Adaptive Compensation And Stability Analysis Of Active Power Filter In Microgrid

Active power filter,as a kind of power electronic device that can dynamically compensate the reactive power and harmonic power in the grid,is widely used in the occasions where it is urgent to improve the power quality due to its advantages such as flexible configuration,low loss and small size.APF is essentially a grid-connected inverter,which is equivalent to a current source type output converter.Compared with the general grid-connected inverter,APF not only needs to track the fundamental current,but needs a harmonic current control loop.Therefore,the design and stability analysis of its controller are different from those of the general gridconnected inverter.APF can dynamically adjust its output when the operating condition of load changes to ensure neither under compensation nor overcompensation.When the load is too heavy and the power to be compensated exceeds the rated output of APF itself,the output of APF shall be limited to ensure reliable and stable operation while maximizing the capacity of the device.Micro power grid structure is an effective way to optimize energy supply structure today,and the power quality problems exist in micro grid and large power grid,such as low power factor,high THD.APF is adopted to improve the power quality as the mainstream way,but the micro grid with the internal impedance considered,will cause interference on the stability of the control of APF and affect compensation performance.Firstly,the mathematical model of APF is deduced.Based on the instantaneous power balance method,the transfer function of active current to DC bus voltage is obtained.Based on the transfer function,the voltage outer loop PI controller is designed.The current inner loop controller with PI and repetitive parallel compound structure is designed and its stability is analyzed.Secondly,the maximum compensation capacity of APF is theoretically calculated.Under the limitation of APF capacity,an adaptive harmonic current compensation method based on second-order generalized integral and recursive discrete Fourier transform is proposed to ensure the safe and stable operation of the device while giving full utilization rate of capacity.Finally,the stability of APF operating in micro grid is studied,and the influence of the damping on the output admittance matrix model of APF is analyzed and the output admittance matrix model with repetitive controller is also established.The stability of cascade system of APF and weak grid is studied by means of generalized Nyquist stability criterion.