Transformers at low load and impact on power factor

[Mark Chris Lewis]

Guys hi,

Looking for a quick and simply response to a technical issue regarding overall site power factor.

Situation. 3km of 132kv cable feeding a 56MVA transformer (132/11.5/11.5). 30 metres of 11kv cable connected to a 11kv switch panel. The load at the 11kv switchable is 3MVA. Local meter also states as pf of 0.95. However upstream (3 kms away) DNO meters are reading 0.6.a) does this sound right if so, is it down to the lightly loaded transformer.

Lastly'I had a loadflow analysis done in house which kicks out a figure of 0.95 at supply and load end! Confused.

 

[davidbeach]

Leading or lagging? What your loadflow analysis isn't accounting for is the capacitance of the cable. If you disconnect the high-side of the transformer and leave the cable energized, you will see current into the cable with a power factor very close to 0 leading.

 

[Mark Chris Lewis]

David hi thanks for the response. The 0.6 of figure at the source end is lagging. This figure is what I expected. Because of the mag current. Yet the loadflow model suggests it should be 0.95. which I don't understand. The issue is that meter operator is certain that their reading of 0.6 is correct, but our loadflow model says different!

 

[mgtrp]

The charging current for 3km of 132kV cable should swamp the magnetizing current of the 56 MVA transformer. It should ruin your site power factor even when operating under load. Therefore, I'd be fairly sceptical of your load flow results showing the same power factor at the DNO and at the load--perhaps check the cable data that you've entered.

Similarly, through, a lagging power factor measured by the DNO doesn't sound right either. They should be seeing a leading power factor due to the cable, unless there's some other load/equipment connected that you've not mentioned.

Cheers,
mgtrp

 

[Mark Chris Lewis]

Thanks.v