Micom 633 trafo Diff Relay

[bdf5526]

Hi all,

Once again I need a advice from gurus here. we need to prove the transformer differential setting to our client. Relay used to protect 2 winding trafo. Diff trip works fine when we inject only at HV winding (winding a)relay CT terminals (single pahse injection and 3 pahse injection). And it is also works exactly when inject on LV (winding b)side relay CT terminals, during our secondary injection test. During this secondary injection test we proved the tripping current as per the setting in the relay.

Trafo data as follows:
25Mva
21kv/6.7kv (winding 'a'/ winding 'b')
21kv side CT ratio= 1000:1
6.7kv side CT ratio= 2500:1
Vector Group of trafo: Dny11
HV side FLA: 687 Amp and relay Ireff = 687/1000 =0.687
LV side FLA: 2154 Amp and relay Ireff = 2154/2500= 0.861

Tripping setting for Idiff> is 0.3 x Ireff

HV side relay CT terminla injected a bit more than (0.3 x (0.687)/1) = 0.206 Amp (after vector group matching factor kompensation ) and the relay triped

LV side relay CT terminal injected a bit more than (0.3 x (0.862)/0.58) = 0.445 Amp (after vector group matching factor kompensation ) and the relay tripped.


Now the client wants to see the stability of the differential. Well we were hoping the relay will not trip if we inject value above the tripping value on both side of the HV and LV winding withe tripping current which the relay tripped. We inject 30 degree lead of current at terminal Hv side at the relay. Surprisingly the relay tripped. We do not know how we can prove this stability of the differential.

Do we need to inject 3 phase on both side with HV side 30 degree lead to prove the stability?
Do we really prove this stability? during the current injection on both side to prove teh stabiliy we injected only on phase L1. could be this is the reason?

Thanks for the explanation guys

 

[rasevskii]

@bdf5526:

You should do a primary test. By injecting on the HV side with the LV side shorted, using a 400V 3 phase supply from a genset, you will get about 20% of rated current (assuming 10% impedance of the trafo) That would be around 140A. With all CTS and the relay in service postion, the relay should be stable. Measure all secóndary currents and the spill current to confirm all is correct. Then inject on the wrong side of each primary CT on each phase at a time, the relay should operate.
(If you have bushing CTs this will not be possible, however.)

Shorting/unshorting of the CT secondaries to obtain a trip under above conditions may or may not work, depending on how the Micom circuitry operates. Dont do this with the test current ON...

This is the usual way to test trafo diff protections on site.

BTW was that Dyn11 not Dny11 ?

rasevskii

 

[bdf5526]

Hi rasevskii,

Thanks for the reply. It is a Dny11.

We are testing the protection relay and all the secondary injection done from relay terminal at relay cubicle.

Soon maybe I will try to inject it with 3 phase current supply and have monitor the differential. But I do not know if it might work.


Energizing the transformer will be in 1 month time and client wanted to have this differential proved.

Or maybe I could change the configuration of the transformer in the relay to vector group '0' and try to ionject current with same phase angle? Than I need to calculate the new equvalant Idiff trip setting value with vector group '0' matching factor.