Distance Relay GF Protection on Impedance Gnded System

[nightfox1925]

We have short transmission lines fed from an upstream power transformer with a 200A NGR secondary system.

The consultant has provided Ground Impedance settings for the distance relays on the load end substation. Is the ground impedance settings on the distance relays going to work or is applicable if we have a 200A NGR system?

If there is a L-G fault on the system, the fault current is further limited also by the arc resistance plus the tower footing resistance (assuming an insulator flashover occurs). Then the total fault resistance may all be well outside the impedance circle.

I got doubts on these though a general guidance will be appreciated since the GF arc resistance effects on distance relays on a system with NGR is complicated for me to understand correctly.

Our distance relay is a SEL311C

 

[buskos]

If I understand well the neutral point of Transformer is earthed by impedance te reduce earth fault at value 200A. Than the line from transformer to bus load is short line.I dont know level of voltage for line. Of course distance protection will consider Arc resistance and resistance of tower wich depence from country and standard is approximately 15 Ohm. The arc resistance you have to consider by warrington formula. You have to selected not MHO charachteric of relay but independente selection of R and X that mean rectangular characeteristike.

 

[davidbeach]

Operation of a ground distance element is not affected by the presence or absence of an NGR provided there is enough fault current for the fault detectors to turn on.

 

[ePOWEReng]

Ground MHO will work with NGR. Try modeling this in a fault program and see where the fault fault out on the MHO. The presence of NGR doesn't make a difference.

 

[ePOWEReng]

Sorry I meant to see where the "fault plots out on the MHO".

 

[nightfox1925]

Does this mean that the ground impedance settings should be set higher than the ground fault resistance + arc resistance + tower resistance so that when an arc flash over occurs along the suspension insulator to ground, the ground distance will be able to pick this up?

 

[ePOWEReng]

It appears that there are some security issues for the MHO ground element in this application. SEL has a good technical paper on this subject "Application Considerations for Distance Relays on Impedance-Grounded Systems".