Ungrounded Single Wye Cap Bank with Neutral VT

[Pepwa33]

Hi there,

I have a 132/33kV Dyn Transformer (transformer neutral earthed via NER) supplying an Ungrounded Single Wye 8MVAr Cap Bank with a neutral VT (and various other feeders). The Ungrounded Cap Bank has a neutral VT for neutral overvoltage (59N) protection.

I was hoping someone could help me understand the following:

Suppose we have an "A" phase to ground fault on the 33kV bus, we could measure this full neutral voltage displacement across the NER (via bus VTs or a neutral VT across the NER). What I am battling to grasp, is what effect this "A" phase fault has on the Capacitor Bank's neutral voltage. I can appreciate that this "A" phase bus fault will shift the Cap Bank's neutral voltage, but i cant draw this or even picture it - I am confused.

1) Do the Transformer's Neutral voltage and Cap Bank's Neutral voltage differ?
2) Can someone show me how to draw the sequence network for this scenario? How would the ungrounded Cap Bank be drawn in this example?

Thanks in advance.

 

[RRaghunath]

I guess the capacitor neutral (to earth) voltage will be same as the voltage across NER.
All the three phase voltages at transformer end as well as at the capacitor bank terminals will also be same (ignoring voltage drops in connections).
The rise in voltage of unfaulted phases could affect the capacitor currents and cause over heating. Hope the same is taken in to account while procuring the capacitor bank.

 

[waross]

Assuming that the NER is 10 Amps, the effect will be similar to a 10 Amp phase to neutral load.
The load is connected to the X0 terminal by the ground connection rather than by the neutral circuit conductor.
The phase voltages and the neutral voltage to ground will now be referenced to the grounded phase.
eg: A phase to ground = 0 Volts
Draw your normal star vector sketch, but show A phase as the ground reference.
Insulation must be suitable for 33 kV to ground (neutral insulation for 19 kV to ground.)

Bill
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