Genearator Transformer without Tap Changer 1

[powering2002]

Last January, there was a serious problem in our generator transformer. Relay 87 T and Pressure Relief Devices operated. After investigation, we had found that the problem was in the Off Load Tap Changer. There was a flashover on the TC. We found some carbon material on the TC phase W tap position no 4. After some consideration, we decided to bypass the tap changer to avoid the same problem on the future. it means that we have no tap changer on the transformer anymore. One of the consideration of bypassing the TC is that our generator transformer to the INFINITE BUS (The voltage remains constant, not like in distribution network). Another consideration is that we can increase the generator voltage using Excitation. Our question is:
1. Was Our Action (BYPASSING The Tap Changer) correct?
2. What are the disadvantages of bypassing the TAP CHANGER of Gen. Transformer?
3. What is the Effect of changing the TC position of Gen. Transformer which is connected to the Infinite Bus?

 

[ScottyUK]

The infinite bus voltage remains constant? In a textbook maybe, but not on a transmission system.

1) Technically - depends on your operating conditions. Contractually - probably not, because most grid connections require a tapchanger. I'm slightly surprised that this is an off-circuit tap selector.

2) The biggest limitation will be the restriction on ability to export reactive power. On a day when the system voltage is high you may not be able to export any reactive power and you may even end up running at a leading power factor because your AVR will either hit the maximum excitation limiter or the terminal voltage will be higher than acceptable and you'll hit the V/Hz limiter.

3) i) You can maintain the machine terminal voltage at a constant level which is normally needed to prevent over-fluxing in the GSU transformer core (also the generator itself, but the GSU usually saturates first). ii) You can flex the reactive export of the machine without significantly varying the generator terminal voltage. iii) You can accommodate abnormal system conditions and continue operation. iv) You comply with the grid code, if you have one.


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If we learn from our mistakes I'm getting a great education!

 

[prc]

Generator transformers are normally provided with OCTCS(off circuit Tap-Changer)except in Germany and some other countries where they go for OLTC.So where ever OCTCs are used, day to day voltage control is done through AVR of generator and rarely tap chnager is used.This some times creates formation of carbon coke at tap changer current carrying contacts,consequent overheating and run down condition.Reason for such coke formation is not clear,suspected to be due to suphur in oil.One way to avoid such situation is to operate OCTC during shut down running tap changer from one end to other end of tap range.

Considering the above, many utilities are now eliminating tap changers in GTs to get higher reliabilty through simple winding construction.Then it is a good practice to go for a working flux density lower than normal say 5-10 % so that transformers can be operated at overvoltages continuously( to meet system requirements) without excess losses.

This trend is there for critical high voltage interconnecting auto-transformers also. In these on-load tap changers are always at high voltage and always a weak link from reliability angle.Italy has dispensed OLTC in their new 400/220 KV auto-transformers.

Considering above, your action is perfectly OK.Please check from old logs how many times OCTC was operated.Most probably it will be Nill and you can be sure that you are on right path.

 

[ScottyUK]

Interesting comments as always, prc. Thanks.

The UK is definitely biased toward OLTC, as is most (all?) of the Middle East, France, Germany and the Northern / Western European countries. My impression was that north amercia tended to use OLTC as well, but I might be very wrong in that respect.

I imagine that the utilities which are dispensing with the tapchanger to save cost must have alternate means of reactive power control because the generator is unlikely to be able to fully utilised. The capability of the machine to operate in the highly lagging region is severely compromised without a tapchanger.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi.
Scotty, usually GSU with off load tap changer.
Once per...years, tap is changed.
In indusrial applications, yes, used with OLTC.

But, as I know, in Austri used GSU with OLTC.
Best Regards.
Slava

 

[ScottyUK]

"Once per...years, tap is changed."

In the UK... a couple of times per shift is not unusual!


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If we learn from our mistakes I'm getting a great education!

 

[dpc]

FWIW, I've never seen a load tap changer on a large GSU in the US.

David Castor

http://www.cvoes.com

 

[electricpete]

Ditto dpc.

At our plant, there are other sources of reactive power that can be adjusted (including switched shunt reactors etc) and the switchyard is maintained at relatively constant voltage.

We have not changed taps ever, which I assumne is similar to the original poster (?). But if we "bypassed" the NLTC, that would amount to changing taps..... there would be something done somewhere to compensate to keep switchyard voltage constant without pushing the generator excitation system outside its range.

Another effect for plants that have low-voltage generator circuit breakers... they often backfeed through the GSU to auxiliary transformer to supply plant loads during outage. You now have a different transformation ratio than you used to. For ours I think NLTC is on high side and if it were bypassed I presume that means minimum turns on high side.... the plant bus voltage during outage backfeed would be higher....unless there is a On-load TC on the auxiliary transformer.

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[m3ntosan]

ScottyUK,

There is no tap changer on 400/275 kV, 1100 and 750 MVA autotransformers in the UK.

In Eastern Europe for instance there are some old Russian type units 400 MVA 400/220 kV with ULTC similar to phase-shifters and quad-boosters, changing the voltage and also the power flow direction by changing the voltage angle across the unit.
Does anyone know if there’s a particular name for this kind of tap changer?


May you grow up to be righteous, may you grow up to be true...

 

[ScottyUK]

m3ntosan,

"There is no tap changer on 400/275 kV, 1100 and 750 MVA autotransformers in the UK."

There are no generator transformers of that configuration in the UK...


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If we learn from our mistakes I'm getting a great education!

 

[m3ntosan]

ScottyUK,

Sorry about that, but I was talking about autotransformers … in 400/275 kV substations as you can see below.

"There is no tap changer on 400/275 kV, 1100 and 750 MVA autotransformers in the UK."

I guess I was off-topic a bit; It wasn’t my idea to challenge what you stated above, just to let you know what the case is with the transmission side of the power system.


May you grow up to be righteous, may you grow up to be true...

 

[ScottyUK]

No problem, I agree with you.


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If we learn from our mistakes I'm getting a great education!

 

[powering2002]

prc,
thanks for your support
We operate two units of Steam Power Plant. Both Generator transformers don't have any TC again (Both TC had been bypassed).

 

[rcw retired EE]

In the 60 or so power plants I have worked on, less than 10 have had Under Load Tap changers on the GSU. The only issues we found were hitting the maximum reacative capability values during perfomance tests without changing taps.

Most units with auto tap changers were in lcoations that needed it, like Pakistan, India and Algeria.