Figure 7 presents the voltage waveforms at the GSU highvoltage terminal for the GSU energization. Very-Fast Transient Overvoltages The switching of GIS breakers and disconnects can produce very-fast transient (VFT) phenomena in the gas insulated 600 500 400 300 200 100 0 -100 0. 1 5 0. 09 5 0. 09 0. 08 5 0. 08 0. 07 5 0. 07 0. 06 5 0. 05 0. 06 -200 Time (ms) (a) 600 500 400 300 200 100 0 -100 1 0. 5 09 09 0. 0. 5 08 08 0. 0. 5 07 07 0. 0. 5 06 0. 05 0. 06 -200 5 V_GIS_Cable_GSU2 (kV) Study scenarios used in lightning overvoltage simulations assume that lightning strikes an overhead line conductor near a substation fence and that it travels to the GSU through a cable. The resulting transient overvoltages at the GSU terminal and at the cable termination are evaluated and compared with the lightning impulse BIL of the GSU and the cable to determine insulation adequacy. These evaluations are made for several network configurations. More circuits at a substation mean more paths along which a lightning surge can travel and dissipate. Therefore, GSU terminal transient overvoltages are expected to decrease as more circuits are connected and in service at a particular substation. Maximum transient overvoltages are anticipated for configurations with fewer circuits connected. As shown in Figure 6, the simulation results indicate the maximum phase-to-ground voltage at the GIS-to-cable interface (576-kV peak) and at GSU2 (533-kV peak) for the 230-kV system. Lighting overvoltage studies also determine the energy discharged in arresters and compare it with the energy-discharge capability of the arrester to confirm that the arrester location and rating are adequate to mitigate expected transient overvoltages. V_GSU2 (kV) Lightning Overvoltages 0. equipment. This model includes a detailed substation representation with transmission lines emanating from the substation. In general, switching surges are not considered to be a design concern for nominal operating voltages up to 230 kV; therefore, insulation coordination studies for 230-kV substations focus only on the transient overvoltages caused by lightning surges. On the other hand, for 345-kV and highervoltage substations, both lightning and switching surges are considered for overvoltage evaluations. Time (ms) (b) Switching Overvoltages july/august 2019 figure 6. The overvoltages at the (a) GSU2 terminal (V_ GUS2) and (b) GIS-cable interface (V_GIS Cable_GSU2). (p.u.) Switching overvoltages become important for systems whose nominal system voltage is 345 kV or higher. Simulations of GSU energization quantifies the risk of system overvoltages for the system configuration with the selected synchronous condenser. The switching studies can be performed for two operating scenarios: 1) no residual flux in the transformer windings 2) residual flux in the transformer windings. Simulations without models of residual flux determine the maximum overvoltage at the GSU terminals and are used to apply the safety margin to the transformer's basic switching impulse level. The residual flux scenarios are simulated to determine the maximum transient voltage dip. The voltage performance following a transformer energization can be evaluated against required reliability standards such as North American Electric Reliability Corporation (NERC) Standard TPL-001_WECC_CRT-2.1 (System Performance Criterion Under Normal Conditions, Following Loss of a Single BES Element, and Following Extreme BES Events, 1 April 2012). V GSU1 HV 1.75 1.50 1.25 1 0.75 0.50 0.25 0 -0.25 -0.50 -0.75 -1 -1.25 -1.5 -1.75 0.008 0.01 0.012 Phase A 0.014 0.016 Time (s) Phase B 0.018 0.02 Phase C figure 7. The phase-to-ground voltage profile at the GSU1 terminal for GSU1 energization. ieee power & energy magazine 43