Distribution Network Overview
A power network that receives electrical energy from the transmission grid or regional power plants and distributes it to users locally or through tiered distribution facilities is called a distribution system. Based on the characteristics of the power supply area, it can be divided into urban distribution networks and rural distribution networks; based on the type of distribution lines, it can be divided into overhead distribution networks, cable distribution networks, and hybrid overhead-cable distribution networks.
The power distribution system is a component of the power system, consisting of substations, high-voltage distribution lines, distribution transformers, low-voltage distribution lines, and corresponding control and protection equipment. Distribution lines are further divided into overhead lines and underground cables. Underground cables are generally used in large cities (especially city centers), tourist areas, and residential areas.
The primary distribution network is the network between the substation's outgoing lines and the distribution transformer, typically operating at 6–10 kV, and is also known as the high-voltage distribution network. Primary distribution networks have two wiring configurations: radial and ring. The secondary distribution network is a system composed of lines and components between the secondary outgoing lines of the distribution transformer and the user's service line; it is also known as the low-voltage distribution network.
The ownership of high- and low-voltage power distribution assets is complex, and inconsistencies may exist between asset ownership and operation and maintenance. The number of power distribution assets is substantial, with a certain amount of equipment directly invested in by users, meaning the assets do not belong to the power supply company. Some users have transferred ownership of their power equipment assets to the power supply company; some users entrust the power supply company to operate and maintain their power equipment assets, paying maintenance fees; and some users, due to various factors, do not pay maintenance fees. Assets for which users have paid maintenance fees should not be included in actual asset statistics.
Power distribution work involves a lot of emergency repair work. Due to the large number of low-voltage lines, cables and transformers, the operating costs are high. The main task is to repair them after an accident.
01 Distribution Network Structure
1.1 Overhead lines and power grid
In rural and mountainous areas, medium-voltage overhead power distribution lines generally adopt a tree-like radial power supply method due to their low and dispersed load density, long power supply lines, and small conductor cross-sectional area.
In urban and suburban areas, medium-voltage overhead power distribution lines are generally constructed using a radial ring network. When substation equipment and lines are under maintenance or malfunction, non-maintained or non-faulty lines can be switched to other power supply lines to improve power supply reliability and operational flexibility.
1.2 Cable distribution network
According to urban planning, cable lines should be used in areas with high load density, bustling areas, areas with high power supply reliability requirements, residential areas, areas with special requirements for urban appearance, and areas where narrow streets make it difficult to solve the problem of overhead line corridors.
02 Reactive Power Compensation for Distribution Networks
Transformers, motors , and other electrical equipment require a significant amount of reactive power from the system during operation. Severe insufficiency or improper configuration of reactive power in the system can lead to voltage drops, increased equipment losses, and low utilization rates. Reactive power compensation in the distribution network can improve the reactive power distribution, increase the power factor, improve voltage quality, avoid long-distance reactive power transmission, reduce distribution network line losses, and increase the power supply capacity of the distribution network. Parallel compensation is generally used as the compensation method.
The power factor for electricity customers should be 0.85 or 0.90 or higher, and the power factor for agricultural electricity should be 0.80 or higher. For electricity users with 10kV power supply of 100kVA or higher, the power factor should preferably be 0.95 or higher; for other electricity users, the power factor should be 0.90 or higher.
Reactive power compensation is generally based on the principle of local balance, and adopts two forms: decentralized compensation and centralized compensation.
03 Distribution network line loss
3.1 Basic Concepts of Line Loss
Within a power supply area, electrical energy is supplied to customers through various stages of the power grid, including transmission, transformation, and distribution. During the transmission and distribution of electrical energy, each component of the power grid incurs a certain amount of energy loss, commonly referred to as line loss (or technical line loss). The amount of energy lost by all components of the distribution network within a given time period (day, month, quarter, year) is called the line loss of the distribution network, and the percentage of line loss to the total 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 Classification of Line Losses
Line loss rate is divided into statistical line loss rate and theoretical line loss rate. Statistical line loss rate is calculated based on the actual line loss measured by power supply and sales meters. Theoretical line loss rate is calculated based on the load conditions of the power network and the parameters of the power supply equipment. Statistical line loss includes both technical line loss and management line loss.
Technical line losses are unavoidable in the power distribution process, including losses in transformer windings and distribution line conductors, transformer iron losses, and dielectric losses in capacitors and cable insulation.
Management line loss is the loss of electrical energy caused by management reasons, including the comprehensive error of the electricity meter, the failure to read the meter at the same time, the missed reading and the wrong reading, the use of electricity without a meter and the theft of electricity, leakage caused by poor insulation or dirt of the live equipment, and the loss of electricity caused by short circuit accidents in the distribution network.
