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9.1 Transmission Line Switching Operations
9.2 Series Capacitor Bank Applications
9.3 Shunt Capacitor Bank Applications
9.4 Shunt Reactor Applications
Switching surges occur on power systems as a result of instantaneous changes in the electrical configuration of the system, and such changes are mainly associated with switching operations and fault events. These overvoltages generally have crest magnitudes which range from about 1 to 3 pu for phase-to-ground surges and from about 2 to 4 pu for phase-to-phase surges (in pu on the phase to ground crest voltage base) with higher values sometimes encountered as a result of a system resonant condition. Waveshapes vary considerably with rise times ranging from 50 μs to thousands of μs and times to halfvalue in the range of hundreds of μs to thousands of μs. For insulation testing purposes, a waveshape having a time to crest of 250 μs with a time to half-value of 2000 μs is often used.
The following addresses the overvoltages associated with switching various power system devices. Possible switching surge magnitudes are indicated, and operations and areas of interest that might warrant investigation when applying such equipment are discussed.
9.1 Transmission Line Switching Operations
Surges associated with switching transmission lines (overhead, SF6, or cable) include those that are generated by line energizing, reclosing (three phase and single phase operations), fault initiation, line dropping (deenergizing), fault clearing, etc. During an energizing operation, for example, closing a circuit breaker at the instant of crest system voltage results in a 1 pu surge traveling down the transmission line and being reflected at the remote, open terminal. The reflection interacts with the incoming wave on the phase under consideration as well as with the traveling waves on adjacent phases. At the same time, the waves are being attenuated and modified by losses. Consequently, it is difficult to accurately predict the resultant waveshapes without employing sophisticated simulation tools such as a transient network analyzer (TNA) or digital programs such as the Electromagnetic Transients Program (EMTP).
Transmission line overvoltages can also be influenced by the presence of other equipment connected to the transmission line—shunt reactors, series or shunt capacitors, static var systems, surge arresters, etc. These devices interact with the traveling waves on the line in ways that can either reduce or increase the severity of the overvoltages being generated.
When considering transmission line switching operations, it can be important to distinguish between “energizing” and “reclosing” operations, and the distinction is made on the basis of whether the line’s inherent capacitance retains a trapped charge at the time of line closing (reclosing operation) or whether no trapped charge exists (an energizing operation). The distinction is important as the magnitude of the switching surge overvoltage can be considerably higher when a trapped charge is present; with higher magnitudes, insulation is exposed to increased stress, and devices such as surge arresters will, by necessity, absorb more energy when limiting the higher magnitudes. Two forms of trapped charges can exist—DC and oscillating. A trapped charge on a line with no other equipment attached to the line exists as a DC trapped charge, and the charge can persist for some minutes before dissipating (Beehler, 1964). However, if a transformer (power or wound potential transformer) is connected to the line, the charge will decay rapidly (usually in less than 0.5 s) by discharging through the saturating branch of the transformer (Marks, 1969). If a shunt reactor is connected to the line, the trapped charge takes on an oscillatory waveshape due to the interaction between the line capacitance and the reactor inductance. This form of trapped charge decays relatively rapidly depending on the Q of the reactor, with the charge being reduced by as much as 50% within 0.5 s.
Figures 9.1 and 9.2 show the switching surges associated with reclosing a transmission line. In Figure 9.1 note the DC trapped charge (approximately 1.0 pu) that exists prior to the reclosing operation (at 20 μs). Figure 9.2 shows the same case with an oscillating trapped charge (a shunt reactor was present on the line) prior to reclosing. Maximum surges were 3.0 for the DC trapped charge case and 2.75 pu for the oscillating trapped charge case (both occurred on phase c).
The power system configuration behind the switch or circuit breaker used to energize or reclose the transmission line also affects the overvoltage characteristics (shape and magnitude) as the traveling wave interactions occurring at the junction of the transmission line and the system (i.e., at the circuit breaker) as well as reflections and interactions with equipment out in the system are important. In general, a stronger system (higher short circuit level) results in somewhat lower surge magnitudes than a weaker system, although there are exceptions. Consequently, when performing simulations to predict overvoltages, it is usually important to examine a variety of system configurations (e.g., a line out of service or contingencies) that might be possible and credible.
FIGURE 9.1 DC trapped charge.
FIGURE 9.2 Oscillating trapped charge.
