Introduction 

due to excellent conductivity, copper bar is one of the high voltage bearing parts of new energy electric drive system. Due to space problems, the gap between high-pressure copper bars or between copper bars and the shell is often very small. In such a case, an insulating material is needed to separate the two.There are many kinds of insulating materials, such as glass, ceramic is a good insulating material, in the electric drive system commonly used in the insulating material is plastic. The broad interpretation of plastic: Monomer as raw materials, through the addition or polycondensation reaction polymerization of macromolecule, the main component is resin.

1.What does high voltage insulation care about?

From the point of view of the application engineer, even if we have not studied the knowledge of high voltage insulation systematically, as long as the electric gap and creepage distance are guaranteed to meet the requirements according to the standards, it is almost guaranteed that the insulation of the high-voltage system designed will not be a big problem. So why are electrical gaps and creepage distances so important?

(1)It's easy to understand how to maintain electrical clearance. When a high-voltage line is opened or closed, a rather spectacular arc is created between the Poles, and when the distance is drawn, the ARC disappears. The arcing occurs because the strong electric field ionizes the air molecules, which is called insulation failure due to the breakdown of air medium. This phenomenon is very good to show that as long as a certain distance, then the two high-voltage components between the air insulation is no problem.

 (2) Why do you need to maintain creepage distance? What is creepage distance? Before answering this question, let's first look at the basic requirements for creepage distance in Gbt4939(information technology equipment safety) : The creepage distance should be sized to ensure that no insulation flashover or insulation breakdown (for example, due to creepage) occurs at a given operating voltage and pollution level. The industry's explanation for flashover is that the breakdown of a gas or liquid dielectric around a solid insulator causes a discharge along the surface of the insulator. The electrical gap is maintained to prevent discharge through the air gap, which means that the creepage distance is maintained to prevent the failure of another type of high-voltage insulation that is different from air breakdown. The results show that the flashover voltage along the solid dielectric surface is much lower than the breakdown voltage of the solid dielectric and the pure air gap with the same distance between electrodes. It should be noted that this involves not only the surface dry, clean characteristics, but also the surface wet, pollution characteristics, obviously, in the latter case of the surface flashover voltage must be lower. This is the fundamental reason why we have to maintain electric climbing distance. After understanding the meaning of the electrical gap and creepage distance, the following two graphs explain the difference in measurement (not too extensive, you can see IEC60950, the solid line is the electrical gap, the dotted line is creepage distance) .

2.Characteristics of high voltage insulating plastics —— electric

As mentioned above, there are two common modes of high voltage insulation failure: Breakdown and Flashover, which is a basis for the analysis of electrical insulation properties of plastic materials. So what are the parameters related to the electrical properties of plastic materials? How does it affect electrical performance? 

(1) Volume resistivity volume resistivity is the electrical impedance per unit volume of a material to the current, used to characterize the electrical properties of the material, the unit is M. Plastic insulators also have a very small current passing through them in an electric field, so volume resistivity is measured by holding the material at a required voltage for a specified period of time and measuring the resulting current. In order to understand the differences between different materials, we first understand the molecular polarity, which is to understand the electrical and thermal properties of plastic materials, very important! When we say a molecule has polarity, we mean that the charge distribution within the molecule is not uniform, or that the centers of positive and negative charges do not coincide. The polarity of a molecule depends on the polarity of its bonds and the way they are arranged. The popular understanding, is the polarity represents the lively, non-polarity is not lively. Therefore, generally speaking, the resistivity of non-polar polymers is slightly larger than that of polar polymers. Benzene, which we are familiar with, is a typical chemical polarity (remember this, we will use it later) , and NH3 is a typical polar molecule.

2. Dielectric strength Dielectricstrength is a measure of a material's ability to resist high voltages without dielectric breakdown. The sample is placed between electrodes and the applied voltage is increased through a series of steps until a dielectric breakdown occurs, the dielectric strength is measured in KV/MM. The greater the dielectric strength, the greater the upper bound on the charge. 

