Insulation coordination of low voltage switchgear

Abstract: in 1987, technical committee 17D of International Electrotechnical Commission (IEC) drafted a technical document named "requirements for insulation coordination in Supplement 1 to iec439", which formally introduced the insulation coordination into low voltage switchgear and control equipment. As far as the actual situation of our country is concerned, in the high and low voltage electrical products, the insulation coordination of equipment is still a big problem, and because the concept of insulation coordination is formally used 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.

Key words: low voltage switchgear insulation materials

Insulation coordination is an important issue related to the safety of electrical equipment products, which has always been paid attention from all aspects. Insulation coordination was first used in high voltage electrical products. In 1987, the technical committee 17D of International Electrotechnical Commission (IEC) drafted a technical document entitled "requirements for insulation coordination in Supplement 1 to iec439", which formally introduced the insulation coordination into low-voltage switchgear and control equipment. As far as the actual situation of our country is concerned, in the high and low voltage electrical products, the insulation coordination of equipment is still a big problem. Some statistics show that in the electrical products of our country, the accidents caused by the insulation system account for 50% - 60%, and because the concept of insulation coordination is formally used 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.

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. Only when the design of the equipment is based on the action strength in its expected life can 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 involves various factors and needs to be considered comprehensively. Its main points are divided into three parts: the first is the service conditions of the equipment; the second is the service environment of the equipment; and the third is the selection of insulation 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. 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, as well as personal safety and the risk of accidents.

1 Classification of voltage and overvoltage, waveform.

a) Continuous power frequency voltage, with constant R, m, s voltage

b) Temporary overvoltage, long duration power frequency overvoltage

c) Transient overvoltage, which lasts for several milliseconds or less, is usually highly damped, oscillatory or non oscillatory.

——Slow wave front overvoltage: a kind of transient overvoltage, usually unidirectional, with peak time of 20 μ s < TP < 5000 μ s and tail duration of T2 ≤ 20ms.

——Fast wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 0.1 μ s < T1 < 20 μ s and a tail duration of T2 ≤ 300 μ s.

——Steep wavefront overvoltage: a transient overvoltage, usually unidirectional, with a peak time of TF ≤ 0.1 μ s, a total duration of < 3MS, and a superimposed oscillation with a frequency of 30kHz < f < 100MHz.

d) Combined (temporary, slow, fast and steep) overvoltage.

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

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 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. In the current situation that the quality of power grid in China is not high, the actual working voltage is more important for insulation coordination when designing products.

The relationship between transient overvoltage and insulation coordination is related to the condition of controlled overvoltage in electrical system. In the system and equipment, there are many kinds of over-voltage, so we should consider the influence of all kinds of over-voltage. In the low-voltage power system, over-voltage may be affected by various factors. Therefore, the over-voltage in the system is evaluated by the statistical method, which reflects a concept of probability of occurrence, and can determine whether it needs to be protected by the probability statistical method Control.

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 supplied by the low-voltage power grid is divided into grade IV. Overvoltage class IV equipment is used in the power supply side of distribution equipment, such as ammeter and current protection equipment. Overvoltage class III equipment is installed in the distribution equipment task, and the use of equipment safety and applicability must meet the special requirements, such as the switch electrical equipment in the distribution equipment. Overvoltage class II equipment is energy consuming equipment powered by power distribution equipment, such as household and similar loads. The equipment of overvoltage class I is connected to the equipment that limits the transient overvoltage to a relatively low level, such as the electronic circuit with overvoltage protection. For the equipment not directly supplied by the low-voltage power grid, the highest possible voltage of the system equipment and the serious combination of various conditions must be considered.

When the equipment is to work in a higher level of over-voltage category, and the allowed over-voltage category of the equipment itself is not enough, it is necessary to take measures to reduce the over-voltage. The following methods can be used.

a) Over voltage protection device

b) Transformers with isolated windings

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

d) Capacitor capable of absorbing surge overvoltage energy

e) Damping devices capable of absorbing surge overvoltage energy

3. Electric field and frequency

Electric field is divided into uniform electric field and non-uniform electric field. In low-voltage switchgear, it is generally considered that it is in the case of non-uniform electric field. As for frequency, it is still under consideration. It is generally considered that low frequency has little effect on insulation coordination, but high frequency has effect, especially on 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 accurate requirements, these factors should also be considered. From the micro environment point of view, the macro environment determines the micro environment, but the micro environment may be better or worse than the macro environment equipment. The different protection level of the shell, heating, ventilation and dust may affect the micro environment. The micro environment is clearly specified in the relevant standards, as shown in table 1, which provides the basis for product design.

（3） Insulation coordination and insulation materials

The problem of insulating material is quite complex. It is different from gas. It is an irrecoverable insulating medium once it is damaged. Even the accidental overvoltage event may cause permanent damage. In the long-term use of insulating material, it will encounter all kinds of situations, such as discharge accident, etc., while the insulating material itself is due to various factors accumulated for a long time, such as thermal stress, temperature and so on Stress, such as temperature, mechanical impact, will accelerate its aging process. For insulating materials, due to the diversity of varieties, there are many indexes to measure the characteristics of insulating materials, but they are not unified. This brings some difficulties for the selection and use of insulating materials, which is the reason why other characteristics of insulating materials, such as thermal stress, mechanical properties, partial discharge and so on, are not considered internationally. The effects of the above stresses on insulating materials have been discussed in IEC publications, which can provide some qualitative guidance for practical application, but the quantitative guidance is not available at present. At present, low-voltage electrical products as a quantitative guide to the insulation material index used more than leakage tracking index CTI value, divided into three groups of four categories, leakage tracking index PTI value. The tracking index of electric leakage is to form the tracking of electric leakage through the dripping of water contaminated liquid on the surface of insulating materials, and give a quantitative comparison.

This index has been applied to product design.

3、 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.

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

The waveform of rated impulse voltage is 1.2/50 μ s.

The output impedance of pulse waveform generator of impulse test power supply should be generally greater than 500 Ω. The determination of rated impulse voltage should be determined according to the application situation, overvoltage category and long-term service voltage of the equipment, and should be based on the corresponding sea voltage Pull out the height to correct. At present, the low-voltage switchgear is not suitable for some test conditions. If the humidity and temperature are not specified clearly, they should also be within the scope of application of the standard for complete sets of switchgear. If the service environment of the equipment exceeds the scope of application of complete sets of switchgear, it must be considered and corrected. 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

Δ T - temperature difference K between actual (Laboratory) temperature and T = 20 ℃

P - actual air pressure, kPa

2. Dielectric test instead of impulse voltage

For low-voltage switchgear, AC or DC test can be used to replace impulse voltage test, but this kind of test method is more severe than impulse voltage test, and it should be approved by the manufacturer.

AC test, in the case of AC, lasted for 3 cycles.

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

4、 General procedure of insulation coordination.

1. Determination of typical overvoltage.

2. Determination of coordination withstand voltage.

3. Determination of rated insulation level.