Analysis of leakage current suppression mechanism of Heric change topology and double parallel Buck inverter topology
1. Heric change topology 1
Figure 1 shows a non-isolated topology based on the Heric structure. This topology moves a freewheeling switch in the Heric topology to a group of bridge arms. Using the unipolar SPWM strategy shown in Figure 2, in the power transmission stage, current flows through S3-S2 (positive direction) and S1-S5-S4 (negative direction);
In the freewheeling phase, the current flows through the S5 parasitic diode-S6 (positive freewheeling), and S5-S6 parasitic diode (negative freewheeling). The working principle is similar to Heric, S1~S4 are disconnected in the freewheeling phase, and the freewheeling is carried out through the switch tubes S5 and S6, so that the photovoltaic panel is separated from the power grid during the freewheeling phase, which is equivalent to putting a large impedance in series in the common mode loop to achieve the purpose of suppressing leakage current. However, compared with the Heric topology, this circuit flows through one more switch tube during the negative half cycle of the power grid, and the conduction loss is slightly larger.
2. Heric change topology 2
Figure 3 shows another non-isolated topology based on the Heric structure. Its structure is to move a group of freewheeling switch tubes in the Heric topology to the same pair of bridge arms, and add diodes D1 and D2, so its leakage current suppression principle is similar to that of Heric. This topology adopts the unipolar SPWM strategy shown in Figure 4. During the power transmission phase, current flows through S1-S5-S4 (positive direction) and S3-S6-S2 (negative direction); in the freewheeling phase, the current flows through S5-D1 (positive freewheeling) and S6-D2 (negative freewheeling). This topology is disconnected during the freewheeling phase S1~S4, which makes the photovoltaic panel separate from the grid, and adopts Path A to suppress high-frequency leakage current.
This topology flows through three power tubes in the power transmission stage. Compared with the Heric topology, the efficiency is slightly lower, but the switching tubes S5 and S6 and the diodes D1 and D2 can be optimized to achieve the purpose of efficiency optimization. As with the H6 topology, more expensive devices, such as silicon carbide diodes, must be selected to further improve efficiency, which increases the cost of the devices.
3. Double parallel Buck inverter topology
In addition to the aforementioned inverters based on the full-bridge topology, there are also literatures on the novel inverter structure of dual-parallel Buck inverters. As shown in Figure 5, it is obtained by combining two unidirectional Buck converters, and has the advantages of unidirectional DC-DC converters without bridge arms and non-working body diodes.
The driving logic of this topology is shown in Figure 6. During the power transmission phase of the positive half cycle of the grid, S1 and S4 are turned on, and the current flows from the DC side through the coupled inductor Li1 to the grid; in the freewheeling phase, the current passes through S5 and diode D5. In the negative half cycle of the power grid, S2 and S3 are turned on, and the current flows from the power grid through the coupling inductor Li2 to the DC side; in the freewheeling phase, the current flows through S6 and diode D6. S5, D5, S6, and D6 provide a freewheeling circuit in the freewheeling phase to ensure that the photovoltaic panel is separated from the power grid. Therefore, this topology is also based on Path A to suppress high-frequency leakage current. Compared with the Heric topology, this topology solves the problem of bridge arm through and improves the reliability of the inverter, but it uses more devices, higher cost, and relatively complex circuit structure.
3. Use common mode filter
A low-cost, small-capacity common-mode filter is installed on the basis of a single-phase full-bridge circuit, so that the filter can be used to increase the common-mode without adding additional switching devices, drive circuits, and system control complexity. The impedance value Z in the loop can suppress the common mode current. The specific structure of the filter mainly includes a set of common-mode inductors or common-mode transformers. However, the methods of common-mode filters to suppress leakage current are only suitable for low-power applications. When the power is large, the volume of the filter will gradually increase and loss Will also improve.
4. Add an isolation transformer
The isolated grid-connected inverter with power frequency or high frequency isolation transformer has better leakage current suppression capability. Essentially, the series connection of the transformer is equivalent to the series connection of a small capacitance (distributed capacitance of transformer primary and secondary side) in the common mode loop, which means that the value of Z in Figure 7 is increased to increase the loop impedance and reduce the leakage current. However, the isolation transformer itself has a large loss, which reduces the efficiency of the system. At the same time, compared with non-isolated inverters, there are problems of large size, heavy weight, and high cost.