Control and stability analysis of DFIG wind system at the load following mode in a DC microgrid comprising wind and microturbine sources and constant power loads

This paper deals with the coordinated control of DFIG wind turbine system and microturbine in a DC microgrid system feeding constant power load (CPL). The DFIG stator and rotor are connected to the DC grid through diode rectifier, buck converter, and frequency converter, respectively. The DFIG wind system depending on the load demand and wind speed operates in the power control mode or load following mode. In the power control mode, the DFIG wind system injects the available wind power to the grid and microturbine sets the DC bus voltage to its reference value. In the load following mode, the DFIG wind system regulates the DC bus voltage to a constant value, and the power injected by the microturbine decreases to a negligible value. Studies show that in the load following mode and under high penetration of CPLs, the study system may become unstable. Hence, this paper, first, develops control structures for the DFIG wind system in the DC microgrid at both operation modes, and then control structure of the DFIG wind system is modified to efficiently stabilize the system under high CPL penetration. Next, performance and stability of the system are examined analytically and by time domain simulations.