Transformer Y-Y Problem

[KHALID AHMED]

Greetings All,



we have loop transformers feeding facilities inside our camp ... We have 6 transformers 13.8kv/480v ... The loop is fed from two sources (Breakers) and in the middle of the loop there is a normally open point dividing the loop into two sections. To make it easier for you let's consider it like this: section 1 The power is coming from the breaker to SF6 switch then to the transformers and the same thing for section 2. All the transformers are delta-Y except one transformer is Y-Y. This is the first transformer in section 1 located after SF6 switch.



The problem:



Always Section 1 of the loop is interrupted with a relay target (earth fault) on the SF6 switch and after deeply investigations we have nothing. The interruption happened three times within 4 months and the results are same.



My Question:



What do you think the problem? Is there something happened before with you for a Y-Y transformer.? Is there a specific test you recommend us to do?

 

[Marmite]

Is the star point of the Yy transformer earthed on both primary and secondary?

Regards
Marmite

 

[waross]

You may have some harmonics on the neutral of the primary wye that the relay is interpreting as an earth fault.

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

 

[krisys]

Disconnect the primary neutral from earth and energise.
Don't forget to update us on the outcome.

 

[waross]

The relay may be too sensitive. You will have harmonic current from the wye point of the Wye:wye transformer.
A step change in voltage on the system or an energization surge from another transformer may be causing the harmonic currents to exceed the set point to the ground fault relay.
There are others more familiar than I with relay settings and techniques to avoid trips from harmonic currents.

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