1. Preface
In 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 electromagnetic compatibility problem of power system has become very prominent. For example, the substation integrated power equipment integrating relay protection, communication and SCADA functions is usually installed near the high-voltage equipment of the substation. The precondition for the normal operation of the equipment 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 such strong current and weak current equipment not only needs to be tested with high voltage and large current, but also needs to pass the electromagnetic compatibility test. When the disconnector of GIS is operated, it can generate fast transient voltage with frequency up to several megahertz. This fast transient over-voltage will not only endanger the insulation of transformer and other equipment, but also spread outward through the grounding grid, interfering with the normal operation of relay protection and control equipment in the substation. With the improvement of power system automation, the importance of EMC technology is increasingly apparent.
According to the definition of the International Electrotechnical Commission (1EC), electromagnetic compatibility (EMC) refers to the ability of the equipment or system to work normally in its electromagnetic environment without causing unbearable electromagnetic interference to anything in the environment. EMC is a new interdisciplinary comprehensive application 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, computer technology, microwave technology, communication technology and network technology, as well as new material applications. The research scope of EMC technology is very wide, covering almost all automation application fields, such as power, communication, radio, transportation, aerospace, military, computer and medical.
All kinds of electrical equipment in the same power system are closely connected and interact with each other through electrical or magnetic connection. The electromagnetic oscillation caused by the change of operation mode, fault, switch operation, etc. will affect many electrical equipment, which will affect the working performance of these electrical equipment, or even damage them. All these indicate that the electromagnetic compatibility problem of power system has become a problem that cannot be ignored.
2. Several Concepts of 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 refers to that the equipment or system can work normally in its electromagnetic environment and does not cause electromagnetic interference to anything in the environment. As a discipline, EMC can be translated as "electromagnetic compatibility". The EMC capability as a device or system can be called "EMC". From the definition, it can be seen that EMC includes two aspects: electromagnetic emission generated by equipment or system will not affect the function of other equipment or systems; The anti-interference capability of the equipment or system is sufficient to prevent the function of the equipment or system from being affected by other interferences.
3）Electromagnetic interference (EMI)
Electromagnetic interference refers to any electromagnetic phenomenon that may reduce the performance of devices, equipment and systems or damage 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) is transmitted through space with the characteristics and laws of electromagnetic waves, but not all devices can radiate electromagnetic waves; Conducted interference (CI) is the interference that propagates along the conductor, that is, there must be a complete circuit connection between the interference source and the receiver.
4）Electromagnetic sensitivity (EMS)
If the sensitivity is high, the anti-interference is low. EMS reflects the anti-interference ability of devices, equipment, or systems from different angles. The lower the sensitivity level (the level when the performance is degraded at the beginning), the higher the sensitivity, and the lower the anti-interference; 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 of electromagnetic interference sources and their transmission characteristics, the harmful effects of electromagnetic interference, the suppression technology of electromagnetic interference, the utilization and management of electromagnetic spectrum, electromagnetic compatibility standards and specifications, electromagnetic compatibility measurement and test technology, electromagnetic leakage and electrostatic discharge, etc.
3. Main EMI 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, industry insiders know that equipment interferes with each other, that is, some equipment is not only vulnerable to various interferences, but also interferes 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 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 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: increase the loss of equipment, increase the temperature rise, and reduce the output and life of equipment; Increase dielectric loss and local discharge in insulation to accelerate insulation aging; Increase the vibration and noise of the motor.
The main influence of harmonics on secondary equipment is to interfere with its normal working state, such as measurement accuracy, action reliability, etc.
In case of fault, distance protection has a greater impact on the interference of harmonics to relay protection devices. The impedance relay is set according to the fundamental wave impedance of the system. The appearance of harmonics, especially the third harmonic, will cause great measurement errors, and may lead to refusal or maloperation in serious cases.
2) Switch operation in primary circuit
It is mainly due to 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 the pantograph.
3) Lightning disturbance
When the lightning strikes the substation in the power grid, the large current will drain into the grounding grid through the grounding point, which will greatly increase the potential of the grounding point. If the secondary circuit grounding point is close to the lightning strike point of large current, the potential of the secondary circuit grounding point will increase accordingly, which will form a common mode interference in the secondary same circuit, causing over-voltage, which will 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 of the digital integrated circuit devices of integrated power equipment in substations or power plants are realized by single chip microcomputer systems. 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, over-voltage 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, thus interfering with the normal operation of the microcontroller system.
Electromagnetic interference can be transmitted from interference source to sensitive equipment in two ways, namely conduction and radiation. Conduction is divided into conductive coupling direct coupling, capacitive coupling electric field coupling and inductive coupling. The radiation is mainly electromagnetic coupling. The interference generated by magnetic field is caused by mutual inductance between conductors. When the current in the secondary circuit suddenly changes, the magnetic flux from the cross link 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, and the greater the interference caused by the 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 conductive and inductive coupling.
4. Measures to suppress electromagnetic interference
In any system, the formation of EMC must meet three basic conditions (called three elements of electromagnetic interference): the existence of interference sources, the existence of receiving units sensitive to interference sources, and the existence of channels to couple energy from interference sources to receiving units.
According to the type and characteristics of electromagnetic interference, shielding, filtering, and grounding methods are generally adopted to suppress electromagnetic interference.
