Metro environment and equipment monitoring system can effectively monitor and control the mechanical and electrical equipment of the station, such as ventilation and air conditioning system, water supply and drainage system, vertical elevator and escalator, low voltage distribution lighting and so on.
The Digital input-output interface of the controller in the environment and equipment monitoring system is easily affected by the AC induction current in series, which affects the normal operation of the whole system. This paper mainly analyses the causes of the AC induction current generated by the interface circuit of the controller in the environmental control system, and gives the concrete solutions and measures. Building Automation System (BAS) is a system which combines communication, computer, industrial control, software technology and network technology. It mainly monitors and controls the mechanical and electrical equipment of metro. Among them, the elevator is monitored but not controlled, and the wind valves and shielded doors of large and small fans are monitored and controlled. Unified and low-voltage distribution systems are monitored and controlled so that they can complete automatic management, ensure that equipment can be transferred to disaster operation mode in time and quickly under the condition of fire, flood, earthquake and other disasters or blocking accidents, and protect the safety of equipment and personnel in Metro stations. Therefore, the normal operation of BAS system for metro operation to It matters. But the BAS system can not control and monitor the fan and wind valve equipment in the actual operation process. The following will analyze the causes of its failure and give solutions. BAS system uses programmable logic controller (PLC) to monitor and control ventilation equipment, low-voltage distribution equipment, water supply and drainage equipment, etc. The I/O module of PLC collects on-site switching input and output signals through external wiring, and requires passive trunk contacts for input and output signals.
There is no power overlap between the control devices to ensure that AC220V or AC380V or other power systems can not be directly connected to the I/O module of the PLC. But in the actual operation process, when the fan air valve is not started, there is an AC 10-50V induction voltage in the input and output lines of PLC. As shown in Figure 1, the induction voltage will rise to AC 70V-120V after the start of the fan air valve and other equipment. PLC can not control and monitor the fan air valve and other equipment normally, and it is liable to produce false alarms and other errors. Work state, more serious will make the PLC input and output interface burned. At present, most of the environmental and equipment monitoring systems of metro companies adopt the same bridge structure with PLC control wire and power cable of fan and wind valve in the process of construction wiring. Although there is cover plate isolation in the middle of the bridge structure, the power cable pair is caused by too close distance and the control circuit of PLC fails to take effective shielding measures. The control circuit generates induction current, especially when the fan and wind valve are working, the current of power cable increases, which leads to the phenomenon of induction current increasing. At present, there is still no good solution to the influence of induced current on the interface of environmental control system controller in domestic metro industry. For the problem of AC and DC induced voltage interference caused by too close installation distance of strong and weak current, thermostatic element the parallel capacitor to ground in the feedback signal line can be used to solve the problem. The AC voltage can be restrained by the characteristic of capacitance separated by direct-to-ground, and the common terminal resistance of the feedback line can be connected to one end and the other end of the resistance can be grounded. This method can reduce the voltage of induction electricity, because capacitive reactance can make it have voltage dividing effect. At the same time, the capacitor has the function of charging and discharging. When the voltage waveform goes down, the capacitor will discharge. This method is a simple step-down measure, simple construction, low cost, but poor load, low safety, capacitors need to be replaced regularly. The piezoresistor is used to connect the input terminals of the PLC in parallel to form the protection of the interface circuit of the PLC, as shown in Figure 3. The above circuit uses varistor to protect the circuit. If the voltage of J2 input point is higher than the rated voltage of varistor RV, the RV resistance will decrease instantaneously. When the current exceeds the rated current of FU1, the fuse of FU1 protects the equipment connected by J1 from being damaged by the inductive AC connected in series. When the fuse of FU1 breaks, the D1 light emitting diode lights up, indicating that This interface voltage is too high, the fuse is disconnected, and the PLC input terminal has no input. Although this method can effectively protect the input endpoint of PLC, but the cost is expensive, it needs to be reconstructed twice in the PLC wiring cabinet to increase the isolation circuit board.
At present, DCS control mode is adopted in urban rail transit environment and equipment monitoring system, and digital input and output mode is adopted in the control and monitoring wiring mode of mechanical and electrical equipment.
If FCS system control mode is adopted and field equipment is controlled by intelligent device, not only can the price of system wiring be saved and the control structure of system be simplified, but also the communication between PLC and intelligent device is RS485. Shielded twisted pair cable is used for communication cable, which can greatly reduce the danger of induction current and AC power entering DC control system.
Through the analysis of the reasons for the induction current in the interface between the metro environment and the equipment monitoring system controller, this paper summarizes three feasible solutions to improve the reliability and safety of the metro environment and equipment monitoring system. Each of the three solutions has its advantages and disadvantages, and can be selected according to different environments and application occasions. Solutions.