Abstract: DCS is the core of the entire control system of a polyester plant. Its stable performance directly affects the safe production of the polyester plant. This paper analyzes the problems existing in the grounding and power supply aspects of the DCS system of a polyester plant and proposes countermeasures. Keywords: DCS, grounding, power supply control 1. Introduction DCS stands for Distributed Control System, also known as a distributed control system. It is a comprehensive high-tech product integrating computer technology, control technology, communication technology, and CRT technology. DCS centrally monitors, operates, and manages the entire process through operator stations, and decentralizedly controls various parts of the process through control stations. It differs from both conventional instrument control systems and centralized computer control systems, combining the advantages of both while overcoming their respective shortcomings. It is increasingly widely used due to its high reliability, flexibility, user-friendly interface, and convenient communication. The polyester plant of Luoyang Shihua Synthetic Fiber Co., Ltd. adopts domestically patented process technology. The esterification and polycondensation reactions use a five-reactor process, resulting in long residence time, stable reaction, easy control of various process parameters, and a large buffer margin in emergencies. The probability of venting, waste discharge, and shutdown is greatly reduced, providing great flexibility in production operations. 2. Overview of the TPS System The polyester plant's DCS system adopts the internationally renowned TPS system from Honeywell, USA. TPS stands for Total Plant Solution, the first automation system to unify the entire plant's business information system and production process control system on a single platform. It consists of the following network types: 1) Plant control network (PCN): An open network system that can access process network data; 2) Local control network (TPN): Also known as LCN, the backbone of the TPS system. Nodes on the PCN access process network data through the TPN; 3) Universal control network (UCN): The process control network of the TPS. Network devices on the TPN (LCN) include: Global System for Mobile Communications (GUS): The human-machine interface of the TPS system, based on the MS NT workstation's Native Window, used for accessing information throughout the LCN system. Plant information is accessed through the PCN network. The GUS platform supports process operation and process engineering configuration and design functions. The Network Interface Module (NIM) provides an interface for LCN network to access UCN network, converting LCN communication technologies and protocols to UCN communication technologies and protocols. The History Module (HM), the TPS file server, supports the storage of TPN network system activities, historical data, and system events for system performance monitoring, adjustment, and troubleshooting. Network devices on the UCN network include: a High-Performance Process Manager (HPM), used to scan and control TPS system process data, featuring a fast internal processor and large memory capacity; and a Safety Manager (SM), used to implement factory safety shutdown policies, which can operate independently or be connected to the TPS system. It is worth noting that the NIM is both an LCN network node and a UCN network node, completing data exchange between the UCN and LCN networks. 3. Polyester DCS Hardware Configuration: The polyester unit, based on production process requirements, is equipped with four GUS workstations, three as operator stations and one as an engineer station. Two redundant HPMs are set up. One is used as the control station for the main polyester unit. Considering that the booster pump of the short fiber unit is closely related to the polyester unit in terms of process, and also due to the high reliability of the DCS system, the other HPM is used as the control station for the melt booster pump of the short fiber unit and the polyester auxiliary system in the engineering design. [align=center] Figure 1. DCS Hardware Configuration of Polyester Unit[/align] The main hardware configuration is as follows: 1) 4 GUS operator stations 2) 1 set of HM history module 3) 2 sets of NIM network interface modules, which are redundant 4) 4 sets of HPM high-performance process managers, which are redundant 5) 2 sets of LCN network, which are redundant 6) 2 sets of UCN network, which are redundant 7) 14 HLAI high-level analog input cards 8) 11 AO analog output cards 9) 13 DI digital input cards 10) 8 DO digital output cards 11) 12) 2 LLMUX multi-channel low-level analog input cards 4. Problems with DCS grounding 4.1 Purpose of DCS grounding In the DCS system, grounding is the main method to suppress noise and prevent interference. If grounding and shielding are correctly combined in design and construction, most noise problems can be solved. Grounding has four basic purposes: 1) To eliminate noise voltage generated when current flows through a common ground impedance; 2) To avoid the influence of magnetic fields and ground potential differences, i.e., to prevent the formation of ground loops. Poor grounding can lead to noise coupling; 3) To prevent loops in shielding and filtering; 4) To ensure safe grounding. 