Research On Harmonic Suppression Strategy Of Converter In Direct Drive Wind Power System

With the problems of environmental pollution in the use of conventional energy,efficient and clean wind energy has developed rapidly in the world.In recent years,China’s wind power technology has made great progress.Direct drive permanent magnet wind turbine without gearbox has the advantages of high energy conversion efficiency and good grid compatibility,so it is widely used.With the rapid development and widespread application of power electronic equipment,harmonic pollution has gradually attracted people’s attention.In addition,grid connected inverter,as the grid connection interface of renewable energy generation,is very important to improve the power quality and reliability of wind power system.However,in the harmonic power grid,the vector control characteristics of grid side inverter are affected by harmonic voltage,resulting in the grid current mixed with low order harmonics,and the system output power appears frequency doubling fluctuation,which may lead to DC bus voltage imbalance.Based on the harmonic harm caused by converter in direct drive wind power system,this paper studies the harmonic suppression strategy of generator side and grid side converter.Firstly,the high-power converter is the largest harmonic source in the system.In the direct drive wind power system,the generator is directly connected with the full power converter.Therefore,the output voltage and current harmonics of the converter are stator harmonics.A large number of harmonics will seriously affect the normal operation of the generator,reduce the efficiency of the generator set and shorten its service life.Due to the use of high-power converter,the stator current of permanent magnet generator contains a lot of low-frequency harmonics.By establishing the harmonic mathematical model in DQ rotating coordinate system,the harmonic suppression link is introduced into the converter double closed-loop vector control system to compensate the 5th and 7th harmonics.At the same time,the peak filter with better filtering characteristics is proposed to replace the low-pass filter to obtain better harmonic current extraction effect.Secondly,in order to improve the adaptability of grid connected inverter under harmonic grid voltage,enhance the stability of the system and improve the output power quality of the system,an improved power control strategy of multiple sampling is proposed.Based on the traditional direct power control,a PIR feedforward harmonic compensator is added.By selecting the current or power feedback signal,two independent control objectives are achieved: 1.Three-phase grid-connected current is smooth and sinusoidal;2.The output active and reactive power is stable without fluctuation.Aiming at the problem of time delay in the DSP digital control process of PWM grid connected inverter,the multi sampling strategy is introduced to solve the problem that the actual resonant frequency of the resonant controller deviates from the theoretical set frequency caused by time delay,and improve the harmonic suppression effect under the two control objectives.Finally,based on Matlab/Simulink platform,the harmonic suppression models of 1.5MW direct drive wind power system and 7k W grid connected inverter system are established respectively for simulation,and compared with other control methods,the effectiveness of the proposed harmonic suppression strategies of machine side and grid side converters is verified.