Research On Key Techniques Of Active Power Filter Based On Predictive Control

With the extensive application of power electronic equipment,the public power grid is seriously polluted,which affects the reliable operation of electric power system.Active power filter(APF)is a novel power quality compensation device with high precision of steady state compensation,fast dynamic response,flexible compensation manners,and less probability having resonance with the grid impedance.APF can comprehensively compensate harmonics,reactive power and negative sequence current,and also can make selective compensation for the above ingredients according to the demand of field compensation.However,there are still some challenges in the practical application of APF.For example,how to accurately detect the reference signal of the compensation component under complex condition;the adaptability of control strategy to calculating delay and control delay;the voltage stabilization control in the process of compensation and soft start pressure of DC voltage;the design parameters of output passive filter is lack of theoretical basis.In order to resolve the above problems,this paper researches the key technologies of APF based on selective compensation.Specific research issues run as follows:For the sake of meeting the needs of accurate detection of fundamental positive sequence active,reactive power and negative sequence current under complex conditions,the paper proposes an improved DFT algorithm based on the reference phase,which can achieve the reconstruction of the fundamental voltage,current and its orthogonal quantity under the ?? coordinate system.The positive and negative sequence components of fundamental voltage and fundamental current are extracted by instantaneous symmetrical component transformation.Based on the instantaneous reactive power theory,the separation of active and reactive components in the fundamental positive sequence current of load is achieved.Considering the frequency fluctuation of power grid,the frequency variation of power grid is tracked on the basis of equal angle interval sampling principle.Compared with the conventional dq detection method,the proposed method eliminates the phase-locked loop and the low-pass filter,which can accurately and timely calculate the fundamental positive sequence active power,reactive current and fundamental negative sequence current.In consideration of the demand of selective harmonic compensation,the frequency adaptive phase-locked loop(PLL)can be used in the sliding window iterative DFT,which can realize the adaptive detection of the specified sub-harmonic frequency and provide the accurate reference signal for the selective compensation of APF.The traditional full compensation technique is not capable of solving the problem of bandwidth limitation of APF and accurately limiting the amplitude for the compensation component.In addition,the maximum duty cycle of the PWM interver is limited by the detection and calculation delay.Though the delayed beat control is an effective method to eliminate the limitation of duty cycle,the control delay is increased.Therefore,this paper proposes a selective compensation strategy based on the dynamic prediction algorithm,which solves the delay problem caused by the delay beat control and enhances the flexibility of the APF compensation.Firstly,the strategy can predict the next reference value of the selected sub-harmonic,reactive power and negative sequence current,and the error correction mechanism is used to improve the prediction accuracy.Then,the reference value of three-phase inverter reference voltage vector is calculated based on Deadbeat control model.Finally,the PWM signal is obtained by a space vector pulse width modulation method to drive the power device and output the compensation current.This paper analyzes the mechanism of DC side voltage fluctuation from the viewpoint of energy conservation,and qualitatively researches on the causes of voltage fluctuation on DC side.The approximate model of the control object is obtained on this basis,and the PI parameters are optimized by using the SISO Design Tool designer.Considering the DC side voltage pressure problem in shock,a novel soft start control method is proposed,which in the PWM pump stage continues to enable the current limiting resistor in the precharge phase and transform the model of control object,so as to improve the dynamic performance of the system response and to achieve steady pressure.The LCL filter is usually used in the switching noise filter,and its parameter design is complex.It is necessary to consider the factors such as inductance minimization,inductance capacitance matching,coordination of compensation stability and rapidity and damping loss.This paper presents a new method of parameter design.Firstly,the frequency characteristic of LCL filter is analyzed,and the total inductance is designed on the basis of the low frequency characteristic;Secondly,design of filter capacitor followes the principle of improving high frequency shunt effect and reducing the reactive current;Finally,using LCL resonant frequency FOSC as variable,inductance in the inverter side and grid side can be selected by using simulation tools.