Distribution network planning is an important work for power supply enterprises. In order to obtain the maximum economic benefits, power grid planning should not only ensure the safety and reliability of the power grid, but also ensure the economic operation of the power grid. Therefore, the main task of distribution network planning is to make a feasible grid development plan under the condition of technical feasibility to meet the demand of load development.

1 load forecasting

The ultimate goal of network planning and design is to meet the demand of load, and the development of load is enough to affect every link of network development. The development steps of power grid planning should be based on load development, and the load data of each feeder can be used to grasp the load development situation, analyze the past load and grasp the load development law. It is necessary to analyze the load, determine the maximum load time and load rate, and get the maximum load time and load value. These data are the basic data for forecasting future load. Distribution network planning can use two commonly used forecasting methods. Extrapolation method is based on the historical data of power consumption areas, assuming that the load development speed is constantly changing, according to the development situation of each period after the original load development speed has passed. In a power-consuming area, the initial load develops rapidly, but the land resources are gradually utilized, the power load tends to be stable, the load development speed changes from large to small, and finally the load reaches saturation or stable development. However, for areas with rapid economic development, the load development speed is not continuous, but leaps and bounds, so there is a certain error in extrapolation. Simulation method and extrapolation method have complementary functions. The simulation method is based on the annual electricity consumption of electricity-consuming areas, and calculates the load forecast value by investigating each type of electricity load and the number of users. No load forecasting method is completely accurate. When the updated load development data is mastered, the original load forecast value must be revised.

2 Determine the system model of the network

Determine the system model of the network, including determining whether the network is powered by overhead lines or cables, determining the cross-section size of wires, network connection mode, load transfer scheme, selection of related equipment in the network, how to transform the network when it meets inappropriate requirements in operation, system protection function, distribution network automation planning, etc.

(1) Overhead lines should be used for power supply in areas with scattered load or slow development. In the case of relatively high load density, rapid development or urban landscaping construction, cable power supply should be adopted.

(2) The selection of conductor section determines the conveying capacity of conductor. It is necessary to select enough wires to ensure that the line meets the requirements of network planning. For example, during the development of load, the cross section of conductor should not be changed frequently. When a line fails, the load of the failed line can be transferred to the adjacent feeder without overload operation. In addition, the selection of conductor section should ensure that the voltage drop at the end of the line is within an acceptable range. When the line has a short circuit fault, it can also withstand the fault current. Therefore, the cross section of the conductor is larger than that required by the maximum load current, but at the same time, the selection of the cross section should conform to the economic principle, and the transmission capacity of the conductor and the engineering investment should be compared.

(3) The planning of flexible connection mode can maximize the function of power supply network. For the overhead line network, the most effective way is to connect the feeder to the outgoing lines of different bus sections of adjacent substations or the same substation at the end of the line, and the two feedback lines are also equipped with segmented load switches and isolating circuit breakers respectively. When a feedback line fails, the faulty section can be isolated by sectioning switch. For wired network connection, two feedback lines can be used to form a backup network, or three feedback lines can be used to form a power supply area, in which two lines are loaded and one line is empty as a backup line for two load lines. Feeder can form a large ring network, and the load of a feeder can also form a small ring network, forming the form of a large ring and a small ring. Switching stations can also be built in areas with dense loads. Substation and switching station are connected through power lines, and then the switching station supplies power to the nearby load, which is used to extend the substation bus to the vicinity of the electric load.

(4) The principle of making load transfer scheme is to narrow the scope of power outage and minimize the power outage time. When a fault is found in the feedback line, it is necessary to find the fault point as soon as possible, and transfer the load before and after the fault point to the adjacent feeder for power supply, so as to isolate the load at the fault point.

(5) Detailed standards have been formulated for all kinds of electrical equipment at home and abroad, which provides a reliable basis for equipment selection. As a distribution network planning, equipment with good operation efficiency, low loss, high reliability and maintenance-free should be selected. For switchgear, it should have the function of network automation, and the equipment should be equipped with network automation equipment first or reserve space for future development. Although some new equipment has high purchase cost, it has high operation reliability, low failure rate and low maintenance cost, and the overall economic benefit is quite ideal.

(6) Distribution network planning is stable with the development of load in the implementation process. When the feeder load exceeds the safe current or there is not enough spare capacity, the feeder should be added to adjust the normal power supply range of the feeder in the power consumption area. At the same time, the content of distribution network planning should be revised accordingly.

