EHV Series Capacitor Banks. A New Approach to Platform to Ground Signalling, Relay Protection and Supervision

Series capacitor banks for transmission systems above 145 kV, are normally erected for technical and economical reasons on fully insulated platforms per phase. Several methods have been used to realize the relay protection for series capacitor banks erected on platforms. One of the first designs used in the 1950's utilized protection relays and auxiliary power equipment on the insulated platforms. Signal communication was carried out by mechanical contact devices, which transmitted on/off signals from platform level to ground. In the late 1960's, the mechanical contact devices were replaced by fibre optics. The protection relays, together with electronic equipment and auxiliary power equipment, were still located on the insulated platforms. However, to give the relay protection of series capacitors the same simplicity as for ordinary substations, a new concept was introduced in the early 1970's. A number of signals were transmitted between EHV potential and ground via a modified current transformer. The relay protection and control equipment was located in the control room. The new Protection and Supervision system includes the advantages and excludes the disadvantages of the foregoing systems. The new system comprises optical current transformers, and a microprocessor-based protection, supervision and control system, located in the control room. Optical fibres connect the current transducers on the platform with the equipment in the control room. Auxiliary power to the current transducers is transmitted by light from ground level.