04 Power supply reliability of distribution network
4.1 Power supply reliability
The power supply reliability of the distribution network is an important part of power system reliability management. It is generally measured by the ratio of the total number of hours of effective power supply to customers to the number of hours in the statistical period.
The ratio of the total decimal time of effective power supply to customers during the statistical period to the decimal time of the statistical period is:
4.2 Factors Affecting Power Supply Reliability
The main factors that cause a decrease in the reliability of power distribution networks include: weak distribution network structure; maintenance and new user additions generally require power outages, resulting in frequent power outages; an incomplete power distribution management system, which makes it difficult to quickly identify, isolate, and restore power supply in the event of a fault; and insufficient system peak-shaving capacity or power supply capacity, which may lead to artificial power rationing under abnormal operating conditions.
05 Distribution Network Planning
Distribution network planning is a comprehensive plan for the development and upgrading of the distribution network over a long period. Its purpose is to increase the power supply capacity of the distribution network through appropriate investment, adapt to the needs of load growth, and improve the power quality of the power grid. Distribution network planning includes the upgrading and expansion of existing distribution networks, as well as the construction of new distribution networks. Distribution network planning is the foundation and an important component of power grid planning.
The planning mainly includes: analyzing the current distribution network layout and load distribution, clarifying the following issues: load forecasting; determining the goals for each planning phase, the principles of the power grid structure, and the standardization of power supply facilities, including the principles for the transformation of medium and low voltage power grids; conducting active and reactive power balance, and proposing requirements for the construction of power supply points; phased overall planning of the power grid structure; determining the geographical locations and line routes of substations, and identifying the engineering projects to be constructed in each phase; determining the scale and requirements of dispatching, communication, automation, etc.; estimating the investment required for each planning phase, and the specifications and quantities of major equipment; drawing the current status and the geographical location wiring diagrams of the distribution network at the end of each planning phase; and compiling the planning specification.
The planning period for power distribution networks should be consistent with the planning period for local national economic development, and is generally defined as three phases: near term (5 years), medium term (10 years), and long term (20 years).
I. Distribution Network Equipment
01 Pole-mounted switch
Medium-voltage pole-mounted switches are typically classified according to their arc-extinguishing method or insulation medium, such as vacuum, SF6, oil, and gas-generating switches; according to their breaking and closing capabilities, such as load switches, circuit breakers, and disconnect switches; and according to their automation capabilities, such as reclosers and sectionalizers. Due to the large number and wide distribution of distribution network equipment, the inconvenience of on-site maintenance, and the ever-increasing demands for power supply reliability, the future development trend of medium-voltage pole-mounted switches is towards the adoption of maintenance-free or low-maintenance switches.
1.1 Pole-mounted load switch
Load switches should generally have the ability to interrupt and close normal load current, circulating current between lines, and charging current of lines or equipment, and should also have the ability to close short-circuit current.
Generally, pole-mounted load switches are used to divide medium-voltage overhead power distribution lines into 3 to 4 sections and install sectionalizing switches. Where conditions permit, each section of the line should also be connected to other power supply lines and connected switches should be installed to meet the needs of transferring loads, quickly restoring power to fault-free sections of the line, or reducing the scope of power outages during maintenance, troubleshooting, and other operations of medium-voltage overhead power distribution lines.
1.2 Pole-mounted circuit breaker
Circuit breakers should generally have the ability to interrupt and close phase-to-phase short-circuit currents. To reduce the impact of transient faults that frequently occur in some line sections on the entire line, or to solve the problem of line ends that cannot be protected by substation relay protection, pole-mounted circuit breakers can be installed in the middle section or at branches of the line to increase reclosing capabilities.
The main difference between a circuit breaker and a load switch is that a circuit breaker can be used to interrupt short-circuit current.
1.3 Pole-mounted disconnect switch
Pole-mounted disconnect switches, also known as knife switches, are used for power outage maintenance, fault finding, and cable testing of line equipment. Opening the pole-mounted disconnect switch isolates the equipment requiring maintenance from other operating lines, providing workers with a visible and obvious break point, ensuring safety during maintenance or testing. The advantages of pole-mounted disconnect switches are low cost, simplicity, and durability. They are generally used as property boundary switches between overhead and non-overhead lines, and as boundary switches between cable and overhead lines. They can also be installed on one or both sides of the line tie load switch to facilitate fault finding, cable testing, and maintenance/replacement of the tie load switch.
