Classification and selection of photovoltaic inverters

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Classification of photovoltaic inverters

There are many ways to classify inverters. For example, they can be divided into active inverters and passive inverters according to the source and flow properties, single-phase inverters and three-phase inverters according to the number of phases of the inverter input AC voltage, and independent control inverters and grid-connected inverters according to different uses. In order to facilitate photovoltaic users to choose inverters, they are generally divided into three categories according to the different occasions where they are applicable: centralized inverters, single-phase/three-phase string inverters and micro inverters.

1. Centralized inverter

The traditional centralized inverter photovoltaic inversion method is to connect all the direct current generated by photovoltaic cells under sunlight in series and parallel, and then use an inverter to convert the direct current into alternating current. The capacity of centralized inverters is between 10 kW and 1 MW. The biggest feature is the high power of the system. It is suitable for large-scale ground photovoltaic power stations or large rooftop power stations with uniform light. The product and technology are highly mature and the cost is low.

However, centralized inverters require good matching between photovoltaic strings and are sensitive to partial shading. Once cloudy, shaded or a single string fails, the efficiency and power generation of the entire photovoltaic system will be affected. In addition, the output voltage and current of different photovoltaic strings are often not completely matched, which will also cause a certain amount of power generation loss.

2. String inverter

String inverters have become the most popular inverters in the international market. Each photovoltaic string (1~5 kW) passes through an inverter, has maximum power peak tracking on the DC side, and is paralleled and grid-connected on the AC side. Many large photovoltaic power plants use string inverters. The advantage is that they are not affected by module differences and shading between strings, and at the same time reduce the mismatch between the best point of the photovoltaic module and the inverter, thereby increasing the power generation. These technical advantages not only reduce system costs, but also increase system reliability. At the same time, compared with centralized inverters, the length of the DC cable of the system using string inverters is reduced, and the shading effect between strings and the loss caused by the difference between strings are minimized.

The investment cost of string inverters is moderate and they are suitable for all types of ground-mounted photovoltaic power stations or BAPV/BIPV. The products are mature and easy to install and maintain.

3. Micro inverter

Micro inverter, also known as component inverter, generally refers to an inverter with a power of less than or equal to 1000 watts and component-level maximum power peak tracking (MPPT) in a photovoltaic power generation system. The full name is micro photovoltaic grid-connected inverter. "Micro" is relative to the traditional centralized inverter. The traditional photovoltaic inverter method is to connect all the DC power generated by all photovoltaic cells under sunlight in series and parallel, and then use an inverter to invert the DC power into AC power and connect it to the grid; the micro inverter inverts each component. Its advantage is that each component can be independently MPPT controlled, which can greatly improve the overall efficiency, and at the same time avoid the DC high voltage and poor weak light effect of centralized inverters. Traditional centralized inverters or string inverters usually have a DC voltage of hundreds of volts or thousands of volts, which is easy to catch fire and difficult to extinguish after catching fire. Micro inverters have a DC voltage of only tens of volts, all in parallel, which minimizes safety hazards. It is mostly used in small photovoltaic power stations.

The disadvantage is that the investment cost is high and the overall efficiency is lower than that of the string inverter. In addition, since it is connected in parallel at the AC side, the complexity of the connection on the AC side and the difficulty of maintenance are increased.

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How to choose inverter for photovoltaic power station

1. Installation scale

Large-scale ground power stations can generally use centralized inverters; while small-scale rooftop power stations, industrial parks or photovoltaic power generation systems in public facilities are not large in scale, and string inverters or micro inverters can generally be selected.

2. Stability and reliability

There are many inverter brands on the market. Many users only focus on price, efficiency and other parameters when choosing an inverter, but ignore the safety and reliability of the inverter. In fact, the inverter only accounts for 11% of the cost structure of the photovoltaic system. However, in the later operation and maintenance of the photovoltaic system, after-sales problems caused by the inverter account for about 50% of the after-sales problems (as shown in the figure below).

Therefore, during the operation and maintenance of the photovoltaic system, the inverter requires the most energy and even the greatest financial resources. High-quality inverters have a long warranty period, a low failure rate, and can save trouble in operation and maintenance; while cheap inverters have a short warranty period, or even no after-sales guarantee, poor stability, and a high failure rate. The worst case scenario is that it may completely break down after a short period of use, and then you have to spend thousands or even tens of thousands of yuan to replace a new inverter. Therefore, you must not try to save money, but must choose an inverter from a reliable brand, and secondly, purchase it from a reliable channel to ensure product quality and after-sales support.

3. Efficiency

Solar inverters on the market have higher efficiency in Europe and America, with the European standard being 97.2%, but they are more expensive. Other domestic inverters have efficiencies below 90%, but they are much cheaper than imported ones. The efficiency of the inverter is very important. The higher the efficiency, the less energy is wasted on the inverter, and more energy is used for electrical appliances. This is especially important when you use a low-power system.

4. After-sales service and design service

It is recommended to purchase brands with good reputation in the current market, because companies with good brand image usually have large investments in technology and maintenance services, can propose design solutions based on user needs, and provide good after-sales service.

5. Load type

When selecting an inverter, you must first ensure that the rated capacity is sufficient to meet the power requirements under maximum load. When the inverter is loaded with multiple devices, the selection of the inverter capacity must consider the possibility of several electrical devices working at the same time, that is, the "simultaneous load factor".

When the electrical equipment is a pure resistive load or the power factor is greater than 0.9, the rated capacity of the inverter can be 1.1 to 1.15 times the capacity of the electrical equipment. For general inductive loads, such as washing machines, refrigerators, air conditioners, high-power water pumps, etc., the instantaneous power at startup may be 5 to 6 times its rated power. At this time, the inverter will withstand a large instantaneous surge. For such systems, the rated capacity of the inverter should have sufficient margin to ensure that the load can be started reliably.

Conclusion

String inverters are a compromise between large centralized inverters and micro inverters, and have the highest cost-performance ratio. Micro inverters have higher safety. In distributed power stations, it is best to use a combination of string inverters and micro inverters, which can effectively control costs and increase the long-term power generation benefits of photovoltaic power stations. With the continuous advancement of technology and the continuous reduction of prices, string and micro inverters will be the mainstream products of distributed and large ground photovoltaic power stations in the future.