Hollysys DCS - the main factor of DCS signal interference

Resistive coupling interference

In the process of work, many factories ignore the problem of wire aging. When a variety of signal lines are transmitted together, other transmission signals will be affected due to the aging and leakage of the insulation material on the wire, and other transmission signals will be interfered with. In addition, not only will the aging of the signal line have an impact on other signal lines, but also some high-power circuits (such as heat generation circuits) will also have great interference with the signal if the line aging occurs during use.

Capacitive inductive coupling interference

Normally, the control cabinet at the controlled site is connected to many signal lines, some of these signal lines use cable troughs, some use cable pipes, especially many signals are routed at the same time, and there is a capacitance distribution between the signals, which will interfere with other signal lines. In particular, alternating magnetic flux is formed around the AC signal line, which in turn forms an electric potential between parallel conductors, causing potential interference signals.

Lightning strikes interfere

In the event of a lightning strike, large electromagnetic interference may be formed around the desired signal, and of course, the introduction of interference paths will also be caused by various grounding wires to a certain extent. There are two main ways of lightning interference: one is the overhead power line, which causes the signal line to be interfered by lightning strike; The other is that the signal cable is struck by lightning, resulting in the formation of capacitive and inductive coupling distribution on the signal line, and then forming a large interference, which damages the equipment and causes personal accidents in serious cases.

Interference with power supply lines

Due to the frequent start-up and shutdown of large equipment in power plants, the switching of large components is also frequent, and instantaneous large alternating magnetic fields are generated when these large electrical equipment are switched on and off. Alternating magnetic fields can interfere with the system by coupling on the signal lines, and these alternating magnetic fields can also cause high-frequency interference to the power lines, and the signal level can also affect the system if the signal level exceeds the specified range. In order to ensure the protection of the system when the large motor is switched, a transformer can be added to the circuit to ensure that the alternating magnetic field will not be generated due to excessive voltage change when switching the motor.

The main cause of interference?

Material and equipment issues

When the signal cable insulation material is aged and the shielding material is damaged, it will be interfered by other electromagnetic interference sources, and the interference will cause measurement errors, and serious interference will cause equipment damage.

The construction is not standardized

In a DCS control system, there are often many signals connected to the DCS at the same time. These signal lines either go through cable trays or cable pipes, resulting in many different types of cables attached to the same path on site. Each of these signals has distributed capacitances that can be used to add interference to other signal lines. At the same time, alternating magnetic fluxes are generated in the surrounding environment where the signal is transmitted, such as power lines, electric motors, generators, power transformers, and relays. These magnetic fields tend to cause large disturbances.

The grounding is unreasonable

A more common grounding accident is that both ends of the signal line are grounded, which will cause large interference due to the difference in ground potential. If both ends of the signal line are grounded at the same time, there may be a large potential difference, which may create a large circulation on the signal line between the two ends. Electromagnetic waves are generated, interfering with signals. In addition, the total grounding resistance of the DCS system does not meet the normal working requirements of the DCS system, etc.

How do you suppress interference?

Ensure that the signal cable is in normal use

In the process of using the signal line, avoid the entanglement of multiple signal lines, and at the same time regularly check the signal line, repair and replace the signal line with insulating fluid and damage, so as to ensure the normal use of the signal line and avoid the interference of leakage and transmission signal.

Lay the cables in layers

The cables that transmit different signals are isolated, and the cables are regulated in a layered way to ensure that the cables will not interact with each other. For the cable that transmits the same signal, the dense entanglement of the line should also be avoided as much as possible, because the computer port can carry out a certain degree of precise analysis of the signal when processing the signal, so if there are more similar signals to interfere with each other, it will have an impact on the judgment of the computer, which will lead to analysis errors in the process of computer work.

Signal line isolation

During the operation of the equipment, the signals can be isolated by cable isolation, so as to ensure that the signals will not affect each other. However, the isolation mode can not only isolate the transmission signal line, but also classify and isolate the signal on the computer side, and the use of this isolation mode can greatly reduce the harm caused by common mode interference to the computer. The isolation amplifier can also be used to completely isolate the input end of the signal from the computer, and the circuit between the blocking interference signals can be formed in this way, and the harm of interference can be suppressed to ensure the normal operation of the system.

The power supply system is isolated

In order to prevent high-frequency electromagnetic interference to the system, a transformer can be installed on the power supply line to restrain the high-frequency electromagnetic generated by the high-power motor switch to avoid affecting the system.

Twisting and shielding

The twisting of the signal wire, that is, the twisted pair instead of the parallel wire, can achieve the effect of suppressing the interference of the magnetic field.

Shielding is the use of metal conductors to insulate components, assemblies and signal lines. Shielding suppresses capacitive coupling noise. A common method is to connect the analog signal by shielding the twisted pair. Often, not only electrical noise interferes with the signal, but also with strong alternating magnetic fields. Therefore, it is necessary to consider not only electrical shielding, but also magnetic shielding, and conductors with superior magnetic permeability (such as iron, nickel, etc.) can be used to achieve shielding measures.

Do a good job of grounding

For the DCS grounding system, it mainly includes two functions: on the one hand, when the signal and power supply enter the DCS system or when it fails, it can bear the overload current and transmit it to the ground; On the other hand, the grounding system can provide a shield to the DCS, thereby isolating electronic noise interference, and can provide a reference zero potential to the entire system. Therefore, the DCS system usually contains two grounds: working and protecting.

Hollysys DCS - the main factor of DCS signal interference

Hollysys DCS - the main factor of DCS signal interference

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