Research On Technologies Of Reactive Power Compensation For LCL-filtered STATCOM

With the increasing presence of the renewable energy sources,the fault tolerance capability of the power grids has been paid growing attention to.Among the devices of the grid,STATCOM(Static Synchronous Compensator)is capable to compensate the reactive power and support the grid voltage,which is benefit to the reliability and fault tolerance capability of the grid.In this dissertation,the important technologies of the reactive power compensation,including the estimation of the grid voltage,the design of the current reference and current tracking,are studied.The contributions of this dissertation are as follows.1)A reduced-order adaptive observer is proposed for the case of the grid under voltage unbalanced fault or the voltage parameter jump faults.This method is capacity to estimate the positive-and negative-sequence components with zero steady-state error.In contrast to most existing estimation schemes of the grid voltage parameters,this method regards the grid voltage as a parametric dynamic system related to an unknown frequency of the grid.This method does not rely on the PLL.As a result,when the phase angle of the grid voltage changes significantly,the proposed method is global stable with a zero steady-state estimation error.2)Considering a more general case that the grid involves harmonic voltage components,a passive observer is proposed for the case of the grid under voltage unbalanced fault or the voltage parameter jump faults.In contrast to most existing estimation methods,this scheme regrads the grid voltage with harmonics as a parametric dynamic system related to an unknown frequency of the grid.And then,a passive observer based on the parametric dynamic system is proposed to estimate the parameters of the grid voltage.Compared to most existing methods,this method is capable to estimate the parameters of harmonic voltages.And also,it can estimate the parameters of the fundamental and harmonic voltage components with a zero steady-state error.3)Considering the shape differences of the grid voltage under different grid faults,such as three phase voltage fault and one phase unbalanced voltage fault,a current reference based on current balance is proposed.In contrast to existing methods,the amplitude of the active power oscillation is introduced as a control parameter and thus the voltage ripple of the DC side capacitance can be easily limited.And in this method,the current balance problem is formulated as an optimal problem with a constraint.The current balance is achieved and thus the maximum delivery capacity of the reactive power is enhanced.4)An output regulation based current tracking controller is proposed for LCL filtered converter.This method can achieve grid side current tracking in the case that the grid involves harmonic voltages.Also,the controller has the capacity of the active damping.Under this controller,the poles of the closed loop system can be configured to various locations in the left half region,which provides greater freedom for the selection of the steady state margin and dynamic performance.In contrast to existing methods,this controller can still achieve a steady-state tracking error in the case that the grid involves harmonic voltage components.