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.