Parallel incomers

[KJLRSA]

I have a strange situation at one of my customers, and was wondering if anyone else has seen this before.

I have just completed load and harmonic recordings at the South Africa national broadcaster. They have the 3 incomers with 2 x bus ties between them. These ties are closed and thus the incomers are all in parallel. The CT's on the incomers are identical, and they are all fed from the utility substation about 1kM from the substation.

The Transformers feeding the system are all identical.

When the customer switches off his capacitor banks (directely connected to the busbar), the current only goes up on the 1 incomer.

I would have thought that because the incomers are in parallel the current would have gone up equally on each incomer. We have tested the system and all breakers are in and there doesnt seem to be a problem with any of the CT's.

The problem transmits to the council metering as well, but they cant tell me what the problem could be..

Please help.

 

[Marmite]

You would need to know the configuration and running arrangement of the utility substation to work out why this is happening. How many transformers, how many bus sections, AVC scheme etc. It may be that they don't run with all bus sections closed at their end. You are only seeing half a picture. Also, are the three circuits identical in size, cable type and length? For security they may run on separate routes and have differing lengths/impedances.
Regards
Marmite

 

[slavag]

Hi.
What Marmite said.
Best Regards.
Slava

 

[isquaredr]

Do the 3-feeders share load equally when the cap bank is in?
Is the load at the bottom end of the ct capacity?
I agree with Marmite cable route length and impeadance play a big part in what you are seeing.

It would be unusual for a supply company to parallel his network entirely through customers switchgear, more likely the 3-feeders are tied together at both ends.

 

[KJLRSA]

As far as I can tell from the customer, this situation is a new one. They have been running correctely for over 20 years and this problem cropped up about 6 months ago.

They have 400/5 CT's and are running about 150A at 11kV on each incomer.

3 incomers have identical current readings when cap bank is in.

They havent thought it through but all three cables run next to each other from one busbar (3 x feeders) in the utility sub.

They might have bus ties between the feeders in the utility sub though. I will enquire about the bus ties changing position in the utility substation.

Thanks