Relay setting for Radial 115kv tranamission line

[Mbrooke]

I'm somewhat unsure what settings would be applicable for a rural transmission line feeding several tap substations with up to 250E primary trafo fuses. Particularly ground settings. Should MHO still be enabled as its technically not needed? Should I implement fast and slow re-closing curves?


When plotting the curves I'm finding out that a transformer ground fault can trip the main line without clearing its local protection first.

 

[bacon4life]

In some cases, the least bad compromise we found was to allow transmission relaying to miscoordinate with substation transformer protection given how rarely transformer faults occur. The line protection should coordinate with the low side feeder relaying even if you can't coordinate it with the high side protection.

 

[Mbrooke]

Yup- and typically when high side protection does activate its from a secondary fault vs a bolted fault internal to the trafo.

 

[RRaghunath]

I would like distance protection to be set to clear all main OHL faults (including tap-offs) instantaneously. Fast auto reclose will ensure the power is restored successfully and fast for all transient faults.
The distance protection Zone-2 with a delay of 250ms (or more as per the local practice) and zone reach set not to overreach in to secondary side of any of the step-down transformers connected to the OHL will ensure the transformer protection will isolate the faulty transformer before the second trip (and reclose) takes place. The distance protection Zone-1 shall be deselected for 2nd trip following the first reclose.
Zone-3 doesn't have a place in this set-up as it will have to be coordinated with secondary side protection which is unlikely to be distance type.

 

[Mbrooke]

Good advice, thank you. I guess I could let both the line trip and the fuse blow, then reclose after.

 

[DM61850]

All the transformers I have seen that were hooked up to transmission had differential. The distance relaying I saw never saw through the transformer and we just lived with the fact that if a hard fault happened near the high side, both would trip. I don't think though that transformers when they fault, fault bolted just due to how sensitive differential can be set. I guess the winding would have to fault to the core and then to another winding.

 

[Mbrooke]

Normally this application would have differential which would trigger a circuit switcher or in older applications either DDT or ground switch. However due to the rural nature and some customer owned substations several of the transformers are protected via fuses on the primary which present a unique protection challenge relative to distribution.


In distribution and radial sub-transmission phase pickup is set to match the load- up to 200% of the maximum load- while ground elements are set to just coordinate with the largest down stream fuse. I'm debating doing the same here.

 

[stevenal]

Sometimes fuse-saving is employed in distribution, and might be employed here as well. Look at RRaghunath's answer again.

 

[Mbrooke]

I know its kind of obvious- but what advantage is there to using MHO over 50/51 elements? RRaghunath is on the right track here btw


FWIW- the taps are at different locations on the line, near the sub and far out. Though the trafo's own impedance is an advantage here allowing for instantaneous tripping at all points of the T line.

 

[stevenal]

More precise reach. Coordinating TC curves for fuse-saving usually involves some compromise.

 

[Mbrooke]

Got it.

 

[jwatty]

Mho is likely easier to coordinate with relays looking into your substation, too. This is why a lot of generators will choose to use mho elements over voltage controlled/restrained overcurrents.