Stator Earth Fault Protection of a Generator

G

Thread Starter

GY

I work in a 210MW thermal Power plant. Here, the Generator is grounded through 50KVA, 15.75KV/240-120V NGT. The stator E/F relay used for 95% to 100% of the stator windings protection has got "Broken Delta arrangement". My question is, how does it differ from "Open Delta configuration"? What are the benefits of using Broken Delta? Please send me more details about this. I'm new to this concept. If possible tell me more details about Broken Delta & Stator E/F detection.
 
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Phil Corso, PE

Responding to GY’s 23-Jul (20:16) query about potential transformer (PT) connections for generator E/F stator protection:

The broken-delta connection uses three single-phase PTs. All three primaries are connected between line and earth (ground!) Their secondaries are delta-connected, except that one corner of the delta is left open-circuited. This arrangement is also called a residually-connected circuit. Its principle benefit is that the residual voltage is much higher than that of the open-delta connection (described in next paragraph!) Thus, it is more sensitive to earth-faults.

The open-delta connection uses two single-phase PTs. They, too, provide 3-phase transformation, but the resultant secondary voltage magnitude is about 1/3 that of the broken-delta arrangement. Furthermore, secondary-circuit failures that could negate proper fault detection are virtually ignored!

Regards, Phil Corso, PE ([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.
 
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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 Elecric Corporation! Sorry, I don't have an ISBN number!

Regards, Phil Corso
 
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