Technically speaking, flexible DC transmission is a new generation of DC transmission technology with voltage source converter as the core. It adopts the most advanced voltage source converter and full control device, which is a replacement upgrade of conventional DC transmission technology. Compared with AC transmission and conventional DC transmission, it can flexibly adjust the voltage of the AC system connected to it while transmitting energy. It has obvious advantages such as better controllability, flexible operation mode and many applicable occasions.
Technical bottleneck of AC grid connection
At present, the use of AC grid connection is the choice of most wind farms connected to the grid. However, there are some technical bottlenecks in wind farms through AC grid connection:
First, the use of AC grid-connected wind farms and connected AC systems must be strictly synchronized in frequency, and the fans are sensitive to AC bus voltage fluctuations at the grid connection. The existing operating experience shows that the voltage fluctuation of the AC system is one of the main reasons for the fan to retreat.
Secondly, in the case of a failure of the AC system, the stable operation of the wind farm often requires the installation of a reactive power compensation device at the bus outlet end, thereby improving the fault ride-through capability of the wind farm. However, this has increased the investment in wind farms, and the compensation device may have an adverse effect on the maximum wind energy capture of the fan and the fan controller itself.
Finally, for offshore wind farms, if an AC cable is used, when the cable length exceeds a certain value, a large inductive reactive compensation device is required, especially for wind farms farther from the shore, in the middle of the line. It is almost impossible to perform reactive compensation.
The use of flexible DC transmission cables theoretically has no distance limitation, so when it exceeds a certain equivalent distance, it is generally greater than 50~100 kilometers. The use of DC grid connection is the most reasonable choice.
Conventional DC transmission problem
Conventional DC requires the commutation voltage provided by the connected AC system, which is more prone to failure of commutation failure, which greatly reduces the ability of the wind farm to operate safely and stably.
Conventional DCs are much larger (more than twice) larger than AC and flexible DC transmission schemes when transmitting the same amount of power, and are therefore not suitable for use in wind farms.
Conventional DC has a higher unit cost than AC and flexible DC transmission when transmitting smaller capacity, so it is not suitable for wind farm integration.
For wind farms, when the wind is not enough to make the fan cut off from the system, in order to supply the load at the wind farm, the system will transmit the active power to the wind farm in a limited way. At this time, reactive compensation may be needed to ensure the stable operation of the system. . Conventional DC does not have the ability to emit reactive power, and it also requires a large number of reactive power compensation devices, which also increases the area of ​​the converter station, so it is not suitable for use in wind farms (especially offshore wind farms).
Flexible DC transmission is of great significance to the development of renewable energy
From the flexible DC transmission technology itself, it can provide good dynamic and reactive support for wind farms, avoid investment in reactive power compensation equipment of wind farms, and provide excellent grid-connected performance to prevent voltage fluctuations of wind farms on AC systems. Affect, and at the same time improve the anti-interference ability of wind farms to system fluctuations. Because it can provide voltage support, it can also greatly improve the low voltage ride-through capability of wind farms in the event of AC system failure. In addition, because flexible DC transmission is not limited by distance, it is also a large foreign long-distance offshore wind farm. The only choice. Based on the above significant advantages, flexible DC transmission has become the internationally recognized best technical solution for wind farm integration.
At present, the unit cost of a flexible DC transmission converter station is about 1.5 times that of conventional DC transmission, but with the improvement of technology and the large-scale application of engineering, the cost is gradually reduced. Especially when the transmission distance is long, the flexible DC transmission scheme is superior in technical economy to AC transmission. Therefore, in the case of the large-scale development and utilization of wind farms (especially offshore wind farms) in China, the large-scale application and application of flexible HVDC transmission technology has significant significance for meeting the needs of China's clean and efficient energy utilization.
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