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Overview of the distribution network
An electric power network that receives electrical energy from the transmission grid or a regional power plant, connects it directly to the user through the distribution facilities locally or gradually, and distributes the electric energy to the user, is called a distribution system. According to the different regional characteristics of power supply, it can be divided into urban distribution network and rural distribution network, and according to the different distribution lines, it can be divided into overhead distribution network, cable distribution network and overhead cable hybrid distribution network.
The power distribution system is a power system, which is composed of substations, high-voltage distribution lines, distribution transformers, low-voltage distribution lines and corresponding control and protection equipment. Distribution lines are divided into overhead lines and underground cables, and underground cables are generally used in large cities (especially in the downtown area), tourist areas, residential areas, etc.
The primary distribution network is a network between the outgoing line of the distribution substation and the distribution transformer, and the voltage is usually 6~10 thousand volts, also known as the high-voltage distribution network. There are two kinds of wiring modes of the primary distribution network: radial and ring, and the secondary distribution network is a system composed of lines and components between the secondary lead line of the distribution transformer and the user's home line, also known as the low-voltage distribution network.
The property rights of high and low voltage distribution assets are complex, and there may be inconsistencies between asset ownership and operation and maintenance. The number of power distribution assets is large, a certain number of equipment assets are directly invested by users, and the asset relationship does not belong to the power supply enterprise, in which some users transfer the property rights of power equipment assets to the power supply enterprise; some users entrust the power supply enterprise to operate and maintain their power equipment assets on behalf of the user, and pay the maintenance fee, and some users do not pay the maintenance fee due to various factors. In the actual asset statistics, the assets and equipment for which the user pays maintenance fees should not be counted.
Power distribution work, emergency accident repair work more, low-voltage lines, cables, transformers due to their huge number, high operating costs, the main work is the accident after the emergency repair.
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Distribution network structure
1.1 Overhead lines and power grids
Due to the small and scattered load density, long power supply lines and small cross-sectional area of conductors in rural and mountainous areas, medium-voltage overhead distribution lines in rural and mountainous areas generally use dendrithing radial power supply.
When the substation equipment and lines are overhauled or faulty, the non-maintenance or non-fault lines can be transferred to other power lines for power supply, so as to improve the reliability of power supply and operation flexibility.
1.2 Cable distribution network
According to urban planning, cable lines should be used in areas with high load density, prosperous areas, areas with high requirements for power supply reliability, residential quarters, areas with special requirements for urban appearance and environment, and areas where narrow streets and overhead line corridors are difficult to solve.
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Distribution network reactive power compensation
Electrical equipment such as transformers and motors need to absorb a large amount of ungenerated power from the system when operating. Serious insufficiency or improper configuration of reactive power supply in the system will cause problems such as voltage reduction, increased equipment loss, and low utilization rate. Reactive power compensation on the distribution network can improve the reactive power distribution of the distribution network, improve the power factor of the power grid, improve the voltage quality, avoid the long-distance transmission of reactive power, reduce the line loss of the distribution network, and increase the power supply capacity of the distribution network. The compensation method is generally parallel compensation.
The power factor of electricity customers should reach 0.85 or 0.90 or above, and the power factor of agricultural electricity consumption should reach 0.80 or above. The power factor of 100kVA and above 10kV power supply should be above 0.95, and the power factor of other power users should be above 0.90.
The reactive power compensation method generally adopts two forms: decentralized compensation and centralized compensation according to the principle of local balance.
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Distribution network line loss
3.1 The basic concept of line loss
Within a power supply area, electric energy is supplied to customers through all aspects of the power grid's transmission, transformation and distribution. In the process of transmission and distribution of electric energy, each component of the power grid must produce a certain amount of electrical energy loss, which is referred to as line loss (or technical line loss). The amount of power lost in all elements of the distribution network in a given period of time (day, month, quarter, year) is called the line loss of the distribution network, and the percentage of line loss in the power supply is called the line loss rate. The line loss rate is a comprehensive economic and technical indicator of the distribution network.
3.2 Line loss classification
The line loss rate is divided into statistical line loss rate and theoretical line loss rate. The statistical line loss rate is calculated by the line loss amount measured by the power supply and sales meters. The theoretical line loss rate is calculated according to the load situation of the power network and the parameters of the power supply equipment. The statistical line loss includes two parts: technical line loss and management line loss.
Technical line loss is unavoidable in the process of power transformation and distribution, including the loss of transformer windings and distribution line conductors, the iron loss of transformer, and the dielectric loss of capacitors and cable insulation.
