Insulation coordination of low voltage switchgear

Abstract: In 1987, the technical document entitled “requirements for insulation coordination in Supplement 1 to iec439″ was drafted by the sub Technical Committee of the International Electrotechnical Commission (IEC) 17D, which formally introduced the insulation coordination into the low voltage switchgear and control equipment. In the current situation of China, in the high and low voltage electrical products, the insulation coordination of equipment is still a big problem. Because of the formal introduction of insulation coordination concept in low voltage switchgear and control equipment, it is only a matter of nearly two years. Therefore, it is a more important problem to deal with and solve the insulation coordination problem in the product.

Key words: Insulation and insulation materials for low voltage switchgear
Insulation coordination is an important issue related to the safety of electrical equipment products, and has always been paid attention to from all aspects. Insulation coordination was first used in high voltage electrical products. In 1987, the technical document entitled “requirements for insulation coordination in Supplement 1 to iec439″ was drafted by the sub Technical Committee of the International Electrotechnical Commission (IEC) 17D, which formally introduced the insulation coordination into the 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. Statistics show that the accident caused by insulation system accounts for 50% – 60% of the electrical products in China. Moreover, it is only two years since the concept of insulation coordination is formally quoted in the low voltage switchgear and control equipment. Therefore, it is a more important problem to deal with and solve the insulation coordination problem in the product.

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. Only when the design of the equipment is based on the strength of the function it bears in its expected life, can the insulation coordination be realized. The problem of insulation coordination not only comes from the outside of the equipment but also from the equipment itself. It is a problem involving all aspects, which should be considered comprehensively. The main points are divided into three parts: first, the use conditions of the equipment; The second is the use environment of the equipment, and the third is the selection of insulation materials.

(1) Equipment conditions
The use conditions of equipment mainly refer to the voltage, electric field and frequency used by the equipment.
1. Relationship between insulation coordination and voltage. In considering the relationship between insulation coordination and voltage, the voltage that may occur in the system, voltage generated by equipment, required continuous voltage operation level, and the danger of personal safety and accident shall be considered.

1. Classification of voltage and overvoltage, waveform.
a) Continuous power frequency voltage, with constant R, m, s voltage
b) Temporary overvoltage, power frequency overvoltage for a long time
c) Transient overvoltage, over-voltage for a few milliseconds or less, usually high damping oscillation or non oscillation.
——A transient overvoltage, usually one-way, reaching a peak value of 20 μ s<Tp<5000 μ Between S, the duration of wave tail T2 ≤ 20ms.
——Fast wave pre overvoltage: A transient overvoltage, usually in one direction, reaching a peak value of 0.1 μ s<T1<20 μ s. Wave tail duration T2 ≤ 300 μ s。
——Steep wave front overvoltage: A transient overvoltage, usually in one direction, reaching peak value at TF ≤ 0.1 μ s. The total duration is less than 3MS, and there is superposition oscillation, and the frequency of oscillation is between 30kHz < f < 100MHz.
d) Combined (temporary, slow forward, fast, steep) overvoltage.

According to the above overvoltage type, the standard voltage waveform can be described.
2. The relationship between long-term AC or DC voltage and insulation coordination shall be considered as 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, we should pay more attention to the actual situation of China’s power grid. In the current situation that the quality of power grid is not high in China, when designing products, the actual possible working voltage is more important for insulation coordination.
The relationship between transient overvoltage and insulation coordination is related to the condition of controlled over-voltage in the electrical system. In the system and equipment, there are many forms of overvoltage. The influence of overvoltage should be considered comprehensively. In low voltage power system, overvoltage may be affected by various variable factors. Therefore, the overvoltage in the system is evaluated by statistical method, reflecting a concept of probability of occurrence, And it can be determined by the method of probability statistics whether protection control is needed.

