The lines that transmit electric energy from power plants to power load centers and the connecting lines between power systems are generally

called transmission lines. The new transmission line technologies we are talking about today are not new, and they can only be compared and

applied later than our conventional lines. Most of these “new” technologies are mature and applied more in our power grid. Today, the common

transmission line forms of our so-called “new” technologies are summarized as follows:

Large power grid technology

“Large power grid” refers to an interconnected power system, a joint power system or a unified power system formed by the interconnection

of multiple local power grids or regional power grids. The interconnected power system is a synchronous interconnection of a small number

of connection points between regional power grids and national power grids; The combined power system has the characteristics of coordinated

planning and dispatching according to contracts or agreements. Two or more small power systems are connected by the power grid for parallel

operation, which can form a regional power system. A number of regional power systems are connected by power grids to form a joint power

system. The unified power system is a power system with unified planning, unified construction, unified dispatching and operation.

The large power grid has the basic characteristics of ultra-high voltage and ultra-high voltage transmission grid, super large transmission capacity

and long-distance transmission. The grid consists of high-voltage AC transmission network, ultra-high voltage AC transmission network and

ultra-high voltage AC transmission network, as well as ultra-high voltage DC transmission network and high-voltage DC transmission network,

forming a modern power system with layered, zoned and clear structure.

The limit of super large transmission capacity and long-distance transmission is related to the natural transmission power and wave impedance

of the line with corresponding voltage level. The higher the line voltage level is, the greater the natural power it transmits, the smaller the wave

impedance, the farther the transmission distance and the larger the coverage range are. The stronger the interconnection between power grids

or regional power grids is. The stability of the entire power grid after interconnection is related to the ability of each power grid to support each

other in case of failure, That is, the greater the exchange power of tie lines between power grids or regional power grids, the closer the connection,

and the more stable the grid operation.

The power grid is a transmission network composed of substations, distribution stations, power lines and other power supply facilities. Among them,

a large number of transmission lines with the highest voltage level and corresponding substations constitute the backbone transmission grid of the

network. Regional power grid refers to the power grid of large power plants with strong peak regulation capacity, such as China’s six trans provincial

regional power grids, where each regional power grid has large thermal power plants and hydropower plants directly dispatched by the grid bureau.

Compact transmission technology

The basic principle of compact transmission technology is to optimize the conductor layout of transmission lines, reduce the distance between phases,

increase the spacing of bundled conductors (sub conductors) and increase the number of bundled conductors (sub conductors, It is an economic

transmission technology that can significantly improve the natural transmission power, and control the radio interference and corona loss at an

acceptable level, so as to reduce the number of transmission circuits, compress the width of line corridors, reduce land use, etc., and improve the

transmission capacity.

The basic characteristics of compact EHV AC transmission lines compared with conventional transmission lines are:

① The phase conductor adopts multi split structure and increases the conductor spacing;

② Reduce the distance between phases. In order to avoid short circuit between phases caused by wind blown conductor vibration, spacer is used to

fix the distance between phases;

③ The pole and tower structure without frame shall be adopted.

The 500kV Luobai I-circuit AC transmission line which has adopted the compact transmission technology is the Luoping Baise section of the 500kV

Tianguang IV circuit transmission and transformation project. It is the first time in China to adopt this technology in high-altitude areas and long-

distance lines. The power transmission and transformation project was put into operation in June 2005, and it is stable at present.

The compact transmission technology can not only significantly improve the natural transmission power, but also reduce the power transmission

corridor by 27.4 mu per kilometer, which can effectively reduce the amount of deforestation, young crops compensation and house demolition, with

significant economic and social benefits.

At present, China Southern Power Grid is promoting the application of compact transmission technology in 500kV Guizhou Shibing to Guangdong

Xianlingshan, Yunnan 500kV Dehong and other power transmission and transformation projects.

HVDC transmission

HVDC transmission is easy to realize asynchronous networking; It is more economical than AC transmission above the critical transmission distance;

The same line corridor can transmit more power than AC, so it is widely used in long-distance large capacity transmission, power system networking,

long-distance submarine cable or underground cable transmission in large cities, light DC transmission in distribution network, etc.

