Study On Novel Permanent Magnet Doubly Fed Induction Generator And Control Strategy

As the increased penetration of wind power generations in power, modern grid codes concerning grid-connected wind turbines are developed, energy and environmental issues is a major strategic issue related to global sustainable development. The efficient use and development of renewable energy has become particularly important. The wind is a renewable energy technology not only saving energy but also protecting environment. However, with the wind powder installed capacity increased, wind power proportion in the power system is growing. The interaction between the wind power system and power network is more and more obvious. As the wind power generation system are mostly located in complex terrain, changeable weather condition in remote mountainous, far away from the grid, and the line is longer, so the voltage fluctuates relatively largely. Because of the randomness of wind speed, resulting in wind power generation systemis easy to generate oscillation fault when faults occur in the grid. In order to solve these problems, in the grid connected wind power generation system must have low voltage ride through capability(LVRT). The control strategy can prevent the wind power generation system from the grid during the grid fault, which ensure the stable operation of power system.A lot of researches concerning doubly fed induction wind power generator were carried out, but the main focus in the research of control strategy on the generator, Innovation and development in the structure of generator is more difficult. From this aspect, the operation performance of doubly fed generator can be improved, it is quite meaningful. This paper mainly studies on the existing double fed wind generator as the foundation, from the two aspects of structure and control strategy of DFIG at the same time, based on structure of DFIG, puts forward a novel permanent magnet doubly fed wind power generator (PMDFIG). Based on the transient mathematical model of the generator, the structure, the control strategy and dynamic performance of PMDFIG are comprehensive studied. According to the structure characteristics of a new type of doubly fed generator, corresponding grid connected control strategy is proposed. The model of doubly fed wind power generator connected to the grid is proposed. The improved the performance is conducive under the stable operation of power grid.The main contents of the dissertation are as follows:(1)Based on the analysis of permanent magnet machine structure and working principle, a novel permanent magnet doubly fed wind power generator was proposed to overcome the disadvantages of the present DFIG. The novel permanent magnet doubly fed wind power generator has a permanent magnet rotor inside a wind rotor. The wind rotor is linked to the shaft, and the built-in permanent magnet rotor is free to rotate against the shaft. One of the notable features of the PMDFIG is that the power density is higher compared to a conventional DFIG. The two-dimensional finite element electromagnetic simulation is conducted. The mathematical model concerning PMDFIG are established in the three-phase stationary reference frame, two-phase stator stationary reference frame and two-phase rotating reference frame at arbitrary angular speed, respectively. The mathematical model concerning the grid side converter are established. The rotor side converter uses double closed loop control strategy oriented the stator voltage vector, which provides a theoretical basis for the effective implementation of control strategy of fault cases permanent magnet doubly fed wind power generation system. Steady simulation analysis verified that the established mathematical model is correct. We also prove that the PMDFIG has superiority than traditional doubly fed induction generation.(2) The operation and control strategy of the PMDFIG wind power system are conducted when network voltage sag conditions. Control design of dynamic modeling and uninterrupted operation ability, Mainly includes:analysis of the dynamic process of PMDFIG when voltage sags, the rotor side converter control strategy considering the excitation current of stator, the load current feed forward compensation and voltage control for grid side converter control strategy.(3) When the grid voltage imbalance is more serious and without corresponding control measures are taken, the generator will have to be out of the grid. Therefore the power grid control strategy will be put into effect. Therefore in the unbalanced fault conditions, it is necessary to study the stability of permanent magnet doubly fed wind generator and grid maintenance. Under unbalanced network voltage condition, by using the method of positive and negative sequence component decomposition, the control model,the power equations and torque equations are deduced. The control target of PMDFIG is carried out under fault conditions, and the effectiveness of the control strategy is verified by simulation.(4)To improve LVRT performance of PMDFIG under severe grid voltage fault, the dissertation designs and develops a new rotor-side converter strategy for a new type PMDFIG. Firstly, the traditional PMDFIG control method of Crowbar is carried out, the influence of the parameters of Crowbar controller are emphasized and the Crowbar control circuit resistance values are determined. A crowbar circuit control model and simulation are analyzed. Under severely grid voltage dip, the control strategy of the permanent magnet doubly fed generator is put forward, excitation current of stator changes of zero torque, Zero torque respectively by traditional Crowbar control and stator excitation current change control strategies are compared and simulated. The simulation results show that, the change of zero torque and stator excitation current control strategy of doubly fed generator can improve low voltage ride through operation ability.