Off load Tap changer on Generator

[HamidEle]

The tap changer on the generator is always off load type, followed by the main distribution transformer with OLTC. Noticed from ETAP motor starting analysis, the off load tap changer can be used to boost the voltage permanetly at the generator output to increase the voltage levels in the downstream system. Wonder it is true in the real world?
Thanks in advance.

 

[ScottyUK]

Most GSU transformers have an on-load tapchanger. Many unit auxiliary transformers have an off-circuit tap selector. That latter is usually acceptable because the station service transformer, if separate, will have on-load tapping capability.


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

 

[HamidEle]

As far as I know, most generator unit transformers have off load tap changer.

 

[ScottyUK]

Well if you already know the answer to your question then what are you asking...?

Unit auxiliary transformers tend to have off circuit tap selectors.
Generator step-up transformers have on-load tapchangers because most transmission system operators require it, either directly or indirectly.


Please note that 'off load' and 'off circuit' are totally different. Off load means live but not carrying current, off circuit means completely disconnected from the system, isolated and made dead. Operating an OCTS with the power on is a recipe for a disaster.


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

 

[electricpete]

I am not familiar with distinction of off-load vs off-circuit.

For our plant, we have LTC on aux transformer, but not on the GSU.

Considering that generator voltage is adjustable, you need an LTC on only one or the other transformer (aux or generator stepup). It makes sense to me that it would be cheaper/easier to put the LTC on the transformer that has lower power throughput, which is the aux.

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(2B)+(2B)' ?

 

[electricpete]

Sorry I didn't read closely - I think Scotty was just describing the wide variety of possible configurations out there.

My experience is limited mostly to one plant. There are GSU's with LTC's?

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(2B)+(2B)' ?

 

[electricpete]

I also see Scotty has answered that question. I guess practices vary widely.

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(2B)+(2B)' ?