Working principle and application prospect of VSC-HVDC

As a new generation of direct current transmission technology, VSC-HVDC is similar in structure to high-voltage direct current transmission, and is still composed of a converter station and a direct current transmission line (usually a direct current cable). The VSC-HVDC (Voltage Source Converter based High Voltage Direct Current Transmission) technology was proposed by Boon-Teck Ooi and others at McGill University in Canada in 1990. A new power transmission technology based on inverters, self-shutdown devices and pulse width modulation (PWM) technology. This power transmission technology has the advantages of being able to supply power to passive networks, without commutation failure, without communication between converter stations, and easy to form multi-terminal DC. system advantages.




1. Introduction to VSC-HVDC


The converter stations at both ends of the flexible direct current transmission system use flexible direct current transmission and are composed of converters, converter and transformer equipment, and converter reactance equipment. The most critical core part is the VSC, which is composed of a rectifier bridge and a DC capacitor. In the system, considering the topological structure of its main circuit and the type of switching devices, the sinusoidal pulse width modulation technology can be used to compare the data of the modulated reference wave with the triangular carrier wave, and the latter data is relatively small. case, triggering the low-side switch to turn on and turn off the low-side. This is mainly due to the fact that both the floating value and the phase can be intelligently adjusted using pulse width modulation technology. Therefore, the amplitude and phase of the fundamental frequency component of the AC output voltage of the VSC can also be adjusted through the pulse width.


2. Principle of VSC-HVDC


Different from the current source converter type high-voltage DC transmission based on phase-controlled commutation technology, the converter in the VSC-HVDC is a voltage source converter (VSC), and its biggest feature is the use of turn-off devices (usually For IGBT) and high frequency modulation technology.


The output active power and reactive power can be independently controlled by adjusting the amplitude of the converter outlet voltage and the power angle difference with the system voltage. In this way, through the control of the converter stations at both ends, the mutual transmission of active power between the two AC networks can be realized. The connected AC system provides reactive power support.


3. Strategic significance of VSC-HVDC


VSC-HVDC (Voltage Source Converter based High Voltage Direct Current Transmission) is an important equipment for building smart grids. Compared with traditional methods, VSC-HVDC has stronger technologies in island power supply, urban distribution network expansion and transformation, AC system interconnection, and large-scale wind farm grid connection. The advantage is a strategic choice to change the development pattern of large power grids.


VSC-HVDC will also face challenges in how to achieve high voltage, high power, use of overhead lines, and hybrid structure DC transmission. Through further research and pilot projects, this technology will be applied in more fields such as large-scale wind farm access system, regional networking to improve power supply reliability, and power grid operation pressure in densely loaded areas.


4. Application prospect of VSC-HVDC


Considering a series of advantages and limitations of VSC-HVDC technology, this technology has been widely used in power plant grid connection, offshore platforms and other fields, and the application direction of this technology in the future development is mainly in the following three directions aspect.


4.1 Application in urban grid capacity expansion and DC power supply. The rapid development of my country's economy and the continuous advancement of urbanization have promoted the further development of urban power grids. At the same time, the load of most urban power grids has also been showing a growing trend. People's requirements for the supply and quality of electric energy are constantly improving.


4.2 Alternative to AC and DC networking. Combined with the general trend of our country, the western region has relatively more resources and less load at the same time. Almost 90% of my country's hydropower is concentrated in the west, while the characteristics of energy and load in the eastern region are just the opposite. It has led to the imbalance of energy and load in our country. Therefore, UHV DC transmission projects are increasing to realize large-capacity and long-distance transportation of electric energy.


There are still certain obstacles in VSC-HVDC technology. In the process of long-distance and large-capacity development, the following difficulties must be overcome: the first is to replace silicon dioxide with silicon carbide, thereby changing the material of VSC, At the same time, it is necessary to enhance the insulation and heat resistance of the packaging material to achieve large-capacity current transportation. The second is to strengthen the optimization and improvement of current DC circuit breakers to break through the above-mentioned faults. If a breakthrough can be made in technology, VSC-HVDC technology may completely replace traditional power transmission technology in the future and undertake the task of long-distance and large-capacity power transmission.


4.3 Application in isolated load power supply. Since the flexible direct current transmission technology can realize direct power supply to the passive network, and there is no corresponding limit on the power transmission, so this technology can be fully utilized in the power supply process of remote isolated charges.


For example, in the Southern Power Grid, for some remote islands or villages, the main grid with isolated loads is generally used, which is relatively difficult for AC power supply, so diesel generators are used to generate electricity locally. Although this method can solve the electricity consumption problem of local residents, it will also bring disadvantages such as large environmental pollution and high energy consumption, which is inconsistent with the goal of building a resource-saving and environment-friendly society in my country. By using flexible direct current transmission technology, the above problems can be better solved, which is also a major application direction of this technology in the future development.

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