Single phase switching as well as three phase switching operations may also need to be considered. On EHV transmission lines, for example, most faults (approximately 90%) are single phase in nature, and opening and reclosing only the faulted phase rather than all three phases, reduces system stresses. Typically, the overvoltages associated with single phase switching have a lower magnitude than those that occur with three phase switching (Koschik et al., 1978).
Switching surge overvoltages produced by line switching are statistical in nature—that is, due to the way that circuit breaker poles randomly close (excluding specially modified switchgear designed to close on or near voltage zero), the instant of electrical closing may occur at the crest of the system voltage, at voltage zero, or somewhere in between. Consequently, the magnitude of the switching surge varies with each switching event. For a given system configuration and switching operation, the surge voltage magnitude at the open end of the transmission line might be 1.2 pu for one closing event and 2.8 pu for the next (Hedman et al., 1964; Johnson et al., 1964), and this statistical variation can have a significantly impact on insulation design (see Chapter 14 on insulation coordination).
Typical switching surge overvoltage statistical distributions (160 km line, 100 random closings) are shown in Figures 9.3 and 9.4 for phase-to-ground and phase-to-phase voltages (Lambert, 1988), and the surge magnitudes indicated are for the highest that occurred on any phase during each closing. With no surge limiting action (by arresters or circuit breaker preinsertion resistors), phase-to-ground surges varied from 1.7 to 2.15 pu with phase-to-phase surges ranging from 2.2 to 3.7 pu. Phase-to-phase surges can be important to line-connected transformers and reactors as well as to transmission line phase-to-phase conductor separation distances when line uprating or compact line designs are being considered.
Figure 9.3 also demonstrates the effect of the application of surge arresters on phase-to-ground surges, and shows the application of resistors preinserted in the closing sequence of the circuit breaker (400 Ω for 5.56 ms) is even more effective than arresters in reducing surge magnitude. The results shown on Figure 9.4, however, indicate that while resistors are effective in limiting phase-to-phase surges, arresters applied line to ground are generally not very effective at limiting phase-to-phase overvoltages.
FIGURE 9.3 Phase-to-ground overvoltage distribution.
FIGURE 9.4 Phase-to-phase overvoltage distribution.
Line dropping (deenergizing) and fault clearing operations also generate surges on the system, although these typically result in phase-to-ground overvoltages having a maximum value of 2–2.2 pu. Usually the concern with these operations is not with the phase-to-ground or phase-to-phase system voltages, but rather with the recovery voltage experienced by the switching device. The recovery voltage is the voltage which appears across the interrupting contacts of the switching device (a circuit breaker for example) following current extinction, and if this voltage has too high a magnitude, or in some instances rises to its maximum too quickly, the switching device may not be capable of successfully interrupting.
The occurrence of a fault on a transmission line also can result in switching surge type overvoltages, especially on parallel lines. These voltages usually have magnitudes on the order of 1.8–2.2 pu and are usually not a problem (Kimbark and Legate, 1968; Madzarevic et al., 1977).
FIGURE 9.5 Voltage magnification circuit.
9.2 Series Capacitor Bank Applications
Installation of a series capacitor bank in a transmission line (standard or thyristor controlled) has the potential for increasing the magnitude of phase-to-ground and phase-to-phase switching surge overvoltages due to the trapped charges that can be present on the bank at the instant of line reclosing. In general, surge arresters limit the phase-to-ground and phase-to-phase overvoltages to acceptable levels; however, one problem that can be serious is the recovery voltage experienced by circuit breakers when clearing faults on a series compensated line. Depending the bank’s characteristics and on fault location with respect to the bank’s location, a charge can be trapped on the bank, and this trapped charge can add to the surges already being generated during the fault clearing operation (Wilson, 1972). The first circuit breaker to clear is sometimes exposed to excessive recovery voltages under such conditions.
9.3 Shunt Capacitor Bank Applications
Energizing a shunt capacitor bank typically results in maximum overvoltages of about 2 pu or less. However, there are two conditions where significant overvoltages can be generated. One involves a configuration (shown on Figure 9.5) where two banks are separated by a significant inductance (e.g., a transformer) (Schultz et al., 1959). When one bank is switched, if the system inductance and bank 1 capacitance has the same natural frequency as that of the transformer leakage inductance and the bank 2 capacitance, then a voltage magnification can take place.
