Abstract: This paper proposes an anti-jamming design concept for an intelligent protection system for underground electrical equipment. From three aspects—suppressing interference sources, cutting off interference propagation paths, and improving the anti-jamming performance of sensitive components—a hardware anti-jamming design scheme is proposed. The concept and method of software anti-jamming design are also presented. Keywords : underground electrical equipment, intelligent protection, anti-jamming. 0 Introduction Intelligent protection devices play a crucial role in protecting the safe and reliable operation of electrical equipment such as motors in coal mines, thus requiring high reliability and stability. Because the controlled objects in intelligent protection systems have high power and high frequency, while the components and thresholds of protection devices are relatively low, coupled with the widespread use of power electronic devices, a large amount of "pollution" is caused to the power system. The interference signals generated by these devices pose a huge threat to the reliable operation of intelligent protection systems. They not only affect the accuracy of various analog voltage and current sampling data, but also damage some components in the device, causing device malfunctions and program deviations. If not handled in a timely and correct manner, the protection device may malfunction or fail to operate, or even cause major accidents. Therefore, improving the reliability and anti-interference performance of intelligent protection devices is of paramount importance. 1. The Impact of Interference on Intelligent Protection Systems 1.1 Sources of Interference Due to the harsh working environment in coal mines, there are strong interference sources. Interference signals may come from outside the protection device (such as connecting strips, switching switch contacts, operating relay contacts, etc.), and they may enter the protection device along various lines, or radiate to the protection device from space in the form of a field. Power supply lines are the main pathways for various surge voltages to enter the power grid. Poor system grounding is also a major cause of interference. Poor insulation of current and voltage transformers, input and output lines, etc., may also introduce interference. Interference in the form of fields mainly occurs near high voltage, high current, and high-frequency electromagnetic fields, coupling into the protection device through electrostatic induction, electromagnetic induction, etc. It may also originate from within the device (such as interference signals generated by the high-frequency clock control signal of a microcomputer on other circuits within the device). Internal interference is mainly determined by the device's structural design, the layout of components and certain circuits, and the manufacturing process. The main coupling methods of interference include electric field coupling, magnetic field coupling, electromagnetic coupling, and conductive coupling. 1.2 Hazards of Interference The impact of interference signals on analog output actuators may lead to malfunctions of the protection device; the impact of interference signals on digital microcomputer chips may cause errors in the transmission of computational data or errors in microprocessor opcodes, resulting in malfunctions or functional impairments in the microcomputer system. The impact of interference on intelligent protection devices is mainly manifested in several aspects, such as deviations of sampled values ​​from actual values, program derailment, damage to microcomputer chips, and computational or logical errors. 2 Basic Ideas of Interference Suppression Technology The anti-interference design of a system can comprehensively adopt a combination of hardware and software methods. The basic idea of ​​hardware anti-interference design is to suppress interference sources, cut off interference propagation paths, and improve the anti-interference performance of sensitive devices; software anti-interference design uses software methods such as digital filtering to suppress interference signals. The organic combination of the two can form a highly efficient and inexpensive anti-interference system. 3 Hardware Anti-interference Design Hardware measures are the first line of defense against interference, generally addressing both prevention and resistance. 3.1 Suppressing Interference Sources Suppressing interference sources means minimizing the du/dt and di/dt of the interference source as much as possible. Reducing the du/dt of the interference source is mainly achieved by connecting a capacitor in parallel across the interference source; reducing the di/dt of the interference source is achieved by connecting an inductor or resistor in series in the interference source circuit and adding a freewheeling diode. Additionally, the following three measures can be taken: adding a freewheeling diode to the relay coil to eliminate back electromotive force interference generated when the coil is disconnected; connecting a 0.01μF to 0.1μF high-frequency capacitor in parallel with each IC on the circuit board to reduce the influence of the power supply on the IC; and minimizing the length of printed lines and avoiding 90-degree bends in the wiring to reduce high-frequency interference emissions. 3.2 Cut off the interference propagation path Cutting off and blocking the coupling channel of interference is one of the effective ways to suppress interference. For interference coupled in the form of "path", the method of cutting off the transmission channel can be adopted; for interference coupled in the form of "field", the distance between the interference source and the sensitive device can be increased, the ground wire can be used to isolate them, and a shield can be added to the sensitive device. The structure of the microcomputer intelligent integrated device is shown in Figure 1. The specific measures taken are as follows: [align=center] Figure 1 Peripheral circuit of the microcontroller system[/align] (1) Analog input channel The intelligent protection device needs to collect analog quantities such as voltage and current of the primary system. In order to prevent the interference signal of the AC circuit from entering the logic part, the primary and secondary coils of the precision transformer are isolated by a shield layer to reduce the distributed capacitance between the primary and secondary. The primary shield layer is connected to the ground to provide a path for common mode interference. The secondary shield layer is connected to the system ground. Therefore, it has a high common mode rejection ratio and can effectively prevent interference in the power grid from entering the system. In addition, a second-order active low-pass filter with anti-frequency aliasing is used to absorb differential-mode surges, so that unwanted high-frequency interference on the transmission line is eliminated or weakened before being sent to the analog-to-digital converter. (2) The switching input channel uses shaping, optocouplers and other methods to limit or cut off interference paths. There is no electrical connection between the primary and secondary sides of the optocoupler. However, it should be noted that the isolated channels must use separate power supplies. (3) In order to prevent strong electromagnetic interference or power frequency voltage from being fed back into the protection system through the output channel, optocouplers are used to isolate electrical signals; the