Magnetising current of a "Buck-Boost" transformer

[BigIanH]

Dear all,

I have looked through the archived questions and cannot seem to find anything suitable (I am a man though...).

Can anyone advise me if the "inrush" (or magnetising current) of a 3 phase/2000kva Buck/Boost voltage optimisation transformer is any different to a standard transformer?
I'm familiar with what I would call "normal" transformers, cast resin or oil filled, but we have no experience with this type.
I believe that it is a "fixed" type, I.E. it does not automatically adjust to changing input voltage, but I need to confirm this.
We are introducing standby diesel generators to this installation where the transformer is used for voltage optimisation of the load and have concerns over the "step load" when we energise from black start.
(it is oversized in relation to the true site load)

Any advice gratefully received!

Ian

"Power is nothing without control"

 

[waross]

A safe method of energizing may be to start the generator with the transformer connected and then close a contactor on the downstream side of the transformer. You may need a UPS to 'carry over' the control voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BigIanH]

Bill,

Thanks.
To clarify, do you mean have the transformer on the alternator (generator) side of the output/load breaker?
This would allow the voltage to build "gently",
Unfortunately this is an impossible situation as the transformer is primarily used supported from the incoming grid supply.
It will only be supported from generator(s) in the event of grid failure, hence it has to be on the "Load" side of an ATS/breaker arrangement.

The alternative is to introduce the generator at a different point on the scheme.

Thanks

Ian

"Power is nothing without control"

 

[waross]

Understood.My suggestion is to add a contactor or automatic switch to the load side of the transformer. Then arrange the ATS to close BEFORE the generator starts. The smoothest way is to bring the voltage up as the generator speeds up. The Under Frequency Roll Off feature of the AVR will help match the voltage to the frequency so that the Volts Per Hertz ratio of the transformer is not exceeded.
This may not be required but may be a "fall back" plan.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BigIanH]

Definitely an option....

Thanks

"Power is nothing without control"

 

[BigIanH]

So, the original question;

Can anyone advise me if the "inrush" (or magnetising current) of a 3 phase/2000kva Buck/Boost voltage optimisation transformer is any different to a standard transformer?
I'm familiar with what I would call "normal" transformers, cast resin or oil filled, but we have no experience with this type.
I believe that it is a "fixed" type, I.E. it does not automatically adjust to changing input voltage, but I need to confirm this.

I cant imagine that there is any difference, but you never know...

Everyday is a school day

"Power is nothing without control"

 

[bacon4life]

Typically voltage regulators are connected as auto transformers. Thus the inrush would be calculated on just the transformer winding rating, not the total throughput of the regulator.

 

[waross]

Further to bacon4life's comments; a 2000KVA auto-transformer with a 10% boost will be equivalent to a 200KVA conventional transformer. (2000KVA x 10% boost, or 2000KVA / 10:1 ratio)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BigIanH]

Thanks for the response guys.

Interestingly, the transformer in question is being used to reduce (Buck) the site voltage.
I'm told it's a 2000 kVA.
How does that effect the calculation??

I'll stick to generators, I understand them better!

"Power is nothing without control"