Unbalanced loading effects

[gokulkrish2]

Hello All,

I have a problem with single phase loading on to a 22.9KV distribution line which i need some help on. Hope you guys can help me.

My System Details:

20MVA 115KV(Wye grouned) - 6.9KV(Delta) transformer connected to a 6.9KV Switchgear will multiple CB's. One of the CB supplies to a 10MVA 6.9KV(Delta) - 22.9KV(Delta) step up transformer which goes as a overhead distribution line. The 22.9KV delta side has a zig zag transformer which dervies a neutral and has a Basler 67N relay which is set to picup at .35A and time dial is 10. The CT ratio on the neutral ckt is 100:5

My Questions:

1. Currently there is no Single phase transformers hooked up on this 22.9KV line. We are right now installing a 5 plex which has a 600A single phase ckt. Our plan is to hook up a 167.5KVA 22.9KV - 120/240V transformer on this line and serve power to the 5 plex. We know this will create an unbalance in the line. But how much of this unbalance will flow through the neutral of the zig zag. What i am afraid of this that all the unbalance will flow through the zig zag and if that is the case 167.5Kva will have 7.3A on the 22.9KV side will is approx .365A and hence basler will pickup. Am i right in doing this assumption.

2. If what i am afraid of is right then what are my options for a solution here?

a. Should i increase the pickup value on the Basler relay? Would it desensitize the ground fault protection on this distribution line?

b. If i do not want to increase the pickup value on the relay what else is my best option. I am thinking of changing the transformer to a 22.9KV-120/208 transformer and balancing the load in the 5 plex. Will this work?

c. Does all the unbalance of the secondary of the transformer carried to the primary side. Does the unbalance reduce/decrease when going through a transformer?

Thanks you all in advance for your help.

Regards,
Gokul

 

[mgtrp]

If you have phase-ground connected loads on only one phase, then all of the current will return through the earthing transformer (presuming that there are no other earthing points on the system). Your options are to raise the pickup on your protection, or to connect the multiplex transformer phase-phase on the primary side. Raising the pickup will decrease the sensitivity of your protection scheme, although 0.35A is a very low pickup setting.

 

[gokulkrish2]

Thanks for the reply mgtrp but I am still not getting it.

Even if i connect the multiplex transformer phase-phase i can only go between 2 phases. Hence there is going to be an unbalance right. Wouldnt that current be flowing in the neutral? Am i missing something here??

Regards,
Gokul

 

[mgtrp]

There will be unbalanced currents, but they won't be through the neutral for a phase-phase connected transformer.

 

[waross]

A line to line connection will not cause neutral current or ground current.
If the 167.5 KVA transformer is connected A to B then the A-B transformer winding will serve about 1/2 the KVA and the B-C and C-B transformer windings will supply about 1/2 the KVA load.
Note that the current through the B-C and C-A windings will be in phase with the current drawn by the 167.5 KVA transformer and so will be either leading or lagging on the B-C and C-A phases.


Bill
--------------------
"Why not the best?"
Jimmy Carter