switching output is implemented through two NAND gates to improve the anti-interference capability of the device. The switching output channel circuit is shown in Figure 2. [align=center] Figure 2 Anti-interference circuit of switching output[/align] (4) Suppression of power supply system interference The actual power supply will cause coupling between various components and assemblies through the internal resistance of the power supply to form an interference source, sometimes even causing low-frequency oscillation. A large electrolytic capacitor and a high-frequency capacitor of 0.01μF to 0.047 pF are connected in parallel at the power input terminal, which can greatly reduce the common impedance of a certain signal spectrum and avoid coupling interference; in addition, a power filter is used to reduce interference from the power grid. (5) Measures to suppress interference at the communication port: Twisted pair communication transmission line is used to suppress common-mode interference from entering the protection device, and an appropriate ferrite ring is connected in series at the communication port to suppress transient disturbances. (6) Other measures: The crystal oscillator and the microcontroller pins are as close as possible, the clock area is isolated by ground wire, the crystal oscillator shell is grounded and fixed; the interference source and sensitive components are kept as far away as possible; the ground wires of the microcontroller and high-power devices are grounded separately; high-power devices are placed at the edge of the circuit board as much as possible to reduce mutual interference. The entire main circuit board of the intelligent protection device is surrounded by a metal box, and the metal box is grounded for shielding. This can eliminate and weaken the distributed capacitance between the electrostatic field and the signal line, and suppress the interference voltage generated by electrostatic induction. 3.3 Improve the anti-interference performance of sensitive devices. Minimize loop area during wiring to reduce induced interference; use thicker power and ground lines to reduce voltage drop and coupling noise; do not leave idle I/O pins of the microcontroller floating; add pull-up or pull-down resistors. Ground or connect other IC idle terminals to power without changing system logic; minimize the use of microcontroller crystal oscillators and select low-speed digital circuits; solder IC devices directly onto the circuit board whenever possible, minimizing the use of IC sockets. 4. Software anti-interference measures Intelligent protection devices are used in underground coal mines. Due to the harsh environment, high voltage, and large current, vacuum contactors frequently engage, causing stray currents in the ground wire to generate strong interference signals. When the interference signal reaches the CPU through the three-wire bus, the CPU cannot execute the program normally, causing chaos. Software anti-interference measures aim to promptly detect interference to the CPU, intercept uncontrolled program flow, and restore the system to normal operation. 4.1 Software anti-interference measures on I/O channels (1) Anti-interference for digital signal input Interference signals are generally very narrow pulses, while digital signals have a longer effective duration. Based on this characteristic, the same digital signal can be sampled multiple times consecutively, and the results of two or more consecutive samplings are considered valid. (2) Anti-interference for digital signal output When relays, contactors, and other devices controlled by digital output circuits operate, the generated arcs can cause strong interference signals, which may change the contents of the output register and cause malfunctions. The most effective software solution is to repeatedly output the same data to the external load. The repetition period should be as short as possible so that the external device does not have time to react to the interference signal before the correct output information is sent, thus preventing malfunctions. (3) Anti-interference for analog signal input For interference in the input channel that has not been completely eliminated by hardware, before the signal data is used, a point of data is sampled multiple times consecutively, and the average value is used as the sampling result for that point. This can reduce the impact of random interference on the sampling results. 4.2 Application of anti-interference of RST instruction in microcontroller The opcode of RST instruction in microcontroller is FF. When the program executes this instruction, it will initialize PSW to 0, PC to 2080, and I/O register to the initial value, and generate a negative pulse on the reset pin. In the software anti-interference design, the FF opcode is sent to the unused memory space. When the program "flies" to the unused program memory unit, it will automatically execute the RST instruction, thereby jumping to 2080 to start executing the program, which helps to repair software faults. 4.3 System reset measures (1) Manual reset Connect the reset circuit to the RESET terminal of the microcontroller. Both the power-on reset circuit and the manual reset circuit can provide a high-level reset signal of more than 1ms to the RESET terminal, so that the uncontrolled CPU is reset and the program automatically starts to execute from 0000H. (2) Program running monitoring system Set up a running monitoring program in the system, and provide independent protection for the CPU through the watchdog timer of chip X5045 and Vcc voltage monitor. When a system failure occurs, when the watchdog timer reaches its programmable timeout limit, or when the power supply voltage Vcc drops below its minimum value, the chip's RESET pin will automatically generate a reset signal immediately upon system power-on or power-off. This prevents system crashes, data write errors, and malfunctions caused by power-on/off or momentary power voltage instability. 4.4 Setting Software Traps Setting software traps in program design to catch "runaway" instruction pointers is a design technique for monitoring whether program execution is abnormal. These are typically placed in empty ROM areas, at the beginning or end of data tables, at unused interrupt vectors in the program, or after statements such as jump instructions, to intercept certain program anomalies. For programs that are confused due to interference, single-byte instructions can straighten out the confused PC pointer, controlling the confusion. The more NOP instructions added, the stronger the capture capability. The author's innovation is based on the research on the sources, propagation paths and reception methods of interference in the protection device of motors in coal mines. The author proposes the idea of ​​anti-interference design for the system and proposes hardware anti-interference design scheme from three aspects: suppressing interference sources, cutting off interference propagation paths and improving the anti-interference performance of sensitive components. The author also proposes the idea and method of software anti-interference design. After practical verification, the hardware and software anti-interference design scheme adopted in the system design has greatly improved the anti-interference performance of the system and made the system maintenance easier. References: [1] Xu Zhihong. Research on intelligent motor controller and protector [J]. Relay, 1998, 22-26. [2] Kong Hui. 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