generator current inbalance

[blanjp]

We have a large westinghouse generator [530 megawatts] that is tripping on current unbalance when we ramp the megawatt load to quickly the relay is set at 1000 amps we have checked the CT'S they seem ok. The generator would not high pot during an outage but ran after the outage until this problem started to occur any ideas?

 

[Zogzog]

Did you check the delays on the realys? What tests did you perform on the CT's?

 

[waross]

When you say "outage" do you mean a routine shutdown during low load demands, or a planned maintenance shut-down?
When you say that the generator "would not hi pot" do you mean that a hi pot test was performed and the generator failed?
If so you may have damaged the generator with the attempted hi pot.
yours

 

[blanjp]

I am an operator but I will try and explain how the electricans tested the CT'S They applied voltage [800V] from the wiring from the control room to the CT'S and checked miliampreges they said to test the CT'S they would have to remove them [big job] We also rolled the wires from one set of CT"S to the other and it was always the B phase indicating high diff. there is no time delay on the relay it is an old GE. And yes the generator failed the HIPOT Test on a major outage Siemens preformed the test

Jeff

 

[GTstartup]

To clarify things I offer the following, please correct me if I am wrong.

It seems that when you say current unbalance you really mean differential. I say this because it seems that you have swapped CT leads from one set of CT's to an other. I am assuming this was a troubleshooting procedure and, according to your post, the fault remained on B phase. If this is the case, it means that, most likely the problem is the relay or the wiring to the relay. If the problem was the generator or the CT the fault would have followed the swap.

The relay should be tested by a qualified technician. Have him apply equal currents at the correct phase angle and ramp them quickly, see what happens.

One word of caution, the generator failed a HIPOT and you are tripping on differential. In the event that you really do have a fault, continuously applying excitation can cause severe damage. Siemens should have made a recommendation on why the generator failed the HIPOT and what might be causing it and what the corrective action should be.

 

[waross]

Hello blanjp;

We also rolled the wires from one set of CT"S to the other and it was always the B phase indicating high diff.

If "B" phase on the relay shows the fault, try changing the relay.
If "B" phase on the generator is causing the problem, you may have a fault in the generator. This would also be indicated by failing the hipot test.

 

[edison123]

If it failed the hipot, how are you now generating voltage ? Was it DC Hipot where the test was stopped before complete breakdown ? Was the earth fault rectified by Siemens or others ?

* Money doesn't grow on tress since the banks own all the branches *

 

[RalphChristie]

How did you rolled wires between the CTs? Did you use the same set of CTs (thus changing wires between the red CT, white CT and blue CT) or did you rolled the wires to another, but similar, set of CTs?
Seems as if the electricians did an excitation/magnetizing test on the CTs.



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[blanjp]

Thanks. the relay was check and is OK but it is interesting you mentioned the wiring to the relay I don't think that has been checked how would we do that? OH and seimens did give a rec they want to sell us a new generator. the unit is off line for an economy outage but when we were running we ran at unity. the diff current would only increase on B phase when we were ramping the megawatt load

 

[GTstartup]

You still didn't clarify if, after the CT wiring swap, the fault remained on Phase B or just that Phase B relay still tripped. This information is important.

Assuming Phase B relay still tripped. Test the wiring by using the same procedure as for the relay test but do it from the CT terminals. No need to disconnect or short the CT's. Of course the Unit must be down. You might have to run some temporary wire to one set of CTs. The definitive differential test is primary injection but for a Unit this size it is not practical.

You say the relay was tested but did they test it by simulating the behavior of the Unit when the trip occurs? In other words inject current and ramp at the same rate.