Research On Modeling And Coordination Compensation Strategy Of Multi-Active Power Filter System

With the popularization of non-linear loads in the power grid and the increasing demand of users for power supply quality,passive and active harmonic suppression technologies have long become a research hotspot.However,with the increase of high-power nonlinear loads such as electric locomotives and large smelting furnaces,the required harmonic compensation capacity is also increasing rapidly.Limited by the device capacity,the traditional three-phase shunt active power filter is difficult to meet the demand for large-capacity harmonic compensation.At the same time,it is necessary to consider that the accuracy of harmonic compensation is limited by the inherent delay of the digital control system and the effect of the delay which will cause a larger phase error for higher harmonics.In view of the problem that the compensation capacity of SAPF is difficult to meet the demand when compensating for high-power nonlinear loads,the current technical methods for APF expansion mainly include multi-level cascade and multiple main circuit structure and hybrid APF etc.Due to the above most of the solutions applied to large-capacity harmonic compensation occasions have problems such as complex structure and cumbersome control and poor effectiveness.Therefore,studying the coordination of compensation among multiple APFs and improving the accuracy of harmonic compensation of a single APF are of great significance to the safe operation of multiple APF systems.In view of the above-mentioned APF compensation capacity and delay impact,this thesis proposes the following strategies:(1)With improving SDFT harmonic current detection and predictive control,APF can compensate for specific multiple harmonics;(2)This thesis establishes multiple APFs mathematical models and coordinate compensation strategies.The main research contents of the thesis are as follows:(1)Aiming at the delay problem of the digital control system in the real-time compensation of harmonics and reactive power in the APF,improvements were made from two aspects: harmonic current detection and compensation current control.The sliding window discrete fourier algorithm(SDFT)in the harmonic current detection link has an inherent delay problem of the fundamental period.An improved SDFT algorithm for specific sub-harmonic extraction is proposed.The algorithm eliminates the useless zeros in the traditional algorithm,which can effectively reduce the inherent delay and the number of sampling points for harmonic detection.At the same time,dynamic predictive control is introduced in the compensation current tracking control link and the predictive error is effectively eliminated by increasing the error correction factor and predicting the reference current at the next moment.Finally,the pulse width modulation technology is used to control the three-phase inverter to generate compensation current,so that the APF can achieve selective harmonic compensation.(2)The mathematical model of the conventional hybrid multi-APF system is established.Based on this model,a mathematical theoretical analysis of its dynamic interaction is carried out.The multi-APF system is composed of different types of modules APF and each module has different LCL filter parameters,switching frequency and current carrying capacity.Based on the analysis of the dynamic interaction of the hybrid multi-APF,it is concluded that increasing the LCL filter inductance can effectively suppress the resonance and coupling between different module APFs.Compared with the current traditional modular APF,the hybrid multi-APF system can provide a wider current tracking bandwidth,fast dynamic response speed and higher capacity utilization.(3)In order to improve the flexibility and redundancy of parallel operation of multiple active power filters,a composite current-limiting strategy is proposed from the harmonic current of the reference compensation.At the same time,based on the relationship between the compensation harmonic current capacity and the APF’s own capacity,the utilization rate of the APF capacity is maximized to carry out the harmonic frequency division compensation distribution and the large-capacity APF compensates for the harmonics with high content of specific orders and the small-capacity APF compensates for the remaining harmonics.