Detailed study on anti-interference requirements of DCS system

Abstract: The important interference factors that restrict the safe, stable and reliable operation of power plant DCS system are power supply, signal classification and cable laying, grounding system design, construction and maintenance problems. By analyzing DCS anti-interference requirements for power supply, signal and grounding Combined with engineering design experience, the three aspects of anti-jamming design are expounded, and some precautions in construction and maintenance are proposed for project reference.

1 Introduction

The Distributed Control System (DCS) has been widely used in large-scale thermal power plants and has gained a lot of experience. For small and medium-sized enterprises to supply thermal power plants or regional thermal power plants, the application of DCS systems is gradually being promoted, but its design, construction and maintenance experience is still There are gaps that need to be constantly explored and summarized.

One of the important problems faced by its DCS system is how to effectively suppress interference and improve the safety, stability and reliability of the control system. Due to the lack of technical reserves and lack of personnel operating experience, small and medium-sized thermal power plants should pay more attention to this point. Stable supply of power supply, correct classification of signals, reasonable laying of cables, and effective implementation of grounding as effective methods to suppress interference and improve system reliability are worthy of extensive discussion and research.

By analyzing the anti-interference requirements of power supply, signal and grounding of small and medium-sized power plant DCS system, combined with engineering experience, the anti-jamming design schemes of these three aspects are expounded, and some precautions in construction and maintenance are proposed for project design. Construction and maintenance reference, in order to achieve effective input of DCS, to ensure safe and stable operation of the power plant.

In order to improve the measurement level and control accuracy of the DCS system of the power plant and ensure the safe, stable and reliable operation of the control system, the thermal control professional must reasonably consider the anti-interference requirements of the DCS system in terms of power supply, signal and ground during the design, construction and maintenance phases.

2.1 Power requirements

Power supply safety is the most basic condition that any power supply system must meet. First of all, in the pre-design, when the power supply and switch models are not selected, fully consider the maximum operating current of all devices, and leave enough margin to increase the equipment in the future. Secondly, when selecting the power line, it is not only required to select a suitable wire diameter according to the current capacity, but also requires sufficient mechanical strength, corrosion resistance, and flame retardancy.

Power supply reliability and continuity are prerequisites for stable operation of DCS. In addition to supplying power to the DCS, the DCS power supply system should not be connected to other systems and equipment. If the independent power supply cannot be realized, it can be implemented by adding a power isolation device (commonly used UPS uninterruptible power supply). The power supply should have two power supplies, one should use AC uninterruptible power supply (UPS), and the other should be from the factory low-voltage working section power supply. The two-way power supply should be equipped with an automatic power supply cutting device, and the cutting time should ensure that the operation of the control system is not affected. Loss or failure of any power supply should not cause any fault, data loss or abnormal operation of any part of the control system; any loss of power should be obtained in the control system.

2.2 Signal requirements

The input and output signal lines of the DCS are one of the ways in which the interference signal enters the DCS. Generally speaking, the weaker the signal, the greater the influence of the interference. The signals should be classified as follows:

Class 1I signals: low-level signals such as thermal resistance signals, thermocouple signals, millivolt signals, and strain signals.

2 Class II signals: 0 to 5 V, 1 to 5 V, 4 to 20 mA, 0 to 10 mA analog input signal; 4 to 20 mA, 0 to 10 mA analog output signal; level/contact type switch Input signal; pulse input signal; resistive load switch output signal with 24VDC less than 50 mA.

Class 3III signal: Switching output signal of a 24V to 48VDC inductive load or a resistive load with a current greater than 50 mA.

4IV type signal: 110VAC or 220VAC switch output signal, the feeder of such signal can be regarded as the problem of power line processing wiring.

Among them, Class I signals are easily interfered, Class II signals are easily interfered, and Class III and IV signals become strong sources of interference at the moment of switching action, interfering with nearby signal lines through the space environment.

In order to effectively reduce the interference between the signals, when the different types of signal cables are connected to the DCS, the design and arrangement of the signal cables should meet the following principles:

1 For Class I signal cables, shielded twisted pair cables are preferred.

2 For Class II signals, shielded cables should be used as much as possible. Among them, shielded cables must be used for the inductive/mode-out signals and input signals used for control and interlocking in Class II signals.

3 For Class IV signals, it is strictly prohibited to bundle the I and II signals together. It should be handled as a 220 V power cable and routed with the power cable. It is recommended to use shielded twisted pair cable when conditions permit.

