Research On Current Control Strategy Of Active Power Filter In Mine Power Grid

The dissertation is supported by Research Fund for the Doctoral Program of Higher Education of China(RFDP),“Research on Harmonic Suppression Technology of Power Grid”(No.20101402120004).Current harmonics is one of the most important indicators to measure power quality and will impair electrical equipment severely.With a large number of variable-frequency drive used underground the mine,power quality is extremely poor due to harmonic pollution.Harmonic generated by variable-frequency drive directly threatens the normal operation of power equipment,and affects the coal mine safety and continuity.Therefore,it is of great importance to study the mine harmonic control method to improve the coal mine power quality and ensure safe production.Owing to the load nonlinear and strong short-time impact characteristics of the coal mine grid,the mine grid voltage,frequency and impedance are susceptible to the load fluctuation.As a result,the ability to suppress the current harmonics is reduced and the compensation performance is restricted when the conventional harmonic current control strategy is applied to the active power filter(APF).For non-ideal features of the mine grid,the goal of this paper is to improve the harmonic compensation precision and dynamic tracking ability,and enhance the frequency adaptability and robustness of the APF.Through the topology optimization,theoretical derivation,system modeling and simulation and experimental verification,the harmonic current control methods of 1140 V and 10 kV power grid are studied.The main contents are as follows.In order to attenuate the switching frequency harmonics and reduce the size of the grid-connected filter,an LCLCL-type shunt active power filter(SAPF)topology is proposed,which consists of a traditional grid-connected LCL filter,and an LC resonant branch connected in parallel with the capacitor.The LC resonant branch occurs in series resonant near the switching frequency,and present high impedance in the low frequency band which will ensure the inverter output current could be transferred to the grid,and present a low impedance in high frequency band so as to switching ripple injected into grid is greatly attenuated.When there is a large load change in 1140 V mine power grid,the grid frequency fluctuating will be occurred.In this thesis the fractional order repetitive control strategy with frequency adaptability has been studied based on LCLCL-type SAPF.Under the condition of ideal power grid,the feedforward compound repetitive control strategy is adopted,which improves the steady-state accuracy and the dynamic performance of current tracking,and effectively reduces the THD of the grid current.This control strategy has also been verified on the experimental platform successfully.Under the condition of a grid with frequency fluctuation,the frequency adaptive repetitive control strategy has been introduced.The design method of optimal fractional delay FIR filter based on Lagrange interpolation and stability criterion of fractional order repetitive control are proposed.Through reasonably selecting the conventional repetitive controller order and the fractional delay filter order,the proposed repetitive controller can achieve satisfactory compensation effect in the variable frequency reference signal conditions.This method is applicable to the control system with a constant delay or a delay which has small update.Under the condition of a grid with the frequent frequency fluctuation,the fractional delay will be changed frequently.A Taylor series expansion based fractional delay FIR filter is proposed.The FIR filter is only related to the fractional delay.As a result,any fractional delay can be achieved without redesigning the fractional delay filter and frequently updating the fractional delay filter coefficients.The new filter greatly improves the stability of the system and achieves high steady-state precision of the high-order filter under the optimal fractional delay scheme.By means of simulation and experimental platform test,it is verified that the fractional order repetitive controller can realize the frequency adaptive harmonic compensation,and the THD is effectively suppressed under the condition of frequency fluctuation.When there is a large load change,the grid impedance will be changed.The changes of impedance will reduce compensation performance of the feedforward repetitive control system.The sliding mode controller can improve the robustness of the system,but the uncertainties of the plant parameter are prone to cause the sliding surface drift,affecting the steady-state compensation accuracy.A sliding mode control strategy with multi-proportional resonant is proposed by introducing multi-proportional resonant terms of harmonic current error in the sliding mode surface.The harmonic current error gain is amplified in the multi-proportional resonant cases,which could eliminate the specified harmonic current tracking error and suppress the grid current THD.This conclusion has also been verified by the simulation and experimental platform separately.Based on the similar internal model principle,a new sliding mode control strategy with repetitive control is proposed to replace the multi-proportional resonant term with the repetitive control,which not only achieves full harmonic compensation and further reduces the steady-state error,but also has frequency adaptability.The experimental results show that the system has good robustness,high steady-state precision and good track capability,and the sliding mode control strategy with repetitive control has higher steady-state precision than the sliding mode control with multi-proportional resonant.A feedforward repetitive control strategy based on a transformerless shunt hybrid active power filter(SHAPF)has been proposed for the 10 kV coal mine power grid.The design methods of passive LC filter parameter and compound current controller are present.The experimental results show that the system has high steady-state precision and the goal of tracking the current accurately in real time has been realized,which effectively improves the power quality of 10 kV mine power grid.