(7) In order to ensure the normal operation of the power grid, a perfect protection system must be established. When the system fails, the fault point will be isolated with the least number of operations to ensure that the non-fault point will be restored as soon as possible. Now commonly used system protection methods are:

(1) Use fuse or overcurrent relay to realize overcurrent protection. When the fuse exceeds the fusing current, it will automatically fuse, thus quickly cutting off the current and protecting the electrical equipment. Fuses are mainly used for transformer protection. Overcurrent relay is used for line protection.

② Ground fault protection is used to eliminate ground fault. For systems grounded directly or through non-adjustable impedance, the secondary winding of the current transformer can be connected to the ground fault relay, or the overcurrent relay and the ground fault relay can be used together. For the system with ungrounded neutral point or grounded through arc suppression coil, the relay can decide whether to control the corresponding circuit breaker according to the basic criterion because the grounding fault will cause the voltage and current asymmetry of the system.

(3) Unit protection, which is used to protect a unit in the system. According to the normal operation of the circuit with the same voltage on both sides, the inflow current and outflow current are the same, and whether there is a fault can be judged by comparing the currents on both sides. However, communication lines are used for unit protection, and it is difficult to collect and compare data completely when the protection lines are too long. Using distance protection method can break through this limitation. In the distance protection scheme, the fault distance is calculated by using the voltage and current of the line according to the principle that the fault distance is proportional to the fault impedance.

④ The method of automatic reclosing device is to control the circuit breaker with relay to perform different tripping and closing sequences. Most faults in the line can be eliminated automatically or temporarily, and the power supply can be automatically restored by using the automatic reclosing device. ⑤ In the power system, sometimes the working voltage far exceeds the rated voltage. For example, when the switch is operated instantaneously or the system is struck by lightning, overvoltage will occur. Strengthening the insulation strength and level of each equipment, or installing overvoltage protection equipment in the network, can reduce overvoltage to a safe level, such as using air gap protection or installing lightning arrester for protection.

(8) The automatic management system of distribution network combines the automatic control system with the management information system by computer network, and establishes the system control and data acquisition system, which provides the foundation for the overall management of network security and economic operation. The main functions of distribution automation system can be divided into four parts. The first is the monitoring and management function of power grid operation, including power grid operation monitoring, power grid operation control, fault diagnosis and analysis, power supply recovery, operation data statistics and reporting. The second is the simulation and optimization function of operation plan, including the operation simulation of distribution network, the preparation of switching operation plan and the optimization of distribution planning at each gateway. The third is the operation analysis and maintenance management function, including analyzing the power grid fault and power supply quality feedback information, determining the weak links of the system and arranging the maintenance plan. The fourth is the function of user load monitoring and fault reporting, including the telemetry of user load and power quality, the control of user metering equipment, user fault reporting and repair processing system.

3 Benefit evaluation

The economic benefit evaluation of distribution network planning includes the comparison between power grid investment and increasing power consumption, as well as the comparison between the required input cost and power grid reliability, line loss rate and voltage qualification rate. The study of investment and income can decide which power supply mode to adopt.

Accelerating power construction provides favorable conditions for regional economic development, but the investment in power grid should be compared with the increased electricity consumption to determine whether these investments are worthwhile. Therefore, the power grid investment should be divided into different periods, and the corresponding power grid investment is also the place where the power consumption develops rapidly, and the place where the power consumption develops slowly is less.

For users, the higher the reliability of power supply, the better, but the investment in the corresponding power grid will also increase greatly. For large power users or important users, in order to ensure higher reliability, power grid investment can be increased, because reducing power outage time can reduce the losses of users and power supply enterprises at the same time. Line loss rate is used to reflect the loss degree of electric energy in the transmission process of power grid, and the loss in public power grid is borne by power supply enterprises. Through the technical transformation of power grid equipment, power supply enterprises can directly obtain economic benefits. In order to ensure the normal operation of power supply equipment and production system, the state has formulated the quality standard of power supply voltage, which stipulates the frequency, amplitude, waveform and fluctuation range of three-phase symmetry of voltage. Stable voltage quality can protect the power supply equipment from damage, so that users can produce normally, compared with which users will get more benefits.