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Analysis of Common Faults in Thermal Protection Systems and Countermeasures Abstract: This paper analyzes and summarizes the common faults in thermal protection systems, focusing on misoperations and failures of such protection systems. It cites and analyzes some classic cases, and proposes measures and countermeasures for analysis; these are of great value in improving the reliability of DCS systems and ensuring the safe, economical, and stable operation of power units. Keywords: Thermal protection, common faults, false operation, failure to operate, analysis, countermeasures. 1 Introduction The thermal control protection system is a very important and essential component of thermal power plants, playing a crucial role in enhancing the reliability and safety of the main and auxiliary equipment in such plants. As a new power plant that was brought online at the end of 2007, the thermal control and protection logic at Datang Gangu Power Plant is not yet fully developed; therefore, thermal control personnel need to continuously communicate, discuss, and improve it. When certain failures occur in the main and auxiliary equipment that could lead to serious consequences, appropriate measures are taken promptly to provide protection, thereby mitigating the impact of the failure, shutting down the equipment for repair, and preventing significant damage to the equipment or even accidents resulting in injuries or deaths. However, when the main and auxiliary equipment are operating normally, if the protection system activates due to its own faults, this leads to the shutdown of those equipment, and in some cases even to a complete power outage. This is known as protective malfunction, and it results in certain economic losses ； When the main and auxiliary equipment fail, the protection system also fails to function, a condition known as protection failure to operate; this leads to the inevitability and escalation of accidents. With the mature development of DCS control systems, the level of thermal automation has continued to increase. Thanks to its significant advantages, these systems have greatly enhanced the reliability, safety, and economic efficiency of plant operations. However, misoperations and failures of thermal protection still occur from time to time. How to prevent DCS system failures as well as incorrect or failed operation of thermal protection systems has become an issue of growing concern in thermal power plants. 2 Classification of common faults in thermal protection systems 1. DCS software and hardware faults ； 2. Failure of thermal control components ； 3. Short circuit, open circuit, or loose connection in the cable wiring ； 4. Power supply failure of thermal control equipment ； 5. Human factors ； 6. Defects exist in design, installation, and commissioning. 3 Analysis of Common Fault Causes in Thermal Protection Systems 3.1 Software and Hardware Failures in DCS Systems As DCS control systems have evolved, in order to ensure the safety and reliability of the units, various important process control stations have been incorporated into thermal protection systems (such as DEH, CCS, BMS, etc.). These stations include shutdown protection mechanisms in case both CPUs fail; as a result, protective actions are sometimes triggered erroneously due to software or hardware failures in the DCS system. The main reasons are faults in the signal processing card, output module, setpoint module, network communication, etc. As early as April 2008, our factory experienced a complete shutdown due to network communication issues – interruptions in data flow led to overload, and since no traffic limit protections were in place, this caused the network to become blocked and the equipment to lose control. 3.2 Faults in thermal control components: A significant proportion of cases involve improper operation or failure of the main and auxiliary equipment due to incorrect signals generated by faults in thermal control components (including those related to temperature, pressure, liquid level, flow rate, valve position, solenoid valves, etc.). In some power plants, such improper operation or failure of thermal protection systems caused by faults in thermal control components accounts for up to half of all such incidents. The main reasons are component aging and unreliable quality; the units operate individually, with no redundancy or identification. 3.3 Short circuits, open circuits, and poor connections in cable wiring: The main causes of protective device malfunction resulting from open circuits, short circuits, and poor connections in cable wiring are improper cable installation (such as using ordinary cables in high-temperature areas), damaged insulation due to aging, water ingress into the terminals, and corrosion caused by humid air. 3.4 Equipment power supply failure: As the automation level of thermal control systems increases, some process control stations in the thermal protection systems are equipped with shutdown protection mechanisms in response to power supply failures. The number of instances of incorrect or failed operation of thermal protection systems due to power supply failures in thermal control equipment is also on the rise. The main reasons are poor contact of the power connectors for the thermal control equipment and an unreliable design of the power supply system. 