We have metering class CTs (on each phase) on the phase & neutral side of our 150MW generator. For calculating the knee point voltage (Vk) for these CTs, we take the lead burden and the burden of the meters. Lead burden is I2r VA, where I is the CT secondary current and r is the cable resistance in Ohms/km.

My question here is that should we consider the return path from the CT or not, i.e should the length of cable be considered L or 2L? Some people advise that in 3 phase circuits it should be L only as the current does not require any return path through neutral, whereas some people advise that current should require a return path and hence it should be 2L.

In protection class CTs the formula for Vk>=2If(Rct+2Rl) or Vk>=50I(Rct+2Rl), here in these formula the lead resistance is inherently considered as twice.

Request kind help on this?

 

Abbs... following are the recommended lengths based on connection method used:

1) 2 wires per CT.
The the total wire length is twice the distance, d, between the CT and protective or metering device(s), presuming the wires are in proximity to one another!

2) 3 wires, plus 4th as common return.
The total length is 1.2 times the distance, d, given above. Again, the presumption is that the wires are in proximity.

Regards, Phil Corso