Research On Converter Control For Wind-Solar-battery Hybrid Power Generation System Under Autonomous Operation In Microgrid

Wind, solar and other intermittent renewable energy directly connected to the utility grid in the traditionally mode are regarded as uncontrollable power supply. Because the renewable energy power system has direct impact on the stability and security of the grid, may decrease the grid power quality, which are connected to the utility at the traditional large scale. Furthermore, in case of electrical grid internal fault; the renewable energy inverter must be immediately disconnected from the grid, the disconnection from the grid affect the renewable energy utilization efficiency. Currently domestic wind power farm has to "abandoned wind" or "shutdown" wind power turbine due to the wind power "inferior" characteristics. Micro-grid as a form of integration of wind power, photovoltaic power generation, and energy storage system has brought hope for the utilization of renewable energy. This paper focuses on the doubly fed induction generator, photovoltaic power, and energy storage interface inverters control problems in the autonomous microgrid. The main work of this paper is as follows(1) At first, the doubly fed generator physical model and mathematical model is established in autonomous operation, comparing the mathematical model of gridconnection doubly fed induction generator with the model in autonomous operation mode, the stator voltage are taken as the control target rather than the stator current which is the control target in the grid-connection generator. The mathematical derivation proves that simultaneous satisfaction of the doubly fed generator stator constant flux and constant frequency is the sufficient conditions to achieve the stable stator voltage.(2) Research on the autonomous doubly fed wind power generator stator voltage control problem, exploring the control strategy which make the doubly fed induction generator support the micro-grid voltage. The starting control strategies of DFIG without load and with load are researched. The stator voltage transient waveform and steady-state accuracy is the key problem for independent operation control. The control strategy of stator voltage loop and the enforced flux oriented is introduced in the dissertation.(3) Research on the ouput characteristics of the energy storage system interface inverter. The bridge leg and inverter output dual voltage close-loop control strategy for the three phases three level inverter with a output voltage natural droop characteristic is proposed. The proposed control strategy abandoned the traditional inverter filter inductor current and the output voltage dual closed-loop control without inductor current feedback. The inverter output filter inductance has not only filtering function, but also is equivalent to the coupled inductor between the inverter and the micro-grid. The dual voltage closed-loop control inverter with output natural droop characteristics can be applied to the multi- inverer parallel operation of microgrid.(4) Research on PV micro-souce interface inverters power sharing problem under the condition that the the low-voltage microgrid line impedance is not longer pure indctive or resistive. In order to solve the power sharing problem caused by the difference among the lines impedance, this paper proposed variable virtual impedance droop control strategy base on the line impedance angle transformation control method. Firstly, the output power equation of voltage type interface inverter connected to microgrid is derived based on the stable AC circuit model,then the factors that affect the inverter output power is analyzed. The derived micro-source inverter ouput power equation shows that the line inductor and resistor value ratio determines the inverter active and reactive power coupling degree.(5) Research on the reactive power droop control strategy applied in the chargeable battery interface three phases three level inverter for energy storage. The reactive power droop control including an integrator is proposed for the the chargeable battery energy storage interface inverter paralleled to the autonomous doubly fed induction generator to improve the power fluctuation of the wind power and PV power. The simulation of the energy storage system in the autonomous wind power generation system was carried out in power charging and releasing mode respectively. The simulation result shows that energy storage system can absorbe the surplus wind power or vice versa。...