Abstract: This paper briefly discusses the basic concept of electromagnetic compatibility, the propagation path of interference source and the anti-interference protection measures, and discusses the electromagnetic compatibility in the power communication network in an inductive manner.
Keywords: Electromagnetic Compatibility; Electromagnetic Interference; Power Network; Power Communication; Interference Way; Protective Measures
0 Preface
In the power system, as the voltage increases and the capacity increases, the power grid itself is a great source of interference. Due to the development of modern technology, electronic technology has gradually penetrated into classic electrical equipment. At the same time, with information technology and With the development of automatic control technology, the power grid is more and more dependent on primary equipment such as communication and automatic control systems, and these equipment are also an object of interference.
Therefore, the construction and operation of the power grid must consider electromagnetic compatibility issues
1. The basic concept of electromagnetic compatibility
Electromagnetic disturbance refers to any electromagnetic phenomenon that may cause the performance of devices, equipment or systems to degrade or cause damage to living or inanimate matter.
Electromagnetic disturbance may be electromagnetic noise, unwanted signals or changes in the transmission medium itself, which may cause equipment or system degradation or damage, but not necessarily cause consequences. Electromagnetic disturbance is only a natural phenomenon. Electromagnetic disturbance is caused by electromagnetic disturbance Consequences of equipment, transmission channel or system performance degradation.
Electromagnetic compatibility (Electromagnetic Compatibility-EMC) refers to the ability of a device or system to work normally in its electromagnetic environment without causing unacceptable electromagnetic disturbance to anything in the environment.
EMC includes two meanings, that is, the electromagnetic emission generated by the equipment or system will not affect the functions of other equipment or systems; and the anti-interference ability of the equipment or system is sufficient to prevent the function of the equipment or system from other interference. Influence.
The occurrence of electromagnetic interference is composed of three parts: interference source, propagation path and interference receptor. It is usually called the three elements of interference. Let's talk about these factors in conjunction with the power system.
2. The source of the Shenli magnetic disturbance of the Shenli system
Power system electromagnetic disturbance sources mainly come from the following aspects:
(1) High-voltage transmission lines
Factors that may cause interference to surrounding things by high-voltage transmission lines mainly include:
1) Corona discharge: due to the excessive potential gradient on the surface of the conductor, when the humidity and air pressure change, it causes discharge into the air and occurs
High-frequency electromagnetic noise. Its spectrum is below 30MHz.
2) Spark discharge: due to the destruction of local insulation on the line, the pollution of insulators; the spark discharge caused by poor contact of metal parts, etc. Its spectrum range can be as high as 100 MHz, and its amplitude may be much larger than corona discharge.
3) Power frequency magnetic field: its strength mainly depends on the carrying capacity of the dry wire. But it decays quickly with the increase of distance.
4) Ground current: For the unbalanced neutral line of the AC three-phase system or the situation where the ground is used as the loop of the DC transmission line and when an accident occurs, the ground current is sometimes quite considerable. If it is not handled properly, it will cause a sharp rise in the ground potential, thereby disturbing or damage the device.
(2) High-power rectifier users
For high-power rectification users such as electric locomotives, smelting, and Shenjie industries, rectification will generate a large number of high-frequency harmonics that interfere with weak current equipment and communication lines
(3) Power Electronics System
Power electronic systems are widely used in high voltage, high current and high frequency systems, such as AC-DC conversion systems for DC transmission: active power factor compensation circuits; frequency conversion adjustment systems; switching power supply devices, etc. (4) Communication information equipment and industrial automatic control system
With the extensive application of communication equipment, embedded microcomputers, computers, information equipment and other equipment in electric power, such as communication, automation and relay protection systems in substations, on the one hand, they are in the environment of high-voltage and high-current electromagnetic fields of electric power. , are susceptible to various electromagnetic interference; on the other hand, they are sources of disturbance, such as improper use of wireless communication equipment in substations will cause malfunction of relay protection equipment. 3 Electromagnetic interference propagation methods and channels
The propagation methods of electromagnetic interference include conduction, coupling and radiation. Generally, there are the following propagation methods:
(1) Radiation coupling: When the frequency of the interference source is high, it will cause radiation of the electromagnetic field. This kind of disturbance has a greater impact on the electronic signal control system.
(2) Capacitive coupling: Capacitive coupling is transmitted through the distributed capacitance between two conductors. The most typical case is that the high-voltage transmission line is close to the communication line in parallel. This coupling only occurs when the electrical distance between the two is close In some cases, as the distance increases, its influence will gradually decrease.
(3) Inductive coupling: When the current of the interference source is very large, the current generates an alternating magnetic field in space, and this magnetic field generates an induced electromotive force in the disturbed circuit, causing electromagnetic interference in the disturbed line.
It must be pointed out that the mutual inductance of inductive coupling decreases very slowly as the distance between the interference source loop and the interfered loop increases, so the interference source of electromagnetic induction can affect the electronic system far away from itself
(4) Resistive coupling: The earth is a natural grounding body, which is full of stray currents of various frequencies. Many electronic and electrical equipment are connected to the ground. Therefore, stray currents will enter the electronic equipment along the grounding wire Or electrical equipment spreads electromagnetic noise, forming electromagnetic interference.
