Abstract: insulation coordination is an important issue related to the safety of electrical equipment products, which has always been paid attention to from all aspects. Insulation coordination was first used in high-voltage electrical products. In China's electrical products, the accidents caused by insulation system account for 50%-60%. It is only two years since the concept of insulation coordination has been formally introduced into low-voltage switchgear and control equipment. Therefore, it is an important issue to properly handle and solve the insulation coordination problem in the product, which should be paid enough attention.
Key words: low voltage switchgear insulation coordination insulation materials
0 Introduction
The low-voltage switchgear is responsible for the control, protection, measurement, conversion and distribution of electric energy in the low-voltage power supply system. Since low-voltage switchgear sets go deep into production sites, public places, residential houses and other places, it can be said that all places where electrical equipment is used should be equipped with low-voltage equipment. About 80% of China's electricity is supplied through low-voltage switchgear sets. The development of low-voltage switchgear sets stems from the material industry, low-voltage electrical appliances, processing technology and equipment, infrastructure construction and people's living standards, Therefore, the level of low-voltage switchgear sets reflects a country's economic strength, science and technology, and living standards from one side.
1 basic principle of insulation coordination
Insulation coordination means that the electrical insulation characteristics of the equipment are selected according to the service conditions of the equipment and the surrounding environment. Insulation coordination can be realized only when the design of the equipment is based on the action strength it bears in its expected life. The problem of insulation coordination not only comes from the outside of the equipment, but also from the equipment itself. It is a problem that involves various factors and must be comprehensively considered. Its main points are divided into three parts: first, the service conditions of the equipment; The second is the operating environment of the equipment, and the third is the selection of insulating materials.
1.1 service conditions of equipment the service conditions of equipment mainly refer to the voltage, electric field and frequency used by the equipment.
1.1.1 relationship between insulation coordination and voltage. In considering the relationship between insulation coordination and voltage, it is necessary to consider the voltage that may appear in the system, the voltage generated by the equipment, the required continuous voltage operation level, and the risk of personal safety and accidents.
① Classification and waveform of voltage and overvoltage.
A. continuous power frequency voltage with constant R, m and S;
B temporary overvoltage, power frequency overvoltage with long duration;
C transient overvoltage, overvoltage with a duration of several milliseconds or less, usually high damping oscillation or non oscillation.
——Slow wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 20 μ sTp5000 μ S, wave tail duration T2 ≤ 20ms.
——Fast wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 0.1 μ sT120 μ s. Wave tail duration T2 ≤ 300 μ s。
——Steep wave front overvoltage: a transient overvoltage, which is usually unidirectional and reaches the peak value with TF ≤ 0.1 μ s. The total duration is 3MS with superimposed oscillation. The oscillation frequency is between 30khzf100mhz.
D combined (temporary, slow front wave, fast front wave, steep front wave) overvoltage.
According to the above overvoltage types, the standard voltage waveform can be described.
② For the relationship between long-term AC or DC voltage and insulation coordination, the rated voltage, rated insulation voltage and actual working voltage shall be considered. During the normal and long-term operation of the system, the rated insulation voltage and actual working voltage shall be mainly considered, which shall not only meet the requirements of the standard, but also consider the actual situation of China's power grid. At present, the quality of China's power grid is not high. When designing products, the actual possible working voltage is more important for insulation coordination.
③ The relationship between transient overvoltage and insulation coordination, which is related to the controlled overvoltage conditions in the electrical system. In the system and equipment, there are many forms of overvoltage, so the influence of various overvoltage should be fully considered. In the low-voltage power system, overvoltage may be affected by various variable factors. Therefore, the overvoltage in the system is evaluated by statistical method, which reflects a concept of occurrence probability, and whether protection control is required can be determined by probabilistic method.
1.1.2 overvoltage category of equipment: according to the service conditions of equipment and the required long-term continuous voltage operation level, the overvoltage category of equipment directly supplied by low-voltage power grid is divided into level IV. Overvoltage class IV equipment refers to the equipment used at the power supply end of the power distribution device, such as electricity meters and front-end current protection equipment. The equipment of overvoltage category III is the task installed in the power distribution device, and the use safety and applicability of the equipment must meet special requirements, such as the switchgear in the power distribution device. Overvoltage category II equipment refers to energy consuming equipment powered by power distribution devices, such as loads for household and similar purposes. The equipment of overvoltage category I is connected to the equipment that limits the transient overvoltage to a relatively low level, such as the electronic circuit with overvoltage protection. For the equipment not directly powered by the low-voltage power grid, the maximum voltage that may occur in the system equipment and the serious combination of various conditions must be considered.
