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Cable differential protection requirements

Cable differential protection requirements

Hi all,

We are currently working on a new project which requires a Flywheel energy storage system to be connected on a 12.6 kV system. The Flywheel energy storage system itself have 4 transformers of 480 VAC/ 12.6 kVAC connected to a main bus. This bus afterwards is tied-in to the main 12.6 kV bus system with a cable about 175 meters long. My question is, does this cable from the new 12.6 kV bus to the existing 12.6 kV requires a cable differential protection? Is there standard indicating the minimum cable length which requires a differential protection?

In advance many thanks for your information.

Kind regards,


RE: Cable differential protection requirements

Required is maybe not appropriate to ask. It seems recommended. You have short leads so a high impedance differential would be easy enough to add, for fairly low cost, and provides high speed that will protect the cables very well. Go for it. You would also add overcurrent backup, though.

RE: Cable differential protection requirements

I have never seen 87 differential protection applied to (just) cables in an industrial setting.
Having said that, have you considered installing 87 protection zone around each transformer. The 87 zone could include the 15kV cables as well. AS JD mentioned above, this would not cost an arm and a leg to provide.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Cable differential protection requirements

Hi GroovyGuy,

Thanks for the info. Below I have included the drawing for the new Flywheel project. Just to mention the upstream existing relay will not be replaced and will be programmed with the required settings. This is however a 50/51 relay. The 4 Transformers however will be equipped with a RET615 (50/51/87T). The main 12.6kV bus is equipped with a REF615 backup relay.

Should the feeder diff. (87L) still be recommended/ required?

Kind regards,


RE: Cable differential protection requirements

Howdy Jairo,
As I stated above, I have never seen 87 protection applied on a MV cable. I would think that 50/51/50N/51N protection for the cable should be more than adequate. If he MV supply has a NGR then the protection should be 50/51/50G/51G.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

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