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Abstract—The application of underground cables and shunt capacitor banks may introduce power distribution system res-onances. In this paper, the impacts of voltage-controlled and current-controlled distributed generation (DG) units to microgrid resonance propagation are compared. It can be seen that a con-ventional voltage-controlled DG unit with an LC filter has a short-circuit feature at the selected harmonic frequencies, while a current-controlled DG unit presents an open-circuit character-istic. Due to different behaviors at harmonic frequencies, specific harmonic mitigation methods shall be developed for current-controlled and voltage-controlled DG units, respectively. This pa-per also focuses on developing a voltage-controlled DG unit-based active harmonic damping method for grid-connected and island-ing microgrid systems. An improved virtual impedance control method with a virtual damping resistor and a nonlinear virtual capacitor is proposed. The nonlinear virtual capacitor is used to compensate the harmonic voltage drop on the grid-side inductor of a DG unit LCL filter. The virtual resistor is mainly responsible for microgrid resonance damping. The effectiveness of the proposed damping method is examined using both a single DG unit and multiple parallel DG units.
. INTRODUCTION
HE INCREASING application of nonlinear loads can led Tto significant harmonic pollution in a power distribution system. The harmonic distortion may excite complex reso-nances, especially in power systems with underground cables or subsea cables [22] and [23]. In fact, these cables with nontriv-ial parasite shunt capacitance can form an LC ladder network to amplify resonances. In order to mitigate system resonances, damping resistors or passive filters can be placed in the distri-bution networks [1]. Nevertheless, the mitigation of resonance
propagation using passive components is subject to a few well-understood issues, such as power loss and additional investment. Moreover, a passive filter may even bring additional resonances if it is designed or installed without knowing detailed system configurations.
To avoid the adoption of passive damping equipment, var-ious types of active damping methods have been developed. Among them, the resistive active power filter (R-APF) [2]–[8] is often considered as a promising way to realize better per-formance. Conventionally, the principle of R-APF is to emu-late the behavior of passive damping resistors by applying a closed-loop current-controlled method (CCM) to power elec-tronics converters. In this control category, the R-APF can be simply modeled as a virtual harmonic resistor if it is viewed at the distribution system level [2]. Additionally, a few mod-ified R-APF concepts were also developed in the recent lit-erature. In [6], the discrete tuning method was proposed to adjust damping resistances at different harmonic orders. Ac-cordingly, the R-APF essentially works as a nonlinear resistor. In [5], the operation of multiple R-APFs was also consid-ered, where an interesting droop control was designed to of-fer autonomous harmonic power sharing ability among parallel R-APFs.
On the other hand, renewable energy source (RES) based dis-tributed generation (DG) units have been adopted to form flexi-ble microgrids and their interfacing converters also have the op-portunity to address different distribution system power quality issues [8], [9], [17]. For current-controlled DG units, the aux-iliary R-APF function can be seamlessly incorporated into the primary DG real power injection function by modifying the cur-rent reference. However, conventional CCM can hardly provide direct voltage support during microgrid islanding operation. To overcome this limitation, an enhanced voltage-controlled method (VCM) [17] was recently proposed for DG units with high-order LC or LCL filters. It can be seen that the control method in [17] regulates the DG unit as virtual impedance, which is dependent on the existing feeder impedance. When the feeder impedance is inductive, this method could not provide enough damping effects to system resonance.
To achieve better operation of grid-connected and islanding microgrids, the paper considers a simple harmonic propagation model in which the microgrid is placed at the receiving end of the feeder. To mitigate the feeder harmonic distortions, a modified virtual impedance-based active damping method that consists of a virtual resistor and a virtual nonlinear capacitor is also proposed. The virtual capacitor eliminates the impacts of LCL filter grid-side inductor and the virtual resistor is interfaced to the receiving end of the feeder to provide active damping