The lines that transmit power from the power plant to the load center and the power lines are generally called transmission lines. The new transmission line technology is actually not new, only relative to our regular lines to compare the application later, most of these "new" technologies are more mature in our grid. The form of commonly used transmission lines in China called "new" technologies is summarized as follows:
Large power grid technology:
The "big power grid" is an interconnected power system, a united power system or a unified power system that is interconnected by a plurality of local power grids or a regional power grid. The interconnected power system interconnects the regional power grids and the national power grids with a few connection points. The united power system has the characteristics of coordinated planning and scheduling according to contracts or agreements. Two or more small power systems connected in parallel with the power grid, you can form a regional power system. A number of regional power systems are connected by a grid that forms a combined power system. A unified power system is a unified planning, unified construction, unified scheduling and operation of the power system.
Large power grid with the characteristics of EHV, UHV transmission grid, large transmission capacity and long-distance transmission, the network consists of high voltage AC transmission network, EHV AC transmission network and UHV AC transmission network, as well as UHV DC transmission network, HVDC constitutes a hierarchical, zoned, structurally clear modern power system.
The limits of super-large transmission capacity and long-distance transmission are related to the natural transmission power and wave impedance of the corresponding voltage level lines. The higher the line voltage level is, the larger the natural power delivered, the smaller the impedance and the longer the transmission distance. Large, the greater the interconnection between the power grids or the regional power grids, the stability of the entire large power grid after networking is related to the ability of supporting each other when the grids are faulty, that is, the greater the switching power of the tie lines between the power grids or the regional power grids, Closely, the more stable the grid operation.
The power grid is a transmission network composed of substations, power distribution stations, power lines and other power supply facilities. A large number of the highest voltage transmission lines and corresponding substations form the backbone transmission grid of the network. The regional power grid refers to the grid with large peaking capacity of large power plants, such as China's inter-provincial six regional power grids, each regional power grids are directly adjusted by the Bureau of large-scale thermal power plants and hydroelectric power plant.
Compact transmission technology:
The basic principle of compact power transmission technology is to optimize the layout of the transmission line conductors, reduce the distance between the phases, increase the spacing between the split wires (sub-wires) and increase the number of split wires (sub-wires), so that the charges are uniformly distributed on the surfaces of the wires, , In order to improve the effective cross-section of the wire, and at the same time, increase the capacitance of the line, decrease the inductance and reduce the impedance of the line. Under the same voltage rating of the transmission line, the natural transmission power can be obviously increased and the radio interference and corona loss can be controlled At an acceptable level, which can reduce the number of transmission lines, the width of the corridor compression lines, reducing land, etc., in order to improve transmission capacity of economic transmission technology.
The basic features of a compact EHV AC transmission line compared to a conventional transmission line are:
â‘ phase conductor multi-split structure, and increased wire spacing;
â‘¡ reduce the distance between the phases, in order to avoid the wind caused by wire vibration phase short circuit, phase spacing rod spacing between the fixed phase;
â‘¢ phase structure with no tower structure.
500kV Luohui â… back AC power transmission line that has adopted the compact transmission technology is the Luoping-Baise section of the 500kV Tian-Guang IV transmission and transformation project. It is the first time that China has adopted this technology in high-altitude areas and long-distance lines . The power transmission project put into operation in June 2005, the current stable operation.
The compact transmission technology can not only significantly increase the natural transmission power, but also reduce the power transmission corridor by 27.4 mu per km, which can effectively reduce the amount of tree felling, young crop compensation and house demolition, with significant economic and social benefits.
Currently, China Southern Power Grid is in the 500kV Guizhou Shi Bing to Guangdong Yin Ling Shan, Yunnan 500kV Dehong and other transmission projects to promote the use of compact transmission technology.
HVDC
HVDC is easy to realize asynchronous networking; it is more economical than AC transmission above the critical distance of transmission; more power can be transmitted than AC by using the same circuit corridor. Therefore, HVDC is widely used in large-capacity long-distance transmission, power system networking, long- Cable or underground city cable transmission, distribution network light DC transmission.
Modern power transmission system is usually composed of UHV, UHV DC transmission and AC transmission with each other. EHV, UHV DC transmission technology with a long distance transmission, transmission capacity, flexible control and scheduling and so on.
For DC transmission projects with a transmission capacity of up to 1000km and a transmission capacity of not more than 3 million kW, a voltage level of ± 500kV is generally adopted. When the transmission capacity exceeds 3 million kW and the transmission distance exceeds 1500km, a voltage level of ± 600kV or above is generally adopted When the transmission distance reaches about 2000km, higher voltage levels need to be considered in order to make full use of the corridor resources, reduce the number of transmission loops and reduce transmission losses.
HVDC technology is the use of high-power power electronic components, such as high-voltage high-power thyristors, SCR can be turned off GTO, insulated gate bipolar transistor IGBT and other components of the rectifier and inverter equipment to achieve high-voltage, long-distance power transmission. Related technologies include power electronics, microelectronics, computer control, new materials for insulation, fiber optics, superconductivity, simulation and power system operation, control and planning.
HVDC transmission system consists of converter valve, converter transformer, DC filter, smoothing reactor, DC transmission line, AC and DC power filter, reactive power compensation device, DC switchgear, protection and control Devices and auxiliary equipment and other components (system) consisting of complex systems. It is mainly composed of two converter stations and a DC transmission line, and two converter stations are connected with the AC system at both ends.
The core technology of DC transmission concentrates on the converter station equipment. The converter station realizes the mutual conversion of DC and AC. The converter station includes the rectifier station and the inverter station. The rectifier station converts the three-phase AC power into DC converter current Station, the inverter station to DC transmission line DC converted into AC converter station. Converter valve converter station is to achieve the exchange of DC and AC core equipment. In operation, the inverter will produce higher harmonics on the AC side and the DC side, causing harmonic interference, instability of the control of the commutation equipment, overheating of the generator and capacitor and interference with the communication system. Therefore, suppression measures need to be taken, In the HVDC transmission system, a filter is set to absorb higher harmonics. The AC side filter not only absorbs harmonics, but also provides some fundamental reactive power. The DC side filter limits the harmonics by using a smoothing reactor.
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