Management line loss is the power loss caused by management reasons, including the comprehensive error of the electric energy meter, the meter reading is not at the same time, missed and wrong reading, the electricity consumption without meter and the theft of electricity, the leakage caused by poor insulation or pollution of live equipment, and the loss of electricity caused by the short circuit accident of the distribution network.
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Distribution grid power supply reliability
4.1 Power supply reliability
The power supply reliability rate of the distribution network is an important part of the reliability management of the power system, which is generally measured by the ratio of the total hours of the customer's effective power supply time to the number of hours in the statistical period.
During the statistical period, the ratio of the total decimal of the effective power supply time to the customer to the decimal of the statistical period is:
4.2 Factors affecting the reliability of power supply
The main factors that cause the reduction of the power supply reliability rate of the distribution network are: the distribution network structure is weak, the maintenance and new users generally need to be cut off, resulting in frequent power outages, the distribution management system is not perfect, once a fault occurs, the judgment, isolation and restoration of power supply is not fast enough, the system peak regulation capacity is insufficient or the power supply capacity is insufficient, and there is a possibility of artificial power rationing under the non-normal operation mode.
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Distribution network planning
Distribution network planning is the overall plan for the development and transformation of the distribution network in the long period of time in the future, and its purpose is to increase the power supply capacity of the distribution network with appropriate investment, meet the needs of load growth and improve the power supply quality of the power grid. The planning of the distribution network includes the renovation and expansion of the existing distribution network, as well as the construction of a new distribution network. Distribution network planning is the foundation and important component of power grid planning.
The planning mainly includes: analyzing the distribution network layout and load distribution status, and the following issues need to be clarified. Load forecasting. Determine the objectives of each phase of the plan, the principles of power grid structure and the standardization of power supply facilities, including the principles of medium and low voltage power grid transformation. Carry out active and reactive power balance, and put forward the requirements for the construction of power supply points. Carry out overall planning of the network structure in stages. Determine the geographical location and line path of the substation, and determine the project of phased construction. Determine the size and requirements for scheduling, communication, automation, and more. Estimate the investment required for each planning period, the specification and quantity of major equipment. Draw the current situation and the distribution network planning geographic location wiring diagram at the end of each planning period. Preparation of planning statements.
The planning period of the distribution network should be consistent with the life of the local national economic development plan, and it is generally stipulated that it is in three stages: short-term (5 years), medium-term (10 years) and long-term (20 years).
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Distribution equipment
01
Pole switch
The classification of switches on medium voltage columns can be divided into vacuum, SF6, oil, gas production switches, etc. according to the arc extinguishing mode or insulating medium; according to the ability to break and close, it can be divided into load switches, circuit breakers, isolation switches, etc.; according to the ability to realize automation functions, it can be divided into reclosers, sectionalizers, etc. Due to the large number of distribution network equipment, wide distribution, inconvenient on-site maintenance, and the continuous improvement of social requirements for power supply reliability, the development trend of medium voltage column switches in the future is to use less maintenance or maintenance-free switches.
1.1 Column load switch
The load switch should generally have the ability to break and close the normal load current, the circulation current between lines, the charging current of the line or equipment, and the ability to close the short-circuit current.
Generally, the load switch on the column is used to divide the medium voltage overhead distribution line into 3~4 sections, install the section switch, and under the condition that each section of the line should also be contacted with other power supply lines, and the contact switch is installed to meet the needs of transferring the load when the power failure maintenance and accident finding and processing of the medium voltage overhead distribution line can be realized, or the power of the fault-free line section can be restored as soon as possible, or the power outage scope of the line can be reduced.
1.2 Pole circuit breaker
In order to reduce the impact of instantaneous faults that often occur in some line sections on the entire line, or to solve the end of the line that can not be protected by relay protection in the substation, a column circuit breaker can be installed in the middle section or branch of the line to increase the recloser.
The main difference between a circuit breaker and a load switch is that a circuit breaker can be used to interrupt a short-circuit current.
1.3 Column disconnector
The column isolation switch, also known as the knife switch, can be used for power failure maintenance, fault finding, cable test, etc. The advantages of the column disconnector are low cost, simple and durable. Generally, it is used as a demarcation switch between the property rights of the empty line and the user, as well as a demarcation switch between the cable line and the overhead line, and can also be installed on one or both sides of the line contact load switch to facilitate fault finding, cable testing and maintenance and replacement of the contact load switch.
