1、 Overview of Distributed Control Systems (DCS)
DCS has the characteristics of strong universality, flexible system configuration, complete control functions, convenient data processing, centralized display and operation, friendly human-machine interface, simple and standardized installation, convenient debugging, and safe and reliable operation. It has a wide range of applications in production fields such as power, petroleum, chemical, metallurgy, and light industry, especially large power generation units, both domestically and internationally. Currently, the most widely used brands in China are:
(1) Foreign brands: ABB from the United States, Westinghouse, Siemens from Germany, Yokogawa from Japan, Hitachi, etc;
(2) Domestic: Guodian Zhishen, Helishi, Xinhua, etc.
The safety and reliability of DCS are crucial for ensuring the safe and stable operation of the unit. If problems occur, it may cause serious damage to the unit equipment and even personal safety accidents. So it is very necessary to analyze various problems that occur during the operation of DCS and take measures to improve the safety and reliability of DCS in thermal power plants.
2、 Fault situation of DCS during production process
Each manufacturer’s DCS has its own characteristics, so the analysis and handling of its fault phenomena are not the same. However, in summary, the obstacles caused by DCS to the unit can be divided into three categories:
(1) System issues, including design and installation defects, software and hardware failures, etc.
(2) Malfunctions caused by human factors, including personnel errors, inadequate management systems, and implementation of execution processes.
(3) DCS malfunction caused by external environmental issues in the system. Abnormal conditions are caused by factors such as high or low ambient temperature, humidity, dust, vibration, and small animals.
2.1 Examples of problems and malfunctions with DCS itself
This type of malfunction is more common in the production process, mainly including system design and installation defects, controller (DPU or CPU) crashes, network disconnection and other faults, operator station black screen, network communication blockage, software defects, low system configuration, and interface problems with other systems and equipment.
2.1.1 Power and grounding issues:
(1) The DCS power supply system of a certain power plant uses ABB Symphony III power supply, but during infrastructure construction, the cabinet installation is still carried out according to the grounding method of Type II power supply, which differs greatly from the grounding technology requirements of Type III power supply. Since the unit was put into operation, there have been multiple DCS module failures, signal jumps, and hardware burns, which are suspected to be related to the grounding system. Similarly, during the construction period of a certain power plant, there were problems with the design, production, and installation of the DCS grounding network. After the DCS system was operated, all temperature measurement points of the thermal resistance thermocouples experienced periodic fluctuations.
(2) A certain factory’s control system on the turbine side failed due to loose power connections.
Lesson learned: DCS does not have a good grounding system and reasonable cable shielding, which not only causes significant system interference, but also makes the control system prone to false signals and module damage. It can be seen that problems with UPS power supply and control system grounding will pose great risks to the safe and stable operation of DCS after the power plant is put into operation. Therefore, the power supply design of DCS system must have reliable backup means, and the load configuration should be reasonable and have a certain margin; The grounding of the DCS system must strictly comply with the technical requirements of the manufacturing plant (if there are no special instructions from the manufacturing plant, DLT774 regulations should be followed). All cables entering the DSC system control signal must use qualified shielded cables and be laid separately from the power cables with good single end grounding.
2.1.2 System configuration issues:
(1) Frequent failures and crashes of the DCS (T-ME/XP system) in a power plant in Zhejiang caused a unit shutdown accident. 7. 8 units (2 * 330MW) experienced a total of 22 DCS system failures and freezes from February 1997 to May, resulting in 8 abnormal trips of the units. Afterwards, there were multiple malfunctions in the operation screen (two times in Unit 8, all six operation stations experienced “black screens”), seriously threatening the safety of the unit. After analysis, it is believed that the DCS system has the following problems: (1) There are problems in the performance calculation software and switch redundancy configuration in the DCS engineering design. (2) Hardware configuration mismatch (including matching and communication issues between T-ME and T-XP systems). (3) Some hardware designs are incomplete. (4) Further analysis reveals that the critical CS275 (lower level T-ME) communication bus has a bottleneck problem due to its high load rate. Under the premise of reasonable configuration, European T-ME/XP system users have generally good usage of the T-ME/XP system.
(2) The DCS used in the automation transformation of the thermal control system of a 200MW unit in a certain power plant was found to have a load rate exceeding 90% and a soft manual operation response close to 1 minute, which was impossible to use due to inaccurate calculation of the load rate of the system configuration and the fact that the technical indicators were close to the allowable limit in order to reduce investment. Additionally, the system had a large number of virtual I/O points during operation, After significant adjustments (system re configuration), this issue was resolved.
(3) A 600MW unit in Northeast China had a low configuration made by the DCS manufacturer due to inadequate description of the isolation properties of the I/O channels in the bidding technical specifications. As a result, a large number of I/O boards were burned during debugging. Later, the isolation method and hardware were changed, and the power plant spent a lot of money, which also offset the original bidding price advantage. In addition, the quality and shielding issues of cables must also be highly valued, and computer specific shielded cables should be used for important signals and control. Many renovation projects have to re lay cables due to cable problems, which affects the construction period.