Another configuration that can result in damaging overvoltages involves energizing a capacitor bank with a transformer terminated transmission line radially fed from the substation at which the capacitor bank is located (Jones and Fortson, 1985). During bank switching, phase-to-phase surges are imposed on the transformer, and because these are not very well suppressed by the usual phase-to-ground application of surge arresters, transformer failures have been known to result. Various methods to reduce the surge magnitude have included the application of controlled circuit breaker closing techniques (closing near voltage zero), and resistors or reactors preinserted in the closing sequence of the switching devices.
Restriking of the switching device during bank deenergizing can result in severe line-to-ground overvoltages of 3–5 pu or more (rarely) (Johnson et al., 1955; Greenwood, 1971). Surge arresters are used to limit the voltages to acceptable levels, but at higher system voltages, the energy discharged from the bank into the arrester can exceed the arrester’s capability.
9.4 Shunt Reactor applications
Switching of shunt reactors (and other devices characterized as having small inductive currents such as transformer magnetizing currents, motor starting currents, etc.) can generate high phase-to-ground overvoltages as well as severe recovery voltages (Greenwood, 1971), especially on lower voltage equipment such as reactors applied on the tertiary of transformers. Energizing the devices seldom generates high overvoltages, but overvoltages generated during deenergizing, as a result of current chopping by the switching device when interrupting the small inductive currents, can be significant. Neglecting damping, the phase-to-ground overvoltage magnitude can be estimated by
(9.1) |
where
i is the magnitude of the chopped current (0 to perhaps as high as 10 A or more)
L is the reactor’s inductance
C is the capacitance of the reactor (on the order of a few thousand picofarads)
When C is small, especially likely with dry-type reactors often used on transformer tertiaries, the surge impedance term can be large, and hence the overvoltage can be excessive.
To mitigate the overvoltages, surge arresters are sometimes useful, but the application of a capacitor on the terminals of the reactor (or other equipment) have a capacitance on the order of 0.25–0.5 μF is very helpful. In the equation above, note that if C is increased from pF to μF, the surge impedance term is dramatically reduced, and hence the voltage is reduced.
Beehler, J.E., Weather, corona, and the decay of trapped energy on transmission lines, IEEE Trans. Power Appar. Syst. 83, 512, 1964.
Greenwood, A., Electrical Transients in Power Systems, John Wiley & Sons, New York, 1971.
Hedman, D.E., Johnson, I.B., Titus, C.H., and Wilson, D.D., Switching of extra-high-voltage circuits, II—Surge reduction with circuit breaker resistors, IEEE Trans. Power Appar. Syst. 83, 1196, 1964.
Johnson, I.B., Phillips, V.E., and Simmons, Jr., H.O., Switching of extra-high-voltage circuits, I—system requirements for circuit breakers, IEEE Trans. Power Appar. Syst. 83, 1187, 1964.
Johnson, I.B., Schultz, A.J., Schultz, N.R., and Shores, R.R., Some fundamentals on capacitance switching, AIEE Trans. Power Appar. Syst. PAS-74, 727, 1955.
Jones, R.A. and Fortson, Jr., H.S., Considerations of phase-to-phase surges in the application of capacitor banks, IEEE PES Summer Meeting, Vancouver, Canada, 1985, 85 SM 400–7.
Kimbark, E.W. and Legate, A.C., Fault surge versus switching surge: A study of transient overvoltages caused by line-to-ground faults, IEEE Trans. Power Appar. Syst. PAS-87, 1762, 1968.
Koschik, V., Lambert, S.R., Rocamora, R.G., Wood, C.E., and Worner, G., Long line single-phase switching transients and their effect on station equipment, IEEE Trans. Power Appar. Syst. PAS-97, 857, 1978.
Lambert, S.R., Effectiveness of zinc oxide surge arresters on substation equipment probabilities of flash-over, IEEE Trans. Power Delivery 3(4), 1928, 1988.
Madzarevic, V., Tseng, F.K., Woo, D.H., Niebuhr, W.D., and Rocamora, R.G., Overvoltages on EHV transmission lines due to fault and subsequent bypassing of series capacitors, IEEE PES Winter Meeting, New York, January 1977, F77 237–1.
Marks, L.W., Line discharge by potential transformers, IEEE Trans. Power Appar. Syst. PAS-88, 293, 1969.
Schultz, A.J., Johnson, J.B., and Schultz, N.R., Magnification of switching surges, AIEE Trans. Power Appar. Syst. 77, 1418, 1959.
Wilson, D.D., Series compensated lines—voltages across circuit breakers and terminals caused by switching, IEEE PES Summer Meeting, San Francisco, CA, 1972, T72 565–0.