Research on anti-interference requirements of DCS system

**Abstract:** The key interference factors that affect the safe, stable, and reliable operation of a power plant's DCS (Distributed Control System) include power supply, signal classification, cable layout, and grounding system design. By analyzing the anti-interference requirements of the DCS system in terms of power supply, signals, and grounding, combined with practical engineering experience, this paper elaborates on the three main aspects of anti-jamming design. It also provides some important considerations for construction and maintenance, offering guidance for future projects to ensure effective DCS implementation and secure, stable plant operations. **Keywords:** small and medium thermal power plant; DCS; anti-interference; grounding **1. Introduction** The Distributed Control System (DCS) has been widely applied in large-scale thermal power plants, accumulating substantial experience. However, for smaller or regional thermal power plants, the adoption of DCS systems is still in its early stages. While the technology is gradually being introduced, there remains a lack of comprehensive design, construction, and maintenance expertise. One of the critical challenges in implementing DCS systems is effectively suppressing interference to ensure the safety, stability, and reliability of the control system. Due to limited technical resources and operational experience, small and medium-sized thermal power plants must pay particular attention to interference control. Ensuring a stable power supply, properly classifying signals, laying cables appropriately, and implementing effective grounding are essential strategies to minimize interference and improve system performance. These aspects require thorough discussion and research to support the successful deployment of DCS systems in such environments. This paper analyzes the anti-interference requirements of DCS systems in small and medium-sized power plants, focusing on power supply, signal handling, and grounding. Based on engineering experience, it outlines design strategies and highlights important considerations during construction and maintenance, aiming to enhance the reliability and performance of DCS systems. **2. Anti-Interference Requirements for DCS Systems** To enhance the measurement accuracy and control performance of the DCS system and ensure the safe, stable, and reliable operation of the control system, thermal control professionals must carefully consider the anti-interference requirements of the DCS system in terms of power supply, signals, and grounding during the design, installation, and maintenance phases. **2.1 Power Supply Requirements** Power supply safety is the fundamental requirement for any power system. During the pre-design phase, it is crucial to select appropriate power sources and switch models, taking into account the maximum operating current of all devices and leaving sufficient margin for future expansion. When choosing power lines, not only should the wire diameter be suitable for the current capacity, but the lines must also have sufficient mechanical strength, corrosion resistance, and flame retardancy. Reliable and continuous power supply is essential for the stable operation of the DCS. The DCS power supply system should be isolated from other systems and equipment if possible. If independent power supply cannot be achieved, a power isolation device, such as an Uninterruptible Power Supply (UPS), should be used. The system should have two power supplies: one from an AC Uninterruptible Power Supply (UPS), and the other from the factory’s low-voltage working section. An automatic transfer switch should be installed to ensure seamless power transition without affecting the control system’s operation. Any power failure should not result in faults, data loss, or abnormal system behavior. **2.2 Signal Requirements** Signal lines are a primary pathway for interference entering the DCS. Weaker signals are more susceptible to interference. Signals can be classified into four categories: - **Class I signals**: Low-level signals such as thermocouple, RTD, millivolt, and strain signals. - **Class II signals**: Analog input/output signals (e.g., 0–5 V, 4–20 mA) and contact-type switches. - **Class III signals**: Switching outputs for inductive or resistive loads above 50 mA. - **Class IV signals**: High-voltage switching outputs (e.g., 110 VAC or 220 VAC). To reduce interference, signal cables must be designed and arranged according to specific guidelines. Shielded twisted pair cables are preferred for Class I signals, while shielded cables are recommended for Class II signals. Class IV signals should be treated like power cables and kept separate from lower-level signals. Class III signals can be routed alongside power lines but must be shielded and maintained at least 15 cm away from Class I and II cables. Proper grounding of shielded cables is essential, and high-power lines should never be bundled with signal cables. **2.3 Grounding Requirements** Grounding ensures a common reference point for the entire system, enhancing safety and reducing interference. There are several types of grounding, including protective grounding, system grounding, shield grounding, and intrinsic safety grounding. Each serves a specific purpose, such as protecting against electrical faults, stabilizing signal references, or preventing hazardous energy from reaching the system. The DCS grounding system consists of grounding connections and grounding devices. Key parameters include connection resistance, ground resistance, and total grounding resistance. Different DCS manufacturers may have varying requirements for grounding resistance, typically ranging from less than 1Ω to less than 5Ω. In cases where the plant’s electrical grounding grid has a resistance of ≤4Ω, the DCS grounding system can be connected to it. However, for higher resistance or special requirements, an independent grounding system is necessary. **References** [1] Chen Bo, Ding Yongjun, Chen Xiaoqiang. Analysis and Suggestion on Grounding Status of DCS System in Zhejiang Thermal Power Plant [J]. Zhejiang Electric Power, 2008, (1): 51–54. [2] Zhang Wei, Niu Yugang. Overview of DCS and Fieldbus [J]. Electrical Automation, 2013, (01): 4–6+46.

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