Summary:Incorporating the characteristics of underground coal mine power supply systems, a high-voltage power supply monitoring system for underground substation has been designed, with a 6kV power circuit as the monitoring target. The system enables real-time monitoring of voltage, current, and temperature of high-voltage distribution switches.
Keywords:Power supply system; data collection; monitoring system
Introduction
Electricity is a crucial power source for underground coal mines. Due to the harsh working conditions, faults such as open circuit, short circuit, overload, and leakage occur frequently. Before these faults arise, a series of symptoms often accompany the high-voltage distribution switch, including abrupt changes in voltage and current, and rising temperatures.
The high-voltage power supply monitoring system designed in this article can upload these features in real-time to the surface monitoring center, reducing the probability of faults occurring. In the event of a fault, it can quickly locate the issue, improving the efficiency of troubleshooting, thereby minimizing property losses caused by power system failures. Through ground computer control, it is possible to operate the opening and closing of switches, enhancing the level of power supply system management. The power supply monitoring system is relatively independent from the power supply system. In the event of a monitoring system failure, it will not affect the high-voltage power supply system, but can still provide real-time monitoring and control of the high-voltage power supply system.
1. High-voltage Power Supply Monitoring System Structural Design
1.1 Structural Design
The structural diagram of the underground coal mine power supply monitoring system is shown in Figure 1. The entire system is of DCS (Distributed Control System) type, featuring a distributed computer monitoring system. It is divided into ground monitoring stations and underground monitoring stations, which communicate via RS-485. Each underground substation is responsible for collecting monitoring information for its own substation, with the collected data stored in the controller cache area of the underground substation. When the ground substation sends an access command to the underground substation,
Figure 1: Structure of the Underground Coal Mine Power Supply Monitoring System
The power station sends all data from its buffer to the ground substation, which can then display the underground monitoring data in real-time on the software monitoring interface. It can also issue opening and closing commands to the underground substation as needed.
1.2 Communication Method Selection
Common communication methods in coal mine shafts include RS-485, CAN bus, RS-232, fiber optic communication, and Zigbee networking. To simplify the structure of the power supply monitoring system and facilitate communication between ground computers and underground substations, we have chosen RS-485 with opto-isolation. This 2-wire system can transmit over a distance of 1500m, meeting the distance requirements from the surface PC to the underground substation. The opto-isolation technology enhances the signal's resistance to interference. For data collectors closer to the substation, RS-232 communication is used, which is a 3-wire system with a transmission distance of 15m.
2. System Hardware Design
2.1 Main Control Chip Selection
The system employs the MSP430F5438 microcontroller, a TI product featuring a 256KiB storage capacity. Notably, it boasts ultra-low power consumption, requiring only a DC voltage of 2.2~3.6V. Built on this low-power foundation, it integrates a wealth of peripherals, including three 16-bit timers, one high-speed 12-bit analog-to-digital converter, UART, SPI, I2C serial communication, WDT watchdog timer, and P1-P10 ports, etc. This chip is applicable to both analog and digital sensor systems. The primary functionality of the coal mining machine's electrical controller can be achieved with the MSP430F5438, while also reducing the system's power consumption.
2.2 Isolated RS-485 Circuit Design
The communication method between the underground substation and the surface monitoring platform is selected as RS-485, with the ADM2587E chip chosen. This chip, a single-power-supplied isolated type from Analog Devices, comes in a SOW-20 package, boasting a transmission rate of 500 Kibit/s and an isolation voltage of 2.5 kV. It is widely used in industrial control and power applications requiring isolated RS-485 communication.
The isolated RS-485 circuit schematic is shown in Figure 2. To reduce the interference from common-mode voltage, lightning strikes, surge voltages, and other factors in the line, the bus is implemented...
Figure 2 Isolated RS-485 Circuit
Protective measures adopted: Connect an RT resistor in series with the VA, VB pins. The resistance value range of this resistor is 4~10Ω. Connect TVS diodes to the VA, VB pins with ground, or alternatively, connect in series a resistor and a zener diode.
