EHV/HV Cable Sheath Earthing: Introduction:In urban areas, high voltage underground cables a re commonly used for the transmission and distribution of electricity. Such high voltage cables have metallic sheaths or screens surrounding the conductors, and/or armour and metallic wires surrounding the cables. During earth faults a pplied to directly earthed systems, these metallic paths are expected to carry a substantial proportion of the total fault current, which would otherwise flow through the general mass of earth, while returning to system neutrals. These alternative return paths must be considered when determining the ex tent of the grid potential rise at an el ectrical plant due to earth faults. For safety and reliable operation, the shields and metallic sheaths of power cables must be grounded. Without grounding, shields would operate at a potential considerably above ground. Thus, they would be hazardous to touch and would cause rapid degradation of the jacket or other material intervening between shield and ground. This is caused by the capacitive charging current of the cable insulation that is on the order of 1 mA/ft of conductor length. This current normally flows, at power frequency, between the conductor and the earth electrode of the cable, normally the shield. In addition, the shield or metallic sheath provides a fault return path in the event of insulation failure, permitting rapid operation of the protection devices. In order to reduce Circulating current and electric potential difference between the sheathings of single core three-phase cables, the sheathing is grounded and bonded at one or both ends of the cables. If the cable is long, double bonding has to be carried out which leads to circulating currents and increased total power loss. Raising the sheath‟s resistance, by decreasing its cross section and increasing its resistivity, can reduce this almost to the level of the core losses. However, in case of an earth fault, a considerable portion of the fault current flows through the increased sheath resistance, creating much higher power in the sheaths than in the faulty core. A simple solution, a conductor rod buried into the soil above or under the cable can divert this power from the sheaths. Cable Screen:(1) Purpose of cable screen: Cable screen controls the electric field stress in the cable insulation. Cable Screen Provides return path for Cable neutral and fault current. If the screen is earthed at two ends than it provides Shielding for electromagnetic radiation. Enclosing dangerous high voltage with earth potential f or safety. (2) Purpose of bonding cable screens at both ends: The electric power losses in a cable circuit are dependent on the currents flowing in the metallic sheaths of the cables so by reducing the current flows in metallic sheath by different methods of bonding we can increases the load current carrying capacity (ampacity) of the cable.
