Transformer impedances

[tommom]

Is there a place I can get an IEC standard impedance for a say, 20 MVA, 110 KV-11 KV transformer, 50 Hz, for use in the Middle East, without laying down 3500 bucks for the IEC transformer collection? Thanks.

 

[dpc]

You can probably get the information from a transformer manufacturer.

 

[lume7006]

Hello,

At least from experience I think that the typical value for the size and voltage should be around 9%.

However, I think you may ask directly to a manufacturer.

In general, the value will depend on several factors like: shortcircuit level desired at low voltage switchgear, regulation voltage and so on.

Best Regards,

 

[wareagle]

I did a Google for "standard impedance for a say, 20 MVA, 110 KV-11 KV " and got a number of hits. Range of impedance was 9% to 13%.

 

[prc]

Transformer impedance is decided by utility based on their past legacy.Since parallel operation is required,they stick to the previous adopted values.Some countries go for high impedance (eg UK ,like 20 %) with extensive tap range to take care of high regulation consequent to higher impedance.Some utilities prefer low impedance,taking care of the higher break capacity required in their breakers and higher Sc withstand capacity necessary in transformers.Manufactueres can give an impedance band of 6-20%, but general range is 9-13 %.

 

[RRaghunath]

Shell specifies 10% impedance for 12.5MVA to 25MVA range. This is also IEC stipulation (IEC76).
It is better to stick to IEC stipulation, unless there is a specific case where the secondary fault levels are not acceptable to the designer.

 

[dipankur]

As per Indian Standard 2026 Part 1, typical value of %Impedance for 12.5 to 25 MVA transformers is 10.0%

IEC might also be the same, because the clause 0.6 of the above standard states that "This standard has been based on IEC Pub 76-1 Power Transformers Part 1 - General"!!