2.3 RS-232 Communication Circuit Design
The communication method between the underground substation and data collector is selected as RS-232, which offers a transmission rate range from 50 bits/s to 38400 bits/s, enabling full-duplex asynchronous communication while retaining the TXD, RXD, and GND lines in the wiring.
In the high-voltage power supply monitoring system of the underground substation, level conversion between TTL and RS-232 is achieved using the MAX3232ESE. The circuit design is as shown in Figure 3, where RX1 and TX1 are connected to the microcontroller's serial port, and RX232-1 and TX232-1 are connected to the data collector's dedicated RS-232 serial ports for data transmission. Additionally, to enhance system expandability, one serial port is reserved for future hardware upgrades.
Figure 3: RS-232 Communication Circuit Principle
2.4 Temperature Measurement Circuit Design
The temperature measurement sensor for the high-voltage distribution switch is selected as the DS18b20 chip, which collects the temperature inside the low-pressure cavity of the switch in real-time and uploads the temperature information to the on-site monitoring center in real-time.
The DS18B20 temperature reading is primarily achieved through timing sequences of reading and writing high and low levels on its DQ pin. To enhance the drive capability of the DS18B20, an pull-up resistor is added to its DQ pin, ensuring it is reliably set to high or low throughout all read and write processes, as shown in Figure 4. During the temperature reading process, it mainly involves initialization sequences, read sequences, and write sequences. All sequences have the host as the master device and the single-wire device as the slave device. Each command and data transmission starts with the host initiating a write sequence. If the single-wire device is required to echo data, after a write command, the host must initiate a read sequence to complete the data reception.
Image 4: Temperature Measuring Device
3. Software Design
3.1 Temperature Measurement Program Design
3.1.1 DS18B20 Temporary Storage Register Distribution
The DS18B20 temperature sensor utilizes a single-bus structure. By reading and writing to the sensor's signal lines, temperature data can be retrieved. The DS18B20's high-speed scratchpad memory consists of 9 bytes, as shown in Table 1. After issuing the temperature conversion command, the converted temperature value is stored in the 0th and 1st bytes of the high-speed scratchpad memory in two's complement format. A microcontroller can read this data via a single-line interface, with the lower byte read first, followed by the higher byte, as shown in Table 1. Temperature calculation: when the sign bit S = 0, directly convert the binary bits to decimal; when S = 1, first convert the complement to the original code, then calculate the decimal value. Table 1 shows a portion of the corresponding temperature values. The 9th byte is a redundancy check byte.
Table 1: DS18B20 Temporary Register Distribution
3.1.2 Temperature Measurement Program Design
In the DS1820 temperature sensing program design, after issuing a temperature conversion command to the DS1820, the program always waits for the DS1820's return signal. If a DS1820 is poorly connected or disconnected, there will be no return signal when the program reads the DS1820. The P6.2 pin of the msp430 microcontroller is connected to the DQ pin of the DS18B20.
3.2 Supervisor Interface Design
The supervisory computer software for the high-voltage power supply monitoring system in coal mine shafts is programmed using VB, primarily based on a visual encoder. It utilizes C programming language and calls different components by adding code attributes, reducing the difficulty of programming. Ultimately, it generates an .exe file for convenient software installation.
The main function of the master station software is to display and store the status information of the distribution switches in the high-voltage power supply system underground through the onshore monitoring platform, facilitating historical data queries and retrieval in case of system failures. The operational functions for the underground high-voltage distribution switches include: opening, closing, and resetting. An operation password must be entered prior to performing distribution switch operations to prevent accidental actions that could cause accidents.
The monitoring interface of the underground high-voltage power supply system is shown in Figure 5, which includes the voltage, current, and temperature data of high-voltage distribution switches at various transformer stations underground. It features historical data query functionality, allowing users to select different dates, times, transformer stations, and switch locations to query information at specific moments.
Figure 5: Monitoring Interface of the Underground High-Voltage Power Supply System
4. Ankerui Acrel-2000Z Power Monitoring System Solution
4.1 Overview
For customer sub-stations (typically 35kV and below), a comprehensive automated monitoring system has been implemented, utilizing microcomputer protection devices, integrated switchgear control and measurement devices, wireless electrical junction temperature measurement products, on-line power quality monitoring devices, distribution room environmental monitoring equipment, and arc protection devices, etc. This system ensures safe operation and management of substation, distribution, and power usage. The monitoring scope includes customer sub-stations, switching stations, transformer stations, and distribution rooms.
