Abstract: This paper introduces an underground power monitoring system for coal mines, which consists of an underground explosion-proof switch and an above-ground computer monitoring system. The underground explosion-proof switch uses a K7M-DR14UE programmable controller as its control core, and the above-ground computer monitoring system is developed using VC++. It can monitor the underground power grid parameters in real time and promptly display and alarm faults such as over/under voltage, phase loss, leakage, overload, and short circuit. Keywords: K7M-DR14UE, Mine Flameproof Feed Switch Controller, Monitored Control System, Cserial Class Abstract: This paper introduces a Power Monitored Control System for Underground Mine, comprising an underground mine flameproof feed switch and a computer monitored control system on the ground. The K7M-DR14UE programmable logic controller is used as the core of the flameproof feed switch. The computer monitored control system is developed from VC++, which can monitor the underground electric network parameters in real time, and detect and alarm overvoltage, undervoltage, open phase, electric leakage, overload, short circuit, etc. in real time. Key words: K7M-DR14UE, Mine Flameproof Feed Switch Controller, Monitored Control System, Cserial Class 0 Introduction With the improvement of mining levels in China's coal mining enterprises, explosion-proof high-voltage vacuum electronic feed switches for underground mines have been widely used. However, there are still many problems with these products in China. To address the problems of complex structure and poor protection performance of current domestic explosion-proof feed switches for mines, a new type of explosion-proof high-voltage vacuum electronic feed switch for mines has been developed. Equipped with a surface computer monitoring system, this constitutes a coal mine underground power monitoring system. This system allows for real-time monitoring and control of various parameters of the underground power distribution equipment from the surface management center, significantly shortening the time required to detect underground power line problems, reducing the workload of administrators, and improving enterprise production efficiency. 1. System Structure The surface computer monitoring system consists of a surface master station monitoring device (industrial control computer), a printer, a PCL-745B dual-port isolated 485 interface card, and a communication line (RS-485 bus). The printer is responsible for printing various reports, and the interface card connects to the RS-485 interface of the PLC in the underground power distribution switch controller via the communication line to complete data communication. This system is responsible for processing and displaying power information from the underground power distribution switch. The system functional structure is shown in Figure 1. Using this system, personnel at the surface management center can observe the parameters of all monitored underground power grids. When a fault occurs, the computer can promptly issue an alarm signal, enabling maintenance personnel to reach the accident site in the shortest possible time to troubleshoot and restore production. This system can perform the following functions: (1) Display and record the parameters of the underground power grid, such as the current, zero-sequence current, line voltage, zero-sequence voltage, insulation signal and phase-sensitive signal of the three phases ABC. (2) Fault alarm and location. When the underground equipment experiences faults such as overvoltage, undervoltage, phase loss, leakage, overload, short circuit, etc., the system's audible and visual alarm function can accurately display the nature and location of the faulty equipment. [align=center] Fig 1 Computer Monitored Control System Block Diagram[/align] (3) Generate daily, monthly and annual power statistics reports for each distribution switch, and print them to facilitate the statistics of power consumption. (4) Implement various query functions, such as querying the historical records of underground power grid parameters, querying reports, querying alarm records, etc. 2. Explosion-proof Distribution Switch for Underground Mines Currently, domestically produced explosion-proof high-voltage vacuum electronic distribution switches typically use discrete components or microcontrollers as their control core, resulting in complex structures, difficult installation and debugging, poor anti-interference capabilities, high failure rates, and the dispersed nature of components significantly impacting the reliability and stability of protection. The displays use ordinary digital tubes or indicator lights, which are not intuitive. Foreign products of the same type also generally use microcontroller systems, but their manufacturing processes are more advanced. However, once a fault occurs, maintenance cannot keep up, affecting production. Furthermore, foreign products are extremely expensive, more than ten times the price of domestic counterparts. To address this situation, a new type of explosion-proof high-voltage vacuum electronic distribution switch for mining has been developed. The core of this device uses an advanced imported industrial programmable logic controller (PLC) and a human-machine interface (GOT) with an LCD display and setting keyboard, forming an intelligent integrated protection controller that makes the distribution device both reliable and safe. It can operate stably in harsh conditions in coal mines, and can control, protect, and meter three-phase AC power supply systems with a rated voltage of 6KV/10KV, a rated frequency of 50Hz, and a rated current of 50A to 800A, where the neutral point is not directly grounded. It also features overvoltage, undervoltage, phase loss, leakage, overload, and short-circuit protection functions. 