Balancing methods of the “3-PHASE” shield wire schemes

Abstract

Insulation of shield wire(s) of HV (115-230 kV) transmission lines and their energization at MV (20-34.5 kV) from a HV/MV transformer station at one line terminal and use of the ground as a MV phase conductor, allows the setting-up of long unconventional MV distribution feeders for 3-phase or single-phase low cost electrification along the route of the HV line. This technique has been applied in various tropical countries and is referred to as the ";Shield Wire Scheme"; (SWS). The ";3-phase"; SWS, that is the most applied scheme, is an inherently unsymmetrical 3-phase system. The paper deals with the methods to reduce the negative sequence voltage at the consumer terminals generally within 1%. Firstly, the paper presents a brief description of the ";3-phase"; SWS for those readers not familiar with this technique. Then the paper deals with the compensating method using a series impedance in the earth return path and a shunt capacitance connected between the two shield wires. A general purpose accurate method is presented for the calculation of the compensating impedances, implemented in a computer program developed by the authors. The results of an application to a ";3-phase"; SWS planned for construction in Western Africa is finally presented.