1、 Foreword
In the power system, with the increase of power grid capacity and transmission voltage, relay protection, power grid control and communication equipment based on computers and microprocessors are widely used. Therefore, the problem of electromagnetic compatibility in power system has become very prominent. For example, the substation integrated power equipment, which integrates relay protection, communication and SCADA functions, is usually installed near the substation high-voltage equipment. The prerequisite for the equipment to work normally is that it can withstand the extremely strong electromagnetic interference generated in the substation under normal operation or accident conditions. In addition, because modern high-voltage switches are often integrated with electronic control and protection equipment, the equipment combined with strong current and weak current equipment not only needs to be tested for high voltage and large current, but also needs to pass the electromagnetic compatibility test. When the disconnector of GIS is operated, it can produce a fast transient voltage with a frequency of up to several megahertz. This fast transient overvoltage will not only endanger the insulation of transformers and other equipment, but also spread outward through the grounding grid, interfering with the normal work of relay protection and control equipment in substations. With the improvement of power system automation level, the importance of electromagnetic compatibility technology has become increasingly apparent.
According to the definition of the International Electrotechnical Commission (1ec), electromagnetic compatibility (EMC) refers to the ability of equipment or system to work normally in its electromagnetic environment without constituting unbearable electromagnetic interference to anything in the environment. EMC electromagnetic compatibility is a new interdisciplinary and comprehensive applied discipline. As an edge technology, it is based on the basic theory of electrical and radio technology, and involves many new technical fields, such as microelectronics technology, computer technology, microwave technology, communication technology, network technology, new material application and so on. The research of electromagnetic compatibility technology covers a wide range, covering almost all automation application fields, such as power, communication, radio, transportation, aerospace, military industry, computer and medical treatment.
All kinds of electrical equipment in the same power system are closely connected with each other and affect each other through electrical or magnetic connections. Electromagnetic oscillation caused by the change of operation mode, fault, switch operation, etc. will affect many electrical equipment, and the working performance of these electrical equipment will be affected or even destroyed. All these show that the electromagnetic compatibility problem of power system has become a problem that can not be ignored.
2、 Some concepts about electromagnetic compatibility
1. Electromagnetic compatibility environment (EME)
It refers to the sum of all electromagnetic phenomena existing in a given place. A given place is space, which refers to all electromagnetic phenomena, including all time and all spectrum.
2. Electromagnetic compatibility (EMC)
EMC means that the equipment or system can work normally in its electromagnetic environment without electromagnetic interference to anything in the environment. As a discipline, EMC can be translated as “electromagnetic compatibility”. The EMC capability of a device or system can be called “electromagnetic compatibility”. It can be seen from the definition that 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 this equipment or system is enough to make the function of this equipment or system not affected by other interference.
3. Electromagnetic interference (EMI)
Electromagnetic interference refers to any electromagnetic phenomenon that may reduce the performance of devices, equipment and systems or cause damage to living substances. It consists of interference source, coupling channel and receiver. According to the way of interference propagation, electromagnetic interference is divided into radiation interference and conduction interference. Radiated interference (RI) propagates through space and in the characteristics and laws of electromagnetic waves, but not any device can radiate electromagnetic waves; Conducted interference (CI) is the interference that propagates along the conductor, that is, the propagation of conducted interference must have a complete circuit connection between the interference source and the receiver.
4. Electromagnetic susceptibility (EMS)
Generally speaking, if the sensitivity is high, the anti-interference degree is low. EMS reflects the anti-interference ability of devices, equipment or systems from different perspectives. The smaller the sensitivity level (the level at which the performance degradation has just begun), the higher the sensitivity, the lower the anti-interference degree; The higher the anti-interference level is, the higher the anti-interference level is, and the lower the sensitivity is. Electromagnetic sensitivity is divided into radiation sensitivity and conduction sensitivity. At present, the hot topics of electromagnetic compatibility (EMC) research mainly include the characteristics and transmission characteristics of electromagnetic interference sources, the harmful effects of electromagnetic interference, the suppression technology of electromagnetic interference, the utilization and management of electromagnetic spectrum, the standards and specifications of electromagnetic compatibility, the measurement and test technology of electromagnetic compatibility, electromagnetic leakage and electrostatic discharge, etc.
