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Medium and low voltage distribution automation and its PLC implementation scheme

Abstract: This paper discusses the necessity of medium and low voltage distribution automation from the requirements of modern society for distribution, the development needs of power enterprises themselves, and the development status of distribution automation at home and abroad; This paper compares several existing schemes of distribution automation, puts forward the solution of using programmable controller (PLC) to realize medium and low voltage distribution automation, and introduces the specific implementation of PLC scheme in detail

key words: power distribution; Automation; With the development of science and technology, the automation of medium and low voltage distribution has become the trend of power system development. As the last link of power transmission and distribution system, the degree of automation of medium and low voltage distribution is closely related to the quality and reliability of power supply and consumption. Therefore, this paper discusses the necessity and implementation scheme of medium and low voltage distribution automation

1 the necessity of realizing medium and low voltage distribution automation

1.1 realizing medium and low voltage distribution automation is the requirement of improving people's quality of life and developing the national economy

in modern society, the quality of power supply not only reflects the quality of life, water level and investment environment of people in a country or region, but also is an important factor affecting economic development, which determines the direction and scale of industrial development. In fact, with the advent of the information age, more and more computer equipment require uninterrupted power supply, which puts forward higher requirements for power supply. Power failure or power restriction will lead to production reduction, and sudden power failure will endanger the important equipment of the factory. Only by realizing the automation of medium and low voltage distribution can we maximize the quality of power supply and meet the needs of people's daily life, work and production

1.2 realizing the automation of medium and low voltage distribution is the need of the development of power enterprises.

realizing the automation of medium and low voltage distribution can improve the quality and reliability of power supply. Realizing the automation of medium and low voltage distribution can reduce the number of faults, narrow the scope of accidents, shorten the time of accidents, and provide an effective basis for restoring power supply, rapid analysis, diagnosis, and reporting the causes of accidents

realizing the automation of medium and low voltage distribution can improve the economic benefits of the whole power system: reduce the labor intensity of maintenance personnel; Reduce operators; Enhance the maintainability of the power system; It is conducive to improving the safety and health level of the equipment and prolonging the service life

realizing the automation of medium and low voltage distribution can improve the management level of the whole electricity. It mainly includes: automatically, accurately and timely providing more detailed and abundant data and information for the computer management of the power system, and the planned power outage and power supply of any user; It can conveniently and intuitively monitor the power consumption and power supply of each user in the overall situation, and realize the overall control

1.3 medium and low voltage distribution is the weak link of distribution automation in China. Although the construction of distribution automation started late in China, it has also been studied and practiced for nearly 20 years and achieved preliminary results. However, most of the research and practice results are carried out at the level of high-voltage distribution (above 35 kV), while the automation of medium and low-voltage distribution (at the level of distribution room) is still blank, with neither overall planning nor a unified technical principle. Moreover, the current longitudinal monitoring is generally limited to the outgoing line of the substation. Before, there was no other monitoring means for the monitoring from the feeder of the substation to the end user, which belongs to the scope of power consumption management, except for the load control of a few large users

2 low voltage distribution automation scheme

2.1 comparison of existing schemes of power system automation

the automation of medium and low voltage distribution (mainly referring to switchyard, switchgear room, switching station) and substation automation have certain similarities. Therefore, analyzing the implementation method of substation automation is of great significance to correctly determine the medium and low voltage distribution automation scheme

the substation automation system consists of five parts: master station, re mote terminal units (RTU), line sensor, remote control SF6 or vacuum switch, and communication cable. Among them, the RTU device is located at the substation site, which can automatically collect various switch state quantities (remote signaling) and analog quantities (telemetry), and transmit them to the master station system of the monitoring center through a special channel; Some RTUs can also perform specific remote control operations according to the intentions and instructions of the monitoring personnel, and return the operation results to the master station system of the monitoring center

judging from the functions that can be realized by substation RTU, substation automation includes three aspects: remote signaling, telemetry and remote control. In addition, some systems can also realize the total electric energy addition function according to the telemetry results. Accordingly, the substation automation system can be divided into two categories: one category only realizes the functions of remote signaling and telemetry, that is, the traditional SCADA system; The latest SCADA system belongs to another category, which should be able to realize all "three remote" functions. These two types of systems correspond to different stages and levels of power system automation

from the perspective of the method of realizing telemetry of substation RTU, there are two implementation schemes of RTU:

a) DC sampling scheme

in principle, the experimental tooling of this kind of RTU device is similar to that of the actual RTU device. Before collecting analog quantity, first use the transmitter to convert AC into DC, and then use rtua/D conversion element to express DC quantity into digital quantity. Its devices are mainly analog circuits, supplemented by a small number of digital circuits. It is characterized by the need for converters and advanced digital processing units (CPU, etc.), which is difficult to reflect the instantaneous changes of analog quantities, unable to carry out harmonic analysis, and the realization of the total electric energy adding function is complex and difficult

