CONSIDERATION OF THE EFFECT OF PRE-INSERTION IMPEDANCES ON THE OVERVOLTAGES PRODUCED BY THE ENERGIZATION OF A SHUNT CAPACITOR BANK

Transcription

1 CONSIDERATION OF THE EFFECT OF PRE-INSERTION IMPEDANCES ON THE OVERVOLTAGES PRODUCED BY THE ENERGIZATION OF A SHUNT CAPACITOR BANK Raymond P. O Leary S u pe rvis i ng Eng i nee r-advan ce Tech no logy S&C Electric Company Chicago, I I Ii nois T58

2 - CONSIDERATION OF THE EFFECT OF PRE-INSERTION IMPEDANCES ON THE OVERVOLTAGES PRODUCED BY THE ENERGIZATION OF A SHUNT CAPACITOR BANK Background The pre-insertion resistor accessory for Circuit-Switchers, offered by S&C Electric Company for a number of years, was designed primarily to limit inrush currents during the energization of capacitor banks. The control of switching surges and the reduction of both audible noise and electrical noise were secondary objectives. Recently, S&C developed a pre-insertion inductor as a replacement for the preinsertion resistor. The unique design and features of the pre-insertion inductor provide improved reliability and operating life, more efficient heat-dissipating capability, and control of inrush currents and switching-surge voltages equal to or better than the pre-insertion. resistor. Several electric utilities recognized years ago the need for controlling switching-surge overvoltages produced at a transformer terminal as a result of energizing a remote capacitor bank. Recently, there has been a general concern expressed by the industry regarding the control of these switching overvoltages. S&C's conversion from the pre-insertion resistor to the pre-insertion inductor has prompted questions from customers as to the effect of the pre-insertion inductor on transient overvoltages produced during capacitorbank energization. The following study describes the overvoltage phenomenon associated with capacitor-bank energization and the mitigating effects of the use of preinsertion resistors and inductors as compared to the use of no pre-insertion impedance. A simplified computer model, shown in Figure A, was developed using only the elements necessary to demonstrate the overvoltage phenomenon. Capacitor-bank size, length of transmission lines, available fault current, and switching sequence of the Circuit-Switcher were all chosen to maximize the phase-to-phase switching overvoltage at the transformer, as experienced during energization of the capacitor bank. The switching overvoltages that will actually appear in the field will very likely be less than those suggested by this study. The intent of this study is simply to compare, on a "worst-case" basis, the volt ages experienced utilizing the pre-insertion inductor to those utilizing a pre-insertion resistor or no pre-insertion impedance. SYSTEM MODEL USED FOR COMPUTER STUDY OF OVERVOLTAGES PRODUCED BY CAPACITOR BANK ENERCIZATIONS L 75 w. FIGURE A -1 -

3 ENERGIZATION OVERVOLTAGE WITH NO PRE-INSERTION IMPEDANCE hase-to-ground Overvoltage Figure 1 shows the phase-to-ground voltage at the capacitor-bank bus before and after energization of the capacitor bank without a pre-insertion impedance. Energization of A and C phases occurs simultaneously at approximately 18 milliseconds. Energization of B phase occurs approximately 3 milliseconds later. Simultaneous closings of A and C phases at equal and opposite voltages were chosen to roduce the maximum phase-to-phase over- Q oltage between A phase and C phase. Copocitor Bonk Eneroizotion. Copocitor-Bank Bus Voltoaes No Pre-insertion Impedance 75 WA Cop. Bank Mils Line to Remota Bus the initial transient, whereas A-phase source voltage is decreasing during the initial transient. Figure 2 shows an expansion of C-phase voltage at the capacitor-bank bus during the time just before and after capacitorbank energization. The voltage on C phase at the remote bus is also plotted. (C phase was chosen for this plot because it has higher phase-to-ground overvoltage than does A phase. The latter, incidentally, responds in a similar manner but with opposite polarity.) Capacitor Bank Energization... Cg Voltages No Pn-inrrtion Impedonn 75 INA Cap. Emk -1 M U)k Lina to Remota Bus -Joo Time (ms) Figure 1 At the instant of capacitor-bank energization, bus voltage abruptly falls to zero due to the large inrush current of the capacitor bank. This abrupt change of voltage injects step voltage waves into any lines connected to the capacitor-bank bus. negative step voltage is transmitted on phase, while a positive step voltage is transmitted on C phase. After the initial drop to zero voltage, the phase voltages recover in a transient sinusoidal fashion. The frequency of this transient is determined primarily by the source inductance and the capacitance of the bank. Because this transient is underdamped, the transient voltage overshoots the source voltage. In this case, C phase overshoots more than A phase because C- 0 phase source voltage is increasing during The collapse in bus voltage on the C-phase remote bus occurs almost.5 milliseconds after the collapse of voltage at the capacitor-bank bus; this time delay is the travel time of the step voltage wave along the 88-mile line to the remote bus. Note that the voltage at the remote bus does not simply collapse to zero, but rather swings almost to the opposite polarity of the bus voltage prior to the collapse. The steeply rising step voltage wave initially sees the remote bus and transformer as a very high surge impedance and consequently almost doubles in magnitude. As time progresses, the step overvoltage decays exponentially with a time constant determined by the X/R ratio of the load connected to the transformer at the remote bus. The almost doubling of the initial voltage wave generates a second steeply rising -2-

