Insulation coordination of low-voltage switchgear

Abstract: In 1987, the 17th Technical Committee of the International Electrotechnical Commission (IEC) drafted a technical document entitled "Supplement 1 to IEC439 Requirements for Insulation Coordination", which formally introduced the insulation coordination problem into low-voltage switchgear and control equipment. 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.

Key words: low-voltage switchgear insulation coordination insulation materials

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 1987, the 17th Technical Committee of the International Electrotechnical Commission (IEC) drafted a technical document entitled "Requirements for Insulation Coordination in Supplement 1 to IEC439", which formally introduced the insulation coordination problem into low-voltage switchgear and control equipment. As far as the actual situation in China is concerned, the insulation coordination of equipment is still a big problem in high and low voltage electrical products. Statistics show that 50% - 60% of the accidents caused by the insulation system in China's electrical products, and because the concept of insulation coordination is formally introduced in low-voltage switchgear and control equipment, it is only a matter of nearly two years. Therefore, it is an important issue to properly handle and solve the insulation coordination problem in products.

2、 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） Service conditions of equipment

The service conditions of the equipment mainly refer to the voltage, electric field and frequency of the equipment.

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

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 s<Tp<5000 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 s<T1<20 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<f<100MHz.

d) Combined (temporary, slow front, fast front, 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 should 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 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. 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.

2. Overvoltage category of equipment

According to the service 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 power grid is divided into Class IV. 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 switching appliances 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.

When the equipment needs to work in a higher level of overvoltage category, but the allowable overvoltage category of the equipment itself is not enough, measures need to be taken to reduce the overvoltage at this place, and the following methods can be adopted.

a) Overvoltage protection device

b) Transformer with isolated winding

c) Multi-branch circuit distribution system with scattered transfer wave passing voltage energy

d) Capacitance capable of absorbing surge overvoltage energy

e) Damping device capable of absorbing surge overvoltage energy

3. Electric field and frequency

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.

（2） 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. 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 protection grades, heating, ventilation and dust of the shell may affect the micro-environment. The micro-environment is clearly specified in the relevant standards, see Table 1, which provides a basis for product design.

（3） Insulation coordination and insulation materials

The problem of insulating materials is quite complex. It is different from gas. It is an insulating medium that can not be recovered once it is damaged. Even accidental overvoltage events may cause permanent damage. In the long-term use of insulating materials, they will encounter various situations, such as discharge accidents, and the insulating materials themselves due to long-term accumulation of various factors, such as thermal stress, temperature, mechanical impact and other stresses, It 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. The influence of the above stress on insulating materials has been discussed in IEC publications, which can play a qualitative role in guiding practical application, but quantitative guidance is not available at present. At present, low-voltage electrical products are mainly used as indicators for quantitative guidance of insulating materials, which are divided into three groups and four categories, namely, leakage resistance tracking index (PTI). The leakage trace index is used to form the leakage trace by dripping the water contaminated liquid onto the surface of the insulating material, and give a quantitative comparison.

A certain amount of indicators have been applied to the design of products.

3、 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.

1. Verify the insulation coordination of equipment with rated impulse voltage test

1.2/50 of 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. Dielectric test to replace impulse voltage

AC or DC test can be used to replace the impulse voltage test for low-voltage switchgear, but this test method is more severe than the 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.

4、 General procedures for insulation coordination.

1. Determination of typical overvoltage.

2. Determination of coordination withstand voltage.

3. Determination of rated insulation level.