The inverter is a converter device that converts direct current into 50 Hz alternating current electric energy, and is one of the core equipment of distributed power sources. The direct current generated by the photovoltaic panel generally needs to be converted into alternating current through an inverter, which is provided to the alternating current load or integrated into the alternating current grid.
There are a variety of implementation schemes for inverters, which are mainly divided into two categories: voltage type and current type. Voltage-type inverters are more common, because the energy storage components in voltage-type inverters are capacitors. Compared with the inductance of energy storage components in current-mode inverters, it has obvious advantages in terms of energy storage efficiency, volume and price of energy storage devices. Inverters can be classified according to topological structure, isolation mode, output phase number, power level, power flow direction, and photovoltaic string mode.
1. Classified by topology
According to the topology classification, it is divided into full-bridge inverter topology, half-bridge inverter topology, multi-level inverter topology, push-pull inverter topology, forward inverter topology, flyback inverter topology, etc. Among them, high-voltage high-power distributed power inverter can adopt multi-level inverter topology, medium-power distributed power inverters can use full-bridge and half-bridge inverter topologies, and low-power distributed power inverters use forward and flyback inverter topologies.
2. Classified by isolation method
According to the classification of isolation methods, there are two types of isolation and non-isolation. Among them, isolated grid-connected inverters are divided into power frequency transformer isolation and high frequency transformer isolation. At the beginning of the development of photovoltaic grid-connected inverters, power frequency transformer isolation was mostly adopted. However, due to its obvious defects in size, weight, and cost, grid-connected inverters with high-frequency transformer isolation have developed rapidly in recent years. Non-isolated grid-connected inverters are gradually gaining recognition due to their high efficiency and simple control. They are now being promoted and applied in Europe, and key issues such as reliability and common mode current need to be solved.
3. Classification by output phase number
According to the number of output phases, there are two types of single-phase and three-phase grid-connected inverters. Generally, single-phase mode is generally used for small and medium power occasions, and three-phase grid-connected inverters are mostly used for high power occasions.
4. Classified by power level
According to the classification of power level, it can be divided into small power inverters with power less than 1kW, medium power grid-connected inverters with power level 1kW~50kW and high-power grid-connected inverters with power level above 50kW. Since the development of photovoltaic grid-connected inverters, the most mature development is the medium-power grid-connected inverters. At present, commercial mass production has been achieved, and the technology is becoming mature.
5. Classification by power flow direction
According to the power flow direction, it is classified into two types: unidirectional power flow grid-connected inverters and bidirectional power flow grid-connected inverters. The one-way power flow grid-connected inverter is only used for grid-connected power generation. In addition to the grid-connected power generation, the two-way power flow grid-connected inverter can also be used as a rectifier to improve the grid voltage quality and load power factor. In recent years, two-way power flow grid-connected inverters have gained attention and are one of the future development directions. In the future, photovoltaic inverters will integrate grid-connected power generation, reactive power compensation, active filtering and other functions. They will realize grid-connected power generation when there is sunlight during the day, and realize reactive power compensation, active filtering and other functions when electricity is used at night.
6. Classified by photovoltaic string method
Photovoltaic grid-connected inverters can be divided into string inverters, centralized inverters and micro inverters according to the combination of photovoltaic panels. This is the most commonly used classification method in the application field.
(1) String inverter
String inverters are becoming the most popular inverters in the international market. It is a design based on a modular concept. Multiple photovoltaic panels are connected in series to form a string according to the rated input voltage requirements of the inverter. They are connected to the grid in parallel through an inverter, and the inverter tracks the maximum power peak at the DC end. There are also string inverters that are connected to multiple strings and perform multi-channel Maximum Power Point Traking (MPPT) control, and their power capacity is about 1 kW ~ 50 kW. They are usually used in photovoltaic systems such as BIPV (Building Integrated Photovoltaic), BAPV (Building Attached Photovoltaic), or Roof Plant, and are therefore also called household or commercial photovoltaic inverters. String inverters are particularly suitable for use in distributed power supplies.
(2) Centralized inverter
Centralized inverters are generally used in large-scale power generation systems such as large-scale workshops, desert power stations, and ground power stations with uniform sunlight. The total system power is large, generally above 100 kW to 1 MW. Multiple photovoltaic strings are connected in parallel to the combiner box, and then connected to the DC input end of a centralized inverter for grid-connected power generation.
(3) Micro grid-connected inverter
A micro grid-connected inverter is a converter device that directly boosts, inverts, and merges the DC power of a single photovoltaic panel into the grid. Generally, the power capacity is less than 1 000 W, so it is called a micro inverter. It has the ability to track the maximum power point at the component level, and can be integrated on the photovoltaic panel assembly as an adapter between a single photovoltaic panel and the grid. This makes the distributed power system plug-and-play, even without professional technical personnel for operation and maintenance. However, the unit power cost of micro-inverters is relatively high and is not suitable for large-scale distributed power use.
The three grid-connected inverters have their own advantages and disadvantages and are suitable for different applications. They need to be selected according to the actual application, and they can also be combined to optimize efficiency.