Implementation of a substation control and protection system based on single chip microcomputer

1 Overview


The safety protection of substation equipment and lines is an important guarantee for maintaining the normal operation of the power grid and the safe use of electricity by users. With the development of electronic technology and computer application technology, the automatic control and protection system with microprocessor as the core is gradually replacing the traditional relay protection device and becoming an important part of the substation. Here, we propose a substation automatic control and protection system based on single-chip microcomputer 8031. The system can realize the functions of accident protection, metering, switching control, real-time display and printing, and database establishment of 35 kV substation, and meet the protection, measurement and control of equipment and lines of 35 kV substation, so as to achieve unmanned or less-manned operation.


2 System hardware structure and function


The system hardware consists of industrial control microcomputer system, communication board, line (equipment) protection metering board, and voltage monitoring protection metering board. The design adopts an upper and lower computer structure. The upper computer is an industrial control microcomputer system. The host is a 486 or above industrial control PC, equipped with keyboard, printer, large-screen display, mouse and other input and output devices; the lower computer (extension) is a line (equipment) protection metering board unit, each unit is an independent single-chip microcomputer acquisition protection system, connected with the upper computer through communication lines; and the communication board plays the role of a bridge between the upper and lower computers, forming a one-to-many communication network with the lower computer through RS485 (block diagram as shown in Figure 1).





Figure 1 System structure diagram


2.1 Industrial control microcomputer system

Complete the human-computer dialogue of the entire system, display the status of each line (equipment) and metering data, protection action information, etc., command and control the opening and closing operations of the circuit breaker, and have password identification and operation repeated confirmation safety operation measures, and can also realize automatic meter reading, protection action record, operation record, data query, automatic generation of various reports, remote control and other functions.

2.2 Lower computer

According to the requirements of relay protection, the entire 35 kV substation must complete: ①10 kV, 35 kV line and main transformer overcurrent, overcurrent speed cut, overload protection and metering measurement of the loop; ②capacitor overvoltage and undervoltage protection; ③main transformer differential protection; ④10 kV and 35 kV bus voltage acquisition measurement and overvoltage and undervoltage protection; ⑤opening and closing operations and other functions. To achieve the above goals, we use 8031 ​​industrial single-chip computer as the core when designing each functional module, supplemented by memory, current (or voltage) sensor, A/D conversion circuit, circuit breaker drive circuit, communication connection circuit, etc., to form current acquisition, protection control board, voltage acquisition, protection control board, main transformer differential protection control board, power factor acquisition board, pulse watt-hour meter electricity acquisition board. The hardware of the acquisition and protection end is similar, and its block diagram is shown in Figure 2.




Figure 2 Block diagram of the acquisition protection terminal


The sensor of the current acquisition control board adopts a step-type sensor, which converts the secondary 0 A to 5 A current of the line (10 kV or 35 kV and main transformer current) transformer into a voltage signal of 0 V to 2.5 V, and converts it into a digital signal through a 12-bit A/D converter for reading by the 8031 ​​single-chip computer. After comparison and judgment with the set value stored in the machine, the overcurrent protection, time-limited quick break, quick break, longitudinal differential and other protection functions of the line or main transformer are completed. For light and heavy gas, oil switches, disconnectors, grounding switches, energy storage switches and other switch quantities, they directly enter the P1 port of 8031, and the corresponding bits of the RAM on the chip are read by 8031. The software determines its status and makes corresponding protection action instructions or anti-misoperation instructions. According to the test, it only takes 300 ms for the overcurrent quick break from the occurrence of the accident to the system responding and driving the corresponding output relay to act. The board also receives the opening and closing instructions of the host computer to drive the corresponding output relay to act. The current acquisition control board is designed to be universal and can be identified by different address codes. The address code of each control board is determined by the hardware jumper on the board or the EPROM solidified fixed value, which can reduce the manufacturing cost, improve interchangeability, and reduce the number of spare parts for users.

The voltage acquisition control board uses a voltage sensor to collect the bus voltage and zero-sequence voltage. The voltage signal is transmitted to the 8031 ​​single-chip computer system for processing by the voltage transformer (sensor) through the A/D conversion circuit. After comparison and judgment with the fixed value, the line over-voltage and under-voltage alarm, capacitor over-voltage and under-voltage protection, grounding alarm and other functions are completed.

The main function of the power factor acquisition board is to collect the power factor signal of the main trunk line to reflect the operating conditions of the power grid with or without reactive power.


The main transformer differential protection control board and the pulse watt-hour meter electricity acquisition board are both special function boards that complete a single function.


2.3 Communication board

The communication control board plays a connecting role in the whole system. It is composed of 8031 ​​single-chip microcomputer, program memory (EPROM), extended data memory RAM, extended serial port 8251, communication port circuit and alarm sound drive circuit. The communication with the lower computer uses the asynchronous serial port of 8031, and adopts MAX485 chip to form a one-to-many communication mode, while the communication with the upper computer (microcomputer) uses the extended serial port and connects with the serial port of the upper computer through RS232. The transmission rate in both directions is 9600 bps, which meets the timeliness requirements of information display. The communication board reduces the time used by the upper computer for communication, so that the upper computer has more time to handle other matters.


3 Software design


The system software includes three parts: microcomputer application software, communication board software and lower computer software.


3.1 Upper computer (industrial microcomputer) software The

upper computer application software is built on the Windows 3.1 and above operating system platform, developed with Borland C++ 3.0 and Foxpro 2.6, and is divided into 7 functional modules, namely: ① System management program; ② Location map display module; ③ Substation wiring diagram; ④ Real-time metering data dynamic display; ⑤ Interrupt service program and accident protection action information display and printing program; ⑥ Communication program; ⑦ Establishment of historical database and report output. Initialization is performed after power-on, including reading initial values, database settings, variable array initialization, serial port initialization, etc. Switching between modules is completed through menu selection; the system management program sends data transmission commands to the lower computer at regular intervals, and the communication program completes the reception and transmission of data, and refreshes the data displayed on the screen in real time; the protection action and alarm signal of the lower computer are processed by the interrupt service program. If an interrupt occurs, the interrupt service program determines the source and type of the interrupt signal and displays it in real time, while receiving and saving the accident data.



4 Conclusion


The distributed substation automatic control system based on 8031 ​​single-chip microcomputer has advanced design ideas and reasonable structure, and has achieved good results in practical applications. According to statistics, from October 1995 when it was put into operation to 1998, the system correctly reflected 6 10 kV line faults, 13 lightning short-circuit faults, 1 two-phase short-circuit fault, 1 35 kV grounding fault, and hundreds of opening and closing operations, which basically reflects the actual operation of the substation.


The system is low-cost and easy to operate, and has broad application prospects in the current power grid transformation.