Reverse power trip on a gas turbine

[berkani]

Dear gentlemen,

we installed a Black Start system in a power plant to start a Gas Turbine in case of Black out.

We made a test yesterday and all the sequence goes perfectly to start the GT, reach the full speed no load, and close the GT breaker.

We close this GT breaker in dead bus bar but after approximately 1 minute of closing the GT circuit breaker, it had tripped due to reverse power protection 32.

We don't know why we had this reverse power as it was no load on the busbar.

Could somebody help us on this subject ?

thanks
Mohamed

 

[ScottyUK]

Phasing problem with either CT or VT circuits? If you get the CT polarity reversed w.r.t the voltage circuits power flow will appear reversed.

This is probably less likely with a multi-function relay. What form is your protection - discrete relays or an integrated type?


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If we learn from our mistakes I'm getting a great education!

 

[berkani]

it's integrated type but we checked the phasing and there is no problem with this.

We made another test with connection to the energised grid and it was definitly no problem

 

[Alex68]

What is the setting of the function 32?

Maybe it was set to trip if the P is ... less than +5%_+10%?
Maybe it was set like a heavy duty turbine, where if the produced active power is zero for a long time, then something is wrong.

Alex68

 

[Marmite]

Other possibilities:-
1.Rated voltage of machine different to rated voltage of VT, leading to incorrect setting calculation.
2. Incorrect phase voltages and currents being measured. ie if VT is ph-e then red phase voltage is parallel with red phase current at unity pf. If VT's are ph-ph then red phase current is used with Yellow/Blue voltage. In this case red phase current is perpendicular to the voltage. The settings of the relay need to account for the manner of connection.
3. Due to the sensitivity of the protection the CT's and VT's need to be accurate and the voltages need to be balanced.
4. Possibility of non electrical non critical trip on prime mover? Reverse power relay being used to trip the set when it begins to motor, rather than just dumping the load and risking overspeed?

Regards
Marmite

 

[davidbeach]

What does the relay sequence of events report have to offer? Which element caused the trip? How is that element set? What does the oscillography from the relay show? Was there reverse power seen by the relay (may or may not correspond to reverse power seen by the generator)? Was there, for instance, a low forward power setting with a 1 minute time delay?

 

[GTstartup]

Was it absolutely no load, or picking up plant auxiliary load as the black start generator is shut down?

This is unlikely but is it possible that the voltage to the relay is fed from the portion of the bus that is still dead and the current fed from the portion that is picking up house load?

This can sometins "confuse" 32 relays.

 

[dpc]

Pickup settings for 32 elements are often extremely low relative to the CT rating. I have often seen trips on reverse power after synchronizing unless a conscious effort is made to pick up at least a small amount of load.

This is a common occurrence in my experience. I would review the settings of the 32 relay - if they are appropriate, I would just try to pick up load right after synch. Sitting at zero load is usually not a good idea.

 

[pwrengrds]

Did the 32R work ok before the black start test? That seems like it's the first question. Were you pushing power out when you tripped? ie Was the generator under load at the time of tripping or was it under no load, just energizing a dead bus? 32R could be a low forward power trip.

 

[slavag]

yes, it's possible.
Not correct setting, instead reverse power used low forward power.
Regards.
Slava