b) AC sampling scheme

this type of RTU device directly uses a/D conversion elements to collect and calculate AC power, without conversion equipment such as transmitters, but requires the cooperation of fast digital processing units to analyze and synthesize the collected data. It can not only reflect the instantaneous change of electricity, but also carry out harmonic analysis, calculate frequency, and simply realize the function of total electric energy. They often use microcomputers (such as 8x86, etc.) with multiple single-chip computers (such as 8051, 8098, etc.) and a large number of a/D conversion circuits to realize the acquisition of switching and analog quantities

at present, with the full development and application of digital technology, AC sampling scheme is a reasonable choice for distribution automation. It is mainly digital circuits, supplemented by a small number of analog circuits, with powerful functions, easy expansion, greater reliability than DC sampling scheme, and low comprehensive cost

2.2 application characteristics of low voltage distribution automation

the medium and low voltage distribution automation system is composed of five parts: master station, remote terminal unit (RTU), line sensor, remote control SF6 or vacuum switch, communication cable, etc. The application of medium and low voltage distribution automation has its own different characteristics:

a) the traditional substation RTU focuses on remote signaling and telemetry in function, but the automation objects of medium and low voltage distribution (switch room, switching station and distribution room) are numerous, the switch operation is frequent, and more attention is paid to remote signaling and remote control functions

b) the automation objects of medium and low voltage distribution are all over the city, rural and other different environments, and are operated by power managers at different levels (including rural electricians). It is also required to have the characteristics of flexible installation, easy operation, professional maintenance free and anti harsh environment

c) RTU applied to medium and low voltage power distribution should have modular structure in function, and the simpler and more reliable the hardware is, the better. It is best to use the same simple hardware. As long as you simply set it up, you can meet the requirements of different occasions and different scales

therefore, it is necessary to develop a new type of RTU, which is different from the traditional structure, to meet the characteristics and needs of medium and low voltage distribution automation

2.3 PLC implementation of medium and low voltage distribution automation RTU

programmable logic con troller (PLC) technology has been quite mature after decades of development. With complete varieties and various functions, it has been widely used in various fields of industrial control. Using PLC to realize the RTU function of medium and low voltage distribution automation can D. the increase of environmental experiments (high and low temperature experiments) can well meet the unique requirements of RTU. In the domestic market, there are PLC products from many famous manufacturers. These products, from simple to complex, are self-contained and can meet the special requirements of different applications. Most middle and low-grade PLC products include discrete point input and output (the number of points can be increased or decreased according to the application), analog sampling input, clock, communication and other functions. Using the ready-made wide sample fixture function of this kind of PLC, the RTU function of medium and low voltage distribution automation can be easily realized. The discrete input point of PLC is used to realize remote signaling, the discrete output point of PLC is used to realize remote control, the analog sampling input of PLC is used to realize telemetering, and the communication function of PLC is used to realize communication with the host. To complete these functions, no additional hardware is required, just simply program the PLC according to the actual situation of the switch room. Not only that, using the analog output function of PLC, it can even realize the remote adjustment of power distribution. For example, adjust the transformation ratio of the voltage regulating transformer, and adjust the voltage and current phase angle of the static var compensation equipment

such a RTU implementation scheme of medium and low voltage distribution automation based on PLC can fully meet the special requirements of medium and low voltage distribution automation. It has the following characteristics and advantages: simple hardware structure, completely maintenance free; The scale can be large or small. Just connect the expansion modules of PLC together to realize the increase of remote control points, remote signaling points and telemetry points; Anti harsh environment; High reliability; Programming to achieve various functions, free of hardware debugging; Low cost

plc scheme includes the following steps in the specific design:

a) obtain operation points. Understand the basic situation of power distribution and the specific requirements of automation, determine the equipment that the system needs to carry out remote control, remote signaling, telemetry, and even remote regulation, and count the specific points of these four signals required by each power distribution room

b) determine the communication scheme. According to the scale and distribution of power distribution, determine the overall design scheme, mainly the design and selection of communication scheme

c) PLC model selection. According to the points of various operations and the determined communication scheme, select the appropriate model of PLC to realize the RTU function

since RTU needs to accept the instructions of the monitoring center and upload the information of power distribution and switchgear, the communication function is the main consideration in selecting PLC

because the operation types and operation points of each switch room and switch cabinet often differ greatly, whether PLC has modular structure and configuration ability, and whether it can flexibly and economically form a variable scale system of input points, output points, measurement points (A/D) and adjustment points (D/a) is another major consideration in selecting PLC model

at present, the products of many manufacturers can meet the requirements of communication and modularization. For example, Siemens' S7-214 series, Mitsubishi's a1s series, Panasonic's higher-level PLC series, etc. According to the specific situation, in a distribution automation project, the PLC of the same manufacturer can be selected for the whole distribution system, or the PLC of different manufacturers can be selected according to the specific situation of the distribution room, so as to make use of each factory