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7 comments
Member

Join Date: Oct 2007
Location: Dubai
Posts: 7

% Impedance of Transformer

03/17/2012 6:44 AM

Dear All.

A customer has specified supply of 220/33-10V Power Transformer with the following parameters

112/82+30MVA (ONAN), 150/110+40 MVA(ONAF)Dyn1yn1 +/-10% OLTC with impedance of 13% (220-33kV), 29%(220-10kV) and 29% (33-10kV) at 110 MVA base.

The transformer supplier's have expressed inability in achieving such impedances stating that these are practically not possible on account of the fact that the Transformer's physical dimensions would become very large.

Can someone from this forum please explain with details as to why the size of the Transformers is getting so large?

regards

Ddubey

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Guru

Join Date: Mar 2007
Location: at the beach in Florida
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#1

Re: % Impedance of Transformer

03/17/2012 10:29 AM

Are you sure the customer wasn't ordering several different transformers?

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Member

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Posts: 7
#5
In reply to #1

Re: % Impedance of Transformer

03/18/2012 12:13 PM

Dear Solar Eagle.

The vector group of the Transformer is Dyn1yn1.

I guess that un equivocally declares that it is a three winding transformer.

The customer did not order several one of them.

Thank you.

regards

Ddubey

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Guru
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#2

Re: % Impedance of Transformer

03/17/2012 1:20 PM

Quote "112/82+30MVA (ONAN), 150/110+40 MVA(ONAF)Dyn1yn1 +/-10% OLTC with impedance of 13% (220-33kV), 29%(220-10kV) and 29% (33-10kV) at 110 MVA base."

Transformer #1 112/82+30MVA (ONAN)

Transformer #2 150/110+40 MVA(ONAF)

Transformer #3 (220-33kV) no rating specified (33-10kV) maybe this Tr goes here?

Transformer #4 (220-10kV) no rating specified

You need sources of 112 kv, 150 kv and 220 kv.

quote "The transformer supplier's have expressed inability in achieving such impedances stating that these are practically not possible." I agree.

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Member

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Location: Dubai
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#4
In reply to #2

Re: % Impedance of Transformer

03/18/2012 12:10 PM

Dear Mr. Wareagle.

It appears that the specification has not been correctly understood.

So plz allow me to explain.

1. We are discussing one Transformer. It is obvious through the vector group description of Dyn1yn1.

2. This Transformer has three windings. ( 220kV, 33kV and 10 kV)

HV winding is rated at 220kV. At ONAN cooling, it is supposed to supply 112 MVA with 82 MVA through 33kV and 30 MVA going through 10kV.

At ONAF it supplies 150 MVA ;110 MVA going through 33kV windings and 40 MVA through 10kV windings.

33kV and 10kV Switchgear are to be connected through MV windings 1 & 2.

Hope this clarifies.

regards

Ddubey

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Guru

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#3

Re: % Impedance of Transformer

03/17/2012 2:41 PM
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Associate

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#6

Re: % Impedance of Transformer

03/18/2012 5:29 PM

The impedance of the transformer has a direct relationship to the physical size. This comes about by the designer being able to "play" with the winding dimensions & core size.

To get a larger impedance in the transformer the designer can increase the height of the coil or make the diameter larger or both. On small pole mounted units it is easy to add in a few extra layers on insulation between the HV & LV windings to get what you want. But in a unit this size it is very delicate balance and to get each of those impedances with what is being asked and it will be a very big transformer. That is if in fact it can be built. It would be expensive as it will have lots of copper & insulation and so also very heavy to transport as the core will also be big.

The customer needs to go back & look at what the transformer is intended to do & if the have fault level problems then they need to review other methods instead of a high impedance transformer. I am sure there is a solution they just need to look at what is more practical & get a transformer designer to help out in this area. They may also find some guidelines in the Standards - IEC is good for that.

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#7

Re: % Impedance of Transformer

03/19/2012 3:23 PM

ddubey

Thank you for the post. It does clear up some misunderstanding

from your initial post.

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