Modern power transmission system is usually composed of ultra-high voltage, ultra-high voltage DC transmission and AC transmission. UHV and UHV

DC transmission technology has the characteristics of long transmission distance, large transmission capacity, flexible control and convenient dispatching.

For DC transmission projects with power transmission capacity of about 1000km and power transmission capacity of no more than 3 million kW,

± 500kV voltage level is generally adopted; When the power transmission capacity exceeds 3 million kW and the power transmission distance exceeds

1500km,the voltage level of ± 600kV or above is generally adopted; When the transmission distance reaches about 2000km, it is necessary to consider

higher voltage levels to make full use of line corridor resources, reduce the number of transmission circuits and reduce transmission losses.

HVDC transmission technology is to use high-power power electronic components, such as high-voltage high-power thyristor, turnoff silicon controlled

GTO, insulated gate bipolar transistor IGBT and other components to form rectification and inversion equipment to achieve high-voltage, long-distance

power transmission. Relevant technologies include power electronics technology, microelectronics technology, computer control technology, new

insulation materials, optical fiber, superconductivity, simulation and power system operation, control and planning.

HVDC transmission system is a complex system composed of converter valve group, converter transformer, DC filter, smoothing reactor, DC transmission

line, power filter at AC side and DC side, reactive power compensation device, DC switchgear, protection and control device, auxiliary equipment and

other components (systems). It is mainly composed of two converter stations and DC transmission lines, which are connected with AC systems at both ends.

The core technology of DC transmission is concentrated on converter station equipment. The converter station realizes the mutual conversion of DC and

AC. The converter station includes rectifier station and inverter station. The rectifier station converts three-phase AC power into DC power, and the

inverter station converts DC power from DC lines into AC power. The converter valve is the core equipment to realize the conversion between DC and AC

in the converter station. In operation, the converter will generate high-order harmonics on both the AC side and the DC side, causing harmonic interference,

unstable control of converter equipment, overheating of generators and capacitors, and interference with the communication system. Therefore, suppression

measures need to be taken. A filter is set in the converter station of the DC transmission system to absorb high-order harmonics. In addition to absorbing

harmonics, the filter on the AC side also provides some fundamental reactive power, DC side filter uses smoothing reactor to limit harmonic.

Converter station

UHV transmission

UHV power transmission has the characteristics of large power transmission capacity, long power transmission distance, wide coverage, saving line

corridors, small transmission loss, and achieving a wider range of resource optimization configuration. It can form the backbone grid of UHV power

grid according to the power distribution, load layout, transmission capacity, power exchange and other needs.

UHV AC and UHV DC transmission have their own advantages. Generally, UHV AC transmission is suitable for grid construction of higher voltage

level and cross region tie lines to improve the stability of the system; The UHV DC transmission is suitable for the large capacity long-distance

transmission of large hydropower stations and large coal-fired power stations to improve the economy of transmission line construction.

UHV AC transmission line belongs to a uniform long line, which is characterized by that the resistance, inductance, capacitance and conductance

along the line are continuously and evenly distributed on the whole transmission line. When discussing problems, the electrical characteristics of

the line are usually described by the resistance r1, inductance L1, capacitance C1 and conductance g1 per unit length. The characteristic impedance

and propagation coefficient of uniform long transmission lines are often used to estimate the operational readiness of EHV transmission lines.

Flexible AC transmission system

Flexible AC transmission system (FACTS) is an AC transmission system that uses modern power electronics technology, microelectronics technology,

communication technology and modern control technology to flexibly and quickly adjust and control power flow and parameters of power system,

increase system controllability and improve transmission capacity. FACTS technology is a new AC transmission technology, also known as flexible

(or flexible) transmission control technology. The application of FACTS technology can not only control the power flow in a large range and obtain

an ideal power flow distribution, but also enhance the stability of the power system, thereby improving the transmission capacity of the transmission line.

The FACTS technology is applied to the distribution system to improve the power quality. It is called the flexible AC transmission system DFACTS of

the distribution system or the consumer power technology CPT. In some literatures, it is called the fixed quality power technology or customized power

technology.