Common problems and improvement measures during DCS installation and debugging

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Common problems and improvement measures during DCS installation and debugging. Judging from the development trend of thermal control technology, decentralized control systems (DCS) will be used in newly built and renovated large machines in the future. The installation and debugging work of DCS starts before the factory receives power and ends when the unit is handed over for production. It runs through the whole debugging work of the unit. It plays a very important role in ensuring that the unit completes the 168-hour trial operation and handover to production with high quality and strives to create a quality project. This article focuses on the common problems and improvement measures during the installation and debugging of DCS. 1. DCS installation requirements 1.1 DCS environmental requirements When installing DCS equipment, the installation environment has a great impact on the operating status of DCS, so great attention must be paid to the environment in which DCS equipment is installed. The installation of DCS equipment must be completed after the indoor decoration, fire protection, air conditioning and other installation work at the installation location have been completed, the environment is clean, the temperature and humidity are suitable, and air conditioning can be put into use when necessary, to avoid future equipment operation risks caused by temperature damage and dust and dirt. Large power electrical equipment should not be used near the computer system, and large-power walkie-talkies, mobile phones, etc. should not be used in the computer room. That is, the ambient magnetic field strength should be limited to the minimum magnetic field strength allowed by the computer system specifications. During the trial operation of Unit 1 of the second phase project of Huaneng Fuzhou Power Plant, signal jitter occurred due to the use of walkie-talkies in the electronic equipment room. 1.2 DCS requirements for grounding The grounding of the control system is to provide a unified, common, ground-zero reference voltage reference point for the entire system. When the power supply or equipment fails, the overload current is withstood through the effective grounding system and quickly transmitted to the earth. It can also provide shielding and eliminate interference for DCS equipment. Therefore, correct grounding is one of the keys to ensuring stable and safe operation of the control. DCS has very strict requirements for grounding. Experience shows that during the commissioning and trial operation stages, many thermal control system failures are caused by improper grounding. In the grounding system, the problems that are more likely to occur are:: (1) The connector is not pressure welded or the welding is not firm, resulting in weak welding ； (2) Bolt connection points loosen due to vibration ； (3) Poor contact due to corrosion at the connection point ； (4) The resistance of the ground electrode increases and the ground electrode is disconnected from the power grid. ； (5) The ground wire wiring is unreasonable. 1.3 Requirements for cable laying. During the commissioning of Unit 1 of the first phase project of Xinjiang Hongyanchi Second Power Generation Co., Ltd., it was found that the voltage at the common end of the DCS terminal cabinet was raised. In addition to the normal operating voltage, there was still an 80-180V AC induced voltage. As a result, some equipment could not operate normally, and even equipment that was operating normally tripped inexplicably. After careful analysis and inspection, it was found that the electric door switch status line shared a cable with the operating power 220V line, resulting in a very high induced voltage on the switch status line. After consultation with each unit, the problem was completely solved by placing the electric door switch status line in one cable and separating it from the operating power supply. Practice has proved that electromagnetic interference has become an important factor affecting the correct action of instrument display, automatic input and protection. Among the many methods of anti-electromagnetic interference, one of the effective measures is to install and lay cables strictly in accordance with the principle of layered discharge of strong and weak current cables. In addition, the direction of the cable should be as far away from and away from heat sources as possible to avoid damage to the cable. 1.4 Requirements for docking and rewiring DCS connection and rewiring are important links in realizing plant-wide equipment automation. In order to ensure the smooth progress of the wiring and wiring work, the following matters must be paid attention to:: (1) All external connection and rewiring work of DCS are completed by the construction unit, and internal connection and rewiring work are completed by the DCS manufacturer. ； (2) The construction unit should notify the commissioning unit to pull out the relevant modules before connecting or changing lines. ； (3) For wiring that has been completed, the construction unit must obtain the consent of the commissioning unit when making changes. 