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.
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.
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.
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.
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
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.
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.
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.