Decoupled Design of Inverter Output Impedance for Parallel Operated Inverters: An Analytical Approach

Abstract

This paper suggests an analytical framework to design the output impedance of a microsource inverter to meet the requirements of proportional load sharing with the permissible range of THD (total harmonic distortion) of the output voltage as per the IEEE 519 standard. In order to achieve the aforementioned objectives, the integral controller is designed in such a way as to make the output impedance of an inverter capacitive in nature. The optimum design of capacitive output impedance at the harmonic frequencies ensures reduced THD levels in the output voltage without affecting proportional load sharing, which is determined by fundamental frequency impedance, ensuring the decoupled design to meet different requirements at the same time. The focus of the work is also laid on the inherent limitations of conventional droop control of inverters and its remedial measures through bolting a regulator on its control loop. The modification of the robust droop control structure for a capacitive inverter is discussed, and small signal stability/PF (participation factor) based analysis is done to ensure the steady state stable operation. In order to enhance the load voltage quality further, a bypassing the harmonic current components strategy is discussed and developed further, which led to THD_v < 5%. Subsequently, the basic stability analysis is conducted to determine the finite parameters of BHC by control system principles. The discussed models and schemes are simulated under both linear and nonlinear load conditions to validate the proposed strategies, and their effectiveness is assessed through a comparative analysis.

Keywords

Power sharing / Power quality / Stability / Total harmonic distortion

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