Wind power grid connection new technology

In layman's terms, a flexible HVDC transmission system is used in the existing power grid, which is equivalent to connecting a valve and power supply in the power grid. It not only can effectively control the electrical energy passing through it, but also isolates the spread of power grid faults. According to the needs of the power grid, it can send or absorb a part of its energy quickly, flexibly, and adjustably. Dr. He Zhiyuan of the China Academy of Electric Power introduced: “This has a certain role in optimizing the distribution of the power grid, enhancing the stability of the power grid, and enhancing the intelligence and controllability of the power grid. ”

Technically speaking, flexible DC transmission is a new generation of DC transmission technology with a voltage source converter as its core. It uses the most advanced voltage source converters and full-control devices, and is a new generation of conventional DC transmission technology. Compared to AC transmission and conventional DC transmission, the energy of the AC system connected to it can be flexibly adjusted while transmitting energy. It has obvious advantages such as better controllability, flexible operation mode, and more applicable occasions.

Technical bottlenecks in the exchange and network At present, the use of AC and grid connection is the choice of most wind farms. However, there are currently some technical bottlenecks in wind farms through exchanges and network integration:

First of all, the use of AC grid connection requires that the wind farm and the connected AC system must strictly maintain the frequency synchronization, and the fan is more sensitive to AC bus voltage fluctuations at the grid connection. Existing operating experience shows that the voltage fluctuation of the AC system is one of the main reasons for the fan to exit the network.

Secondly, in the event of a fault in the AC system, the stable operation of wind farms often requires the installation of reactive power compensation devices at the outgoing end of the bus to improve the fault-tolerant capability of the wind farm. However, as a result, the investment in wind farms has been increased. In addition, the compensation device may adversely affect the maximum wind energy capture of the fan and the fan controller itself.

Finally, for offshore wind farms, if AC cables are used, when the cable length exceeds a certain value, a large inductive reactive power compensation device is required, especially for wind farms far from the shore, in the middle of the line. It is almost impossible to perform reactive compensation.

The use of flexible DC transmission cable theoretically does not have a distance limit, so when it exceeds a certain equivalence distance, it is generally greater than 50 to 100 kilometers, and the use of DC grid connection is the most reasonable choice.

Conventional DC transmission problems Conventional DC requires the connected AC system to provide the commutation voltage, which is relatively prone to commutation failure. This greatly reduces the ability of the wind farm to operate safely and stably.

Conventional DCs are much more than twice as large (more than twice as much) than AC and flexible DC transmission solutions when transmitting the same amount of power and are therefore not suitable for use in wind farms.

When the conventional direct current is used to transmit a small capacity, the unit cost is higher than that of the alternating current and flexible direct current transmission, so it is not suitable for the on-grid connection of the wind farm.

For wind farms, when the wind is not enough to remove the wind turbine from the system, to supply power to the load at the wind farm, the system will transmit active power to the wind farm to a limited degree. At this time, reactive power compensation may be required to ensure stable operation of the system. . The conventional DC does not have the ability to send reactive power, and it still needs a large amount 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 HVDC transmission is of great significance to the development of renewable energy. From the perspective of flexible HVDC technology itself, it can provide good dynamic reactive power support for wind farms and avoid investment in reactive power compensation equipment for wind farms, while providing excellent grid connection performance. Prevent the voltage fluctuation of the wind farm from affecting the AC system and at the same time improve the anti-interference ability of the wind farm against 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 an AC system failure. In addition, since flexible DC transmission is not limited by distance, it is also a large offshore long-range offshore wind farm. The only choice. Based on the above significant advantages, flexible DC transmission has now become the internationally recognized best technology solution for wind farms.

At present, the unit cost of a flexible HVDC converter station is about 1.5 times that of conventional HVDC transmission. However, with the improvement of technology and the large number of applications of the project, its cost is gradually decreasing. Especially when the transmission distance is long, the technical and economical efficiency of using the flexible DC transmission scheme is better than that of the AC transmission. Therefore, with the large-scale development and utilization of wind farms (especially offshore wind farms) in China, large-scale deployment of flexible HVDC transmission technologies has significant implications for meeting the needs of clean and efficient energy use in China.