Modeling and analysis of the inverter type high voltage power supply with duty cycle modulation

The acceleration grid power supply (AGPS) is an important part of neutral beam injector (NBI) system. With the increasing of the NBI power, the requirement for output voltage of the AGPS is becoming higher and higher, even reaching to 1000 kV. As the Inverter type high voltage power supply uses the inverters located at lower voltage side to regulate and cut off the output voltage, it's very suitable for high voltage applications. The advised scheme for the AGPS of ITER NBI system is composed of five stages, each stage is an inverter type power supply with an output voltage of 200 kV. This paper investigates the inverter type power supply based on neutral point clamped (NPC) three phase three level (TPTL) inverter and three phase step-up transformer, which is equivalent to an insulated TPTL dc-dc converter connected with a dual dc link. In order to design a preferable controller for the inverter type power supply, the topology and principles of the TPTL dc-dc converter with duty cycle modulation has been analyzed. Our results indicate that only when the converter operates at a large duty cycle, the output voltage ripple can be limited to ± 5%. So the small signal model of the converter at a duty cycle larger than 2/3 is established. Following the mathematical model, a single loop PID controller has been designed. Simulation results verify the correctness of the theoretical analysis. The system achieves lower output voltage ripple and fast dynamic response.