Current transfer characteristics of control switches in a DC controllable energy dissipation device

To limit the overvoltages of the flexible DC converter valves and absorb surplus energy, a DC controllable energy dissipation device based on metal oxide varistors has been applied for the first time in the Baihetan-Jiangsu ±800 kV hybrid cascaded ultra-high voltage direct current (UHVDC) transmission project. The control switches of the device consist of three types of switches connected in parallel, so the transient current transfer and distribution among the three control switches are critical to the safe operation of the device. Firstly, the operating conditions and current transfer characteristics of the control switches are analyzed. Theoretical calculations and electromagnetic transient simulations are performed on current transfer characteristics under a sinusoidal half-wave current waveform. The results show that after the fast switch is closed, the current in the thyristor switch branch can be rapidly transferred to the fast switch branch, and the thyristor turns off reliably. Furthermore, the current transfer characteristics among the control switches are validated via a synthetic test circuit, and the test results verify the accuracy of theoretical calculations and simulations. In the tests, the thyristor turn-off time is delayed by approximately 5.7 ms compared with the simulation results, mainly due to discrepancies in the nonlinear characteristics between the simulated and actual saturated reactors. This reduces the current transfer rate of the thyristor branch. After adjusting the saturated reactor parameters in the simulation, the thyristor turn-off time in the simulation lags the test result by about 1.8 ms. Finally, the current transfer characteristics between the control switches under engineering operating conditions are verified using the electromagnetic transient model of the Baihetan-Jiangsu hybrid cascaded UHVDC system. Under extreme operating conditions, the current through the fast switch branch accounts for approximately 30% of the total current.