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Application of epoxy resin insulation on electrical equipment

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 In recent years, epoxy resin has been widely used in the power industry as dielectric insulation, such as bushings, support insulators, contact box, insulating cylinders and pole columns, etc. made by epoxy resin in the three-phase AC voltage switchgear. Next, I will give my personal opinions on insulation problems during epoxy resin insulation application.


1. Epoxy resin insulation manufacturing

        Epoxy resin material with powerful cohesion, strong adhesion, and good flexibility, has a series of outstanding merits in the organic insulating material, such excellent thermal curing properties and stable chemical resistance, etc.. It can be made into all kinds of solid materials by epoxy resin casting system, such as Automatic Pressure Gelation Process (APG technology). Owing to the high mechanical strength, excellent arc resistance, high density, smooth surface, the vacuum casting epoxy resin insulation pieces has better cold  resistance, and favorable heat resistance and electrical insulation properties, etc., which is widely used in the power primarily for support and insulation. The table below shows the performance of physical, mechanics, electrical and thermal properties of epoxy resin insulation pieces under the voltage of 3.6-40.5 kV.

        Epoxy resins and additives used simultaneously can achieve application value. Different additives may be selected for different purpose, and commonly used additives has the following categories: ① curing agent. ② modifier. ③ filler. ④ diluent. ⑤ others. Among these additives, the curing agent is indispensable as bonding agent, coatings and potting materials, without which the epoxy resin can not be cured. For various use, performance, and requirements , the requirements of additives for epoxy resins, curing agents, modifiers, fillers and diluents are also different .

        As Insulating end products quality is influenced by raw materials, such as the epoxy resin quality, mold, diefilling, heating temperature and injection pressure and curing time, each manufacturer has standardized technology process to ensure quality control.


2. Breakdown mechanism and optimum proposal of epoxy resin insulation

        Epoxy resin insulation is a solid dielectric, its solid breakdown field strength is higher than the liquid and gaseous medium. Solid dielectric breakdown has a great relationship with the time between the breakdown field strength and voltage. In general, the reaction time t <1 s, is electrical breakdown, reaction time 1 s≤t <hours is thermal breakdown, the reaction time t≥hours is electrochemical breakdown. Although these three breakdown process are different, the consequence is the same, permanent damage to the solid medium. While doing work power frequency voltage test to the switching equipment, testing voltage boost gradually by the regulator, the said insulation pieces to withstand the test voltage together with the main frame. Except the breakdown period of 1 min under specified withstand voltage after successful boost, when a breakdown in any part due to the weak insulation while boosting, which is transient, we call it electrical breakdown. This occasion usually happens to the epoxy resin insulation pieces. Take 40.5 kV vacuum circuit breaker solid enveloped pole for example, the statement below performs an analysis on this issue.

        The so-called embedded pole is individual component that vacuum interrupter and / or the conductive connection, terminals sealed by solid insulating materials. Considering its solid insulation materials are mainly epoxy resin, silicone rubber for electric power and adhesive agent, with which the vacuum interrupter chamber surface is enveloped from the bottom up surrounding the main current by consolidated technics and form the pole, in the manufacturing process, the pole should ensure that may not cause the vacuum interrupter performance degradation or total performance loss, and its surface should be flat and smooth and should not have loose, impurities, bubbles or voids causing reduce the electrical and mechanical properties, nor cracks and other defects. Nevertheless, the rejection rate of 40.5 kV embedded pole finished product is still relatively high, coupled with the damage to the vacuum interrupter, the loss is quite a big problem to so many manufacturers. The reason, the rejection rate was mainly due to the pole can not meet insulation requirements, such as during the 95 kV 1 min power frequency withstand voltage insulation test,  discharge sound or punch-through occurs inside the insulation.

        Based on the principle of high-voltage insulation, electric breakdown process of solid dielectric is similar to gas. When the electron avalanche, formed by the impact ionization, is powerful enough to destroy the lattice structure of the medium, punch-through occurs. Several insulation materials that can withstand the maximum voltage before breakdown has comparatively high inherent breakdown voltage, especially the epoxy resin Eb≈20 kV / mm. However, the uniformity of the electric field has great influence on the insulating properties of solid medium. An excessively strong electric field exists inside may cause fluent insulation breakdown malfunction after a period of operation, even if the insulating materials have sufficient thickness and insulation margin and the puncture test and partial discharge test have passed when delivery. The effect of partial excessively strong electric field is just like tearing the paper. The excessive concentration of stress is sequentially applied to each point of action, and the acting force is less powerful than the paper tensile force resulting in tearing the whole paper. Partial excessively strong electric field inside the organic insulating acts on the insulating materials, causes similar "taper hole" effect, making the insulating materials gradually breakdown. Whereas, this hidden danger, in the early stage, can not be identified by a conventional power frequency voltage withstand test, nor by partial discharge test. What’s more, there is no detection means at present except optimize the manufacturing process. Therefore, the edge of the solid enveloped pole outlet must have circular arc transition and the radius should be large so as to optimize the electric field distribution. Due to the cumulative effect to breakdown caused by area or volume size difference of the epoxy resin, silicone rubber and other solid dielectric, it can lead to different breakdown field strength--solid dielectric with a large area or volume has low breakdown field strength. Therefore, during the progress of pole manufacturing, solid dielectric, like epoxy resin, must be mixed uniformly by the mixing device before epoxy-coating cured to control the dispersion of field effect.

        Meanwhile, the solid medium belongs to non-self-restoring insulation. If each voltage test causes partial damage, which will expand and ultimately lead to the breakdown of the pole after the action of several tests and accumulative effect. Therefore, the pole insulating allowance should be designed bigger  to avoid damage to the pole under the specified test voltage.

        In addition, the air gap due to poor bond or bubbles within solid medium between various of solid medium, under the action of voltage, has higher electric field strength than in the solid medium. the air gap or bubbles breakdown field strength is far lower than solids. Therefore, partial discharge in the bubbles or disruptive discharge in the air gap happens in the solid medium of the pole. To resolve this insulation problem, it is clear that we must start from preventing the formation of air gaps or bubbles: ① Bonding surface can be processed into a uniform matte surface (the surface of the vacuum interrupter) or potholes surface (silicone rubber surface), and it can be bonded effectively using reasonable binder. ② Excellent raw materials and casting equipment ensure insulativity of solid medium.


3. Test on epoxy resin insulation pieces

        Under normal circumstances, the mandatory test items on epoxy resin insulation pieces should include:

        1) Appearance or X-ray detection, dimension measurement.

        2) Environmental test, such as thermal cycling test, mechanical vibration test and mechanical strength test.

        3) Insulation tests, such as partial discharge test, power frequency voltage withstand test.


4. Conclusion

        In conclusion, considering the epoxy resin insulation pieces are widely used nowadays, we should make accurate use of the epoxy resin insulating properties from epoxy insulation pieces manufacturing technics and electric field design optimization in power equipment aspects to apply the epoxy resin insulation pieces to power equipment more perfectly.

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