Solar energy is now being used to generate electricity by numerous solar power plants scattered around the world. Solar inverters have been used to convert the direct current generated from sunlight into alternating current. Grid-tie and off-grid inverters are the two major types that are used, especially grid-tie inverters. But what are they?
Concept: Grid-tie inverters are those that turn direct current into alternating current and can synchronize to interface with a utility line. If direct current voltage is a little bit low, it will raise it by using its alternating current transformer, so as to get the standard alternating current voltage and frequency.
Requirements and features for each inverter:
1.Each inverter is required to have high efficiency.
2.Each inverter needs more reliability.
3.The direct current input voltage must be broad.
4.These inverters help each solar power system sell unused electricity to the utility grid.
5.They should be quickly disconnected from the utility grid when it is being fixed.
6.They don’t need storage batteries.
Transformer type: newer high-frequency transformer, conventional low-frequency transformer and transformerless solar inverters.
Central inverters: They are often used in large-scale PV power plants (over 10kW). Their most prominent features are that each system has high power and low cost. However, they might be affected by the matching of PV modules and shading. If they are affected, the whole PV system’s efficiency and output will also be affected. Meanwhile, the reliability of the whole system is also affected by a PV unit team which doesn’t work well. The newest methods to solving this problem are two: adjusting it through space vector and developing new topography connections of the inverter.
String inverters: Based on the module concept, each PV string (1kW to 5kW) has MTTP at the direct current end and parallels and connects to the grid at the alternating current end. Lots of large-scale PV power plants use string inverters. Its features lie in that their efficiency won’t be affected by the module differences between strings and shading, and that this avoids the mismatches between PV modules’ best points and inverters. Therefore, more electricity can be generated. Today, transformerless string inverters are leaders among other kinds.
Multiple string inverters: These inverters can be installed in the PV power plants which have the capacity of thousands of MW. In addition, they have the features of central and string inverters. In these inverters, different single power peak value tracking and direct current to direct current converters are included. These direct currents are turned into alternating currents through a common direct to alternating inverter and connect to the utility grid. As the length of the direct current cable decreases, the shading between the strings and the losses caused by the distinctions between the strings will be reduced to the minimum amount.
Module inverters: They connect each module with a inverter. At the same time each module has a single maximum power peak value tracking. They are often installed in the power plants with the capacity from 50W to 400W. The total efficiency is lower than that of string inverters.