The Acrel-2000Z Power Monitoring System is a hierarchical distributed substation monitoring and management system developed by Ankorui Electric Co., Ltd. in response to the requirements of power system automation and unattended operation. It is designed for voltage levels of 35kV and below. This system integrates protection, monitoring, control, and communication functions into an open, networked, modular, and configurable system, utilizing power automation technology, computer technology, network technology, and information transmission technology. It is suitable for urban, rural, and user substation transformer stations with voltage levels of 35kV and below. The system enables control and management of substation operations, meets the needs for unattended or minimally attended substation operations, and provides a solid guarantee for the safe, stable, and economic operation of substation facilities.
4.2 Application Areas
Design, construction, and operation and maintenance services for 35kV and below user-end power supply and distribution automation systems applicable to rail transit, industry, construction, schools, commercial complexes, and more.
4.3 System Architecture
The Acrel-2000Z Power Monitoring System employs a layered distributed design, which is divided into three layers: the station control management layer, the network communication layer, and the field equipment layer. The networking methods can be a standard network structure, an optical fiber star network structure, or an optical fiber ring network structure. The choice of networking method is determined by considering various factors such as the scale of electricity usage, distribution of electrical equipment, and the area covered, among others.
4.4 System Function
(1) Real-time Monitoring: Provides a clear display of the distribution network's operational status, real-time monitoring of electrical parameters in each loop, and dynamic surveillance of fault and alarm signals related to distribution loops.
(2) Electrical Parameter Inquiry: Detailed electrical parameters of the circuit can be directly viewed on the primary distribution diagram.
(3) Curve Query: Directly view curves for various electrical parameters.
(4) Operational Reports: Query operating parameters for each loop or equipment over time.
(5) Real-time Alerts: Equipped with real-time alert functionality, the system can issue alerts for events such as changes in remote signal status of distribution circuit, protective operation, and accidental tripping.
(6) Historical Event Inquiry: Stores and manages event records to facilitate users in tracing historical system events and alarms, as well as for query statistics and accident analysis.
(7) Electricity Consumption Statistics Report: The system is equipped with a scheduled meter reading and summary function, allowing users to freely query the electricity usage of any distribution nodes over any time period since the system has been operating normally.
User Permission Management: A user permission management feature has been set up, allowing for the definition of login names, passwords, and operational permissions for users of different levels.
Network Topology: Supports real-time monitoring and diagnostics of communication status of all devices, displaying the entire system's network structure in full.
(10) Power Quality Monitoring: Continuously monitors the power quality and reliability within the entire distribution system range.
(11) Remote Control Function: Allows for remote operation of equipment within the entire distribution system range.
(12) Fault Recording: Automatically and accurately records the changes in various electrical quantities before and after a system fault occurs.
(13) Incident Memoir: Automatically records all real-time stable information around the moment of an accident.
(Web Access: The display page shows an overview of the number of substations, transformers, monitoring points, equipment communication status, power consumption analysis, and event records.)
(15) APP Access: The Device Data page displays the electrical parameter data and curves for each device.
4.5 System Hardware Configuration
5. Conclusion
This article takes the 6kV power loop in coal mine shafts as the research object and designs a high-voltage power supply monitoring system for underground substation of coal mines. It designs an isolation RS-485 communication circuit to achieve two-way communication between the surface PC and the underground substation. In the data collector, the main control chip selected is the ultra-low power consumption MSP430 microcontroller for collecting voltage, current, and temperature data. In the software design, it details the temperature measurement process of the DS19B20 temperature measurement chip and uses VB software to design the monitoring system software of the surface monitoring platform, realizing real-time monitoring and historical data query of the high-voltage power supply system in coal mine shafts.
Reference
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Liu Hong. Coal Mine Safety Testing Technology (Volume 1) [M]. Beijing: Coal Industry Press, 1993: 19-25.
Li Jianwei: Design of High-Voltage Power Supply Monitoring System for Underground Coal Mine Transformer Substations
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