# The principle of common mode current is explained in detail with pictures and texts

The area of ​​the photovoltaic array is generally large, and the parasitic capacitance to the ground can reach 200nF/kWp in a rainy or humid environment. The power switching action of the grid-connected inverter may generate high-frequency voltage changes. Due to the existence of the parasitic capacitance of the photovoltaic array to the ground, a common mode current (also called leakage current) is generated on the parasitic capacitance to the ground. The generation of leakage current will bring about conduction and radiation interference, increase in harmonics and loss of grid-connected current, and even threaten equipment and personal safety.

When the photovoltaic power generation system has poor insulation due to various reasons, and people just touch this part, a flowing current will be generated between the human body and the earth, which is called residual current. When the residual current is greater than 30mA, it will seriously threaten human life. The sum of common mode current and residual current is called differential current. For non-isolated PV inverters, the residual current cannot be measured during operation, only the differential current can be measured indirectly. When a certain sudden change in the differential current is measured, it is considered that the residual current may occur, and the photovoltaic inverter operates according to certain standards.

①When the continuous output rated power of the photovoltaic grid-connected inverter is less than or equal to 30kVA, the differential current reading value is 300mA, which is suitable for fire protection.
② When the continuous output rated power of the photovoltaic grid-connected inverter is greater than 30kVA, the differential current action reading value is 10 mA/kVA. For the isolated photovoltaic grid-connected inverter, due to the existence of the transformer, the leakage current is almost zero, so the magnitude of the residual current can be directly measured.

Taking the negative N of the input DC bus as a reference, the two outputs of the single-phase grid-connected inverter are represented by 1 and 2 respectively. As shown in Figure 1, the common mode voltage Vcm and the differential mode voltage Vdm are respectively expressed by formula (1) And formula (2) said. Defining the full common mode voltage Vtcm is represented by Equation (3).

In the formula, L1 and L2 are the filter inductances that connect the live wire and the neutral wire of the grid voltage respectively, usually L1=L2. Therefore, if both L1 and L2 exist in the circuit, then Vtcm=Vcm.

In order to suppress the magnitude of the common mode current, many novel and practical circuit topologies have appeared. Typical single-stage circuit topologies include single-phase full-bridge inverters, half-bridge inverters, HERIC circuits, H6 circuits, H5 circuits, quasi-H5 circuits, HERIC derivative circuits, hybrid bridge arm circuits, and three-level half-bridge inverters device (NPC circuit), etc. A typical two-stage circuit topology is usually a chopper circuit + a single-stage circuit topology.

For the inverter circuit of unipolar PWM modulation mode, it is characterized in that the main switch is turned off to cut off the electrical connection between the photovoltaic array and the AC grid. At this time, an auxiliary freewheeling path is required so that the grid-connected current no longer flows through the DC. The bus filter capacitor is used to ensure that the common mode voltage basically remains half of the output voltage VPV of the photovoltaic array, that is, the common mode voltage remains unchanged.