Photos courtesy of Jonathan Woodworth
Close-up of failed stator.
Figure 1 is a simplified system schematic that shows that the surge arresters in the system are located downstream of the breaker and not on the generator side.
Figure 2: Surge on high side of GSU( red) and resulting effect on low side of the GSU( green).
Identifying the Source of the Surge
There are a number of possible sources of surges that can damage a power generator or its associated equipment, including:
1. direct lightning strike; 2. lightning surge entering the station along a phase wire; 3. switching surge entering the station on the incoming line; or 4. switching surge generated within the station.
From Figure 1, it would appear that the generator is well protected from the external power system, with the GSU between the incoming transmission line and the generator. Indeed, running a system model confirmed that the GSU does in fact offer significant protection to the generator. Arr _ 0 is the only arrester placed in front of the 69 / 23 kV transformer and offers it protection.
Figure 2 provides the input and output of the GSU with a high current surge just outside the generator substation. With the GSU transformer configured as it was, a fast ringing surge with a maximum of 10 kV could make it onto the generator bus( i. e. the low side of GSU), although it is not likely that a 15 kVrms rated generator would be affected by such a surge.
Experience also suggests that a switching surge entering from the transmission line would be of lower magnitude and therefore also not a likely source of damage to the generator.
The only other possible source is from within the generator switchgear. Indeed, maintenance records revealed that a 15 kV vacuum switch had failed at the same time as the generator. Failure of this fast operating switch could mean that a fast or slow rising surge could have been generated at the switch situated directly in front of the generator. While the surge arrester at the breaker would limit the surge magnitude that travels toward the step-up transformer, it would have little effect on the generator side of the switch. As can be seen from Figure 2, the only component between
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