During transformer back charging 11000/433V , 2MVA ,6.75% , I powered the transformer through a 240A MCCB on the 433V windings by applying 433V from another strong source. I noticed that the first 2 or 3 attempts (charge-trip-reset-charge-trip -reset-charge took about 10s in total ) to back charge caused the MCCB to trip but after that was able to sustain. Can anybody explain why it is able to sustain after repeated attempts ? Has it got anything to do with flux build up? If its due to flux , how much time (roughly) does a transformer core require to be totally removed of flux after the voltage is removed ?

Mohan,

The magnitude of the magnetizing inrush current depends upon a few things:

The level of residual magnetization which is a function of where on the sine-wave it was de-energized,

Where on the sine-wave it is energized this time,

What type of load, if any, is on the non-charging side,

Whether you charge it from the lowside or the high-side.

The question of how long does the residual flux remain is a function of the core material.

The answer to your overall question is...sometimes you get lucky and the transformer was de-energized at just the right moment such that no residual magnetism is "stored" in the core and you re-energize at exactly the right moment so that the inrush is minimal...and sometimes you get really "unlucky" and you try to energize a fully charged transformer at exactly the worst point on the energization sine-wave and the inrush current is 10-20 times greater than you expect.

And don't forget that charging from the low-side may result in doubling the inrush that you'd expect from charging from the hgh-side, all other things being equal.

Alan

Mr Alan

Thanks for the explanation.

Note that its a 3 phase supply that I am applying so there are actually 3 points on the sine wave at which the sine wave is applied. Never could the transformer charge in the first attempt.It was a rapid exercise to charge in 2-3 attempts.
The other question I have here is that is the flux still dying out when I am re applying the voltage causing the inrush current to reduce.?

Could you also elaborate on the last point "charging from the low-side may result in doubling the inrush that you'd expect from charging from the hgh-side, all other things being equal."

Is there any best practise to check a typical 11000/433, 2 MVA tranformer other than back charging. Back charging has sort of assured that the transformer can be charged rather than testing it directly from the 11KV winding side.?

Mr. Mohan Kudua,
You didn't mention which protective relay operated to cause the trips. Since, responding to Mr. Alan Maltz you have acknowledged that fact that in a 3-ph system it is normally not possible to achieve physical "closing" of contacts of circuit breaker for each of the 3 Poles at ZERO cross-over instant of Voltage Sine Wave of the phase corresponding to the pole. Therefore, circuit breaker contacts of only one of the 3-phases can be closed at Zero-cross over instant of Voltage of that phase, Due to this the generator transformer is always prone to cause Differential Protection Relay of one phase or the other despite the Second Harmonic Restraint in the Relay.
In case of large transformers of rating over 250 MVA it is customary to provide "Single Pole" Circuit Breakers so that each Pole has its individual Closing and Opening mechanism. A Power Switching Device (PSD)" is also provided. Function of this PSD is to continuously monitor Voltage Wave and upon receiving "Closing" command, ensure Closing the Poles of Circuit Breaker ONE-BY-ONE according to their sequence of phase rotation such that each Pole Circuit Breaker Contact is Made ON at Zero Cross-over instant of Voltage Wave. This ensures protection of the Transformer concerned from damaging due stresses during charging INRUSH CURRENT.

Mr Anand

This was during commissioning the 2 MVA transfomer.Rather than directly test on 11KV , I back charged it through a MCCB , 240A. The intent was to allow it to rip if there were some internals faults. (Of course we had done all there tests on the transformer and back charging was the last step of our test before we were actually charging it.)There were no other protection systems in place during this back charging.

You mentioned about PSD. I will be googling it but do you suggest some recommended articles on this ?
In the case that I mentioned , it was by pure trial and error that I was able to back charge , first 2 or 3 attempts (charge-trip-reset-charge-trip -reset-charge took about 10s in total ) to back charge caused the MCCB to trip but after that was able to sustain.I was wondering if it is has anything to do with the flux build up and my attempt to recharge ,before any appreciable flux decay, that caused the MCCB to hold on in later attempts.
If its due to flux , how much time (roughly) does a transformer core require to be totally removed of flux after the voltage is removed ?

Mohan,

Mr. Anand's explanation is good as far is it goes, but it ignores the effects of saturation, and it's the degree of saturation of the core that determines the instantaneous level of overall saturation. Unfortunately this editor does not allow pictures or scientific characters so it's nearly impossible to illustrate what's happening but if you can get your hands on a copy of the J&P Transformer Book and go to pgs. 501-513 you will see the effect of closing angle on inrush current, along with an explanation of why low-side energization can result in a doubling of the current over high-side energization.

The time decay of residual magnetism is a topic worthy of a PhD thesis, the opinions are many, and actual tests on large power transformers few and far between. It depends upon many factors and no rules of thumb are forthcoming. A hot transformer will lose its residual magnetism quicker than a cold one, but time constants of days and weeks are the norm, and as we know it takes at least ten time constants to be considered washed out.

Alan

Mr. Alan,
Excellent.
However, whenever I respond I take it that the query pertains to a real life situation, to solve which I try to suggest such approach that can be comprehended by the person raising the query.
In our CCPP we are operating 7 nos. Units each with 525 MVA Generator Transformers (20/400 kV), the Units being each equipped with Bus Duct mounted GCBs and single pole operating SF6 Circuit Breakers (One Unit feeds into extended Switchyard for which we have installed 400 kV GIS also with single pole operating CBs. During start up of these units we always take Aux. Power through back charging of the Generator Transformers from 400 kV Side. Each Unit is equipped with Power Switching Device (PSD) for closing the 400 kV Circuit Breakers during back charging. Obviously, after FSNL and building up generator Voltage (actually each unit is equipped with totally automatic start-up and loading facility such that it requires a Single Click on DCS Screen to start any Unit and to come up to set load in Combine Cycle).
If the power plant about which the enquirer is asking is either fictitious or totally buggered up in design & engineering, it is near impossible to put up step-by-step suggestions about how to solve the problem.
And I do not consider the queries as Research Topics, therefore, ;limit my comments to simple practical explanations based more on real life experiences rather than ROCKET SCIENCE. Where, I feel that I must mention too much of theory, I always mention concerned URLs so that one can refer to them rather than depend upon my giving him/her detailed technical lectures.
I know fully well that there are other like you, perhaps more knowledgeable who too contribute and often I also enhance my learning through such highly enlightening discourses.

Dear Mr. Mohan Kudua,
It would be helpful if you inform me your email ID so that I am able to send some factual details about the PSDs that we are using for many transformers.
Where are you physically based? If you are in India, please also inform me your Cellular/landline telephone no. so that we can talk.
However, I have to take out time from my busy schedule. Therefore, please expect some delays in my responses, nonetheless I will try ASAP.