Design of high-voltage insulation parts for new energy electric drive systems

Due to its excellent conductivity, copper busbar is one of the high-voltage bearing parts of new energy electric drive system. Due to space problems, the gap between high-voltage copper busbars or between copper busbars and housing is often very small. In this case, an insulating material is needed to separate them.

There are many kinds of insulating materials, such as glass and ceramics, which are very good insulating materials. The commonly used insulating material in electric drive systems is plastic.

A broad explanation of plastic: A high molecular compound formed by polymerization of monomers through addition or condensation reactions, the main component of which is resin.

What should we pay attention to when it comes to high voltage insulation?

From the perspective of an application engineer, even if we have not systematically studied high-voltage insulation knowledge, as long as we ensure that the electrical clearance and creepage distance meet the requirements according to the standards, we can basically ensure that there will be no major problems with the insulation of the designed high-voltage system.

So why are clearance and creepage distances so important?

1. Maintaining electrical clearance is easy to understand. At the moment when the high-voltage line is opened and closed, a spectacular arc will be generated between the two poles. When the distance is increased, the arc disappears. The arc is generated because the strong electric field ionizes the air molecules, which is the so-called insulation failure - air dielectric breakdown. This phenomenon is a good example of how the air insulation between two high-voltage parts will be fine as long as a certain distance is maintained.

2. Why do we need to maintain creepage distance? What is creepage distance? Before answering this question, let's take a look at the basic requirements for creepage distance in GBT 4939 (Safety of Information Technology Equipment):

Creepage distances should be dimensioned so that no insulation flashover or insulation breakdown (e.g. due to tracking) occurs at the given working voltage and pollution degree.

Here comes a strange term - flashover. The industry defines flashover as the phenomenon of discharge along the surface of a solid insulator when the gas or liquid dielectric surrounding the solid insulator is broken down. The electrical clearance we mentioned earlier is to prevent discharge through the air gap, that is, to maintain creepage distance is to prevent another type of high-voltage insulation failure that is different from air breakdown.

Experiments show that the flashover voltage along the surface of solid dielectrics is not only much lower than the breakdown voltage of the solid dielectric itself, but also much lower than the breakdown voltage of pure air gaps with the same inter-electrode distance. It should be noted that this not only involves the characteristics when the surface is dry and clean, but also the characteristics when the surface is wet and contaminated. Obviously, the surface flashover voltage in the latter case must drop lower.

This is the fundamental reason why we need to maintain creepage distance.

After understanding the meaning of electrical clearance and creepage distance, the following two pictures are used to explain the difference in measurement between the two (without going into details, if you are interested, you can check IEC 60950, the solid line is the electrical clearance and the dotted line is the creepage distance).

Creepage distance is greater than clearance

Creepage distance is equal to clearance

High Voltage Insulation Plastic Properties—Electrical

As mentioned earlier, there are two common modes of high-voltage insulation failure: breakdown and flashover, which is a basis for analyzing the electrical insulation properties of plastic materials. So what are the parameters related to the electrical properties of plastic materials? How do they affect the electrical properties?

** 1. Volume resistivity**

Volume resistivity is the resistance of a material to electric current per unit volume. It is used to characterize the electrical properties of a material and its unit is Ω·m.

Plastic insulating materials will also have a small current flowing through them under an electric field, so the volume resistivity test method is to keep the material under the required voltage for a specified time and measure the current generated.

In order to understand the differences between different materials, let us first understand molecular polarity, which is the key to understanding the electrical and thermal properties of plastic materials and is very important!

When we say a molecule is polar, we mean that the charge distribution within the molecule is uneven, or that the centers of positive and negative charge do not coincide. The polarity of a molecule depends on the polarity of the bonds within the molecule and how they are arranged.

In layman's terms, polarity means activity, and non-polarity means inactivity. Therefore, generally speaking, the resistivity of non-polar polymers is slightly greater than that of polar polymers. Benzene, which we are more familiar with, is a typical non-polar molecule (remember this conclusion, it will be used later), and NH3 is a typical polar molecule.