3. Comparative Tracking Index (CTI) Comparative Tracking Index (CTI) refers to the maximum voltage that can withstand 50 drops of electrolyte (0.1% aqueous ammonium chloride solution) without the formation of tracking. We can simply understand the ability of plastic to resist pollution. If you are familiar with the previous explanation of creepage distance, you will understand why the creepage distance is related to CTI. It's okay to forget. Let's go over the rest of this. The results show that the flashover voltage along the solid dielectric surface is much lower than the breakdown voltage of the solid dielectric and the pure air gap with the same distance between electrodes. It should be noted that this involves not only the surface dry, clean characteristics, but also the surface wet, pollution characteristics, obviously, in the latter case of the surface flashover voltage must be lower.

The whole failure process: 

(1) firstly, when the surface of the material is damp and dirty + the electric field is big enough, there will be leakage current on the surface 

(2) under the joule heat of the electric current, the water is evaporated, the gap formed by the separation of the liquid film on the surface (called the drying zone)

(3) when the liquid film between the drying zone is formed, the electric field strength between the liquid film reaches the Electric Field Strength, which leads to the discharge

 (4) the heat generated by the discharge makes the surface of the material locally carbonized

 (5) due to the high conductivity of the carbonized product, the electric field density here is concentrated on the carbonized part, which causes a repetition of the discharge, produces more carbides around it, forms a carbonized conductive circuit, and extends toward the electrode, resulting in a short circuit in addition to the above mentioned parameters, the Electric Properties of plastic are also arc resistance (dry burning) , corona resistance, partial discharge resistance (insulation materials contain impurities, pores, resulting in partial discharge) and so on.

3.Characteristics of high voltage insulating plastics -- machinery

The insulating material must first meet the high-voltage insulation requirements, followed by the system to meet the mechanical performance requirements. Mechanical failure of plastic materials is easy to understand, the common failure modes are broken, broken, broken. Before discussing the performance parameters, we first take a look at the common stress-strain curve of polymer materials.

(1)Tensile strength (tensile strength)

 indicates the resistance of the material to maximum uniform plastic deformation. During the testing process, the maximum tensile stress is tensile strength in MPA2. Elongation at break the percentage of total elongation to the original gauge length of the material after tensile fracture. In Engineering, materials with ≥5% are often called plastic materials, and those with ≤5% are called brittle materials. In other words, the greater the elongation at break, the more brittle the material. Properties of high voltage insulating plastics —— Thermal properties we have discussed many key parameters of plastic materials, such as volume resistivity, dielectric strength, tensile strength, etc. . The values in the physical property tables provided by the material suppliers are generally measured at room temperature, also does not include the material endurance aging (in the use of polymer materials, as a result of heat, oxygen, water, light, microorganisms, chemical media and other environmental factors, the chemical composition and structure of polymer materials will have a series of changes, physical properties will be correspondingly bad, such as hair hard, sticky, brittle, color change, loss of strength, and so on, these changes and phenomena are called aging) . 

(2)Therefore, in the process of material selection, in addition to look at the physical properties table at room temperature, more attention should be paid to the material key parameters in different temperatures and environments after aging performance. When it comes to aging, we first need to understand a very important index of the material —— Glass transition temperature (Tg) , which is the temperature corresponding to the transition from a glassy state to a high elastic state. Glass transition is an inherent property of amorphous polymer materials and a macroscopic reflection of the transition of polymer movement. Popular understanding, glass transition temperature below, plastic is "frozen" , almost no aging; glass transition temperature above, plastic materials began aging, the higher the temperature, the more serious aging. Of course, a rise in temperature is not all bad; sometimes it increases certain parameters of the material, such as tensile strength, which is of concern to us.

