33kV Earthing / Auxiliary Transformers

[RRaghunath]

We have 132/33kV transformers YNd connected with ZNyn connected earthing / auxiliary transformers (EATs)providing system earthing at 33kV level as well as 415V auxiliary power supplies for the substation.
The EAT is directly connected to the 33kV terminals of the power transformer through a disconnectible link. The connection is through short cables. The transformers are experiencing failures for various reasons and client wishes to provide fuses on the primary of EAT in order to minimise the effect of a fault in the EAT primary on 33kV system and allow the operator some time for planned shutdown of the power transformer (it is well understood that the 33kV system cannot be left in service without neutral earthing for long.)

Would you please share your opinion / experiences on the subject.

Thanks in anticipation. Raghunath

 

[busbar]

Above all, it seems that effort spent determining why the grounding transformer is failing should be the first order of business. There may be some non-obvious voltage/current harmonics that should located and reduced before anything else.

When the grounding transformer is taken out of service, understand resultant effects on cable, switchgear and transformer insulation, potential transformers and surge arresters.

The grounding-transformer connection to the 132/33kV power transformer should be gang operated for protective-relay or manual isolation to limit a resonant effects.

The relaying scheme has to be able to be reconfigured for the ungrounded-mode operation.

Low-side neutral shift during high-side faults on the of the ZNyn bank may have undesired effects.

 

[jbartos]

Suggestion:
Reference: IEEE Std 143-1991 IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, (Green Book),
Page 27 Par. 1.4.5. Ground-Fault Neutralizer (Resonant Grounding). This method of grounding is used primarily on systems above 15kV (i.e. good for 33kV system), consisting largely of overhead transmission or distribution line.