Abstract: This paper proposes a design scheme for a dual-power dual-fan intelligent protection control system based on a microcontroller. The composition and working principle of the system are briefly introduced, with a focus on analyzing the complementary control strategy and start-up control strategy. Field tests show that the system can accurately and reliably achieve automatic switching between the main and backup fans, and can reduce the impact of starting inrush current on the power supply. Keywords: Mine; Dual-power dual-fan; Microcomputer protection; Control strategy 0 Introduction Currently, in China's coal mine ventilation systems, dual-power dual-fan is a relatively efficient and safe fan configuration. Its automatic switching device is a key piece of equipment in the system, directly related to the safe operation of the entire system. Therefore, the effectiveness and reliability of the protection and control methods of the dual-power dual-fan automatic switching device are crucial to its safe operation. Currently, most operating dual-power dual-fan systems use relay control, which has limited functionality, poor reliability, and low control accuracy. Especially in the event of an accident, it cannot automatically take emergency measures, seriously affecting the safe operation of the equipment. Therefore, this paper proposes a novel dual-power dual-fan intelligent protection control system based on a microcontroller. This system utilizes CAN bus technology and an adaptive complementary control strategy to easily detect various operating parameters of the dual-power, dual-fan system. When a fan malfunctions or operates abnormally, it can take corresponding fault-handling measures in real time and issue warning messages. It can accurately and reliably achieve automatic switching between the main and backup fans; when one fan fails and stops, the other fan automatically starts, ensuring uninterrupted underground air supply. Sequential starting of multiple fans avoids damage to equipment caused by excessive starting current when multiple devices start simultaneously. 1. Composition of the Dual-Power, Dual-Fan Protection and Control System The structure of the dual-power, dual-fan protection and control system is shown in Figure 1. The system includes two protection and control systems: a master system and a slave system. The control core adopts a dual-CPU structure, with four functional modules: communication, LCD display, human-machine interface, and control and protection. An 8-bit AVR microcontroller acts as the master computer, responsible for LCD display, human-machine interaction, and CAN bus communication; a 16-bit DSPIC microcontroller acts as the slave computer, responsible for real-time data acquisition and processing, executing protection algorithms, and protecting and controlling the fans. This structure improves system real-time performance, clarifies CPU division of labor, and increases efficiency. Dual power supplies from the power grid independently power the master and slave protection control systems. The master and slave systems complement each other, ensuring uninterrupted operation of the air supply system. Simultaneously, the master and slave protection control systems each control the operation of two fans. Since the master and slave protection control systems are two independent yet complementary systems, each system must understand not only its own status but also the status of the complementary system. Therefore, communication between the master and slave control systems is necessary. Because the dual-power, dual-fan protection control system must strictly guarantee continuous air supply underground, the slave system must immediately start operating when the master system shuts down. CAN bus, as a software communication method, may fail to allow the complementary system to receive frames indicating the other's operating status immediately due to the complex and variable underground working environment or delays inherent in the software protocol. This is unacceptable from the perspective of the air supply system's reliability and continuity. Therefore, this system adopts a hardware-based complementary system communication method. This communication method uses one auxiliary relay on both the master and slave sides as a "handshake signal," as shown in Figure 2. Z-JZ-1 and Z-JZ-2 are normally closed contacts of the master auxiliary relay, and F-ZJ-1 and F-ZJ-2 are normally closed contacts of the slave auxiliary relay. Zflag and Fflag are system status detection signals. The master/slave auxiliary relays open and close in sync with the master/slave circuit breakers to notify each other of their current status. The system default detection signal is high level indicating that the master/slave is in the closed operating state, and low level indicating that the master/slave is in the open state. The advantages of this communication method are its simplicity and reliability. The fan control system can effectively detect the status of the complementary system in a short time, thereby determining the control strategy of this system. 2 System Complementary Control Strategy The working environment of the dual-power dual-fan protection control system requires reliable operation to strictly ensure the continuity of underground air supply. This requires that, regardless of whether all fans are in normal operation or some fans are in a faulty state, the control system must work with its complementary system to determine the optimal ventilation control strategy. Table 1 shows the complementary control strategies of the dual-power dual-fan protection control system under different states, which maximizes the utilization of the fans that are not faulty, ensuring the continuity of underground ventilation. In the table, master fault or slave fault includes the failure of any fan in the master or slave system, as well as the power outage of the master or slave system; a single-circuit fault in the master or slave system is assumed to be a fault in master 1 or slave 1. 2.1 Master Protection Control System Program Flow Generally speaking, the master protection control system, as a commonly used system for underground ventilation, receives external input system start commands and controls the entire complementary system to start operation. It ensures the implementation of the control strategy by controlling the Zflag signal change and detecting the Fflag signal. Figure 3 shows the program flowchart of the master protection control system. The auxiliary relay of the master protection control system opens and closes with the opening and closing of the main circuit breaker, and the Zflag signal change is controlled by a normally closed contact. A Zflag signal transition from low to high indicates the start of the main unit, and a transition from high to low indicates the stop of the main unit. When the main unit is running, various protection algorithms of the protection control system are activated to protect the operating fans from various faults. Once a fault is detected in the fan, the main circuit breaker is first disconnected to cut off the fan power supply, a fault alarm is issued, and fault information is uploaded; simultaneously, the auxiliary relay is disconnected, and the system enters the open standby state. When the main unit is in open standby, the protection control system monitors the Fflag signal status in real time. If the Fflag signal remains low for a certain period or transitions from high to low, the main protection control system performs a self-check. If no fault has occurred in the fans controlled by the system, or if not all of them have failed, the main protection control system immediately starts the fans that have not failed and enters the closed operation state. 