2. Overvoltage category of equipment
According to the equipment conditions, the long-term continuous voltage operation level required will be directly divided into IV class by the overvoltage category of the power supply equipment of low voltage grid. The equipment of overvoltage category IV is the equipment used at the power supply end of the distribution device, such as ammeter and current protection equipment of the previous stage. The equipment of class III overvoltage is the task of installation in the distribution device, and the safety and applicability of the equipment must meet the special requirements, such as the switchgear in the distribution device. The equipment of overvoltage class II is the energy consuming equipment powered by distribution device, such as the load for home use and similar purposes. The equipment of overvoltage class I is connected to the equipment which limits the transient overvoltage to a very low level, such as electronic circuit with over-voltage protection. For equipment not directly supplied by low voltage grid, the maximum voltage and serious combination of various situations that may occur in system equipment must be taken into account.
When the equipment is to work in the situation of higher level overvoltage category, and the equipment itself does not have enough allowed overvoltage category, measures shall be taken to reduce the overvoltage at the place, and the following methods can be adopted.
a) Over voltage protection device
b) Transformers with isolated winding
c) A multi branch circuit distribution system with distributed transfer wave passing through voltage energy
d) Capacitance capable of absorbing surge overvoltage energy
e) Damping device capable of absorbing surge overvoltage energy

3. Electric field and frequency
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. The frequency problem is still under consideration. Generally, low frequency has little influence on insulation coordination, but high frequency still has influence, especially on insulation materials.
(2) The relationship between insulation coordination and environmental conditions
The macro environment where the equipment is located affects insulation coordination. From the requirements of current practical application and standards, 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 is more accurate requirements, these factors should 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. The different protection levels, heating, ventilation and dust of the shell may affect the micro environment. The micro environment has clear provisions in relevant standards. See Table 1, which provides the basis for the design of the product.
(3) Insulation coordination and insulation materials
The problem of insulating material is quite complex, it is different from gas, it is an insulation medium which can not be recovered once damaged. Even the accidental overvoltage event may cause permanent damage. In the long-term use, insulation materials will encounter various situations, such as discharge accidents, etc. and the insulation material itself is due to various factors accumulated for a long time, such as thermal stress Temperature, mechanical impact and other stresses will accelerate the aging process. For insulation materials, due to the variety of varieties, the characteristics of insulation materials are not uniform, although there are many indicators. This brings some difficulty to the selection and use of insulating materials, which is the reason why other characteristics of insulation materials, such as thermal stress, mechanical properties, partial discharge, etc., are not considered at present. The influence of the above stress on insulation materials has been discussed in IEC publications, which can play a qualitative role in practical application, but it is not yet possible to do quantitative guidance. At present, there are many low voltage electrical products used as quantitative indicators for insulating materials, which are compared with CTI value of leakage mark index, which can be divided into three groups and four types, and the resistance to leakage mark index PTI. The leakage mark index is used to form leakage trace by dropping the water contaminated liquid onto the surface of insulation material. The quantitative comparison is given.
This certain quantity index has been applied to the design of the product.

3. Verification of insulation coordination
At present, the optimal method to verify insulation coordination is to use impulse dielectric test, and different rated impulse voltage values can be selected for different equipment.
1. Verify the insulation coordination of equipment with rated impulse voltage test
1.2/50 of rated impulse voltage μ S wave form.
The output impedance of impulse generator of impulse test power supply should be more than 500 generally Ω, The rated impulse voltage value shall be determined according to the use situation, overvoltage category and long-term use voltage of the equipment, and shall be corrected according to the corresponding altitude. At present, some test conditions are applied to the low voltage switchgear. If there is no clear stipulation on humidity and temperature, it should also be within the scope of application of the standard for complete switchgear. If the equipment use environment is beyond the applicable scope of the switchgear set, it must be considered to be corrected. The correction relationship between air pressure and temperature is as follows:
K=P/101.3 × 293( Δ T+293)
K – correction parameters of air pressure and temperature
Δ T – temperature difference K between actual (Laboratory) temperature and T = 20 ℃
P – actual pressure kPa
2. Dielectric test of alternative impulse voltage
For low voltage switchgear, AC or DC test can be used instead of impulse voltage test, but this kind of test method is more severe than impulse voltage test, and it should be agreed by the manufacturer.
The duration of the experiment is 3 cycles in the case of communication.
DC test, each phase (positive and negative) respectively applied voltage three times, each time duration is 10ms.
1. Determination of typical overvoltage.
2. Coordinate with the determination of withstand voltage.
3. Determination of rated insulation level.
4. General procedure for insulation coordination.

Post time: Feb-20-2023