Research Of Technologies On Medium Voltage Active Filter

With the development of modern industry,a large number of power electronic products are continuously put into use,and harmonic pollution in the power grid is increasing.As a new type of power electronic device that improves power quality,active power filter can not only compensate the harmonics in the power grid,but also compensate for reactive power and solve the three-phase imbalance problem.At present,the research on active filters used in low-voltage systems is becoming more and more perfect,while the research on active filters in medium-and high-voltage systems is relatively scarce.Therefore,this paper studies the related technologies of active filters for medium-voltage networks.The working principle,compensation function and topology of active power filter are introduced.The advantages and disadvantages of different multi-level converters for medium voltage active filters are compared and analyzed.Because of the high economy,the simple structure and the convenient control,the H-bridge cascaded multilevel active filter is selected as the research object of this paper.The ip-iq command current detection algorithm and the synchronous rotation dq coordinate transformation command current detection algorithm for three-phase unbalance are analyzed in detail.It is pointed out that the dq algorithm can not only detect and track the different frequency harmonics separately,but also detect the effective values and the initial phases angle of the voltage and current with the different frequency.Considering that the low-pass filter(LPF)in the ip-iq and dq detection methods cannot simultaneously meet the demands of the filtering accuracy and response speed,it is adopted to use the moving average filter(MAF)instead of the traditional LPF.For the case of grid voltage imbalance distortion,phase-locked loop analysis is carried out,and an improved dq coordinate transformation phase-locked loop technique is adopted,that a MAF filter is added to the q-axis channel to filter out harmonic and negative-sequence interference signals,improving the accuracy of phase lock.The PWM modulation schemes of different multi-level converters are studied.It is pointed out that the single-pole frequency-frequency carrier phase-shifting SPWM modulation technology,with a large number of output levels and a high equivalent switching frequency,is simple to implement.However,the carrier frequencies of the units are the same.At higher switching frequencies,switching losses are greater.In order to further reduce the switching loss,this paper proposes a hybrid SPWM modulation strategy,that is,if N cells are cascaded,one of the cells is separately unipolar multiplying SPWM modulation with the higher carrier frequency,and the remaining N-1 cells adopt single-pole frequency-frequency carrier phase-shifted SPWM modulation with the lower carrier frequency.Thus,the APF can reduce the switching loss more effectively under the premise of satisfying the harmonic compensation requirement.For the current closed-loop tracking control strategy,this paper adopts the composite control strategy of PI controller and repeated controller in parallel,making full use of the dynamic response capability of PI controller and the steady-state accuracy of repeated controller.Regarding the DC-side voltage control strategy,this paper analyses the absorption power between APF three phases under balanced and unbalanced grid voltage,and indicates that the three-stage DC-side voltage control strategy based on zero-sequence voltage injection can maintain the stability of APF DC side total voltage,the balance of phase-to-phase DC-side voltage as well as the balance of DC-side voltage between the units.Integrated device withstand voltage,device cost,computer performance and other factors,when simulating,the grid voltage is 6k V medium voltage and the cascaded multi-level active power filter cascade unit is six,and the output filter uses single inductor.Finally,this paper uses MATLAB/Simulink software to build the model and carry out simulation analysis.The results show that the various control and modulation schemes proposed in this paper are feasible.