Impedance relaying through a transformer
Impedance relaying through a transformer
(OP)
Ok, lets say you need to provide secondary short circuit and internal fault protection for a 115kv - 34.5kv D-Y transformer on a radial transmission line supplying a rural distribution substation. Would it be better to to use step distance relaying with an over reaching zone through the transformer or over current elements instead? Anything special to consider when adding the positive sequence impedance of the transformer to the positive sequence impedance of the transmission line or is it way more complicated?
The transformer in question is 25/35/40 MVA with an R of 0.01363 and an X of 0.43186
{Yes I know people will ask why not send a DTT signal to the supply station when differential kicks in, but I'd like to have redundant protection if the PLC fails}
The transformer in question is 25/35/40 MVA with an R of 0.01363 and an X of 0.43186
{Yes I know people will ask why not send a DTT signal to the supply station when differential kicks in, but I'd like to have redundant protection if the PLC fails}






RE: Impedance relaying through a transformer
RE: Impedance relaying through a transformer
RE: Impedance relaying through a transformer
As for setting it, impedance is impedance. Put the transformer into your fault model and place a fault on the low-side bus. If the transformer is tapped from a line where one end is a much stronger source than the other, you may find that you need to let the strong end clear before the weak end can do anything. It can be a bit of a double-edged sword, look at a bunch of cases and see what seems to work and what might present an undesired level of risk.
RE: Impedance relaying through a transformer
FWIW the 115kv line is radial, but the source does vary in strength depending on generation dispatch.
RE: Impedance relaying through a transformer
For radial lines into transformers it becomes even easier. We try to set a zone 1 that covers the entire line but doesn't reach through any of the transformers. If one of the transformers faults the line and the transformer should trip together and the transformer should be gone when the line recloses. Then you can have a distance element that reaches through the transformers without the worry about how many additional lines it covers seeing as how there aren't any more lines beyond the far end. Those are also the lines where the ground distance can be set the most sensitively since there's no worry (in our system where all the transformers in question are Dyn1 type transformers) that block low-side ground faults from being seen by the ground distance elements on the high-side.
RE: Impedance relaying through a transformer
RE: Impedance relaying through a transformer
Just a consideration, as 120 cycles for a zone 2 would be unacceptable in my case. Unless you are referring to a zone 3 on looped lines. I guess this delay is what made me confused.
There are 34.5kv lines beyond the far end, so any fault on the 34.5kv system must coordinate with what ever reaches through the transformer. Ie, a 34.5kv bus fault or 34.5kv line fault right in front of the substation must be cleared by the 34.5kv breakers before the 115kv protection elements decide to trip.
RE: Impedance relaying through a transformer
But in this case I'd like to have zone 2 reach through the transformer and trip for faults right in front of the transformer.
The transformer does have differential, but it relies on PLC for DTT. If the PLC fails, I'd still like to have a last resort fail safe.
RE: Impedance relaying through a transformer
Or use zone 1 reach your transformer, and zone 2 for the 80%.
If you had communications, set up a POTT scheme.
RE: Impedance relaying through a transformer
RE: Impedance relaying through a transformer
Since the transformer protection trips the remote line end, there is no harm in reaching into the transformer with zone 1 as Cranky suggested.
RE: Impedance relaying through a transformer
But my question is, what if I set zone 1 to 125% of the line and Zone 2 at 70-80 cycles?
RE: Impedance relaying through a transformer
RE: Impedance relaying through a transformer
Last thing regarding the furthest reach, I just add 100% of the positive sequence line impedance + 100% of the transformer positive sequence impedance x 1.25 and I am set with my zone 2 distance value? I saw 80% mentioned, but in all honesty I am a bit clueless how that number works out.