As the high-voltage insulation parts in the electric drive system, in some cases, we will need to consider the material's oil resistance. Because of the characteristics of polymer molecules, oil solvent molecules can penetrate into the interior. The main factors affecting the permeability are material structure, glass transition temperature, crystallization properties, so in the assessment of whether the oil-resistant plastic materials can start from the following two points. At the same temperature, the lower the glass conversion temperature, the easier the oil penetration occurs in the non-crystalline region. Therefore, materials with higher crystallinity are not easy to penetrate, but the cost of obtaining the aging data is relatively high, therefore, the material supplier will not be every brand will be aging data test. So in the absence of these aging data, we typically use the heat deflection temperature (HDT) to represent the short-term heat capacity of a material. Thermal deformation temperature is a popular term for load deflection temperature. It is a method of measuring the rigidity of plastics at high temperature: under a certain load, it is heated continuously at a certain speed until the temperature at which the specimen is displayed to indicate the amount of deformation.

4.Properties of high voltage insulating plastics -- additive

In order to meet different performance requirements, plastic materials often add a variety of additives, which is like cooking with a variety of spices in general. Plastic material is the most common additives, but also the most familiar is the glass fiber (inorganic non-metallic materials, can effectively improve the tensile strength of materials) , that is, the material behind the letters GF. There are many other additives, such as heat stabilizers/antioxidants/blowing agents/flame retardants. There are two major problems with additives. Take glass fiber as an example, GF40 material is more expensive than GF30 material because of its high glass fiber content and high tensile strength? In fact, the price of glass fiber is much lower than plastic substrate. Glass fiber more, less substrate, material cost is low, in addition through glass fiber to improve tensile strength, other parameters will become worse, such as plasticity. This kind of side effect, we need to be careful to choose.

5. Flame Retardant for electric drive systems?

Before we get back to that, let's take a look at some of the flame retardants other than V0. UL94 is the most widely used flammability standard for plastic materials. It is used to evaluate the ability of a material to extinguish after being ignited. UL94 has 12 fire protection grades: HB, V-0, V-1, V-2,5VA, 5VB, VTM-0, VTM-1, VTM-2, HBF, HF1, HF2. Here we focus on HB and V 0. HB is the lowest flame retardant rating in the UL94 standard. The test method is to clamp one end of the test piece horizontally at 45 ± 2 degrees, take an iron net, fix it horizontally at 10 ± 1 mm below the test piece, turn the Bunsen Burner (flame height about 20 mm) into 45 degrees, move it to the other end of the test piece and contact the sample 6mm, and Time 30 ± 1 seconds to move away, to test piece burning to 25 mm mark, start another timer. If burning to 25mm mark within 30 ± 1 seconds, start another timer and remove the bunsen burner. The V 0 test is also known as the 50w vertical burn test because the amount of energy burned is around 50w. The test method is to remove the bunsen burner at a speed of 300 mm/s, at least 150 mm from the sample, and to record the first time of spontaneous combustion. After the spontaneous combustion stopped, the second combustion was carried out immediately. After 10 ± 0.5 seconds of combustion, the Bunsen burner was removed, and the time of the second spontaneous combustion and the red-hot time after the flame was extinguished were recorded.

According to the test specification and the requirement limit, the flame retardancy of V0 is higher than HB, which is one of the reasons why the new energy automobile industry has the flame retardancy of V0. So for such requirements, the original HB flame-retardant materials can not meet the new requirements, measures must be taken to improve the flame retardancy, the most common method is to add flame retardant. So we can see that the same broad category of materials, such as PPA, have different brands of HB flame retardant and V0 flame retardant. At this stage of technology, the side effects of flame retardant additives are huge compared with glass fiber, not only the tensile strength and toughness will be greatly reduced (especially at the weld line) , and most of the flame retardant is easy to decompose at high temperature, releasing acidic substances, it corrodes metal. The burning of new energy vehicle has been the focus of attention, which brings great social impact, which promotes the attention to improve the flame retardancy of plastic parts in the industry. To get back to the question: Do New Energy Electric Drive Systems Need v 0 Flame Retardant? Only in the fuel, fuel, ignition source three conditions at the same time, fuel can occur combustion. For the electric drive system, generally IP67 seal level, the internal parts to metal-based, from this point of view, the electric drive system due to the failure of flame-retardant materials caused by the severity, frequency, detection is relatively low. For Design, of course, is the process of excellence, there is no best choice, only better choice. Weighing price, performance, manufacturing, and so on, at this stage, I think the electric drive system technology from the engineering point of view is not necessary to flame retardant as a mandatory requirement, of course, there is the ability of V0 is of course the best.

