Monday, February 22, 2010

Earth Fault Analysis Response Draft

Generator Tripped by Power Reverse Relay.
When CPP bus is paralleled with grid system thru transformer TR1A both voltages are the same. Fig.1 indicate the system arrangement. Please corrected if it is wrong.
During motor starting, the voltage generated by DG is drop due to slow response from AVR or Governor to maintain CPP busvoltage, while the voltage from grid is fixed and maintain voltage as it is.
Grid sees the Generator as a motor due to generator voltage lower than grid voltage and then power reverse relay operated.
AC3 & AC8 Fdrs Tripped.
As stated by post(er) that Sensitive Earthfault relay connection is using residual connection for both fdr protection. Residual connection relay may operate due to ground fault condition, unbalance current on each phase and unbalance saturation on CTs as relay input.
I think the relay operate due to unbalance current on each phase flowing in the primary circuit during motor starting. I try to explain this situation by Fig.1.
The unbalance current in the secondary circuit are shown in Fig.2 and Fig3.

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