Topology of string inverter

String-connected on grid inverters generally have transformer-less non-isolated, power-frequency transformer-isolated, and high-frequency isolation types.

The commonly used topologies are as follows.

①Single-phase on grid inverter with Boost

The front stage of this on grid inverter circuit uses a Boost circuit for boosting, while the maximum power point tracking control is performed on the output of the photovoltaic panel, and the latter stage uses a full bridge inverter circuit. The introduction of the booster circuit expands the working voltage range of the on grid inverter and increases the power generation time of the photovoltaic system. The boost part limits the power capacity, so it is suitable for low-power systems. In order to improve the MPPT problem of multiple strings connected in parallel, some inverters have extended multiple Boost boost devices, so that multiple strings can be tracked separately for MPPT, and the boost inductance and switching devices can also be reduced. Capacity requirements. As shown in Figure 1.

Figure 1 Single-phase on grid inverter with Boost boost

②Single-phase high-frequency isolation on grid inverter

The high-frequency isolation topology has the characteristics of high efficiency, small size, light weight and no need for power frequency transformer isolation, and is suitable for single-phase low-power photovoltaic power generation systems. as shown in picture 2.

Figure 2 High-frequency isolated on grid inverter

③Three-phase two-level on grid inverter with Boost

This topological structure is suitable for situations where the power is relatively large and three-phase balanced grid connection is required. The topological structure chart is shown as in Figure3.

Figure 3 Three-phase on grid inverter with Boost boost

④Three-level on grid inverter

The three-phase three-level on grid topology has the advantages of low output voltage harmonic content, low du/dt and low electromagnetic radiation, and is suitable for high-voltage and high-power applications. As shown in Figure 4.

Figure 4 Three-phase three-level on grid inverter

Topology of centralized inverter

Centralized inverters generally do not include the DC/DC part, and the multiple strings of batteries on the DC side are converged through the combiner box and then connected to the inverter through the inverter bridge to be converted into AC power and fed into the grid. As shown in Figure 2-7. At present, the mainstream centralized inverters on the market generally adopt a two-level topology. In order to improve the utilization rate of DC voltage, the AC voltage level is generally reduced. The conventional method is to use line

The voltage is 270V, which is increased to a high voltage of 10kV and above by a step-up transformer, and then connected to the transmission line. The centralized inverter also has a three-level structure, and the topology is basically the same as the string three-level. At present, there is a topological structure that puts the DC/DC part in the confluence part on the market. It can realize the MPPT function and avoid the energy loss caused by the imbalance between the strings.

Topology of micro inverter

①Flyback

The front and back stages of the inverter are respectively a flyback DC/DC converter and a full-bridge voltage source inverter. The AC output voltage is higher than the peak voltage of the power grid, and it can work in CCM mode, which can reduce losses and improve inverter efficiency. The topological structure is shown as in Figure6.

Figure 5 Topological structure of centralized on grid inverter
Figure 6 Flyback type miniature on grid channel converter

②Push-pull

The push-pull structure is conducive to making full use of the transformer core and has a higher power density, but it is difficult to achieve soft switching control. The topological structure is shown in Figure 7.

Figure 7 Push-pull miniature on grid inverter

③ Series resonance type

The structure has a complete full-bridge inverter function and LC resonance function, which promotes the improvement of efficiency. As shown in Figure 8.

Figure 8 Series resonant micro on grid inverter