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Transformer impedance
3

Transformer impedance

Transformer impedance

(OP)
Is there any way to change transformer % impedance by some external means? (Like by putting some external impedance in circuit)  

If not then why?

RE: Transformer impedance

No. Adding impedance external to the transformer does not change the impedance of the transformer, it changes the circuit impedance.
Regards
Marmite

RE: Transformer impedance

2
Yo may use an air core reactor in series with the transformer to increase the reactance of the circuit reduce the Available Short Circuit Current downstream of the reactor. This will not affect the ASCC at the transformer terminals but it will allow lower rated switchgear to be used downstream.
The poor man's version is to calculate the cable impedance and use enough length of cable to reduce the ASCC at the switchgear.
I have worked to specs that had wording such as:
The minimum length of cable used to feed unit subs shall be 100 feet. Excess cable shall be doubled back in the cable tray.
Does this solve your issue?
To split hairs, the PU impedance of a transformer may be changed by changing the KVA base. The actual impedance doesn't change so if the KVA base is changed, the PU value changes.  

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

RE: Transformer impedance

What the voltages and the size of the xmfr by the way?
 

RE: Transformer impedance

How about adding a frequency converter either side of the transformer?  I presume that would change the impedance based on x = 2(pi)fL? blllttt

RE: Transformer impedance

(OP)
Thanks for all your replies. Transformer is 20MVA 66/22kV.

My intention was to ask the manufacturer some sort of arrangement (like tapping some part of winding and the bring it out of body as some contacts where we can add some Independence by some means that can change the %Z value. May be I am thinking a aloud but my aim was to device a method where engineers can play with %Z at site/ substations and there is no need to go back to manufacturer for any future alterations.
  
Thanks,

ITE

RE: Transformer impedance

Playing with impedance in a working substation may have unintended implications for protection and control. If you must change impedance on the fly, consider an external, tapped, air core reactor with shorting contactors to change settings.
Air core reactors are often used to reduce the available fault current to a switchgear lineup.

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

RE: Transformer impedance

Why are we discussing this? You don't play around with 20MVA, 66kV equipment or any other electrical equipment for that matter.

What Marmite said was correct, to the point and adequate.

Rafiq Bulsara
http://www.srengineersct.com

RE: Transformer impedance

Quote:

(Like by putting some external impedance in circuit)
I have installed a number of air core reactors reactors to do just this.
Your point is well taken, Rafiq, we weren't playing and this is not something to play with. Design it, install it, and forget it.

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

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