Accurate power sharing and harmonic mitigation scheme for parallel operation of single-phase voltage source inverters
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ORIGINAL ARTICLE
Accurate power sharing and harmonic mitigation scheme for parallel operation of single‑phase voltage source inverters Minh‑Duc Pham1 · Hong‑Hee Lee1 Received: 30 August 2020 / Revised: 11 September 2020 / Accepted: 14 September 2020 © The Korean Institute of Power Electronics 2020
Abstract Inaccurate reactive power sharing and voltage distortion at the point of common coupling (PCC) always exist in a parallel inverter system due to feeder impedance mismatches and nonlinear load disturbances. An enhanced inverter control scheme through an output impedance adjustment is proposed to address these issues. Inverter output impedance is regulated considering fundamental and harmonic frequencies, and accurate power sharing and improved PCC voltage quality are realized at the steady state. In addition, the compensation scheme is easily applied in multiple parallel inverters without any prior knowledge of the feeder impedance. Experimental results with three parallel inverters validate the feasibility of the proposed control scheme. Keywords Harmonic mitigation · Stand-alone operation · Renewable energy system · Voltage source inverter
1 Introduction Distributed generators using renewable energy resources have been widely used in recent years. When several inverter-based distributed generators are connected in parallel, they operate as a voltage source inverter (VSI) to stabilize the critical load voltage. Figure 1 shows a configuration of a parallel VSI system that operates independently from the utility grid. In a stand-alone operation, a traditional droop controller enables the decentralized regulation of the inverter voltage and frequency to share the real and reactive powers autonomously [1, 2]. Nevertheless, an inverter with a droop controller has a small inertia, and it is sensitive to the feeder impedance differences among inverter units. The mismatched feeder impedance makes the voltage drop among inverters unequal, which reduces the accuracy of reactive power sharing [3]. Furthermore, when the nonlinear load is connected at the point of common coupling (PCC), its harmonic currents cause a voltage drop across the grid side inductors and distort the voltage at the PCC bus [4, 5]. Many harmonic compensation techniques have been reported to address these problems. In [6], the traditional * Hong‑Hee Lee [email protected] 1
Department of Electrical Engineering, University of Ulsan, Ulsan, Korea
active power filter was used to improve the PCC voltage quality. However, it is difficult to set up in a stand-alone AC system, because inverters are scattered with long distances [7]. To remove the power filter, a harmonic compensation function was added to all inverters to mitigate the PCC voltage harmonics [8], and the virtual resistance control strategy was presented to enhance the PCC voltage quality [9, 10]. However, these control approaches increase the voltage drop at the inverter output and reduce the PCC voltage quality. In addition, the negative virtual harmonic impedance was introduced t
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