Investigation On The Predictive Current Control Of DFIG System Under Unbalanced Grid Voltage Conditions

Nowadays, energy shortage and environmental crisis has become one of the world’s major problems, it is of necessity to find a kind of clean, renewable and environment-friendly new energy alternative to traditional fossil energy. Among various kinds of new energy technologies, wind power generation system based on the doubly-fed induction generator (DFIG) has become increasingly popular due to its mature technology and vast potential for future development. In practical systems, when unbalanced sags occur, the DFIG operating performance will be greatly affected, mainly causing unequal heating in the stator windings and the torque and power pulsations fluctuated at double the electrical frequency, which could force a disconnection between the DFIG-based wind turbines and the grid. Hence, it is necessary to find a suitable control strategy for DFIG system under unbalanced grid.This paper is aimed at the predictive current control (PCC) strategy of DFIG systems under unbalanced grid voltage conditions. The proposed PCC scheme was lucubrated from the aspects of theoretical research, simulation analysis and experimental verificationCurrently, linear PI controllers were mostly employed for the control of the DFIG system. PI controller gains excellent steady-state performance, but the dynamic response is slow and the control effect depends greatly on the PI parameters. In order to overcome the drawbacks of the PI controller, this paper presented a novel control scheme for DFIG system by replacing the PI controllers with PCC controllers. Firstly, the proposed PCC control scheme was applied for the control of DFIG system under ideal grid conditions, both the DFIG’s rotor-side converter (RSC) and grid-side converter (GSC) were modeled under synchronous rotating coordinate. And then, the PCC strategy was expanded to the control of DFIG system under grid voltage imbalance. Under unbalanced grid voltage conditions, the proposed predictive current controller applied in both RSC and GSC was developed in the aβ coordinate frame. Thus, the current control algorithm of the DFIG system was simplified as it would not be necessary for the actual rotor currents and the GSC output currents to be decomposed into positive and negative sequence components. Based on PCC, the DFIG system could achieve different control targets, including eliminating torque ripples, keeping the output currents of the whole system symmetrical and eliminating active power pulses of the whole system.However, in practical systems, predictor errors were introduced into the control system due to relatively low sampling frequency, and control delays caused by sampling and computation would lead to control error of the system. In order to eliminate control errors and optimize the operation performance of the DFIG system, detailed compensation methods were studied and implemented to improve the performance of the control scheme.The proposed PCC strategy featured excellent steady-state and transient response under unbalanced grid voltage conditions. Simulation and experimental results had verified the control strategy and shown good consistency with theoretical analysis.