Transmission ground distance relaying with distribution underbuild

[stevenal]

How do you handle the mutual coupling?

1. Ignore it.
2. Carefully model the distribution circuit(s) and mutual(s) in your fault program.
3. Model just the distribution neutral in your fault program.
4. Measure the line impedances including the mutual coupling with test equipment.
5. Use actual fault data to estimate the parameters.
6. Avoid using ground distance. (In favor of?)
7. Something else. (What?)

Thanks for sharing.

 

[System Protection]

I model it all in the line constants program that I use as its not much more effort to add the underbuild.

But I don't input the mutual impedance between transmission conductors and underbuild into my fault sim program as the distribution system is not modeled.

Definitely would recommend modeling the neutral conductor though.

I usually set my ground distance reach quite a bit less than the phase distance reach to account for modeling errors.
Coordination is much nicer with ground distance; I wouldn't recommend going without.

If you have transfer trip or a differential then you could use ground distance as a backup only if you had concern over your model.

Verifying faults with known origin can be used to verify your model; there is an SEL paper on it somewhere.

 

[stevenal]

SP,

How much less?

Yes we are using differential protection, but wish to have step distance protection if/when the differential channels fail.

Davidbeach please chime in.

 

[System Protection]

Typical would be 85% reach for Phase distance and 70% reach for Ground distance for Z1.

 

[davidbeach]

The distribution phase conductors have very little to do with line impedance, but that neutral is just line any other "shield" wire. Include it in your parameter calcs the same way you would an overhead shield. Also, don't blindly assume rho earth is the conventional 100 ohm meters. I find that 10, here, gives better results. Others have found that on essentially solid granite something more like 1000 gives better results. Our "real" Z0 is around 80% of what would be calculated just looking at the three transmission conductors and assuming 100 for rho earth. But it's a line variable, not a line constant. Soil moisture plays a big part in the actual value; Z0 in August and Z0 in March can be very different. Calculate the March value so you don't over reach and accept the shorter reaches in August.

Where the mutual coupling between the underbuilt distribution and the overbuilt transmission gets really fun is when a distribution ground fault induces circulating 3I0 is a transmission loop. Can totally confuse ground directional elements that expect a common source for the I2 and I0 currents since the I0 is just induced and the I2 comes from the low-side of the delta-wye transformer that's feeding the ground fault.