When a single-phase grounding occurs in a neutral point grounding substation, the strong short-circuit current will cause the ground potential of the substation to rise. At this time, as long as the electronic equipment and electrical equipment are grounded, the stray electrical noise (poor difference) in the ground will be provided Intruded channels, especially high-frequency interfering noise, are more likely to travel around.
4 Main measures to reduce electromagnetic interference
The method of reducing electromagnetic interference is aimed at the three elements of interference, starting from suppressing the source of interference, cutting off the propagation path of interference, and improving the anti-interference ability of the interference receiver, and taking various measures, mainly including shielding, filtering and grounding.
(1) shielding
Shielding is a measure to prevent or reduce the interference caused by the transmission of electromagnetic energy in space by using shielding materials. Through shielding, the energy of interference can be effectively reduced. It can be widely used in all aspects of reducing the three elements of interference. The shielding effect and The conductivity and magnetic permeability of the shielding material, the structure of the shielding body, the distance from the interference source, the nature of the interference field (electric field or magnetic field), and frequency are related to the magnetic field shielding, and the attenuation through the shielding material plays a major role. It is related to The characteristics of the material are directly related to the thickness, and the shielding effect of ferromagnetic materials is better.
(2) Filtering
Filtering is a means to deal with electromagnetic compatibility problems in the frequency domain. Through filtering, conductive electromagnetic disturbance can be suppressed. Filters can be divided into two types: signal selection filter and electromagnetic disturbance suppression filter according to the type of signal they process. The main function is to select the signal of the frequency we need. The electromagnetic disturbance suppression filter is to pass useful frequency signals in the filter. The signals higher and/or lower than these frequencies are suppressed or blocked. The common ones are power line filters, signal line control filters, etc. pass waver
(3) Grounding
The concept of grounding is relatively broad. It is not only an anti-interference measure, but also an important measure for safety. Correct grounding can suppress
It can control the influence of external electromagnetic interference and prevent electronic and electrical equipment from emitting electromagnetic waves to the outside; and wrong grounding often introduces very serious interference, and even makes electronic and electrical equipment unable to work normally.
In a narrow sense, grounding is to connect the equipment to the ground and lead the interference current into the ground (the aforementioned shielding and filtering must be well grounded. If it is not grounded, it will not work, and there will be no shielding and filtering.) Broadly speaking, Grounding includes the concept of establishing a reference potential in the equipment, which is the equipotential point or equipotential surface in the circuit system.
The main purpose of grounding is to:
1) Establish a low-impedance path with the ground, so that large currents (such as lightning strike current) can flow directly into the ground without affecting the circuit. Normal operation of equipment and personal safety:
2) To establish a low impedance between the shell of the equipment and the zero potential, and when there is a leakage current in the equipment, it will not endanger personal safety?
3) Prevent the accumulation of electrostatic charges;
4) Establish a common equipotential point (or surface) in the circuit or in the equipment so that there is a common reference potential: 5) The circuit (DC, low frequency or high frequency current) needs to pass through the ground wire to form a loop. Good The grounding design can make each current flowing through the ground wire independent of each other, or its influence can be suppressed.
(4) Other measures
In addition to the above-mentioned main measures such as shielding, filtering, and grounding, anti-interference also includes: isolation, which is used to prevent the interference signal between the primary circuit and the ground from being transmitted to the secondary circuit; voltage equalization, to equalize the potential difference between points in the use area : Reasonable wiring, except that all connecting lines (signal lines, power lines and grounding lines, etc.) ; Parallel laying of wires should pay attention to the symmetry of the geometric space of the wires to reduce the sensitivity to electromagnetic interference.
5 Anti-interference measures for power communication equipment
The communication equipment in the power system (in fact, including all electronic equipment), especially the equipment in the substation, is in a very harsh electromagnetic environment, and all live equipment is a source of interference (disturbance). The wire is a radiation source. Therefore, anti-interference measures should be considered,
(1) Electromagnetic Compatibility Requirements for Equipment
The equipment is the source and the receptor of electromagnetic disturbance, and its own electromagnetic compatibility is the most important factor. According to the definition of compatibility, it includes two aspects, that is, how to improve the immunity of the equipment and prevent electromagnetic leakage. For users For example, we must pay attention to the electromagnetic compatibility performance of the equipment when selecting equipment. At present, there are many international organizations related to EMC. The most important ones are TC-77 and CIS2PRTC-77. Study the relevant standards and regulations on electromagnetic compatibility between electrical equipment in low-voltage networks. CISPR is an organization established to study the prevention of broadcast reception, communication, etc. The equipment puts forward suggestions on the limit value of electromagnetic interference. Many domestic standards are formulated with reference to their standards. When purchasing communication equipment, including switching power supplies and uninterruptible power supplies (UPS) for communication, the electromagnetic compatibility index should be taken as a Indispensable indicators, pay attention to the adoption of equipment that meets the above criteria.
