Enhanced Current Reference Calculation to Avoid Harmonic Active Power Oscillations

Abstract

Energy storage systems play a key role in the rise of distributed power generation systems, hence there is great interest in extending their lifetimes, which are directly related to DC current ripple. One of the ripple sources is the low-frequency active power fluctuations under unbalanced and distorted grid voltage conditions. Therefore, this paper addresses a multifrequency control strategy where the harmonic reference currents are calculated to reduce harmonic active power oscillations. The stationary reference frame (StRF) approach taken here improves the precision and computational time of the current reference calculation method. Additionally, in order to ensure safe converter operation when a multifrequency reference current is provided, a computational efficient peak current saturator is applied while avoiding signal distortion every time step. If the injected current harmonic distortion is to be minimized, which is a feature included in this work, the peak current saturator is a necessary requirement. Active power ripple is reduced even with frequency variations in the grid voltage using a well-known frequency-adaptive scheme. The simulation and experimental results prove the optimized performance for the control objective: power ripple reduction with minimum current harmonic distortion.Energies 2019, 12, 4075 2 of 21 torque pulsations [9]. Besides, on doubly fed induction generators (DFIGs) for wind turbines, active power oscillations are related to electromagnetic torque ripples that increase the mechanical stress on the turbine system [10].The harmonic power oscillations in three-phase three-wire AC electrical systems come from the presence of distorted or unbalanced grids. Therefore, the control strategy aims to regulate the instantaneous active power at a constant value by injecting the suitable currents into the grid. The current reference could be calculated by means of the active and reactive power theory, first introduced in Reference [11]. From that theory, many works have dealt with the regulation of active power oscillations when grid imbalances [12,13] or grid faults [14][15][16] are faced by the power converter. However, these methodologies only remove the second harmonic power ripple due to the appearance of fundamental negative sequence (FNS) voltage and its interaction with the fundamental positive sequence (FPS) current. In distorted grids, there are higher even harmonic power ripples due to other harmonic sequence voltages (−5, +7, −11, +13). Hence, an effective multifrequency current reference calculator is required by the algorithm to accomplish the power ripple removal.The presented control objective has an unavoidable characteristic in three-phase three-wire systems-the elimination of active power oscillations implies more reactive power fluctuations [13,16]. Previous publications such as References [17][18][19] claimed the elimination of both active and reactive power oscillations, simultaneously. However, this approach in three-wire systems leads to the injection of zero-sequence...

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An active parallel power decoupling circuit with a dual loop control scheme for micro‐inverters

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