Growatt's high-frequency isolated photovoltaic inverter leads the world

In photovoltaic power generation systems, we often encounter these problems: when it rains, the grid-connected leakage current switch trips, and when the weather clears up, it automatically returns to normal, and no matter how we check, we can't find the problem. In fact, this problem has nothing to do with the components, inverters and other equipment, but is mainly caused by non-isolated photovoltaic systems.

In the solar power generation system, there is a parasitic capacitance between the photovoltaic module and the ground. In a humid environment or rainy days, the parasitic capacitance will increase. The parasitic capacitance forms a common-mode loop with the photovoltaic power generation output grid system. The parasitic capacitance to the ground can form a resonant path with the filter elements in the grid-connected inverter and the grid impedance. When the frequency of the common-mode current reaches the resonant frequency point of the resonant loop, a large leakage current will appear in the circuit. This common-mode current will increase the system loss and affect the normal operation of the inverter. It will also inject a large amount of harmonics into the grid, causing safety problems; when the system detects that the leakage current is too large, the inverter will stop working.

Schematic diagram of ground leakage current of non-isolated photovoltaic grid-connected inverter

In silicon-based thin-film photovoltaic power generation systems, in order to prevent corrosion of the conductive layer TCO of the components, the negative pole of the component must be grounded. In order to prevent the common-mode voltage to ground from exceeding the system voltage and suppress the common-mode interference of the distributed capacitance of the photovoltaic array panels to the ground on the inverter control circuit, transformer isolation must be used for inverter grid connection.

For the above reasons, more and more applications require photovoltaic grid-connected inverters to achieve electrical isolation. Isolated photovoltaic grid-connected inverters effectively improve the electrical safety of the photovoltaic side and eliminate the common-mode current problem in photovoltaic grid-connected systems. According to the operating frequency of the transformer, isolated photovoltaic grid-connected inverters can be divided into power frequency isolation type and high frequency isolation type.

The power frequency isolation photovoltaic grid-connected inverter is the most commonly used structure in photovoltaic power generation systems. Its topology is based on the non-isolated grid-connected inverter, and a power frequency transformer is added on the grid side. Its weight accounts for about 50% of the total weight of the photovoltaic grid-connected inverter, which has become a major obstacle for the inverter to reduce the system volume and increase the power density. In addition, the power frequency transformer also causes a large loss to the inverter, increasing the cost of the power generation system and the difficulty of transportation and installation.

Under the condition of constant voltage and current, the number of turns of the primary and secondary windings of the transformer is inversely proportional to the operating frequency, and the cross-sectional area of ​​the iron core is also inversely proportional to the operating frequency. The higher the operating frequency of the transformer, the fewer turns of the windings of the primary and secondary sides of the transformer will be correspondingly reduced, and the required area will also be reduced, so that a smaller iron core can be selected. Therefore, increasing the operating frequency of the transformer has become an effective way to reduce the volume and weight. The high-frequency isolation transformer has the advantages of light weight and small size. The circuit structure of the high-frequency isolation photovoltaic grid-connected inverter is shown in the figure. The DC power emitted by the photovoltaic array is flipped by the full-bridge inverter on the input side into an AC pulse voltage with a high-frequency pulse width, and isolated and transmitted through a high-frequency transformer. After the high-frequency pulse voltage reaches the secondary side of the transformer, the negative half-cycle pulse voltage is reversed by the diode rectifier bridge, rectified into DC power, and then filtered by the large electrolytic capacitor on the intermediate DC side to form a stable DC voltage on the intermediate side. Finally, it is inverted into AC power by the post-stage full-bridge inverter on the output side and connected to the grid.

High frequency isolation photovoltaic inverter topology diagram

Although high-frequency isolated inverters have many advantages, the addition of DC/AC inverter, high-frequency transformer, AC/DC rectifier three-level topology makes the electrical structure complex, with many components and very high technical difficulty. If the design is not good, it will cause poor system reliability, low efficiency, high cost, large size, heavy weight and other disadvantages, so many manufacturers stop at it. After years of dedicated research, Shenzhen Growatt has developed three high-frequency isolated grid-connected inverters, model Growatt2000HF, Growatt3000HF, Growatt5000HF. By adding soft switching circuits, using low-power switching tubes, increasing switching frequency, using high-quality magnetic materials and other improvement measures, the inverters have greatly improved in performance and efficiency. Compared with non-isolated inverters, the volume and weight of these three Growatt inverters are not much different, and the highest efficiency can reach 96.5%. Compared with the inverter with power frequency isolation, the volume is reduced by more than 30%, the weight is reduced by 40%, and the efficiency is increased by 5%.

Growatt's high-frequency isolation inverter, with the negative pole of the photovoltaic input terminal directly connected to the ground, is very effective in preventing TCO corrosion of the conductive layer of thin-film modules and PID of crystalline silicon modules. It is currently widely installed in photovoltaic systems with negative grounding requirements, such as Hanergy's silicon-based thin-film module system, and crystalline silicon module systems installed in humid environments with PID resistance requirements. The global installation volume has reached 30,000 sets, ranking first, far exceeding the second place. From the operating situation, the inverter performance is very stable, the power generation is high, the TCO corrosion of thin-film modules and the PID suppression effect of crystalline silicon modules are very significant, and the attenuation of the modules is very small.

Shenzhen Growatt New Energy Co., Ltd. was established in May 2010 with a registered capital of RMB 100 million. It is a national high-tech enterprise focusing on the research and development, production, sales and service of new energy equipment such as solar inverters, energy storage inverters and their monitoring systems. The company has been committed to being a world-class new energy solution provider and serving all mankind. Since its establishment, Growatt's inverters have shipped more than 3.5GW worldwide, exported to nearly 80 countries and regions in six continents including Europe, America, Australia, Asia, Africa and Latin America, and have been ranked first in exports in China for four consecutive years. It is a world-leading inverter system supplier known for "high power generation, high returns and high intelligence".