cable Deterioration factors


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We are going to install 90 MVA transformer in our plant (11/132 KV)For the 11 KV side we have decided to lay XLPE cables as that is only suitable for the site conditions and for the transformer location.

if i select a 400 sqmm 3 core XLPE aluminum cable How many runs we have to lay. The maximum current flow of the cable will be 4100 Amps.

I came to know that the deterioration is less when we lay the cable in Air (i.e on cable trays) instead of under ground.

Some people are advising me to lay single core cable instead of 3 core cable to avoid deterioration due to cable capacitance as three phases are going in the same cable. Can any one clarify this point.

I feel that 3 core, 400 sqmm XLPE, Aluminum armoured cable of 12 runs (4 runs in a tray with spaced apart at a distance equivalent to diameter of cable) in 3 GI trays may be suitable.

Distance between TFR and panel will be 100 mtrs.

Pl verify this and inform corrections.
Sriram’s 27-Jan-10 (06:28) queries... please clarify the required Ampere value. 4,100-A, is adequate for only 78 MVA?

Regards, Phil Corso (cepsicon [at] aol [dot] com)

Thanks for the response.The load on the transformer will be around 70 MVA. We may go for an 80 MVA transformer.

Sriram... three major derating factors, when using 3 single conductors in a three-phase system, are:

1) ambient temperature. What is the prevailing ambient temperature in your installation?

2) configuration, that is, trefoil (triangular in arrangement) or flat (side by side, or one above the other.)

3) position, in groups or sets of 3, arranged horizontally or vertically on cable-trays.

In general trefoil is more efficient because it virtually eliminates transposing conductors, which for 400-mmq cable can impact installation cost.

The statement that derating is due to capacitive effects is untrue. In fact, flat-formation results in asymmetrical distances between phase conductors, hence, unbalanced phase-phase inductive-reactances.

Regards, Phil Corso