GE gas turbine with 20 MW capacity has BASLER brushless ecxcitation system. when we energize the grid transformers, the AVR changes to MVR and we have to reset and change to AVR again. the capacity of transformers is 40MVA, (13.8/123)kv

the turbine sometimes trip by overflux. Any reason for this switching? thanks

 

Just as points of clarification, it seems that you don't synchronize across the breaker between the generator and the transformer.?.?.?

Do you synchronize across the breaker on the high side of the transformer?

Is there even a breaker between the transformer and the generator and the transformer?

Do you build voltage up on the generator terminals and then close the breaker onto the generator, with the breaker on the high-side of the transformer open?

Or, if there's no breaker between the generator and the transformer, is the generator connected directly to the transformer and the transformer is energized at the same time as the generator terminal voltage is built-up?

Has this problem been occurring since commissioning or since the exciter regulators were replaced with the Basler regulator?

Did it just start occurring, or recently start occurring?

Does the same thing happen to all the units (how many of them are there?)?

markvguy

 

Generator of gas turbine has output of 13.8kv and its circuit braker already connected in parallel with another gas turbine generator (both are GE frame 5). We always face this problem when we energize any step-up or step-down transformer in sub-station.The capacity of these transformers are 2x25MVA &4x40MVA. On GDP we rececive 'negative sequence 1st'alarm and on BASLER excitation panel the alarm of over-excitation is activated before the switching to manual position.

The load of GT's during that time is about 3MW on each unit and they face the same problem always.

 

Responding to meshal's 13-Jan 5:12 pm comment, "... any step-down or step-up transformer... !" Do uou mean that any of the six transformers can be synchronized on either their 13.8kV or 123kV side?

 

Further to my 13-Jan-07 response:

Actuation of the negative-sequence relay is a clear sign of current unbalance. And with the asymmetrical nature of transformer energization, actuationof the 1st level alarm is easily explained! Furthermore, an appropriate corrective course-of-action can be implemented.

Can you provide additional information, especially that which is related to manual vs auto synchronization:
1) Is manual-synch always successful, regardless of trafo size?
2) Is auto-synch, except for alarm, always successful?
3) Does alarm occur with every auto-sync attempt? Or is it random?
4) Has alarm ever occured with manual-sync?
5) Have units ever tripped for neg-sequence?
6) Finally, can you provide some info as to relay settings?

Regards, Phil Corso, PE ([email protected])

 

The generator has no step-up transformer after M.C.B., which is already closed with load. (Our power plant supplies some direct 13.8kv outgoing feeders with no step-up transformers and some of these feeders are the low tension side of sub-station step-up transformers for long distance power transmission purposes.) Our power plant is the only source of power of this sub-station.

When these transformers are energized, the problem of AUTO to MANUAL happens and we observe high MVAR values.

Regarding negative sequence alarm, the 1st stage setting is 0.31A and 2 sec. time delay, but it will not trip the unit.

 

Meshal, your 3-Feb-1:07am reply gave the current set-point of the negative sequence relay as 0.31A.

1) What is the corresponding primary current (or CT ratio?)
2) What is generator (I^2)t characteristic?

Regards, Phil Corso, PE ([email protected])