I. Introduction
In recent years, the scale and number of petrochemical enterprises have continued to expand and increase, and instrumentation systems have developed rapidly towards networking and intelligence. Instrumentation equipment generally has low insulation strength, poor overvoltage and overcurrent tolerance, and sensitivity to electromagnetic interference. If the instrument equipment is directly struck by lightning or a lightning strike occurs in the vicinity, the lightning overvoltage, overcurrent and pulsed electromagnetic field will reach the instrument equipment through the power supply line, instrument signal wire, cable trunking, threading pipe and other channels, threatening the instrument equipment Normal operation and safe operation.
If the protection is improper, it will cause the instrumentation equipment to malfunction, and the instrumentation equipment will be permanently damaged. In severe cases, it may cause casualties and production accidents.
Therefore, the design of modern petrochemical instrument system must attach great importance to the design of lightning protection.
2. Review of Lightning Protection of Petrochemical Instrument System
At present, when designing petrochemical instrument systems in China, basically no lightning protection is considered, but foreign countries have had nearly 20 years of research and use experience in this area. Some domestic petrochemical plants often suffer from lightning strikes, which paralyzes the control system and causes the equipment to stop, resulting in huge economic losses. Therefore, some remedial lightning protection measures were taken, mainly the shunt method, which is to use a surge protector SPD (Surge ProtecTIveDevice) in the signal or communication circuit of the instrument system and the power supply part of the system to limit transient overvoltage And split the inrush current. However, an SPD can only provide protection for a certain part of the circuit; for example, an SPD installed in the DCS control room can only provide protection for the card channel of the DCS; an SPD installed in the output of the field transmitter can only protect the transmitter Provide protection. If all I / O channels are equipped with SPDs, the cost will rise significantly; then, if two SPDs are added to each circuit, the SPD itself will also fail (this has been proven in production practice), The failure rate of the instrument system will be greatly increased.
Therefore, some domestic petrochemical plants can only partially use SPD in relatively important occasions, and the protection is also limited to field instruments or control room DCS, PLC, etc., and the overall lightning protection of the instrument system is not truly achieved.
3. Harmful form of lightning strike to the instrument system
Lightning strikes can be divided into direct lightning strikes and induction lightning strikes in form. The possible forms of damage to instrument systems are divided into the following types:
Direct lightning strike
Lightning directly hits the field instrumentation equipment or the pipeline connected to it, which usually damages the sensor module of the instrument and may damage the electronic circuit board of the transmitter. When lightning current flows into the ground along the instrument bracket, it generates a strong induced magnetic field, which can be coupled to the electronic equipment such as DCS in the control room through the signal transmission line and damage the electronic equipment such as DCS.
2. Induction lightning strike
(1) Electrostatic induction. When the thundercloud comes, a large amount of charge accumulates on the ground objects, especially the conductors, and the discharge occurs. If the discharge current enters the field instruments and electrical equipment, the equipment will be damaged.
(2) Electromagnetic pulse radiation. The lightning current generates an electromagnetic field in the space around its channel, radiates electromagnetic waves outward, and is coupled to computers, instruments and field instruments in the control room, as well as various types of metal conductors, generating induced electromotive force or induced current, causing equipment failure and damage. The control system is malfunctioning.
3. Lightning overvoltage intrusion
Direct lightning strikes or lightning induction may cause overvoltages on wires or metal pipes. This lightning overvoltage may introduce high potential into the instrument system along various metal pipes, cable ducts, and cable lines, causing interference and damage.
4. Counterattack
When the lightning protection device is connected to the lightning, a powerful instant lightning current flows into the grounding device through the down conductor. Due to the presence of the earth resistance, the lightning charge cannot be quickly discharged to the ground, which will inevitably cause a local ground potential rise (possibly hundreds of kilovolts). If the grounding body of the instrument control system does not have a sufficient safety distance from this point, a discharge will occur between them, causing a countercurrent, which can directly penetrate the insulating part of the electrical appliance, which will cause interference or even damage to the instrument control system.
Fourth, the main measures of instrument system lightning protection
The measures for the management of lightning damage to intrusive instrument systems are multi-faceted, mainly including flashover, shunt, voltage equalization, grounding and shielding. These measures must be comprehensively applied in order to truly achieve the lightning protection of the instrument system. The current lightning protection measures adopted by the petrochemical instrument system are as follows:
1. Flash
The protection against direct lightning strikes is mainly achieved by the lightning protection devices of the building. The lightning protection of the field instrument system should be designed together with the lightning protection measures of the surrounding oil storage tanks and other equipment.
