Phase distance relay overreach 1

[HamburgerHelper]

Am I thinking about this correctly? If you have an arrangement with a delta-wye-grounded transformer feeding a plant and a line to ground fault happens on the low side, the phase distance relaying on the high side can overreach to the low side due to load current on the other two phases causing the distance relaying to overreach.


As - phase A secondary - As fault causes fault currents on Aprimary and Bprimary
Bs - phase B secondary - Bs load causes currents on Bprimary and Cprimary
Cs - phase C secondary - Cs load causes currents on Cprimary and Aprimary


Total -

Aprimary + Bprimary (fault current)
Bprimary + Cprimary (Bs load current)
Aprimary + Cprimary (Cs load current)

End result - Aprimary + Bprimary is higher with the load and might cause the phase distance relaying to overreach.

 

[Mbrooke]

Higher load (fault) does not automatically result phase distance over reach regardless the situation. In my calculations the impedance of most transformers is such that Zone 2 and even zone 3 does not over reach. In fact when you want step distance relaying to reach through a transformer you must factor in (add) the transformer's impedance, which usually results in zone 2 and zone 3 reaches that go into many other lines and substations.


But- to be sure more information is needed regarding your system.

 

[HamburgerHelper]

It picked up on a long reaching zone that shouldn't have been able to quite see through the transformer. Most transformers are small enough that they have enough impedance to prevent the phase distance relay from seeing all the way through. I was trying to figure out what was causing it to see further than it should on paper. The relay pickup'd up correctly on what it saw so it isn't a relay mis-operation. Load though on the other phases could cause it to overreach?

 

[Mbrooke]

Has the transformer and line impedance been properly calculated? You say a long reaching zone, is this zone 3? How long is the time delay for that impedance zone?

 

[JJHorak]

The way impedance relays looking into xfmrs are are set, load current is not really a problem. Assume xfmr Z = 9% per unit, and system impedance 1% on the xfmr base, for arguement and also a reasonable set of values. Through fault current is 10 x the load (self cooled MVA).
An impedance relay looking into a xfmr is usually not set to about half the impedance of the xfmr, so it will not see through the xfmr or trip for secondary faults or inrush currents. e.g. the impedance relay is set to 5% of the base impedance of the xfmr, so you need 20 per unit current for it to trip. The load current is negligible in this perspective.

J. Horak, P-R Engineering, Colorado

 

[davidbeach]

Hi John, haven't seen you around in many a year.

Some zones are set to not see through the transformer. Other zones, with longer delays, may be intentionally set to reach through to provide last ditch back up protection.

 

[cranky108]

Actually treating the transformer as a line section with something like a POTT scheme is not a bad idea for backup protection. Thus you can use distance or directional over-current relays for this, depending on the conditions and preferences. Just need to keep in mind that the ground fault will not act the same as a line.

 

[Mbrooke]

It can also be done without a communication assisted tripping system, provided the zone reaching into the transformer has a long enough time delay (ie, 65 cycles), and still a good idea to keep that long delay if you loose communications. While I do not know the details, to me a long reaching zone trip would be from an insufficient time delay. The transformer/GSU local protection must operate first and give enough time for breaker failure and all other elements before a long reaching zone makes the trip command.

 

[JJHorak]

My only notable experience with impedance relays seeing through a xfmr are in large auto-xfmrs where there are line relays on the remote side of the xfmr that can be relied upon to trip in less than a second (likely instantaneously in the reach of the xfmr relay). I seem to recall a case where a zone 3 relay on a tapped line would reach well into a distribution xfmr, and we had to work with the load to check what they had on the secondary.

J. Horak, P-R Engineering, Colorado