Balance Voltage Relay (ANSI 60) 3

[billcasper5968]

Hi Guys,

I just want ideas on this;

I was previously assigned to power plant where the protection scheme of the Voltage Balance Relay (ANSI 60) was used to block other relays that utilizes voltage (e.g. undervoltage) and issue an alarm. Recently I was transferred to another plant. The protection scheme seems weird to me since the ANSI 60 issues a trip (triggers the ANSI 86) and opens the excitation breaker.

I could not find any literature on the web that supports the use of VBR as direct tripping. Can anybody provide a rationale of this scheme? I know that this is not a wrong scheme...I just want to know why?

Thanks

 

[Mbrooke]

Is this to prevent the generator from motoring? To me this would make more sense then alarming.

 

[davidbeach]

How did the two plants power the excitation system? If you're using a rotating exciter that doesn't need line power you can continue generating even with a VT problem, but if your excitation power comes from the source the 60 is monitoring you're going to want to get it off line as soon as you can.

 

[rcw retired EE]

If your voltage regulator can only connect to one VT and that VT fails, the regulator sees no voltage and goes to full excitation, possibly damaging something with the overvoltage. Tripping prevents that.

If the system has dual voltage regulators or a means to swap VT's, the 60V relay can switch the VT's and alarm. If you trust your operators and need the generator to keep running, then you block some protection and alarm (like your first plant).

On one cost effective plant (cheap owner) we believed the 51V relay would trip the generator if we lost the VT signal and went to full excitation. We did not need a 60 function.
When the generator VT fuse blew, all metering went out, but the generator stayed online at full excitation. Voltage was high but the output was limited to just below the 51V operating point by the line impedance to the utility substation. Fortunately, the regulator melted and caused a trip before the generator was seriously damaged. We added over excitation protection after that.

 

[billcasper5968]

Hi Mbrooke, davidbeach, rcwilson,

Guys thank you so much for the very fast response to my post. Every idea is very much appreciated and of great help to me.

@ Mbrooke – In my previous plant this was alarm and blocking of other elements only. No tripping.

@ davidbeach – In both installation, it utilizes PMG and “rotating exciter”.

@ rcwilson – I think your first paragraph is correct. It made sense why the excitation breaker is trip during triggering of the VBR.

Again, thanks and more power.