2.2 Slave Protection Control System Program Flow The slave protection control system generally serves as a backup system for underground ventilation, receiving external start signals. It is not only activated as a backup system when a fan controlled by the main protection control system fails. The auxiliary relay of the slave protection control system opens and closes along with the main circuit breaker. A normally closed contact controls the Fflag signal; a low-level to high-level transition indicates slave startup, and a high-level to low-level transition indicates slave shutdown. The slave protection control system's program flow is similar to the master system's and will not be repeated here. 3 System Startup Control Strategy 3.1 Inrush Current Analysis During System Startup Based on the above analysis, a dual-power dual-fan protection control system can control two fans, which share one power supply. However, in actual field operations, there may be more than one ventilation duct, requiring multiple protection control systems to control more than two fans for ventilation. These master protection control systems may share one power supply, while their complementary slave protection control systems share another, resulting in multiple fans connected to a single power supply. Fans are induction motors, and their starting current inrush is significant, equal to the fan's stall current, approximately 5 to 7 times its rated current. Assuming N fan loads are connected to a single power source, each fan has a rated current of IN, if these N fan loads start simultaneously, they will generate an inrush current of N×(5~7)IN to the power source, which can easily cause low voltage in the power system. To prevent this from happening, certain measures must be taken regarding fan startup. Since the startup inrush current of a single fan has a relatively small impact on the power source, appropriate delay measures can be taken to start multiple fans sequentially, keeping the current impact on the power source during startup at a low level. 3.2 System Startup Control Strategy Analysis Taking a power supply system with one power source connected to four protection control systems and driving eight fans as an example, we analyze the control strategy when the power source 1 fails and the eight fans stop, and the eight fans controlled by the four complementary slave protection control systems immediately start up and maintain the underground air supply. Assuming that the rated current of each fan is IN, the functional relationship between the fan current and time is as follows: I=f(t)E(t) (1) Where: E(t) is the step function. The total current Isum on the power line is given by the formula: f[sub]i[/sub](t) represents the relationship between the current of the i-th fan and time, which is roughly the same as the relationship between the current and time of a typical AC motor; t[sub]i[/sub] represents the starting time of the i-th fan. Power systems are generally equipped with protection devices that automatically trip in the event of a short-circuit fault. The short-circuit protection setting for the fan, i.e., the induction motor, is generally set at more than 8 times its rated current. Therefore, when setting the short-circuit protection setting of the power system, the short-circuit protection threshold current is generally set to the total current generated when all 8 fans are operating at their rated current, plus the current generated when 1 fan experiences a short-circuit fault. Therefore, the short-circuit protection current threshold of the power system is set to (7+8×1)IN=15IN. The goal of the dual-power dual-fan protection control system startup control strategy is to adjust the startup time t[sub]1[/sub]～t[sub]8[/sub] of each fan so that it meets the condition at any time: Isum＜15IN (3) Since the startup process of the fan is generally short, and the above objective function involves 8 variables, it is difficult to solve. Therefore, the condition can be simplified. That is, it is assumed that when the i-th (i>2) fan receives the startup command, the i-1-th fan is still in the startup process, and the fan current f[sub]t-1[/sub](t)>IN. The i-2-th and earlier started fans can be assumed to be in the startup completed state, and the fan current can be directly replaced by IN. Therefore, the condition of the system startup control strategy can be changed to f[sub]i[/sub](0) equals the stall current of the fan. So equation (4) can be further simplified to since each condition is only related to 2 time parameters. This greatly simplifies the system control strategy. 3.3 Specific Implementation of System Startup Control Strategy The dual-power dual-fan intelligent protection control system implements the startup control strategy as follows: A time interval t_i-t_n is pre-determined, and the first fan is set. The first fan starts immediately upon receiving the startup signal. Simultaneously with the startup of any i-th fan, the system's internal clock begins counting. After a time interval t_i+1-t_i, a startup signal allowing the (i+1)-th fan to start is sent via the CAN bus. The (i+1)-th fan then starts immediately upon receiving this signal. This system uses time-based criteria to control fan startup, replacing the common method of using current-based criteria, due to the requirement for continuous underground air supply. If current-based criteria are used to control fan startup, the starting current is very large. At this time, the current transformer used for current detection may be in a non-optimal linear detection range, and the A/D conversion chip may also be at its maximum value due to excessive current. These factors will cause errors between the current result calculated by the algorithm within the microcontroller and the actual current result. This error will cause the process of using current criteria to determine the start-up conditions to take longer than the process of using time criteria to determine the start-up conditions, thus reducing the real-time performance of the system. 4 Conclusion This paper introduces the design of a dual-power dual-fan intelligent protection control system. The complementary control strategy and start-up control strategy adopted by the system can realize the timely switching of the master and slave, ensure the uninterrupted operation of the air supply system, and reduce the impact on the power supply caused by the simultaneous start-up of multiple fans. After on-site debugging and testing, the system has achieved good results, ensuring the continuity of underground air supply and greatly increasing the safety factor of the underground ventilation system. The next improvement direction of this intelligent protection control system is to adjust the control strategy in real time based on environmental changes (such as changes in air volume, gas concentration, etc.) using intelligent control technology, so that the system can better adapt to the requirements of the production environment and further improve the safety factor. References: [1] Liu Hongwen, Wang Hanqing. Modification method of dual-fan automatic conversion device [J]. 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