4.1 Interference transmission channel suppression
4.1.1 Shielding can be divided into electric field shielding, magnetic field shielding and electromagnetic shielding. Generally, electromagnetic shielding is adopted to prevent interference generated by alternating electromagnetic fields. Shielding has two purposes: a. to limit 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 ground the inductive source and the inductor with a metal partition to suppress parasitic capacitive coupling and realize electric field shielding. For strong electric field interference, it is better to use high conductivity metal cover for grounding.
b. Magnetic field shielding method
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 field, high magnetic conductivity materials can be used as shields to realize magnetic field shielding, but the shielded components 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, electric field shielding and magnetic field shielding are required to be conducted simultaneously. However, the high frequency magnetic field protection of ferromagnetic materials is limited to below 100kHz. For higher frequency magnetic fields, special measures should be taken. In order to prevent magnetic leakage from gaps and holes, the gaps should be reduced or the gap depth should be increased as much as possible. The holes should be covered with metal covers. If there are protruding metal shafts, they must be reliably grounded or waveguide attenuators should be installed.
When the magnetic field to be shielded is very strong, the shielding material will be saturated. Once the saturation occurs, the shielding effectiveness will be lost. In this case, double-layer shielding can be used, and the first layer is made of low permeability material, which is not easy to saturate; 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.
4.1.2 Filtering
Filtering technology is an effective measure to filter power interference. Interference caused by power pollution is the most common. With the rapid development of electronic technology, the application of switching power supply is increasingly 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 electromagnetic interference as much as possible, it is also required to make the power supply, load impedance and corresponding element impedance of the filter as close as possible at the cut-off frequency, and follow two basic principles: a. The series inductance of the filter should be connected to a low impedance power supply or a low impedance load; B. The parallel capacitor of the filter shall be connected to the high impedance power supply or 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 important. For example, when the filter 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.
4.1.3 Grounding
Grounding is one of the basic technical requirements for circuit, equipment, and system work, and also one of the most basic methods to prevent interference. Because grounding can make the interference current in the circuit return to the ground, correct grounding can effectively suppress the influence of interference signal 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 mutually complementary. For example, the reliable grounding of equipment can prevent electrostatic interference and reduce the shielding requirements of equipment; Good electromagnetic shielding can effectively prevent electromagnetic radiation interference, and the requirements for filter circuits can be appropriately relaxed. Considering the overall effect, a good grounding can reduce the energy of interference frequency; Shielding can isolate the coupling path of electromagnetic radiation and reduce the radiation energy; The filtering can attenuate the interference energy transmitted through the power supply.
4.2 Time separation
The time-sharing rule is to turn on the interfering device and the interfered device in different time periods to avoid the simultaneous use of interfering devices in the same time period.
4.3 Frequency management measures
Frequency management includes frequency control, frequency modulation, digital transmission, and photoelectric conversion. Frequency control means that equipment with the same frequency in the equipment shall not be used together, and attention shall be paid to the double frequency interference between them. Frequency modulation technology is to use frequency to modulate the equipment twice 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 can, because photoelectric signals have very high signal-to-noise ratio and anti-interference ability.
4.4 Spatial separation
Selection of location and location, isolation of natural buildings, angle control of equipment installation, vector direction control of electric field and magnetic field. That is to say, the avoidance and unblocking technology shall be adopted, the natural isolation formed by buildings shall be reasonably used, the appropriate installation location and direction shall be selected, and the interference caused by equipment with poor electromagnetic compatibility shall be controlled to the maximum extent. For example, when installing the monitor, the direction of the transmitting and receiving bracket must be reasonably selected, and it must 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:
5.1 Electromagnetic environment assessment
The electromagnetic interference level (amplitude, frequency, waveform, etc.) that the equipment may be subjected to during operation shall be estimated by means of measurement or digital simulation. For example, the movable electromagnetic compatibility test vehicle is used to measure various interferences generated by high-voltage transmission lines or substations, or the transient electromagnetic field that may be generated is digitally simulated through electromagnetic transient calculation program. Electromagnetic environment assessment is an important part of EMC technology and the basis of anti-interference design.
5.2 EMI coupling path
Find out the path through which the electromagnetic interference generated by the interference source reaches the interfered object. Interference can be divided into conducted interference and radiated interference. Conducted interference refers to the interference caused by electromagnetic interference propagating through power lines, grounding wires and signal lines to the object. For example, interference generated by lightning impulse source transmitted through power line. Radiation interference refers to the interference transmitted to sensitive equipment through electromagnetic source space. For example, radio interference or television interference generated by transmission line corona belongs to radiation type interference. Studying the coupling way of interference is of great significance for formulating anti-interference measures and eliminating or suppressing interference.
5.3 Evaluation of electromagnetic immunity
Study the ability of various sensitive equipment and meters in the power system, such as relay protection, automatic device, computer system, electric energy metering instrument, to withstand electromagnetic interference. Generally, the test is used to simulate the possible interference in operation and test whether the tested equipment will cause 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 anti-interference measures taken. With the wide application of various automation systems and communication systems in power systems, and with the trend of integrating strong current equipment and 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 power systems.
6. Concluding remarks
With the wide application of power system automation equipment and the progress of technology, the electromagnetic compatibility problem is becoming more and more prominent. It is urgent to promote the existing and mature electromagnetic compatibility technology, establish a perfect test and testing system and inspection standards, and study new problems and new directions of electromagnetic compatibility in power system application technology. In the design and application of automation engineering, electromagnetic interference can be eliminated and the stability and reliability of equipment can be improved if the electromagnetic compatibility of equipment is fully considered and various technical measures and management methods are adopted.
