Abstract
This technical article comprehensively explores the application of
epoxy resin and the Automatic Pressure Gelation (APG) injection process in the manufacturing of current transformers (CTs). By detailing the material properties, process steps, and quality control measures, it aims to provide a practical guide for engineers and manufacturers seeking to optimize CT production efficiency, insulation performance, and long - term reliability.
1. Introduction
Current transformers play a crucial role in power systems by stepping down high - current values for measurement, protection, and control purposes. Ensuring their reliable operation requires high - quality insulation materials and precise manufacturing processes. Epoxy resin, renowned for its excellent electrical insulation, mechanical strength, and chemical resistance, has become the material of choice for CT encapsulation. When combined with the APG injection process, it enables the production of CTs with consistent quality, minimal voids, and enhanced performance.
2.1 Material Properties
Epoxy resin used in CT manufacturing typically exhibits the following key properties:
- Electrical Insulation: High dielectric strength (usually >20 kV/mm) and low dielectric loss tangent ensure effective isolation of high - voltage components within CTs, preventing electrical breakdown and partial discharge.
- Mechanical Strength: Adequate tensile, compressive, and flexural strength help CTs withstand mechanical stresses during installation, operation, and transportation.
- Thermal Stability: A wide operating temperature range (e.g., - 40°C to 155°C) allows CTs to function reliably in diverse environmental conditions. Epoxy resin also has a relatively low coefficient of thermal expansion, reducing the risk of thermal stress - induced cracks.
- Chemical Resistance: Resistance to moisture, oils, and common chemicals in power systems safeguards the internal components of CTs from degradation over time.
2.2 Formulation
The
epoxy resin used in APG processes is often a two - component system: the base resin and the hardener. The base resin is typically a bisphenol - A or bisphenol - F epoxy, while the hardener can be an anhydride - based compound. Additives such as fillers (e.g., alumina trihydrate for flame retardancy), coupling agents (to improve adhesion between the resin and other materials), and accelerators (to control the curing rate) are also incorporated into the formulation to meet specific performance requirements.
3. Automatic Pressure Gelation (APG) Injection Process
3.1 Process Principle
The APG injection process combines the advantages of pressure - assisted filling and controlled gelation. It operates under a closed - loop system where the epoxy resin mixture is heated to a suitable viscosity level (usually around 100 - 500 mPa·s) and then injected into a pre - heated mold under pressure (typically 0.3 - 0.8 MPa). The pressure ensures complete filling of the mold cavities, even those with complex geometries, while the controlled temperature and pressure environment facilitate a uniform gelation and curing process. This results in a high - density, void - free encapsulation.
3.2 Advantages over Traditional Processes
Compared to traditional casting or potting methods, the APG process offers several significant advantages:
- Reduced Voids: The pressure - assisted filling minimizes the entrapment of air bubbles, leading to a lower void content (usually <0.5%) in the final product.
- Improved Dimensional Accuracy: Precise control of temperature, pressure, and injection speed ensures consistent product dimensions, reducing the need for post - processing adjustments.
- Enhanced Production Efficiency: Shorter cycle times (ranging from 30 minutes to 2 hours depending on part size) due to faster filling and curing processes increase overall manufacturing productivity.
4. Step - by - Step CT Manufacturing Process
4.1 Preparation of Raw Materials and Components
- Epoxy Resin Mixing: The base resin, hardener, and additives are accurately weighed and mixed in a high - shear mixer to ensure a homogeneous mixture. Proper mixing is crucial to avoid curing defects, and the mixture is then degassed under vacuum to remove any entrapped air.
- CT Core and Windings: The CT core, typically made of high - permeability magnetic materials such as silicon steel or amorphous alloys, is assembled with the primary and secondary windings. The windings are carefully wound to meet the required turns ratio and electrical performance specifications. Insulation tapes or films may be applied to the windings for additional protection.
4.2 Mold Design and Preparation
- Mold Design: The mold is designed based on the CT's geometry and size. It is usually made of high - quality steel or aluminum alloy to withstand the injection pressure and temperature. The mold has cavities that match the shape of the CT, with provisions for inlet and outlet ports for the epoxy resin injection.
- Mold Preparation: Before use, the mold is thoroughly cleaned and coated with a release agent to prevent the cured epoxy from sticking to the mold surfaces. The mold is then pre - heated to a temperature slightly higher than the injection temperature of the epoxy resin (e.g., 80 - 100°C) to ensure a smooth filling process.
4.3 APG Injection
- Injection: The degassed epoxy resin mixture is transferred to the APG injection machine. The machine pumps the resin into the pre - heated mold at a controlled pressure and speed. The injection process is closely monitored to ensure that the mold is completely filled without any overflow or incomplete filling.
- Gelation and Curing: After injection, the mold is maintained at a specific temperature and pressure for a certain period to allow the epoxy resin to gel and cure. The gelation stage is when the resin starts to harden, and the curing stage further cross - links the polymer chains to achieve the desired mechanical and electrical properties. The curing time and temperature depend on the specific epoxy resin formulation, typically ranging from 120 - 160°C for 1 - 3 hours.
4.4 Demolding and Post - Processing
- Demolding: Once the curing process is complete, the mold is cooled down to a suitable temperature (usually below 60°C) to facilitate demolding. The release agent helps in easily separating the cured CT from the mold without causing any damage to the product.
- Post - Processing: After demolding, the CT may undergo post - processing steps such as trimming excess resin, sanding rough surfaces, and applying a protective coating if required. These steps enhance the product's appearance and further improve its performance in some cases.
5. Quality Control and Testing
5.1 In - Process Quality Control
- Material Inspection: Before mixing, the epoxy resin, hardener, and additives are inspected for their physical properties (such as viscosity, density) and chemical composition to ensure compliance with the specifications.
- Process Parameter Monitoring: During the APG process, critical parameters such as injection pressure, temperature, and resin flow rate are continuously monitored and recorded. Any deviation from the set values can lead to quality issues, and immediate corrective actions are taken.
5.2 Final Product Testing
- Electrical Testing: CTs undergo a series of electrical tests, including insulation resistance measurement (using a high - voltage megger to check for any insulation breakdown), dielectric withstand voltage test (applying a high voltage for a specified time to ensure the insulation can withstand operating voltages), and partial discharge test (detecting any small electrical discharges within the insulation, which can be a precursor to failure).
- Mechanical Testing: Mechanical tests such as impact resistance, vibration resistance, and tensile strength tests are conducted to ensure that the CT can withstand the mechanical stresses it will encounter during its service life.
- Dimensional Inspection: The dimensions of the CT are measured using precision instruments to ensure they meet the design specifications. Any dimensional variations can affect the CT's performance and compatibility with other equipment.
6. Conclusion
The combination of epoxy resin and the APG injection process provides an efficient and reliable method for manufacturing current transformers. By carefully controlling the material properties, process parameters, and quality control measures, manufacturers can produce CTs with high - performance insulation, excellent mechanical strength, and long - term reliability. As power systems continue to evolve towards higher voltages, smarter grids, and increased renewable energy integration, the demand for high - quality CTs manufactured using advanced processes like APG will only increase. Continuous research and development in epoxy resin formulations and APG process optimization will further enhance the performance and competitiveness of CT products in the global market.
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