1C31194G03 空间的辐射电磁场（EMI）主要是由电力网络

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1C31194G03 空间的辐射电磁场（EMI）主要是由电力网络

空间的辐射电磁场（EMI）主要是由电力网络、电气设备的暂态过程、雷电、无线电广播、电视、雷达、高频感应加热设备等产生的，通常称为辐射干扰，其分布极为复杂。若PLC系统置于所射频场内，就回收到辐射干扰，其影响主要通过两条路径：一是直接对PLC内部的辐射，由电路感应产生干扰；而是对PLC通信内网络的辐射，由通信线路的感应引入干扰。辐射干扰与现场设备布置及设备所产生的电磁场大小，特别是频率有关，一般通过设置屏蔽电缆和PLC局部屏蔽及高压泄放元件进行保护。

2.2.2来自系统外引线的干扰

　　主要通过电源和信号线引入，通常称为传导干扰。这种干扰在我国工业现场较严重。

　　（1）来自电源的干扰

　　实践证明，因电源引入的干扰造成PLC控制系统故障的情况很多，笔者在某工程调试中遇到过，后更换隔离性能更高的PLC电源，问题才得到解决。

PLC系统的正常供电电源均由电网供电。由于电网覆盖范围广，它将受到所有空间电磁干扰而在线路上感应电压和电路。尤其是电网内部的变化，入开关操作浪涌、大型电力设备起停、交直流传动装置引起的谐波、电网短路暂态冲击等，都通过输电线路传到电源原边。PLC电源通常采用隔离电源，但其机构及制造工艺因素使其隔离性并不理想。实际上，由于分布参数特别是分布电容的存在，隔离是不可能的。

　　（2）来自信号线引入的干扰

　　与PLC控制系统连接的各类信号传输线，除了传输有效的各类信息之外，总会有外部干扰信号侵入。此干扰主要有两种途径：一是通过变送器供电电源或共用信号仪表的供电电源串入的电网干扰，这往往被忽视；二是信号线受空间电磁辐射感应的干扰，即信号线上的外部感应干扰，这是很严重的。由信号引入干扰会引起I/O信号工作异常和测量精度大大降低，严重时将引起元器件损伤。对于隔离性能差的系统，还将导致信号间互相干扰，引起共地系统总线回流，造成逻辑数据变化、误动和死机。PLC控制系统因信号引入干扰造成I/O模件损坏数相当严重，由此引起系统故障的情况也很多。

1C31194G03 空间的辐射电磁场（EMI）主要是由电力网络

The radiation electromagnetic field (EMI) in space is mainly generated by power networks, transient processes of electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., usually called radiation interference, and its distribution is extremely complex. If the PLC system is placed in the RF field, the radiation interference is recovered, and its impact is mainly through two paths: one is the direct radiation inside the PLC, which is interfered by the circuit induction; It is the radiation to the network within the PLC communication, and interference is introduced by the induction of the communication line. Radiation interference is related to the field equipment layout and the size of the electromagnetic field generated by the equipment, especially the frequency, and is generally protected by setting shielded cables and PLC local shielding and high voltage release elements.

2.2.2 Interference from external leads of the system

It is mainly introduced through power and signal lines and is often referred to as conducted interference. This kind of interference is more serious in the industrial field of our country.

(1) Interference from the power supply

Practice has proved that the interference caused by the power supply caused a lot of PLC control system failure, the author encountered in a project debugging, after replacing the PLC power supply with higher isolation performance, the problem was solved.

The normal power supply of the PLC system is powered by the power grid. Due to the wide coverage of the grid, it will be subject to all space electromagnetic interference and induced voltage and circuit on the line. In particular, the changes in the power grid, the incoming switch operation surge, the start and stop of large power equipment, the harmonics caused by AC and DC transmission devices, the short-circuit transient impact of the power grid, etc., are transmitted to the original side of the power supply through the transmission line. PLC power supply usually uses isolated power supply, but its mechanism and manufacturing process factors make its isolation is not ideal. In practice, isolation is impossible due to the presence of distributed parameters, especially distributed capacitors.

(2) Interference introduced from signal lines

All kinds of signal transmission lines connected with the PLC control system, in addition to the effective transmission of all kinds of information, there will always be external interference signal intrusion. There are two main ways of this interference: one is through the transmitter power supply or the power supply of the shared signal instrument into the grid interference, which is often ignored; Second, the signal line is interfered by electromagnetic radiation induction in space, that is, the external induction interference on the signal line, which is very serious. The interference introduced by the signal will cause the abnormal work of the I/O signal and the measurement accuracy will be greatly reduced, and the components will be damaged in serious cases. For the system with poor isolation performance, the signals will interfere with each other, causing the bus backflow of the common ground system, resulting in logical data changes, misoperation and crash. PLC control system due to signal interference caused by I/O module damage is quite serious, resulting in a lot of system failures.