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This paper presents a current controller for a transformerless single-phase two-level inverter grid connected by means of an LCL output filter and subject to a sever harmonic distortion in the grid voltage signal. The controller indirectly controls the grid-side current by controlling the inverter-side current. The inverter-side current controller is simpler to implement, moreover, it is shown that a damping injection can be done by a simple proportional action of the measured current. Instrumental for the proposed scheme is the proper design of the inverter-side current reference, which must guarantee that the grid-side current reaches its corresponding reference, i.e., a pure sinusoidal signal with the appropriate phase-shift. It is shown that the inverter-side current has to be distorted as much as necessary to guarantee that the grid-side current reaches its reference. The controller includes a harmonic compensation mechanism, therefore, it is aimed to properly perform under harmonic distortion operation. Numerical tests are presented to assess the performance of the proposed scheme. Experiments are under development and will be presented in the final version of the paper.

This paper presents a current controller for the single-phase grid-connected modular multilevel converter of 2n + 1 levels. The controller is designed based on the mathematical model of the converter. It has the aim of injecting an almost pure sinusoidal current with a proper phase, despite the low-frequency harmonics present on the grid voltage. Also, a loop for the circulating current is developed to keep this variable close to a constant value for proper internal operation of the converter. The overall current controller comprises injected and circulating current loops. Out of these loops, a voltage reference for each converter arm is built, which is then used in a phase-shift carrier-based pulse-width modulation to generate the switching sequences. These are then used to command the modules conforming the converter. Experimental results in a 7-level modular multilevel converter (n = 3) operating under harmonic distortion in the grid voltage are obtained from a hardware-in-the-loop test bench to validate the performance of the proposed current controller. n, number of cells in one arm of MMC; L, inductance of each arm; i P , i N , upper and lower arm currents; E, DC-link voltage value; v S , distorted grid voltage; e P , e N , total inserted voltage for the upper and the lower arms; v Ci , i-th (i ∈ 1, … , 2n) capacitor voltage of MMC; u i , i-th switching signal of MMC; C, capacitors' capacitance; z i , capacitors' scaled energy; i 0 , injected current to the AC side; e D , difference between the produced voltages of the lower and the upper arms; i T , circulating current; e T , total voltage produced by the upper and lower cells; z P , z N , upper and lower cells' scaled energy; z T , total scaled energy stored in the upper and the lower cells; z D , difference between the scaled energy of the upper and the lower cells; 0 , fundamental frequency of the grid voltage; i * 0 , reference for the injected current; P 0 , constant load power reference; v S,RMS , RMS value of the grid voltage;v S,1 , estimation of the grid voltage fundamental component; t, time; i * T , reference for the circulating current; e * D , e * T , steady-state expressions for e D and e T ;ĩ 0 , error of the injected current loop; D , harmonic disturbance in the injected current loop; R D , damping gain of the injected current loop;̂D, estimate for the harmonic perturbations grouped in D ; k, odd harmonics number; D,k , estimation gains for the resonant filters; s, complex variable; f 0 , fundamental frequency (in Hz) of the grid voltage; T kr , desired response time for each harmonic component; i0

This study presents a model-based controller for a hybrid power filter (HPF) to reduce the current harmonic distortion in a three-phase system for a general operation case considering unbalanced and distorted source voltages and load currents. The HPF comprises an active power filter (APF) grid connected by means of an LC passive filter. This topology is aimed to reduce the current rating handled by the APF, representing a cost effective solution. The controller design is split into two stages, a first one to control the fundamental component of the system dynamics, and to regulate the DC voltage; and a second stage to compensate the harmonic distortion caused by a non-linear load. For this, the harmonic compensation scheme forces the injection of the required harmonic current to the point of common coupling. The proposed scheme keeps the very familiar proportional-resonant control structure. The compensation of the load reactive current is left to the passive filter. Experimental and simulation results are presented to exhibit the benefits of the proposed control solution.

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