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Common Problems in DCS Installation and Commissioning and Improvement Measures. Publication date: 2006-7-25. Author: Zhang Lie. Abstract: From the perspective of the development trends in thermal control technology, distributed control systems (DCS) will be used in newly built and upgraded large-scale systems in the future. The installation and commissioning of DCS start before power is supplied for plant use and continue throughout the unit’s commissioning process, playing a vital role in ensuring that the unit completes its 168-hour trial operation successfully, is handed over for production, and becomes a high-quality project. This article focuses on the common problems that arise during the installation and commissioning of DCS, as well as the corresponding solutions. I. Installation requirements for DCS 1.1 Environmental requirements for DCS When installing DCS equipment, the installation environment has a significant impact on its operational performance; therefore, great attention must be paid to the environment in which the DCS equipment is installed. The installation of DCS equipment must begin only after the interior finishing, fire protection systems, air conditioning, and other related installations at the installation site have been completed, the environment is clean, the temperature and humidity are appropriate, and air conditioning is available if necessary. This is to avoid potential issues in the equipment’s operation caused by temperature damage or dust accumulation. Large-powered electrical equipment should not be placed near computer systems, and high-power walkie-talkies, mobile phones, etc. should not be used in computer rooms; in other words, the intensity of the magnetic field in the environment must be kept below the minimum level permitted by the specifications for computer systems. During the commissioning of Unit 1 in the second phase of Huaneng Fuzhou Power Plant, signal fluctuations occurred due to the use of walkie-talkies in the electronic equipment room. 1.2 Requirements for DCS grounding: The grounding of the control system is intended to provide the entire system with a unified, common reference voltage level based on the earth as zero. In the event of a power supply or equipment failure, the overload current is absorbed by an effective grounding system and quickly conducted to the ground. It can also provide shielding for DCS devices to eliminate interference. Therefore, proper grounding is one of the keys to ensuring stable and safe operation of the control system. DCS has very strict requirements regarding grounding. Experience shows that during the commissioning and trial operation phases, many faults in thermal control systems are caused by improper grounding. In grounding systems, common problems include: (1) weak soldering or improper welding at the connections, resulting in incomplete soldering ； (2) The bolt connections become loose due to vibration ； (3) Poor contact at the connection point due to corrosion ； (4) The resistance of the grounding electrode increases, and the grounding electrode becomes disconnected from the power grid ； (5) The ground wire wiring is unreasonable. 1.3 Requirements for cable installation During the commissioning of Unit 1 in the first phase of Xinjiang Hongyanchi No.2 Power Generation Co., Ltd., it was found that the common terminals of the DCS cabinet had an elevated voltage level; in addition to the normal operating voltage, there was also an alternating current induced voltage of 80–180 V, which prevented some equipment from functioning properly, and even caused normally operating equipment to trip without any apparent reason. After careful analysis and inspection, it was found that the switch status line of the electric door experienced a very high induced voltage because it shared a single cable with the 220V operating power line. After consulting with various departments, the issue was finally resolved by placing the switch status wire for the electric door in a separate cable, apart from the power supply for operation. Practice has shown that electromagnetic interference has become an important factor affecting the proper display of instruments, as well as the correct operation of automatic switching and protection systems. Among the various methods for resisting electromagnetic interference, one effective measure is to install and lay cables strictly in accordance with the principle of separating strong-current and low-current cables. Additionally, the cable should be routed as far away from heat sources as possible to avoid damage to the cable. 1.4 Requirements for wiring connections and modifications: The work of connecting and modifying wires in a DCS is an important aspect in achieving automation of all equipment in the plant. To ensure the smooth progress of wiring connections and modifications, the following points must be noted: (1) All external wiring connections and modifications for the DCS are carried out by the construction party, while internal wiring connections and modifications are done by the DCS manufacturer ； (2) The construction unit shall notify the commissioning unit to remove the relevant modules before connecting or modifying the lines ； (3) For wiring that has already been connected, the construction party must obtain the approval of the commissioning party before making any changes. 