Parameters, characteristics, classification and development trends of photovoltaic grid-connected inverters

As the interface device between photovoltaic cells and the power grid, photovoltaic grid-connected inverter converts the electrical energy of photovoltaic cells into AC power and transmits it to the power grid. It plays a vital role in photovoltaic grid-connected power generation systems, and photovoltaic grid-connected inverters are developing towards high efficiency, high power density, high reliability and intelligence.

PV grid-connected inverter parameters　
　
1. Output capacity: single-phase 0.5-100KVA, three-phase 2-200KVA
　　2. Output voltage: AC220 ± 3%, 380 ± 3%
　　3. Output frequency: 50Hz ± 0.05
　　4. Waveform distortion rate: <5%
　　5. Power factor: 0.8
　　6. Overload capacity: overload 150%, 10S

PV grid-connected inverter characteristics　　

1. Maximum efficiency 98.6%, European efficiency 97.5, MPPT efficiency up to 99.9%
　　2. Modular design, easy to install and maintain
　　3. Automatically select output power according to input conditions (100%, 80%, 60%, 40%, 20%, 0%)
　　4. Filter out self-harmonics
　　5. Low voltage ride-through function
　　6. Over/under voltage, over/under frequency and over-current protection
　　7. Active and passive combination of anti-islanding
　　8. Reactive power is adjustable, power factor range is 0.95 ahead to 0.95 behind
　　9. Adapt to harsh power grid environment, working frequency 50/60±4.5Hz

Photovoltaic grid-connected inverter classification

1. Classification by isolation method

It includes two types: isolated and non-isolated. Isolated grid-connected inverters are further divided into industrial frequency transformer isolation and high-frequency transformer isolation. At the beginning of the development of photovoltaic grid-connected inverters, industrial frequency transformer isolation was mostly used. However, due to its obvious defects in size, weight and cost, high-frequency transformer isolation grid-connected inverters have developed rapidly in recent years. Non-isolated grid-connected inverters have gradually been recognized for their high efficiency and simple control. They have begun to be promoted and applied in Europe, but key issues such as reliability and common-mode current need to be resolved.

2. Classification by output phase number

It can be divided into two categories: single-phase and three-phase grid-connected inverters. Single-phase is generally used in small and medium power occasions, and three-phase grid-connected inverters are mostly used in large power occasions. According to the power level, it can be divided into small power grid-connected inverters with a power of less than 1kVA, medium power grid-connected inverters with a power level of 1kVA~50kVA, and large power grid-connected inverters above 50kVA.

3. Classification according to power flow

It is divided into two categories: unidirectional power flow grid-connected inverter and bidirectional power flow grid-connected inverter. Unidirectional power flow grid-connected inverter is only used for grid-connected power generation, while bidirectional power flow grid-connected inverter can be used as a rectifier in addition to grid-connected power generation to improve grid voltage quality and load power factor. In recent years, bidirectional power flow grid-connected inverter has begun to gain attention and is one of the future development directions.

4. Classification by topological structure

The topologies currently used include: 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 and high-power photovoltaic grid-connected inverters can adopt multi-level inverter topology, medium-power photovoltaic grid-connected inverters mostly adopt full-bridge and half-bridge inverter topologies, and small-power photovoltaic grid-connected inverters adopt forward and flyback inverter topologies

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In recent years, with the policy support of Spain, Germany, the United States, and Japan for their own photovoltaic industries, the global sales of photovoltaic inverters have increased year by year, and photovoltaic inverters have entered a stage of rapid growth. However, the global photovoltaic inverter market is currently divided up by several international giants. Europe is a pioneer in the global photovoltaic market, with a complete photovoltaic industry chain and photovoltaic inverter technology in a world-leading position. SMA is the world's earliest and largest photovoltaic inverter manufacturer (with a market share of more than 50% in Germany), accounting for about one-third of the global market share, and Fronius is second. The top seven manufacturers in the world occupy nearly 70% of the market share.

At present, the domestic photovoltaic grid-connected inverter market is relatively small. There are many domestic inverter manufacturers, but there are not many inverter manufacturers dedicated to photovoltaic power generation systems. However, many domestic companies have been researching the inverter industry for many years and have a certain scale and competitiveness. However, there is still a large gap between the quality and scale of inverter technology and foreign companies. At present, the large-scale manufacturers include Hefei Yangguang, Beijing Kono Weiye, Beijing Suoying, Zhicheng Champion, Nanjing Guanya, Shanghai Yingweili New Energy Technology Co., Ltd., etc. At present, the output of these companies for photovoltaic systems is increasing year by year.

Although the domestic market is relatively small, the huge development space and potential of the photovoltaic power station market in the future will bring historical opportunities for domestic enterprises. At present, domestic photovoltaic inverters are mainly occupied by brands such as Sungrow, Aisma, and KACO. Most foreign companies enter the domestic market through agency channels. Due to the difficulty in providing after-sales service, the overall market share is not high. Statistics in 2008 show that Hefei Sungrow Power Supply Company occupies more than 70% of the photovoltaic inverter market share, and almost all high-power products of key domestic photovoltaic projects are domestic products.

From a technical perspective, there is still a certain gap between domestic companies and foreign advanced levels in terms of conversion efficiency, structural technology, degree of intelligence, stability, etc. At present, my country is at the same level as foreign countries in small-power inverter technology, and high-power grid-connected inverters still need further development.