Abstract: In 1987, Subcommittee 17D 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. middle. 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 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.

Key words: low-voltage switchgear insulation and insulation materials

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 1987, Subcommittee 17D of the International Electrotechnical Commission (IEC) drafted a technical document called "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 in our country is concerned, in the high and low voltage electrical products, the insulation coordination of the equipment is still a big problem. Statistics show that the accidents caused by the insulation system account for 50% of the electrical products in our country. -60%, and because the concept of insulation coordination is 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 properly handle and solve the problem of insulation coordination in products.

2. 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. The insulation coordination can only be achieved when the design of the equipment is based on the strength of the action to be endured 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) Conditions of use of the equipment

The use conditions of the equipment mainly refer to the voltage, electric field and frequency used by 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, 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, long duration power frequency overvoltage

c) Transient overvoltages, overvoltages of a duration of a few 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 20μs<Tp<5000μs, and the wave tail duration T2≤20ms.

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

——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 30kHz<f< between 100MHz.

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

From the above-mentioned overvoltage types, standard voltage waveforms 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. 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, more attention should be paid to 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 conditions of the 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.

2. Overvoltage category of equipment

According to the use 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 being installed in the power distribution device, and the safety and applicability of the equipment must meet the special requirements, such as the switchgear in the power distribution device. 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.

When the equipment needs to work in a higher-level overvoltage category, and the equipment itself has insufficient overvoltage category, it is necessary to take measures to reduce the overvoltage at the location. The following methods can be used.

a) Overvoltage protection device

b) Transformers with isolated windings

c) Multi-branch circuit power distribution system with distributed transfer wave through voltage energy

d) Capacitors that can absorb 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 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.

(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 the altitude, the daily air pressure change has been ignored, and 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 regulations, see Table 1, which provides a basis for product design.

(3) Insulation coordination and insulation 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 an accidental overvoltage event may cause permanent damage. In the long-term use of insulating materials, it will encounter Various situations, such as discharge accidents, etc., and the insulating 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. The influence of the above stress on insulating materials has been discussed in IEC publications, which can provide some qualitative guidance for practical application, but quantitative guidance cannot be achieved at present. At present, low-voltage electrical products are mostly used as indicators for quantitative guidance of insulating materials, compared with the CTI value of the tracking index, which are divided into three groups and four categories, and the PTI value of the tracking resistance index. The Tracking Index gives a quantitative comparison to the formation of a tracking trace by water-contaminated droplets falling to the surface of an insulating material.

This quantitative indicator has been practically applied to the design of products.

3. 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.

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

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. Dielectric test instead of impulse voltage

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

4. General procedures for insulation coordination.

1. Determination of typical overvoltage.

2. Match the determination of withstand voltage.

3. Determination of rated insulation level.