# Common-Mode Voltage Analysis and Reduction for the Quasi-Z-Source Inverter with a Split Inductor
## 1 Introduction  
The interest in renewable power generation systems has been increasing with no signs of slowing down in recent years worldwide. The photovoltaic (PV) arrays are particularly attractive as a renewable source for distributed generation with the advantages of relatively small size, noiseless operation and simple installations [1]. Due to the low-voltage and intermittent output characteristics of the PV arrays, power electronics are essential interfaces to deliver solar energy to the grid or residential applications with boosting capability. The conventional topologies for PV applications either employ a high-frequency or a low-frequency transformer for galvanic isolation between the PV arrays and the grid, which results in higher cost, larger volume and lower efficiency. Recently, the transformerless alternatives have been receiving more and more attention by addressing the aforementioned issues. However, the leakage current may appear due to the PV array parasitic capacitance to the ground. Leakage currents bring many concerns, e.g., electromagnetic interference issues, fault activation of detector circuits and harmonic currents [2; 3; 4]. Therefore, the leakage current should be suppressed within a certain level according to the standards like the IEC 62109-2 [5]. The fluctuations between the grid and PV array, i.e., common-mode voltage (CMV), will induce large leakage currents flowing through the parasitic capacitors in a transformerless PV system [6; 7]. To tackle the leakage current issue, various topologies and modulation methods have beenproposed [8; 9; 10]. The basic principle of the topology modification is to introduce new paths to isolate the PV array from the grid, e.g., a highly efficient and reliable inverter concept (HERIC). The CMV amplitude depends on the choice of the switching vectors in the same topology. Hence, there are various pulse width modulation (PWM) methods that can reduce the CMV, such as discontinuous pulse width modulation (DPWM), active zero state pulse width modulation (AZSPWM), near state pulse width modulation (NSPWM) and remote state pulse width modulation (RSPWM) [11; 12; 13]. While the conventional PWM methods make the most use of all the vector states, the modified PWM methods can reduce the CMV with active vector states and partial zero vector states to generate the output. DPWM adopts all the active vector states and partial zero vectors to eliminate the CMV. RSPWM uses the active vector states, e.g., odd vectors or even vectors, which lead to the same CMV amplitude. NSPWM adopts the adjacent active vector states to generate the output that meets the reference requirement. Furthermore, the CMV can also be alleviated by changing the traditional control schemes [14; 15; 16; 17; 18; 19]. In terms of topology, the quasi-Z-source inverter (qZSI) has been considered as an alternative to the conventional two-stage two-level inverter [20; 21; 22; 23; 24]. The qZSI can operate as a single-stage converter by utilizing the shoot-through (ST) state. Due to its advantages, like a continuous input current and a common ground point between the input and the DC-link, the qZSI can be an attractive candidate for renewable applications with a wide input range. A typical three-phase grid-connected qZSI is presented in Figure 1. As a transformerless inverter, the CMV fluctuations in the qZSI can also lead to large leakage currents. The high-frequency components in the CMV of the qZSI are much larger than those in the conventional two-stage transformerless inverter when the ST states were adopted [25]. When the prior-state-of-the-art CMV reduction strategies for the conventional voltage source inverter (VSI) are applied to the qZSI, the high-frequency components are still large, which results from the ST state. To address this issue, various strategies have been proposed. In [26], the CMV in the qZSI with different modulation methods was analysed in detail. A modified PWM method, which only utilizes odd vectors and the ST vector, was proposed in [27].