Active Power Filter Based On Active Impedance And Robust Repetitive Control For Power Harmonics

Nowadays,nonlinear loads have been increasing in the distribution system from time to time.This is mainly due to the nonlinear loads have gaining attention from customers and utility for many types of applications.They are found in applications such as in the domestic household appliances,information technology,office equipment,emerging electric transportation sectors;grid-connected power electronic devices,etc.The nonlinear loads may be in three-phase or single-phase loads.These nonlinear loads cause current harmonics,reactive power burden,excessive neutral line current,and phase unbalancing in the single-phase and/or three-phase four-wire distribution system.Harmonics affect the accuracy,life,rating,and efficiency of the devices that are connected at the same point of common coupling(PCC)of the distribution system.Although the kVA rating of the single-phase nonlinear is small,its cumulative harmonic contribution is huge.Furthermore,its zero-sequence harmonics currents will be added together in the neutral line.Although passive power filters have a limitation in harmonic compensation,it has still used as harmonic compensation in the single-phase or three-phase system.Moreover,passive power filters have a crucial role for building a hybrid active power filter(HAPF).HAPF combines the advantage of the passive filter with an active power filter.Following this advantage,the author reported an active impedance synthesized approach with the adoption of a new interpretation using a proportional-resonant control strategy and the passive LC filter.The active impedance hybrid active power filter was modelled in the MATLAB Simulink toolbox and later verified with the experiment on the TMS320F28335 core processor.The nonlinear loads are current-source type,voltage-source type,and mixed type nonlinear loads.The active impedance enables actively tuned the passive LC filter towards the active short circuit for multiple characteristic harmonic frequencies.This virtual multiple parallel impedance is synthesized in series with a smaller rated active power filter.The virtual impedance provides multiple parallels tuned harmonic frequencies over the single-tuned passive power filter.It enables to have a lower APF rating,and lower power losses.This impro ved harmonic voltage damping,current harmonic reduction(from 60.86%to 4.94%),and power factor correction from lagging/leading to about unity.It uses the filter fundamental current information to compensate for the required type of reactive power of the loads.Therefore,the reported controller can be taken as a solution to extend the passive filters into multiple tuned dominant harmonics frequencies hybrid active power filter(HAPF).This research has a direct practical benefit for power quality(PQ)compensation in the case of a single-phase smart grid.Furthermore,this active impedance hybrid power filter can be taken as a contribution to an application area with single-phase highly variable reactive loads and distortion such as in the residential,and single-phase loads center.It has also a potential advantage to avoid a regular field retuning the passive filter in the industry.In many countries,the single-phase and/or the three-phase nonlinear loads are usually connected to the upstream with the delta-star transformer by the outgoing feeder lines.Nowadays,nonlinear loads are contributing a challenging scenario and becoming a source of current harmonics,reactive burden,and excessive neutral current in the distribution system.Unlike the balanced three single-phase linear loads,balanced three single-phase nonlinear loads cause an unbalance and excessive neutral current.Therefore,in the case of nonlinear loads,the conventional phase balancing mechanism could not achieve a balanced phase system.In this regard,the subject needs further investigation and researches to alleviate the current problem.Fortunately,the power harmonics,disturbance,and its reference are dominantly odd-harmonic frequency.The repetitive control is a suitable control strategy for the periodic signal to improve the control performance of the active power filter.However,the conventional repetitive controller can easily loss its high gains characteristics for a slight frequency deviation from the target frequency.Moreover,the insertion of repetitive control highly affects the nonperiodic performance of the controller.This is due to the repetitive control has tight control characteristics.The relationship between performance and increment of the orders of repetitive control has to be investigated against the robustness.Therefore,the author reported second-order odd repetitive control for the three-phase four-wire active power filter for harmonic compensation,reactive power correction,reduction of neutral current,and load balancing.The detail MATLAB simulation and experimental investigation were conducted for the second-order odd-harmonic repetitive controls.The second-order odd-harmonic repetitive control had compared with the first-and third-order odd-harmonics repetitive controls.The second-order odd-harmonic repetitive control was found with deeper and wider notches at the odd fundamental frequencies.This alleviates the performance degradation of the repetitive control.As a result of this,the second-order odd RC compensates the total current harmonic distortion THDi from 87.42%to about 3.52%for unbalanced single-phase nonlinear loads.At the same time,it also compensated the neutral current from the phase current level to a negligible current level.In the case of the balanced three-phase nonlinear loads,it achieved a total current harmonic reduction from 49.45%to 3.43%,and for unbalanced three-phase nonlinear loads from 50.55%to 3.24%.The experimental results have also verified that it would have a promising solution for compensation harmonics circulating in the three-phase four-wire system.Particularly,the reduction of neutral current has a direct practical benefit in the service life of distribution transformer,safety concerns,and failure of sensitive or medical devices that are connected with the three-phase four-wire distribution system.On the other hand,the multilevel pulse-width-modulation has advantages on its higher efficiency,better harmonic characteristics,and smaller output voltage steps as compared to the two-level voltage-source-converter(VSC).Among the multilevel converter,the three-level T-type topology is much preferred for its reduced number of switching semiconductors than neutral point clamped(NPC).Furthermore,the three-level T-type converter had the potential advantage for harmonic noise reduction and more converter voltage steps.On the other side,the robustness of the repetitive control can be improved by modifying its sensitivity function.Squaring the sensitivity function of the repetitive control widens the gain at the peaks and the notches.Therefore,the author investigated and combined the potential advantage of a modified sensitively function of repetitive control with the three-level T-type converter.The combined result provides a better harmonic compensation,reactive power correction,neutral current reduction,and phases load balancing in a three-phase four-wire active power filter.The three-level T-type converter was modeled in MATLAB with detailed simulation and later verified with the experimental investigation.The investigations were conducted for different nonlinear loads:underbalanced and balanced nonlinear loads.The adopted controller reduces current harmonic distortion from 82.5%to 3.97%on the three-level T-type converter of balanced current type-source nonlinear loads.The experimental and simulation results also indicated that the proposed controller can improve the unbalanced load current waveforms and the excessive neutral current.Therefore,the modified repetitive control with the three-phase three-level T-type converter had a potential advantage in a domestic application area.Furthermore,it is suitable for resident application due to its low harmonic noiseTherefore,to sum up the main points,the thesis contributes an investigation of the active power filter topology as well as the control performance.The T-type converter configuration and the active impedance-based hybrid compensation approach which combines the advantage of the passive filter and active power filter would have a practical benefit for APF topological structure.At the same time,modifying the order or the internal model(IM)structure of repetitive control(RC)has also benefit for the performance of the active power filter control to compensate current harmonics,reactive power,and/or excessive and unbalancing current compensation.These would be the author's future research direction that applied in the local power industry.