02
Cut-out fuses
10kV cut-out fuse (generally installed on the high voltage side of the distribution transformer on the column to protect the 10kV overhead distribution line from the fault of the distribution transformer.) There are also long lines in rural areas and mountainous areas that install drop-out fuses at the end of the line or at the branch of the line that the relay protection of the substation cannot reach for protection.
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Low-voltage fuses
The general sequence of the shutdown operation of the on-column distribution transformer is to stop the low voltage first. Usually transformers with a capacity of more than 30kVA are equipped with a low-voltage knife switch, and the fuse is installed on the knife switch, which is an open fuse, which can be collectively called a fuse disconnector (see Figure 4-20). When the knife switch is pulled, there is a visible, obvious break point. When the transformer of 30kVA and below is out of operation at light load, the high-voltage drop-out fuse can be directly operated, and the low-voltage side can not be equipped with a knife switch, and the low-voltage fused piece is installed on the porcelain base (commonly known as fly insurance).
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Surge arrester
A lightning arrester is an appliance that connects between the power line and the earth, makes the thundercloud discharge to the earth, and protects the electrical equipment. When lightning overvoltage or operating overvoltage comes, it will be rapidly discharged to the ground, and when the voltage drops to the normal voltage of the generator, transformer or line, the discharge will be stopped to prevent the normal current from flowing to the earth.
Metal oxide arresters (also known as zinc oxide arresters) can generally be divided into two categories: no gap and series clearance. Due to the increasing use of gapless zinc oxide arresters, and the excellent performance of the lightning arresters has been achieved.
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Reactive power compensation device
Overhead distribution line reactive power compensation is usually divided into two types of equipment: medium voltage reactive power compensation and low voltage reactive power compensation, and low voltage reactive power compensation is generally used. The installation location of medium-voltage reactive power compensation equipment is generally in a place where the load is relatively concentrated, or about 3/4 of the length of the line, and it is generally connected to the line with a fixed capacity. The installation position of low-voltage reactive power compensation equipment (see figure below) is generally at the outlet of the distribution transformer on the column.
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Box transformer, box substation, cable branch box
With the development of urbanization, the density of electricity load continues to increase, the implementation of one meter per household has greatly increased the allocation of electricity capacity and electricity consumption per household, and the original public power distribution room and power line have been unable to meet the requirements; As a result, it has led to the development and implementation of miniaturized box transformers, box-type substations (and cable single-ring network power supply technologies in public places.
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Distribution transformers
Distribution transformer is an electrical equipment used for electric energy conversion, which can convert one voltage and current alternating current into another voltage and current alternating current of the same frequency.
The distribution transformer is a device that converts voltage, and its main components include the body, voltage regulating device, oil tank and cooling device, protection device, and insulating sleeve.
The distribution transformer should not be overloaded, should be operated economically, the maximum load current should not be less than 60% of the rated current, and the special transformer for seasonal electricity (agricultural irrigation, etc.) should stop running in the no-load season.
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Distribution network-specific equipment
8.1 Cement square rods
In order to avoid the use of pull wires, cement square rods with good bending resistance are often used at the terminal poles, tensile or corners of the distribution network.
8.2 Insulated wires
With the development of society, the power supply area of bamboo, buildings and corrosion seriously affect the safety of distribution lines, in order to reduce the risk of short circuit, power outage, personal shock casualties caused by foreign objects, more and more insulated wires with external insulation layer are used in the distribution network.
8.3 Insulated tension clamps
Due to the insulated wire with an insulating layer, a special insulated tension clamp with better grip is often used to withstand the wire tension.
8.4 Grounding Rings
In order to be able to check the electrical grounding on the line where the insulated wire is erected, a grounding ring is specially installed on the insulated wire of the distribution network in a certain place.
叁
Automation of distribution networking
Distribution network automation is the use of modern electronic technology, communication technology, computer and network technology to realize the normal operation of the distribution system and the modernization of monitoring, protection, control and distribution management in the event of an accident. With the deepening of social modernization and reform and opening up, power users have higher and higher requirements for power quality and power supply reliability, and voltage fluctuations and short-term power outages will cause huge losses. Therefore, it is necessary to realize distribution automation in the distribution network in combination with the transformation of the power grid, so as to improve the management level of the distribution network and provide high-quality power for the majority of power users without interruption. The functions of distribution network automation include data acquisition and control (SCADA) of the distribution network, feeder automation (FA, i.e., fault location, isolation, power supply restoration in non-fault sections), load management, geographic information system (AM/FM/GIS), distribution application analysis (PAS), etc.
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