02 Drop-out fuse
10kV drop-out fuses are generally installed on the high-voltage side of pole-mounted distribution transformers to protect 10kV overhead distribution lines from transformer faults. In rural and mountainous areas, drop-out fuses are also installed at the end of long lines or at line branches where the relay protection of the substation cannot reach.
03 Low-voltage fuse
When shutting down a pole-mounted distribution transformer, the general sequence is to first shut down the low-voltage side. Transformers exceeding 30kVA are typically equipped with a low-voltage knife switch, with the fuse mounted on the switch. This is an open-type fuse, generally referred to as a fuse-type disconnector (see Figure 4-20). Opening the knife switch reveals a visible and obvious circuit break. For transformers of 30kVA and below, when shutting down under light load, the high-voltage drop-out fuse can be operated directly. A knife switch is not required on the low-voltage side; the low-voltage fuse is mounted on a porcelain base (commonly known as a flying fuse).
04 Surge Arrester
A surge arrester is a device connected between a power line and the ground to protect electrical equipment by allowing lightning to discharge into the ground. When lightning overvoltage or operational overvoltage occurs, it rapidly discharges into the ground; when the voltage drops to the normal voltage of the generator, transformer, or line, it stops discharging to prevent normal current from flowing into the ground.
Metal oxide surge arresters (also known as zinc oxide surge arresters) can generally be divided into two categories: gapless and those with series gaps. Gapless zinc oxide surge arresters are becoming increasingly widely used and have achieved excellent performance in operation.
05 Reactive Power Compensation Device
Reactive power compensation for overhead power distribution lines is generally divided into two categories: medium-voltage reactive power compensation and low-voltage reactive power compensation equipment. Low-voltage reactive power compensation is now more commonly used. Medium-voltage reactive power compensation equipment is generally installed at locations with concentrated loads, or approximately 3/4 of the way along the line, and is typically connected to the line with a fixed capacity. Low-voltage reactive power compensation equipment (see diagram below) is generally installed at the outlet of the pole-mounted distribution transformer.
06. Prefabricated substation, prefabricated substation, cable branch box
With urbanization, electricity load density is constantly increasing. The implementation of individual meters for each household has led to a significant increase in the average electricity capacity and consumption per household, making existing public distribution rooms and power lines insufficient to meet the requirements. Furthermore, the development of municipal construction has necessitated the undergrounding of overhead power lines along major urban roads. This has prompted the development and implementation of miniaturized prefabricated substations, box-type substations, and single-ring cable power supply technologies in public areas.
07 Distribution Transformer
A distribution transformer is an electrical device used for power conversion. It can convert alternating current (AC) energy of one voltage and current into alternating current energy of another voltage and current of the same frequency.
A distribution transformer is a device that transforms voltage. Its main components include the transformer body, voltage regulating device, oil tank and cooling device, protection device, and insulating bushing.
Distribution transformers should not be operated under overload conditions and should be operated economically. The maximum load current should not be lower than 60% of the rated current. Special transformers for seasonal power use (such as those for agricultural irrigation) should be shut down during off-load seasons.
08 Distribution Network Specific Equipment
8.1 Cement square pole
To avoid using guy wires, concrete square poles with good bending resistance are often used at the terminal poles, tension poles, or corners of the power distribution network.
8.2 Insulated wires
With social development, trees, bamboo, buildings, and corrosion in the power supply area seriously affect the safety of power distribution lines. In order to reduce the risk of short circuits, power outages, and electric shock injuries caused by foreign objects, insulated conductors with external insulation layers are increasingly used in power distribution networks.
8.3 Insulated Tension Clamp
Because insulated wires have an outer insulation layer, special insulated tension clamps with better grip are often used to withstand the tension of the wires.
8.4 Grounding ring
In order to enable voltage testing and grounding on the lines with insulated conductors, grounding rings are specially installed at certain locations on the insulated conductors of the distribution network.
II. Distribution Network Automation
Distribution network automation utilizes modern electronic, communication, computer, and network technologies to modernize the monitoring, protection, control, and management of power distribution systems during normal operation and in case of accidents. With the deepening of social modernization and reform and opening up, electricity users have increasingly higher requirements for power quality and supply reliability. Voltage fluctuations and short-term power outages can cause significant losses. Therefore, it is necessary to implement distribution automation in the distribution network in conjunction with power grid upgrades to improve the management level of the distribution network and provide uninterrupted, high-quality power to a wide range of electricity users. The functions of distribution network automation include Supervisory Control and Data Acquisition (SCADA), feeder automation (FA, i.e., fault location, isolation, and power restoration to non-faulty sections), load management, geographic information systems (AM/FM/GIS), and distribution application analysis (PAS), among others.
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