Overview of leakage current suppression technology and ways of leakage current suppression
Overview
The photovoltaic power generation system is usually composed of a battery panel array and a power conversion part (inverter). Placing a low-frequency isolation transformer between the inverter and the grid can achieve electrical isolation between the grid and the photovoltaic array, ensure personal safety, and achieve voltage matching and suppression of the DC component of the grid current. However, the low-frequency transformer increases the volume, weight, and cost, and reduces the conversion efficiency. Inserting a high-frequency transformer in the front stage of a photovoltaic system is an alternative measure, which reduces the volume, weight and cost to a large extent, but makes the power conversion more complicated and does not significantly improve the system efficiency. For non-isolated grid-connected inverters, since they do not contain high-frequency or low-frequency transformers, this type of inverter has the advantages of high conversion efficiency, small size, light weight and low cost, and has received extensive attention at home and abroad.
However, non-isolated inverters also bring electrical connections between photovoltaic panels and the large power grid. The high-frequency switching of power devices results in the generation of high-frequency common-mode voltage, which forms a high-frequency common-mode current in the loop composed of the inverter, the parasitic capacitance of the battery board, and the power grid. In practical applications, the large area of the photovoltaic panel causes its parasitic capacitance to the ground to be non-negligible (the schematic diagram of the distributed capacitance of the photovoltaic cell to the ground is shown in Figure 1), and due to the consideration of inverter efficiency optimization, the inverter’s own impedance is low, and the grid itself is also very low. This leads to common-mode loop leakage current that is often not negligible. High-frequency leakage current will not only bring conduction and radiation interference, increase the harmonic content of the grid current and system loss, but also endanger the safety of related equipment and personnel. Therefore, the suppression of common-mode current is the primary problem that non-isolated photovoltaic grid-connected inverters need to solve. At present, there are a series of low-frequency leakage current photovoltaic grid-connected inverter topologies, some of which have been widely used, but the systematic analysis of the leakage current suppression mechanism of grid-connected inverters is still relatively rare.
This category article starts from the common-mode equivalent circuit of the bridge-connected inverter, establishes a more complete common-mode analysis model, and applies the model to two types of grid-connected inverter topologies, namely, full-bridge and half-bridge photovoltaic grid-connected inverter structures. Finally, from the perspective of improving the reliability of grid-connected inverters, the improvement measures of full-bridge and half-bridge inverter topologies are given.
Leakage current suppression approach
It can be seen from the equivalent circuit in Figure 2 that the magnitude of the leakage current of the bridge inverter topology depends on the equivalent impedance Z and CPV of the common mode loop and the common mode equivalent voltage vCM and vCM-DM. Among them, vCM depends on the working mode of the circuit, and vCM-DM depends on the symmetry of circuit parameters and parasitic parameters. Considering that the distributed capacitance CPV in the circuit is related to the installation and grounding of the battery board, CPV is not controllable, but the common mode impedance Z is variable and controllable, then the simplest unified common-mode equivalent circuit of the single-phase bridge-type grid-connected inverter as shown in Figure 3 is obtained.
As can be seen from Figure 3, due to the existence of the distributed capacitance CPV in the loop, if there is a common-mode equivalent voltage and the common-mode voltage changes, a leakage current will be formed. Although the distributed capacitance is limited in size and the leakage current generated by the low-frequency varying common-mode voltage is limited, the high-frequency component of the common-mode voltage will cause a larger leakage current. It can also be seen from this figure that when the common-mode equivalent voltage is a constant value or the common-mode voltage changes at high frequency, the impedance Z is infinite, which can theoretically suppress high-frequency leakage current.
Based on the above analysis and the equivalent model shown in Figure 3, two types of leakage current suppression approaches can be summarized.
(1) Configure the common-mode circuit to show high impedance when the common-mode equivalent voltage changes at a high frequency. The specific methods are as follows:
[Path A] Under the premise of symmetrical circuit parameters (voltage vCM-DM is a constant value), change the common mode impedance Z so that Z always maintains a high impedance when vCM changes at high frequencies;
(2) Through proper circuit parameter matching or design of proper sinusoidal pulse width modulation (SPWM) strategy, the common mode equivalent voltage can be kept constant. There are two ways:
[Path B] Under the premise of symmetrical circuit parameters (voltage vCM-DM is a constant value), adopt a suitable SPWM strategy to make vCM a constant value;
[Path C] Through the circuit parameter matching, the sum of the voltage vCM and the voltage vCM-DM is a constant value.