Research On Control Technology Of DFIG Under Grid Faults And Grid Voltage Unbalance Conditions

As the converter only manages the rotor power, it can be rated at slip rated power, which can achieve advantages of small size, light weight, low cost, and small losses compared to direct-drive wind power systems with a full scale power converter, nowadays, doubly fed induction generator (DFIG) based wind power system is dominant above 1WM wind power system and it is an essential aspect of wind power technology. With the development of large scale wind power, the capability of low voltage ride through(LVRT) and resisting grid voltage unbalance for grid connected wind power system, Transient behavior of DFIG, LVRT and unbalanced control technology are studied in this paper. The primary contents and original contributions of this dissertation are as follows:The transient characteristic of DFIG under grid faults is very important for analysis of its behavior during grid and LVRT control, now the transient characteristic under symmetrical grid fault with rotor side terminal shortcircuit is clear, but the theoretical analysis of electromagnetic under asymmetrical grid faults is lack,based on the study of symmetrical grid fault, the theoretical analysis of electromagnetic under asymmetrical grid faults are studied in this paper,The mathematical models under different reference frame of DFIG are established, based on its space vector model, the transient behaviors of DFIG are derived and analyzed, the simulation results verify the theoretical derivation. It concludes from expressions of flux, current, power and torque that the negative component is the root cause of active power, reactive power and torque pulsation.Crowbar resistor selection is crucial for carring out effective protection with traditional crowbar protection scheme.Its range can be gotten with mathematical derivation based on a series of approximations and specified running condition,therefore, it has errors and its specific value is not given, in addition, the derivation of crowbar range is not based on the extreme running condition and it not also considered its influence on power and torque.In this paper, a selection method of crowbar resistor is proposed, firstly, considering influence of the running conditions on rotor current, the extreme running conditions are confirmed with simulations, then, under such condition, according to consistence that limiting rotor current is simultaneously limiting power and torque oscillation, the value of crowbar resistor is decided by considering the use of crowbar limits the rotor current at the most, its decision also considers the voltage drop cross crowbar resistor is lower than the maximum value of output voltage of rotor side coverter, the simulative results validated effectiveness of LVRT with crowbar resistor selecte with such method, It concludes that the current of stator and rotor, the power and the torque can be restrained at most after optimization.The DC and negative component of stator flux can induce very large voltage at rotor terminal, so in order to avoid overcurrent at rotor side, rotor side converter must output a very large voltage to balance with it, but the large voltage can exceed the maximum output voltage of rotor side converter much more; demagnitizing control strategy is based on the control of machine side convter itself, its effectiveness is limited by the severity of grid fault and the leakage resistance of DFIG, the magnetizing current under deep dip may exceed the limit of rotor sid converter current, which can damage the converter, in light of such defect, an improved LVRT control strategy for rotor side converter is proposed, the margin of the output voltage of rotor side converter is used to introduce virtual resistance, which can decrease the required demagnitzing current during grid faults; load current forward control strategy is employed for grid side converter during grid faults, because the power loss of input filter is ignored in this strategy, which leads to a large flucuation of DC link voltage during grid faults, in this paper, an improved control strategy of grid side convter is proposed in which the power loss of input filter is not ignored and the computation of the forward current is with convter side power instead of grid side input power, the simulative results validated the effectiveness of rotor sided control strategy and grid side control stragtey, even in the case of severe grid faults, the improved control strategies can still effectively decrease overcurrents on rotor side converter and large fluctuation of DC link,hence, the LVRT capability is improved.Positive and negative component extraction is the pre-conditions of unbalance control, which can be employed with low pass filter or noch filter, however, the filter is an independent part in control system, the decomposed speed is slow and it leads to a large delay,which can influence the stablity,in addition, in order to gurantee the decomposed accuracy, the filter order must be high, but the higher filter order, the worse the system stablity, so the use of filter is also increase the difficulty of PI parameter selection of controller, to vercome the shorcomings of filter, a fast positive and negative extrcation method is adopted, based on it, an improved machine side strategy and an improved grid side control stragtey are proposed, the simulation validated the effectiveness of the fast extraction method and the improved unbalanced control stragtegy, It concludes that the improved machine side and rotor side unbalanced control strategies can effectively realize the specified control targets.