Underground Cables

Underground cables are employed for transmission and distribution of electric power where it becomes impracticable to make use of overhead construction. Such locations may be congested areas where right of way cost would be excessive or local ordinances prohibit overhead lines for reason of safety, or around plants and substations or crossings of wide bodies of water which for various reasons would not permit the overhead crossings. The type of cables used will depend upon voltage and service requirement. Recent improvements in design and manufacture have led to the development of cables suitable for use at high voltages. This has made it possible to use underground cables for transmission of electric power for short or moderate distances.

Underground cables consists of one central core or a number of cores (two, three or four) of tinned stranded copper conductors (sometimes use of aluminium conductor is also made) insulation from each other by paper or varnished cambric or vulcanized bitumen or impregnated paper. A metallic sheath of lead or alloy or of aluminium is provided around the insulation to protect it against ingress of moisture. The initial heavy cost is the only factor which discouraged the use of underground cables for the purpose of transmission and distribution of electric power.

Construction of underground cable

The type of cable to be used at a particular location is determined by the mechanical considerations and the voltage at which it is required to operate. According to voltage these are classified as low-voltage cables for operating voltage up to 1kv, high-voltage cables for operating voltage up to 11KV, super tension cables for operating voltage up to 33 KV, extra high tension cables for operating voltage up to 66KV and extra super voltage power cables for operating voltage beyond 132KV.

The current rating of the cables apart from the above factors also depends on a large number of other factors such as method of cable laying employed, spacing between the cables, number of cores and the thermal conductivity of the soil. Cable manufactures employ multiplying factors to take care of each of them. An approximate indication of the current carrying capacity for Indian conditions, which is based on a maximum conductor temperature of 80 °C for 11KV cables and above. The ground ambient temperature of 30 °C applies to most places in India.

The following multiplying factors are used:

1. For a ground temperature of 30 80 °C = 0.88
   2. For 3,300 and 6,600 V cables = 0.95
   3. For 11,000 V cables = 0.9
   4. For 22,000 V cables = 0.8
   15cm 30cm 61cm
   5. For 2 cables 0.84 0.88 0.92
   6. For 3 cables 0.75 0.80 0.85

Construction of underground cable

Selection of Cables

For selection of cable of correct size and type for a particular application, the factors to be considered are:

1. System voltage

The type of operating system such as dc (2-wire or 3-wire) or ac (Single-phase, three-phase), earthed or unearthed and operating voltage such as 415/240 V, 11, 33, 66, 132 KV etc.

2. Current carrying capacity

The current rating is the most important factor.

3. Permissible voltage drop

The cable should be selected of such a size that the voltage drop in the cable is within permissible limits.

4. Short circuit ratings

For selection of low tension cables this factor is not important but for high voltage cables, this factor is the most important. It is because in high-voltage systems the fault current (phase-to-phase or phase-to-earth) is very high. The cables should be so selected that it can withstand the stresses and the resulting increase in temperature caused by the maximum short-circuit current produced by the phase-to-phase short circuit up to a period of 1 second.

5. Economic consideration

The cables should also give minimum operating cost (power losses, the interest on capital cost and depreciation).

Mechanism of Breakdown of Cable

There are two ways in which breakdown of cables usually occurs. One way is by a progressive coring and tracking, which always starts from the core or sheath, and ultimately bridges the electrodes. Another way is by thermal instability which occurs due to rapid increase in power factor with the rise in temperature. A marked difference between the methods of breakdown is that coring, once it occurs, will continue until the cable breaks down, another the time duration may be considerable for complete action. In thermal instability, however, no damage occurs until just before breakdown, so that if the load is reduced before breakdown the cables will not have suffered any permanent change. A very common occurrence is for coring to start and then introduce thermal instability at the centre of coring.

Causes of Failure of Underground Cables

1. The most common point of failure is at the cable sealing box mostly due to poor workmanship of the cable jointer when the end was sealed.
   2. Another most common cause is the mechanical puncturing of the lead sheathing of a cable, such as by a crowbar, especially in industrial installations where excavation and building operations are carried on in areas having several underground cables.
   3. The cable may also get damaged due to vibration fatigue or overheating.

Advantages

1. Less subjected to damage from severe weather conditions (mainly lightning, wind, freezing).
   2. Underground cables need a narrower surrounding strip of about 1-10 meters to install up to 30 m for 400 KV, whereas an overhead line requires a surrounding strip of about 20-200 meters wide to be kept permanently clear for safety, maintenance and repair.
   3. Underground cables pose no hazard to low flying aircraft or to wildlife.

Disadvantages

1. Undergrounding is more expensive. In highly urbanized areas the cost of underground transmission can be 10-14 times as expensive as overhead.
   2. Underground cables locations are not always obvious, which can lead to unwary diggers damaging cables or being electrocuted.
   3. Underground cables are more subjected to damage by ground movement.

Comparison between overhead and underground systems

1. Maintenance cost of underground system is very low in comparison with that of overhead system.
   2. Underground system is free from interruption of service on account of thunderstorm, lightning and objects falling across the wires.
   3. In underground system there is no interference to communication circuits.
   4. Underground system cannot be operated above 66 KV because of insulation difficulties but overhead system can be designed for operation up to 400 KV or higher even.

Conclusion

Underground cables offer an affordable and justifiable solution for critical parts and in some cases the entire length, of overhead high voltage power lines. With appropriate technology used in appropriate places, the environment impact of underground cables can be minimised.

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