Application of the hottest surge protection measur

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The application of surge protection measures in process control system (I)

today, with the rapid development of computer technology and network technology, especially the rapid development of distributed technology and the wide application of Internet and Intranet, the number of various networks and stations based on PC, hub, router and various servers has increased dozens of times, and the network economy marked by e-commerce has not only flooded major businesses and enterprises, It has also reached thousands of households. As we all know, the high-speed development of PC and networking technology is entirely dependent on the high-speed development of IC technology. Today, 600MHz, 0.18 micron process technology adopts aluminum conductive layer and low capacitive reactance silicon fluoride, and its minimum working voltage is between 1.1V and 1.6V. It can not only integrate more transistors per unit area, improve operation speed, improve power frequency, but also greatly reduce the energy consumption of the system. However, the progress of any technology is at the expense of other performance. The development of IC technology is no exception. The improvement of its integration and the reduction of energy consumption bring about the reduction of its ability to withstand overvoltage and external heat. Experiments have proved that one tenth of a joule of heat is enough to damage the transistors in the integrated circuit, and the heat generated by a pen falling on the desktop can reach 20 joules. It is conceivable that when the surge with energy of hundreds of joules and current amplitude of thousands of amperes invades, the threat to computers, switches and other instruments and equipment dominated by integrated circuits will be how serious

China has a mature understanding and specifications for lightning protection, lightning protection and grounding protection of buildings, but the protection of power supply and instruments of process control system is still in its infancy. Designers have no specifications and precedents to follow, and the majority of users lack experience in use. More and more users have suffered varying degrees of losses. Foreign countries have nearly 20 years of research and application experience in this field. Now we will talk about some specific application schemes of surge protective devices in combination with foreign application experience in this field

1. Application scheme of surge protector

1.1 overview:

surge usually refers to the phenomenon of sudden overvoltage and overcurrent in the circuit due to lightning stroke, power supply voltage fluctuation, electrostatic discharge, electromagnetic field interference and excessive ground potential difference. The strong electromagnetic field interference caused by direct lightning or induction lightning is often the main reason for the formation of strong surges. The rising edge of its current waveform is abnormally steep, forming a strong impact on the instrument; In addition, the imbalance of external potential often brings various forms of counterattack current, which is common in process systems with decentralized monitoring and control points

due to the scattered on-site detection and control points of the field control system, different field meters, such as transmitters and flow meters, are usually grounded directly or indirectly at their respective sites through pipelines, supports, etc. their signal working circuit is connected to the instrument ground in the control room, and there may also be electrical protection ground and lightning protection ground at the end of the control room. Potential differences are inevitable between different grounding, if due to lightning Problems will occur when the potential difference increases sharply due to short circuit and other reasons

nowadays, many customers often confuse surge and interference. The impact of interference signal on equipment is a kind of high-frequency clutter, and the impact of surge on equipment is physical destruction or accelerated aging. Shielding cables or armored cables are used for the protection of interference signals, while surge protection is the installation of surge protectors. Although they are not completely irrelevant, they play a very different role. Basically, the surge protector adopts the principle of equipotential, discharging the surge current to the ground in advance. The signal surge protector usually adopts gas discharge tube (GDT), thermistor, surge diode and zener diode. Power surge protector mainly adopts varistor (MOV), metal gap, etc. The different quality of components and design methods will lead to great differences in performance

due to many reasons, complex process and unpredictability, it is difficult for us to accurately calculate the amplitude, location, frequency and propagation direction of surge. According to the research and application experience of foreign countries in recent 20 years. We can basically get some application rules

when we need to implement surge protection for our system in practical applications, we mainly consider two aspects: one is the signal or communication circuit of the system; The second is the power supply of the system

1.2 application scheme

1.2.1 signal circuit

if the communication cable leads to the outdoor, such as PLC, DCS and other control circuits. Direct lightning or induction lightning that occurs outdoors often forms very strong surges in the circuit and rushes indoors. If most signal circuits are located indoors, such as computer networks. The surge on the circuit is mainly caused by electrostatic discharge or electromagnetic wave intrusion. The more high-frequency electromagnetic wave, the stronger its penetration ability. For the protection of these communication circuits to expand the rehabilitation medical equipment and fitness equipment industry, series signal surge protectors are mainly used. We need to consider the working voltage, load current, working frequency and cable connection mode of the circuit signal. To ensure that it will not affect the work of the system, achieve the effect of protection, and make the implementation of the project as simple as possible. Because the signal part of the system has a low ability to withstand overvoltage, we need to pay special attention to the limit voltage of the surge protector, which determines the voltage (limit voltage) at the output end of the surge protector when discharging surge current. When applied in highly exposed environments (such as when the outdoor transmitter communicates with the indoor i/o card), when the test waveform is 6kv/3ka (8/20ms composite waveform), the optimal limiting voltage should be less than 2.5 times the normal working voltage

