Analysis and processing of frequency converter interference to DCS

1 Introduction

In August 2008, the catalyst feed regulating valve signal of a fluorine company's DCS system (distributed control system) intermittently gave an OOP (output signal line broken) alarm. At the same time, the regulating valve was fully opened instantly, causing abnormal fluctuations in the catalyst flow rate, seriously affecting the safe and stable operation of the device.

2 Fault Cause Analysis and Treatment

2.1 Failure Cause Analysis

Since the oop alarm appeared, it was naturally suspected that the signal line was broken. The signal line was checked and the line was found to be intact. First, it was determined that the DCS card was faulty. The DCS card at the alarm point was replaced, but the fault was still not solved. At this time, we realized that there must be interference, which affected the DCS signal. In order to reduce interference, the instrument grounding was modified, but the fault was still not solved. After checking the DCS signal point, only the signal line from the inverter to the DCS was the most suspicious. To confirm that the interference source was generated by the inverter, we passed the output cable of the inverter through the ferrite ring, and kept the signal line from the inverter to the DCS as far away from the input and output lines of the inverter as possible and crossed them. Through the above measures, the oop alarm frequency of this point on the DCS decreased to a certain extent. This confirms that the interference source comes from the inverter. Then the next measure is to minimize the interference of the inverter.

In order to eliminate the interference of the inverter to the DCS signal, we took the following measures: thicken the grounding wire and place the grounding point as close to the inverter as possible; at the same time, lay the signal line from the inverter to the DCS through a steel pipe on the inverter outlet side, and ground the steel pipe reliably. Through the above measures, the frequency of DCS OOP alarms has been significantly reduced, but the inverter interference has not been completely eliminated. Could it be a fault with the inverter? For this reason, we replaced the inverter, and the interference signal was immediately eliminated. Just when we thought the problem was completely solved, the fault appeared again a month later. So far, except for the signal line from the inverter to the DCS, all other components have been replaced. So, we replaced the shielded signal line, and the fault was completely solved.

2.2 Measures to reduce the interference of inverter on DCS signal

This fault is caused by the long distance between the inverter and the DCS (200m) and the poor quality of the original shielded control line, which causes the inverter high-frequency signal to pass through the control line to the DCS and affect the nearby signal lines. At the same time, the following measures can effectively reduce the interference of the inverter:

(1) Reasonable wiring

It can significantly reduce the strength of interference signals. When wiring, the control lines of various devices should be kept as far away from the input and output lines of the inverter as possible. In terms of space, the control lines should cross the input and output lines of the inverter as much as possible, preferably vertically.

(2) Weakening interference sources

Connecting a reactor or filter is costly for low-power inverters. We use a low-cost electromagnetic interference suppression method: pass the motor cable through the ferrite ring to increase the impedance of the wire passing through, thereby preventing the electromagnetic interference current from passing through. If the wire is wound around the ferrite ring several times, the total inductance and impedance value will increase with the square of the number of turns. The motor cable can pass 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) Shield the line

The connection line between the inverter and the motor should be inserted into a metal tube as much as possible, and the metal tube should be grounded. Whether the signal line shielding layer is connected to the common end or the ground, it can only be done at one end, and never 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 line as possible; the grounding wire used for the inverter must be separated from the grounding wires of other equipment; it must be absolutely avoided to connect the grounding wires of all equipment together before grounding; the grounding terminal of the inverter cannot be connected to the "neutral line" of the power supply.

3 Conclusion

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