Transformer Impedance

Percent of impedance of a transformer means that, the maximum operation current will flow in the secondary winding if the secondary side is shorted circuited line to line while the primary voltage is raised from 0V to a point which is then compared to secondary voltage. Let's say 5% of 400V. It means 20V.

Therefore, the impedance is directly proportional with the transformer's secondary voltage.

Your question is ambiguous. When you say "% impedance" are you trying to solve a problem in power system analysis using the "per unit" system? If so then you need to know the base KV and KVA of the system and the transformer. If you're unsure what that means then maybe you want to know if the impedance of a transformer varies with voltage, short answer- not when it is operated within its ratings, it is a function of the physical configuration of the windings, laminations, etc. KVAR losses will vary slightly with terminal voltage within specs, but not enough to affect most calculations. If this is a "theoretical"; i.e., homework, question then consult any handbook on transformers, we don't do homework here.

Thanks for the reply.

actually i am doing transformer kVAr loss compensation study where we want to know the variation in kVAr loss with +/-10% voltage variation of the transformer

Thank in advance

You have not made it clear the situation

1. Transformer on fixed tap, ingoing voltage change +/- 10% OR
2. Transformer taps automatically changing to accommodate +/- 10% change of ingoing voltage. OR
3. If voltage variation is at input or output of transformer (by output loading change).
4. The size of the secondary load and its daily and seasonal variations. Also Ambient temperature, insolation, windspeed variations which affect oil temperature and winding temperature.
5. The pattern of voltage variations coincident with the daily/seasonal load and climate variations.
   

I suggest you need tests on a representative transformer (in service), leading to a validated theoretical model of the transformer losses. You then apply the nominal voltage, load and climate variations etc to the theoretical model, using the chosen pattern of ambient variations.

Apologies, you wrote kVAr losses, so I²r losses will have an effect on KVAr which is negligible, but the remarks about real voltage and load variation patterns are relevant, since I am sure there is a cost benefit factor in your study.

Msamad,

"For a transformer, the voltage ratings and the % impedance are fixed values...".

For transformers which have OLTC, during the design stage, we will also take it for grant?

I think we're getting a clearer picture of the goal of the study. It sounds like you are trying to select a standard design for a distribution transformer that is going to be installed in multiple locations, or a few very large transformers such as Generator Step Up (GSU). In either case the term "KVAr losses" can cause confusion because KVArs are lossless except for the I**2 R losses that they cause by the flow of the current through the windings as a result of the cyclical magnetizing/demagnetizing of the core, eddy current flow in the laminations, internal support structures, tank, etc., all of which show up as heat and noise from the unloaded transformer. The seasonality effects of environmental temperature are negligible since the resistivity change of the materials varies so little under these conditions (we're not concerned with the external temperature since that only affects the dissipation rate of the energy which in turn determines the temperature rise of the transformer). However the no load kW loss, which does have a fuel cost associated with it, varies according to the square of the voltage, from 0.9 squared to 1.1 squared or from 81% to 121% of the no load loss at the rated/nominal voltage. Now go apply that to your diurnal/seasonal voltage profiles and integrate the cost over a year. Isn't the per unit system handy?!

I have been kicked 4 times for being "off-topic", but there were two original questions :-

1. How does % impedance vary with transformer voltage? Since it has been stated by vinodk in #8 that the service is wind generation from 0.69 up to 33 kV, it is clear "variation of impedance with design voltage" is not the issue. Also it is clear from posts, that for a given transformer, the variation in series impedance is negligible.
2. How are kVAr losses affected by % impedance and variation of voltage?

For the second question, one has to address what "kVAr losses" means. In AC power systems, generation of lagging VArs is usually considered + and good (because all the essential transformers and motors draw lagging current which must be supplied). There are two "losses" of lagging kVar in the transformer, first, the lagging magnetising current (which will be proportional to voltage). Secondly, the series impedance (mostly inductive reactance), which will make the lagging power factor worse. If one is using induction generators, the whole generator/transfo system is a "kVAr loss". The actual amount under varying voltage/wind conditions will need an equivalent circuit for the system, transformer and machine and my post #9 was intended to help with this