Abstract: Insulation coordination is an important issue related to the safety of electrical equipment products, which has always been paid attention to by all aspects. Insulation coordination was first used in high-voltage electrical products. In China's electrical products, accidents caused by insulation systems account for 50%-60%, and due to the formal introduction of the concept of insulation coordination in low-voltage switchgear and control equipment, it is only a matter of the past two years. Therefore, the correct handling and solving the problem of insulation coordination in products is a more important issue and should be given sufficient attention.

Keywords: low voltage switchgear insulation coordination insulation material

0 Introduction

The low-voltage switchgear assembly is responsible for the control, protection, measurement, conversion and distribution of electric energy in the low-voltage power supply system. As low-voltage switchgear sets go deep into production sites, public places, residential houses and other places, it can be said that all places using electrical equipment should be equipped with low-voltage equipment. About 80% of China's electric energy is supplied through low-voltage switchgear sets. The development of low-voltage switchgear sets stems from material industry, low-voltage electrical appliances, processing technology and equipment, infrastructure construction and people's living standards, therefore, the level of low-voltage switchgear from one side reflects a country's economic strength and science and technology, living standards.

1 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, and can only be achieved when the design of the equipment is based on the strength of 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 considered comprehensively. Its main points are divided into three parts: one is the use conditions of the equipment; the other is the use environment of the equipment, and the third is The selection of insulating materials.

Conditions of use of 1.1 equipment The conditions of use 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 occur 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, s voltage;

B temporary overvoltage, longer duration of power frequency overvoltage;

c Transient overvoltages, overvoltages of a duration of a few milliseconds or less, usually highly damped oscillatory or non-oscillatory.

-- Slow front overvoltage: A transient overvoltage, usually unidirectional, with a peak time between 20μsTp5000μs and a tail duration T2 ≤ 20ms.

-- Fast wavefront overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 0.1 μsT120μs and a tail duration T2 ≤ 300 μs.

-steep wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of Tf≤ 0.1 μs, a total duration of 3ms, and superimposed oscillation with an oscillation frequency of 30 kHzf100MHz.

D joint (temporary, slow front wave, fast wave front, steep wave front) overvoltage.

According to the above-mentioned type of overvoltage, a standard voltage waveform can be described.

The relationship between long-term AC or DC voltage and insulation coordination should consider the rated voltage, rated insulation voltage and actual working voltage. In the process of normal and long-term operation of the system, the rated insulation voltage and the actual working voltage should be considered, which should not only meet the requirements of the standard, but also pay attention to 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.

The relationship between transient overvoltage and insulation coordination, which is related to the conditions of the controlled overvoltage in the electrical system. In the system and equipment, there are many forms of over-voltage, to fully consider the impact of a variety of over-voltage, in the low-voltage power system, over-voltage may be affected by a variety of factors, so, the system of over-voltage is assessed by statistical methods, reflecting a probability of occurrence of the concept, and can be used to determine whether the need for protection and control.

1.1.2 Over-voltage category of equipment According to the operating conditions of the equipment and the required long-term continuous voltage operation level, the over-voltage category of the equipment directly supplied by the low-voltage grid is divided into level IV. Over-voltage category IV equipment is used in the power distribution device power end equipment, such as electricity meters and pre-current protection equipment. Over-voltage category III equipment is installed in the power distribution device of the task, and the use of safety and applicability of the equipment must meet the special requirements, such as the power distribution device in the switch electrical appliances. Overvoltage Class II devices are energy-consuming devices powered by electrical distribution devices, such as household and similar loads. Overvoltage Class I devices are connected to devices that limit transient overvoltages to a relatively low level, such as electronic circuits with overvoltage protection. For equipment that is not directly supplied by the low-voltage network, the maximum voltage that may occur in the system equipment and the serious combination of various conditions must be considered.

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The electric field is divided into uniform electric field and non-uniform electric field. In low-voltage switchgear, it is generally believed that it is in the case of non-uniform electric field. The frequency problem is still under consideration. It is generally believed that low frequency has little influence on insulation coordination, but high frequency still has influence, especially on insulating materials.

The relationship between 1.2 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. The daily air pressure change has been ignored, and the factors of temperature and humidity have also been ignored. However, if there are more accurate requirements, these factors should also be considered. 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. Different protection levels, heating, ventilation and dust of the shell may affect the micro environment. The micro environment is clearly defined in the relevant standards, which provides a basis for product design.

1.3 insulation coordination and insulation material The problem of insulation material is quite complicated. It is different from gas. It is an unrecoverable insulation medium once it is damaged. Even accidental overvoltage events may cause permanent damage. In long-term use, insulation materials will encounter various situations, such as discharge accidents, etc., while insulation materials themselves are due to various factors accumulated for a long time, such as thermal stress and temperature, stress such as mechanical impact will accelerate its aging process. For insulating materials, due to the diversity of varieties, the characteristic indicators of insulating materials are many, but not uniform. This brings certain difficulties 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 not considered in the world.

2 Verification of insulation coordination

At present, the preferred method of verifying the insulation coordination is to use the impact dielectric test, and different rated impact voltage values can be selected for different equipment.

2.1 The rated impulse voltage test is used to verify the insulation of the equipment and the waveform of the rated impulse voltage is 1.2/50μs.

This waveform is used to simulate transient over-voltage and atmospheric over-voltage, including over-voltage generated by on-off and off-switch of low-voltage equipment. The output impedance of the pulse waveform generator of the impact test power supply should generally be greater than 500 Ω. The determination of the rated impact voltage value should be determined according to the application of the equipment, over-voltage category and long-term use voltage of the equipment, and should be corrected according to the corresponding altitude. Present low-voltage switchgear assemblies for certain test conditions. Such as humidity, temperature does not make a clear provision, but also should be in the scope of the standard switchgear, such as the use of the equipment environment beyond the scope of the switchgear, it must be considered to amend. The correction relationship between air pressure and temperature is as follows:

K = P/101.3 × 293(ΔT 293)

K -- Correction parameter of air pressure and temperature

ΔT -- 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 sets, 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 shall be approved by the manufacturer.

AC test, in the case of AC, the duration is 3 cycles.

DC test, each phase (positive and negative) applied voltage three times, each duration of 10ms.

As far as the actual situation of our country is concerned, in the high and low voltage electrical products, the insulation coordination of the equipment is still a big problem, and because the concept of insulation coordination is formally quoted in the low voltage switchgear and control equipment, it is only in the past two years. Therefore, it is an important problem to correctly handle and solve the problem of insulation coordination in products.

References:

IEC439-1 low-voltage switchgear and controlgear assemblies-Part 1: Type-tested and partially type-tested assemblies [S].

IEC890 Temperature rise of low-voltage switchgear and control equipment packages through partial type test by extrapolation [S].