Which are the typical insulation materials for power electronics?

The properties of insulating materials are enumerated and discussed as under^[1]:

1. Electrical Properties
2. Thermal Properties
3. Chemical Properties
4. Mechanical Properties

1. Electrical Properties of Insulating Materials^{[2, 3]:}

 Dielectric Constant (Permitivity)

A dielectric is a material which has poor electrical conductivity but inherits an ability to store an electrical charge (due to dielectric polarization). The dielectric constant is a measure of the electrostatic energy stored in the insulating material per unit volume under one unit of voltage gradient (is a measure of the charge retention capacity of a medium). It is dependent also a temperature, moisture, exposure frequency and other factors

This property is defined as the ratio of the electric flux density in the material to that produced in free space by the same electric force, or the ratio of the permittivity of the substance to the permittivity of the free space is the dielectric constant.

Volumen resistivity

Measures how strongly a plastic material opposes the flow of electric current through a volume of cubic specimen. The lower the resistivity the higher the conductivity (electric charges meet weak resistance to circulation). It is also known as electrical resistivity, bulk resisitivity, specific electrical resistance, specific volume resistance or simple resistivity. Volumen resistiviy is measured in units: Ohm.cm

• Below 10⁵cm; the material is considered as conductive
• Above 10⁹cm, the material is considered as an electrical insulator

Surface resistivity

Surface resistivity is the resistance to leakage current along the surface of an insulating material. The resistance offered by insulating material to the electric current is the composite effect of volume and surface resistances, which always act in parallel:

• Volumen resistance is the resistance to leakage of the current passes through a body of the material. It depends largely on the nature of the material
• Surface resistance, is the resistance to leakage along the surface of the material and is largely a function of surface finish and cleanliness.Dielectric strengthIt is one of the most important factors in selecting a resin for various conformal coating, potting and encapsulation applications in the electrical and electronic industries, where excellent electrical insulation properties are a must. Dielectric strength can be defined as the maximum voltage required to cause a dielectric breakdown through the product. In other words, it is the measure of the insulating strength of a material or is the voltage per unit thickness at wich material conduct electricity. The higher the value, the more electrically insulating material is. Unit for dielectric strength is kV by mm or cm. Most plastics have good dielectric strengths, in order of 100 to 300 kV/cm. The measurement of dielectric strength is usually carried out either by:
  • Short-time method
  • Slow-rate of rise method
  • Step-by-step method.

  Dielectric loss factor (loss tangent)

  Is a measure of the loss of energy in a dielectric material through conduction, slow polarization currents, and other dissipative phenomena.

  

Arc resistance

The aim of this test method is to make a relative distinction between solid electrical insulating materials. The ability of the test specimens to resist an arc at a high voltage but with a weak current in the vicinity of the insulating surface is investigated (usually stated in terms of the time required to render the material electrically conductive). The test focuses on the time until tracking paths start to form.  Failure of the specimen may be caused by heating to incandescence, burning, tracking, or carbonization of the surface

The test is intended to Approximate service conditions in alternating-current circuits operating at high voltage with currents generally limited to less than 0.1 ampere.

Hot Wire Ignition Test (HWI)

 This is a test where an electrically heated resistance wire is wound around a horizontally arranged bar specimen. By doing this, you simulate the overheating of wires which can be an ignition source. Events like this can occur in electrical applications such as a transformer. Components can become abnormally warm caused by the malfunctioning of the device

Glow wire

IEC glow-wire ignition temperature (GWIT)

The GWIT is the temperature which is 25 K (30 K between 900 C and 960 C) higher than the maximum temperature of the tip of the glow-wire, which does not cause ignition of a test specimen of a given thickness during three tests.

Glow Wire Testing

The Glow Wire Test measures the ignition resistance of plastic materials. The setup of this test simulates the conditions that are present when a glow wire (current conductor) makes contact with an insulation material. This situation may be caused by the malfunction of an electrical application. The glow wire test apparatus consists of a loop of a heavy gauge (10-14 AWG) nickel-chromium resistance wire, a thermocouple, and a sample mounting bracket. To perform the test, an electrical current is passed through the nickel-chromium loop. For every test there will be a predefined temperature at which the test will be performed. Then, the specimen is brought into contact with the wire for 30 seconds. The test is passed if after withdrawal, the sample displays no flame or glowing, or if so, it is self-extinguishing after 30 seconds.

