If the DCS control system is not maintained at ordinary times, it will easily affect the stability of performance and greatly reduce its service life. Therefore, it is usually necessary to improve the system margin, diagnostic software, test system load, and so on, to prevent DCS from malfunctioning during operation. So what problems do DCS have during the operation?
Communication network failure
Communication network faults generally occur at the contact bus, local bus, or due to incorrect address identification.
Node bus failure
The transmission medium of the node bus is generally a coaxial cable, and some use a token signal transmission method, and some use a multi-channel transmission and contention bus signal transmission method with collision detection. Regardless of the method used, any interruption in the trunk of the bus can cause communication failures of all stations and their sub-devices on the bus.
At present, the general method to prevent such failures is to use a dual redundant configuration to avoid the impact of a bus failure and affect the overall situation, but this does not fundamentally avoid the occurrence of failures, and once a bus failure, deal with It is very easy to cause another bus fault and the consequences are very serious. An effective method should be to prevent bad bus contact or open hand.
One of the more successful system node bus layouts. The connection of the coaxial cable is not in front of the communication module, but behind the module, so that when dealing with the failure of the communication module during system operation, it can avoid accidentally touching the coaxial cable and causing the network cable to be disconnected. At the same time, in addition to its special inspection, the coaxial cable will not be touched at any time, which can prevent loosening of the plug of the coaxial cable and increase the possibility of failure. In addition, a coaxial cable inspection and replacement management system should be established to replace or treat the contact resistance before its contact resistance increases to affect communication.
In-place bus failure
A local bus or field bus generally refers to a data communications network consisting of twisted pairs. Because the connected equipment is a primary component or control device that is directly related to the production process, the working environment is harsh, the failure rate is high, and it is susceptible to misoperation by maintenance personnel and affects the production process. In addition, the bus itself can cause communication failures for various reasons. An effective way to prevent such failures is to properly handle the connection between the local bus and the local equipment. When disassembling the equipment, it must not affect the normal operation of the bus. The bus branch should be installed in a place that is not easily touched. At the same time, the local bus is best to use dual redundant configuration to improve the reliability of communications.
Address identification error
Whether it is an in-place component or a bus interface, once its address is identified incorrectly, it will inevitably cause disruption of the communication network. Therefore, it is necessary to prevent the failure of the address identification of each component and prevent human misoperation or mistaken modification. When the system is expanded, it should generally be performed when the system is stopped. In particular, systems using token-based communication methods, any increase or decrease in the work of components must be released to the network when the system is out of service, so as to avoid unpredictable consequences.
hardware malfunction
According to the different functions of each hardware, the DCS system can be divided into man-machine interface faults and process channel faults. The human-machine interface mainly refers to the engineer station, operator station, printer, keyboard, mouse, etc. used to realize the human-machine contact function; the process channel mainly refers to the local bus, I/O channel, process processor, primary component or control device, etc. . The human-machine interface is composed of multiple workstations with the same function. When one of them fails, as long as it is processed in time, the monitoring operation of the system will not be affected. When the process channel fault occurs on the local bus or primary device, it will directly affect the control or detection function, and the consequences are serious.
Human interface failure
The failure of the human-machine interface is often caused by failure of the ball marking operation, failure of the control operation, crash of the operator station, abnormal function of the membrane keyboard, and failure of the printer.
The abnormal operation of the ball mark is generally due to the aging or contamination of the internal mechanical device for a long time, so that the contact cannot be reliably turned on or off, or the cable is not firmly connected with the host, so that it is only necessary to replace the check.
The failure of the control operation is due to the ball flag's operation signal can not change the state of the process channel. On the one hand, the process channel hardware itself may be faulty. On the other hand, the software defect of the operator station itself may be caused. When the equipment overload or the open process window is excessive, Causes no response. After the function of the inspection process is normal, the operator station should be checked, restarted if necessary, and the operator station initialized.
There are many reasons why the operator's station crashes. This may be due to a failure of the hard disk or card, a defect in the software itself, or a failure of the cooling fan to cause the host to overheat, or the load is too heavy. Can first check the temperature of the host itself, followed by an alternative method to check the hard disk, the host card, etc. to determine the fault part.
