Generator voltage output mismatch

[stratford]

Dear all, on site we have a 170MVA generator with 15kV voltage output, Y connection, earthed through transformer and resistor.
On this generator we notice from the first day of operation a consistent difference on the terminal voltage, meaning that on normal operation we measure phase A 14.99kV, phase B 15.15kV, phase C 15.18KV.
This is noticed during full speed no load (when the generator circuit breaker is open) and when the generator is synchronized to the grid via the step-up transformer.
Can you give me some points of searching or testing?
Voltage is measured on the terminal box of the generator (before the star connection) and on the brekaer side.

 

[edison123]

From OP's 24th March post

"About the VT's used their are from the same batch at each generator. Fir the ST generator, 15kV raing, the same difference in measurements apply for both the VT's located at the start point and those located at line side of the generator circuit breaker."

Doesn't this rule out the PT's as a problem since both sets of PT's show the same voltage difference ?

I must say I am surprised by the amount of current variation for such a small voltage deviation.

Would the 3 winding trafo have to do anything with this ?

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[waross]

When I asked about the burden, I was not referring to the VT/PT ratings but to the actual connected load. Any indicators, relays, or devices connected to the VT secondaries that do not present a balanced load may cause errors.
As well as "rolling" the VT primaries, it may be well to roll the connections on the VT secondaries. This should pick up any difference in cable impedances and any difference in individual loading.
respectfully

 

[scottf]

JBinCA-

Maybe my wording wasn't clear.

VTs are accuracy tested at 2 points...0 burden and full burden rating. The accuracy changes linearly between these points as a function of applied burden. The 2 points are going to be near the extremities of the accuracy class because the longer the load line, the more cost-effective the unit is to produce (i.e. smaller core and/or less turns).

What this means is that while there is some variation from unit to unit, it is not possible to have 2 units (with the same connected burden) and have one unit giving -0.3% and one unint giving +0.3%. The difference between 2 units with the same connected burden is not going to be very much.

Waross-

Rolling the secondaries is a good idea, but only to see if there is some problem or inequality in the connected burdens. The lead impedance (or difference in lead impedance) is not going to be of any significance, in that the lead impedance of a VT circuit is normally negligable in terms of the VA burden presented to the VT.

 

[waross]

Hi scottf;
In a perfect world I agree with you.

the lead impedance of a VT circuit is normally negligable in terms of the VA burden presented to the VT.

and with one reading high the wiring is probably good, but with one indication low, as it is here, there may be a possibility of a connection developing high resistance, or some other unequality in the connection wiring.
When trouble shooting, I sometimes sometimes try to look at each part off the circuit or system and ask myself,
"What can be expected from abnormal operation of this part?"
respectfully

 

[scottf]

waross-

Agreed. Could be a high resistance connection issue.

 

[waross]

scottf
Yes. This is probably not the problem, but rolling the connections should detect any problems with the wiring and remove the wiring from further suspicion.
respectfully