3.5 Human factors: Most protection malfunctions caused by human factors result from mistakes such as thermal workers entering the wrong bay, misidentifying terminal block connections, sending incorrect or missing forced signals, and improper use of multimeters. In March of this year, the thermal control personnel in our team were dealing with the issue of large fluctuations in the air volume of the mixed air supply for Mill B in Unit 1 during operation; sometimes this volume would suddenly drop to zero. They planned to force the setting for the air volume measurement point of Mill B’s mixed air supply and then purge the air ducts. However, they mistakenly applied this forced setting to the reference point following the Mill B mixed air supply volume, while the original measurement point was not affected by this setting. Moreover, it was precisely this reference point that was used for inter-network communication. As a result, Mill B stopped operating, causing a drop in load of over 50,000 units. 3.6 Defects in design, installation, and commissioning. Many units experience malfunctioning or failure of their thermal protection systems due to quality defects in the design, installation, and commissioning of the thermal control equipment. For example, on December 16, 1997, a serious accident of severe water shortage occurred in the boiler at Qinhuangdao Power Plant. Although there were various causes for this incident, the environmental temperature of the drum level transmitter played a role in causing measurement errors; the actual temperature was 130°C, whereas the design value for temperature compensation was 50°C. As a result, the level reading was inflated by 108 mm, which prevented the low-level protection system from activating ； In the event of a failure in the monitoring system for Pump A, which is used for boiler water circulation, no alternative measures were taken, resulting in a loss of protection. Due to the use of a three-out-of-three protection logic, when the boiler water circulation was disrupted, Pumps B and C stopped operating due to low differential pressure, while Pump A only issued an alarm for low differential pressure without stopping operation. This prevented the MFT from activating, ultimately leading to a serious accident involving the catastrophic rupture of large areas of the water wall. For another example, Shanghai Boiler Factory once led the steam and water sampling tube of the drum to a connecting vessel (referred to as a balance vessel), and then led a steam and water side sampling tube for the differential pressure level gauge from the middle section of this vessel, thereby reducing the range of the level gauge from 1270 mm to 860 mm. This design is clearly flawed, as it not only affects the accurate measurement of the drum water level but also impacts the reliable generation of low-water-level trip signals, potentially leading to accidents in the boiler. Boiler accidents caused by this reason have occurred in some power plants. This explicitly prohibits sampling using this method in the \"Twenty-Five Key Requirements for Preventing Major Accidents in Power Generation.\" In the construction and installation of some power plants, the furnace negative pressure sampling tubes are not installed in accordance with the design; due to an insufficient inclination angle or an installation that is horizontal and downward, these tubes become severely clogged with ash, making it impossible to take measurements, and as a result the furnace negative pressure protection system fails to function. 4 Measures and Countermeasures for Common Faults in the Thermal Protection System Since thermal control equipment monitors all parameters of the thermal system and thermal equipment, and these systems are not only interconnected but also interdependent, a fault in any one of them can trigger a shutdown signal via the thermal protection system, resulting in unnecessary economic losses. Therefore, how to improve the reliability of protection systems is a very important and urgent task. 4.1 Adopt redundant design as much as possible. Redundant design for the power supply and DPU in process control stations has become standard; in our plant, all DPU units employ a 1:1 redundancy design. The power supply for certain protection devices (such as trip solenoids) also needs to be monitored. Redundancy should also be provided for some important thermal signals, and effective monitoring and evaluation should be carried out on the signals from sensors that are sampled from the same source. The measurement channels for critical sensors should be located on different cards in order to reduce risks and improve reliability. For the on-site sampling holes at key measurement points, it is also advisable to use multiple independent sampling points as much as possible in order to improve reliability and facilitate fault resolution. A sampling method with multiple points in parallel requires consideration for improvement. For example, among the three flow measurement points for the feed water in this