## 1 Introduction  
With the wide application of distributed generations (DGs), such as photovoltaic and wind turbines in microgrids, power electronic technology has also rapidly developed [1; 2; 3]. Among them, the three-phase voltage-source pulse width modulation (PWM) converter has the advantages of high efficiency, low harmonics, simple topology, adjustable power factor, and various control methods. It is mainly used as the interface connected between the DG and the bus or the AC bus and the DC bus [4; 5]. However, because the transmission efficiency of a single converter is constrained by semiconductor devices, switching frequencies, and other conditions, it cannot meet the actual demand of power conversion. To solve this problem, the modular parallel operation of converters has been applied in conditions of low voltage and high current [6]. The parallel operation of the power module can increase the system capacity and conversion efficiency as well as improve the stability and reliability of the grid [7; 8; 9]. However, due to the different hardware parameters between the parallel converters, their switching action cannot be fully synchronized, which will generate zero-sequence voltage. The voltage soon acts on the equivalent resistance between the converters to generate zero-sequence circulating current (ZSCC) [10]. The circulating current will increase the switching loss, reduce the efficiency of the system, increase the probability of converter failure and shutdown, and even destroy the whole system [11; 12]. In order to further solve the problem of circulating current in parallel systems, scholars in China and abroad have put forward some active methods. [PERSON] utilized a voltage source converter consisting of gate turn-off thrustors (GTOs) to connect the energy storage system to the AC bus [13]. In this converter, the active and reactive power can be managed independently and quickly by controlling the on/off of the current. [PERSON] et al. explained that the reason there is a path for ZSCC in the three-phase converter parallel system shown in some studies is because they ignore the magnetic couples caused by the three-pole reactors and the three-pole transformers [14]. Therefore, it is obvious that the addition of three-pole reactors and transformers at the output of the three-phase converter can block this path, which effectively suppresses the ZSCC. [PERSON] et al. designed a photovoltaic inverter that connects multiple parallel converters to inductors and transformers to reduce the zero-sequence voltage [15]. [PERSON] et al. showed that adding a coupled inductor to the output of paralleled converters can reduce the circulating current. However, the authors also noted that the system is sensitive to low-order harmonics, and adopting proportional resonance (PR) control to improve the scheme can therefore result in better performance [16]. All the above methods take the measure of hardware isolation to block the current path to eliminate the circulating current. Although it is simple and easy, it requires the connection of additional hardware, which will increase the weight, volume, and cost of the system and reduce the system efficiency. Therefore, more and more modulation and control strategies have been proposed. [PERSON] indicated that the zero-sequence problem is mostly caused by zero-sequence low-frequency harmonics. He therefore proposed a ZSCC control method based on harmonic elimination pulse width modulation (HEPWM) [17]. Similarly, a new selective harmonic elimination pulse width modulation (SHE-PWM) control technique was developed by [PERSON] and [PERSON], which can eliminate the ZSCC generated by paralleled modular voltage-source inverters (MVSIs) [18]. Although these modulation methods can reduce low-frequency and high-frequency circulating current, the high switching frequency will increase the switching loss and limit conversion efficiency. [PERSON] put forward a multicarrier PWM technique for parallel three-phase active front-end converters (AFECs) [19].