2.1 Distribution Switch Working Principle: The system principle block diagram is shown in Figure 2. The line protection function of this distribution switch requires the acquisition of eight signals from the high-voltage transformer: three-phase current, zero-sequence current, one line voltage, zero-sequence voltage, insulation, and phase-sensitive signals. These eight signals pass through a low-voltage transformer, are then rectified and filtered into DC signals, enter an A/D converter, and finally enter the PLC to control the circuit breaker's opening output. [align=center]Fig 2 Feed Switch Operation Principle Block Diagram[/align] The PLC calculates the incoming voltage signal. If the calculated voltage is greater than 1.2 times the rated voltage, an overvoltage warning is displayed, and the overvoltage protection will activate within 10 seconds; if the calculated voltage is less than 0.8 times the rated voltage, an undervoltage warning is displayed, and the undervoltage protection will activate within 10 seconds; if the calculated three-phase imbalance is greater than 0.7, a phase loss warning is displayed, and the protection will activate within 10-20 seconds. If any phase in the current calculation is within 6-10 times the rated current, a short circuit warning is displayed, and the short circuit protection will activate within 0.2 seconds; if any phase in the current calculation is within 1.2-6 times the rated current, an overload warning is displayed, and the overload protection will activate according to the inverse time characteristic. If a leakage fault occurs in the line, the leakage protection will activate based on the phase sensitivity and voltage signal, disconnecting the line. 2.2 Hardware Design The PLC used is an LG K7M-DR14UE programmable controller. The K7M-DR14UE is an economical PLC from LG's Master-K series, suitable for small machinery. It is DC 24V, with 8 inputs/6 relay outputs, and AC 85～264V power supply. Its processing speed reaches 0.4µs/step, requiring no expansion. The main unit can perform multiple functions: (1) It allows the main unit to capture pulse inputs with a pulse width of 10µs, and can accept single-phase 100kHz and two-phase 50kHz pulse signals, suitable for various applications requiring fast response; (2) The input has a filtering function (0～1000ms), which can prevent malfunctions caused by input signal fluctuations or external interference; (3) It has built-in RS-232C and RS-485 ports, which can be connected to devices such as computers or touch screens, and can be connected 1:N between K7M-DR14UE systems. Because A/D conversion is required, the PLC needs to be expanded with A/D conversion modules. Two G7F-AD2A modules are selected here; these are 12-bit precision, 4-channel A/D conversion modules with a maximum speed of 1ms/channel, and their performance fully meets the system requirements. The eight signals are converted by the A/D converter, and the results are stored in the PLC's internal registers. To achieve three-phase active power metering, the Belling BL0952 is used. The BL0952 is a three-phase active power metering chip with high linearity, good output stability, low power consumption, and good batch consistency. The chip outputs a pulse signal from pin 24 (F1), which enters the PLC, is converted to numerical values ​​to obtain the system power, and then sent to the human-machine interface for display. The well-ground computer monitoring system is based on a systematic, master-station integrated design concept. It uses a reliable Advantech industrial control computer as a platform, VC++ 6.0 as the development tool, and database technology to process and display the collected electrical energy information. Data from the well surface and underground are transmitted via an RS-485 bus. Currently, the monitoring system monitors eight downhole power distribution switches (see Figure 1). 3. Software Design The software is divided into the software design for the downhole explosion-proof power distribution switch controller and the software design for the surface computer monitoring system. 