Photovoltaic grid-connected inverter
The electric energy conversion device in the photovoltaic power generation system is a core component. In photovoltaic grid-connected applications, grid-connected inverters are usually used to realize the power transfer from photovoltaic cells to the grid. The composition of the grid-connected inverter mainly includes the inverter power circuit, the grid-connected filter and the grid-connected controller. Among them, the power circuit topology of the photovoltaic inverter has an important impact on the cost, efficiency, quality of the grid current, safety and reliability of the photovoltaic power generation system.
According to whether it is equipped with an isolation transformer, photovoltaic inverters can be divided into power frequency isolation type, high frequency isolation type and non-isolation type.
1. Power frequency isolation type
The grid-connected inverter with power frequency transformer first converts the DC power of the photovoltaic cell into AC power through DC/AC conversion, and then, the power frequency transformer is connected to the power grid (Figure 1) to achieve electrical isolation between the power grid and the battery panel, to ensure personal safety, and to provide voltage matching and suppression of the DC component of the grid current. However, the low-frequency transformer increases the system volume, weight and cost, and reduces the conversion efficiency. Grid-connected inverters with power frequency isolation are generally used in high-power or high-power three-phase and single-phase systems.
2. High frequency isolation type
The disadvantages of power frequency isolation transformer in terms of volume, weight and cost limit its application in small and medium power photovoltaic grid-connected inverters. The method of inserting a high-frequency isolation transformer in the DC/AC conversion link can also achieve electrical isolation and voltage matching between the grid and the battery panel, and at the same time can greatly reduce the volume, weight and cost of the transformer. According to different combinations of high-frequency transformers and conversion links, it can be divided into a conversion structure with a DC link, a conversion structure with a pseudo DC link, and a conversion structure without a DC link, as shown in Figure 2.
The introduction of high-frequency isolation transformers has greatly enriched the topology of small and medium-power grid-connected inverters, which are widely used in household single-phase photovoltaic grid-connected occasions. However, due to the more complex system transformation links, system efficiency may be affected.
The non-isolated grid-connected inverter (Transformerless Grid-Connected Inverter, TLGCI) structure does not contain transformers (including high frequency and low frequency). It has the advantages of high conversion efficiency, low volume, weight and low cost, and has been valued by scientific researchers from all over the world and sought after by the industry. Figure 3 (a) and (b) respectively show two types of transformerless grid-connected inverters: single-stage and two-stage. The two-stage grid-connected inverter can adapt to a wide input voltage range, and the two-stage system is easy to optimize and control by stages, so the whole system design is very convenient; for single-stage grid-connected inverters, since the minimum input voltage is not lower than the peak value of the grid voltage, the single-stage grid-connected inverter is suitable for higher photovoltaic string voltage, and because the number of power conversion stages is small, the system conversion efficiency is higher.