Investigation On The Performance Of Doubly Fed Induction Generator Under Unbalanced Grid Fault Condition

With the growing of the environmental problem, the wind power generation is considered to be the most useful and effective energy. But the increase of the demand of energy will result in the increase of the rate of the generator of wind power generation, which will influence the grid when it happens to be fault. So the grid have proposed very strict code of the operation of wind power generation. First, the power quality of wind power generation should be good enough to connect to the grid. Secondly, the wind power generation should obtain the Low Voltage fault Ride Through(LVRT) capability. Therefore, the theory and control strategy of the variable-speed constant-frequency(VSCF) performance of Doubly Fed Induction Generation(DFIG) based back-to-back PWM converters have been discussed and investigated.Firstly, the performance of DFIG based back-to-back PWM converters in which the conventional PI controller is applied are investigated and experimented under unbalanced grid voltage fault condition. The experimental results show that the conventional control scheme can not be suitable for the wind power generation on this fault condition. In this thesis, two brand new control strategies are investigated and applied both in the grid side and rotor side converters.As in the grid side converter, the mathematical models in the positive synchronous reference frame(SRF) and in the negative synchronous reference frame have been proposed as well as the power model. In order to improve the fault ride through(FRT) capability, three control targets have been proposed and investigated. The dual-PI controller, which is applied in the positive SRF to regulate the positive sequence component of grid current and applied in the negative SRF in order to regulate the negative sequence component of grid current, has been emphasized. The simulation result shows that the dual-PI current controller can obtain good performance on the condition of grid voltage fault. But the control scheme requires the decomposition of the positive and negative sequence components of the grid current, which will introduce considerable time delay in the current control loop. As a result, the dynamic properties will be deteriorated. Another control strategy, which named proportional resonant (PR) current controller, is proposed to overcome the shortage of the dual-PI controller. The PR controller is applied in the stationaryαβreference frame. So there is no decomposition of current in the current loop. Owing to this ,the dynamic properties of the converter improve. The experimental results will show that two control scheme both obtain good performance on the unbalanced grid voltage fault condition, but the PR controller obtain better performance in the transient progress during the period of the grid fault.As to the rotor side convenor, the conventional PI current controller will not obtain good performance under unbalanced grid voltage fault condition any more because of the unbalance of stator voltage and current and rotor current. So the detailed DFIG mathematical models in the positive SRF and negative SRF have been investigated. Four control targets have been proposed in order to improve the LVRT capability. The dual-PI rotor current controller and PR rotor current controller have been investigated and compared both in the control scheme and in the experimental result. And finally, the experiment results have been drawn that PR controller obtain better dynamic properties.In the end of the thesis, the designed and development of a 15kW rated hardware system have been proposed. And the experimental results will show that the control schemes of the grid side converter and rotor side converter can co-operated, and validate the theory proposed aboved.