I. Photovoltaic Inverter
(I) What is a photovoltaic inverter?
The difference between a photovoltaic (PV) inverter and a general inverter lies in its maximum power point tracking (MPPT) function and low-voltage ride-through capability for grid safety. Distributed PV inverters are a category of PV inverters in terms of application, and are currently widely used in PV poverty alleviation and residential PV systems.
(II) Application of Photovoltaic Inverters
Distributed photovoltaic (PV) power generation includes grid-connected, off-grid, and multi-energy complementary microgrid applications. Grid-connected PV systems are mostly used near users for self-consumption; off-grid systems are mostly used in remote and island areas, utilizing their own power generation and energy storage systems to directly supply power to loads; and off-grid systems form multi-energy complementary microgrids with other power generation methods, such as hydropower, wind power, and solar power, which can operate independently as a microgrid or be connected to the grid. With its excellent and stable performance, it has won the favor of many users.
II. String Inverter
(I) What is a string inverter?
With a power rating of less than 30KW, it uses low-current MOSFETs as power switches. The topology employs a two-stage power electronic device conversion process: DC-DC-BOOST boost and DC-AC full-bridge inverter. The protection rating is typically IP65. It is compact and can be installed outdoors on a wall-mounted basis.
(II) Application of String Inverters
Components, DC cables, inverters, AC power distribution, power grid.
III. Advantages and disadvantages of distributed photovoltaic inverters and string photovoltaic inverters
Most inverters used in distributed photovoltaic systems are string inverters, as is the case with Growatt's distributed projects. According to Growatt's market installation data, string inverters have multiple MPPTs, are small in size, lightweight, and easy to install. Due to the complex terrain in the central and eastern regions, which are mostly mountainous and hilly, string inverters can ensure the continuous and stable operation of photovoltaic power stations in high-temperature and high-humidity environments, such as those for fishery-solar integration and agriculture-solar integration, while reducing maintenance costs and potential risks.
Photovoltaic power stations use special photovoltaic cables with the following characteristics: high temperature resistance and voltage resistance up to 1000V; corrosion resistance and weathering resistance; insulation layer 2-3 times thicker than ordinary cables. Photovoltaic power stations need to determine the cable diameter according to the current value of the string. Distributed power stations use a single string directly connected in series with the inverter. Generally, the current value of a single string does not exceed 9A, so the second type is used.
(a) Advantages
1) Relatively easy to coordinate.
2) There will be additional subsidies such as "Beautiful Countryside" and "Photovoltaic Poverty Alleviation".
(II) Disadvantages
1) Urban rooftops have unclear property rights and many irregular roof shapes; rural rooftops have small usable areas and unclear load-bearing capacity.
2) Low electricity prices;
3) The electricity load curve does not match the output curve of the photovoltaic project.
(III) Advantages and disadvantages of string inverters
String inverters, as the name suggests, directly convert the direct current (DC) generated by photovoltaic (PV) modules into alternating current (AC), which is then boosted and connected to the grid. Therefore, the power output of these inverters is relatively small. PV power plants typically use string inverters with a power output of 50kW or less.
(1) Advantages of string inverters:
1. Unaffected by differences between modules in the same string and by shading, it also reduces the mismatch between the optimal operating point of the photovoltaic module and the inverter, maximizing power generation;
2. MPPT has a wide voltage range, allowing for more flexible module configuration; it also provides longer power generation time in areas with cloudy, rainy, or foggy conditions.
3. Small size, small footprint, no need for a dedicated server room, and flexible installation;
4. Low self-power consumption and minimal impact from malfunctions.
(2) Problems with string inverters:
1. Power devices have small electrical clearances, making them unsuitable for high-altitude areas; they also have many components integrated together, resulting in slightly lower stability.
2. Outdoor installations are easily affected by wind and sun, which can cause the casing and heat sink to age.
3. A larger number of inverters increases the overall failure rate and makes system monitoring more difficult;
4. The design without an isolation transformer has slightly lower electrical safety and is not suitable for thin-film module negative grounding systems.
