Abstract: in 1987, technical committee 17D of International Electrotechnical Commission (IEC) drafted a technical document entitled "requirements on insulation coordination in Supplement 1 to iec439", which formally introduced insulation coordination 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 is formally used in low-voltage switchgear and control equipment, it is only in recent two years. Therefore, it is an important problem to correctly handle and solve the insulation coordination problem in products.

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 from all aspects. Insulation coordination was first used in high voltage electrical products. In 1987, technical committee 17D of International Electrotechnical Commission (IEC) drafted a technical document entitled "requirements on insulation coordination in Supplement 1 to iec439", which formally introduced insulation coordination 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 the accidents caused by insulation system account for 50% - 60% of the electrical products in China, and it is only in recent two years that the concept of insulation coordination is formally used in low-voltage switchgear and control equipment. Therefore, it is an important problem to correctly handle and solve the insulation coordination problem in products.

2、 Basic principle of insulation coordination

Insulation coordination means that the electrical insulation characteristics of the equipment are selected according to the service conditions and surrounding environment of the equipment. Insulation coordination can be realized only when the design of the equipment is based on the action strength it bears in the expected life. The problem of insulation coordination is 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. The main points are divided into three parts: the first is the service conditions of the equipment; the second is the use environment of the equipment; the third is the selection of insulation materials.

（1） Service conditions of equipment

The operating 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, the possible voltage in the system, the voltage generated by the equipment, the required continuous voltage operation level, as well as the risk of personal safety and accident should be considered.

1. Classification and waveform of voltage and overvoltage.

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

b) Temporary overvoltage, long duration power frequency overvoltage

c) Transient overvoltages, over voltages of several milliseconds or less in duration, are usually highly damped oscillatory or non oscillatory.

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

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

——Steep front overvoltage: a transient overvoltage, usually in one direction, with a peak time of TF ≤ 0.1 μ s, a total duration of less than 3MS, and a superimposed oscillation with an oscillation frequency of 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. 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 working voltage is more important for insulation coordination.

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 various forms of overvoltage. It is necessary to comprehensively consider the influence of various overvoltage. 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, which reflects a concept of probability of occurrence, and can be used to determine whether protection is needed Control.

2. Overvoltage category of equipment

According to the operating 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. The equipment with overvoltage category IV is the equipment used in the power supply end of power distribution equipment, such as ammeter and front level current protection equipment. The equipment with overvoltage category III is the task to be installed in the power distribution device, and the safety and applicability of the equipment must meet special requirements, such as the switch appliance in the power distribution device. Overvoltage category II equipment is energy consuming equipment powered by power distribution devices, such as domestic 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.