Distance Protection with Zero Ph. Sequence Throttling

[Vegemite]

There is this radial 132 kV line which is part of a solidly earthed transmission network. It is a teed radial line thus:
- source = substation A
- tee point = T located 16 km from A
- first load substation = B is 0.6 km from T
- second load substation = C is 8.5 km from T.
There is no generation at either B or C and T is simply the tee-off point on the line. The only significant load on the line is a 15 MW cycloconverter at sub B 11 kV which I am told is non-regenerative.

There is a YNd 132/11 kV transformer at sub B with a solidly earthed Y winding. The transformers at C are YNynd but I believe this makes no difference to my question.

We want zone 2 to reach to 120% of the length A - C. For earth faults at C the following currents flow:
- 1200 Amps from A to C
- 800 Amps from B to C (this is the throttling current).
For phase faults all current flows from A and none from B.

The formula generally recommended to account for a source of generation (or grid infeed) at B is:

Zone 2 = 1.2[Za-t + {Zt-c {(Ia+Ib)/Ia}] where:
- Ia is the fault current from substation A for a fault at C
- Ib is the fault current from substation B for a fault at C
- Za-t is the line impedance from sub A to the tee
- Zt-c is the line impedance from the tee to sub C.

In applying such a formula to the earth reach point should I be using all zero phase sequence quantities or a mixture of zero phase sequence and positive phase sequence?

PS Thank you jghrist and rlv for your helpful advice and information about the earth resistivity of salty areas.

 

[alehman]

Unless this is a ground relay, positive sequence only. Ground protection must be handled separately.

 

[Bung]

Another reminder of why I always tell our planning people that tees are impossible to protect. They don't believe me, but they are usually not willing to argue and instead take me at face value on it!

It is likely to do funny things to the RCF, so the earth fault distance element is always going to be a bit of a compromise. All you can really do is model it and calculate the "Z seen" for a number of different cases (including faults along the lines) and try to reach an RCF figure that works. If you only have transformers at B and C you might be lucky.


Bung
Life is non-linear...

 

[mc5w]

For a wye primary delta secondary transformer you do not need to earth the primary neutral - by all means it should be isolated from ground and the primary system neutral.

Same for wye primary wye secondary delta tertiary transformers - DO NOT ground the primary neutral but do ground the secondary neutral.

If you have to do so, ground the primary neutral using a resistor or air core inductor with a rather hefty rating.

3-wire wye primary (transformer neutral fully insulated) 3-wire or 4-wire delta is a common distribution step down here in the US.

Mike Cole, mc5w@earthlink.net