(2) Anti-interference measures for equipment
After the equipment is selected, how to correctly install and use it in the current environment is the next problem to be solved. The anti-interference measures of the equipment should first consider the shielding. The important communication room itself should have a shielding layer to reduce the radiation in the air Coupled disturbance; followed by the use of DC switching power supply and UPS to isolate the AC interference signal from the power supply. From a practical point of view, the most important thing is the grounding problem. At present, the grounding method adopted by communication equipment
1) Single point grounding
Single-point grounding is to combine the "ground" of each device on a manifold or a busbar, and then connect the grounding lead to the ground grid. Single-point grounding can make all devices use one point as a reference point, with stable The potential of the ground grid can eliminate the interference caused by various circulating currents to the equipment due to the potential difference of the ground grid.
2) Multi-point grounding
Multi-point grounding is to connect each grounding point in the system directly to the ground grid conductor closest to it. As shown in Dong 2, the multi-point grounding system is relatively simple, which can reduce the high-frequency standing wave phenomenon that may appear on the grounding lead. However, multi-point grounding will cause ground loops between devices, and the potential difference of the ground network may cause circulation to interfere with the devices.
3) Shared ground
Shared grounding means that the grounding of the communication system in the substation is connected with the grounding grid in the substation. Its main feature is that it has the same reference potential. Poor, which is beneficial to the safety of people and equipment. The main problem of the common ground type is the influence of various strong electric disturbance ground currents in the power system under normal or accidental conditions
4) separate grounding
Separate grounding means that except for lightning protection and substations sharing the same grounding network, the grounding of communication and other electronic equipment is all connected to a specially laid grounding pair. Ground to solve the interference of conductance coupling. However, attention should be paid to the connection of signal lines, and the problems of potential induction and low potential introduction in the event of network failure.
There are two laying methods for special grounding: one is to lay the grounding grid far away from the main grounding grid of the substation, and use high-voltage insulated wires to connect the grounding bus in the computer room with the special grounding grid. The main problem is the introduction of low potential. This grounding method is more suitable for substation reconstruction. The other is to lay it in the main grounding network of the substation. The potential difference between the network and the main network is not large, which can greatly reduce the interference between the two. For new substations, it can be considered.
(3) Anti-interference measures for connection lines
The connection line connects the equipment, the user and the power supply, and is an indispensable part of the communication system. The electromagnetic interference signal superimposed on it will be directly transmitted to the inside of the equipment, causing various interferences to normal work and even damage to the equipment.
The anti-interference measures of the connection line mainly adopt shielding, grounding and isolation, and reasonable wiring. Pay attention to the following aspects:
1) Using twisted pair cable
For communication majors, twisted-pair cables are the most commonly used cables. For other majors, this is not the case. In substations, cables from communication equipment to other professional special equipment are often designed and constructed by other majors, and this problem is often ignored. Cause The equipment is not working properly. In fact, the twisted pair cable uses the twisting of the wire pair to improve the balance of the two wires to the interference source, and reduces the electromagnetic interference generated between the wires by electrostatic coupling and electromagnetic induction. It is a good secondary signal cable.
2) Grounding of cable sheath and spare core
Everyone knows the function of cable sheath grounding, but the grounding of spare cores is often neglected. Experiments have proved that in the substation, the grounding of the spare core of the plastic cable can reduce the induced voltage to about 30%~40%.
3) Separate cables for different level signals
The transient signal of one core wire in the cable can be coupled to another core wire through the capacitance or inductance between the core wires. At this time, neither the cable sheath nor the spare core wire grounding can effectively weaken this interference. Effective The best method is to put the wires of different types of circuits in different cables
4) Try to use the cable trench
The cable trench below the ground is generally close to the conductor of the grounding grid or has a grounding wire in the cable trench, which is a natural shielding place. Communication cables should be routed in the cable trench as much as possible
5) Direction of communication (secondary) cables
The electromagnetic field distribution in the substation is very complicated, so the direction of the communication cable in the substation should be radial to avoid the formation of loops.
6) The location of the communication (secondary) cable
The high-voltage busbar in the substation is a strong interference source, and the entry point of the transient current, such as a lightning arrester, has a large high-frequency transient current passing through the ground point of the lightning rod, which will cause the ground potential near the ground point to rise. Therefore, the communication cable is laid out It should be as far away from the above places as possible, and the parallel length with the busbar should be minimized
7) Using signal isolation
If the above measures cannot be used or there is still a large interference after the adoption; or the transmitted signal is very important (such as a relay protection signal), the anti-interference ability is very high, and the isolation device can be used for isolation. The commonly used isolation device is an isolation transformer. For weak electric signals, photoelectric isolation can be used, and the isolation effect is better.
6 Conclusion
The problem of electromagnetic compatibility is a very complicated problem. This paper briefly describes and analyzes the problems of electromagnetic compatibility in power communication and the measures that should be taken.