D represents the additional contribution to the apparent power by the harmonics 5 .4. 2 Power factor: displacement and true Power factor (PF) is a ratio of useful power to... not a new phenomenon on power systems Concern over distortion has ebbed and flowed a number of times during the history of ac electric power systems Scanning the technical literature of the 1930s and 1 940 s, one will notice many articles on the subject At that time the primary sources were the transformers and the primary problem was inductive interference with open-wire telephone systems The forerunners... work For this reason it is called imaginary or reactive power since no power is dissipated or expended It is expressed in units of vars In the sinusoidal case, the reactive power is simply defined as V1I1 Q ϭ S sin ␪1 ϭ ᎏ sin ␪1 ϭ V1rms I1rms sin ␪1 2 (5.7) which is the portion of power in quadrature with the active power shown in Eq (5.6) Figure 5 .4 summarizes the relationship between P, Q, and S in... adjustable-speed drives have a nearunity displacement power factor, but the true power factor may be 0.5 to 0.6 An ac-side capacitor will do little to improve the true power factor in this case because Q1 is zero In fact, if it results in resonance, the distortion may increase, causing the power factor to degrade The true power factor indicates how large the power delivery system must be built to supply a . this 1 64 Chapter Four Figure 4. 42 Energizing a capacitor and transformer simultaneously can lead to dynamic overvoltages. –2 –1 0 1 2 0 200 40 0 600 800 Phase A Time (ms) Voltage (V pu) Figure 4. 43. for most power system problems. 4. 9 References 1. Electrical Transmission and Distribution Reference Book, 4th ed., Westinghouse Electric Corporation, East Pittsburgh, Pa., 19 64. 2. Electrical. Equations (5.3) and (5 .4) sim- plify to Eq. (5.2). The active power P is also commonly referred to as the average power, real power, or true power. It represents useful power expended by loads