Investigations On Optimized Control Strategies Of DFIG-based Wind Power Generation System Under Distorted Power Grid

The doubly fed induction generator(DFIG)has occupied the majority wind power market due to its outstanding merits of variable-speed constant-frequency operation mode,adjustable power factor,small rating of power converters and so on.Since the stator windings of DFIG are directly connected to the power grid,the operation performance of DFIG is susceptible to the grid voltage conditions.When the grid voltages are nonideal that contains unbalanced or harmonic components,stator current harmonics,power pulsations,electromagnetic torque oscillations and dc-link voltage fluctuations would appear as a result,which not only deteriorate the power qualities of DFIG system generated to the power grid,but also threaten the safe operation of DFIG system.Thus,it is of practical significance to investigate the optimized control strategies of DFIG under nonideal power grid conditions.This paper takes DFIG as the research object and the following innovative research results have been achieved by theoretical analysis,control scheme design and experimental research.1.For the grid side converter(GSC)of DFIG.the improved three-vector based predictive control strategies are proposed.The conventional three-vector based model predictive direct power control(MPDPC)strategy select the three converter vectors according to the angular position of the grid voltage vector,which would result in the negative duration times of these vectors in some region and power together with current ripples appear.This paper proposes two methods to improve the steady performance of the conventional three-vector based MPDPC strategy.The first method select the three converter voltage vectors based on the power errors rather than the angular position of the grid voltage vector.Thus,the predicted duration times are ensured to be positive.The second method still selects the voltage vectors based on the angular position of the grid voltage vector.By adopting the optimized output voltagte vector sequence,the pre-selected voltage vectors can be transformed into more suitable ones once the duration times are negative.Both methods can suppress the power and current ripples.Based on the improved three-vector based MPDPC strategy,the three-vector based MPDPC and model predictive current control(MPCC)strategies of GSC under unbalanced and low order harmonic grod voltage conditions are investigated.The qualities of output currents can be improved.2.For the rotor side converter(RSC)of DFIG,the model predictive stator current control(MPSCC)strategy is proposed.Unlike the MPDPC and model predictive rotor current control(MPRCC)strategies,the proposed MPSCC strategy directly controls the stator currents.Thus sinusoidal and balanced stator currents can be obtained even when the grid voltages contain unbalanced and harmonic components.In order to decrease the calculation complexity of the predictive control strategy,the low-complexity algorithm has been designed for the MPSCC strategy,which makes it suitable for implementing the predictive control strategy in general purpose control chips.Before connected to the grid,stator voltages that are identical to the grid voltages can be established on the basis of the proposed virtual stator currents concept.Thus the smooth grid connection process can be obtained.3.When the.power grid contains both low order and high order harmonic components,the space vector modulation direct power control(SVM-DPC)strategies based on repetitive control(RC)regulators are proposed.No sequential extractions are needed through direct-RC approach for the control variables and sinusoidal stator currents,smooth powers or smooth electromagnetic torque can be obtained.From the point of system security,the independent control strategy of GSC and RSC is proposed to realize smooth electromagtic torque and smooth dc-link voltage.Since the power qualities are determined by GSC and RSC simultaneously,the' coordinated control strategy is proposed,where the GSC would output the same amplitude but oppsite sign grid currents of the stator currents and the overall currents would be sinusoidal.