I have been investigating a series of mal-ops on a protection system. It is a diferential system on a 13.8KV to 6.6KV 12MW delta/delta transformer.
I have checked all the normal things, secondary injected relays, checked CT outputs etc but cant find anything.
I am not used to delta/delta windings, could this cause mal-ops?
The CT's are star connected on both sides, they are earthed at only one point. The CT ratios are correct for the transformer ratio.
The circuit has given problens since new.
Any help or advice would be appreciated.
Thanks in advance.

Arthur, a few questions for you:
. are the differential relays mechanical or electronic?
. the CTs are usually sized for nominal tap. What tap is used on the transformer, and is it an auto tap change scheme?
. do the trips occur with normal load, or is it a stability issue with a large fault on the secondary of the transformer?
. do you have a value of current (primary or secondary) that causes maloperation?


Regards,

Alan.

Alan
The relays are Alstom MBCH12 wired one per phase. Electronic relays.
There is no tap changer.
The mal ops are not linked with any LV fault.
There dosent seem to be any particular load that causes trips.
The load is mainly 6.6KV motors
The 6.6KV system runs earth free with an earth fault monitoring system.
The 13.8KV system has resistance earthing with the max earth fault limited to 40A.
The transformer and cables have resistances to earth in Gohms at 5KV.
The tripps have been on different relays, so there is not one faulty relay. There could be a faulty batch of relays though.
The relays have circuits for inrush restraint and over fluxing. These ciruits are not normaly tested with simple secondary injections. I am considering injecting the relays with an Omicron or similar to prove these circuits.

Arthur,

Testing with an Omicron or equivalent is a good idea, as it will allow you to check that the restraint is working correctly. I don't have an MBCH manual to hand, but it would be worth checking that you have the correct value of restraint set, as well as confirming that the rating / class / kneepoint of the CTs you are using are suitable for the relay. If any of the CTs are saturating with load, that could give you a trip too.

Regards,

Alan.

Thanks Alan
I have the manual for the relays.
The CT's are all class X
If one CT saturates that should not cause a trip if everything else is set up correctly, However I have advised the client that mag curves and primary injections would be worth while.
As the circuit it now re-energised they are loath to have another shutdown.
Thanks for your advice.

Arthur

I take it the relays have been set up to match the transformer? Do you have access to the final settings sheets and calcs to compare?
Differential can also be tripped by phase shift, is there any thing that could be causing that?
Can't remember if the MBCH has any stabilising resistors or metrosils, they may cause issues.
If they don't want to de-energise would it be possible to install a disturbance recorder on the CT secondaries and get some readings?

Kind regards

Donald Lane

When the transformer is loaded the various currents into the relay can be easily measured. It will present a clear picture of the ratio and phase shift requirements. The heavier the loading the better is the assessment.

I have however 2 questions.

1. Is there a tertiary winding?

2. Have you verified the connections of all secondary current circuit from CTs to relays?

Regards
Chris Chew

Gents,
Thanks for your thoughts.
There is no tertiary winding.
I have tonged the CT wiring to check current distribution.
The MBCH does not require metrosils.
The transformer is Dd0 so there is no phase shift in the transformer. The loads are mostly motors so there could be some phase shift during starting.
The disturbance recorder sounds like a good idea, I will look into fitting one.
Thanks