HV/MV transformer impedance

[kokotiko]

It seems that a high transformer (HV/MV)impedance protects the MV busbars from short-circuits in the HV Grid by limiting the short-circuit current.
Accordingly a high impedance value should be desirable in all cases.

What are the parameters influencing the increasing of a transformer's impedance?
Why can't I find on the market (easily) 150kV/20kV transformers rated at 40-60MVA with an impedance of 25% or 30% ?

What is the financial and technical trade-offs in choosing high impedance tramsformers e.g. going from a 15% transformer to a 25% one with the same rating.

 

[ScottyUK]

Higher impedance results in poorer regulation of the secondary voltage which can cause problems during load changes such as motor starts. Higher impedance transformers usually require (expensive) on-load tap changers with high numbers of taps to maintain the secondary voltage at somewhere near setpoint.


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

 

[magoo2]

As Scotty points out, voltage regulation suffers if you go too high in impedance. Once you try to load up a high impedance transformer, in addition to LTC, you probably need a fair amount of switched capacitor banks to be able to hold the voltage within normal limits.

Since transformer impedance is largely reactance, as you load up the transformer, you consume a lot of reactive load (I^2 X).

Admittedly the short circuit duty goes down with high impedance transformers, but the voltage control is more difficult and the need for more reactive supply increases.

 

[waross]

Look at air core reactors to limit your short circuit current. Higher transformer impedance means higher losses and poorer regulation.

Bill
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"Why not the best?"
Jimmy Carter

 

[bacon4life]

In addition to poor regulation, per Sankar's tutorial, "higher impedance gives more winding eddy and tank stray losses and also hot spot problems."

http://www.transformerscommittee.org/info/F01/Tutoria_sankarl.pdf

Transformer impedance is caused by flux linking only one winding and not the other. Normally the windings would be concentric tubes with the spacing between LV and HV determined by voltage stress,etc. In a very high impedance transformer, the windings would need to be further apart to allow leakage flux. The larger diameter of the windings thus increase the resistance losses and increase the amount of materials needed and therefore increases the cost of the transformer.