Overview Distributed Control System (DCS) is a product of advancements in computer and automation technologies. As unit capacity and parameters continue to increase, vast opportunities for DCS applications have emerged. The widespread use of DCS systems provides strong support for the safe and economical operation of units. The control method of a unit comprehensively reflects its level of automation. Early thermal power plants had low levels of automation and low personnel skill levels, primarily employing a decentralized, localized operation method. Monitoring and operation were dispersed across the site. This control method was only suitable for low-parameter, small-capacity units, resulting in high labor intensity and poor safety. From the mid-1950s onwards, the development of local control entered a stage. Due to the common operation mode of the main control system for generating units, local centralized control methods such as boiler, turbine, and feedwater deaeration were adopted accordingly. In the 1960s, electronic computer technology was applied to the monitoring and control of thermal power plants, resulting in centralized control of the turbine, boiler, and electrical systems. In the early 1970s, programmable logic controllers (PLCs) were developed. In the 1980s, with the development of high technologies such as control technology, computer technology, communication technology, and CRT technology, distributed control systems (DCS) were developed abroad and gradually applied to thermal power plants. In the 1990s, due to the development of microprocessors and their very large-scale integrated circuits (VLSI) technology, DCS, along with computer technology, fault-tolerant technology, and human-machine interface technology, [further development occurred]. The emergence of window technology, interactive graphics, standardized data communication networks, and the development of artificial intelligence have propelled DCS towards integration and openness. DCS is widely used in China's power, petrochemical, and food industries. This is mainly due to its strong versatility, flexible system configuration, comprehensive control functions, convenient data processing, centralized display and operation, user-friendly human-machine interface, simple and standardized installation, convenient debugging, and reliable operation. With the development of China's power industry, DCS has become ubiquitous equipment, increasingly widely used in both large and small heating units. Correspondingly, the daily maintenance of this equipment requires thermal control personnel to change their traditional mindset and adapt to the needs of modern development. This paper describes several years of production maintenance experience. 1. Daily Maintenance Work 1.1 Process channel failures are most commonly caused by I/O card malfunctions. Common troubleshooting and handling of I/O card failures involves system diagnostics, channel replacement, or replacing with a spare part. However, damage caused by aging internal components or other reasons is generally difficult for thermal control personnel to diagnose. I/O card repairs are typically handled by the manufacturer, as current thermal control maintenance personnel lack the expertise to perform repairs like those for conventional instruments. Furthermore, manufacturers are increasingly producing integrated I/O cards, necessitating the purchase of spare parts. Fortunately, these failures are more frequent during the commissioning phase and occur less frequently during normal operation. Sometimes, primary component or control equipment failures are not directly detected by operators; thermal control personnel are only notified after an anomaly or alarm is triggered. This places higher demands on the skills of both maintenance and operation personnel. Operation personnel must provide detailed descriptions of the pre- and post-failure states to facilitate quick and accurate troubleshooting by thermal control maintenance personnel, minimizing the escalation of the problem. Additionally, many DCS manufacturers advertise hot-swap card replacements; therefore, control personnel must take strict safety precautions when replacing cards during operation to prevent system or load changes, especially with digital cards. 1.2 Reports of operator station crashes have been reported for both domestic and imported equipment. The causes are varied, including hard drive or card failures, excessive cooling fan load, and sometimes human error. These incidents are most likely to occur when modifying control logic, installing software, restarting the equipment, or forcing protection signals. The consequences can range from minor equipment malfunctions to severe equipment shutdowns. Restart times after a crash vary between manufacturers, ranging from tens of seconds to several minutes. Human error accounts for a significant proportion of unsafe incidents in thermal engineering and requires close attention to minimize human-caused failures. 1.3 Abnormal ball bearing operation is generally due to long-term operation, aging, contamination, unreliable continuity, or loose cable connections. These require replacement and inspection. 1.4 Control operation failure occurs when the ball bearing's operation signal fails to properly change the process channel state, causing operational inefficiency. This is caused by two factors: software defects and hardware malfunctions. For such defects, the usual practice is to check the process channel function and then check the operator station, performing a restart if necessary. 1.5 For membrane keyboards, malfunctions are mainly caused by poor keyboard contact, loose signal cables, erroneous keyboard operation by the host, or incomplete startup. Different situations require different solutions. 1.6 Printer malfunctions are generally due to configuration issues. Such faults should be addressed by checking the printer settings and ensuring the hardware is functioning correctly. Weak reporting software functionality manifests primarily as printer crashes caused by printing reports and SOEs, or discrepancies between the printer's SOE recording time and actual data; inability to return to historical curves after browsing SOE prints; and inconsistent SOE time sequences, sometimes with significant deviations. This can delay accident analysis and sometimes mislead the analysis direction. SOE problems are related to both unreasonable system design, where SOE points are not fully concentrated on a single DPU, and inadequate consideration in system hardware and software design. Analysis suggests that this type of fault mainly stems from imperfect overall considerations for the power plant. Insufficient attention to detail in smaller aspects can lead to various malfunctions. This situation requires serious attention; no detail should be overlooked. A thorough discussion with the manufacturer is necessary to identify and improve the system, ensuring it better serves production. 1.7 Power Failure: Power failures are a common problem, including inadequate fuse configuration, failure of backup power to automatically switch on, protection malfunctions caused by power fluctuations, and poor contact at connectors leading to no power. Troubleshooting power failures is relatively straightforward. First, carefully verify the fuse configuration and capacity to ensure they are functioning correctly. Second, the UPS (Uninterruptible Power Supply) is crucial; it must guarantee normal system power supply during power fluctuations, and redundancy and backup should be considered. 1.8 Interference-induced faults: Interference is mainly caused by grounding issues, switching of backup power supplies, and high-power wireless communication devices such as mobile phones and walkie-talkies. Additionally, interference signals within the DCS system itself may also be caused. Therefore, grounding issues in DCS systems are receiving increasing attention, especially in the power industry, where the starting and stopping of high-power electrical equipment can interfere with DCS control signals, causing unnecessary faults. To prevent interference signals from entering the system, strict adherence to shielding and grounding requirements and methods is essential. Signal lines should be kept away from interference sources, and measures should be taken to prevent power fluctuations. During unit operation, manual switching between master and slave process processors should be avoided as much as possible unless absolutely necessary to prevent interference. If switching is unavoidable, measures should be taken to first switch control to manual mode to avoid affecting the unit's operating conditions. High-power wireless communication equipment should be strictly prohibited in key areas such as electronic equipment rooms and engineering workstations. 2. Operation Management: DCS system operation management refers to system inspection, activation and deactivation of thermal protection, and supervision and management of DCS hardware and software. 2.1 Software backup management: Application software (database) should be backed up promptly, and even minor changes should be recorded. Modifications to the database should be saved to the engineering workstation and also to a floppy disk or other hard drive. However, backup disks should not be used beyond their expiration date to prevent data loss. 2.2 Software inspection and functional testing: Inspections should be conducted according to general computer equipment methods, primarily checking the settings of various levels of permissions: The use of non-DCS software is strictly prohibited; unauthorized personnel are strictly prohibited from configuration. 2.3 The activation and deactivation of thermal protection should strictly adhere to the work permit system. When repairing a single operating piece of equipment, proper isolation measures must be taken to prevent cascading failures in related equipment. To address the above common faults and avoid their occurrence, a strict maintenance, repair, and periodic inspection system should be established. DCS equipment inspection cards should be filled out carefully, and minor defects should be promptly identified and addressed to nip faults in the bud. Operation logs should be maintained to strengthen management.