Researchers from Beijing Kono Technology Co., Ltd. and Beijing Jianheng Certification Center, including Wang Zhe and Liu Limin, wrote an article in the second issue of Electrical Technology in 2015 to explore a tandem medium and high voltage photovoltaic array and system implementation. The purpose of the method is to solve the problem that the number of components in series is limited by the withstand voltage of the photovoltaic module being ≤1000V. In order to increase power generation, a large number of PV arrays are currently connected in parallel using a number of converging devices, which has the disadvantages of a large number of cable applications, large transmission currents, multiple auxiliary devices, and serious losses.
The solution proposes a series of high-voltage square-wave array solutions for photovoltaic power generation systems, which uses the maximum power point tracking of MPPT, DC/DC, safety monitoring, high-voltage isolation (isolation voltage> Uxmax), etc. Features: High-voltage isolation of a number of photovoltaic strings, followed by series connection of high-voltage photovoltaic arrays in series. This solution eliminates the bus and step-up transformers and improves the input voltage of inverter and DC equipment. Transmission current, increase power generation power, reduce transmission loss and equipment failure rate, and meet the re-series of power of different power PV string, realize the purpose of high voltage transmission of PV DC.
Photovoltaic power plants have been developing rapidly under the government's policy incentives, and installed capacity has reached 20 GW. The capacity of photovoltaic power stations depends on the number of photovoltaic panels. Photovoltaic modules constitute the module units of photovoltaic power generation systems through photovoltaic strings/arrays. The typical number of series applications is up to 20~22, depending on the withstand voltage of the components. The string voltage of the external components in series is ≤1000V.
In order to increase the amount of power generation, two methods are adopted at home and abroad:
The first is to use a large number of PV strings/arrays to increase the current in parallel by increasing the current through the DC-convergence equipment to increase the power output. The AC output is reversed through the inverter. This method is also referred to as the centralized type. This method requires that the string performance parameters are similar, but the MPPT maximum power point of the string cannot be tracked automatically, resulting in partial power loss. For photovoltaic power plants with complex construction conditions and limited area, such as roofs, barren hills, etc., in order to satisfy the equal string voltage of each string, the string design must be taken into account when selecting strings, resulting in waste of resources.
The second is to use a string inverter, which can directly input 2~3 strings into the string inverter to realize independent MPPT maximum power point tracking of each string, and then parallel string string inverter. Inverter AC output, this type of power is relatively small (<30kW), in order to increase power, multiple units of series inverters are output in parallel by the AC bus devices. See Figure 2 for details. The two types are in fact the parallel connection between strings, and there are many problems such as large current transmission lines and bus equipment losses, number of cables, and many faults. At the same time, two types of inverter devices are current-type devices, and there is also a problem of large loss.
Moreover, due to the parallel input structure of the PV string string, the input DC voltage is limited to <1000v. At present, the output of the centralized inverter is mostly 270V and 330V, and is boosted by the transformer to the grid. The string inverter outputs multiple inverters in parallel through the transformer and outputs them to the grid. There are also problems of increased losses and costs.
At present, there are basically two methods for boosting the output of photovoltaic systems. One is that the photovoltaic strings are connected in parallel (parallel square arrays), and the output is boosted by the AC transformers of the inverter. Most of them are centralized; the second one is the PV array and the parallel inverter is the inverter. The exchange and convergence are then boosted by the transformer and are mostly string strings. The essence is still to increase the output power of the photovoltaic array by using a large number of strings in parallel (parallel arrays).
In response to the above problems, this paper proposes a solution for tandem high-voltage photovoltaic arrays (compared to the current centralized and string-type application of photovoltaic arrays), and increases the number of photovoltaic strings, increases the output voltage of the array, and reduces the output current to meet higher requirements. Grade transformerless grid connection, increase the energy output of PV arrays under low light conditions, reduce the number of cables and devices, use high voltage topology inverters or DC equipment, reduce loss of lines, inverters or DC equipment, reduce cables, Converging the cost of equipment and transformers to increase the efficiency of photovoltaic string generation and adapt to the needs of distributed, large-scale photovoltaic power plants and future medium- and high-voltage DC transmission.
in conclusion
To interpret the national new energy development, the development of safe, stable, dispatchable, multi-energy complementary high-voltage AC and DC power plants is the future direction. In this solution, a series of high-voltage photovoltaic square arrays is used, which eliminates the need for a large number of bus devices and grid-connected transformers, improves the input voltage of inverters and DC equipment, reduces the transmission current, and optimizes and increases the power generation, and reduces transmission lines and equipment. Losses and failures, while satisfying the re-series of power of different power PV strings, realizes the purpose of PV DC medium and high voltage transmission.
With the national policy to promote the rapid growth of the domestic photovoltaic market, the scale of the power station is becoming larger and smarter, accelerating the demand for photovoltaic power plant technology innovation, integrating photovoltaics with new technologies, new materials, new equipment, new solutions and multi-technology integration. Group string high-voltage isolation power conditioning module technology extension and future power station thinking: In the PV string string high-voltage isolation power conditioning module to increase energy storage function to achieve energy storage with a tandem photovoltaic array see Figure 14
Utilize “modular multi-level technology†to control photovoltaic string power units (photovoltaic strings + modules) to realize inverter devices with energy storage, no confluence, no transformer, no large-scale equipment room, and integral zero To realize a unitized multi-level star or angular series medium- and high-voltage photovoltaic power station system, see Figs. 15 and 16.
The same technology is extended to implement multi-energy complementary (wind, light, water, biological, energy storage, etc.) compensation adjustment between different characteristics of the power supply, connecting multiple energy sources and multiple modules to form several different energy power units, and connecting each power unit in series. Connecting, utilizing the features of system power unit redundancy, different output power and other modularity, combining energy storage, multi-energy complementary and inverting to form a unitized multi-energy complementary series high-voltage micro-grid system to realize wind power, Solar power generation, hydropower, coal power and other characteristics of the power between the compensation adjustment, to solve the new energy output randomness and volatility and other issues.
The future of smart photovoltaic power plants is changing with each passing day, and today’s thinking is tomorrow’s reality.
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