Abstract: The problem of insulation coordination is an important issue related to the safety of electrical equipment products, which has always received attention from all aspects. Insulation coordination was first used in high-voltage electrical products. In my country's electrical products, accidents caused by insulation systems account for 50%-60%, and the concept of insulation coordination has been officially introduced in low-voltage switchgear and control equipment. It is only a matter of the past two years. Therefore, it is a relatively important issue to correctly handle and solve the problem of insulation coordination in products, which should be given enough attention.
Key words: low-voltage switchgear insulation coordination insulation material
0 Preface
The low-voltage switchgear is responsible for the control, protection, measurement, conversion and distribution of electrical energy in the low-voltage power supply system. Since low-voltage switchgear goes deep into production sites, public places, residential buildings and other places, it can be said that low-voltage equipment should be equipped wherever electrical equipment is used. About 80% of my country's electrical energy is supplied through low-voltage switchgear. The development of equipment comes from the material industry, low-voltage electrical appliances, processing technology and equipment, infrastructure construction and people's living standards, so the level of low-voltage complete switchgear reflects a country's economic strength, science and technology, and living standards from one side.
1 The basic principle of insulation coordination
Insulation coordination means that the electrical insulation characteristics of the equipment are selected according to the conditions of use of the equipment and the surrounding environment. Insulation coordination can only be achieved if the design of the equipment is based on the strength of the action that it will withstand during its expected life. The problem of insulation coordination comes not only from the outside of the equipment but also from the equipment itself. It is a problem that involves various factors and must be considered comprehensively. The main points are divided into three parts: one is the use conditions of the equipment; It is the choice of insulating material.
1.1 Conditions of use of the equipment The conditions of use of the equipment mainly refer to the voltage, electric field and frequency used by the equipment.
1.1.1 The 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 of voltage and overvoltage, waveform.
a continuous power frequency voltage, with constant r, m, s voltage;
b Temporary overvoltage, power frequency overvoltage of longer duration;
c Transient overvoltage, an overvoltage of a duration of several milliseconds or less, usually highly damped oscillatory or non-oscillating.
——Slow wave front overvoltage: a transient overvoltage, usually unidirectional, the time to reach the peak value is between 20μsTp5000μs, and the wave tail duration T2≤20ms.
——Fast wave front overvoltage: a transient overvoltage, usually unidirectional, the peak time is 0.1μsT120μs, and the wave tail duration T2≤300μs.
——Steep wave front overvoltage: a transient overvoltage, usually unidirectional, the time to reach the peak value is Tf≤0.1μs, the total duration is 3ms, and there is superimposed oscillation, and the oscillation frequency is between 30kHz and 100MHz.
d Combined (temporary, slow wave, fast wave front, steep wave front) overvoltage.
From the above-mentioned overvoltage types, standard voltage waveforms can be described.
②The 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, it is also necessary to consider the actual situation of my country's power grid. In the current situation that the quality of my country'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 condition of controlled overvoltage in the electrical system. In the system and equipment, there are various forms of overvoltage, and the influence of various overvoltages should be fully considered. In the low-voltage power system, the overvoltage may be affected by various variable factors. Therefore, the overvoltage in the system It is assessed by statistical methods, which reflects a concept of probability of occurrence, and can be used to determine whether protection control is required by means of probability and statistics.
1.1.2 Overvoltage category of equipment According to the operating conditions of the equipment and the required long-term continuous voltage operation level, the overvoltage category of the equipment directly powered by the low-voltage grid is divided into level IV. Overvoltage category IV equipment is the equipment used at the power supply side of the power distribution device, such as electricity meters and front-end current protection equipment. Overvoltage category III equipment is the task of installation in power distribution equipment, and the safety and applicability of equipment must meet special requirements, such as switching appliances in power distribution equipment. Equipment of overvoltage category II is energy-consuming equipment supplied by power distribution units, such as household and similar loads. Overvoltage category I equipment is connected to equipment that limits transient overvoltages to fairly low levels, such as electronic circuits with overvoltage protection. For equipment that is not directly powered by the low-voltage grid, the highest possible voltage of the system equipment and a serious combination of conditions must be considered.
The 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. Regarding the frequency problem, it is still under consideration. It is generally believed that low frequency has little effect on insulation coordination, but High frequencies still have an effect, especially on insulating materials.
1.2 The relationship between insulation coordination and environmental conditions The macro environment of the equipment 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, and the daily air pressure change has been ignored. , the factors of temperature and humidity have also been ignored, but 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 protection levels of the shell, heating, ventilation, and dust may affect the micro-environment. The micro-environment is related to The standard has clear provisions, which provides a basis for the design of the product.
1.3 Insulation coordination and insulating materials The problem of insulating materials is quite complicated. It is different from gas. It is an insulating medium that cannot be recovered once it is damaged. Even accidental overvoltage events may cause permanent damage. In use, various situations will be encountered, such as discharge accidents, etc., and the insulation material itself will accelerate its aging process due to various factors accumulated for a long time, such as thermal stress, temperature, mechanical shock and other stresses. For insulating materials, due to the diversity of varieties, although there are many characteristic indicators for measuring insulating materials, they are not uniform. This brings a certain degree of difficulty to the selection and use of insulating materials, which is why other characteristics of insulating materials, such as thermal stress, mechanical properties, partial discharge and other indicators, are temporarily not considered from the international arena.
2 Verification of insulation coordination
At present, the preferred method for verifying insulation coordination is to use the impulse dielectric test, and different rated impulse voltage values can be selected for different equipment.
2.1 Use the rated impulse voltage test to verify the insulation coordination of the equipment The waveform of the rated impulse voltage is 1.2/50μs.
Use this waveform to simulate transient overvoltage, atmospheric overvoltage, and also include overvoltage generated by switching on and off of low-voltage equipment. The output impedance of the pulse waveform generator of the impulse test power supply should generally be greater than 500Ω, and the rated impulse voltage value should be determined. , should be determined according to the use occasion of the equipment, the overvoltage category and the long-term use voltage of the equipment, and should be corrected according to the corresponding altitude. Current low-voltage switchgear sets are subject to certain test conditions. If the humidity and temperature are not clearly specified, they should also be within the applicable scope of the complete switchgear standard. If the use environment of the equipment exceeds the applicable scope of the complete switchgear, it must be considered for correction. The correction relationship between air pressure and temperature is as follows:
K=P/101.3×293(ΔT+293)
K——the correction parameter of air pressure and temperature
ΔT——The temperature difference K between the actual (laboratory) temperature and T=20℃
P——actual air pressure kPa
2.2 Dielectric test in place of impulse voltage For low-voltage switchgear, AC or DC test can be used instead of impulse voltage test, but this kind of test method is more severe than impulse voltage test, and the consent of the manufacturer should be obtained.
The AC test, in the AC case, lasts 3 cycles.
In the DC test, each phase (positive and negative) is applied with voltage three times, and the duration of each time is 10ms.
As far as the actual situation in our country is concerned, in high and low voltage electrical products, the insulation coordination of equipment is still a big problem, and because the concept of insulation coordination is officially introduced in low-voltage complete switchgear and control equipment, it is only a matter of nearly two years. year thing. Therefore, it is a relatively important issue to properly handle and solve the problem of insulation coordination in products.
References:
[1] IEC439-1 low-voltage complete switchgear and control equipment part one: type test and partial type test complete sets of equipment [S].
IEC890 Use extrapolation method to check the temperature rise of low-voltage switchgear and control equipment through partial type test[S].
