A fluorination company somewhere reported that their DCS system (distributed control system) catalyst feed control valve signal intermittent OOP (output signal line disconnection) alarm, and the control valve was fully opened instantly, causing abnormal fluctuations in the catalyst flow rate, seriously affecting The device operates safely and stably.
Failure cause analysis and treatment
- 1. Failure cause analysis
Since an OOP alarm appears, it is natural to suspect that the signal line is broken. I checked the signal line and it turned out to be intact. First, it was determined that the DCS card was faulty and the DCS card at the alarm point was replaced, but the fault was not resolved. Only then did we realize that there must be interference that would affect the DCS signal. In order to reduce interference, the instrument grounding was modified, but the fault remained unresolved. After checking the DCS signal points, only the signal line from the frequency converter (hereinafter referred to as -VFD) to DCS is the most suspicious. In order to confirm that the interference source is generated by the VFD, we pass the VFD outlet cable through the ferrite ring, and cross the signal line from the VFD to the DCS with the input and output lines of the VFD as far as possible. Through the above measures, the frequency of OOP alarms on the DCS at this point has been reduced. This proves that the source of interference comes from the VFD. Then the next step is to minimize the interference of the VFD.
In order to eliminate the interference of the VFD on the DCS signal, we have taken the following measures: thickening the ground wire to make the ground point as close to the VFD as possible; at the same time, the signal line from the VFD to the DCS is laid through the steel pipe on the outlet side of the VFD, and the steel pipe is reliably grounded. Through the above measures, the frequency of OOP alarms in DCS has been significantly reduced, but VFD interference cannot be completely eliminated. Could it be that the VFD failed? For this reason, we changed to a VFD, and the interference signal was eliminated immediately. Just when we thought the problem was completely resolved, the problem occurred a month later. At this point, all components have been replaced except for the signal line from the VFD to the DCS. So, we replaced the shielded signal line and the fault was completely resolved.
- 2. Measures to reduce VFD interference on DCS signals
The fault was due to the long distance (200m) between /kloc-0 and DCS and the poor quality of the original shielded control line, which caused the VFD high-frequency signal to pass through the control line to DCS and affect nearby signal lines. At the same time, the following measures can better reduce VFD interference:
(1) Reasonable wiring
It can weaken the strength of interfering signals to a considerable extent. When wiring, keep the control lines of various devices as far away from the input and output lines of the VFD as possible. In space, the control lines should cross the input and output lines of the VFD as much as possible, preferably vertically.
(2) Weaken interference sources
The cost of connecting a reactor or filter for a low-power VFD will be very high, so we use a low-cost electromagnetic interference suppression method: passing the motor cable through a ferrite ring to locally increase the impedance of the passing wire to prevent electromagnetic interference Electric current passes through. If the wire is wound several turns around the ferrite ring, the total inductance and impedance values will increase as the square of the number of turns. The motor can pass through the ferrite ring three times. But it should be noted that the ground wire connecting the motor and VFD should be left outside the ring.
(3) Shielded lines
The connecting wire from the VFD to the motor should be inserted into the metal tube as much as possible, and the metal tube should be grounded. Whether the shielding layer of the signal line is connected to the common end or the ground, it can only be connected at one end, not both ends.
(4) Accurate grounding
The grounding wire should be as thick as possible and the grounding point should be as close to the VFD as possible; the grounding wire should be as far away from the power cord as possible; the grounding wire used by the VFD must be separated from the grounding wires of other equipment. Absolutely avoid connecting the ground wires of all equipment before grounding; the ground terminal of the VFD cannot be connected to the “neutral line” of the power supply.
The interference of VFD will affect the operation of other equipment, so anti-interference measures for VFD must be taken to ensure the safe and reliable operation of the production system.
A fluorination company somewhere reported that their DCS system (distributed control system) catalyst feed control valve signal intermittent OOP (output signal line disconnection) alarm, and the control valve was fully opened instantly, causing abnormal fluctuations in the catalyst flow rate, seriously affecting The device operates safely and stably.
Failure cause analysis and treatment
- 1. Failure cause analysis
Since an OOP alarm appears, it is natural to suspect that the signal line is broken. I checked the signal line and it turned out to be intact. First, it was determined that the DCS card was faulty and the DCS card at the alarm point was replaced, but the fault was not resolved. Only then did we realize that there must be interference that would affect the DCS signal. In order to reduce interference, the instrument grounding was modified, but the fault remained unresolved. After checking the DCS signal points, only the signal line from the frequency converter (hereinafter referred to as -VFD) to DCS is the most suspicious. In order to confirm that the interference source is generated by the VFD, we pass the VFD outlet cable through the ferrite ring, and cross the signal line from the VFD to the DCS with the input and output lines of the VFD as far as possible. Through the above measures, the frequency of OOP alarms on the DCS at this point has been reduced. This proves that the source of interference comes from the VFD. Then the next step is to minimize the interference of the VFD.
In order to eliminate the interference of the VFD on the DCS signal, we have taken the following measures: thickening the ground wire to make the ground point as close to the VFD as possible; at the same time, the signal line from the VFD to the DCS is laid through the steel pipe on the outlet side of the VFD, and the steel pipe is reliably grounded. Through the above measures, the frequency of OOP alarms in DCS has been significantly reduced, but VFD interference cannot be completely eliminated. Could it be that the VFD failed? For this reason, we changed to a VFD, and the interference signal was eliminated immediately. Just when we thought the problem was completely resolved, the problem occurred a month later. At this point, all components have been replaced except for the signal line from the VFD to the DCS. So, we replaced the shielded signal line and the fault was completely resolved.
- 2. Measures to reduce VFD interference on DCS signals
The fault was due to the long distance (200m) between /kloc-0 and DCS and the poor quality of the original shielded control line, which caused the VFD high-frequency signal to pass through the control line to DCS and affect nearby signal lines. At the same time, the following measures can better reduce VFD interference:
(1) Reasonable wiring
It can weaken the strength of interfering signals to a considerable extent. When wiring, keep the control lines of various devices as far away from the input and output lines of the VFD as possible. In space, the control lines should cross the input and output lines of the VFD as much as possible, preferably vertically.
(2) Weaken interference sources
The cost of connecting a reactor or filter for a low-power VFD will be very high, so we use a low-cost electromagnetic interference suppression method: passing the motor cable through a ferrite ring to locally increase the impedance of the passing wire to prevent electromagnetic interference Electric current passes through. If the wire is wound several turns around the ferrite ring, the total inductance and impedance values will increase as the square of the number of turns. The motor can pass through the ferrite ring three times. But it should be noted that the ground wire connecting the motor and VFD should be left outside the ring.
(3) Shielded lines
The connecting wire from the VFD to the motor should be inserted into the metal tube as much as possible, and the metal tube should be grounded. Whether the shielding layer of the signal line is connected to the common end or the ground, it can only be connected at one end, not both ends.
(4) Accurate grounding
The grounding wire should be as thick as possible and the grounding point should be as close to the VFD as possible; the grounding wire should be as far away from the power cord as possible; the grounding wire used by the VFD must be separated from the grounding wires of other equipment. Absolutely avoid connecting the ground wires of all equipment before grounding; the ground terminal of the VFD cannot be connected to the “neutral line” of the power supply.
The interference of VFD will affect the operation of other equipment, so anti-interference measures for VFD must be taken to ensure the safe and reliable operation of the production system.
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