Best Option for Overcurrent Protected Taps on Distance Protected Line

[111R]

On a line protected by instantaneous distance protection along the majority of the line, what is the best option for coordination with overcurrent protection on multiple taps with breakers feeding transformers at the end of a short line with negligible impedance?

For high side transformer faults on the taps, the distance protection will see these faults because the impedance is less than that of 80-90% of the main line, but it's not desirable to trip the entire line for a fault that can be interrupted by the breaker on the tap.

Is the best option to put instantaneous overcurrent protection on the taps and then delay the distance protection with a set delay of 6-10 cycles to allow clearing of the tap without tripping the entire line?

Thanks

 

[davidbeach]

On our system we'll let the line trip while the transformer trips; the line will then hold on the reclose and the transformer will be locked out. Transformer faults are rare enough that most operations of the line will still be for line faults, not for transformer faults.

 

[111R]

Do you have any issues with the distance protection clearing the fault prior to the instantaneous overcurrent picking up and sending the trip command to the tap breaker? Therefore, the tap breaker would never open and the fault would still be present on distance relay reclose?

 

[davidbeach]

Nope. The instantaneous overcurrent on the transformer has about 3.5-4 cycles to make a trip decision. The zone 1 distance relay at the line source shouldn't be reaching farther into the transformer than the instantaneous overcurrent on the transformer.

 

[111R]

I'm not really concerned about the distance relay picking up for transformer faults, but faults on the short 1/2 mile radial line down to the transformer. Since there's a separate breaker on this radial feed at the tap, it would be preferable for the breaker to open for a fault on this radial line segment rather than the entire main line which has multiple taps.

Since the impedance down this radial line is less than the impedance to 80% of the long main line, it will be seen by the distance relay as being a Zone 1 fault. So, there will be a race condition between the distance relay zone 1 and the instantaneous overcurrent on the tap unless the distance relay is delayed to allow the overcurrent to trip first.

 

[davidbeach]

Oh, that's not a configuration we've used. There's no interrupting device at the tap point on our system, just at the transformer itself. You're going to need some form of communication aided tripping scheme to sort things out or let you main line wait to trip.

 

[Mbrooke]

Is it really necessary to protect this single line tap? 1/2 mile is not the much exposure or risk. Ie, let faults on the tap clear the main line as well. Often the next substation down down line effected has an alternate or companion line to draw from. Around here that is what is frequently done. If it is absolutely necessary that a tap line by isolated for a fault, a motorized SCADA switch is used. After the fault the switch is opened and the line remotely re-closed.