plant, two of them come from the same sampling point; as a result, when processing one of these measurement points, the protection mechanism related to the other point is activated, leading to the shutdown of the feed water pump. 4.2 Try to use thermally controlled components that are technologically mature and reliable. As the level of automation in thermal control increases, the requirements for the reliability of thermal control components also rise. Therefore, using mature and reliable thermal control components plays a vital role in enhancing the overall reliability of DCS systems. In line with the demands of thermal control automation, investments in thermal control equipment are continuously increasing; it is crucial not to sacrifice long-term benefits for the sake of saving short-term costs. With reasonable investment, it is essential to choose local thermal control equipment of good quality and performance. To improve the overall reliability of the DCS system as well as the reliability and security of the protection systems. 4.3 Optimize the protection logic configuration. Optimizing the configuration of protection logic is of great significance for improving the reliability and safety of protection systems, as well as reducing the rates of false operations and failures in thermal control protection systems. Considering the actual conditions of our plant, due to construction issues, the measurement points for the primary air and mixed air flow in the coal mill often experience fluctuations as a result of dust accumulation in the pipelines, which leads to the coal mill shutting down on multiple occasions. After logical optimization, by multiplying the primary air-mixed air pressure signal with the primary air-mixed air volume signal, such false activations were eliminated. 4.4 Improve the hardware quality of DCS and the self-diagnosis capabilities of its software. Striving to improve the quality of the software and hardware in DCS systems, as well as their self-diagnosis capabilities, plays a vital role in preventing failures in advance and mitigating their impact. 4.5 Strict quality control is applied to design, construction, commissioning, and maintenance. Improving the quality of design, construction, commissioning, and maintenance of thermal control equipment is of great long-term significance for enhancing the reliability of thermal control protection. 4.6 Strictly control the environmental conditions between electrons. Temperature, humidity, dust, and vibration have a significant impact on thermal control electronic devices. Strictly controlling the environmental conditions surrounding the electronics can extend the service life of thermal control equipment and improve the reliability of the system’s operation. In particular, the use of electronic communication devices must be strictly prohibited to prevent accidental signal transmission. 4.7 Improve and enhance the working environmental conditions of on-site thermal control equipment. The working environment of field equipment is generally very harsh; improving and enhancing the working conditions of such equipment plays a crucial role in increasing the reliability of the entire system. For example: Local equipment junction boxes should be kept as sealed as possible to prevent rain, moisture, and corrosion ； On-site equipment should be kept as far away as possible from heat sources, radiation, and interference ； Local equipment (such as transmitters, process switches, etc.) should be installed inside the instrument cabinet as much as possible; if necessary, measures such as anti-freezing heating should be applied to the sampling pipes and within the cabinet. 4.8 Strictly implement the regular maintenance system. Properly manage the maintenance of equipment for major and minor overhauls of the units, identify potential issues with the equipment in a timely manner, and keep the equipment in good working condition. Carry out regular maintenance and testing. During shutdown, the protection system is thoroughly inspected and overhauled, along with rigorous protection tests being conducted. 4.9 Strengthen technical training to improve the technical skills and fault handling capabilities of thermal control personnel. 5 Conclusion With the rapid development of the power industry and high-tech technologies, power generation equipment is becoming increasingly automated and intelligent, making the safety and reliability of such systems ever more important. However, no matter how advanced the equipment is, from a reliability perspective, it is absolutely impossible to achieve absolute reliability (i.e., no failures). Therefore, in a sense, being “faulty” is absolute. However, there is no inevitable connection between failures and accidents, and failures can be prevented. The key lies in detecting and identifying them as early as possible, and then preventing, mitigating, controlling, and eliminating them to avoid further escalation of the failure. Strive to achieve a 100% correct operation rate for thermal protection systems, thereby serving as the final safeguard for the safe operation of thermal equipment. This is the highest goal pursued by our designers, installers, debuggers, and maintenance personnel.