Transformer 30/0.4kV unbalanced currents 2

[LMSANZ]

Hi,

We have a distribution transformer 30/0.4kV 1.6MVA, which has the primary currents unbalanced. Secondary currents looks balanced +/- 80A. The transformer supply to MCC which has motor an single phase loads.

We've made insulation test 5000V 1 min, on primary side and the results are GOhms.

What can be the reason?

See attached picture.
Regards

 

[LMSANZ]

I found this paper from IEEE, where it seems that the protection 51N can start detection false trips due to the unbalance currents. So it seems to me that 51N zero current calculations don't distinguish between ground fault or unbalance current?

 

[HamburgerHelper]

Relaying can't distinguish residual load caused by load imbalance or a fault. You have to make the distinction by setting the pickup so that it never operates under imbalanced load.

Zero sequence current can't pass through the delta on the high side so all of your residual current is going to be coming up the ground on the wye low side.

You are going to see negative sequence currents and voltages on the highside due to imbalance. Positive and negative sequence quantities will be sourced from the high side through the transformer to your load. With a mixture of positive and negative sequence, the phases will all add up to zero but the angles between the phases will no longer be 120 degrees.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[waross]

Does the transformer have taps? A wrong tap setting on one phase may be the cause.
It's possible that one lead is on the wong tap.
A more remote possibility is that the taps were connected wrong at the factory.

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

 

[LMSANZ]

Well, the transformer has taps, +/- 2.5%, 5%, but the tap is common for the three phases.

I understand that Zero sequence can't pass through delta winding, in the case of earth fault in secondary. But what happen if there is a starting fault in any of the primary windings of the transformer? Zero current will be present at primary line currents, at swgr level? . The relay is detecting residual current, but don't know if due to the unbalanced currents (bad CT in phase L1) or sth else.

 

[rubron]

@7anoter4 In your calculation, whv/wlv=30/(0.4/sqrt(3)=129.9, but I would say it is rather whv/wlv=30/0.4=75 since, to my knowledge, the ratios of the transformers always are given referring to line-to-line values.

 

[waross]

The voltage on each of the wye connected secondary windings is 1/1.73 of line to line voltage.

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