Grading across a transformer

I wanted to verify my understanding was correct, that owing to the delta/star winding of most LV transformers, the fault level seen by the HV protection is 2/3 the fault level(converted to current at the primary voltage) as the fault experienced at LV for both 2phase and 3phase faults.

I came across this info in a power system commissioning book published by theIEE; however am confused that this does not appear to be taken into consideration by software packages.

If correct, then it would suggest the HV time current curve should be moved tothe right of the LV curves when they are displayed together and you are assessing discrimination across a transformer  with the fault immediately downstream on the main LV protective device...




asked 9/26/2012
Rhys Jones2
Rhys Jones

2 Answers

You are a wee-bit incorrect, I am afraid. In a delta connected primary of a delta/star trafo., only the phase current (i.e.) the current thro the windings would be about 56% of the line current. But, the protection system, even on the HV side, would only see the line current, which would be reduced at the HV side, by the turns ratio of the trafo.

Why then, there should be a problem in co-ordination?
answered 4/10/2013 EE65 19
EE65

Not sure where 2/3 comes from; it doesn't sound right to me. I think the primary fault level would be related to turns (voltage) ratio of the transformer. If you have for example a 240 MVA fault on the secondary, it will still be seen as a 240 MVA on the primary. The difference will be in the level of current (the secondary current being much bigger and the actual value being dependent on the voltages).

If you can post an extract from the book, I can try to work out where to 2/3 came from.

On protection curves, you are right - you do need to make sure all the curves are related to the same voltage (which would mean shifting any HV curves to the right on an LV plot).
answered 9/30/2012 Steven McFadyen 246
Steven McFadyen

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