Flexible DC transmission is a new type of power transmission technology that uses voltage source converters for power conversion and transmission. It can improve the transmission level of renewable energy and enhance the flexibility and stability of the power grid. Flexible DC transmission refers to high-voltage DC transmission technology based on voltage source converters (VSC-HVDC). Its main feature is the use of voltage source converters (VSCs) composed of fully controlled power electronic devices, replacing the current source converters (CSCs) based on semi-controlled devices in previous DC transmission. The main components of a flexible DC transmission system include voltage source converters, converter transformers, commutation reactors, DC capacitors, and AC filters. In a typical double-ended flexible DC transmission system structure, a three-phase two-level VSC based on IGBTs is often used. The AC sides of the VSCs on both sides are connected in parallel to different AC systems, and the DC sides are connected through DC transmission lines or cables.

Flexible DC transmission (DC) is a new generation of DC power transmission technology with broad application prospects. It boasts advantages such as black-start capability, independent decoupling control of active and reactive power, the absence of inherent synchronization and stability issues in AC power grids, the ability to achieve large-scale power flow regulation and control, and significant support for renewable energy generation. It represents a crucial direction for power grid technology upgrading and construction. Flexible DC transmission technology has broad application prospects and excellent technical advantages in numerous fields, including safe grid connection and efficient transmission of new energy power generation, long-distance power transmission, DC grid construction, and smart grids. Since 2011, my country has successively completed several flexible DC transmission projects, including Nanhui, Nan'ao, Zhoushan, Xiamen, Luxi back-to-back, Chongqing-Hubei back-to-back, Zhangbei, and Wudongde, with the Baihetan-Jiangsu project under construction.

DC support capacitors are one of the key components of converter valves, accounting for approximately 6% of their cost. Flexible DC support capacitors, also known as DC-LINK capacitors, are essential components for IGBTs. By providing support for the DC voltage and absorbing ripple current, they stabilize voltage and current, thus ensuring the stable operation of the IGBT. Referring to photovoltaic inverters, assuming a 30% gross profit margin for converter valves and direct material costs accounting for 95%, with film capacitors accounting for 9% of direct material costs, then film capacitors account for approximately 6% of the converter valve cost. In recent years, with the decline in converter valve costs, their proportion of project construction costs has decreased from 17.9% to around 8.6%. Therefore, it is estimated that film capacitors account for approximately 0.5% of the total investment in flexible DC transmission projects.

Flexible DC support capacitors are reliant on imports. Previously, the technology for flexible DC support capacitors was only mastered by foreign companies, and domestic companies basically relied on imports for this type of product. In the Zhoushan five-terminal flexible DC project, the Xiamen flexible DC transmission demonstration project, and the Chongqing-Hubei flexible DC back-to-back interconnection project, all of which have been completed and put into operation, foreign companies' products were used. Domestic manufacturers have completed the research and development and industrialization of key technologies for flexible DC transmission capacitors and their film materials. Related products have reached the leading level domestically and advanced level globally, and can completely replace imports.