4 wave of lesser magnitude but same polarity as the initial incoming step wave. This second wave is transmitted back down the line to the capacitor-bank bus. Since the capacitor bank looks like a short circuit to such a steeply rising wave, a third wave is generated and transmitted back down the line to the remote bus; this third wave is of opposite polarity to the original step wave. When the third wave reaches the remote bus, it again is approximately doubled in magnitude. However, at this point, it is the same polarity as the transient voltage on C phase. The net result is that these voltages add to yield an overvoltage-to-ground of 3.5 per unit. Phase-to-Phase Overvoltage A phase is subject to the same kind of transient voltages as C phase, except for slightly lower magnitudes and opposite polarity. The net result is that A-phase line-to-ground voltage approaches 3 per unit, but of opposite polarity to C-phase line-to-ground voltage. C-phase to A- phase voltage therefore approaches 6.5 per unit, as shown in Figure 3. coo m- m- 8 ica I -1m,o -m- -m WA Cap. Bank 0 UH. Liru to Run&. 8". 3.6 PU L I- /\ A 1 / / _- / \/ C#-4 Voltogs at the Remote Bua -6.5 W... ENERGIZATION OVERVOLTAGE PRE-INSERTION RESISTOR Phase-to-Ground Overvoltage I WITH A Figure 4 corresponds to Figure 1 of the base case, except that a 40-ohm pre-insertion resistor is utilized during the energi- 2w "i Copocitor Bonk Energizotion... Capacitor-Bank Prc-insertion Resistor 75 MVA Cop. Bank 88 Mile Line to Remote Bus -3w i lima (m.) y ' ' 20 ' ', ' ' 25 ' Figure 4 Bus Voltoges ', ' Jo ' ' ' * zation of the capacitor bank. In this case, bus voltage at the capacitor-bank bus does not collapse to zero. The extent to which the bus voltage collapses is dependent upon the ratio of the resistance of the preinsertion resistor to the resultant surge impedance of the transmission lines connected to the capacitor-bank bus. In this case, there are three transmission lines of approximately 380 ohms each in parallel, yielding an effective surge impedance of 125 ohms. The capacitor-bank bus voltage thus drops to a value determined by the ratio of 40 ohms divided by the sum of 125 ohms plus 40 ohms -- or 25% of system voltage at the time of energization. This reduction in the collapse of bus voltage manifests itself as a reduction in the step voltage wave injected into the system. Use of the pre-insertion resistor results in the capacitor-bank sinusoidal transient - recovery voltage being nearly critically damped, so that very!ittle overswing of capacitor-bank bus voltage occurs. During the transient period, there are small step discontinuities of the bus voltage at the capacitor bank. These discontinuities occur because voltage waves returning from the remote bus see the 40-ohm pre-insertion resistor rather than the very low surge impedance of the, capacitor bank. The reflection at this bus is a negative wave somewhat less in magnitude than the incoming wave, resulting in -3-