1.5 Requirements for the power supply system: Some small systems often overlook this aspect; in fact, some control systems have only one power supply circuit. For example, the FSSS of Unit 1 at the Xinjiang Hongyanchi No. 2 Power Plant had only one power supply; it was only under strong insistence from the commissioning unit that the manufacturer made improvements to the power supply system. The most basic requirement of the thermal control system for the power supply system is that there must be two power supplies, with the ability to switch between them seamlessly. Only in this way can the normal operation of the control system be ensured. II. Main issues in DCS commissioning 2.1 Static commissioning of the DCS 2.1.1 The first step in the DCS commissioning process is to supply power to the DCS. If the power connection is not done carefully, it may cause damage to the modules, CPU, and even the main unit. Key points to note before receiving power: (1) Check the insulation of the power supply circuit ； (2) Is the system’s ground resistance meeting the requirements? ； (3) The power switches for each circuit should be in the off position ； (4) All functional modules should be in the removed position ； (5) The waveform of the power supply should be checked when necessary. Key points to note during power reception: (1) The sequence of powering on should start from the main power switch, followed by the cabinet power switch, and then proceed step by step to the modules and sub-modules ； (2) Power supply should be applied to each cabinet and each system separately; only after the power supply to one cabinet/system has been checked and normal operation is confirmed can power be supplied to the next cabinet/system ； (3) If any problems occur during the power reception process, the operation should be stopped immediately. The cause should be analyzed and identified, and the fault resolved before resuming power reception. 2.1.2 During debugging, module burnout is a common issue. Although there are many reasons for the burnout of modules, the main cause is the intrusion of high-voltage current from the external circuit due to wiring errors or other on-site issues. To prevent the module from being damaged, the following precautions should be observed: (1) Before testing the module’s I/O channels, first disconnect the external wiring of the module, release the power fuse of the module, then insert the module and reattach the power fuse. After the module testing is completed, the module must be released. (2) Before conducting the transmission test between the DCS and external devices, first check that the wiring from the terminal cabinet to the external circuits is correct. Use a multimeter to verify the grounding status of all back pins on the corresponding modules of this equipment, as well as the AC and DC voltages relative to ground; there should be no intrusion of high-voltage currents. After checking that there are no issues, insert the module for testing; once the testing is complete, remove the module. Repeat the above steps if another device on that module requires transmission. The module can only be placed in the inserted position after all devices on it have undergone transmission tests. (3) Modules that have not undergone transmission or system debugging should not be in the inserted position. (4) Try not to plug or unplug modules while they are powered on. (5) Strengthen coordination between installation and commissioning to avoid overlapping work on the same system. 2.2 Dynamic Commissioning of DCS 2.2.1 The Problem of Computer Crashes During the commissioning of Unit 1 at Guohua Zhungeer Power Generation Co., Ltd. and Unit 1 at Ningxia Guodian Shizuishan Power Generation Co., Ltd., crashes of the AP in the SIEMENS TXP system occurred. The main reasons are: (1) A large number of modifications were made during the debugging process, resulting in a significant amount of garbage accumulating within the AP ； (2) Some functional areas are improperly allocated, resulting in excessive computer load, etc. Therefore, during the dynamic commissioning of the unit, no modifications to the configuration should be made whenever possible; if modifications are indeed necessary, approval from the commissioning command center must be obtained before proceeding. The configuration changes are recorded and carried out by the DCS manufacturer. Additionally, since the TXP system generates some junk files during operation, system administrators are required to clean the system regularly. 2.2.2 Defect Handling During the 168-hour commissioning of Unit 1 at Huaneng Shang’an Power Plant, one member of the installation unit’s thermal control debugging team went to the site to handle an issue with one temperature sensor of the air supply fan; accidentally, he disconnected the wire connected to another, working temperature sensor, which caused the unit to trip. It is evident that defect handling during the unit’s operation must be handled with great caution. Before addressing a defect, a defect handling form should be filled out first; contact the operators and obtain approval before proceeding with the resolution. 2.2.3 Signal isolation: During debugging in many field applications, it has been found that isolators must be installed in the control circuits of all on-site electric control actuators for valve regulation. The main reason is that the power ground for the commands in the DCS is directly connected to the earth, whereas the ground wire of the electric control valve is not grounded; this power mismatch prevents the valve from operating properly. After installing the isolator, the power supply of the DCS becomes floating, matching that at the local site. Common examples include the fan impeller blades, the exhaust fan stator blades, and the primary fan guide vanes, all of which require the installation of isolators. 