Single-core cables are the most commonly used cables for underground high-voltage transmission systems. This is because 3-core high-voltage transmission cables, mainly above 33kV, are not easily transportable due to their very thick insulation and weight. A single-core cable is lighter than a three-phase or 3-core cable. The insulation of these single-core cables is covered with a metallic sheath. This metallic sheath acts as an electrical shield. The metallic sheath of the cable must be grounded at least at one point to fulfill its electrical shielding function. Grounding the sheath confines the electric field produced by the high voltage conductors within the insulation. Therefore, it reduces the degradation of the insulation. Also, the metallic sheath provides a return path to the current during faults. Normally, we do not earth both ends of a metallic sheath. Although both-end bonding provides better shielding, but it has many disadvantages.
If both or more than one ends or points of a cable sheath is grounded or earthed, there will be a continuous circulating current in the sheath even in normal loading condition of the cable. Due to this induced circulating current in the sheath, there will be a continuous ohmic loss in the cable which makes the cable hot. Therefore, the cable gets derated. Although, both ends bonding reduces the voltage developed across the cable sheath due to induction, but due to induced current, there will be a derating of the cable.
To overcome this disadvantage of both-end bonding, single-end bonding and cross-end bonding come into picture. In single-end bonding, one end of the sheath is in floating condition. The floating or open end of the cable sheath may suffer from dangerously high voltage even under normal loading condition. For single-end bonding, the sheath voltage rises as the distance from the earthed end increases. The high induced voltage problem, mainly during a faulty condition, may be overcome somewhat by using sheath voltage limiters and earth continuity conductor (cable).
Here in this article, we shall discuss the basic theory of earth continuity conductor or cable (ECC).
The earth continuity cable plays a vital role in both single bonding and cross-bonding systems. This is essential to provide a return path to the fault current to the substation earthing system, for functioning of the protective relays. This acts just as the overhead GSS earth wire running through the tip of the towers.
Suppose there is no ECC used in an underground cable. In the event of a fault, the entire fault current shall return through the sheath only, which may cause a significant rise of sheath voltage. But if ECC is there, the fault current gets an alternative path to flow towards the substation earthing grid. Therefore, the intensity of fault current in the sheath is reduced, thereby reducing voltage rise beyond limits. So, it is considered as a backup path for the fault current to return to the source earthing system.
Therefore, in short, we can conclude two main purposes of ECC:
- As this conductor provides an extra path to the fault current to return, fault current through the sheath gets reduced, hence the probability of rising dangerously high sheath voltage during fault is minimized. That means it limits the touch and step voltages.
- Like overhead GSS earth wire, it provides a return path to the fault current to reach the substation earthing system for an underground cabling system.
Key Aspects of an Earth Continuity Conductor
- The earth continuity cable must be of copper as per recommendations. This is because of its high conductivity and durability.
- Copper-clad steel is another alternative, which has higher mechanical strength. But copper is most commonly used for this purpose.
- The cross-section of the conductor is finely stranded to increase effective cross-section. The fine strands of the ECC are twisted together to reduce the proximity effect, hence to increase further the effective cross-section. Twisting also increases the mechanical flexibility of the ECC.
- Insulation is mainly used in earth continuity conductors to provide a protective layer on the conductors.
- The cross-section of the conductor must ensure the short-circuit current rating of the system. Suppose, for a 220kV system, the sheath circuit current rating is 40kA for 3 seconds, therefore the conductor (ECC) must be capable of withstanding that short-circuit current for that duration.