An optimized bidirectional buck–boost​ converter for DC bus voltage stabilization in new generation poloidal field power supply

Abstract

In the Tokamak device of controllable nuclear fusion, the poloidal field power supply (PFPS) provides the dynamic magnetic field inside the device. The power level of full-controllable devices has been improved with the rapid development of high-power semiconductor technology, which promotes the upgrade of poloidal field power supply. The PFPS adopts an AC–DC–AC circuit structure, which makes the load coil operate in four quadrants. Because the load is a superconducting magnet coil, the power of the load coil will change for a period of time to meet the demand of the internal magnetic field of Tokamak, resulting in the instability of DC bus voltage. This paper proposes an optimized bidirectional buck–boost active storage unit (ASU) with unidirectional inductor current, which can quickly make the bus voltage stable. In the circuit structure, the direction of the inductor current is unidirectional, which avoids the slow response of the inductor current in the reverse direction of the traditional buck–boost converter; And the inductor has a controllable freewheeling circuit, which can reduce the disturbance of inductor current to bus charging. The number of inductors is small, which avoids the large volume of traditional dual-inductor DC–DC converter; Compared with the traditional buck–boost converter, the buck–boost ASU with unidirectional inductor current improves the energy utilization of the bus capacitor. By establishing the mathematical model of the circuit topology, the dual closed-loop controller and modulation strategy are used to control the stable operation of the system. The simulation results show that the circuit structure can balance the bus voltage well and keep stable, when the load power changes suddenly.