Source-Current-Detection-Based Control Schemes For Active Power Filter

With the development of the energy internet and new energy,more power electronic equipments are applied in the grid.The power quality of the system is becoming an important problem.A shunt active power filter(APF)is viable solution for the harmonic compensation and it is widely used.Currently,the load-current-detection-based control schemes are widely used for the APF.The error caused by the harmonic detector in the load-current-detection-based control schemes may impact on the compensation reference current and worsen the compensation performance of the APF.The souce-current-detection-based control schemes become an important research topic,which have excellent performance,no need of the harmonic detector,high precision and less computation.This paper intensively discusses the power balance(PB)method,one cycyle control(OCC)method and the associated key issues.The main contributions are as follows:(1)This paper proposes a dual close loop control scheme based on the source current detection to improve the compensation accuracy of the APF,which overcomes the effect of the harmonic detector owing to removing this harmonic detector.In this paper,load current as disturbance,the mathematic model has been developed based on source currents by the variable substitution.The frequency characteristics of load current in rotating coordinate system and dynamic characteristics of DC voltage are studied based on the mathematic model.On the basis of system power balance,the fluctuation characteristic of the DC side voltage and the effect of the equivalent resistance on the APF have been discussed.The corresponding controllers have been designed based on the internal model to form the dual closed-loop control strategy.The simulated and experimental results validate the feasibility of the proposed control strategy.This control strategy improves the compensation performance of the APF,and it can selectively compensate specific harmonic.(2)This paper proposes a modified OCC scheme to improve the performance of the three-phase shunt APF under the non-ideal supply voltage conditions which solves the problem that the traditional OCC is easily affected by the distortion voltage.In this paper,the operation characteristic of OCC-based APF under the non-ideal supply voltage has been analyzed and the mathematical equation between the source current and distorted voltages has been derived.The control equation of modified OCC has been developed by distortion voltage decomposition and variable substitution.The proposed OCC is realized based on the control equation and the positive sequence filter.The compensation performance is verified by simulations and experiments.The modified scheme improves the compensation performance of the OCC under the distorted voltages,no need of phase locking loop(PLL)and complicated current controller,and greatly reduces the complexity of the control system.(3)This paper proposes an improved OCC for APF to compensate only harmonic components of load currents which solves the problem that the traditional OCC can not compensation harmonic alone and effectively reduces the compensation capacity of the APF.The power components in the system have been presented and the compensation characteristic of the OCC-based APF has been analyzed based on the instantaneous power theory.On this basis,the optimal compensation principle has been presented.The reference signals of the OCC control core have been reconstructed and the associated control equations have been developed.The positive sequence filter and stationary reference frame controller has been designed,and the proposed control strategy is realized by the phase angle relation between the current and load current.The simulated and experimental results in the paper validate the feasibility this proposed strategy.This control scheme guarantees the good performance of harmonic compensation alone,broadens the application prospects of the OCC,and avoids the complex current controllers and the coordinate transformations.(4)The prototype of the dual closed-loop-based APF based on the source current detection is implemented in the laboratory.This paper studies the key techniques for the prototype: soft start scheme,suppressing the shock current and dc voltage caused by the starting and load sudden changes,current protection strategy in case of the insufficient capacity for the APF.The techniques ensure that the system runs smoothly when starting and operates stably under the abnormal conditions such as load sudden change and insufficient compensation capacity.The reliability of the system is improved,and the practical application value of the control strategy is verified.