Photos courtesy of Jonathan Woodworth
Surge pack with 18 kV arresters and. 25 µ F 3-phase surge capacitor.
Landfill methane gas power plant with 8 x 1 MW generators in parallel. here were in the switchgear itself, not the generator.
Figure 6 is the simplified schematic for the system.
Since the generators are isolated from the 34.5 kV collector system, they are well protected from switching and incoming lightning surges. The switchgear, however, located in a common control room, was not well protected and failed.
Again, a transient study quickly demonstrated that, if a lightning event occurred within a few spans of the generator, it would travel to the riser pole – the power station’ s first line of defense. The arresters here would normally clamp the voltage to an acceptable level across the underground cable. But, with a 25 ohm ground on the pole, the voltage at the pole top actually soon increases to significant levels( as seen in Figure 7).
With no additional arresters or capacitors in place at this small 1 MW power plant, the riser pole arresters at the entrance of the station do not in themselves offer adequate protection in the event there is a strike near the incoming line. As per Figure 7 depicting an average strike of 31 kA near the station, voltage levels at the switchgear easily exceed its 171 kV BIL.
However, with an arrester installed at the plant’ s main breaker( PT1 on the schematic), switchgear voltage is reduced to 140 kV – offering a respectable margin compared to its 171 kV BIL. The addition of a capacitor in this case, would not have made much difference( see Fig. 8).
Once voltage levels are properly clamped at the switchgear, it is also important to look at the voltages at the end of the‘ daisy-chained’ generators. Figure 9 illustrates that if no arrester is mounted at the end of the underground line, the voltage will double and again exceed the system’ s BIL. To mitigate this, an elbow arrester can be mounted at the end point and the voltage level is then held to 80 kV( also shown in Figure 9.
Landfill Methane Gas Generator
Failed potential transformer in generator switchgear.
Figure 6: Landfill methane gas generator schematic.
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