Investigations On Direct Power Control Of DFIG Wind Power System Without Phase-Locked Loop Under Nonideal Power Grid

With the fast development of the global energy,wind power generation systems based on the doubly fed induction generators(DFIGs)have been widely employed.However,the unbalanced and distorted voltage at the point of common coupling(PCC)will deteriorate not only the operation reliability of DFIG system but also the injected power quality,which may not meet the grid requirements.Thus,it is of great importance to investigate the control strategies with high performance for DFIG under nonideal grid voltage conditions.However,most of control strategies proposed present are implemented in the synchronous reference frame based on the phase-locked loop(PLL),whose behavior will worsen with the phase detection error under the nonideal voltage conditions.Thus,the control performance of DFIG and even the system stability will be severely threatened.Therefore,this paper investigates an improved direct power control(DPC)strategy without PLL for DFIG systems under nonideal grid voltage conditions,which is implemented in a virtual reference frame with fixed angular speed.With mathematical model analysis,control scheme design and experimental validations,the following research results have been achieved.1.Based on the mathematical models established in the virtual dq reference frame,an improved DPC strategy without PLL involved is proposed for grid-connected voltage source inverters under distorted grid voltage conditions.In this way,the PLL can be avoided and its potential phase detection failures under adverse voltage conditions can be eliminated.A second-order vector integrator(SOVI)is integrated with DPC to implement the direct-resonant control,achieving three different control targets of smooth active power,smooth reactive power or sinusoidal output current,alternatively.Meanwhile,in order to guarantee the harmonic suppression ability against grid frequency variation,a frequency estimation algorithm based on the resonant controller is designed,and it has been proved that harmonic distortions can be precisely eliminated with real-time tracking ability.2.For the rotor side converter of DFIG,the DPC strategy without PLL is also proposed under distorted grid voltage conditions.With the DFIG mathematical models established in the virtual dq reference frame,the pulsation characteristics of the stator powers,currents and electromagnetic torque have been analyzed under distorted voltage conditions.In order to improve the operation performance of DFIG system,with the accurate grid frequency obtained by the frequency estimation algorithm,the frequency adaptive SOVI is integrated with DPC to directly eliminate the harmonic components of currents,powers or electromagnetic torque alternatively.3.When the grid voltage encounters both unbalanced and harmonic conditions,the mathematical models of rotor-side converter and the grid-side converter of the DFIG system have been analyzed in the virtual dq reference frame,so that a collaborative DPC strategy without PLL is proposed for DFIG systems under unbalanced and distorted voltage conditions.Based on a frequency adaptive dual-frequency SOVI(DFSOVI),the direct-resonant control can be implemented to promote the operation performance of DFIG and the power quality,where smooth electromagnetic torque is achieved by the rotor-side converter,and the grid-side converter serves as the compensation for the stator-side output to guarantee the sinusoidal and balanced output current.