5 0 1 ess-t han-perfect cancel lation of the two waves. Figure 5 corresponds to Figure 2 of the base case. With the pre-insertion resistor, the initial collapse of voltage at the remote bus is driven by a step voltage wave of approximately 75% the magnitude of the base case for no pre-insertion impedance. This results in a voltage doubling of a smaller wave and therefore the extent to which the voltage collapses beyond zero is much smaller. 0 "1 88 Mile Line to Remote Bus "1 I Capacitor Bank Energization... C-$ Voltages Pm-insertion Resistor 75 LNA Cap. Bonk Remote Bus Phase-to-Phase Overvoltage Figure 6 corresponds to Figure 3 of the base case. The effect of the pre-insertion resistor has reduced the phase-to-phase overvoltage by approximately 38%, from 6.5 per unit down to 4.0 per unit. Capacitor Bank Eneraization Voltaae I, Pre-insertion Resistor 75 LNA Cop. Bonk gml "! 88 Mile Line to Remote Bur Jw s - 1w OI e -1w >. -_ ---_- -3w-. C+-A+ Vdtoge at the Remote Bus -5M -'-I I 1 \/ _A " D,, -Y I - w o,...,,...,,..,.,,.,,,..., I,,,,,,,,,, a lime (m*) Figure 6 e Figure --i -'w)..,,t,.,,,,,.,,,,,.,.,,.,,., In lima (mr) 5 ENERGIZATION OVERVOLTAGE PRE-INSERTION INDUCTOR Phase-to-Ground Overvoltage WITH A As in the case of no pre-insertion impedance, a second voltage wave is generated and transmitted back down the line toward the capacitor-bank bus. Upon reaching this bus, it is reduced somewhat in magnitude by the effect of the 40-0hm pre-insertion resistor and is also changed in polarity and transmitted as a third wave down the line toward the remote bus. This wave is again doubled upon reaching he remote bus. However, it is important e o note that with the pre-insertion resistor, the initial step voltage wave has been reduced due to the action of the resistor and subsequently the third wave has also been reduced. Further, the sinusoidal transient voltage has been considerably reduced due to the damping effect of the resistor. The net result is that the peak C-phase line-to-ground voltage at the remote bus is reduced to 2.2 Der unit. amroximately 63% of the I.. molt age experienced with io pre-insertion impedance. Figure 7 corresponds to Figure 1 of the base case. In this instance, however, energization of the capacitor bank occurs through a pre-insertion inductor. As is the case with the pre-insertion resistor,' the extent to which voltage collapses at the capacitor-bank bus is reduced, largely due '" Capacitor Bank Energization... Capacitor-Bank Pre-insertion Inductor 75 MVA COP. Bonk aa Mile Line to Remote us Bus Voltages ----,,,,,,,,,,,,,, ~,,,,,,,,,, 5 $ Time (ms) Figure 7-4-

6 ~.~ to the impedance of the pre-insertion inductor. In fact, because the inductor has a very high surge impedance, there is no abrupt step change in bus voltage, but rather the decay of voltage is limited to the natural frequency of the pre-insertion inductance resonating with the capacitor bank. As in the base case, the capacitorbank bus voltage recovers in an oscillatory fashion with a high-frequency sinusoidal wave form. Because the initial drop of voltage at the capacitor-bank bus is significantly lower than that of the base case, the magnitude of the transient sinusoidal voltage is reduced. Since the pre-insertion inductor has low resistance, the transient sinusoidal voltage is not significantly damped as it is with the pre-insertion resistor. Figure 8 corresponds to Figure 2 of the base case. In this expanded form, one can note more clearly that the bus voltage does not collapse abruptly, but rather falls at a moderate rate. Because. the bus voltage falls at a moderate rate, a ramp voltage wave is transmitted down the line which, upon reaching the remote bus, does not double. At this lower rate of change of voltage, the remote bus transformer acts like a high -- but not infinite -- impedance. Thus, the second voltage wave generated at the remote bus is of lower magnitude than either the base case or the pre-insertion resistor case. Consequently, upon returning to the capacitor-bank bus, Capacitor Bonk Energizotion _ Pn-inmrtmn Inductor 75 MVA Cap. Bank m] 88 #le Line to Remote Bur Cg Voltages yz-z--i -1.9 PU -um Time (ms) the third voltage wave generated is of substantially lower magnitude. The result is that peak C-phase line-to-ground voltage at the remote bus is approximately 54% of that with no pre-insertion impedance, as ' compared to 63% for the pre-insertion resistor. Phase-to-Phase Overvoltage Figure 9 corresponds to Figure 3 of the base case. With the pre-insertion inductor, the phase-to-phase overvoltage between C phase and A phase is approximately 3.3 per unit, or 51% of that for no pre-insertion impedance -- as compared to 62% for a pre-insertion resistor. Copocitor Bonk Energizotion. 9-9 Voltoge Prc-insertion Inductor 75 WA Cap. Bank 88 Mile Line to Remote Bus 1 3W g. -. 1w,/-, f -lwe.- I- / \/- - ~, / '-3w&i-L'i%2ge ot :le Rem,!, Bus '(,,.3.3'% -7w Time (mr) 0 m 3 Figure 9 There is a significant benefit to the fact that this overvoltage is characterized by a ramp function rather than a step function, as produced by no pre-insertion impedance or a resistor. Under a steeply rising transient wave, the voltage distribution across a transformer winding will be initially determined by stray capacitances rather than the inductance of the winding, creating stress concentrations at the end of the winding. The lower rate of change of voltage produced by the pre-insertion inductor will be distributed more evenly across the entire winding of the transformer. Figure 8-5-

