Verification of a Vertical Stabilization Converter for a Superconducting Coil by Using a Real-Time Simulator

In this paper, verification of operation and control of a vertical stabilization (VS) converter for a superconducting coil by using a real-time digital simulator (RTDS) is described. The VS converter supplies voltage (±1000 V) and current (±22.5 kA) to the superconducting coil for plasma current VS, and four-quadrant operation must be achieved without a zero-current discontinuous section. To remove any zero-current discontinuous sections, circulation current control is required, but it is hard to analyze and design it because of the characteristics of thyristor nonlinearity. In this paper, delay time of the thyristor and processor is implemented using a zero-order hold, and an equivalent model is designed and analyzed. Circulation current control and difference current control are similarly analyzed because both equivalent circuits are the same, and are satisfied with a 54° phase margin and a 11 ms step response. In addition, gamma control is designed to restrict the firing angle to prevent commutation failure and commutation failure is prevented by the proposed gamma control, even if a short circuit or sag occurs in the grid. To verify the feasibility of the VS converter, a real Zynq-based controller is assembled and a power system, including the VS converter, is implemented in an RTDS for the hardware-in-the-loop simulation. Moreover, the small-scaled VS converter is built and tested. Experimental results are well-matched to simulation results and analysis.