Location of sheath voltage limiters (SVLs) used for accessory protection to assure the insulation coordination of cable outer sheath, sectionalising joints and terminations of high voltage cable systems

摘要

The CIGRE Electra Brochure No. 283 analyses the transient sheath overvoltage of atmospheric origin in cross bonding configurations and presents an analytical equation to evaluate these overvoltage. Reasonable surge values are obtained when the SVL bonding lead is a concentric cable, but in practice the bonding lead is a mix of an unipolar cable length and concentric cable length and in this case transient overvoltage can exceed by far the withstand insulation level of outer sheath cable and accessories. Furthermore, the expression presented by the CIGRE Electra Brochure No. 283 cannot be applied directly to the insulation level between the sheath and earth at sectionalizing joints with one side of joint connected directly to earth and the other side protected with a SVL. Nor can be applied to the insulation level between sheath and earth in cable terminations installed at: dead-end towers, outdoor substations or gas-insulated substations (GIS). In this paper different models are established and the analytical equations for the transient sheath-overvoltage of atmospheric origin are deduced, as well as the equations for the maximum admissible bonding lead length between accessories and in order to ensure an appropriate protection margin for the insulation of the cable outer sheath and accessories. The equations are established for different scenarios: a) sectionalizing joint in a cross bonding configuration; b) sectionalizing joint with one side of joint connected directly to earth and the other side protected with a SVL; c) GIS terminations protected by a SVL and d) outdoor termination protected by a SVL. These equations have been ratified by ATP software simulations. The expressions deduced in this paper were applied to the power grid owned by REE which has been growing since January 1st of 2006 when REE started assuming the control of the Spanish cable systems maintenance.