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Ross Burns on Trip on differential voltage relay

By: Ross Burns [390] 5 Stars
Date: Tuesday, June 20, 2017
Time: 11:12:25 AM

Maybe you can tell me what you mean by differential voltage relay? There should be a device number associated with your protection relay in the switchgear room such as 27 for a Undervoltage Relay or 87B for Differential Protection for primary phase fault protection for AC Generator using current transformers. Or a 60 for Voltage balance protection?

So maybe you are asking about a device number 87 known as a Differential -Current- Relay protection? The device 87 is a must have on a Gas Turbine Simple Cycle AC Generator.

Regardless of the prime mover such as a combustion turbine or gas turbine the AC generators have a level of protection that is consistent to the country, history of facility or power station and age of the installation as well as philosophies.

My thoughts are a 87B Differential Relay protection trip is a serious fault and at least a generator main breaker trip will occur. On large stations a backup protection relay can be employed known as a 87U Differential Relay that extends protection to the Generator and Step-up Power Transformer, so with a device like 87U the scheme is to rachet-up to a full generator trip, which includes the excitor or field as well as the generator main breaker, and switchgear trip to protect the power step-up transformer. This means the combustion turbine will overspeed and hopefully just momentarily.

Should a device number 87 Differential Relay activate then it will be without time delays and be instantaneous. So, we can see a 87 device is a must have, and on a Gas Turbine Simple Cycle AC Generator a 87U activation will trigger a AC generator trip with the power step-up transformer.

In decision is likely to include an orderly manual emergency shutdown on the gas turbine, for the scheme to do so is likely to be carefully weighed against the risk of not having an auto-emergency shutdown on the gas turbine from a 87 generator trip. Either way spinning the combustion turbine or gas turbine with a 87 trip is not productive since a serious electrical fault is present that needs investigation.

Perhaps, we can hear from others who have different philosophies from their country on protection relays that result in a gas turbine trip resulting from a 87 activation.

Ross

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