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Parallel operation of distribution transformers causing EF trip

Parallel operation of distribution transformers causing EF trip

Parallel operation of distribution transformers causing EF trip

(OP)
Hi everyone,

We have a situation where we have troubles with parallel operation of 11/0,4 kV transformers.
The transformers are identical, same manufacturer, same ratings and impedance and operating on same tapping. Both are Dyn5 connected.
During parallel operation we see difference in HV side currents between phases up to 15% of the load, equal on both transformers. This has caused a trip of earth fault protection on HV side. When either one of the transformers is disconnected the phase currents are balanced immediately, so it is the parallel operation that causes the current imbalance (not the load).

Could the imbalance be caused by different regulation between the transformers in only one or two phases? If so, what could explain the different regulation?
If there was an difference in impedance between the transformers it would have same effect on all phases, not only one or two, correct?



Best regards

jkristinn

RE: Parallel operation of distribution transformers causing EF trip

If the high side is truly delta, those EF relays can't see any current without a fault on the transformer. The phase currents may well have different magnitudes, but they are also not exactly 120 degrees apart either. The three currents have to vectorially sum to zero.

A bad CT or a loose connection might get you there, but both in parallel would seem to make things better (less current) than with one transformer.

Something about the actual installation doesn't match the sketch. Hard to say which of many possibilities. Too bad you don't have a smarter relay with all three currents brought in individually so that you'd have an event record to see what really happened.

RE: Parallel operation of distribution transformers causing EF trip

What is your current reading at the primary side? What is the current rating of your primary CT's? Looks like when you split your load in 2, less current goes through the CT's and some precision is lost. May be the GF settings are too sensitive?

RE: Parallel operation of distribution transformers causing EF trip

Two issues here;
1> Unequal currents.
2> Relay tripping.
You may have slightly different phase impedances in the connections to the bus bars. If one transformer is connected close to a large load and the other is connected some distance from the load connetion, you may have unequal reactance in the bus bars. Given the low Ohmic impedance of a transformer that size, it wouldn't take much difference in phase to phase bus or cable impedance to affect current sharing between the transformers.
Check the operation of your relay.
We expect the relay to respond to a non-zero sum of the phase currents.
However there are some relays that respond to unequal currents. These relays are common in refrigeration applications.
If a phase unbalance relay has mistakenly been installed in place of a ground fault relay, it could trip on a 15% current difference.

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

RE: Parallel operation of distribution transformers causing EF trip

You said the power transformers are identical, but what about the CTs?

The two issues might have a single cause. Ampere's Law works both ways, so a CT secondary poor connection can affect the primary. Not much affect when operating independently, but it doesn't take much added impedance to divert the current when a parallel path is available.

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