Power transmission and distribution, a power term, includes three aspects, namely power transmission, power transformation, and power distribution.
Among them, power transmission refers to the transmission of electric energy. Through power transmission, power plants that are far apart (up to thousands of kilometers) are connected with load centers, so that the development and utilization of electric energy can transcend geographical restrictions; power transformation refers to the use of certain equipment. The process of changing the voltage from low level to high level (boost) or from high level to low energy level is the distribution means of distributing power to users in the area of power consumption, directly serving users.
1. Introduction to power transmission and distribution
The concept of power transmission and distribution includes three aspects, namely power transmission, power transformation and power distribution. Among them, power transmission refers to the transmission of electric energy. Through power transmission, power plants that are far apart (up to thousands of kilometers) are connected with load centers, so that the development and utilization of electric energy can transcend geographical restrictions; power transformation refers to the use of certain equipment. The process of changing the voltage from low level to high level (boost) or from high level to low energy level is the distribution means of distributing power to users in the area of power consumption, directly serving users.
Both transmission and distribution facilities include equipment such as substations and lines. All transmission equipment are connected to form a transmission grid. The network composed of distribution equipment from the transmission network to the user is called the distribution network. They are also sometimes referred to as transmission and distribution systems. The power transmission system and power distribution system, together with the power plants and electrical equipment, are collectively referred to as the power system. Various electrical equipment in the power system have their own rated voltages, which constitute the voltage level of the entire power system. The transmission voltage is higher than the distribution voltage. The division of transmission and distribution voltage boundaries is not fixed and varies with the area covered by the grid and the size of its capacity.
The "Guidelines for Urban Power Network Planning and Design" issued by my country stipulates that: 35~110kV is the high-voltage distribution voltage; 10kV is the medium-voltage distribution voltage; 220/380V is the low-voltage distribution voltage. Before the 220kV transmission network is formed in some areas, 110kV (or even 35kV) can also be used as the transmission voltage. As far as the vast majority of power grids in my country are concerned, 220kV is the high voltage transmission voltage (HV), 330kV and 500kV are the extra high voltage transmission voltage (EHV), and higher transmission voltages are under research and demonstration.
2. Transmission and distribution of electricity
According to the nature of the transmission current, power transmission is divided into AC transmission and DC transmission.
In the 1880s, DC transmission was first successfully realized. However, because the voltage of DC transmission was difficult to continue to increase under the technical conditions at that time, the transmission capacity and benefits were limited.
At the end of the 19th century, DC transmission was gradually replaced by AC transmission. The success of AC transmission ushered in a new era of electrified society in the 20th century.
At present, three-phase AC transmission is widely used, and the frequency is 50Hz (or 60Hz). Since the 1960s, there have been new developments in DC transmission, which is combined with AC transmission to form an AC-DC hybrid power system.
The basic process of power transmission is to create conditions for the transmission of electromagnetic energy along the direction of the transmission line. The power transmission capacity of the line is governed by the various laws of the electromagnetic field and the circuit. With the earth potential as the reference point (zero potential), the line conductors must be under the high voltage applied by the power supply, which is called the transmission voltage.
The maximum transmission power determined by a transmission line after comprehensive consideration of various factors such as technology and economy is called the transmission capacity of the line. The transmission capacity is roughly proportional to the square of the transmission voltage. Therefore, increasing the transmission voltage is the main technical means to realize large-capacity or long-distance transmission, and it is also the main symbol of the development level of transmission technology.
In the process of power transmission, the level of transmission voltage depends on the transmission capacity and transmission distance. The general principle is: the larger the capacity and the farther the distance, the higher the transmission voltage. There are ten grades of long-distance transmission grades: 3, 6, 10, 35, 63, 110, 220, 330, 500, and 750.
From the perspective of the development process, the transmission voltage level increases by about two times. When the power generation increases to about 4 times, a new higher voltage level will appear. Generally, the transmission voltage of 220 kV and below is called high-voltage transmission, the transmission voltage of 330-765 kV is called ultra-high voltage transmission, and the transmission voltage of 1000 kV and above is called UHV transmission. The approximate range of transmission voltage, transmission capacity and transmission distance is listed in the table. Raising the transmission voltage can not only increase the transmission capacity, but also reduce the cost of transmission, reduce the consumption of metal materials, and increase the utilization rate of line corridors. As of 1987, the AC transmission voltage used in the world reached 765 kV. 1150 kV UHV AC transmission has already been tested in industry. The largest direct current transmission project that has been built has a transmission voltage of ±750 kV, a transmission distance of 2,400 kilometers, and a design transmission capacity of 6 million kilowatts.
3. Transformation of power transmission and distribution
Power transformation refers to the process of changing the voltage from low level to high level (boost) or from high level to low level (step down) through certain equipment in the power system. The rated voltage of the generator in the power system is generally below (15-20) kV. Commonly used transmission voltage levels are 765 kV, 500 kV, 220-110 kV, 35-60 kV, etc.; distribution voltage levels are 35-60 kV, 3-10 kV, etc.; Electrical appliances have high-voltage electrical equipment with a rated voltage of 3-15 kV and low-voltage electrical equipment such as 110 volts, 220 volts, and 380 volts. Therefore, the power system is to connect parts of different voltage levels through substation to form a whole.
