Cable arrangement

[eejacky]

Dear all experts,

I have a question regarding the 132kV underground cable arrangement. In commom practice, trefoil and flat formation are usually adopted in cable laying, but which one is better? What is their advantages and disadvantages?

Also, I have read a page talking about cable arrangement. It mentioned that VERTICAL ARRANGEMENT OF THREE PHASES CABLE MAY DERATE THE CABLE. What do they mean?

 

[peebee]

Triplexed (I think this is the same as trefoil) is usually nicer because the impedance will be symmetrical, and capacitance will be lower. It can be difficult to do with larger cables, though.

In a vertical arrangement, the lower cables will heat the upper cable leading to a derating of the upper cable (which implies a derating of all three cables).

 

[jghrist]

Flat arrangement allows spacing between phases, reducing mutual heating. Trefoil minimizes shield losses, but if you use single point grounding or cross-bonding, there is no advantage here. I think you will find that flat is more common. My experience is somewhat limited and dated, but this is the only way I've seen it done at transmission voltages. Vertical has the shield loss disadvantages of flat, but with more mutual heating for the same spacing and the requirement for deeper trenches.

 

[RRaghunath]

Eejacky,

All single core cables are preferably laid in trefoil formation. 132kV class cables I believe are necessarily single core cables.

This is because in trefoil formation the flux due to individual phase currents sums up to zero and thus leaving no residual flux externally that otherwise could link with the metal around the cables (such as trays, supporting metal or ducting) causing problems.