Small Landfill Gas Generator in Central U.S.
A second example of how important good surge protection is at power plants also involves a smaller generator, this time located at a large landfill where collected methane is used to power 8 reciprocating engines and 8 x 1 MW generators. The power system in this case is a typical overhead 35 kV network with a well grounded four wire star system and the failures 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.
One last consideration in such a case is how to increase the reliability of the riser pole where tariff metering is often found. The transient model shows that this can be accomplished by tying the riser pole ground to the station ground. Since station ground grids are generally below 1 ohm, this is the optimal means to manage riser pole reliability.
Given the increase in distributed generation throughout the world, great care must be taken to mitigate both internally and externallygenerated surges. As in these two examples, several ‘rules of thumb’ can be applied:
1. Install arrester-capacitor combinations in front of generator windings to offer protection to these especially vulnerable windings.
2. Tie the pole ground of the first pole (where the tariff metering is installed) to the station grid if possible. If not possible, ensure it has the lowest possible ground resistance possible.
3. Apply arresters to protect switchgear from outside surges that inevitably will come in on the export line.
4. Consider the end point on collector systems since the voltage will double and easily cause even a relatively low incoming surge to become one that leads to insulator failure. Apply end point arresters to avoid this.
5. If in doubt, adding an arrester is the most cost effective surge damage insurance.
6. Whenever necessary, conduct a study that will result in a detailed understanding of the transient environment for all types of surges.