Transformer Inrush Current Rised to Voltage Dip

Yes - the transformer manufacturer can. Please advise the outcome of the telephone call that was made to them when this problem first occurred. What were the recommendations that dropped out of that discussion, please?

This is not something that can be resolved by a student, or intern.

You'll need an electrical engineer who understands transformers and how they work.

Have that person contact the manufacturer of the unit and discuss the problems.

ONLY a qualified engineer can resolve this issue.

Things to consider and investigate:

Applied Primary Voltage is greater than 33KV.

Surge arrestors and/or filter capacitors are not properly sized.

Transformer windings turn-to-turn ratio not correct. (Caused by faulty transformer winding or bad assembly process.)

Shorted transformer winding caused by electrical fault or high impact.

Primary and secondary are reversed.

Who determined what your protective relay settings (including the differential relay) should be to begin with? You should talk to that person first. And what justification do you have for changing the differential relay setting the way you did (except that it prevented the relay from tripping)? Arbitrarily changing relay settings can have catastrophic results when the relay is supposed to pick up for a fault but doesn't because someone incorrectly set it! You will be working for a long time to pay for a transformer of that size if it fails because of your ignorance and capriciousness.

Are the CTs correctly connected? Wrong polarity or even phase cross-over/wrong ratio?

Did anyone do CT polarity & connection checks?

You write "rated 15/20MVA with 33Kv/6.6Kv voltage rating". Where is the 11 kV? - at a too-small generator with step-up to 33kV?

67model

Sir

yes we are having GTG of 29MW ( at present it is loaded at actove power4.26MW and reactive power 2.2MVAR ) it is at 11KV . This 11KV has been stepped up to 33KV and distributed further. IN this distribution one of the transformer 15/20 MVA has been charged.

One of might observation is as GTG is less loaded and possessing very less rective power, as and when a transformer of 15/20MVA charged its reactive power is getting shooted to 6 or 7 MVAR. Is this might be any cause for voltage dip. please explain me in this regards also.

Why do you find this unusual, after all a transformer is a big inductor, and inductors draw reactive VARs during charging.

20MVA transformer inrush is big for a 29MW machine.

Typically, the AVR of 29MW GTG is selected to be in 'PF' control mode.

If you can change the selection to 'Voltage' control mode, it improves the speed of AVR response and thus reduce the Voltage dip/duration.

Further, please ensure/confirm that the field forcing circuit (through compounding CTs or otherwise) is commissioned and in service. This boosts the field current instantaneously (as soon as it senses the inrush current in generator power cables) and plays important role in minimising such voltage dips during large motor starts or transformer energisation.

Dear Venu,

Thank you for the additional information. I think the system is like the sketch drawing below :-

I have put in typical impedances for the generator and transformer. Note that X'' for the generator [and the 2% volt drop] applies only for the first cycles of a load application. The settled value after a few seconds will depend on the generator "synchronous reactance", usually Xs on its data sheet, if the excitation is a fixed field current and typically would be a voltage fall of about 20% of 11kV for load MVA of 6 to 7.

It seems your voltage regulator is having some effect, although much better is usual.

Further information would help about the following :-

1. How do you know the voltage dip is 0.8 kV? If you are looking at an ordinary moving iron "pointer" meter it may overshoot and swing about before settling to a steady value - this gives a dip exceeding the true value.
2. What is the settled "steady state" voltage & current value at 33 kV after the 15/20 MVA transformer charging [I assume 11.0 kV before transfo charging and you have ammeter & voltmeter in the HV feed to the 15/20 MVA transfo]?
3. What was the current at 11 kV with no load at 33 kV [that is the steady magnetizing current of the 11/33 kV transfo]??
   
4. How long before charging current and generator voltage become steady after closing breaker C.B. ?
5. What is the final current into the 15/20 MVA transfo? I assume there was no secondary load on this transfo during your test.
6. What happens to the 33 kV voltage when C.B. is closed?

I note that in a system like this it would often be the 33 kV voltage which is fed back to the generator automatic voltage regulator.

Finally, I write again my opinion that the tripping of the differential relay is due to a wrong wiring somewhere. Your present setting of 1.5 In gives little protection.

It would help if you give the manufacturer and type of the differential relay.

67model