Discussion on the Insulation Coordination Principle and Verification of Low Voltage Complete Switchgear

Abstract: The insulation coordination problem is an important issue related to the safety of electrical equipment products, and has always received attention from all parties. Insulation coordination is applied in high-voltage electrical products. In my country's electrical products, accidents caused by insulation systems account for 50%-60%. In addition, the concept of insulation coordination has only been formally introduced in low-voltage complete switchgear and control equipment in the past two years. Therefore, it is a relatively important issue to correctly handle and solve the insulation coordination problem in products, and sufficient attention should be given.

Keywords: low-voltage switchgear, insulation coordination, insulating materials

0 Introduction

Low-voltage complete switchgear is responsible for the control, protection, measurement, conversion and distribution of electric energy in low-voltage power supply systems. Since low-voltage complete switchgear is used in production sites, public places, residential houses and other places, it can be said that all places where electrical equipment is used should be equipped with low-voltage equipment. About 80% of my country's electricity is supplied by low-voltage complete switchgear. The development of low-voltage complete switchgear originates from the material industry, low-voltage electrical appliances, processing technology and equipment, infrastructure construction and people's living standards. Therefore, the level of low-voltage complete switchgear reflects a country's economic strength, science and technology, and living standards from one aspect.

1 Basic principles of insulation coordination

Insulation coordination means selecting the electrical insulation characteristics of the equipment according to the use conditions and surrounding environment of the equipment. Insulation coordination can only be achieved when the design of the equipment is based on the intensity of action it will withstand 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 involving various factors that must be comprehensively considered. The key points are divided into three parts: one is the use conditions of the equipment; the second is the use environment of the equipment, and the third is the selection of insulating materials.

1.1 Use conditions of equipment The use conditions of the equipment mainly refer to the voltage, electric field, and frequency used by the equipment.

1.1.1 The relationship between insulation coordination and voltage. When considering the relationship between insulation coordination and voltage, we should consider the voltage that may appear in the system, the voltage generated by the equipment, the required continuous voltage operation level, and the danger 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, power frequency overvoltage of longer duration;

c transient overvoltage, overvoltage of several milliseconds or shorter duration, usually highly damped oscillation or non-oscillation.

—— Slow wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 20μsTp5000μs, and a wave tail duration of T2≤20ms.

—— Fast wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 0.1μsT120μs, and a wave tail duration of T2≤300μs.

——Steep wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of Tf≤0.1μs, a total duration of 3ms, and superimposed oscillation, with an oscillation frequency between 30kHzf100MHz.

d Combined (temporary, slow wave front, fast wave front, steep wave 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 the 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, it is also necessary to pay attention to the actual situation of my country's power grid. In the current situation where the quality of my country's power grid is not high, when designing products, the actual working voltage that may occur is more important for insulation coordination.

③ The relationship between transient overvoltage and insulation coordination, which is related to the conditions of controlled overvoltage in the electrical system. There are many forms of overvoltage in systems and equipment, and the impact of various overvoltages must be considered. In low-voltage power systems, overvoltage may be affected by various variable factors. Therefore, the overvoltage in the system is evaluated by statistical methods, reflecting the concept of occurrence probability, and whether protection control is needed can be determined by probabilistic statistical methods.