Research On Voltage Ripple Suppression Methods Of DC Microgrid Under Unbalanced Conditions

Microgrid,which is based on distributed generations,can promote large-scale access of renewable energy,and can improve the reliability of power supply for different types of loads.Therefore,it will be an important part of the smart grid in the future.As one type of microgrid,DC microgrid has gained extensive attention for a number of structural advantages.Under unbalanced grid voltage and unbalanced load,second order pulsation components caused by negative components of the AC side will be brought to the DC side power and DC bus voltage,which deteriorate the power quality of DC microgrid and also have influences on normal operation of the equipment.Hence,it is essential to study the DC voltage ripple suppression methods of DC microgrid under unbalanced conditions.In this thesis,concrete analyses are conducted and DC voltage ripple suppression methods are designed under unbalanced grid voltage and unbalanced load.The main work and achievements are as follows:(1)Based on the influence analyses of unbalanced grid voltage and unbalanced load on DC microgrid,it is stated that DC voltage pulsation component should be eliminated locally,that is,when the DC voltage pulsation component is caused by unbalanced grid voltage,it should be eliminated by improving the control strategy for DC/AC converter;when the DC voltage pulsation component is caused by unbalanced load,then a compensation device is proposed to absorb the pulsating component of DC power and suppress the DC voltage ripple.(2)A control strategy of the grid-connected VSC under unbalanced grid voltage is proposed based on sliding mode control(SMC),which is aimed at eliminating the second order pulsation power component of the grid,thus suppressing the DC bus voltage ripple.The proposed control strategy can achieve satisfactory performance with well-smoothed DC voltage,but the grid-connected current is distorted.Super-twisting algorithm of the second order SMC is employed to avoid chattering,and stability along with robustness of the proposed control system are analyzed.(3)A super-capacitor(SC)based DC bus voltage ripple suppression device is proposed under unbalanced load,and a current control strategy of the SC is designed adopting SMC approach to quickly and precisely track the reference current.The proposed control strategy has significant effects on DC bus voltages under various unbalanced load conditions.And stability along with robustness of the proposed control system were analyzed.Furthermore,considering the most serious condition of unbalanced load,parameters design of the super capacitor based compensation device is given,which can can avoid waste of the capacitance.(4)Considering cost and reliability of the system,an improved control strategy of the DC bus voltage ripple suppression device is proposed introducing a nonlinear disturbance observer.The improved device can not only do without real-time data collection of the DC microgrid,but also effectively suppress the DC voltage ripple.Besides,it can coordinate with the grid-connected VSC to improve quality of the DC voltage as well as the grid-connected current under unbalanced grid voltage.