1.5 Requirements for power supply system Some small units often ignore this aspect, and even some control systems only have one power supply. For example, the FSSS of Unit 1 of Xinjiang Hongyanchi No. 2 Power Plant only has one power supply. Later, under the strong request of the commissioning unit, the manufacturer improved the power supply system. The most basic requirement for the power supply system of the thermal control control system is that there must be two power supplies, and the two power supplies can be switched normally. In this way, the normal operation of the control system can be guaranteed. 2. Main issues in DCS debugging 2.1 Static debugging of DCS 2.1.1 The first thing to do during the DCS debugging process is the power receiving work of the DCS. If the power receiving work is not done carefully enough, it may cause damage to the module, CPU, or even the host. Main things to pay attention to before receiving electricity: (1) Check the insulation of the power circuit ； (2) Whether the system’s grounding resistance meets the requirements ； (3) Each branch power switch should be in the off position ； (4) All functional modules should be in the pulled-out position ； (5) The power supply waveform should be checked if necessary. Main things to pay attention to when receiving electricity: (1) The power receiving sequence should be carried out step by step from the main power switch, cabinet power switch, to modules and sub-modules. ； (2) Power should be received on a cabinet-by-cabinet and system-by-system basis. Only after the power measurement of one cabinet/system is completed and returns to normal can the next cabinet/system receive power. ； (3) If a problem occurs during the power receiving process, the power receiving work should be stopped immediately. The cause will be analyzed and found out, and the power reception will be restored after the fault is eliminated. 2.1.2 During the debugging process, modules are often burned out. Although there are many reasons for module burnout, the most important reason is the intrusion of strong electricity from the external circuit due to incorrect wiring or some local reasons. In order to prevent the module from burning, the following matters should be paid attention to:: (1) Before testing the module I/O channel, first disconnect the external wiring of the module, loosen the power fuse of the module, then insert the module and install the power fuse. After the module testing work is completed, the module must be pushed out. (2) Before the DCS and external equipment transmission test, first check that the wiring between the terminal cabinet and the external circuit is correct. Use a multimeter to check the grounding conditions of all back pins of the corresponding module of the equipment and the AC and DC voltages to the ground. There should be no strong electricity flowing in. After checking that there are no problems, insert the module for testing. After the test, push the module out. Repeat the above work if another device on the module needs to be driven. The module can be placed in the insertion position only after all the equipment on the module has been tested for transmission. (3) Modules that have not undergone transmission or system debugging should not be in the insertion position. (4) Try not to plug or pull out modules while the power is on. (5) Strengthen the coordination of installation and debugging to avoid cross-operation of the same system. 2.2 Dynamic trial operation of DCS 2.2.1 Computer crash problem During the commissioning process of Unit 1 of Guohua Zhungeer Power Generation Co., Ltd. and Unit 1 of Ningxia Guodian Shizuishan Power Generation Co., Ltd., AP crash problem in the SIEMENS TXP system was encountered. The main reason is: (1) A lot of modifications were made during the debugging process, which caused a lot of garbage to accumulate in the AP. ； (2) The allocation of some functional areas is unreasonable, the computer is overloaded, etc. Therefore, during the dynamic trial operation of the unit, no modifications to the configuration should be made as much as possible. If modifications are really needed, they must be submitted to the trial operation headquarters for approval before proceeding. Modified configurations are recorded and completed by the DCS manufacturer. In addition, since the TXP system will automatically generate some junk files during operation, the system administrator is required to clean the system regularly. 2.2.2 Defect treatment During the 168-hour trial operation of Unit 1 of Huaneng Shang'an Power Plant, a person from the thermal control debugging class of the installation unit went to deal with a temperature measurement point of the air blower on the spot, and accidentally removed the line of another good temperature measurement point, causing the unit to trip. It can be seen that defects must be dealt with very carefully during the operation of the unit. Before handling defects, you should fill in the defect handling form, contact the operating personnel and obtain approval before proceeding. 2.2.3 Signal isolation During many on-site debugging processes, it was found that all electric doors with local electric adjustment actuators need to be equipped with an isolator in the command loop. The main reason is that the power supply commanded in the DCS is directly connected to the earth, while the ground wire of the electric door is not grounded, resulting in a power mismatch and preventing the door from operating normally. After the isolator is installed, the power supply of the DCS is floating, which is consistent with the local one. Common ones include supply fan blades, induced draft fan stator blades, primary fan guide blades, etc., all of which need to be equipped with isolators. 