4 For Class III signals, it is allowed to be routed together with the 220V power line (ie, the same as the Class IV signal), or it can be routed together with the I and II signals. However, in the latter case, the type III signal must be shielded cable, preferably shielded twisted pair cable, and more than 15cm away from the I and II type signal cables.

5 The shield of the shielded cable must be grounded only one point and grounded well.

6 It is absolutely forbidden to bundle high-power switch output signal lines, power lines, power lines and other cables in parallel with I and II type signal cables that directly enter the DCS system.

7 It is absolutely forbidden to use some of the core wires of a multi-core cable for transmitting Class I or Class II signals, and some core wires for transmitting Class III or Class IV signals.

8 It is forbidden to distribute several core wires of the same signal in different cables (such as three-wire thermal resistance).

2.3 Grounding requirements

Grounding provides the entire system with a uniform, common reference reference point with zero ground. When the power supply or equipment fails, the overload current is withdrawn through an effective grounding system and quickly transmitted to the ground to ensure the safety of personnel and equipment. In addition, the grounding system provides shielding and interference cancellation for DCS equipment. The Dcs system has two grounding methods: protective grounding and working grounding (logic grounding, shield grounding, and intrinsically safe grounding). 1 Protective earthing (CG): Protection measures taken to prevent static charge accumulation in the device casing and to avoid personal injury. 2 System grounding: Make the DCS signal establish a reference point. The reference point is grounded to keep the reference point at a stable potential to ensure the reliability and accuracy of signal transmission. 3 Shield Ground (AG): Also called analog ground, it can shield the interference received by the live signal transmission to improve signal accuracy. 4 intrinsically safe grounding: the grounding of the Zener safety barrier. When the current and voltage of the intrinsically safe loop are too large, the fuse of the safety barrier is blown, limiting the energy of the ignition source in the hazardous area, and achieving intrinsic safety protection.

DCS grounding system consists of two parts: grounding connection and grounding device. The grounding connection includes grounding wires of different natures, grounding busbars, grounding trunking lines, grounding summing boards, and grounding trunks; the grounding device includes a total grounding plate, a grounding main trunk, and a grounding pole.

In order to properly design the DCS grounding system, the concept of connection resistance, ground resistance and grounding resistance needs to be introduced. Connection resistance: the sum of the resistance of the conductor and the connection point from the ground terminal of the device to the total grounding plate; the resistance of the grounding electrode to the ground: the ratio of the potential of the grounding electrode to the current flowing through the grounding electrode; the grounding resistance: refers to the grounding The sum of the pole-to-ground resistance and the connection resistance of the total grounding plate, the grounded mains, and the grounded mains.

At present, each DCS manufacturer has different requirements for the grounding system, but the difference mainly lies in the requirement of the grounding resistance value and whether the grounding system is completely independent. For example, Honeywell's TPS system requires the grounding system's grounding resistance to be zero. 4 ~ 5Ω, if there is no intrinsic safety barrier, the grounding resistance is less than 5Ω; Westinghouse and ABB DCS require the grounding resistance to be less than 1Ω; and the DCS requirements of the company, when the grounding resistance of the electrical grounding grid of the plant is less than When it is equal to 4Ω, the DCS grounding grid can be directly connected to the electrical grounding grid of the plant.

In view of the inconsistency between different requirements of DCS manufacturers and industry specifications, combined with the actual experience of the project, the design and construction recommendations of the DCS grounding system are as follows:

1 When the grounding resistance of the electrical grounding grid of the power plant is ≤ 4Ω, the DCs total grounding plate can be connected to the electrical grounding grid in a single point. The distance between the grounding point of the DCS system and the electrical grounding pole is not less than 5Ω, and the distance from the grounding pole of the lightning protection grounding system is not Less than 10 Ω.

2 When the grounding resistance of the electrical grounding grid of the power plant is large or the DCS manufacturer has special requirements, the DCS grounding system should be set independently, and the grounding resistance should be less than O. 5Ω or as determined by DCS manufacturer requirements.

3 The Ansina type safety barrier grounding system should be set independently, the grounding resistance is less than 1Ω, and the connection resistance of the grounding main trunk is less than 0.1Ω. The intrinsically safe grounding should be kept independent, and the distance between the electrical grounding grid of the power plant or other instrumentation system grounding grid should not be less than 5Ω.

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