Continue to explore the application of flexible power transmission technology

On November 25, the three innovation teams of Sutong GIL multi-terminal flexible DC transmission, line flow controller, and Nanjing central urban area back-to-back flexible DC transmission held an annual plan formulation meeting to clarify the next phase of the joint research plan for the new generation of flexible power grid. These three innovation teams were established by the Economic and Technological Research Institute of State Grid Jiangsu Electric Power Co., Ltd. in conjunction with equipment manufacturers, universities, and design institutes.

As the power grid continues to develop, Jiangsu power grid has shown problems such as uneven distribution of power flow in each line, tight power transmission resources from northern Jiangsu to southern Jiangsu, and imbalance between substation capacity distribution and load development. Flexible power transmission technology, with its characteristics of flexible and precise adjustment of power grid power flow and voltage, provides a new idea for solving the dilemma of traditional power grid.

"In traditional power systems, electric current is naturally distributed. As a representative of the new generation of power system technology, flexible transmission technology has equipped power transmission with a precise dispatcher, which can achieve completely precise control of electric current and effectively improve the transmission capacity and safety and stability of the power grid." said Gao Zhengping, director of the Planning and Development Research Center of Jiangsu Economic Research Institute.

In recent years, Jiangsu Economic Research Institute has actively explored the application of flexible transmission technology, and has participated in the Nanjing West Ring Network 220 kV unified power flow controller (UPFC) demonstration project, Suzhou Southern Power Grid 500 kV UPFC demonstration project, Jiangsu offshore wind power flexible DC access system project, etc., and has continuously explored the application of flexible transmission technology.

Explore UPFC's upgraded version - IPFC

Flexible power transmission includes flexible AC transmission and flexible DC transmission. The typical representative of flexible AC transmission technology is the unified power flow controller (UPFC). This technology can control the current and voltage of AC transmission lines and has broad application prospects in load-intensive areas and new energy transmission channels.

"UPFC is known as the intelligent navigation system of the power grid. At present, Jiangsu has built and put into operation the Nanjing West Ring Network UPFC project and the Suzhou Southern Power Grid UPFC demonstration project." Cai Hui, a full-time staff member of the Planning and Development Research Center of the Jiangsu Economic Research Institute, said, "We have proposed an upgraded version of UPFC - the inter-line power flow controller (IPFC), and have carried out relevant research."

The Nanjing West Ring Network UPFC project was completed and put into operation in December 2015, successfully solving the problem of uneven power transmission in the region and increasing the power receiving capacity of the West Ring Network area by about 600,000 kilowatts. In December 2017, the Suzhou Southern Power Grid UPFC Demonstration Project achieved flexible and precise control of the power flow of the 500 kV power grid, which can increase the power consumption capacity of the Suzhou power grid by about 1.3 million kilowatts.

"IPFC is equivalent to a combination of multiple UPFCs. It can control the power transmission of multiple transmission lines at the same time. It is a more advanced flexible AC technology and there is currently no precedent for its application," said Cai Hui.

Since September this year, Jiangsu Economic Research Institute has conducted in-depth research on the technical principles and application scenarios of IPFC, and simulated and analyzed the feasibility of applying IPFC in the cross-river transmission channel of Jiangsu power grid.

At present, Jiangsu has a total of four 500 kV cross-river channels and one 1000 kV GIL cross-river channel. According to simulation calculations, as the load of Jiangsu power grid continues to increase, the problem of uneven distribution of power flow will continue to worsen, and some cross-river channels may be overloaded while others may be underloaded.

"After calculation, IPFC can effectively balance the transmission power of five lines and increase the transmission capacity by about 3 million kilowatts, which is equivalent to replacing a 500 kV cross-river transmission line. At the same time, the power grid flow is optimized, increasing Jiangsu's new energy absorption capacity by about 8 million kilowatts." Cai Hui said.

Explore the multi-scenario application of flexible power transmission technology

"Conventional DC transmission has been widely used in large-scale long-distance power transmission, but conventional DC can only be used for point-to-point transmission, and the transmission power must be constant, so it cannot be used in other scenarios." Sun Wentao from the Planning Center of Jiangsu Economic Research Institute said, "Flexible DC transmission technology solves this problem."

At the beginning of this year, the Development and Reform Commission of Jiangsu Province approved 24 offshore wind power projects with a total scale of 6.7 million kilowatts. At the same time, the Jiangsu Economic Research Institute organized relevant research institutions to launch a research project on flexible direct current access for offshore wind power in Jiangsu.

"If AC transmission is used, long-distance submarine cable transmission will bring serious overvoltage problems. Traditional DC transmission cannot adapt to the fluctuating characteristics of wind power generation. Flexible DC transmission technology makes it possible for large-scale and long-distance offshore wind power to be connected to the power grid." Sun Wentao said.

On August 13, the Jiangsu Power Grid Offshore Wind Power Transmission Planning Research Report compiled by the Jiangsu Economic Research Institute was officially approved and implemented. On October 16, the Jiangsu offshore wind power flexible direct current transmission plan passed the expert review.

At the same time, Jiangsu Economic Research Institute also undertook the State Grid Corporation of China's guidance project "Research on Flexible Transmission Network Planning Evaluation Methods and Key Application Technologies". The project proposed a set of quantitative indicators for evaluating the application of flexible transmission devices and established a planning evaluation system, which can effectively guide the scenario application, site selection and capacity determination, and technical and economic analysis of flexible transmission devices in the planning of power grids, and realize the application of flexible DC transmission in urban distribution networks.

"Take the power grid in the central urban area of ​​Nanjing as an example: there are government departments and commercial customer loads distributed near Xinjiekou, and it is very difficult to upgrade the power grid. For this reason, we proposed a solution of flexible DC transmission." Zhang Wenjia, deputy director of the Transmission Planning Office of the Planning Center of Jiangsu Economic Research Institute, said, "If we follow the traditional idea, it will require the construction of new cable channels in the core area of ​​the city, which will require huge investment and is almost impossible to implement. The flexible DC transmission solution can be upgraded based on the existing transmission corridors to achieve a comprehensive improvement in the reliability of power supply of the power grid."

Through simulation calculations, the technical team concluded that during the maintenance of some lines, this solution can improve the reliability of the regional power grid by 50% and reduce the maintenance power outage area by 30%.