A Modified Reference of an Intermediate Bus Capacitor Voltage-Based Second-Harmonic Current Reduction Method for a Standalone Photovoltaic Power System

Abstract

A typical configuration of a standalone photovoltaic power system consists of PV arrays, storage units, a front-end dc–dc converter, a bidirectional dc–dc converter, and a dc–ac inverter. This paper addresses the second-harmonic current (SHC) issue in the front-end dc–dc converter and the bidirectional dc–dc converter when they regulate the intermediate dc bus voltage for the downstream single-phase inverter. The propagations of SHC under different operation modes of the power system are studied, based on which, a method that could make the intermediate bus capacitor fully provide the SHC induced by the dc–ac inverter is proposed. In the proposed method, a modified reference of the intermediate bus capacitor voltage is obtained by adding the desired second-harmonic voltage fluctuation to the original dc voltage reference. By the means of letting the control loop of the dc–dc converter track the modified reference of the intermediate bus voltage, the SHC in the dc–dc converter is well suppressed. The proposed method is easy to be applied in each operation mode of the power system, and it is proved to have the advantage of suppressing the SHC effectively without sacrificing the dynamic performance of the dc–dc converters. The SHC suppressing mechanism of the proposed method is analyzed from the viewpoint of output impedance and a proportional-integral-resonant controller is employed to further enhance the SHC suppressing ability. Finally, a 6-kW photovoltaic power system is built in the laboratory, and the experimental results verify the effectiveness of the proposed method.