** 2 Dielectric strength

Dielectric strength is a measure of a material's ability to withstand high voltages without dielectric breakdown. It is measured by placing a sample between electrodes and increasing the applied voltage in a series of steps until dielectric breakdown occurs. The unit is kV/mm. The greater the dielectric strength, the greater and stronger the upper limit of the ability to bind charge.

** 3 Comparative Tracking Index (CTI)

The relative tracking index refers to the maximum voltage value at which the surface of a material can withstand 50 drops of electrolyte (0.1% ammonium chloride aqueous solution) without forming a leakage trace. We can simply understand it as the ability of plastics to resist pollution.

If you still remember the previous explanation of creepage distance, then you will understand why creepage distance is related to CTI. If you forget, it doesn’t matter, let’s review the previous content.

Experiments show that the flashover voltage along the surface of solid dielectrics is not only much lower than the breakdown voltage of the solid dielectric itself, but also much lower than the breakdown voltage of pure air gaps with the same inter-electrode distance. It should be noted that this not only involves the characteristics when the surface is dry and clean, but also the characteristics when the surface is wet and contaminated. Obviously, the surface flashover voltage in the latter case must drop lower.

After reviewing, let's come back to continue the discussion. Why is it called the tracking index and what is tracking? This is because polymer insulation materials have a special electrical damage phenomenon, that is, the surface of polymer insulation materials will experience electrical tracking degradation under certain conditions and lead to electrical tracking damage. The failure model is shown in the figure below, which is a special flashover.

The whole failure process:

1. First, when there is moisture and dirt on the surface of the material and the electric field is large enough, leakage current will be generated on the surface.

2. Under the Joule heat of the electric current, the water is evaporated, and the gap formed by the separation of the surface liquid film (called the drying zone)

3. At the moment the dry zone is formed, the field strength between the liquid films reaches the discharge field strength, resulting in discharge

4. The heat generated by the discharge causes local carbonization of the material surface

5. Due to the high conductivity of the carbonized product, the electric field density here is concentrated in the carbonized part, causing repeated discharges, generating more carbides around it, forming a carbonized conductive path, and extending toward the electrode, eventually leading to a short circuit.

In addition to the above parameters, other parameters related to the electrical properties of plastics include arc resistance (dry burning), corona resistance, partial discharge resistance (insulating materials contain impurities, pores, etc., which lead to partial discharge), etc.

High Voltage Insulation Plastic Properties—Mechanical

The insulation material must first meet the high voltage insulation requirements, and secondly meet the system mechanical performance requirements. The mechanical failure of plastic materials is relatively easy to understand, and common failure modes include breaking, cracking, and breaking. Before discussing performance parameters, let's first look at the common stress-strain curves of polymer materials.

1. Tensile strength

Tensile strength characterizes the resistance of the material to the maximum uniform plastic deformation. During the test, the maximum tensile stress in the process of stretching the specimen to fracture is the tensile strength, unit MPa

2 Elongation at break

The percentage of the total elongation of the material after it breaks in tension to the original gauge length. In engineering, materials with δ≥5% are often called plastic materials; materials with δ≤5% are called brittle materials. In other words, the smaller the elongation at break, the more brittle the material.

High Voltage Insulation Plastic Properties - Thermal Properties

We have discussed many key parameters of plastic materials before: volume resistivity, dielectric strength, tensile strength, etc. The values ​​in the physical property table provided by the material supplier are generally measured at room temperature, and do not include the durable aging of the material (during the use of polymer materials, due to the combined effects of environmental factors such as heat, oxygen, water, light, microorganisms, and chemical media, the chemical composition and structure of polymer materials will undergo a series of changes, and the physical properties will also deteriorate accordingly, such as hardening, stickiness, brittleness, discoloration, loss of strength, etc. These changes and phenomena are called aging).