Research On Several Issues On Half Quasi-Z-source Wind Power Conversion System With D-PMSG

Wind power generation is an important energy strategic deployment in China.The development of wind power generation is very meaningful to save energy and protect the environment.And it is important for our country under the new round of global energy change and economic competition.The converter is the core device in the wind power generation system(WPGS).It not only directly affects the reliability and stability,but also the efficiency and cost of the whole system.Compared with the traditional voltage fed PWM inverter,voltage fed impedance-source inverter can advantageously use the shoot through states to boost voltage in a single stage without extra power switch.The application of the impedance-source inverter to the permanent magnet direct drive WPGS provides a new way to further improve the reliability of the grid-connection and reduce the system cost.This paper studies on the impedance-source wind power conversion system through the theoretical analysis,software simulation and experiment through the financial support from the National Natural Science Fundation(51377050).The research is focus on the novel half quasi-Z-source inverter topology,the input power factor correction of three-phase diode rectifier,the ripple suppression of half quasi-Z-source network,operation and control strategy of half quasi-Z-source wind power conversion system under the asymmetric fault of AC grid.The detail contents are as follows:This paper proposes a novel half quasi-Z-source inverter(H-qZSI)for grid-connected wind power conversion system.The operation principle of H-qZSI is discussed in detail during shoot-through and non-shoot-through state.The relationship between the boosting factor and the shoot-through duty ratio of the quasi-Z-source inverter is analyzed.It is concluded that the average output voltage of the quasi-Z-source network is the steady voltage value of the impedance-source capacitor.The voltage gain,switching voltage stress and switching current stress of the three-phase voltage-source quasi-Z-source inverter under different modulation strategies are discussed.The small signal model of the quasi-Z-source network is established,and the transfer function of the DC input voltage and the shoot-through duty ratio to the impedance-source network inductance and the impedance-source network capacitance is deduced.The influence of the impedance-source network inductance and the impedance-source network capacitance on the dynamic performance of the system is analyzed.Finally,the full range power control of the half quasi-Z-source wind power conversion system is realized.The generator stator current gets distorted with unacceptable levels of total harmonic distortion(THD),because impedance-source wind power conversion system using the three-phase diode rectifier.The stator current harmonics will cause increasing losses and torque ripple,which reduce the efficiency and stability of the system.In this paper,the reason of the stator current’s high THD phenomenon is analyzed for the traditional impedance-source wind power conversion system and half quasi-Z-source wind power conversion system in detail.It is found that the stator current harmonics of PMSG in half quasi-Z-source wind power conversion system are effectively suppressed compared with the traditional impedance-source wind power conversion system.Then,the input power factor of the half quasi-Z-source wind power conversion system is analyzed,and a control method with periodic shoot-through duty ratio is introduced,which can further improve the input power factor of the system.The introduced impedance-source network,ncluding the inductors and the capacitors,increases the size and weight of the system greatly.There are difficulties in equipment installation and transportation.At the same time,the system power density is also reduced.In this paper,the formation mechanism of the impedance-source inductor ripple current and impedance-source capacitor ripple voltage are analyzed under the six-stage boost space vector pulse width modulation(SVPWM)in detail.Compared with the quasi-Z-source wind power conversion system,the half quasi-Z-source wind power conversion system has a lower impedance-source network ripple.Then,an improved six-stage boost SVPWM method for half quasi-Z-source wind power conversion system is proposed.The effect of improved six-stage boost SVPWM method is verified under the inductor current ripple optimum and the capacitor voltage ripple optimum.Asymmetrical grid voltage will introduce negative sequence current components on the grid side and cause large voltage ripple of half quasi-Z-source capacitors.It will lead to the torque ripple of generator.This paper proposes a novel control method based on super-capacitor to improve the capability of asymmetric fault ride through in half quasi-Z-source wind power conersion system with the control objective to maintain the symmetrical grid-connected current.The stream of system power,in the condition of unbalanced grid voltage,is discussed in detail in this paper.In order to limit the voltage rise and suppress the 2-order voltage ripple of half quasi-Z-source capacitors,the throughput power of super-capacitor is controlled by proportional-resonant controller to track the output power subtraction between generator and half quasi-Z-source inverter.The 2-order voltage ripple of half quasi-Z-source network and the negative-sequence component of grid-connected current are significantly suppressed under the asymmetrical grid voltage fault.