4.2 Honeywell's requirements for TPS grounding 4.2.1 Safe grounding 1) The grounding electrode should be connected to the grounding busbar of the TPS system PDP cabinet using a cable with a cross-section of 100mm² or more. 2) The safety grounding system connection below the grounding busbar should use a cable with a cross-section of 20mm² or more, and the cable should have a yellow/green striped insulating sheath. 3) The connection length from the grounding electrode to any terminal (operating console or cabinet) in the grounding system should not exceed 152M. 4) This grounding system should be dedicated to the TPS equipment and should not be mixed with other unrelated equipment. 5) For non-intrinsically safe systems, the grounding resistance of the grounding stake should not exceed 5 ohms. 6) For intrinsically safe systems (using Zener barriers), the grounding resistance of the safety ground (ACG) should not exceed 0.1 ohms. 4.2.2 Master Reference Ground 4.2.2.1 General Requirements for the Master Reference Ground 1) This grounding system should be dedicated to the TPS and be an independent grounding system, not shared with other unrelated equipment. In the TPS system, it is also called the logic ground. 2) For non-intrinsically safe systems, the grounding resistance should not exceed 5 ohms. 3) For intrinsically safe systems (using Zener barriers), the grounding resistance of the master reference ground (MRG) should not exceed 1 ohm. 4) The distance between the master reference grounding stake and the safety grounding stake and other underground metal objects should be greater than 3 meters to ensure effective isolation. 4.2.2.2 Connection of the Master Reference Grounding System 1) The system grounding adopts a star connection. Depending on the system size, auxiliary star plates can be hung under the main star plate, but the cascading of star plates should not exceed two levels. 1) The grounding of the HPM cabinet should be directly connected to the star board, and should not be connected by a chain. 2) The main reference grounding electrode should be connected to the main star board in the control room using a cable with a cross-section of 100 mm2 or more, and the cable length should be controlled to about 15M. The cable should have a green insulating sheath. 3) The connection of the grounding system below the main star board should use a green insulating sheath cable with a cross-section of 20 mm2 or more. 4) The connection length from the main reference grounding electrode to any end of the HPM cabinet in the grounding system should not exceed 152M. 4.3 Engineering Design Requirements 4.3.1 Safety Ground 1) The cable diameter from the safety grounding electrode to the grounding busbar is designed to be 16 mm2 by the design institute. 2) The cable diameter below the grounding busbar is designed to be 16 mm2 by the design institute. 4.3.2 The original design of the main reference grounding electrode by a certain design institute required the grounding resistance to be less than 1Ω, and the cable from the MRG busbar to the grounding electrode was 16 mm2. During on-site construction, the construction unit, in an effort to save time, only installed three grounding electrodes, and the burial depth was insufficient. More seriously, the electrodes were too close to the main electrical grounding network of the polyester unit and the 2000KVA 10KV transformer (the closest grounding electrode was only 5m from the transformer). The electromagnetic interference from the transformer severely affected the main reference grounding system. After the DCS was powered on on July 26, 2006, a large amount of noise was generated on the DCS's UCN cable monitoring, and the daily cumulative number increased extremely rapidly, seriously affecting the normal operation of the DCS system. On August 12, a UCN network fault occurred, with the fault symptom being "HPM ALIVE". The fault persisted after a power outage, but later resolved itself. On August 23, NIM15 malfunctioned at 6:02 AM, resulting in the loss of all data from the GUS operator station. Investigation revealed that a terminating resistor on the TAP head of a UCNA cable at NIM15 was faulty. After replacement, the system recovered. The cause was determined to be that the main reference grounding system did not meet the hardware manufacturer's requirements. 