Reliability measures of PLC control system in the

Reliability measures of PLC control system in power station

1 Characteristics of industrial environment of power station

there is a very strong electromagnetic field in the space of power station, the voltage of generator is as high as thousands of volts, the current is as high as hundreds of amperes or even thousands of amperes, and the output voltage of switching station is as high as tens of kilovolts or hundreds of kilovolts. Due to the limitations of site conditions (such as the transformation of old equipment), sometimes hundreds of meters of strong current cables and PLC signal cables cannot be effectively separated, and even can only be laid in the same cable trench. The strong electric interference generated during the connection and disconnection of high voltage and large current may produce strong induced voltage and induced current on the PLC input line, which is enough to make the light-emitting diode in the optocoupler at the PLC input end light up, make the anti-interference effect of the optocoupler invalid, and cause PLC malfunction. If the PLC in a hydropower station works normally when there is no generator in the station, and the generator often malfunctions after starting and running, it can be observed that the LED at the PLC input point sometimes flashes when there is no input signal

in addition to invading PLC through the input end of PLC, the interference signal may also invade PLC through the power supply of PLC

different from the general industrial environment, the relay and actuator (such as circuit breaker, contactor and solenoid valve) in the relay control system of the power station use DC 220V power supply, which should be fully considered when designing the output circuit of PLC

2 anti interference isolation measures

plc uses optocouplers, small relays in output modules, optoelectronic thyristors and other devices to realize the isolation of external switching signals. The analog i/o module of PLC generally also adopts optoelectronic coupling isolation measures. In addition to reducing or eliminating the impact of external interference on the system, these devices can also protect the CPU module from the harm of high voltage flowing into the PLC from the outside, because it is generally unnecessary to set interference isolation devices outside the PLC

if the optocoupler at the PLC input cannot effectively resist interference, a small relay can be used to isolate the switching value signal introduced into the PLC input with a long line in the power station. The working current of the light-emitting diode in the optocoupler is only a few milliamps, while the coil pull-in voltage of the small relay is tens of milliamps. The energy generated by the strong electric interference signal through electromagnetic induction is generally impossible to pull in the relay for isolation. Some systems need to use multiple pairs of contacts of external signals. For example, a pair of contacts is used to provide input signals to PLC, a pair of contacts is used to provide switching signals to the upper computer, and a pair of contacts is used for indicator lights. Using relays to transfer input signals can not only provide multiple pairs of contacts, but also realize the isolation of strong current interference signals. The input signal from PLC in the switch cabinet and the input signal not far from the switch cabinet generally do not need to be isolated by relay

in order to improve the anti-interference ability, the external signal of PLC and the serial communication line between PLC and computer can also be isolated by optical fiber or the communication interface with optocoupler. This method is more suitable for fire-proof and explosion-proof environments

3 treatment of power supply

power supply is one of the main ways for interference to enter PLC. Pressure adaptive oil source such as power supply can improve the efficiency by about one time compared with constant voltage source. Interference is mainly generated through the impedance coupling of power supply lines, and various high-power power power consumption and power generation equipment are the main interference sources

if PLC uses AC power supply, in the case of strong interference or high reliability requirements, an isolation transformer with shielding layer and low-pass filter can be added to the AC power input end of PLC. The isolation transformer can suppress the external interference from the power line and improve the capability of high-frequency common mode interference. The shielding layer should be reliably grounded

the low-pass filter can absorb most of the "burrs" in the power supply. For example, L1 and L2 are used to suppress the high-frequency differential mode voltage, L3 and L4 are used to indicate that the wires with equal length are wound reversely on the same magnetic ring, and the magnetic flux generated by 50Hz power frequency current in the magnetic ring offsets each other, so the magnetic ring will not be saturated. The magnetic flux generated by the common mode interference current in the two wires in the magnetic ring is superimposed, and the common mode interference is blocked by L3 and L4. C1 and C2 are used to filter common mode interference voltage, and C3 is used to filter differential mode interference voltage. R refers to the varistor, whose breakdown voltage is slightly higher than the maximum voltage when the power supply works normally, which is usually equivalent to an open circuit. In case of peak interference pulse, it will be broken down, and the interference voltage will be clamped by the varistor, whose terminal voltage is equal to its breakdown voltage

the high-frequency interference signal is not coupled through the winding of the transformer, but transmitted through the distributed capacitance between the primary and secondary windings. Winding a shielding layer between the primary and secondary windings and grounding it together with the iron core can reduce the distributed capacitance between windings and improve the ability to resist high-frequency interference. No matter how much is said, it is just a piece of paper

power filter products can also be directly selected. For example, the power filter of Beijing Zhongshi company has good common mode filtering, differential mode filtering performance and high-frequency interference suppression performance, and can effectively suppress the interference between lines and between lines and ground. Its products can be used for AC single-phase, three-phase power supply and DC, and the experimental report can also be programmed and printed according to the format required by users; Power supply

in the power system, if the 220V DC power supply (battery) is used to supply power to the PLC, the interference from the AC power supply can be significantly reduced, and the normal operation of the PLC can be guaranteed when the AC power supply disappears. In the power input terminal of some PLCs (such as FX series PLC of Mitsubishi Corporation), there is a diode rectifier bridge that directly rectifies 220V AC power supply, and the rectified and filtered DC voltage is sent to the switching power supply in the PLC. The input voltage range of switching power supply is very wide. This kind of PLC can also use 220V DC power supply. When using AC power supply, each diode of the rectifier bridge only bears half of the load current, and when using DC power supply, two diodes bear all the load current. Considering that the power input current of PLC is very small, the rectifier diode generally has a large margin in the design. If this kind of PLC uses DC 220V power supply voltage, there will be no problem. After long-term industrial operation, the above scheme is proved to be feasible

4 reliability measures of PLC output terminal

the contact operating voltage range of relay output module is wide, and the conduction voltage drop is small. Compared with transistor type and bidirectional thyristor type modules, it has a strong ability to withstand transient overvoltage and overcurrent, but the action speed is slow. When the output of the system does not change very frequently, the relay type output module is generally selected. The contact of the third small relay in the PLC output module is very small, and the arc breaking ability is very poor, so it cannot be directly used in the DC 220V circuit of the power station