Abstract
In the earlier time, Taiwan’s industrial power systems had been fed from Taipower system with overhead transmission lines. Lightning strikes on these lines sometimes caused factory production failures. To reduce lightning outages, in particular of the science park areas, the overhead lines were replaced by Taipower with underground cables. With lightning damages reduced, some high-tech factories recently however encountered arrester failures and are thus planning to dismantle the existing arresters installed to protect the 11.4kV (or 22.8kV) resin-type transformers inside the factory. The decision to dismantle the exiting 11.4kV (or 22.8kV) arresters actually requires in-depth evaluation of the overvoltage characteristics of the high-tech industry power system. For this purpose, the author simulates the transient overvoltage of a 69kV factory power system, focusing on the switching surge, by assuming no lightning can penetrate into the 11.4kV (or 22.8kV) system. The simulation makes use of the Electromagnetic Transient Program (EMTP) which is based on the trapezoidal rule for solving the transient equations. The solution process is thus efficient but can incur numerical instability. To avoid numerical instability, adequate models should have been selected, and are accordingly selected in this study, for the simulated power system components which include the 69kV underground cable fed to the factory, gas-insulated substation, 11.4kV in-plant overhead cables, transformers, arresters, static capacitors, and the ideal, statistical or systematic switching models for representation of circuit breakers. After model selection, the author then estimates model parameters and simulates the circuit breaker switching sequence of high-tech factories. The simulation applies the statistical switching models. Thus the probability distribution for the voltage peak of switching surge can be evaluated. The simulation results show that, at the primary side of 11.4kV resin-type transformer, most of the peaks are within the insulation withstand level of transformer, but the surge can be capacitively coupled to transformer’s secondary side with the voltage peak exceeding the insulation withstand level of the secondary system. By accounting for the risk of the secondary system failure, and if the factory owner can not bear this risk, it is suggested the arresters remain as where they are to continue their protecting of the 11.4kV resin-type transformers.