the generator and breaker is 10 meters of underground cable.
Figure 4 shows a typical magnitude and waveshape of a surge that could be generated by the pre-strike of a vacuum switch. The green trace in the figure is the waveshape curve when it reaches the generator; which looks like an open circuit and causes the amplitude to double to close to 50 kV. The circuit impedance and short distance to the end of the circuit cause a ringing surge at that point. Because generator windings have among the lowest dielectric strength of all power equipment components, a 35 to 40 kV fast rising surge could cause damage due to a winding to case fault. This is what actually occurred.
Transient modeling showed that if an arrester was installed at the generator( as in Figure 3), the surge magnitude would be reduced a few percentage points as shown by the blue trace in Figure 4. But it would still have significant magnitude. Therefore, in this case, arrester protection alone would likely not have been adequate. However, with a surge capacitor installed in front of the generator, along with the arrester, surge magnitude would be significantly reduced and made harmless to the generator( as per Figure 5).
Installation of surge packs at this generator resulted in no further failures for more than 3 years, even though vacuum switches have continued to be an issue.
Figure 3: Generator with cap and arrester installed at the generator input.
Figure 4: Red curve is fast-rising pre-strike on vacuum switch. Green is how surge looks at the generator, with no protection. Blue is with arrester protection at the generator only.
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
Figure 5: Generator protection with arrester and surge capacitor combination.
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