Control Strategy Of DFIG In Distributed Generation System Based On Voltage Source Output Mode

Nowadays, with the extensive research on renewable energy generation technologies, control strategies of doubly fed induction generator (DFIG) wind system based on current output mode have been thoroughly studied. But the current output mode of DFIG relies on a relatively stable grid and that without doubt will limit the application of DFIG in distributed generation system, a power system form which has enjoyed worldwide popularity recently.Aiming at achieving DFIG’s flexible and effective penetration into distributed generation system, firstly, this paper applies indirect stator flux orientation (ISFO) strategies to the control of rotor side converter to establish its voltage source output characteristics. Along with the specially designed pitch angle control algorithms, ISFO can fit well for the autonomous working mode of DFIG. Secondly, taking into account that the distributed generation system generally consists of multiple parallel connected units, the droop characteristics is introduced into the control of DFIG, which lays the base of power assignment mechanism when DFIG need to cooperate with other units. And also, the droop control enables the distributed generation system with multiple parallel connected generation units to work under autonomous mode. While at the same time, to test the coordination of ISFO and droop, the small-signal mathematical analysis model of DFIG’s electrical part is constructed. Then the influence of critical control parameters on DFIG control system has been studied, which provides a necessary criterion for the stability analyze. After that, to realize the gird-connected working mode and seamless switch of distributed generation system, by reference to the secondary power system adjustment method, operating point regulation mechanism when islanded is achieved by feeding back the voltage frequency and amplitude of the point of common coupling (PCC), and also, output power adjustment mechanism when grid-connected is achieved by feeding back each generator’s output power. In all this process, the control algorithm of DFIG keeps a good consistence and there is no change on the hardware structure. Based on the secondary adjustment method, the maximum power point tracking (MPPT) of DFIG is provided to enhance the efficiency of wind system in distributed generation system when grid-connected, And of course, DFIG persists on voltage output mode.To dealing with imbalanced conditions, a revised ISFO is come up with to make DFIG suitable for an autonomous imbalanced local load. Also, considering that DFIG’s collaboration with other distributed generation units may be interrupted by imbalance when islanded, negative sequence droop is designed, which forms a virtual conductance and leads to the change of negative sequence resistance circuit. And eventually, negative sequence current is well assigned and the PCC voltage balance is well preserved under an extremely unbalanced load.The control strategies mentioned of above need to be verified, so with the help of MATLAB/Simulink, simulating models involve either a single DFIG or a whole distributed generation system is built. And the simulating results well testified those algorithms. At last, the hardware experimental platform of DFIG is established, and together with the energy storage platform and the distributed generation system central controller, a series of experiments are conducted, which validate the proposed control strategies of DFIG in a distributed generation system under various working mode.