Abstract
M.Tech.
Point on wave switching has been used extensively for shunt capacitor bank switching. This is due to its ability to switch capacitor banks on-line at the zero crossing of the voltage waveform, which in the case of a mechanically linked circuit breaker will be achieved by staggering the links 120 electrical degrees. The use of point on wave switching using a 3-phase mechanically linked SF6 circuit breaker has been found to be unreliable as a result of drift in the mechanical linkages when the circuit breaker continues to operate. Due to this drifting of the mechanical linkages, circuit breakers ultimately switch beyond the zero crossing, thus generating undesired transient voltage. This work is based on an investigation carried out on 132 kV eThekwini Municipality Substation in Durban North, where the capacitor bank recorded a high unbalance within six months of commissioning. The capacitor bank was consequently isolated from the system as a result of unbalance. The data used in this work is sourced from on-site measurements and recordings using ImpedoGraph units with recordings over a seven-day period, where transient voltage of up 102kV are observed, resulting in transients of 1.18 p. u. Simulation runs were conducted using the Electromagnetic transient program, the simulation results observed at peak voltage is 124.4kV at resulting in transients of up to 1.6 p.u . Both the on-site measurements and simulation test results showed that the circuit breaker’s mechanical linkages had drifted and caused these devices to switch on beyond the zero crossing of the voltage waveform. The results observed in simulation tests, which incorporated a pre-insertion resistor, showed that when the point on wave switching is combined with a pre-insertion resistor, transient voltage can be mitigated.