Parelled cables and step distance relaying 1

[Mbrooke]

Can paralleled cables (242kv XLPE, 1,500cu) be protected via a single CT or does each cable need its own CT? I'm thinking that theoretically it will work if the impedance is set to account for only one cable, however I am not sure if mutual coupling will cause over or under reaching. Each 3 phase set is cross bounded on the ground shield and each phase in its own conduit.

 

[HamburgerHelper]

I don't think you can set it to account for one cable. I think you have to take a look at the situation with a sliding fault and pick a reach that is conservative enough to prevent over reaching. It can over reach or under reach depending on where the fault is. This will be due to mutual coupling and infeed from the other paralleled line. I haven't done this with underground cables but for overhead lines this is what I have to do. Having one ct covering paralleled bundles also makes determining the location of the fault very hard.

 

[davidbeach]

I'd want a good model to show what currents and voltages will be seen at the relay for various fault types/locations. With one CT you'll have a non-linear progression of apparent impedance from one end to the other, you may also find that some in-zone faults have a higher apparent impedance than some faults beyond the zone. The length of the cable run, relative to source strength, can also affect how well it works.

 

[Mbrooke]

@Hamburgerhelper: I know with overhead lines you can get away with it by joining (jumping) both sets together about every 1/4 mile. But I don't think thats possible with underground cables?


@DavidBeach: Well said, and I will take this into account. If studies were to demonstrate this idea as impractical; would using 2 CTs (one per cable) externally paralleled (at the test switches) make any difference over summing each CT internally within the relay?

 

[davidbeach]

I don't see how two CTs externally summed would be any different than one CT. Two sets of CTs, each connected to a separate relay, and both relays tripping the same breaker might produce more definitive results. Whether the results are better enough to justify the added cost is a different consideration.

 

[Mbrooke]

I might go the two relay route. In your experience (assuming you've encountered it), when POCOs parallel cables, which is typically done?

 

[HamburgerHelper]

Mbrooke,

Yeah, you can jumper them. Our group ask for it, and sometimes we get it. We don't get them installed every quarter mile when we do get them.

 

[Mbrooke]

Underground cables or just overhead? Sorry

 

[HamburgerHelper]

Overhead.

 

[Mbrooke]

Roger that

 

[davidbeach]

Nope, not had to deal with it. Some overhead lines, and enough jumpers to get acceptable performance. Haven't faced that with UG cables.

 

[Mbrooke]

Can I have your job?

 

[davidbeach]

No. ;-)

 

[Mbrooke]

I will probably go the two relay route.