2.2.4 Logical issues (1) For signals indicating \"high/low water level in the steam drum\" and \"high/low furnace pressure\", an MFT signal should be generated, with a delay of 2–5 seconds being advisable. Its main purpose is to prevent the signal from being interfered with, and moreover, adding a short delay will not cause any harm to the unit. (2) In the MFT logic, an MFT trip is often triggered when both fire detection cooling fans fail to operate. There is also an interlock relationship between these two fans; the most basic principle is that if one fire detection cooling fan fails, the other fan will start automatically. However, if one fan trips, it can lead to a situation where both fans trip momentarily during the process of starting the other fan, and this may result in an MFT occurrence. Therefore, a delay needs to be added to the MFT signal generated when \"both flame detection cooling fans trip.\" The duration of this delay should be the shortest time sufficient to ensure proper interlocking between the two fans. Similar situations exist for devices such as sealed fans and air preheaters, which, if they trip, can affect the operational safety of other equipment; careful attention must be paid to the logic involved in these devices. (3) One of the main tasks of the thermal control team during the commissioning shift is to identify and analyze problems that occur after equipment trips. To quickly and accurately identify the source of the problem, a \"first-failure logic\" has been established for all critical auxiliary machines, primarily covering devices with numerous tripping conditions such as the MFY logic, ETS logic, feed pump tripping logic, and coal mill tripping logic. This will facilitate accident analysis and enable the unit to start up quickly. Additionally, once the issue is identified, it should be reported to the on-duty project manager as soon as possible. 2.2.5 Other common issues (1) Issues with the display of the drum water level. In many sites, it is observed that under rated operating conditions, there is a significant discrepancy between the drum water level shown by the DCS and that indicated by the electrical contacts, with a difference of 50–100 mm. The values displayed in the DCS, after pressure and temperature corrections, should be the most accurate. Inaccurate readings can be caused by the electrical contacts not being adjusted, as well as potential contamination of the electrodes that may lead to damage. (2) Communication between the PLC and DCS, between the bypass system and DCS, and between the DEH and DCS often experiences interruptions. This needs to be taken into account during on-site debugging. (3) During the commissioning of Unit 1 at Ningxia Guodian Shizuishan Power Generation Co., Ltd., power outages to the PLC control cabinet of the circulating water pump occurred frequently. After careful investigation, it was found that the common terminal of the PLC input signals is grounded. Once grounding occurs in the on-site cables or contacts, the fuse blows, resulting in a power loss for the entire PLC cabinet. Later, the live and ground wires of the input circuit power supply in the I/O cabinet were reversed, which prevented the fuses from blowing. (4) Issue of bypass system protection activation. Based on the experience from bypass systems in various power plants, due to issues such as improper design of piping and support systems, as well as inadequate design or operation of drain and HVAC pipelines, there have been numerous instances of severe vibration in the pipes or even damage to them when the bypass valves were opened quickly. Therefore, the quick-open interlocks adjacent to many power plants are in the disabled state. For example, during the commissioning of the bypass system of Unit 1 at Shizuishan Power Plant, the rapid opening of the high-pressure bypass caused a \"false\" high water level in the boiler drum, which led to an MFT. (5) Flame detection issue. The quality of the flame detector is crucial for ensuring that flame protection can be put into use. The flame detectors currently in use have several issues; primarily, there are serious problems with these detectors ‘seeing’ the flame incorrectly or failing to detect it at all, and the output signals of these detectors become unstable as the burning conditions change. As a result, some of the main protection systems of the boiler cannot function properly. To solve this problem, the flame detector must be continuously adjusted during debugging in order to achieve the most optimal results. For oil flame detectors, it is better to \"miss\" a flame rather than \"detect\" an unauthorized one; in other words, the stability of these detectors should be improved by reducing their sensitivity, so as to prevent fuel from being injected into the furnace when it has not ignited. For coal flame detectors, it is better to have them detect the flame even if there is some interference, rather than missing it; in other words, the sensitivity of these detectors should be increased appropriately, as the furnace temperature is usually high during coal powder injection, and the coal powder almost always catches fire. Additionally, the \"peek\" display logic of the coal flame detector can be slightly modified: the CRT will display that a flame is detected by the coal flame detector only when there is an \"AND\" operation between the signal indicating the operation of the coal feeder and the signal indicating that a flame has been detected by the detector’s probe; otherwise, it will indicate that no flame is present. It addresses the issue of \"peeking\" in the display aspect from a control logic perspective, thereby avoiding misjudgments by operators to some extent. The above methods have been successfully applied at Baoji No. 2 Power Plant, Huaneng Fuzhou Power Plant, Xuzhou Pengcheng Power Plant, and others.