Abstract: The insulation coordination problem is an important issue related to the safety of electrical equipment products, and has always received attention from all aspects. Insulation coordination was first applied in high-voltage electrical products. In China's electrical products, accidents caused by insulation systems account for 50% -60%, and it is only in the past two years that the concept of insulation coordination has been officially introduced in low-voltage switchgear and control equipment. So, correctly handling and solving the insulation coordination problem in products is a relatively important issue that should be given sufficient attention.
Keywords: Low voltage switchgear, insulation coordination, insulation materials
0 Introduction
Low voltage switchgear is responsible for the control, protection, measurement, conversion, and distribution of electrical energy in low-voltage power supply systems. Due to the deep penetration of low-voltage switchgear into production sites, public places, residential areas, and other locations, it can be said that low-voltage equipment should be equipped in all places where electrical equipment is used. About 80% of China's electricity is supplied through low-voltage switchgear. The development of low-voltage 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 switchgear reflects a country's economic strength, scientific and technological capabilities, and living standards from one aspect.
Basic principles of insulation coordination
Insulation coordination refers to selecting the electrical insulation characteristics of equipment based on its usage conditions and surrounding environment. Insulation coordination can only be achieved when the design of the equipment is based on the strength of the expected service life. The issue of insulation coordination not only comes from the external environment of the equipment but also from the equipment itself. It is a problem that involves various factors and needs to be comprehensively considered. Its key points are divided into three parts: first, the operating conditions of the equipment; The second is the usage environment of the equipment, and the third is the selection of insulation materials.
1.1 Equipment usage conditions The equipment usage conditions 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, it is necessary to take into account the voltage that may occur in the system, the voltage generated by the equipment, the required continuous voltage operation level, as well as the risk of personal safety and accidents.
① Classification and waveform of voltage and overvoltage.
A continuous power frequency voltage with constant voltages of r, m, and s;
B Temporary overvoltage, prolonged power frequency overvoltage;
Transient overvoltage, lasting for a few milliseconds or less, typically characterized by high damping oscillations or non oscillations.
——Slow wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of between 20 μ s and 5000 μ s, and a tail duration T2 ≤ 20ms.
——Fast wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 0.1 μ s to T120 μ s and a tail duration 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 superimposed oscillations with oscillation frequencies between 30kHz and 100MHz.
Joint overvoltage (temporary, slow front wave, fast front wave, steep front wave).
According to the above overvoltage types, a 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 mainly considered. In addition to meeting the requirements of the standards, it is also important to consider the actual situation of China's power grid. In the current situation where the quality of China's power grid is not yet 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 is related to the controlled overvoltage conditions within the electrical system. In systems and equipment, there are various forms of overvoltage, and the impact of each overvoltage should be comprehensively considered. In low-voltage power systems, overvoltage may be affected by various variable factors. Therefore, the overvoltage in the system is evaluated through statistical methods, reflecting a concept of probability of occurrence, and can be determined whether protection control is needed through probability statistical methods.
1.1.2 The overvoltage category of equipment is classified as Level IV based on the long-term continuous voltage operation level required by the equipment's usage conditions, which are directly supplied by the low-voltage power grid. Equipment with overvoltage category IV is used at the power supply end of distribution equipment, such as electric meters and front-end current protection devices. Equipment with overvoltage category III is installed in distribution equipment, and its safety and applicability must meet special requirements, such as switchgear in distribution equipment. Equipment with overvoltage category II is energy consuming equipment powered by distribution equipment, such as household and similar loads. Equipment with overvoltage category I is connected to devices that limit transient overvoltage to a relatively low level, such as electronic circuits with overvoltage protection. For equipment that is not directly powered by the low-voltage power grid, it is necessary to consider the maximum voltage that the system equipment may experience and the serious combination of various situations.
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The electric field situation can be divided into uniform electric field and non-uniform electric field. In low-voltage switchgear, it is generally considered to be in a non-uniform electric field situation. Regarding the frequency issue, it is still under consideration. It is generally believed that low frequency has little effect on insulation coordination, but high frequency still has an impact, especially on insulation materials.
1.2 The relationship between insulation coordination and environmental conditions. The macro environment in which the equipment is located affects the insulation coordination. From the current practical application and standard requirements, changes in air pressure only consider changes in air pressure caused by altitude, and daily air pressure changes have been ignored. Temperature and humidity factors have also been ignored. However, if there are more precise requirements, these factors should still be considered. From a micro environmental perspective, 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 all affect the micro environment. The micro environment is clearly defined in relevant standards, which provides a basis for product design.
1.3 Insulation coordination and insulation materials. The problem of insulation materials is quite complex. Unlike gases, it is an irreparable insulation medium once it is damaged. Even accidental overvoltage events may cause permanent damage. In long-term use, insulation materials will encounter various situations, such as discharge accidents, and the insulation material itself will accelerate its aging process due to various factors accumulated over time, such as thermal stress, temperature, mechanical shock, and other stresses. For insulation materials, due to the diversity of varieties, although there are many characteristic indicators for measuring insulation materials, they are not unified. This brings certain difficulties to the selection and use of insulation materials, which is why other characteristics of insulation materials, such as thermal stress, mechanical properties, partial discharge, etc., are not currently considered internationally.
Verification of insulation coordination
The current preferred method for verifying insulation coordination is to use impulse dielectric testing, and different rated impulse voltage values can be selected for different equipment.
2.1 Use the rated impulse voltage test to verify the insulation of the equipment with a waveform of 1.2/50 μ s for the rated impulse voltage.
This waveform is used to simulate transient overvoltage, atmospheric overvoltage, as well as overvoltage generated by the connection and disconnection of low-voltage equipment. The output impedance of the impulse test power pulse waveform generator should generally be greater than 500 Ω. The determination of the rated impulse voltage value should be based on the equipment's usage scenario, overvoltage category, and long-term operating voltage, and should be corrected according to the corresponding altitude. At present, low-voltage switchgear is subject to certain testing conditions. If there are no clear regulations on humidity and temperature, but they should still be within the scope of application of the complete set of switchgear standards. If the operating environment of the equipment exceeds the scope of application of the complete set of 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 - Correction parameters for air pressure and temperature
Δ T - the temperature difference K between the actual (laboratory) temperature and T=20 ℃
P - Actual air pressure kPa
2.2 Dielectric testing as an alternative to impulse voltage testing can be used for low-voltage switchgear, but this type of testing method is more rigorous than impulse voltage testing and requires the approval of the manufacturer.
The communication experiment lasts for 3 cycles under communication conditions.
DC test, apply voltage to each phase (positive and negative) three times, with a duration of 10ms each time.
As far as the current situation in China is concerned, insulation coordination of equipment is still a major issue in high and low voltage electrical products. Moreover, the formal introduction of insulation coordination concept in low voltage switchgear and control equipment is only a matter of the past two years. So, correctly handling and solving the insulation coordination problem in products is a relatively important issue.
References:
[1] IEC439-1 Low voltage switchgear and control equipment - Part 1: Type testing and partial type testing equipment [S]
IEC890 uses extrapolation method to check the temperature rise of low-voltage switchgear and control equipment passing partial type tests [S]