Underground cables are used for power applications for transmission and distribution where it is impracticable, difficult or dangerous to use the overhead line. Such locations may be congested areas where right of way cost would be excessive or local ordinances prohibited overhead lines for reason of safety, or crossing of wide bodies of water which for various reasons would not permit the overhead crossing. The type of underground cable to be used at a particular location is determined by the voltage at which it is required to operate. Underground cables are classified and used at a particular location as per their voltage rating. Low voltage cables operate at voltage up to 1kv, high voltage cables operate at voltage up to 11kv, super tension cables operate up to 33kv, extra high tension for up to 66kv, super voltage cables for operating voltage beyond 132kv.
Underground cable consists of one central ore or number of cores (two, three or four) of tinned stranded copper conductors. Sometimes use of aluminum conductor is also made. Paper or varnished cambric or vulcanised bitumen or impregnated paper are employed for the isolation from each other. A metallic sheath of lead or alloy or aluminum is provided around the insulation to protect it against ingress of moisture. The initial heavy costs the only factor which discourage the use of underground cables for the purpose of transmission and distribution of electrical power.
New Tends in Undergrounding
Over the last few years underground cables are becoming more interesting for higher power rating. Although underground cables are generally more expensive then overhead transmission lines but due to several reason these are widely preferred today is:
- Limited space of new and existing substations with no possibility to expand and the necessary right of way for the transmission corridor.
- Increase customer awareness such as visual impact of transmission line, nature preservation etc.
- Increased reliability for the transmission grid. This is especially true for the distribution grid as cable connections can be made very reliable, easy to monitor and diagnose and most of all, are not prone to extreme weather events.
Countries which have an almost exclusive medium voltage cable distribution grid, like Singapore and the Netherlands, have a very reliable public power supply within average customer outage rate of less than 30 minutes a year.
Some technical issues that used to be addressed and take care of, such as change within grid as it behaves more capacitive instead of inductive due to increase in high voltage cable connections in the transmission grid; limited transmission capacity present around 1000MVA for a single cable compared to overhead line connections; and the reliability of long stretches of such high power of high power transmission cable with many cable joints is point of attention.
Underground electricity cables are typically only used in the US, Europe and Australia. These focus on low medium voltage networks (200V – 20KV). Some countries have achieved in undergrounding between 10 per cent to 20 per cent of their high voltage cables, while other European countries are less than 10 per cent for extra high voltage average is around 2 per cent.
- LT cables: low-tension cables with a maximum capacity of 1000 v.
- HT- cables: High-tension cables with a maximum of 11kv.
- ST cables: Super-tension cables with a rating of between 22kv to 3kv.
- EHT cables: Extra high-tension cables with rating of between 33kv and 66kv.
- Extra super voltage cables: with a maximum voltage rating beyond 132kv.
- Belted cables: Maximum voltage of 11kv.
- Screened cables: Maximum voltage of 66kv.
- Pressure cables: Maximum voltage of more than 66kv.
- Underground cables need to be insulated against surrounding soil.
- On large lines the methods of electrical and heat insulation become more important.
- For 400kv lines, each conductor needs to be encased in oil filled sleeves.
- Each conductor needed to be placed in a 2m deep trench, resulting in up to 12 trenches for a 400kv line.
- Width of excavation may be anything between 15 and 30m depending on the technology used.
- Underground cables are reported to be more reliable but outages are more difficult to fix, as it is harder to find the fault.
- Routine maintenance of underground cables is much lower in the initial 10 years of operation, maintenance costs can rise steeply thereafter.
- The lifespan of underground is shorter, in some cases it is reported to be half that of overhead cables.
- The underground cables have low voltage drop, low chance of developing faults, and hence low maintenance costs.
- Underground cables pose no hazard to low flying aircraft or to wildlife.
- It does not provide any kind of visual pollution most particularly in congested areas.
- Underground cables are more expensive to manufacture, and their cost may vary depending on the construction as well as the voltage rating.
- Underground cables locations are not always obvious, which can lead to unwary diggers damaging cables or be electrocuted.
- Underground cables are more subjected to damage by ground movements.
Benefits of Underground Cables
- Low maintenance
- Small voltage drops
- Not easy to steal, make illegal connections
- Poses no danger to wildlife or low flying aircrafts
- Suitable for congested areas where overhead lines may be difficult or impossible to install.