3.2 There are problems
3.2.1 Operation errors and errors
The substation operator is the direct executor who guarantees the operation of the substation. Since the operation of substation is characterized by a large number of maintenance equipment, the probability of abnormalities and obstacles is high, the work is tedious and tedious, and it is easy to cause the staff to relax in thought. Therefore, in the operation of substation, any irregular behavior may affect the safety of the power grid, Stable operation, and even cause major accidents.
3.2.2 Inadequate preventive measures
Enterprise managers have not implemented measures to prevent misuse accidents. For example, the on-duty staff can use the key of the mechanical padlock of the grounding knife switch mechanism box without the permission of the workshop management personnel. In the implementation of dangerous point analysis and control measures, there is a phenomenon of mere formality and inspection, and it has not really been implemented.
3.2.3 Lack of employee safety training
The pertinence and effectiveness of employee training are not strong, the safety quality and professional skills of employees are difficult to meet the job requirements, and the safety system, requirements and measures have not really been penetrated into the daily work of employees.
3.2.4 The equipment is aging and cannot be updated in time
From the factory to the installation and use of electrical equipment, as time goes by, it gradually ages and exceeds the service life. The hidden dangers of the equipment threaten the safety of the substation operation. The equipment involved in the substation operation has not been remodeled in time during the use process, which is a potential hidden danger of safety accidents.
4. Distribution of power transmission and distribution
Power distribution is a link in the power system that is directly connected to users and distributes electric energy to users. The distribution system consists of distribution substations, high-voltage distribution lines, distribution transformers, low-voltage distribution lines and corresponding control and protection equipment.
4.2 Power supply mode
4.2.1 AC power supply mode
Commonly used AC power supply methods in power distribution systems are:
18.104.22.168 Three-phase three-wire system. Divided into delta connection (0.4kv motor and lighting) and star connection (for low-voltage three-phase 0.23kv industrial lighting distribution and 0.23kv three-phase motor).
22.214.171.124 Three-phase four-wire system. Used for 0.4kv/0.23kv low-voltage power and lighting hybrid power distribution.
126.96.36.199 Three-phase two-wire one-ground system. It is mostly used for rural power distribution (because of safety issues, it has long been eliminated).
188.8.131.52 Single-phase single-wire system 27.5kv. Commonly used in electric railway traction power supply.
184.108.40.206 Single-phase two-wire system 0.23kv (obtained from three-phase four-wire system). It mainly supplies electricity for residents.
4.2.2 DC power supply mode
Commonly used DC power supply methods for power distribution systems are:
220.127.116.11 Two-wire system. It is used for power supply of urban trolleybus, subway locomotive, mine traction locomotive, etc.
18.104.22.168 Three-wire system. Supply power plants, substations, power distribution stations for their own use, secondary equipment, electrolysis and electroplating.
The primary power distribution network is the network from the outgoing line of the distribution substation to the entrance of the distribution substation (or power distribution station). In China, it is also called high-voltage power distribution network. The voltage is usually 6-10 kV, and most cities use 10 kV power distribution. With the increase of urban load density, 20 kV power distribution scheme has been adopted. The main part of the primary distribution line drawn from the distribution substation is called the main line. The part branched off from the main line is called a branch line. Distribution transformers are connected to branch lines. There are two types of wiring in the primary distribution network: radial and ring.
The secondary power distribution network is a system composed of lines and components between the secondary lead-out line of the distribution transformer and the user's home line, also known as the low-voltage power distribution network. In addition to the radial and ring wiring methods, important users in the city can use double-circuit wiring. Urban areas with a high density of electricity loads use grid-style wiring. This kind of network is powered by multiple primary distribution main lines, and after stepping down through distribution transformers, it is connected to the secondary distribution network through low-voltage fuses. Since the adjacent primary distribution transformers in the secondary system are connected to different primary distribution trunk lines, it is possible to avoid power outages in the downtown area due to primary distribution line failures.
According to the structure of distribution lines, there are overhead lines and underground cables. Overhead lines can be used in rural areas and small and medium-sized cities, and underground cables should be used in large cities (especially downtown areas), tourist areas, and residential areas.
5. Current status of power transmission and distribution
As the basis of contemporary power supply, power distribution network plays a very important role in national economic construction and development. In the process of building the power distribution network, in order to achieve the purpose of effective transmission of power in the power supply, it is necessary to set up a corresponding level of power transmission network in areas that do not pass through. Through the construction of power transmission networks, the demand for power in different regions can be met. Power supply promotes the economic construction and development of the region. Based on the differences in the electrical properties of different power sources, the construction of my country's power distribution network has effectively adapted to the changing grid environment, and can achieve the transmission of different types of electrical energy by flexibly changing the nature of electrical energy. With the gradual deepening of research on power network construction technology, my country has begun to explore effective ways of power network construction. By studying different forms of power network construction and comparing the feasibility of different construction schemes, the most effective power distribution network construction has been selected. Way. By comparing a number of power network construction schemes, combined with the current situation of power development in my country, it has become the most effective way to build power grids by combining old and new means to build power distribution networks. The development of new and old power distribution network construction projects in my country effectively promotes the progress of my country's power transmission level, but also puts forward certain requirements for my country's economic development and power supply industry construction. Optimization is the primary task facing the construction and development of my country's power distribution network.