3.1 Power Distribution Switch Controller Software Design The controller software utilizes ladder logic and a modular design approach. The 1.2 times overload protection sampling value of the switching device is taken from the B-phase current. When the data in the register is greater than the 1.2 times overload setting value, the PLC outputs a delayed signal to control the circuit breaker to trip, and the human-machine interface displays the overload fault screen. The short circuit protection sampling value is taken from the A and C phase current values. When either value is greater than the instantaneous trip setting value, the protection operates and displays the short circuit fault. In each scanning cycle, the PLC compares the three-phase current values, finds the maximum current value, and then subtracts the data of the other two phases respectively. When the difference is greater than 70% of the set current value, the circuit breaker is controlled to trip after a 15-second delay, and the phase loss fault screen is displayed. In each scanning cycle, the PLC compares the line voltage sampling protection value with the rated value. When the sampling value is lower than 80% of the rated voltage value or higher than 120% of the rated voltage value, the circuit breaker is controlled to trip, and the undervoltage/overvoltage fault screen is displayed. When a leakage fault occurs, the PLC detects the zero-sequence voltage signal, generates a phase-sensitive signal after passing through the phase-sensitive circuit, the fault occurs, the protection operates, the circuit breaker trips, and the leakage fault screen is displayed. The software flow is shown in Figure 3. 3.2 Inoue Computer Monitoring System Software Design The monitoring system software mainly consists of two parts: a user management interface program and a data communication software program. The management interface is designed to be as simple as possible for easy operation by management personnel. In addition to completing normal data communication, the communication software design must also be able to automatically detect communication faults and alarm signals on each communication line in order to promptly identify and eliminate faults. The implementation of this program mainly addresses the serial communication problem under Windows. The Windows system completely takes over hardware resources and does not allow users to directly control serial port interrupts; instead, data exchange must be performed through device drivers provided by the Windows operating system. [align=center] Fig.3 Soft Frame Chart of the Programmable Logic Controller[/align] This system uses the VC++ multi-threaded serial port programming tool Cserial class to solve this problem. The Cserial class is provided by a third party. Its basic principle is to encapsulate low-level communication functions in the VC class library using multi-threading technology, making each port independent and accurately realizing serial communication. To respond to various events in a multi-tasking execution mode, a thread function needs to be defined in the CserialPort class library. The CserialPort class handles reading, writing, and serial port monitoring. When a response is detected on the serial port, it sends a message to notify the program. The thread written using it can prevent the main program from blocking. UINT CserialPort ::Comm Thread(L PVOID p Param) { CserialPort *port = (CserialPort *) p Param; port->m_bThreadAlive = True; if (port->m_hComm) PurgeComm(port->m_hComm, PURGE_RXCLEAR | PURGE_TXCLEAR | PURGE_RXABORT | PURGE_TXABORT); return 0; } 4 Conclusion This monitoring system consists of two parts: above ground and underground, and its development was also divided into two steps. First, a power distribution switch was developed. Its controller, based on the characteristics of electrical equipment in Chinese coal mines, incorporates advanced international intelligent technologies and structural forms. It uses a PLC control system instead of discrete electronic components or microcontrollers, achieving control, protection, and metering of the underground power supply system. It features high performance, high reliability, strong stability, and a fast development cycle. After nearly two years of field operation, it has already brought good economic benefits to the enterprise. The computer monitoring system utilizes a dedicated communication interface on the K7M-DR14UE programmable controller. This system communicates with the ground management center host via a 485 bus, allowing the management center to monitor and control various parameters of the underground power distribution device in real time. Since the computer monitoring system is relatively new, it needs further improvement, and its reliability requires further verification. The author's innovation lies in the development of the underground explosion-proof power distribution switch, which incorporates advanced international intelligent technologies and structural forms. Using a PLC control system instead of discrete electronic components or microcontrollers shortens the equipment development cycle and enhances the reliability of the device. References [1] Hou Shuming, Xu Demin, Xu Hualong. Method for obtaining communication modes between PLC programming port and host computer [J]. 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