Impedance relaying through a transformer

[mbrooke]

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}

 

[NewtonLaw]

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}

I would think the cost of simple transformer differential directly on the transformer and the closest OCPD's would give the least cost, best solution for internal transformer fault wrap in simple overcurrent on the CTs to trip low side protection. Zone distance relaying with permissive overreach is more prone to miss-coordination in my experience and poor at transformer internal fault protection. Hope this helps.

 

[mbrooke]

I would think the cost of simple transformer differential directly on the transformer and the closest OCPD's would give the least cost, best solution for internal transformer fault wrap in simple overcurrent on the CTs to trip low side protection. Zone distance relaying with permissive overreach is more prone to miss-coordination in my experience and poor at transformer internal fault protection. Hope this helps.

True, but the closest OCPD is 2 miles at the supply substation. There will be a differential relay that sends a DTT signal down the line, but if that fails and the supply breaker does not "see" the fault Id rather not think of the outcome. But none the less your input does help :happyyes:

 

[rian0201]

id prefer overcurrent relays.


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[Bugman1400]

You can set distance, OC, or both. If your main sub feeding the radial has SCADA alarms for the loss of the PTs for distance then, using distance only is preferred. The distance allows for stepped coverage of the line and backup coverage of the entire xfmr. It would also be independent of how strong your source is at the main sub. The OC protection is beneficial because it doesn't require the PTs (or if you don't have SCADA alarm for a loss of PT) and you can set the pickup to some x percentage of your xfmr rating or known load. IOW, your xfmr may be rated at 40MVA but, you know that it is under-loaded the whole year to a max of 22MVA, for example. The downside to this is that it is dependent on a stable source. If for some reason during the year the main source sub is weak and the xfmr is supplying a light load, a lowside or mid-xfmr fault may go undetected by the OC protection.

 

[mbrooke]

You can set distance, OC, or both. If your main sub feeding the radial has SCADA alarms for the loss of the PTs for distance then, using distance only is preferred. The distance allows for stepped coverage of the line and backup coverage of the entire xfmr. It would also be independent of how strong your source is at the main sub. The OC protection is beneficial because it doesn't require the PTs (or if you don't have SCADA alarm for a loss of PT) and you can set the pickup to some x percentage of your xfmr rating or known load. IOW, your xfmr may be rated at 40MVA but, you know that it is under-loaded the whole year to a max of 22MVA, for example. The downside to this is that it is dependent on a stable source. If for some reason during the year the main source sub is weak and the xfmr is supplying a light load, a lowside or mid-xfmr fault may go undetected by the OC protection.

Great answer and makes perfect sense!

Should I do one zone of protection or multiple zones like with looped lines?

 

[mbrooke]

id prefer overcurrent relays.


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125% of the OFAF FLA? Should I consider negative sequence?

 

[Bugman1400]

Great answer and makes perfect sense!

Should I do one zone of protection or multiple zones like with looped lines?

A lot depends on the line length and the type of xfmr protection. If the line is short and there are diff relay on the xfmr, I would use 1 zone to reach half way through with no delay and let both the line clear and the xfmr clear. If the fault is in the xfmr then its diff should clear and allow the line to reclose successfully. I would also provide a 2nd zone to reach all the way through the xfmr to act as a backup and use a 60-90 cyc delay. If the line is longer then you could set up a typical underreaching zone 1 w/ no delay, an overreaching zone 2 w/ 20 cyc delay, and a 3rd zone to act as a backup.

 

[mbrooke]

A lot depends on the line length and the type of xfmr protection. If the line is short and there are diff relay on the xfmr, I would use 1 zone to reach half way through with no delay and let both the line clear and the xfmr clear. If the fault is in the xfmr then its diff should clear and allow the line to reclose successfully. I would also provide a 2nd zone to reach all the way through the xfmr to act as a backup and use a 60-90 cyc delay. If the line is longer then you could set up a typical underreaching zone 1 w/ no delay, an overreaching zone 2 w/ 20 cyc delay, and a 3rd zone to act as a backup.

Question, why would one scenario with zone 2 involve 60-90 cycles and the other only 20 cycles? My understanding is that any zone the reaches through the transformer must coordinate will all protection on the 34.5kv bus and system.

 

[Bugman1400]

For a long line, say 10 miles, that is terminated by a xfmr, a traditional Zone 1 (0 cyc) and Zone 2 (20 cyc) makes sense. This means 8-9 miles is covered by Zone 1 and the last 1-2 miles is covered by Zone 2. If the line is short, say 0.5 miles, you don't get the same coverage benefit. Also, you may not be able to set a Zone 1 because you may be below the minimum setting parameters of the relay. So, you are left with setting a Zone 2 at some percentage of the xfmr (25, 50, 75%). If you don't have local bkr failure at the xfmr that can send a transfer trip back to the local end then you can set a backup zone to look through the xfmr with a 60-90 cyc delay.

 

[mbrooke]

For a long line, say 10 miles, that is terminated by a xfmr, a traditional Zone 1 (0 cyc) and Zone 2 (20 cyc) makes sense. This means 8-9 miles is covered by Zone 1 and the last 1-2 miles is covered by Zone 2. If the line is short, say 0.5 miles, you don't get the same coverage benefit. Also, you may not be able to set a Zone 1 because you may be below the minimum setting parameters of the relay. So, you are left with setting a Zone 2 at some percentage of the xfmr (25, 50, 75%). If you don't have local bkr failure at the xfmr that can send a transfer trip back to the local end then you can set a backup zone to look through the xfmr with a 60-90 cyc delay.

I mused with a few different scenarios, and I am considering a zone 1 set at 125% of the line and then a 60 cycle zone 2 that reaches through the transformer. 60 cycles to allow 34.5kv relaying to clear first.

Does this sound doable/ok/kosher from you experience?

Also, I am confused about the 25,50,75 percentage. I assumed it would be 125% of the transformers positive sequence impedance.

 

[Bugman1400]

Check the protection at the xfmr end. If the xfmr, the lowside bus, and close-in to the feeder can be cleared quickly for a fault in those zones then, your Zone 2 reach and 60 cyc delay should be adequate.

Some utilities set the Zone 1 or Zone 2 to reach only part of the xfmr. This provides a quick backup to the xfmr relaying. Because of the extra reach, it also provides protection for high impedance faults on the T-line.

 

[mbrooke]

Check the protection at the xfmr end. If the xfmr, the lowside bus, and close-in to the feeder can be cleared quickly for a fault in those zones then, your Zone 2 reach and 60 cyc delay should be adequate.

Some utilities set the Zone 1 or Zone 2 to reach only part of the xfmr. This provides a quick backup to the xfmr relaying. Because of the extra reach, it also provides protection for high impedance faults on the T-line.

Thanks!

I plan on relaying through the whole thing as you mentioned and to let bus faults clear in case of 34.5kv relaying failure.