[Popular Science] Classification of Photovoltaic Inverters

What is the use of photovoltaic inverters?

The result of the century-long battle between Edison and Tesla determined that this world is a world of alternating current. Except for a very small number of DC systems, the power transmission part and the application end part of the photovoltaic system are both AC. As the core device for converting photovoltaic direct current into alternating current (DC-AC), the photovoltaic inverter is an indispensable part of the photovoltaic system.

What’s more, the photovoltaic inverter has another function that is no less important than DC-AC conversion – ** power point tracking MPPT.

If it is a grid-connected photovoltaic system, the inverter has another important function to adjust the phase, frequency and voltage of the alternating current to coordinate with the power grid.

Classification of photovoltaic inverters

According to the scale and application scenario, photovoltaic inverters can generally be divided into three types: micro inverters, string inverters, and centralized inverters.

1. Micro inverter

Each photovoltaic module corresponds to a micro inverter. The MPPT function and the inverter function are realized at the module end, and the general power application range is the power range of the mainstream module. The application scenario is mainly a small household system.

Advantages: The advantages are optimization, monitoring and control of individual components, especially in scenarios where the power generation conditions of each component are different (shading greater than 25% throughout the year, different directions in the east, west, south and north), each component can be optimized separately, avoiding the barrel principle caused by the series and parallel connection of components (the performance of the entire system is lowered by a single problematic component); component-level monitoring can achieve precise operation and maintenance, for example, if a component has a problem, it can be precisely located, checked and replaced; if some regions (such as the United States) require the system to be powered off in an emergency, the micro-inverter can achieve separate control of power off; second, the micro-inverter also avoids the danger of high DC voltage output.

Disadvantages: One inverter for each component corresponds to a large increase in components. The result is high cost and high error rate. At the same time, since the inverter must be close to the component, it adds considerable difficulty to high-altitude operation and maintenance. It is generally believed that micro-inverters have advantages for systems less than 3-5 kilowatts, and for systems larger than this scale, micro-inverters give way to string inverters.

2. String inverter

Each PV module string corresponds to a string inverter. The MPPT function and the inverter function are realized at the string end, and the general power range is between kilowatts and tens of kilowatts. The general application scenario is small and medium-sized household and commercial systems, which are generally applicable from several kilowatts to several megawatts.

Advantages: Whether it is an energy storage system or a grid-connected system, these systems only need to communicate with one inverter, avoiding the trouble of multi-head communication, which is the advantage of the string inverter. The coordination of phase, frequency, voltage and power grid is the forte of the string inverter. The number of components is small, and the operation and maintenance are performed on the ground, which are advantages compared with the micro inverter.

Disadvantages: There is no way to perform fine optimization, monitoring and control of components, and it is slightly incompetent in the face of complex terrain and shading.

Regarding the advantages and disadvantages of string inverters and micro inverters, the hot spot in the recent market segment is the combination of component optimizer + string inverter, that is, the MPPT function is realized at the component end, and the inverter function is realized at the string end. The cost and functionality of this solution are between micro inverters and string inverters.

3. Centralized inverter

The giant among inverters, the general power range is from hundreds of kilowatts to megawatts. The corresponding application scenarios are business-level and large commercial-level photovoltaic power stations, which are generally above megawatts. Generally, photovoltaic strings are connected in parallel and then input into the centralized inverter.

Advantages: For large systems, the number of centralized inverters is the least, so the corresponding cost is the lowest. It is directly connected to the power transmission and transformation system and integrated with the data acquisition and monitoring system (SCADA) of large power stations, which is the most friendly to the power grid.

Disadvantages: As the name of centralized inverters suggests, this design lacks control within a small range. Since a large number of photovoltaic strings are connected to an MPPT after being connected in parallel, there is basically no control over the output within the range of components or even the range of strings. It is particularly noteworthy that this giant large photovoltaic power station can be spread out. From a meteorological perspective, it may be exaggerated to say that “sunrise in the east and rain in the west”, so the power generation conditions of the local part of the power station may vary greatly. In this case, the centralized inverter will be slightly powerless, and it may be necessary to seek compensation solutions from the perspective of power station topology to reduce unstable factors.

Conclusion

According to statistics, the power generation loss caused by various faults and errors in each unit of the photovoltaic system can account for almost 1% of normal power generation. As a power electronic device, the inverter is destined not to be as tough as the photovoltaic module. The failure rate can basically account for more than 40% of the entire power station failure, and the power generation loss caused by the failure can account for more than 35% of the overall loss.

Therefore, choosing a high-quality and correct type of inverter is the key to achieving high returns in the power station. There is no inverter that is suitable for all situations. When choosing what kind of inverter, you must carefully analyze the actual application scenario.