I. Introduction
In the process of grid-connected photovoltaic power generation, the output current will be distorted due to the switching characteristics of the inverters, resulting in harmonics. The frequency of the harmonic current component is related to the number of pulses in the inverter. The higher the number of pulses in the inverter, the higher the lowest frequency of the harmonic component [1]–[2]. The emergence of harmonics, the use of electrical equipment in a non-ideal working environment, power system is a kind of interference and pollution[3]–[4]. At present, the main measures for harmonic suppression of grid-connected photovoltaic inverters are active harmonic suppression and passive harmonic suppression[5]–[6]. Active suppression starts from the electrical equipment itself, and reduces the harmonic generation of the electrical equipment by improving the PWM control technology[7]–[8]; passive suppression compensates the harmonic of the electrical equipment by adding passive filter and active filter device to avoid the harmonic component entering the network[8]–[9].