Energization Study of Parallel Transformers Adjacent to SVC and 500 kV Transmission Line

摘要

The author's utility is performing substantial construction at existing substations and of new substations to facilitate construction of an LCC-HVDC link. During this construction temporary substations must be built and AC transmission line re-terminations must occur. These result in many construction stages of the project. One stage is when an auto-transformer at a substation must be test-energized while a nearby auto-transformer at a temporary substation is in service. The transformer at the temporary substation is connected to a critical AC transmission line for system stability and generation/load balance. Therefore, obtaining an outage on the transformer at the temporary substation is not feasible. Upon successful test-energization of the auto-transformer at the permanent substation, the critical AC transmission line will be re-terminated at the permanent substation. The temporary substation will subsequently be salvaged and the equipment will be used elsewhere in service territory of the author's utility. This paper documents the efforts to study the test-energization of a 500/240 kV auto-transformer in the vicinity of a 240 kV SVC, 500 kV AC transmission line, parallel 500/240 kV auto-transformer, and three 240 kV AC circuits. The goal of the study is to determine whether the transformer energization will result in undesirable overvoltages and/or system stability conditions. PSCAD/EMTDC is used to model this equipment and perform all simulations. The following equipment modeling aspects are discussed: surge arrester characteristics, surge arrester energy absorbing capability, non-linear transformer saturation, transformer protection relays, SVC control, and harmonic impedance of the AC network. Variation of parameters is considered for the following aspects to arrive at eight simulation scenarios: powerflow on AC network, transformer remanent flux, and point-on-wave closing of the transformer circuit breaker. Simulations are conducted to determine the worst-case transformer inrush currents resulting from trans-former energization. The harmonic current injection from the saturated transformers interacts with the non-linear harmonic impedance of the AC network to result in instantaneous overvoltages. At the same time, RMS under-voltage is observed because of the large inductance added to the system. The SVC output controls the positive sequence voltage however this also increases the stress on surge arresters. Transformer differential protection is analyzed for excessive fundamental currents and protection blocking based on second harmonic current content of the inrush current. The phenomenon of parallel sympathetic inrush is observed in the parallel 500/240 kV transformer and SVC transformer. The study results show, for all simulations, the surge arresters are not overloaded and the transformer protection relays do not mis-operate.