Research On Modular Parallel System Of LCL Type Active Power Filters

In high power applications,single active power filter（APF）limited by its capacity cannot compensate all the harmonics.And this demands multiple APFs to run in parallel to improve the compensation capacity in modular form.The performance of the APF parallel system is determined by the control techonology of a single APF and the control technology of APF parallel system.However,the existing APF parallel system still has room for improvement in both aspects.And this thesis will unfold the research on these two aspects.A single shunt APF is essentially a grid-tied converter.A faster current controller is conducive to improving APF’s performance.In this thesis,the state space model of LCL-filtered grid-tied converter is built first,and a special pole-placement approach termed optimal deadbeat（ODB）control is proposed,which uses a pole inside the unit circle to offset the zero outside the unit circle.Compared with other current controller,the ODB control has the widest bandwidth namely responses fast,which can improve the performance of APF greatly if applied.The corresponding grid voltage feedforward section and state observer are introduced to eliminate the influence of grid voltage on state variables.Discriminating the real values and the rated values of the physical components results in the 6^th-order state equations of the whole system.According to the sensitivity analysis against the both side inductances of LCL filter,it is found that the ODB control can tolerate a great amount of parameter drifts of LCL filter.Relevant simulations and experiments are conducted and prove the correctness and practicality of ODB control.The basic principle is that for a PR controller,the influence of other pole-zero pairs is ignored when determining the position of a pole-zero pair.For resonant controller,when determining the position of a pole,it is assumed that the other poles are located on the unit circle.The advantages and disadvantages of PR filter and R filter are discussed.Aiming at the shortcoming that the R filter has a 90°phase lag at high frequency band,an improved method is proposed,and an improved R filter is obtained.The control of APF needs to extract harmonic components from current.And a good filter can extract the harmonics correctly and quickly assuring the stability of the system meanwhile.In this thesis,the continous domain models of proportional resonant（PR）controller and resonant（R）controller are introduced.Then,the PR filter and the R filter are designed by the z-domain zero-pole configuration method,so that the amplitude at the resonance frequencies are 0d B.Their basic principles are that for a PR filter,the influence of the other pole-zero pairs is ignored when determining the position of a pole-zero pair.For R filter,when determining the position of a pole,it is assumed that the other poles are located on the unit circle.The advantages and disadvantages of PR filter and R filter are compared.Aiming at the shortcoming that the R filter has a 90°phase lag at high frequencies,an improving method is proposed,and an improved R filter is obtained.There are still some problems and shortcomings in the existing APF parallel schemes.For example,in the schemes without central controller or interconnections,APFs cannot share the harmonics in proportion to their capacities.As for the schemes with central controller or interconnections,its robustness is rather weak,any communication fault or APF failure will affect the stable operation of the whole system.Besides,the operation of adding or cutting an APF is rather complex.To deal with these problems,this thesis proposes a novel modular APF parallel system scheme.The scheme is based on feedback type APF,where a single APF adopts the ODB control to regulate the output current as the current inner loop,adopts an improved resonance filter to extract harmonics,and adopts proportional control to regulate the grid current outer loop.Their respective ks should in proportion to their own capacities to ensure that their output currents are in proportion to their capacities.All APFs have the same connect ions and controllers,namely the same statuses.One APF can be connected to or disconnected from the system without changing other APFs,thereby changing the system’s capacity.So,this scheme can modularize APF.The parameter k_max is the maximum value of k,and decides the maximum number of APFs that can be connected to the system.The k_max is determined by the current outer loop,which includes the current inner loop,proportional controller and the filter.Relevant simulations and experiments are conducted and verify the parallel scheme’s correction and practicability.