In 1987, the 17D sub-technical committee of the International Electrotechnical Commission (IEC) drafted a technical document entitled "Supplement 1 to IEC439 on Insulation Coordination Requirements", 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, 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.
The key words: low-voltage switchgear insulation with insulation materials

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 1987, the International Electrotechnical Commission (IEC) 17D Technical Committee drafted a technical document entitled "Supplement 1 to IEC439 on Insulation Coordination Requirements", 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, in high and low voltage electrical products, the insulation coordination of equipment is still a big problem. Statistics show that in our country's electrical products, accidents caused by insulation system account for 50%-60%, and because the concept of insulation coordination is formally quoted in low voltage switchgear and control equipment, it is only a matter of the past two years. Therefore, it is an important problem to correctly handle and solve the problem of insulation coordination in products.

II. 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 (I) 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 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 voltage and overvoltage classification, waveform.
A) continuous power frequency voltage, with constant r, m and s voltage
B) Temporary overvoltage, power frequency overvoltage of longer duration
c) Transient overvoltages, overvoltages of a duration of a few milliseconds or less, usually highly damped oscillating or non-oscillating.
-Slow wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time between 20μs-- Fast wavefront overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 0.1 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 a superimposed oscillation with an oscillation frequency of 30kHzd) Combined (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.

2 long-term AC or DC voltage and insulation with the relationship, to consider the rated voltage, rated insulation voltage, the 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.

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 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.

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 power supply equipment directly from the low-voltage grid is divided into Grade 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, as well as the use of equipment safety and applicability must meet the extraordinary requirements, such as power distribution devices 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.
When the equipment is to work in a higher level of overvoltage category, and the equipment itself is not enough to allow the overvoltage category, it is necessary to take measures to reduce the overvoltage, the following methods can be used.
a) Overvoltage protection devices
B) Transformer with isolated winding
c) A multi-branch circuit power distribution system with decentralized transfer wave through voltage energy.
d) Capacitor capable of absorbing surge overvoltage energy
e) Damper 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 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.

Relationship between (II) 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 precise 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 stipulated in the relevant standards, as shown in Table 1, which provides a basis for product design.

(III) 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 accidental overvoltage events may cause permanent damage. Insulating materials will encounter various situations in long-term use, such as discharge accidents, etc. The insulating materials themselves are due to various factors accumulated for a long time, such as thermal stress, temperature, mechanical impact and other stresses, it will accelerate the 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. The effects of the above stresses on insulating materials have begun to be discussed in IEC publications, which can provide some qualitative guidance for practical applications, but quantitative guidance is still not available. At present, low-voltage electrical products as a quantitative guide to the index of insulation materials used more than the leakage tracking index CTI value, divided into three groups of four categories, leakage tracking index PTI value. The tracking index gives a quantitative comparison of the tracking mark formed by the drip of water-containing water on the surface of the insulating material.
This quantitative indicator has been applied to the design of the product.

Three. 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.

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

The rated impulse voltage is 1.2/50μs waveform.
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. Dielectric test instead of impulse voltage

For low-voltage switchgear sets, 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 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.

Four. General procedures for insulation coordination.

1. Determination of typical overvoltage.
2. Cooperate with the determination of withstand voltage.
3. Determination of rated insulation level.