# Novel Common-Mode Current Suppression Method in Transformerless PV Grid-Connected System
## 1 Introduction  
The PV grid-connected inverter is an indispensable part of grid connected PV power generation systems. It should be pointed out that the isolated transformers of line frequency or high frequency are used as inverters in the traditional PV system. The line frequency transformer is installed on the ac output side, which is bulky and inconvenient to install and decreases the efficiency of the system. Although the high-frequency transformer reduces the size of PV system, it is installed on the dc side, which increases the complexity of main circuits, reduces the system stability, and affects the system efficiency. To overcome these shortcomings and improve the efficiency of PV system, the transformerless inverters are widely used in the residential PV system with small capacities [1]. Although the volumes have been greatly reduced in the transformerless inverter, the efficiency has been improved, and the costs has been decreased. The parasitic capacitance, which is between the PV modules and the ground as well as the grid, forms the CM circuit due to the lack of galvanic isolation between the PV systems and grids. Therefore, the CM current in the PV system has a negative influence on the safety of the whole system. At present, there are two main methods to suppress the CM currents: (1) An inverter topology with the characteristics of CM current suppression and an improved modulation mode are adopted. A Space Vector Pulse Width Modulation (SVPWM) method is proposed to suppress the CM current [2]. The full bridge topology, with dc bypass and full bridge topology with ac bypass [3], and the topology with CM current suppression [4] have a decoupling effect on the grid side and dc side and ensure the constant CM voltages during the freewheeling period of PWM method. The Single-phase clamp topology [5] can also suppress CM currents. (2) The Electro Magnetic Interference (EMI) filter is added to the ac side [6]. The impedance in the CM loop can be increased, which inhibits the CM current flowing into the grid and effectively suppresses the CM loop current. The current standard for the gridsrequires that the CM current should be less than 30 mA [7]. Based on the above two methods, the CM current can be generally suppressed to be less than 30 mA. However, there are many high frequency components in the CM current flowing through the power grid; the power quality is decreased and the great electromagnetic interference to the inverter is introduced such that the stability of the whole system is reduced and the internal lifetime of the inverter is shortened. To prevent the electric shock accident due to excessive CM current, the leakage protection device is installed on the output side of inverters. Because of the changes in the external environment, especially for the parasitic capacitance value change on rainy days, the change of the CM current in the high frequency component will lead to the action of leakage protection device, which makes the system stop working and often brings unnecessary losses to the users. The distributed PV system will be widely used in future. If many grid connected and residential PV systems stop working at the same time, the voltage drop of the grid will appear such that the stability of the power grid will be greatly reduced. In recent years, some scholars have proposed to set CM internal loop at the output side of the system to suppress CM current. In [8], a CM internal loop is proposed by connecting the midpoint of ac side voltage to the dc side voltage point. In [9], a new modulation method is proposed for optimally controlling the power switches of transformerless single-phase full-bridge PV inverters to reduce CM current. In [10], the RC absorption branch is added between the output terminal and the dc negative bus to form a single CM internal loop. However, there is no further analysis of CM current in [8; 9; 10], and there is no comparative analysis of the effect of CM internal loop on the high frequency component of the CM current.