The insulation coordination principle and verification of low-voltage switchgear

Abstract: 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 China's electrical products, accidents caused by insulation system account for 50% - 60%, and because the concept of insulation coordination is formally introduced in low-voltage switchgear and control equipment, it has only been nearly two years. Therefore, it is an important issue to properly handle and solve the insulation coordination problem in the product, which should be given enough attention.

Key words: low-voltage switchgear insulation coordination insulation materials

0 Introduction

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 areas 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 originates 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 to select the electrical insulation characteristics of the equipment according to the service conditions and surrounding environment of the equipment. Insulation coordination can only be realized when the design of the equipment is based on the action strength it bears in 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 involving various factors and needs to be considered comprehensively. Its main points are divided into three parts: first, the service conditions of the equipment; The second is the use 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 of 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, power frequency overvoltage with long duration;

C Transient overvoltage, which lasts for several milliseconds or less, is usually highly damped oscillation or non-oscillation.

——Slow-wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 20 μ sTp5000 μ Between s, the 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 kind of transient overvoltage, which is usually unidirectional, and the time to reach the peak value is Tf ≤ 0.1 μ s. The total duration is 3ms, with superimposed oscillation, and the oscillation frequency is between 30kHz and 100MHz.

D Combined (temporary, slow front, fast front and steep front) overvoltage.

According to the above overvoltage types, the standard voltage waveform can be described.

② The relationship between long-term AC or DC voltage and insulation coordination shall consider rated voltage, rated insulation voltage and actual working voltage. During the normal and long-term operation of the system, the rated insulation voltage and actual working voltage should be considered, which should not only meet the requirements of the standard, but also take into account the actual situation of China's power grid. In the current situation that the quality of China's power grid is not high, the actual possible working voltage is more important for insulation coordination when designing products.

③ The relationship between transient overvoltage and insulation coordination is related to the conditions of controlled overvoltage in the electrical system. In the system and equipment, there are many forms of overvoltage. The impact 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, reflecting a concept of probability of occurrence, and the need for protection and control can be determined by the method of probability statistics.

1.1.2 The overvoltage category of the equipment is classified as Level IV according to the service conditions of the equipment and the required long-term continuous voltage operation level. Equipment of overvoltage category IV refers to the equipment used at the power supply end of the power distribution device, such as the ammeter and the previous current protection equipment. The equipment with 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 class II equipment refers to energy-consuming equipment powered by power distribution devices, such as household and similar loads. Equipment with overvoltage category I is connected to equipment that limits transient overvoltage to a relatively low level, such as electronic circuits with overvoltage protection. For equipment that is 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.

|<12>>

The electric field is divided into uniform electric field and non-uniform electric field. In the low-voltage switchgear, it is generally considered that it is 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 insulation 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 atmospheric pressure only takes into account the change of atmospheric pressure caused by altitude. The daily atmospheric pressure change 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 protection grades, heating, ventilation and dust of the shell may affect the micro-environment. The micro-environment is clearly defined in relevant standards, which provides a basis for product design.

1.3 The problem of insulation coordination and insulating materials is quite complex. It is different from gas. It is an insulating medium that cannot be recovered once it is damaged. Even if an accidental overvoltage event occurs, it may cause permanent damage. In the long-term use of insulating materials, it will encounter various situations, such as discharge accidents, and the insulating materials themselves 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. For insulating materials, due to the diversity of varieties, there are many but not uniform indicators to measure the characteristics of insulating materials. 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 characteristics, partial discharge and other indicators, are not considered in the world 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 Use the rated impulse voltage test to verify that the insulation coordination of the equipment is 1.2/50 of the rated impulse voltage μ Waveform of s.

This waveform is used to simulate transient overvoltage and atmospheric overvoltage, and also includes overvoltage generated by switching on and off of low-voltage equipment. The output impedance of impulse test power pulse waveform generator shall be generally greater than 500 Ω. The determination of rated impulse voltage value shall be determined according to the use of the equipment, overvoltage category and long-term service voltage of the equipment, and shall be corrected according to the corresponding altitude. At present, 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 scope of application of the complete set of switchgear standards. If the operating 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 parameter 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 instead of impulse voltage test, but this kind of test method is more severe than impulse voltage test and should be approved by the manufacturer.

The duration of AC test is 3 cycles under AC condition.

For DC test, each phase (positive and negative) is applied with voltage three times, each time lasting for 10ms.

As far as the actual situation of our country is concerned, the insulation coordination of equipment is still a big problem in high and low voltage electrical products, and since the concept of insulation coordination has been formally introduced in low-voltage switchgear and control equipment, it has only been nearly two years. Therefore, it is an important issue 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 uses extrapolation method to check the temperature rise of low-voltage switchgear and control equipment that have passed part of the type test [S]