in addition, manufacturers often mention the response time of surge protective devices. We believe that this parameter is actually the same as the residual voltage. The IEC standard has made a detailed introduction to the limiting voltage. An obvious fact is that if the response time of the protector is slow, resulting in the surge current has passed, its output voltage (residual voltage value) is high, which is certain; However, it can not be said that if the reaction time is short, the residual pressure value will be low, because it also involves the problem of product design. Moreover, a surge protector will use a variety of devices with different performance and response time. We cannot use the device with the fastest response time to represent its overall performance. In practical applications, the surges entering the circuit are very different. When the same device encounters surge current with different rise time, its response time is also different. We can definitely think that the response time of surge protective device is different every time it works in practice

1.2.2 power circuit

it varies greatly according to the scope of application. For those highly exposed environments, such as GS, which is almost impossible to achieve, m base station and outdoor microwave communication facilities are under the threat of direct lightning strike. For various types of buildings, such as intelligent building, control power room, etc. Due to various direct lightning protection measures taken around its power supply system, such as lightning rod and grounding, these facilities have discharged most of the lightning current energy. Therefore, the surge threat caused by direct lightning strike has been greatly reduced. After entering the indoor distribution system, the surge can often be reduced to a lower range due to the shunt and attenuation of the circuit

therefore, the implementation plan can be deployed and implemented according to the actual situation and the budgeted cost. For example, for very important power users, primary protection can be implemented in the main power system, sub power system and end-user location, If the residual voltage value of the product used in the primary protection in the next step is too high, it may also need to consider the secondary protection and even the final protection.

due to the characteristics of its use environment, the internal design of the power surge protector should not only have overcurrent fusing protection, but also perfect overheating fusing protection, so as to prevent overheating from causing equipment fire after enduring overvoltage for a long time (there have been many fires and even explosions in the use of power surge protective devices from foreign manufacturers). This is because most power surge protectors use zinc oxide varistors internally, so the mandatory standard ul14492nd in North America has clear requirements for this. Most manufacturers of surge protective devices use general-purpose sheet MOVs with a diameter of 16mm and a nominal value of 3KA or 6.5ka provided by ordinary mov manufacturers. By superimposing these small MOVs, the surge resistance of surge protective devices can reach dozens or hundreds of Ka. This superposition method greatly reduces the production cost because it can be purchased in large quantities in the market. However, this design method also brings insurmountable defects to surge protective devices. First of all, due to the nonlinear characteristics of MOV itself, the superposition method cannot ensure that its actual anti surge ability can reach the design level, and the error may be large. The heat resistance and pressure resistance of each mov are different. At the same time, the small mov has poor heat dissipation capacity, which greatly increases the possibility of surge protector failure and failure. And unlimited stacking is impossible. The superposition design method makes the limiting voltage increase quickly. For example, when the test current (8/20ms composite waveform) increases from 3KA to 10kA, the limiting voltage will increase linearly from about 800V to 2000V. Such a high limiting voltage often leads to the aging of our distribution facilities and electrical equipment faster. If a large mov with a diameter of 40mm and 3 terminals is used, although this large mov has considerable difficulties in the design and production of 8 product certificates, stacking and transportation, it can ensure that the surge carrying capacity of the equipment meets the design standard (one piece provides 90ka surge carrying capacity). At the same time, its heat dissipation capacity is greatly improved, and the important index of limiting voltage is also greatly improved. The test performance is a flat curve, For example, when the test current (8/20ms composite waveform) rises from 3KA to 10kA, the limit voltage only rises from about 800V to about 1000V. Therefore, if this mov is adopted, when using surge protective devices in practice, there is no need to set up multi-level protection. Although the performance is good, the defect is that the cost is high

2. Application of surge protective devices in the field of intrinsic safety

in the field of process control, factories often have DCS, ESD, FG and other systems, but these systems are always under the threat of surge and instantaneous overvoltage caused by nearby power failure or lightning strike. These instantaneous overvoltages also affect the equipment in dangerous and safe areas, which are connected together through various systems. Various electronic devices used on site, such as flow rate, liquid level and temperature transmitters, are also not immune. Therefore, many factories find ways to add lightning protection devices to their equipment. However, if electronic and electrical systems are used in explosive gas environment, they need to pass various certifications first, which makes the problem more complicated. This paper describes the complex process when surge protection devices and certified intrinsically safe systems are used in hazardous areas. The working characteristic of surge protector is that it can only work locally to provide protection. For example, the surge protector installed in the cabinet can only protect the communication cards in the cabinet. If

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