Optimization Of Current Control For Three Types Of Shunt Active Power Filter

It has been nearly half a century since Shunt Active Power Filter（SAPF）was proposed in the 1970s.Its technical principles have matured and products in low-voltage distribution networks have also been marketed.However,some users hope to have customized SAPF for their power quality problems,such as comprehensive SAPF that can suppress harmonics and damp resonance at the same time,high switching frequency SAPF based on Si C devices to suppress high-frequency harmonics,multi-level SAPF directly connected to the medium and high voltage grid to save transformer cost and so on.Although these SAPF structures and applications are different,they all have similar current control.By studying the new problems in different scenarios,it has common benefits on the optimization of SAPF current control.Therefore,this article takes the three types of SAPFs:multifunctional integrated type,high switching frequency type,and modular multilevel type as the starting point to study resonance damping,stability analysis,high-frequency design,and bandwidth expansion related issues.For the type Ⅰ comprehensive SAPF with reactive power compensation capacitors located upstream,after the capacitor bank is partially switched,the system resonance frequency changes.At this time,SAPF resonance damping current control cannot adaptively track the resonance frequency.Existing research would like to inject disturbance current,and find the resonance frequency by calculating the larges response of the point of common coupling（PCC）voltage or the grid-side current,which is to obtain largest parallel impedance.This method is susceptible to the equivalent resistance on the grid side.If the equivalent resistance is too large,the maximum impedance is not obvious.Therefore,this paper proposes SAPF-based 120°square wave current injection resonance frequency detection method.The resonance frequency is determined by calculating the ratio of the PCC voltage and the grid-side current product to the square of the injected current changes the most,which is the resonance energy increases the most.The proposed method is verified to be more accurate and sensitive.So that the SAPF damping control can better track changes in resonance frequency.For the type Ⅱ comprehensive SAPF with the reactive power compensation capacitor located downstream,when SAPF tries to suppress harmonics,the resonance frequency will drift and the SAPF will be unstable.The study found that the type II comprehensive SAPF detection current contains the parallel capacitor current,and the system transfer function has extra closed-loop.The SAPF current control model used in previous analysis is often simplified as a constant or first-order element,which has limitations in explaining the high-frequency drift of the resonance frequency.Therefore,this article adding the harmonic extraction to the SAPF current model,which can more accurately obtain the stability conditions of the harmonic suppression coefficient and damping gain of each harmonic.At the same time,to reduce the number of zeros and poles of the harmonic extraction model,a method using the equivalent of bandpass filter transfer function to replace the transfer function of the Recursive Discrete Fourier Transform（RDFT）is proposed,which does not affect the stability analysis of the system.Based on the analysis results,this article points out that the instability is caused by the harmonic suppression after the resonance frequency.Finally,a comprehensive control method of harmonic suppression and resonance damping based on frequency band is proposed to stabilize the SAPF current control.For the high switching frequency SAPF using wide bandgap devices such as Si C,when the sampling frequency of current control is low,there is a problem of limiting current control to deal with wide frequency harmonics.The single-thread current digital control will restrict the further increase of sampling frequency due to the slower DSP processing speed.Therefore,based on comprehensive SAPF current control,this paper proposes a dual-core multi-threaded current control method with 100k Hz switching and sampling frequency SAPF.When the sampling frequency becomes higher,the 100k Hz SAPF current control closed-loop can track the 5^th～50^th harmonic current with the PI controller,eliminating the repetitive controller.As for the wide frequency characteristic harmonics extraction in a single sampling period,this paper proposes the abc-frame multi-resolution and dq-frame centralized RDFT method,both of which can effectively broaden the harmonic extraction range of SAPF without sacrificing the accuracy and enhance the 100k Hz SAPF wide frequency harmonic current control capability.For the Modular Multilevel Converter-SAPF（MMC-SAPF）based on carrier phase shift modulation,the sampling frequency with regular sampling method might be low and the current control bandwidth is not enough for harmonic suppression.Therefore,this article discusses the current control bandwidth expansion by quadruple sampling on five-level MMC-SAPF which remains switching frequency unchanged.At the same time,considering that most MMC-SAPF adopts hierarchical control in actual projects,the difficulty of multi-sampling application lies in the synchronization of state updates in each controller.Therefore,this paper studies the quadruple sampling current control timing of the master-slave hierarchical control structure and proposes a method of using the master-slave serial communication to calculate the fixed time delay of the sub-controller that helps each module to update the duty cycle synchronously.After the equivalent sampling frequency of the low switching frequency MMC-SAPF is increased,its current control bandwidth has been effectively expanded,which can track low-frequency harmonic current references and reduce the output waveform distortion.The current control studies of three types of SAPFs have been verified by relevant experiments on their respective prototypes.