Absence on zero sequence polarising voltage 1

[HiSet]

Folks recently we had maloperation of a directional blocking scheme.The directional element on the 22kV faulty outgoing feeder did not pickup due to absence of zero sequence polarising voltage even though the relay did record single phase to ground fault current of 500Amp.Could any body suggest possible scenarios where Io could flow without accompaniny Vo ?

 

[Bung]

What is the fault current as a proportion of the total available fault current? Was there enough Vo to allow the directional element to work - if you don't have enough volts the relay won't respond. Have you got a calculated expected value you can compare against?

Bung
Life is non-linear...

 

[jbartos]

Question: It relay electromechanical, electronic - solid state or digital/numerical? This is to narrow down causes.

 

[DiscoP]

Are you using a 5 limb VT ?
3 limb VTs will not produce any Vo

 

[HiSet]

Sorry for not responding earlier.The relay in question is ABB make REF541 and computes Vo internally by vector addition of three single phase voltages Vo= 1/3{Van+Vbn+Vcn}.Single phase to ground fault level on 22kV bus is 400MVA.This perticular fault however was high z.

 

[jbartos]

Suggestion: Visit

http://www.abb.com/GLOBAL/SEITP/sei...trialIT+EMS+-+Power+Management+Handout+A4.pdf

where ABB applied several REF541 on 6kV feeders

 

[jwerthman]

If you are quite close to the zero sequence source, resulting in a low zero sequence source impedance, then the zero sequence voltage may be very low even though the zero sequence current may be very high. As you move the fault away from the zero sequence source the zero sequence fault current will decrease and the zero sequence voltage at the fault will increase. If you are close to the source, it is generally good to use zero sequence current polarizing or negative sequence polarizing if your relay will allow.

 

[jbartos]

Suggestion: The power distribution system grounding schemes should not be overlooked and the relay properly applied to their parameters.

 

[schweiko]

The earth fault protection needs for correct directional discrimination a a minimum Vo voltage. (Typical 50 mV).
I see that you have a very high fault resistance (only 500 A fault current). According to your information the "normal" ground fault level should be 400 MVA. This denotes a very high fault resistance.
In such cases the measured Vo = 1/3 (Vr+Vs+Vt) could be very low, not reaching the minimum required polarizing level.
I suggest for such cases to set the protection for "cross polarization", that means, for phase r to ground fault the s-t voltage shall be used for directional discrimination ( unfaulted phases are less affected by voltage drops caused by the short circuit current).
Best regards