Membrane keyboards are used on most operator stations. Its main function is to quickly retrieve process graphics, allowing operators to quickly monitor process parameters. When the membrane keyboard is incorrectly configured, the keyboard is in poor contact, the signal cable is loose, or the keyboard is misstarted when the host starts up, the startup may be incomplete. This may cause the function to be abnormal and should be handled according to different situations.
The printer is not working due to the configuration. At the same time, after the printer is screened, the printing function will also be disabled. In addition, the hardware failure of the printer itself may cause some or all of the functions of the printer to be abnormal, and the settings and hardware of the printer should be re-checked and processed.
Process channel failure
Most of the process channels have I/O card faults or local bus faults. One reason is that the I/O card itself is working for a long time and the components are aging or damaged. In addition, due to the external signal grounding or the strong electrical signal degrading the card, it can also cause channel failure. Nowadays, generally, the card itself adopts good isolation measures. Under normal circumstances, it will not cause the expansion of the fault. However, once such a fault occurs, it will directly cause the abnormality of the process control or monitoring function. Therefore, it is necessary to promptly identify the cause of the failure and promptly replace the card.
The failure of a primary component or control device can sometimes not be directly detected by the operator. Only when the parameters are abnormal or the alarm occurs, attention is paid.
Failure to control the processor (process handler) generally generates an alarm immediately, causing the operator's attention. Now the control processor basically adopts a 1:1 redundant configuration, in which one of the failures will not cause serious consequences, but the failed machine should be handled immediately. In the process of processing, you must not mishandle the normal processor, otherwise serious consequences will occur.
Human failure
In the course of maintenance or troubleshooting of the DCS system, human misoperation sometimes occurs, which can happen to those who frequently perform system maintenance or newly participate in system maintenance. Generally, misoperation events are most likely to occur when modifying control logic, downloading software, restarting a device, or forcing a device or protecting a signal. At worst, some measuring points and equipment abnormalities are caused, while the unit or major auxiliary equipments are suspended. The consequences are very serious. In power plants that use DCS, faults caused by human error operations account for a large proportion of unsafe incidents in the thermal engineering profession.
electricity failure
There are also many problems in the power supply. For example, the standby power supply cannot be self-injected, the insurance configuration is unreasonable, and the DPU power supply internal failure causes the power supply to be interrupted. The fluctuation of the regulated power supply causes the protection to malfunction and the bad contact of the plug leads to no output of the regulated power supply; The whole system of the system has all the input signals or all the external power supplies for the whole cabinet through insurance. The control power supply is neither connected to the UPS nor redundant.
SOE is not working properly
The conclusion of SOE plays an important role in the analysis and judgment of accidents. However, in reality, SOE is not recorded or the recording time does not conform to the actual situation when many power plants generate protection actions. For example, if the SOE event sequence recollection time of the ×× power plant #1 unit does not correspond to the actual trip time, the SOE time cannot be returned after printing and browsing. The first trip reason cannot be reflected first in the event sequence, and the SOE time sequence data cannot be set. problem. Some power plants in the analysis of several accidents found that the timing of the SOE conclusions and the historical curve in the timing of deviation, sometimes even reversed timing, specifically in the same point in the historical curve and SOE occurred inconsistent time, and sometimes large deviations This will delay the process of accident analysis and sometimes mislead the direction of accident analysis. The SOE problem is related to the unreasonable design of the system, the fact that the SOE point is not completely centralized on one DPU, and also related to the poor consideration of the system hardware and software design.
Interference caused by failure
There are also many examples of interference caused by DCS failures. D-channel system interference signals may come from the system itself, it may also come from the external environment.
Because different DCS systems have strict rules for grounding, once the grounding resistor or grounding method fails to meet the requirements, the efficiency of network communication will be reduced or the possibility of bit error will be increased, causing some of the functions to be abnormal. Network yo.
Power quality also affects the stable operation of the system. The power supply for the DCS system must ensure the stability of the voltage, but also ensure that when one-way power supply fails, it can be switched to the other power supply without interference, otherwise it will interfere with the system operation.
Switching between the main/standby processors of the process control processor can also sometimes cause interference.
In addition, high-power radio communication equipment such as mobile phones and walkie-talkies can easily cause interference and endanger the operation of the system.
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