3、 Main electromagnetic interference modes and transmission routes
The formation of electromagnetic compatibility of power equipment is mainly due to the increase of power equipment in all walks of life, the extensive use of wireless communication equipment, electric equipment and high-frequency equipment in the surrounding environment, and the increasing electromagnetic interference between equipment. According to the electromagnetic compatibility of power equipment, people in the industry know that equipment interfere with each other, that is, some equipment is not only vulnerable to various interferences, but also interfere with other equipment. In fact, many devices have electromagnetic compatibility, but the interference between them has not been clearly detected, but these potential threats have affected the safe operation of power equipment. Of course, the electromagnetic compatibility of the equipment also includes the potential safety hazards caused by electromagnetic leakage. Electromagnetic leakage refers to the leakage of useful information. Although they are weak electromagnetic signals, for some malicious attackers, once they are interested in some information, they can easily use modern means to intercept, amplify, decrypt or decode to obtain information.
Electromagnetic interference mainly includes the following:
1. Harmonic interference
The influence and harm of harmonics on primary equipment are mainly shown in the following aspects: increasing the loss of equipment, increasing the temperature rise, reducing the output and service life of equipment; Increase the dielectric loss and partial discharge in the insulation, and accelerate the aging of the insulation; Increase the vibration and noise of the motor.
The main influence of harmonics on the secondary equipment is to interfere with its normal working state, such as the accuracy of measurement, the reliability of action and so on.
In the case of fault, the distance protection has a greater impact on the interference of harmonic to relay protection devices. The impedance relay is set according to the fundamental impedance of the system. The occurrence of harmonics, especially the third harmonic, will cause great measurement error, which may lead to rejection or misoperation in serious cases.
2. Switch operation in primary circuit
It is mainly the operation of circuit breakers, disconnectors, etc. in the power network, which causes overvoltage of capacitor banks, no-load transformers, reactors, motors, etc., and electromagnetic interference from pantographs.
3. Lightning interference
When lightning strikes the substation in the power grid, the large current will be discharged into the grounding grid through the grounding point, which will greatly increase the grounding point potential. If the grounding point of the secondary circuit is close to the grounding point of the large lightning current, the grounding point potential of the secondary circuit will increase accordingly, which will form common mode interference in the secondary same circuit, cause overvoltage, and even cause insulation breakdown of the secondary equipment in serious cases.
4. Interference of secondary circuit itself
The interference of the secondary circuit itself is mainly generated by electromagnetic induction. Many digital integrated circuit devices of integrated power equipment in substations or power plants are realized by single-chip microcomputer system. Because the devices on the printed circuit board (PCB) in the system are powered by DC power supply, and there are many large inductance coils in the DC circuit, when switching operation, overvoltage will appear at both ends of the coil, which will induce induced voltage and induced current that are not conducive to the normal operation of the secondary equipment, causing interference to the devices on the PCB, thereby interfering with the normal operation of the single chip microcomputer system.
There are two ways to transmit electromagnetic interference from interference source to sensitive equipment, namely, conduction and radiation. Conduction is divided into conductivity coupling, direct coupling, capacitive coupling, electric field coupling and inductive coupling. The radiation is mainly electromagnetic coupling. Interference generated by magnetic field is caused by mutual inductance between conductors. When the current in the secondary circuit changes suddenly, the magnetic flux linked to the secondary circuit also changes, and then the interference voltage is induced. The greater the amplitude and frequency of the transient current in the primary circuit, the stronger the magnetic connection between the primary circuit and the secondary circuit, the greater the interference caused by inductive coupling. The interference of power system is mainly transmitted to low-voltage equipment through TA, CVT and transmission cables, followed by high-frequency radiation coupling. The main coupling forms are conductivity and inductance coupling.
4、 Measures to suppress electromagnetic interference
In any system, the formation of EMC must meet three basic conditions (called the three elements of electromagnetic interference): the existence of interference sources, a receiving unit sensitive to interference sources, and a channel to couple energy from interference sources to the receiving unit.
According to the types and characteristics of electromagnetic interference, shielding, filtering and grounding methods are generally adopted to suppress electromagnetic interference.
