Cascaded Voltage-Control Method for a Diode-Front-End-Based Variable-Speed Synchronous Generator in a DC Power System

This paper proposes a novel cascade voltage-control method for DC variable-speed synchronous generators with a diode rectifier. In order to emphasize the necessity of the proposed method, the light-load voltage fluctuation problem that affects the conventional control method is analyzed based on a simplified excitation system model. In this analysis, two operation modes of the diode-front-end rectifier are presented: the rectifier mode and the discharging mode. Between the two modes, integral windup in the discharging mode is considered to be the main cause of light-load DC voltage fluctuations in the conventional voltage-control method, and a range of parameters to avoid the discharging mode is designed considering the characteristics of DC variable-speed synchronous generators. However, because there are tradeoffs between the response time and stable operation in the conventional method, a cascade voltage-control method is proposed to improve the performance in both modes. In the proposed method, the voltage-control loop is divided into an inner and outer voltage-control loop. The inner loop indirectly controls the DC voltage with fast dynamics while the outer loop ensures stable and accurate operation. When applying the proposed method, no windup occurs in the discharging mode, eliminating the cause of light-load voltage fluctuations. The proposed cascade voltage-control method is verified through both a PSIM-based simulation and experimentally.