7 Figure 10 shows the phase-to-phase overvoltages of Figures 3, 6, and 9, all perimposed on the same graph for easier 916 mparison I%t%rnote Bus ra -1 Y- o u Mu -ma Copocitor Bank Energization.._ C#-A# At the Remote Bus Voltage P lima (m.) figure 10 time to the peak of the sinusoidal transient voltage, many reflections can occur and, due to the damping at each reflection, the ramp or step wave part of the transient can be damped to zero. In this situation, the inductor will exhibit a greater net peak transient voltage than the pre-insertion resistor due to the fact that the sinusoidal transient voltage is not significantly damped by the inductor. As an example, refer to Figure 11, in which the transmission line length was reduced to 4 miles. In this case, the ramp or step waves have been attenuated to nearly zero by the time the sinusoidal transient voltage reaches a peak. The inductor circuit generates 2.3 per unit overvoltage versus 2.0 per unit for the resistor circuit. Note, however, that the initial step change in voltage for the resistor is substantially greater than the ramp change for the inductor. EXTENSION OF RESULTS TO OTHER SYSTEM CONFIGURATIONS Capacitor Bank Energizotion... C#-A# At the Remot. Bun 73 UiA Cap. Bonk 4 uib Uns to Run& &n "7 Voltage As stated earlier, the model used for is study, shown in Figure A, was chosen for illustrative purposes to yield the highest phase-to-phase overvoltages. This criterion is met by choosing the transmission line length such that two travel times on the transmission line equal one-half the period of the sinusoidal transient voltage produced by the capacitor bank oscillating with the rest of the system. This ensures that the third voltage wave arrives at the remote bus exactly at the peak of this sinusoidal transient voltage such that the voltage, the sinusoidal transient ~ ~ and the ~ ramp or ~ step wave e volt-, age all add at the same point in time. If the transmission line were half of the length used in this model, then four travel times would elapse before the peak of the sinusoidal transient voltage would occur. Because of the extra reflections involved and the damping which occurs at each reflection, the ramp or step wave would be of lower magnitude at the point where maximum voltage occurs and therefore the eak overvoltage would be reduced In the limit, as the transmission line length becomes very short, during the --[ la d 5 %m (mm) Figure 11 A smaller capacitor bank would result in a higher-frequency sinusoidal transient voltage, such that the time-to-peak of the transient voltage would be shorter. This shorter time-to-peak would require a shorter transmission line to fit the criterion of two travel times equaling the time to reach peak transient overvoltage. Similarly, a stiffer system (one with higher available fault current) at the capacitorbank bus would correspond to a smaller system inductance which would also have the effect of increasing the frequency of the sinusoidal transient voltage. -6-

8 CONCLUSIONS As has been demonstrated, both the pre-insertion resistor and the pre-insertion inductor do an effective job of controlling overvoltages produced by capacitor-bank energization. But the pre-insertion inductor offers an important additional benefit in controlling phase-to-phase switchingsurge overvoltages produced at remote apparatus as a result of energizing a capacitor bank. Unlike the steeply rising stepfunction overvoltage experienced when using the pre-insertion resistor, or the, even higher magnitude step-function overvoltage experienced when using no preinsertion impedance, the overvoltage experienced when using the pre-insertion inductor is characterized by a moderately rising ramp function. This is significant in that connected apparatus, particularly transformers, are subjected to much reduced voltage-stress concentrations. -7-