Overexcited power transformer 1

[mk44]

I am an Operations Planning Engineer working for a large utility.
I wonder if someone could help me to solve a puzzle on high voltage high power transformer.

I wonder if a large system transformer (1200 MVA) is excited to between 130 and 150 percent of rated voltage (500/230 kV) for 1 to 2 sec due to external system conditon, will the transformer differential protection operate to trip the bank?

I undersstand the core will most likely be driven into saturation and the bank will be generating lots of odd numbered harmonics but I wonder if the difference between the primary and the secondary current be large enough to operate the differential relays.

I have just read a recently published article from a relay manufacturer and it says it is possible.

I personally think it is possible too.

Comment?

mk44

 

[PowerfulStuff]

Sounds like a creative thought! If the transformer is operating above rated voltage (ignoring insulation problems) and hence saturated the core of the transformer there should be two consequences-

1/ The Magnetising impedance of the transformer would get much lower, drawing more reactive current

2/ Leakages flux would become much higher, heating any nearby magnetic components

I wouldn't expect that either of these would result in current high enough to trip differential protection - maybe someone else has seem more?

 

[Skogsgurra]

I take it that the differential protection works on difference between primary and secondary p.u. current. Then it is absolutely clear that overexcitation of the primary results in a difference in Ip and Is - so the differential should trip. Quite normal, I would say.

If it some other differential protection you are talking about, then I am not at all sure what's going on.

Gunnar Englund

http://www.gke.org

 

[waross]

mk44

I understand the core will most likely be driven into saturation

I think this may be your answer. If the overvoltage is sufficient to drive the core into saturation the primary current will become disproportional to the secondary current. The question is, not will it happen but at what actual voltage will it happen.
yours

 

[jnims]

It would depend on the relay. Many microprocessor based relays operate only on the fundamental component of the current.

 

[jghrist]

Some transformer protection relays, such as the SEL-387, have a 5th harmonic blocking feature that could be used to block tripping. Overexcitation produces 5th harmonic currents.

 

[subtech]

Sure it's possible.
A differential trip would depend on two things.
1. Amount of primary current into transformer due to over-excitation.
2. Settings that are applied to differential relay(s) that are protecting said transformer.

Without knowing more specific information about the over-voltage event and the relay settings however, we are all just guessing on the odds of a differential operation.
The documentation that came with your trans. may very well show some info on over-excitation tests that were performed prior to shipment to you. A thorough study of that info and a review of the relay settings for your diff relay would give a much better idea of the likelyhood of a trip during an event such as the one you described.

 

[stevenal]

I believe you will find that differential current caused by overexcitation will still contain a very large amount of the fundamental frequency component, so simply filtering the harmonics out is insufficient. You must actively restrain against tripping based on the percentage of 5th or other odd harmonics present. You may be able to set the slope to avoid tripping on this condition, but then your settings will be less sensitive during the non-overexcited state. Also, most relays also provide an unrestrained setting as well, so if a high current fault occurs during the overexcitation condition you still have instantaneous protection.

 

[ausphil]

I think whether the diff protection tripped is probably a secondary issue, my main one would be how do I retrieve a transformer that has got up and walked around the yard, how far have my connecting conductors stretched and how far away could the groaning be heard. Also the fact that you will have had severe burning between the laminations of the core, which will probably render it useless anyhow.

Even slightly overfluxed transformers make a hell of a noise for that time when they are energised, let alone 50% overvoltage. Anyone that has been in a yard when a transformer has been energised on the wrong tap can attest to the racket made.

 

[stevenal]

Protection against (rather than restraint from) overexcitation can be provided by a volts/hertz relay or function. But is 2s at 150% something to worry about? That's faster than most breakers will clear.
The easiest method of retrieval is to use a crane. If fastened to the foundation, it is less likely to walk away.

 

[amps21]

The CRGO used now a days has very high temperature withsatnd capability. Proper core cooling design should help a little. Interlaminar coating to get worn out by 2sec of Overexcitation
does not seem likely but the core shall become very hot and surely gas.

 

[stevenal]

Depending on the BIL, a 230kV transformer may have been tested up to 395kV at nominal frequency for 1 minute. See C57.12.00 and C57.12.90. Takes time as well as heat to cook a turkey.

 

[amps21]

Stevenal,

I think what u are refering to is Applied over voltage test. This does not involve the core inductively. (capacitively only). Hence the core shall not saturate. Correct me if i am wrong. (I dont have the standards at this time.)

 

[stevenal]

Yes, the applied over voltage test. Tested winding to grounded other windings, core and tank. You're correct, no core induction involved. Proves the winding insulation can handle the over voltage and nothing more.

 

[stevenal]

But is 2s at 150% something to worry about? That's faster than most breakers will clear.

Did I really type that? I read s, typed s, and was thinking cy. Where's that edit button? So just how far can a 1200MVA transformer walk in 2s?

 

[RalphChristie]

Source: J&P Transformer Book, twelfth edition, Martin J. Heathcote.

Generator transformers may be subjected to sudden load rejection due to operation of the electrical protection on the generator. This can lead to the application of a sudden overvoltage to the terminals connected to the generator. Very little documentary evidence is in existence concerning the likely magnitude of the overvoltage since monitored full-load rejections on generators are not the type of tests which are carried out every day. GEGB carried out some testing in the 1970s and concluded that the likely magnitude was of the order of 135% of normal voltage and this might persist for up to one minute. As a result of this testing the GEGB spesification for generator transformers contained a clause requiring that this level and time of overvoltage should be withstood without damage. Manufacturers of generator transformers maintained that before the overvoltage had reached this level the core would become saturated and would thus draw a very large magnetising current at a very low power factor which would have the effect of pulling the generator voltage down. The feeling was that the end result would be a magnitude of about 125% volts persisting for something less than one minute. IEC 76, Part 1, specifies that the generator transformer subject to load rejection should withstand 140% volts for 5 seconds. This is probably pessimistically high but for an optimistically short duration.

A sudden loss of load may cause a 10 to 20% increase in voltage at the input terminals of a transformer until such time as tapchangers or other system voltage control equipment is able to respond. This might briefly lead the transformer into saturation with a resulting large increase in exciting current which will be seen only by the input line CTs. Exciting currents resulting from saturation have a waveshape which has a period during each cycle for which the current remains close to or at zero. Newer types of relays can detect this in a similar manner to that used to identify magnetising in-rush current, and are able to remain inoperative to this "over excitation current". (Some older relays may not be able to do this, and can operate during such conditions)







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[mk44]

Thanks everyone for your information and suggestion.