Study On Compensation Characteristics And Control Of Low-Capacitance Cascaded H-Bridge STATCOM In Unbalanced Power Grid

In the context of the "dual carbon" goal,distributed power sources such as photovoltaic and wind power are widely connected to the power grid,coupled with the widespread use of nonlinear and impulsive loads in industry,the stability of power grid voltage is becoming increasingly prominent.Cascaded static synchronous compensator(STATCOM)is a typical reactive power compensation device suitable for medium and high voltage distribution networks.The research on the compensation characteristics and control methods of traditional cascaded H-bridge STATCOM has become mature,so this article focuses on reducing the DC side capacitance value to achieve the goal of reducing costs and improving power density.The research object of this article is the low capacity star cascaded H-bridge STATCOM(LC-STATCOM).A mathematical model and power model of STATCOM were established in the abc and dq coordinate systems.Based on the mathematical model,a feedforward decoupling strategy with positive and negative sequence separation was adopted to achieve independent control of the active and reactive components of the device’s output current in an unbalanced power grid.Based on the power model,the mechanism of voltage imbalance on the DC side of STATCOM was analyzed,and feedforward control was added to the traditional negative sequence current injection method to improve dynamic performance.At the same time,a submodule capacitor voltage control based on active vector superposition was studied to achieve three-layer balance of STATCOM DC voltage.Subsequently,the mechanism of STATCOM capacitance reduction was studied,which was achieved at the cost of significantly increasing the fluctuation amplitude of the DC side voltage in exchange for capacitance reduction.However,this brought about the problem of reducing the compensation range under inductive conditions.In response to this issue,this article establishes a relationship between the capacitance value of LC-STATCOM,the fluctuation value of capacitance voltage,and the compensation current under inductive/capacitive conditions.Under the constraint of limit modulation,the relationship between the connection reactance per unit value,capacitance value,and compensation range of the system impedance is established by standardizing the connection reactance in the relationship equation.It is found that properly increasing the connection reactance within the per unit value of the connection reactance is less than 0.236 can significantly reduce the capacitance value.Based on this theory,an optimized capacitance value method under full range compensation is proposed,and the relationship curve between the capacitance/inductive reactive current compensation range and the normalized impedance of the connection inductance under constant capacity value is drawn.The significant fluctuation of the second harmonic voltage on the DC side is the most direct problem with LC-STATCOM.This makes it easy for the device to trigger overvoltage protection on the DC side during high-voltage crossing and output a large amount of third harmonic current under unbalanced power grids.Therefore,this thesis studies a DC side voltage peak control method based on SOGI,which replaces traditional average control and can effectively avoid DC side overvoltage.In addition,traditional filtering methods cannot balance the filtering effect and voltage loop bandwidth requirements.This thesis designs a closed-loop adaptive predictive control filtering algorithm,which deduces the analytical expression of the DC side second harmonic signal based on the model and predicts the size of the second harmonic signal for filtering.At the same time,a closed-loop control loop is established to real-time correct the amplitude and phase of the predicted second harmonic signal,making the system more robust,The filtered harmonic signal is very precise and will not affect signals in other frequency bands.The simulation proves that the filtering algorithm has good suppression effect on the third harmonic of the grid side,and its dynamic performance is better than traditional filtering methods.Due to the large pulsating voltage on the DC side,the capacitance of LCSTATCOM,which has a small margin for capacitance,is more prone to aging,leading to a decrease in device safety.In response to this issue,this article studies two capacitor state monitoring methods for LC-STATCOM;One is to improve the closed-loop adaptive predictive control filtering algorithm mentioned earlier,which achieves monitoring of the average capacitance of each converter arm while filtering,without the need for additional algorithms.The other method is based on LC-STATCOM capacitor second harmonic voltage for capacitor state monitoring,which can achieve efficient online monitoring and fault location of all capacitor capacitance values without the need for current sensors.The Lyapunov second method is used to prove the stability of the proposed control system,and a method for selecting loop controller parameters is provided.