From an electrical engineering perspective the greatest issue for the underground network is the cable rating assigned to feeders and the assumption made on the installation of conduits and the cables carried within the ducts.
The feeders are designed generally to the maximum limits of the cables carrying capacity for peak loads with a maximum operating temperature of 90 degrees Celsius. The reduction in maximum capacity due to conduits being installed and touching each other as opposed to the 50mm separation for 11kV Copper 3 Core 240mm XPLE cable for various conduit banks is as follows:
- 4 Conduits: 3% reduction for each feeder in the bank
- 6 Conduits: 5% reduction
- 8 Conduits: 7% reduction
- 10 Conduit: 8% reduction
- 12 Conduits: 9% reduction
The data is based on a number of assumptions including a backfill material with a minimum thermal rating of 1.2 degrees Celsius M/W, maximum depth of 1.2m, even when the conduits are touching the void between conduits is filled with backfill material etc.
If the void is not filled or if washed beach sand is used to bed the conduits, the reduction in carrying capacity can and most probably will double.
Below are some sample reports from CYMCAP. CYMCAP is a cable rating program used by various electricity authorities throughout the world, primarily for underground transmission projects. Two scenarios are provided, the first being a bank of twelve conduits each containing 11kV Al 3 Core 240mm XLPE cable and the second being a bank of four. The reports show the heat dissipation and the cable ampacity and temperature. The bank of twelve shows a reduction in cable capacity of 5% and the bank of four shows a reduction of 3 to 4%. The engineers responsible for the installation and the network responsible for the ongoing power delivery are unlikely to have taken this reduction into consideration.
The cost implication for power distribution companies due to the reduction in capacity of the cable alone (excluding maintenance costs) significantly outweighs the minimal capital outlay for the spacer and improved power delivery quality.
CYMCAP Test Results
3 x 4 LAYERS OF CONDUITS
- 1-rating-3×4 - 50mm-spacing-isotherm Isotherm indicates, with proper spacing heat dissipation will occur so that cable rating is maintained.
- 2-rating-3×4 - no spacing - isotherm This Isotherm demonstrates the concentration of heat when no provision for spacing is made.
- 3-rating-3×4-50mm-spacing The results of the test with spacing - 153 amps are possible at cable rating temperature.
- 4-rating-3×4 - no spacing The results with no spacing - 146 amps only is possible under the same conditions.
2 x 2 LAYERS OF CONDUITS
- 5- rating-2×2-50mm-spacing-isotherm Similarly to above example proper spacing allows heat dissipation.
- 6 -rating-no spacing-isotherm Heat is concentrated with no spacing.
- 7 -rating-2×2-50mm-spacing It is possible to deliver 229 amps with spacing at maximum cable rating temperature.
- 8 -rating-no spacing power delivery is restricted to 221amps at cable rating if spacing is not used.
Clearly, power delivery is reduced if no spacing is allowed for.