6.Characteristics of high voltage insulating plastics -- material selection

A wide variety of plastic materials, plastic products to BASF pyramid as an example (different companies, there are some differences) , up to more than 40 kinds.

PPS is a polyether plastic, the formula gives the first impression is just YTHT, stable like a "turtle Shell" , this "Turtle Shell" is benzene, benzene has different properties from saturated compounds, not Easy to add, not easy to oxidation, carbon ring abnormal stability, these properties are known as aromatic (non-polar or weak polarity) . PBT also has a "Shell" , which should also be very stable? Yes, PBT technically known as poly (butylene terephthalate) is an aromatic polyester that also has the aforementioned aromaticity. PA6, PA66 is often mentioned in our lives Nylon Class (polyamide commonly known as Nylon (Nylon)) . If the aromatic material is "stuffy" character, sedate; then nylon is a "cheerful" character, all-inclusive. Perhaps some readers have noticed that, of the five common plastics mentioned earlier, why are there only four formulas, and one more? Where's PPA? Ppas are special and sometimes called "PA6T" . PPA is a blend of polymers formed by condensation of isophthalic acid, terephthalic acid, adipic acid and hexanediamine. It is a semi-crystalline and semi-aromatic Nylon. The most common blend is PA6T/66, which is both aromatic and nylon, so ppas are excellent in overall performance and, of course, not cheap, special nylon after all. So the electric drive system insulation structure parts should choose what material? The intuitive understanding of cracking is that the material is too brittle (fracture elongation is small) , easy to occur under temperature alternation and high stress. Therefore, the Brittleness of materials is a key parameter in the design and selection of insulation structure. As mentioned earlier, Nylon is a naturally aromatic material with excellent toughness. Given its brittle failure, is it not possible to use aromatic materials? But the thermal stability of nylon is inherently inferior to aromatic materials, considering the aging failure, is it not possible to use nylon? The metaphor may be extreme, but the answer to the previous question comes naturally. When the material can not be changed, then a reasonable structural design can circumvent the lack of material itself. When the structure can not be adjusted, then reasonable material selection can avoid the design deficiencies. In the actual work process, we have also encountered a good toughness parts cracking, also encountered a bad toughness parts cracking. There are many solutions, such as increasing thickness is a good choice, of course, the replacement of a more flexible material is more commonly used quick solution.

7.Design Standard for high voltage insulation

Next, let's talk about high voltage insulation design standards. That is to say, what are the limits of those parameters. Regarding the electric gap and creepage gap, the new energy electric drive system standard interpretation and development in the public name "the Cobbler Laboratory" : the electric gap and creepage distance, has carried on the systematic collection to the existing national/industry standard, you can refer to. It is important to note, however, that the limits in these standards are not hard and fast metrics, but rather references. Some parameters we can easily limit according to the system working condition, for example, according to CAE analysis on the tensile strength of materials, according to the highest temperature of the system on the material HDT requirements. But volume resistivity, dielectric strength, elongation at break and other parameters? Returning to the previous discussion of Brittleness of materials, Brittleness of materials leads to cracking, but Brittleness is only a property. How Brittle would it be to fail? For the same broad group, PPS can increase toughness by adding additives, and PPA may decrease toughness by adding additives. The development of high-pressure and light-weight electric drive system provides opportunities for the application of plastic materials. 

Source: Naughty Jinx Public Car materials expert Mr. Yu bin

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