Stability Testing of Tansformer REF Protection Scheme

Stability testing of REF schemes

Stability tests of REF schemes are done to ensure that all wirings and CT polarities are connected correctly. For the REF protection of a winding, 4 CTs are involved, three phase and one neutral (except for auto transformers where sometimes 6 CTs, 3 phase and 3 neutral may be present). Normally during commissioning time single phase current test sources are available at site. So the test procedure using a single phase current source is shown. The figure below shows a typical REF scheme and setup for performing a single phase stability test (REF stability test).

Fig. 1 : Single phase stability test setup for transformer REF protection scheme

In this case the transformer is a DELTA / STAR transformer with Dyn1 configuration is shown. The REF protection is provided for the star winding (Winding 2). The steps involved in the stability testing are listed below;  The sin single phase ase cu current ent sou source is con conn nect ected betw etween een an any on one ph phase ase (“ (“C” phase ase is is >  The > >

> >

> > > >

shown in figure) and the neutral near the grounding point.  The  The cor corresp espond onding ing phase ase to phase ase term erminal inal is temporar oraril ily y shor shortted on the delt elta winding (“BC” in this case).  The  The stab stabil iliz izin ing g resis esisttor is also also temporar oraril ily y shor shortted (or (or set set to zer zero ohm ohms). s). This This is just just to ensure that continuous currents do not flow through it while testing, as some times the testing process may take quite some time. (It is just a good practice and not mandatory). Inject a current of magnitude of alteast Is*CTR (where “Is” is the relay setting and CTR is the CT ratio).  To inje inject ct this cur current ent the sou source shou should ld have ave a minim inimum VA cap capacit acity y of I 2Z, where I is the injected current and Z is the impedance of the transformer in ohms referred to the winding 2 side. Measure the current at Ip, In and Id locations (refer to figure 1). If the CT and wiring connections are correct, Ip should be equal to In and Id should equal to zero (Ip=In and Id=0). Repeat the testing for the other two phases also correspondingly changing the short on winding 1. Remember to remove the temporary shorting after the test is completed.