Influence of Different Types of Short Circuit in Transmission Line on the Rotor Loss and Temperature Rise of Synchronous Condenser

Synchronous condenser (SC) can provide the reactive power for the converter station of Ultra High Voltage Direct Current (UHVDC) to keep the stability of the system voltage and prevent DC commutation failure. When the drop of the AC system voltage results from the short circuit of the transmission line, the SC can provide a large amount of reactive power for the power grid. However, the high reactive power and large current of the SC will increase the loss and heat of the SC. By coupling the electromagnetic field and temperature field models of the SC with the model of the power grid, the rotor losses and temperature distributions of the SC are calculated under the single-phase, two-phase, and three-phase short-circuit faults in the transmission line. The ability of the SC to withstand different types of short circuits of the power system is studied. In this paper, the field-circuit coupled time-stepping finite element model of the SC is established and the dynamic processes of the SC under three different short-circuit faults of transmission line are calculated. Then the loss densities of the rotor slot wedges and small teeth under the different short-circuit faults of transmission line are calculated, and the loss densities in different positions of the rotor are compared and analyzed. Finally, the temperature fields of the rotor core and slot wedges are calculated, and the rotor maximum temperature rises are obtained. It is revealed that the relationship between the duration of the SC and the type of short-circuit fault under the consideration of the maximum withstand temperature of the rotor. The results of the loss calculation show that the loss densities of the upper layers of the rotor are larger than that of the middle and lower layers during the short-circuit faults, and the rotor loss densities decrease along with the increase of the radial depth; the loss densities of the slot wedges are higher than that of the small teeth, and the loss density of the large teeth are the maximum. According to the temperature field of the SC under the single-phase, two-phase and three-phase short-circuit faults of the transmission line, the highest temperature rises of the rotor slot wedges are 32.102℃, 25.962℃ and 17.31℃, respectively; those of the small teeth are 26.039℃, 21.331℃ and 15.108℃, respectively; and those of the large teeth are 84.03℃, 76.3℃ and 22.728℃, respectively. The temperature of the rotor large teeth is a key factor that restricts the operating ability of the SC. With the consideration of the maximum permissible temperature of the rotor core, the ability of the SC to withstand different types of short-circuit faults is obtained. The durations of the SC under the single-phase and two-phase short circuit are 3.62s and 3.97s, respectively; the highest temperature of the SC under the three-phase short circuit would not reach the maximum permissible temperature of the rotor core. The conclusions can be drawn as follows: ① The maximum loss density of the SC appears on the surface of the rotor large teeth and the loss density of the rotor slot wedges is larger than that of the small teeth. ② The rotor loss of the SC is mainly distributed on the rotor surface during the transient process due to the skin effect, the rotor loss under single-phase and two-phase short circuit is larger than that under three-phase short circuit. ③ The temperature rises of the rotor large teeth are larger than those of the rotor slot wedges and small teeth, the temperature rise of the rotor large tooth is the main factor that restricts the transient operating ability of the SC. ④ The negative sequence current under the asymmetric short circuit results in the larger rotor loss and temperature rise, therefore, the duration of the SC under asymmetrical short-circuit faults of transmission line is significantly smaller than that under the symmetrical short-circuit fault. © 2023 Chinese Machine Press. All rights reserved.