1. Interference transmission channel suppression
(1) Shielding can be divided into electric field shielding, magnetic field shielding and electromagnetic shielding. Generally, electromagnetic shielding is used to prevent interference caused by alternating electromagnetic fields. Shielding has two purposes: A. limiting the leakage of electromagnetic energy radiated in the equipment to the outside; b. Prevent external radiation interference from entering the equipment and interfering with the normal operation of the equipment.
a. Electric field shielding method
The simplest measure is to use a metal diaphragm to ground between the induction source and the inductor to suppress parasitic capacitance coupling and realize electric field shielding. If the electric field interference is strong, the grounding effect of metal cover with high conductivity is better.
b. Magnetic field shielding method
The magnetic field is divided into low-frequency magnetic field and high-frequency magnetic field, and different measures should be taken for different magnetic fields. For low-frequency magnetic fields, high magnetic conductivity materials can be used as shields to realize magnetic field shielding, but the shielded components and parts shall not have gaps in the direction parallel to the magnetic field to avoid magnetic leakage. For high-frequency magnetic field, due to the existence of electric field component and magnetic field component, it is required to use electric field shielding and magnetic field shielding at the same time. However, the anti high-frequency magnetic field of ferromagnetic materials is limited to below 100kHz, and special measures need to be taken for higher frequency magnetic fields. In order to prevent magnetic leakage of slots and holes, the slots should be reduced or the depth of slots should be increased as much as possible, and the holes should be covered with metal covers. If there are protruding metal shafts, they must be reliably grounded or equipped with waveguide attenuators, etc.
When the magnetic field to be shielded is strong, the shielding material will be saturated. Once saturated, the shielding efficiency will be lost. In this case, double-layer shielding can be used. The first layer is made of low permeability material, which is not easy to be saturated; The second layer is made of high permeability material, but it is easy to saturate. The first layer of shielding first attenuates the magnetic field to an appropriate strength, so that the second layer of shielding will not be saturated, and the high permeability material can give full play to the shielding effect.
(2) Filtering
Filtering technology is an effective measure to filter out power interference. Generally speaking, the interference caused by power pollution is the most common. With the rapid development of electronic technology, the application of switching power supply is becoming more and more popular. Therefore, from the perspective of eliminating electromagnetic interference generated by switching power supply, EMI filter should also be considered. The design of EMI filter is different from that of traditional filter. In addition to attenuating the high frequency of electromagnetic interference as much as possible, it also requires that under the cut-off frequency, the impedance of power supply, load and the impedance of corresponding elements of the filter should be as close as possible, and follow two basic principles: a. the series inductance of the filter should be connected to the low impedance power supply or low impedance load; b. The shunt capacitor of the filter should be connected to a high impedance power supply or a high impedance load. In this way, the practical application effect of EMI filter can be improved.
The correct installation method of the filter is also very important. If it is installed on the circuit board, the electromagnetic interference directly enters the filter, which will reduce the filtering effect, so the filter must be shielded.
(3) Grounding
Grounding is one of the basic technical requirements of circuit, equipment and system work, and it is also one of the most basic methods to prevent interference. Because grounding can return the interference current in the circuit to the earth, the correct grounding can effectively suppress the influence of interference signals on other equipment.
The three basic methods of grounding, filtering and shielding can enhance the electromagnetic compatibility of electromagnetic equipment, which can be implemented separately or complementary to each other. For example, reliable grounding of equipment can prevent electrostatic interference and reduce the requirements of equipment for shielding; Good electromagnetic shielding can effectively prevent electromagnetic radiation interference, and the requirements for filter circuits can be appropriately relaxed. Considering the overall effect, good grounding can reduce the energy of interference frequency; Shielding can isolate the way of electromagnetic radiation coupling and reduce the energy of radiation; Filtering can attenuate the interference energy transmitted through the power supply.
2. Time separation
The principle of time sharing is to turn on the interfering and interfered equipment in time periods, and avoid using mutually interfering equipment at the same time.
3. Frequency management measures
Frequency management includes frequency control, frequency modulation, digital transmission and photoelectric conversion. Frequency control means that the equipment with the same frequency in the equipment should not be used together, and attention should be paid to the frequency doubling interference between them. Frequency modulation technology is to use frequency secondary modulation to avoid interference frequency. Digital transmission refers to the conversion of analog signals into digital signals for transmission, so that various interferences can be prevented to the greatest extent. Enterprises can try photoelectric conversion and photoelectric transmission technology if they have conditions, because photoelectric signals have very high signal-to-noise ratio and anti-interference ability.