The electric field is divided into uniform electric field and non-uniform electric field. In the low-voltage switchgear, it is generally considered to be in the case of non-uniform electric field. As for the frequency, it is still under consideration. It is generally considered that the low frequency has little impact on the insulation coordination, but the high frequency still has an impact, especially on the insulating materials.
1.2 relationship between insulation coordination and environmental conditions the macro environment in which the equipment is located affects the insulation coordination. From the current practical application and standard requirements, the change of air pressure only takes into account the change of air pressure caused by altitude, the daily change of air pressure has been ignored, and the factors of temperature and humidity have also been ignored. However, if there are more precise requirements, these factors should also be considered. From the perspective of micro environment, the macro environment determines the micro environment, but the micro environment may be better or worse than the macro environment equipment. Different degrees of protection, heating, ventilation and dust of the shell may affect the micro environment. The micro environment is clearly specified in relevant standards, which provides a basis for product design.
1.3 insulation coordination and insulating materials the problem of insulating materials is quite complex. It is different from gas. It is an insulating medium that cannot be recovered once it is damaged. Even an accidental overvoltage event may cause permanent damage. Insulating materials will encounter various situations in long-term use, such as discharge accidents. However, due to various factors accumulated for a long time, such as thermal stress and temperature, Mechanical impact and other stresses will accelerate its aging process. As for insulating materials, due to the diversity of varieties, there are many indexes to measure the characteristics of insulating materials, but they are not unified. This brings some difficulties to the selection and use of insulating materials, which is the reason why other characteristics of insulating materials, such as thermal stress, mechanical properties, partial discharge and other indicators, are not considered internationally at present.
2 verification of insulation coordination
At present, the preferred method to verify insulation coordination is to use impulse dielectric test. Different rated impulse voltage values can be selected for different equipment.
2.1 the rated impulse voltage of the insulation coordination of the equipment verified by the rated impulse voltage test is 1.2/50 μ S waveform.
This waveform is used to simulate transient overvoltage and atmospheric overvoltage, as well as the overvoltage generated by switching on and off of low-voltage equipment. The output impedance of impulse test power supply pulse waveform generator shall generally be greater than 500 Ω. The determination of rated impulse voltage value shall be determined according to the equipment use occasion, overvoltage category and long-term service voltage of the equipment, and shall be corrected according to the corresponding altitude. At present, some test conditions for low-voltage switchgear assemblies. If the humidity and temperature are not clearly specified, they should also be within the scope of application of the complete set of switchgear standards. If the service environment of the equipment exceeds the scope of application of the complete set of switchgear, it must be considered for correction. The correction relationship between air pressure and temperature is as follows:
K=P/101.3 × 293( Δ T+293)
K -- correction parameters of air pressure and temperature
Δ T -- temperature difference K between actual (Laboratory) temperature and t=20 ℃
P - actual air pressure kPa
2.2 dielectric test instead of impulse voltage for low-voltage switchgear, AC or DC test can be used to replace impulse voltage test, but such test method is more severe than impulse voltage test and shall be approved by the manufacturer.
AC test, under AC conditions, the duration is 3 cycles.
For DC test, apply voltage to each phase (positive and negative) for three times, each time lasting for 10ms.
As far as the actual situation in China is concerned, the insulation coordination of equipment is still a big problem in high-voltage and low-voltage electrical products. It is only two years since the concept of insulation coordination has been formally introduced in low-voltage switchgear and control equipment. Therefore, it is an important problem to properly handle and solve the insulation coordination problem in products.
reference:
[1] Iec439-1 low voltage switchgear and controlgear assemblies Part 1: type test and partial type test assemblies [s]
Iec890 using extrapolation method to check the temperature rise of low-voltage switchgear and control equipment [s]
Scanning code
