Application of Epoxy Resin Insulators in Power Equipment

Application of Epoxy Resin Insulators in Power Equipment

In recent years, insulators with epoxy resin as dielectric have been widely used in the power industry, such as bushings, supporting insulators, contact boxes, insulating cylinders and poles made of epoxy resin on three-phase AC high-voltage switchgear. Columns, etc., let’s talk about some of my personal views based on the insulation problems that occur during the application of these epoxy resin insulation parts.

1. Production of epoxy resin insulation
Epoxy resin materials have a series of outstanding advantages in organic insulating materials, such as high cohesion, strong adhesion, good flexibility, excellent thermal curing properties and stable chemical corrosion resistance. Oxygen pressure gel manufacturing process (APG process), vacuum casting into various solid materials. The epoxy resin insulating parts made have the advantages of high mechanical strength, strong arc resistance, high compactness, smooth surface, good cold resistance, good heat resistance, good electrical insulation performance, etc. It is widely used in the industry and mainly plays the role of support and insulation. The physical, mechanical, electrical and thermal properties of epoxy resin insulation for 3.6 to 40.5 kV are shown in the table below.
Epoxy resins are used together with additives to obtain application value. Additives can be selected according to different purposes. Commonly used additives include the following categories: ① curing agent. ② modifier. ③ Filling. ④ thinner. ⑤Others. Among them, the curing agent is an indispensable additive, whether it is used as an adhesive, coating or castable, it needs to be added, otherwise the epoxy resin cannot be cured. Due to different uses, properties and requirements, there are also different requirements for epoxy resins and additives such as curing agents, modifiers, fillers and diluents.
In the manufacturing process of insulating parts, the quality of raw materials such as epoxy resin, the mold, the mold, the heating temperature, the pouring pressure, and the curing time have a great impact on the quality of the finished product of the insulating parts. Therefore, the manufacturer has a standardized process. Process to ensure the quality control of insulating parts.

2. Breakdown mechanism and optimization scheme of epoxy resin insulation
Epoxy resin insulation is a solid medium, and the breakdown field strength of solid is higher than that of liquid and gas medium. solid medium breakdown
The characteristic is that the breakdown field strength has a great relationship with the time of voltage action. Generally speaking, the breakdown of the action time t < 1 s is electrical breakdown, and the breakdown of the action time of 1 s ≤ t < several hours is thermal shock. The breakdown of the action time t ≥ several hours is electrochemical breakdown. Although these three breakdown processes are different, the consequence of breakdown is that the solid medium is permanently damaged. When we do the power frequency withstand voltage test for the switchgear, during the process of uniform boosting of the test voltage through the voltage regulator, the above-mentioned insulating parts are subjected to the test voltage together with the host, except that the specified withstand voltage is maintained for 1 min after the voltage is successfully boosted. Except for the breakdown, when any part is in the process of boosting due to weak insulation, the breakdown is instantaneous, and such breakdown should be judged as electrical breakdown. This situation is often encountered on epoxy resin insulating parts. The following is an example of a 40.5 kV vacuum circuit breaker solid-sealed pole to analyze this problem.
The so-called solid-sealed pole refers to an independent component composed of a vacuum interrupter and/or a conductive connection and its terminals packaged with a solid insulating material. Since its solid insulating materials are mainly epoxy resin, power silicone rubber and adhesive, etc., the outer surface of the vacuum interrupter is encapsulated in turn from bottom to top according to the solid sealing process. A pole is formed on the periphery of the main circuit. In the production process, the pole should ensure that the performance of the vacuum interrupter will not be reduced or lost, and its surface should be flat and smooth, and there should be no looseness, impurities, bubbles or pores that reduce electrical and mechanical properties, and there should be no defects such as cracks. . Despite this, the reject rate of 40.5 kV solid-sealed pole products is still relatively high, and the loss caused by the damage of the vacuum interrupter is a headache for many manufacturing units. The reason is that the rejection rate is mainly due to the fact that the pole cannot meet the insulation requirements. For example, in the 95 kV 1 min power frequency withstand voltage insulation test, there is a discharge sound or breakdown phenomenon inside the insulation during the test.
From the principle of high-voltage insulation, we know that the electrical breakdown process of a solid medium is similar to that of a gas. The electron avalanche is formed by impact ionization. When the electron avalanche is strong enough, the dielectric lattice structure is destroyed and the breakdown is caused. For several insulating materials used in the solid-sealed pole, the highest voltage that the unit thickness can withstand before breakdown, that is, the inherent breakdown field strength, is relatively high, especially the Eb of epoxy resin ≈ 20 kV/mm. However, the uniformity of the electric field has a great influence on the insulating properties of the solid medium. If there is an excessively strong electric field inside, even if the insulating material has sufficient thickness and insulation margin, both the withstand voltage test and the partial discharge test are passed when leaving the factory. After a period of operation, insulation breakdown failures may still occur frequently. The effect of the local electric field is too strong, just like tearing paper, the excessively concentrated stress will be applied to each action point in turn, and the result is that the force far less than the tensile strength of the paper can tear the entire paper. When a locally too strong electric field acts on the insulating material in the organic insulation, it will produce a “cone hole” effect, so that the insulating material is gradually broken down. However, in the early stage, not only the conventional power frequency withstand voltage and partial discharge test tests could not detect this hidden danger, but also there is no detection method to detect it, and it can only be guaranteed by the manufacturing process. Therefore, the edges of the upper and lower outgoing lines of the solid-sealed pole must be transitioned in a circular arc, and the radius should be as large as possible to optimize the electric field distribution. During the production process of the pole, for solid media such as epoxy resin and power silicone rubber, due to the cumulative effect of the area or volume difference on the breakdown, the breakdown field strength may be different, and the breakdown field of a large area or volume may be different. Therefore, the solid medium such as epoxy resin must be mixed evenly by mixing equipment before encapsulation and curing, so as to control the dispersion of the field strength.
At the same time, since the solid medium is non-self-recovery insulation, the pole is subjected to multiple test voltages. If the solid medium is partially damaged under each test voltage, under the cumulative effect and multiple test voltages, this partial damage will be expand and eventually lead to pole breakdown. Therefore, the insulation margin of the pole should be designed to be larger to avoid damage to the pole by the specified test voltage.
In addition, the air gaps formed by the poor adhesion of various solid media in the pole column or the air bubbles in the solid medium itself, under the action of the voltage, the air gap or the air gap is higher than that in the solid medium due to the higher field strength in the air gap or bubble. Or the breakdown field strength of bubbles is much lower than that of solids. Therefore, there will be partial discharges in the bubbles in the solid medium of the pole or breakdown discharges in the air gaps. To solve this insulation problem, it is obvious to prevent the formation of air gaps or bubbles: ① The bonding surface can be treated as a uniform matte surface (surface of vacuum interrupter) or a pit surface (surface of silicone rubber), and Use a reasonable adhesive to effectively bond the bonding surface. ②Excellent raw materials and pouring equipment can be used to ensure the insulation of the solid medium.

3 Test of epoxy resin insulation
In general, the mandatory type test items that should be done for insulating parts made of epoxy resin are:
1) Appearance or X-ray inspection, size inspection.
2) Environmental test, such as cold and heat cycle test, mechanical vibration test and mechanical strength test, etc.
3) Insulation test, such as partial discharge test, power frequency withstand voltage test, etc.

4 Conclusion
In summary, today, when epoxy resin insulation is widely used, we should accurately apply epoxy resin insulation properties from the aspects of epoxy resin insulation parts manufacturing process and electric field optimization design in power equipment to make epoxy resin insulation parts. The application in power equipment is more perfect.