4. Spatial separation
Location and location selection, isolation of natural buildings, equipment installation angle control, electric field and magnetic field vector direction control. That is, take the technical treatment of avoidance and dredging, make reasonable use of the natural isolation formed by the building, select the appropriate installation location and direction, and control the interference caused by equipment with poor electromagnetic compatibility to the greatest extent. For example, when installing the monitor, the direction of the transmitting and receiving bracket must be reasonably selected, and it should be as far away from the elevator, TV and computer as possible.
5、 Main contents of EMC Research
The main contents of electromagnetic compatibility of power system include:
1. Electromagnetic environment assessment
Estimate the electromagnetic interference level (amplitude, frequency, waveform, etc.) that the equipment may be subjected to during operation by means of actual measurement or digital simulation. For example, a movable electromagnetic compatibility test vehicle is used to measure various interferences generated by high-voltage transmission lines or substations, or a transient electromagnetic field that may be generated is digitally simulated through an electromagnetic transient calculation program. Electromagnetic environment evaluation is an important part of electromagnetic compatibility technology and the basis of anti-interference design.
2. Electromagnetic interference coupling path
Find out the path through which the electromagnetic interference generated by the interference source reaches the interfered object. Generally speaking, interference can be divided into conduction interference and radiation interference. Conducted interference refers to the interference caused by electromagnetic interference propagating to the object through power lines, grounding wires and signal lines. For example, the interference generated by the lightning impulse source transmitted through the power line. Radiated interference refers to the interference that spreads through the space of electromagnetic source to sensitive equipment. For example, the radio interference or television interference generated by the corona of the transmission line belongs to the radiation type interference. Studying the coupling ways of interference is of great significance for formulating anti-interference measures and eliminating or suppressing interference.
3. Electromagnetic immunity evaluation
Study the ability of various sensitive equipment and instruments in the power system, such as relay protection, automatic devices, computer systems, electric energy metering instruments, to withstand electromagnetic interference. Generally, tests are used to simulate the possible interference in operation, and test whether the tested equipment will have maloperation or permanent damage when the equipment is as close to the working conditions as possible. The immunity of the equipment depends on its working principle, electronic circuit layout, working signal level, and the anti-interference measures taken. With the wide adoption of various automation systems and communication systems in the power system, and with the trend of integrating strong current equipment with strong current equipment, how to evaluate the ability of these equipment to withstand interference, study practical and effective test methods, and formulate evaluation standards will become an important topic of electromagnetic compatibility technology in the power system.
4. Anti interference measures — generation and coupling of electromagnetic interference
It is impossible for sensitive equipment to completely avoid electromagnetic interference. Therefore, the most economical and reasonable solution is to apply anti-interference measures to sensitive equipment. For example, it is inevitable that the power dispatching building is struck by lightning. However, the safe operation of the access system and dispatching automation system can be ensured by correct grounding, shielding and isolation measures. Studying effective, economical and applicable anti-interference measures is also an important task in the field of electromagnetic compatibility in the future.
5. Power quality
The 36 Academic Committee of the international grid Conference (electromagnetic compatibility of power systems) has also included power quality control in the scope of electromagnetic compatibility to study the impact of frequency changes, harmonics, voltage flicker, voltage dips, etc. on the performance of user equipment.
Public doubts about the possible harmful effects of power frequency electromagnetic fields on human health have become an important constraint to the development of high-voltage transmission in some countries. People are familiar with the harmful effects of ionizing radiation, such as X-rays and gamma rays, on human health. Nonionizing radiation, including whether low-frequency electromagnetic fields have harmful effects on biological systems, especially human health, has always been an open question.
6、 Concluding remarks
With the wide application of power system automation equipment and the progress of technology, the problem of electromagnetic compatibility is becoming more and more prominent. The top priority of power system application technology is to promote the existing and mature electromagnetic compatibility technology, establish a perfect test and test system and inspection standards, and study new problems and new directions of electromagnetic compatibility. In the design and application of automation engineering, electromagnetic interference can be eliminated and the stability and reliability of equipment can be improved as long as the electromagnetic compatibility of equipment is fully considered and various technical measures and management methods are adopted.
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