Neutral Displacement

[neps3]

Hi

The VT used on a 11kV generator SWBD bus has 11/SQRT3 / 110/SQRT3 // 110/SQRT3. The third winding is an open delta used for neutral displacement relay. Usually generators have 11/SQRT3 / 110/SQRT3 // 110/3. However, a gas turbine set have the 110/sqrt3 for the open delta.

What setting does these relay usually have. Is it 10V or 20V or in terms of % 5% or 10%. When a phase is down the voltage can rise to 3 times.

Any comments please.

 

[busbar]

 
ANSI/IEEE Std C37.101-1993 Generator Ground Protection §5.6 Method VI: High-resistance grounded (grounding-transformer grounded) ..uses three distribution transformers whose primary windings are connected to the generator leads in a wye configuration, while the secondaries are connected in broken delta configuration with a resistor. These transformers must have their primary voltage rating equal to the line-to-line voltage of the generator.

Appendix A discusses voltage-relay settings as a function of percentage of stator winding protected, VT-primary fuse coordination {and assumedly limiting false relay operation.}

 

[neps3]

busbar

Thank you. I do not have the said standard.

Thank you kindly anyway.

 

[RRaghunath]

If the VT secondary is rated 110/sqrt3 (in place of usual 110/3) the voltage seen by relay during earth fault will be proportionately higher i.e. 173% of 110V. To protect 95% of the generator windings with a 110V input rated relay, you may set the relay at 5.5Vx1.73 = 9.5 or 10V.

The higher expected voltage to the relay (at the time of fault) may not worry as these relays have short time withstand capability that is much higher.

Trust the above is useful.

 

[neps3]

Mr.Raghunath

Thank you for your valued contribution. Agree with you.

Thanks

Nath