Station Service Transformer (SST)


Thread Starter


In Thermal Power plants, Station Service Transformers (SST) are normally solidly grounded. But Generators are grounded through NGT. Why? Can anyone pls tell me the reason behind it?

Phil Corso, PE

Responding to GY’s 23-Jul (20:13) query… it has to do with equipment replacement cost(s) associated with electrical failure! Please note that collateral damage (harm to personnel or lost revenue) is omitted in the following discussion:

Many agree that electrical failures begin as earth-faults! Then, how does economics influence the earthing method? Furthermore, why are they different for different voltage levels? Discussion then centers on solid vs some form of limited earth-fault current scheme:

A) Solidly-Earthed (e.g.: SST)
The typical Station Service Transformer’s (SST) secondary fault levels are in the order of thousands of kVA. Earth-fault currents resulting from solidly-earthed neutrals are high enough to operate fuses and circuit breakers protecting low voltage cables and utilization apparatus. Separate earth-fault protection devices are not necessary, except when fault currents are too low!

B) Limited Earth-Fault Earthing System (e.g.: NGT)
Generators and other apparatus installed at higher voltage levels are exposed to much greater fault energy… in the order of thousands of MVA. Earth-fault currents could damage iron structures in generators, motors, and transformers, so that they can’t be repaired, but instead must be replaced… at great cost! Hence, some method of current limiting, like NGT (Neutral Grounding Transformer) or NGR (Neutral Grounding Resistor) is beneficial!

GY, additional information or detail can be furnished if required.

Regards, Phil Corso ([email protected])
Dear Mr. Phil,

Thank you very much for your kind reply. I got some idea about Broken-delta and Open-delta configurations. Kindly send me more details if possible.

Thank you once again.

Phil Corso, PE

The reference I recommend is my teacher, Professor J. Lewis Blackburn's text, "Silent Sentinels" first printed in 1957! It was subsequently reproduced as "Applied Protective Relaying" by the Relay-Instrument Division Of Westinghouse Electric Corp!

Sorry, I don't have an ISBN number!

Regards, Phil Corso