Abstract: insulation coordination is an important issue related to the safety of electrical equipment products, and has always been paid attention to from all aspects. Insulation coordination was first used in high voltage electrical products. In China, the accident caused by insulation system accounts for 50% to 60% of the electric products in China. It is only two years since the concept of insulation coordination is formally quoted in low voltage switchgear and control equipment. Therefore, it is a more important problem to deal with and solve the insulation coordination problem in the product correctly, and enough attention should be paid to it.
Key words: insulation and insulation materials of low voltage switchgear
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. As the low-voltage switchgear goes deep into the production site, public place, residential and other places, it can be said that all places where electrical equipment is used shall be equipped with low-voltage equipment. About 80% of the power energy in China is supplied through low-voltage switchgear. The development of low-voltage switchgear is derived from material industry, low-voltage electrical appliances, processing technology and equipment Infrastructure construction and people’s living standards, so the level of low voltage switchgear reflects the economic strength, science and technology and living standard of a country 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 and surrounding environment of the equipment. Only when the design of the equipment is based on the strength of the function it bears in its expected life, can the insulation coordination be realized. The problem of insulation coordination not only comes from the outside of the equipment but also from the equipment itself. It is a problem involving all aspects, which should be considered comprehensively. The main points are divided into three parts: first, the use conditions of the equipment; The second is the use environment of the equipment, and the third is the selection of insulation materials.
1.1 the use conditions of equipment the use 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, the voltage that may occur in the system, voltage generated by equipment, required continuous voltage operation level, and the danger of personal safety and accident shall be considered.
① Classification of voltage and overvoltage, waveform.
A. continuous power frequency voltage, with constant R, m, s voltage;
B. temporary overvoltage, power frequency overvoltage for a long time;
C transient overvoltage, over-voltage for a few milliseconds or less, is usually high damping oscillation or non oscillation.
——A transient overvoltage, usually one-way, reaching a peak value of 20 μ sTp5000 μ Between S, the duration of wave tail T2 ≤ 20ms.
——Fast wave pre overvoltage: a transient overvoltage, usually in one direction, reaching a peak value of 0.1 μ sT120 μ s. Wave tail duration T2 ≤ 300 μ s。
——Steep wave front overvoltage: a transient overvoltage, usually in one direction, reaching peak value at TF ≤ 0.1 μ s. The total duration is 3MS, and there is superimposed oscillation, and the frequency of oscillation is between 30kHz and 100MHz.
D. combined (temporary, slow forward, fast, steep) overvoltage.
According to the above overvoltage type, the standard voltage waveform can be described.
② The relationship between long-term AC or DC voltage and insulation coordination should consider rated voltage, rated insulation voltage and actual working voltage. In the normal and long-term operation of the system, the rated insulation voltage and actual working voltage should be considered. In addition to meeting the requirements of the standard, we should pay attention to the actual situation of China’s power grid. In the current situation that the quality of power grid is not high in China, when designing products, the actual possible working voltage is more important for insulation coordination.
③ The relationship between transient overvoltage and insulation coordination is related to the condition of controlled over-voltage in the electrical system. In the system and equipment, there are many forms of overvoltage. The influence of overvoltage should be considered comprehensively. In low voltage power system, overvoltage may be affected by various variable factors. Therefore, the overvoltage in the system is evaluated by statistical method, reflecting a concept of probability of occurrence, And it can be determined by the method of probability statistics whether protection control is needed.
1.1.2 the over-voltage category of equipment shall be divided into IV class directly from the overvoltage category of low-voltage power grid power supply equipment according to the long-term continuous voltage operation level required by the equipment use conditions. The equipment of overvoltage category IV is the equipment used at the power supply end of the distribution device, such as ammeter and current protection equipment of the previous stage. The equipment of overvoltage class III is the task of installation in the distribution device, and the safety and applicability of the equipment must meet the special requirements, such as the switchgear in the distribution device. The equipment of overvoltage class II is the energy consuming equipment powered by distribution device, such as the load for home use and similar purposes. The equipment of overvoltage class I is connected to the equipment which limits the transient overvoltage to a very low level, such as electronic circuit with over-voltage protection. For equipment not directly supplied by low voltage grid, the maximum voltage and serious combination of various situations that may occur in system equipment must be taken into account.
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Electric field is divided into uniform electric field and non-uniform electric field. In low voltage switchgear, it is generally considered to be in the case of non-uniform electric field. The frequency problem is still under consideration. Generally, low frequency has little influence on insulation coordination, but high frequency still has influence, especially on insulation materials.
1.2 the macro environment of the equipment related to the insulation coordination and environmental conditions affects the insulation coordination. From the requirements of the current practical application and standards, the change of air pressure only takes into account the change of air pressure caused by altitude. The daily air pressure change has been ignored, and the factors of temperature and humidity have also been ignored. However, if there is more accurate requirements, the air pressure will be changed according to the requirements of the standards, These factors should also be taken into account. From the micro environment, the macro environment determines the micro environment, but the micro environment may be better or worse than the macro environment equipment. The different protection levels, heating, ventilation and dust of the shell may affect the micro environment. The micro environment has clear provisions in relevant standards, which provides the basis for the design of the products.
1.3 the problems of insulation coordination and insulation materials are quite complex. It is different from gas, and it is an insulating medium that can not be recovered once damaged. Even the accidental overvoltage event may cause permanent damage. In the long-term use, the insulation materials will encounter various situations, such as discharge accidents, The insulation material itself will accelerate its aging process due to various factors accumulated for a long time, such as thermal stress, temperature, mechanical impact and other stresses. For insulation materials, due to the variety of varieties, the characteristics of insulation materials are not uniform, although there are many indicators. This brings some difficulty to the selection and use of insulating materials, which is the reason why other characteristics of insulation materials, such as thermal stress, mechanical properties, partial discharge, etc., are not considered at present.
2. Verification of insulation coordination
At present, the optimal method to verify insulation coordination is to use impulse dielectric test, and different rated impulse voltage values can be selected for different equipment.
2.1 the insulation matching rated impulse voltage of the equipment is 1.2/50 by the rated impulse voltage test μ S wave form.
The output impedance of impulse generator of impulse test power supply should be more than 500 generally Ω， The rated impulse voltage value shall be determined according to the use situation, overvoltage category and long-term use voltage of the equipment, and shall be corrected according to the corresponding altitude. At present, some test conditions are applied to the low voltage switchgear. If there is no clear stipulation on humidity and temperature, it should also be within the scope of application of the standard for complete switchgear. If the equipment use environment is beyond the applicable scope of the switchgear set, it must be considered to be corrected. 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 pressure kPa
2.2 for low voltage switchgear, AC or DC test can be used to replace impulse voltage test for dielectric test of alternative impulse voltage, but this kind of test method is more severe than impulse voltage test, and it should be agreed by the manufacturer.
The duration of the experiment is 3 cycles in the case of communication.
DC test, each phase (positive and negative) respectively applied voltage three times, each time duration is 10ms.
In the current situation of China, in the high and low voltage electrical products, the insulation coordination of equipment is still a big problem. Because of the formal introduction of insulation coordination concept in low voltage switchgear and control equipment, it is only a matter of nearly two years. Therefore, it is a more important problem to deal with and solve the insulation coordination problem in the product.
Reference:
[1] Iec439-1 low voltage switchgear and control equipment – Part I: type test and part type test complete equipment [s].
Iec890 check the temperature rise of low voltage switchgear and control equipment through some type test sets by extrapolation method.
Post time: Feb-20-2023