Glow-wire flammability index (GWFI)

The GWFI is the highest test temperature during three tests for a test specimen of a given thickness, at which one of the following conditions is met. Flames (or glowing of the test specimen) extinguish within 30 s after removal of the glow-wire, + no ignition of the wrapping tissue placed underneath the test specimen. No ignition of the test specimen.

CTI Tracking index

This method is used to assess the relative resistance of insulating materials to tracking. In this test, a contaminate liquid is slowly dripped between two electrodes on the surface of the material. By adding this contamination, electrical conduction between the two electrodes is started and carbonization of the polymer slowly occurs. Once the carbonization occurs and the electrical current exceeds the threshold set on the test, the test is halted, and the voltage is decreased. This continues until sufficient amount of data is gathered to interpolate the number of contaminate drops necessary to achieve the electrical current threshold.

High Voltage Tracking resistance (IPT)

This method is used to assess the susceptibility to tracking of insulating materials that are exposed to high voltages outdoors.

Distance Through Insulation

When it comes to high voltage safety certifications, the following three distances are of utmost importance to determine insulation ratings of semiconductor components: clearance, creepage, and DTI. Clearance is the shortest distance in air between two conductive parts and creepage is the shortest distance along the surface of a solid insulation material between two conductive parts across the isolation barrier. DTI, on the other hand, is the shortest distance within an insulating material interposed between two conductive parts. In other words, DTI is the distance inside a solid insulation whereas clearance and creepage are distances outside the solid insulation.

Solid Insulation is defined as insulation consisting entirely of solid material. The intrinsic material characteristics of solid insulation directly impact its insulation behavior. As the electric strength of solid insulation is considerably greater than that of air, the distances through solid insulation are much smaller than the clearance so that high electric stresses result. In insulation systems, gaps or voids may occur between electrodes and insulation and between different layers of insulation. Partial discharges can occur in these voids at voltages far below the level of puncture and this influences the service life of the solid insulation. As opposed to air, solid insulation is not a renewable insulating medium and therefore high voltage peaks which may occur infrequently can have a very damaging and irreversible effect on solid insulation. This situation can occur while in service and during routine high-voltage testing. The physical and geographical location of the equipment can affect the insulation system significantly. Environmental factors such as altitude, temperature, vibrations and humidity require consideration to ensure that the insulation remains reliable over the life time of the equipment.

Pollution Degree

Pollution Degree 1: applies where there is no pollution or only dry, non-conductive pollution. The pollution has no influence. Normally, this is achieved by having components and subassemblies adequately enclosed by enveloping or hermetic sealing so as to exclude dust and moisture.

Pollution Degree 2:  applies where there is only non-conductive pollution that might temporarily become conductive due to occasional condensation. It is generally appropriate for equipment covered by the scope of this standard.

Pollution Degree 3: applies where a local environment within the equipment is subject to conductive pollution, or to dry non-conductive pollution that could become conductive due to expected condensation.

2. Thermal Properties of Insulating Materials:

Thermal Conductivity:

The thermal conductivity of a material is a measure of its ability to conduct heat (see Thermal Conductivity Post).

Thermal diffusivity:  In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the rate of transfer of heat of a material from the hot end to the cold end (see Thermal diffusivity  Post).

Thermal Expansion: Thermal expansion is called the increase in length, volumen or some other metric dimension that a physical body suffers due to the increase in temperature by any means. On the other hand, thermal contraction is the decrease in metric dimensions due to a decrease in temperature (see Thermal Expansion Post).

3.-Chemical Properties of Insulating Materials^[1]:

Resistance to External Chemical Effects:

Insulating materials should be resistant to oils or liquids, gas fumes, acids and alkalies. The materials should not undergo oxidation and hydrolysis even under adverse conditions.

