The question of Infeed : Zonal Protection

[mutimuti]

The book (Horrowitz) says even if an infeed exists, we can ignore it for the purpose of Setting Zone one, but must consider it for Zone 2 & 3. Why? Whats the latest trend?

 

[bacon4life]

Typically zone 1 is only set to protect 80-90% of the line, so there cannot be any infeed. When using setting that reach beyond the remote bus, then the portion of line beyond the remote bus will have voltage drop due to both the fault current from the local breaker, and the infeed current from the remote bus.

 

[mutimuti]

In this example the infeed considered a tap somewhere along the protected line. Why cant this be done?

 

[bacon4life]

By tap, do you mean a load that would have no fault contribution, or a generator that turns it into a three terminal line?

 

[mutimuti]

Yes a generator that turns it into a 3 terminal line. Go on.

mutimuti

 

[davidbeach]

On a three terminal line you will have infeed issues beyond the tap and need to account for it in determining your zone 1 settings.

 

[RalphChristie]

mutimuti:

Are you refering to the book Power System Relaying by Horowitz and Phadke?

underreaching zones are set with the infeeds removed from consideration, and overreaching zones are set with the infeeds restored

http://i16.tinypic.com/2hyfozc.jpg

Consider the relay to be located at Bus 1

When the tap current is an infeed (also contributing to the fault) the apparent impedance is greater than the correct value, due to the fact that both lines will contribute to the fault.

E1 = Z1.I1 + Zf.(I1 + I2)
E1 = Voltage at bus 1


Z apparent = E1 / I1 = Z1 + Zf.(1 + I2/I1)

If we set zone 1 setting to be 85% of line length 1 - 2, many faults inside the zone of protection will appear to be outside the zone, and the relay will not operate. This condition must be accepted, since when the tap is out of service, the correct performance of the relay is restored.

Hope it helps

Regards
Ralph

Above is taken from the book "Power System Relaying", with a few changes been made. It seems for me to be almost self-explaining.





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