Implementation of an unconventional voltage slide scheme

Eskom experienced a voltage collapse condition in part of its subtransmission network. The network consists of a number of subtransmission substations that are interconnected and fed from three transmission substations. The voltage collapse condition occurred after a protection operation cleared a fault on one of the 132 kV lines feeding the network from one of the transmission substations. Network studies have shown that the source is relatively weak and that a voltage collapse will occur again if any one of a number of lines from the source transmission substations is tripped out due to a fault. From a solution point of view, constructing additional lines feeding into the network from the transmission substations would solve the problem. However, the load in the area is of such a nature that it is not possible to economically justify the construction of new lines. The paper finally focuses on one specific solution that was selected as being the most appropriate and cost effective. In summary, this solution is an undervoltage load-shedding scheme. The system studies showed that a conventional definite-time undervoltage load-shedding scheme would not provide much benefit. A unique undervoltage algorithm was developed so that load could be shed at selected substations based on the voltage depression at that substation, while still supplying a large part of the network. This unique algorithm was required because voltage at specific substations greatly differs depending on the various contingencies of incoming feeders tripping. The algorithm uses inverse time to shed load based on the severity of the voltage collapse at a specific substation. A modern numerical relay enabled the user to freely program unique algorithms, and could also be used to perform tripping. The paper discusses this solution and its implementation.