2. Pressure equalization
When a lightning strike occurs, there will be a transient potential increase in the path through which the lightning transient current passes, resulting in a transient potential difference between the path and the surrounding metal object. If this transient potential difference exceeds The dielectric strength between the two will cause breakdown discharge of the medium. This breakdown discharge can directly damage the instrument equipment and also generate electromagnetic pulses, which can interfere with the normal operation of the instrument system. In order to eliminate the breakdown discharge between the lightning transient current path and the metal object, all metal enclosures, frames, metal equipment of the production equipment, facilities, equipment, components and metal enclosures of the instrument control room of all field instruments, The metal facilities are connected together and connected to the lightning protection grounding system of the instrument control room to form a perfect equipotential connection.
3. Ground
At present, there are two main measures for the grounding of domestic petrochemical instrument systems: floating ground and multi-point grounding.
(1) Floating ground means that the working ground of the instrument is kept insulated from the grounding system of the building, so that the electromagnetic interference in the building grounding system will not be conducted into the instrument system, and the change of the ground potential has no effect on the instrument system. However, due to the protective grounding of the instrument shell, when the lightning is strong, a high voltage may appear between the instrument shell and the internal electronic circuit, breaking the insulation gap between the two, causing damage to the electronic circuit.
(2) Grounding refers to the separation of the working ground and protective ground of instruments, DCS, PLC and other equipment. The outstanding advantage of this grounding method is that it can be grounded nearby and the parasitic inductance of the ground wire is small. However, if a strong lightning wave enters the system through the protective ground, the electronic circuit will also be damaged due to high voltage. Since the above two grounding methods can not meet the needs of lightning protection, you can consider connecting the protection ground to the working ground and accessing the lightning protection grounding system. The problem can be solved.
4. Shield
Petrochemical instrument systems use a large number of semiconductor devices, integrated circuits and signal-transmitting cables. Transient electromagnetic pulses generated by lightning strikes can be directly radiated to these components, or transient overvoltage waves can be induced on power supply or signal lines. Invading electronic equipment along the line, making the electronic equipment malfunction or damaged. The use of a shield to block or attenuate the energy propagation of electromagnetic pulses is an effective protective measure. The lightning protection shield of the instrument system mainly includes three aspects: control room shield, field instrument shield, signal wire and power wire shield.
(1) Control room shielding
The control system in the control room is the heart of the instrument system and is very sensitive to the electromagnetic pulses generated by lightning. Special attention should be paid to its shielding. The instrument control room should be a closed structure without windows, electrically connect the structural steel intersections in the house walls, and weld to the metal door frame to form a shielding cage with door openings, and make another protective ground ring around the wall around the room (Connected to lightning protection ground), grounding ring and shielding cage for effective electrical connection.
(2) Field instrument shielding
The field instrument can use metal instrument box (hood) to realize lightning shielding. The instrument box (hood) should be connected to other on-site metal facilities to achieve equipotential connection, and connected to the lightning protection grounding system.
(3) Signal line and power line shield
In order to prevent lightning electromagnetic pulses from inducing transient overvoltage waves on signals or power lines, all signal lines and low-voltage power lines should use cables with metal shields. For transient overvoltage protection, the shielding layer of the signal line or power line needs to be grounded at multiple points along the line or at least at the first and last ends of the line. When multi-point grounding is adopted, the shielding layer between each grounding point forms a loop along the line, and the electromagnetic field of low-frequency interference current may partly pass through the shielding layer to generate low-frequency interference in the core-sheath circuit of the cable. It is required that the shielding layer can only be grounded at a single point along the line. In order to prevent the low-frequency interference caused by multi-point grounding, the cable can be inserted into a metal tube or a double-shielded cable can be used. The layer can be grounded at one end, which not only ensures safety, but also helps to suppress low-frequency interference.
5. Diversion
Shunting is an effective measure for lightning protection. Because there are too many instrument circuits, it is impossible to use SPD in each instrument circuit. SPD or lightning arresters must be selectively installed in important circuits and system power circuits.
V. Overview
In order to achieve the lightning protection of the petrochemical instrument system, the entire production device should be designed according to the principle of equipotential connection. Comprehensive considerations should be taken from the control room, field instruments, instrument signals and power lines. , Grounding, shielding and other measures require electrical, construction, automatic control and other professional cooperation to achieve, in addition to considering system security, but also consider the cost of investment and economic operation.
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