2.2.4 Logic issues (1) When "drum water level is high or low" and "furnace pressure is high or low", it is best to add a delay of 2 to 5 seconds to the MFT signal. Its purpose is mainly to prevent the signal from being interfered, and adding a short delay will not cause harm to the unit. (2) In the MFT logic, MFT often occurs when "both fire detection cooling fans trip", and there is also an interlocking relationship between the two fire detection cooling fans. The most basic point is that the trip of one fire detection cooling fan will start the other fan. However, when one turbine is tripped and another turbine is started, both turbines will trip in an instant, which may lead to MFT. Therefore, it is necessary to add a delay to the MFT signal of "both fire detection cooling fans tripped". The length of the delay should be the shortest time to ensure the normal interlocking of the two fans. Similar situations include sealing fans, air preheaters and other equipment that will affect the safe operation of other equipment after tripping. The logic of these equipment must be rigorous. (3) One of the main tasks of the thermal control major during the trial operation is to find and analyze problems after the equipment trips. In order to quickly and accurately find the problem, it is recommended that all important auxiliary machines have "first out logic", including MFY logic, ETS logic, water supply pump trip logic, coal mill trip logic and other equipment with many trip conditions. In this way, accident analysis will be facilitated and the unit can be started quickly. In addition, problems should be reported to the project manager on duty as soon as possible after they are identified. 2.2.5 Other common problems (1) Display problem of drum water level. In many sites, there will be a large deviation between the DCS display of the drum water level and the display of the electrical contact under rated working conditions, reaching 50 to 100 mm. The display in the DCS corrected for pressure and temperature should be the most accurate. However, the electrical contacts have not been corrected and the electrodes may be contaminated or damaged, resulting in inaccurate display. (2) The communication between PLC and DCS, between the bypass system and DCS, and between DEH and DCS are often interrupted. This should be paid attention to during on-site debugging. (3) During the commissioning process of Unit 1 of Ningxia Guodian Shizuishan Power Generation Co., Ltd., the PLC control cabinet of the circulating water pump often lost power. After careful search, I found that the common terminal of the PLC input signal is the ground wire. Once the local cable or contact is grounded, the fuse will blow, causing the entire PLC cabinet to lose power. Later, the fire and ground wires of the input circuit power supply of the I/O cabinet were reversely connected to avoid burning the fuse. (4) The problem of bypass system protection investment. Based on the experience of bypass systems in multiple power plants, due to improper design or operation of pipelines and support and hanger systems, drainage, and HVAC pipelines, strong vibration and even damage to pipelines have occurred many times when the bypass is quickly opened. Therefore, many power plant bypass quick-opening interlocks are in a cut-off state. For example, during the trial operation of the bypass system of Unit 1 of Shizuishan Power Plant, MFT was caused by a "false" high water level in the boiler drum due to high bypass speed. (5) Fire detection issues. The quality of the flame detector is the key to whether flame protection can be put into use. There are some problems with the flame detectors currently used, mainly due to the serious "peeping" or "missing sight" phenomenon of the flame detector and the unstable output signal of the flame detector as the combustion conditions change, resulting in the failure of part of the main protection of the boiler to be put in normally. To solve this problem, the flame detector must be continuously adjusted during the debugging process in order to achieve the most ideal effect. It is better to "miss a look" than to "peep" the oil flame detector, that is, to improve the stability of the oil flame detector and appropriately reduce the sensitivity to prevent fuel from being sprayed into the furnace. It is better to "peep" than "miss" the coal flame detector, that is, to appropriately increase the sensitivity of the coal flame detector, because the furnace temperature is generally already high when the powder is added, and the pulverized coal can almost burn. In addition, the "peep" display logic of the coal flame detector can be slightly modified: After the corresponding coal feeder operation signal and the coal flame detector probe detect the fire signal are "ANDed", the coal flame detector detects fire on the CRT, otherwise it displays no fire. Solve the "peeping" phenomenon in the display from the control logic, and avoid the operator's misjudgment to a certain extent. The above methods have been well applied in Baoji Second Power Plant, Huaneng Fuzhou Power Plant, Xuzhou Pengcheng Power Plant, etc.