Inverter to DCS Interference Handling Solution - Fuji Electric

The signal of the catalyst feed control valve of a company's DCS system (distributed control system) intermittently appears OOP (output signal line break) alarm, and the control valve is fully opened instantly, resulting in abnormal fluctuations in catalyst flow, which seriously affects the safe and stable operation of the device.

Fault cause analysis and treatment

1. Analysis of the cause of the failure

Since it was an OOP alarm, it was naturally suspected that the signal line was broken, and the signal line was checked, and the result was that the line was intact. First, it was judged that the DCS card was faulty, and the DCS card at the alarm point was replaced, but the fault was still not solved. At this point, we realized that there must be interference that was affecting the DCS signal. In order to reduce interference, the meter grounding was modified, but the fault remained unresolved. After checking the DCS signal point, only the signal line sent to the DCS by the inverter is suspicious. In order to confirm that the interference source is generated by the inverter, we pass the outlet cable of the inverter through the ferrite ring, and the signal line from the inverter to the DCS is as far away and cross as far away as possible from the input and output lines of the inverter. Through the above measures, the OOP alarm frequency at this point on the DCS has decreased to a certain extent. This confirms that the source of interference is coming from the frequency converter. The next step is to minimize the interference of the drive.

In order to eliminate the interference of the inverter on the DCS signal, we have taken the following measures: thicken the grounding wire and bring the grounding point as close to the inverter as possible; At the same time, the signal line of the inverter to the DCS is laid through the steel pipe on the outgoing side of the inverter, and the steel pipe is reliably grounded. Through the above measures, the frequency of OOP alarms in DCS is significantly reduced, but the inverter interference is still not completely eliminated. Could it be a malfunction of the inverter? For this purpose, we replaced the frequency converter and the interfering signal was eliminated immediately. Just when we thought it was a complete solution, the fault appeared a month later. So far, the fault point has not been replaced except for the signal line from the inverter to the DCS, and the other components have been replaced. As a result, the shielded signal cable was replaced, and the fault was completely resolved.

2. Measures to reduce the interference of the inverter to the DCS signal

The fault is due to the long distance (200m) from the inverter to the DCS, and the poor quality of the original shielded control line, which causes the high-frequency signal of the inverter to affect the nearby signal line through the control line to the DCS. At the same time, the following measures can better reduce the interference of the inverter:

(1) Reasonable wiring

It can weaken the strength of the interference signal to a considerable extent, and the control line of various equipment should be as far away as possible from the input and output lines of the inverter when wiring. In terms of space, the control line should be crossed with the input and output lines of the inverter as much as possible, and it is better to cross vertically.

(2) Weaken the source of interference

Access reactor or filter, for small power inverter, the cost is high, we use a low-cost electromagnetic interference suppression method: the motor cable passes through the ferrite ring, so that the impedance through part of the wire is partially increased, preventing the electromagnetic interference current from passing. If the wire is wound around the ferrite ring several times, the total inductance and impedance values will increase with the square of the number of turns. The motor cable can be passed through the ferrite ring three times. However, it should be noted that the grounding wire connecting the motor and the inverter should be left outside the ring.

(3) Shielding the line

The connecting line between the inverter and the motor should be penetrated into the metal pipe as much as possible, and the metal pipe should be grounded. Whether the signal line shield is connected to the common end or to the ground, it can only be carried out at one end, and must not be connected at both ends.

(4) Accurate grounding

The grounding wire should be as thick as possible, and the grounding point should be as close to the inverter as possible; The grounding wire should be as far away from the power cord as possible; The ground grounding wire of the inverter must be separated from the grounding wire of other equipment; It is necessary to avoid connecting the grounding wires of all equipment together and then grounding; The ground terminal of the inverter cannot be connected to the "neutral line" of the power supply.

The interference of the inverter will affect the operation of other equipment, so the anti-interference measures of the inverter must be done well to ensure the safe and reliable operation of the production system.

At the same time, the control valve is fully opened instantly, causing abnormal fluctuations in the flow rate of the catalyst, which seriously affects the safe and stable operation of the device.
Inverter to DCS Interference Handling Solution - Fuji Electric

Inverter to DCS Interference Handling Solution - Fuji Electric

Inverter to DCS Interference Handling Solution - Fuji Electric

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