Hi all, Yesterday we encountered a 1

[SyedUmairAli]

Hi all,
Yesterday we encountered a problem in our gas turbine generator that its one phase voltage increased from 11kV to 17kV in 2.5secs but othr two phases voltages were stable at 11kV. We have checked the excitation current trend but no abnormality found. Kindly help me to to rectify the problem...

 

[racookpe1978]

Did YOU call YOUR generator supplier?
Did YOU call YOUR power company's technical representatives and YOUR power company's engineering staff?

Good Lord, you didn't even tell us what company made the unit, didn't say what size or specification it was, and didn't say it you shut it down safely, whether the problem went away, or whether the problem remains, but has remained stable!

 

[waross]

It sounds as if you have lost the load or a significant part of the load on one phase.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SyedUmairAli]

We have called the supplier they will come for detail analysis...
GT tripped safely on over voltage protection...
GT started normally and on load.
10MW gas turbine of turbomach..

 

[GerH]

Was the generator connected to a grid or load at the time. If so
Could the over-voltage come from the load side, and not the generator ?
However it would be very unusual for a Grid or load to only impact one phase.

You mention 11kV as as the 1-phase voltage. What is the normal Line-to-Line
voltage of the generator. I wonder if 11kV is L-L, not L-N ?

What is the primary winding arrangement of the VT's 3 x single phase?

Is the same VT used for both over-voltage and measurement. If so maybe
it is a VT problem.

 

[waross]

Alternators need more excitation under load than they need at no load..
Many alternators need about twice the excitation current under full load than under no load.
Some of us have used batteries to excite generators for testing. A typical alternator may require 12 Volts ecitation to produce near rated voltage at no load.
The same machine will need around 24 Volts excitation to produce near rated voltage at full load.
The AVR looks at the terminal voltage and adjusts the excitation to maintain rated voltage at the terminals.
However, the AVR can set only one level. The AVR cannot adjust the excitation to individual phases.
The AVR may monitor the voltage on one phase, or it mat take an average of the three phase voltages.
You have about 54% voltage rise. That may easily be caused by the loss of some load on one phase.
OR
Check your instrumentation for a bad measurement.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[GerH]

Check the phase current logs at the same time the voltage rise was observed.
If the current did not change then it is a voltage measurement fault.

 

[SyedUmairAli]

We have checked complete three phase current trend but there was no variation at all...

 

[SyedUmairAli]

GT was ON load about 8MW...

 

[waross]

We have checked complete three phase current trend but there was no variation at all...

An indicated 50% voltage rise with no variation in current?
Check your PTs and relays.
You most likely have an instrumentation issue.

Bill
--------------------
"Why not the best?"
Jimmy Carter