Water uptake (Moisture absorption):

Water lowers the electrical resistance and dielectric strength. With its absorption certain chemical and mechanical effects may result e.g., swelling, warping and corrosion.

Effect of Water and Tropical Tests:

Water directly lowers electrical properties, such as electrical resistance and dielectric strength. The water may be transmitted through an outside coating and cause damage inside; it may be directly absorbed by an insulating material; it may cause a chemical change of insulation itself; or it may drastically lower the surface resistance of an insulator. The effect of water absorption on electrical properties may be determined by measuring dielectric strength, insulation resistance, or power factor after immersion in water or during exposure at high humidity.

4.-Mechanical Properties of Insulating Materials:

VICAT

This test method is used to assess the dimensional stability of plastics as a function of temperature and determine the Vicat softening temperature.

The Vicat softening temperature (VST) is the temperature at which a standard indenter penetrates 1 mm into the surface of a plastic test specimen under a constant load when the temperature is increased at a uniform rate.

Tensil

This test method is used to assess the behaviour of plastics when subjected to uniaxial tensile stress.

The advantage of the tensile test is that even ductile materials can be tested to complete break point. The elasticity modulus (E modulus) serves as a parameter for comparing different materials and is a measure of stiffnes.

This test method is used to assess the behaviour of plastics when subjected to uniaxial tensile stress.

The advantage of the tensile test is that even ductile materials can be tested to complete break point. The elasticity modulus (E modulus) serves as a parameter for comparing different materials and is a measure of stiffness.

Hardness:

SHORE A AND SHORE D

These test methods are used to determine the hardness of plastics and elastomers.

Shore hardness is a material parameter for elastomers. The Shore hardness apparatus consists of a spring-loaded indenter whose flexible indentation depth is a measure of the material’s Shore hardness; the hardness is measured on a scale from 0 to 100. A high number means a large hardness.

Shore A is specified for softer elastomer measurements using a needle with a blunted point. Shore D is specified for harder elastomer measurements using a needle that ends with a 30° point angle and is not blunted

5.-Flammability Properties of Insulating Materials^[2]:

Low Oxygen Index (LOI):

This method is used to determine the flammability of plastics with the help of the limiting oxygen index.

The test calculates the minimum oxygen concentration that will just about support combustion of the test specimen in a mixture of oxygen and nitrogen. The results are referred to as the limiting oxygen index (LOI). Flammability  UL 94 V (Vertical):

This method is used to determine the UL 94 V-0, V-1 and V-2 flammability ratings. The test evaluates both the burning and afterglow times and dripping of the burning test specimen.

Pre-treatment:

2 days / 23 °C / 50 % relative humidity

7 days / 70 °C / hot air oven

Flame height     20 mm

Flame application time 2 x 10 s

The second flame application time begins as soon as the first burning time ends.

Flammability UL 94 5V

This method is used to determine the UL 94-5VA and -5VB flammability ratings.

The test evaluates both the flammability of the test specimen and any holes that are formed in sheets.

Pre-treatment:
2 days / 23 °C / 50 % relative humidity

7 days / 70 °C / hot air oven

The table shows how the test results of the HAI and the HWI can be combined with the UL94 V-ratings.  What we see is that materials with higher flammability requirements must score better in the HAI and HWI test.

6-Aging^{[2, 3]:}

The aging of natural and artificial polymeric materials is a natural phenomenon in metals, glass, minerals and other inorganic materials. The main environmental parameters influencing the degradation of polymeric materials is daylight combined with the effects of temperature, moisture and oxygen. These act as the main parameters of stress for outdoor weathering.

Secondary factors of weather

Atmospheric pollutants (e.g., sulfur dioxide, nitrogen oxides, hydrocarbons, etc.), in combination with solar radiation, can also be responsible for severe damage. Acid-base induced chemical changes may also be responsible for much pollution-caused damage

Javier Echave

PhD. Chemical Engineer

References

[1] https://www.engineeringenotes.com/electrical-engineering/insulating-materials/properties-of-insulating-materials-electrical-engineering/42073

[2]https://www.ulttc.com/en/

[3]https://www.coursehero.com/file/24107873/chapter-4-insulating-materialspdf/

Images

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