Tap position VS Impedance

The percentage impedance of a transformer is the volt drop on full load due to the vector sum of the winding resistance and leakage reactance, expressed as a percentage of the total voltage.


So, if one shifts the secondary taps by 10%, although the voltage shifts by 10%, so does the length of wire in the winding - so the winding resistance also changes, and to a first order, there is no great change in the percentage impedance. To second order there is some effect, as although winding resistance is linear with no of turns, self inductance is not.


Note that if it was a primary side tap that was shifted, then the effect would be a little stronger  than pro-rata with the voltage change , as then the volts per turn are changing, rather than the no of secondary turns. In that case,  when the volts per turn rise (shortening of primary winding) at the same time both the primary coil resistance and reactance are falling - the copper losses in a transformer are minimised if the magnetic core is made to work hard,  (more volts per turn) however magnetic (iron) loss is maximised in that case, so there is a soft trade-off.

If you need to measure percentage impedance, it is worth realising that  the % impedance  is also the fraction of the normal terminal voltage  to cause the full-load current to flow under short circuit conditions (*), and this is an easier test of the copper losses in transformer, as no big dummy loads are needed, as only the loss power, not the load power, needs to be provided. A variable power supply is needed (and for testing 11kV to 400V transformers, the 400V LV mains is about 4%, so mains powered  variacs up to about 125% of normal mains voltage make a good test rig, ) and then only a few kW are required to test transformers that may be in service up to megawatt levels. The magnetic losses can also be tested, separately, by running the transformer at full terminal voltage (energising from either primary or secondary side) but with no load.

(*) In other words, percentage impedance of a transformer is the percentage of the rated voltage applied at the primary winding  to circulate rated current in the secondary winding when the output terminals are shorted.


hope this helps.



PS

Note that distribution transformers with adjustable tappings normally have them on the LV side, where the effect on Z% is small

Primary tappings are more normally encountered on the mains side of equipment designed to be sold in more than one country, and selecting the wrong one either has a devastating effect, if the magnetic core saturates and a destructive current flows, or with the error in the other direction, greatly reduces the max load the transfomer can supply.

 

The reason behind my question is that i am looking into specifying a transformer which happens to be 11kV/433V. I understand that the normal LV voltage for the UK is 400/230. So i am looking into shifting the taps. 

MG:

OMS, I have had a 415V transformer previously, however it is far more better to have a design package which includes for some custom made parts from a specific manufacturer rather than 2-3 different providers with a nightmare of coordination and discussions with them, i just know it wont work.


Surprisingly the 433V transformer is also a bit smaller which helps with the transformer room clearances which are already a problem.

Just a minor point, but the tap changer acts invariably on the HV side winding (and for this voltage transformation, will be off load or off circuit changing).

Apologies - it seems my earlier post was incorrect, OMS deals with more of this HV stuff  for his day job, I have met such things but occasionally, so he will be better informed.

Note then the impedance will vary a bit as the tap changes, but even so only at the 5 % of 5% sort of level. (you cannot alter the volts per turn much, or the core losses go out of whack)


(However, 1960 era transformers with manually bolted taps can be found on the LV side, as at the University they were just that. Equally that transformer caught fire in 1995 apparently, so maybe that is an obsolete way of doing it or it was a  special design for that installation - perhaps physics depts just have some funny requirements)