Under frequency Element 1

[TWD82]

I've been reading this forum since I got out of school (about a year ago). I've learned alot from everyone here, hopefully I can start contributing as I become more competent in the power field, for now I have a question I would like advice on. Recently we had a main breaker trip on underfrequency (load shedding scheme) during an arcing feeder fault (before the feeder breaker could trip on time-overcurrent). The event report showed the frequency dropped to 58.5 Hz, so somehow the arcing fault "tricked" the relay into thinking the frequency had dropped. Is this caused by harmonics generated in the arcing fault? Would moving the PT inputs from the low side of the transformer (grounded wye) to the high side (delta) smooth out the waveform somehow and help the relay from being tricked again? The relay is an SEL 351.

 

[RRaghunath]

The underfrequency elements can maloperate when voltage dips during faults. The relays normally include low voltage block element that can prevent the relay maloperation (sensing it wrongly as under frequency).
You may like to Check the relay literature and if it includes the element, check whether the same is set / enabled.

 

[jghrist]

On the SEL-351, the frequency elements are supervised by undervoltage. From the 351S manual:

...Relay Word bit 27B81 asserts to logical 1 and blocks the frequency element operation if any voltage (VA, VB, or VC) goes below voltage pickup 27B81P. This control prevents erroneous frequency element operation following fault inception.

Check that 27B81P is set properly.

 

[TWD82]

Sure enough, this voltage restraint was set at 33%, we are gonna move it up to 85%.
Thanks

 

[rbulsara]

All well and good, but I would rather trip on arcing fault at some point than not at all. The overcurrent may not always operate under arcing fault currents as they could be less than OC setting and could cause tremendous damage.

 

[jghrist]

I wouldn't depend on an underfrequency relay for arcing fault protection. It would be both unreliable and insecure. You can't depend on an underfrequency relay to misoperate for arcing faults (especially low current ones) even if you don't use undervoltage blocking. If you don't use undervoltage blocking, you may operate faster than overcurrent relaying and lose coordination, tripping more than just the affected line.

 

[rbulsara]

jgrist:

I am not saying, use UF as arc fault detection, but if there is a fault, it is better to trip then not.

 

[SHR1969]

What is the time delay setting for the under frequency element?