Differential Relay Protection

[buddy91082]

Coworker recommended installing differential relay protection on a 12kv feeder that is a mile long. I typically see differential protection on transformers. Anyone know the why one might use it on a feeder?

Thanks
b

 

[davidbeach]

Are there sources at both ends? If not, is there a particular need for fast clearing? But in any case, differential provides fast zoned protection.

 

[buddy91082]

No, source is at one end, load on the other end.

 

[rasevskii]

Differential protection for long HV cables is known as a pilot wire protection. In addition to the main cable itself, a two core small gauge cable is laid parallel to it (can be in the same trench or elsewhere) to compare the currents in one end with those at the other end. A special type relay is used at each end. Search for "pilot wire protection". Made by many electrical manufactureres for many decades. Example Reyrolle "Solkor" protection.

Also used for short OHL where the pilot cable can be included on the towers or underground.

rasevskii

 

[davidbeach]

Today line differential is much more likely to involve digital communications between relays at both ends then it is to have a pilot wire carrying current between the two ends.

 

[cranky108]

I believe you can still buy a pilot relay, but I would recommend the digital.

And there are other options, but a differential (or pilot) is about the only way to be sure of your zone.

 

[slavag]

As David saied, fast cleraing of fault, not needed any selectivity study.

 

[ThePunisher]

As David said. We normally call this as Line Differential protection. Using digital relays on both ends of the line interconnected via fiber optic communication. Oftenly, we take the transformer differential up to the transformer primary and take the line differential CT to overlap the transformer differential primary CT.

In this way, not only that we have fast fault clearing, we are also able to segregate the fault sensing and therefore provide indication whether the fault is within the transformer or within the cable feeder.