4.3.3 Grounding System Rectification In response to the identified problems, we focused on rectifying the location, construction, and wire diameter of the grounding grid. According to the DCS manufacturer's requirements, the main reference grounding grid should be far from major electromagnetic interference sources. We carefully considered the site selection. Due to site limitations, to meet both the requirements for distance from interference sources and grounding resistance, the original three grounding electrodes were discarded, and a new site was chosen. A total of seven grounding electrodes were installed, interconnected in a triangular mesh. This ensures that any grounding electrode remains connected to the grounding grid even if any connecting iron corrodes or breaks. During this grounding grid renovation, the company assigned dedicated personnel to supervise the entire construction process. The requirements included a minimum driving depth of 3m for the grounding electrodes, a 3m spacing between electrodes, L40 galvanized steel connecting iron, a minimum trench depth of 800mm, and welding on all four sides with asphalt coating for corrosion protection. Because the diameter of each level of grounding wire was too thin and did not meet the hardware manufacturer's requirements, all levels of grounding wire were replaced with grounding copper wire conforming to Honeywell specifications. After the rectification of the above projects, the operation of the DCS has greatly improved. The UCN cable network noise monitored by the GUS engineer station has dropped from about 200-500 per day before the renovation to 0, and there has been no noise since, providing a reliable guarantee for the future operation of the DCS. 5. DCS Power Supply System According to the design institute's design and the requirements of Honeywell, the power supply for each part of the DCS is supplied by the UPS. The UPS backup time is 30 minutes, ensuring the normal power supply of the DCS system during power outages or power fluctuations, and meeting the power needs of the field instruments and the DCS system during plant shutdowns. The HPM uses a redundant regulated power supply to ensure that if any power module fails, the other redundant power supply will work normally and will not affect the operation of the HPM. However, due to inadequate consideration in the design and hardware integration, there are many problems in the power supply section of the entire DCS system. The following figure is a diagram of the entire DCS power supply system. [align=center]Figure 2. Polyester DCS Power Supply Diagram[/align] 5.1 DCS Power Supply Problem Analysis 1. All equipment in the DCS system is powered by a single UPS. If the UPS fails or the UPS main power distribution switch QF01 trips, the entire DCS system will lose power. The so-called redundancy of NIM, HPM, GUS, etc., will be ineffective, and the system will be paralyzed. For example, in the summer of 2006, a reforming unit in a refinery lost control and was forced to shut down due to a UPS failure that stopped supplying power to the DCS. 2. The two mutually redundant NIM power supplies, which are crucial to the operation of the DCS, are powered by the same air switch QF06. If QF6 trips due to a short circuit inside the NIM cage box or a fault in the air switch, NIM15/16 will lose power simultaneously. All data exchange between the field and GUS will be impossible, and the entire DCS system will be paralyzed. 5.2 DCS Power System Rectification Measures As the above analysis shows, the main problem with the DCS power supply lies in the fact that, despite hardware redundancy, the main power supply or power switch is shared, rendering the hardware redundancy ineffective in the event of a power failure. The rectification plan is shown by the dotted line in the figure. The power supplies for the redundant hardware components HPM09/10, HPM11/12, and NIM15/16 of the DCS are respectively powered by UPS and mains power. HPM09, HPM11, and NIM15 are powered by UPS, while HPM10, HPM12, and NIM16 are powered by mains power via a regulated power supply. Since these devices are mutually redundant, this ensures that the DCS can continue to operate normally even if any part of the UPS and its circuit breaker, mains power and its regulated power supply, or any part of the three sets of redundant equipment fails, truly achieving power redundancy and equipment redundancy. This solution requires the UPS and mains power to be drawn from two separate busbars in the low-voltage distribution system, maximizing the guarantee of normal power supply. This meets the requirements for the dual-circuit split-operation systems currently used in the power grids of most continuous production chemical plants. The rectification plan is shown in Figure 3, with the dotted line indicating the modification section. [align=center] Figure 3. Polyester DCS Power Supply Renovation Plan[/align] 6. Conclusion The stable and reliable operation of the DCS system plays a crucial role in the safe, stable, long-term, and high-quality production of the polyester plant. Grounding and power supply are two aspects that most design institutes and hardware integrators often overlook during engineering design and installation. However, these two aspects are indeed crucial for the stable operation of the DCS. After the rectification, the DCS operates stably, and the concern of DCS paralysis caused by UPS failure is completely resolved.