In a power system, the oil-immersed transformer is the heart, and the transformer bushing is a critical “arterial connection point.” It safely brings the internal high-voltage conductor out of the tank while maintaining reliable insulation to ground. A substandard bushing can cause equipment failure and outages, or in severe cases, lead to explosions. Therefore, knowing how to select a qualified oil-immersed transformer bushing is essential for procurement and maintenance engineers. The quality of its external insulation—the Porcelain Shell—and its internal insulating core—often the Porcelain Bushing in capacitive-type bushings—forms the foundation of its performance and longevity.
1. Understanding the Core Components: Porcelain Shell vs. Porcelain Bushing
First, let’s clarify these two key industry terms:
- Porcelain Shell (瓷套/瓷外壳): This typically refers to the visible, sheds-shaped external porcelain insulator. Its primary functions are to provide sufficient external creepage distance, withstand environmental stresses like pollution, moisture, and UV rays, and contain the internal insulating oil pressure.
- Porcelain Bushing (瓷质绝缘芯子/瓷导管): In oil-impregnated paper (OIP) capacitive bushings, this specifically denotes the cylindrical porcelain tube that houses the capacitive core (layers of conductor foil and insulation paper). It is not only part of the main insulation but also a key structural component for mechanical support and sealing. It is filled with insulating oil and protected by the outer Porcelain Shell. Its quality directly impacts the dielectric strength and hermetic reliability of the entire bushing.
Selecting a qualified bushing requires rigorous evaluation of both these Porcelain components.
2. Five Key Selection Criteria for Qualified Porcelain Components
1. Material & Formulation: High-Strength Electrical Porcelain is Fundamental
Qualified porcelain must be made from high-alumina or high-strength electrical-grade ceramic. This material offers:
- High Mechanical Strength: Withstands mounting torque, electromagnetic forces during operation, and external stresses from weather.
- Excellent Electrical Properties: High volume resistivity and breakdown strength ensure reliable insulation.
- Superior Thermal Stability: Resists cracking or deformation under transformer load cycles and temperature variations.
- Good Hydrophobicity & Aging Resistance: Especially crucial for the Porcelain Shell exposed to the elements.
2. Manufacturing Process: Uniformity and Density are Critical
- Dry Isostatic Pressing: This advanced molding technique ensures uniform density and absence of internal flaws in both the Bushing and Shell, yielding far higher mechanical strength than traditional slip-casting.
- Precision High-Temperature Firing: Controlled sintering curves guarantee a stable crystalline structure for optimal performance. The surface glaze should be smooth and even to facilitate rain washing and resist pollution.
3. Electrical Performance & Design
- Creepage Distance & Shed Design (for Porcelain Shell): The shell must have adequate creepage distance and an optimized shed profile (e.g., alternating sheds) based on the site’s pollution severity to prevent flashover.
- Wall Thickness & Uniformity (for Porcelain Bushing): As a pressure-bearing component, the porcelain tube must have uniform wall thickness meeting design pressure requirements. Any inconsistency is a potential weak point.
4. Rigorous Testing & Inspection
Manufacturers must provide authoritative type test and routine test reports. Key tests include:
- Mechanical Tests: Bending withstand load test, internal pressure test (verifying the strength of both Bushing and Shell).
- Electrical Tests: Power frequency dry/wet withstand voltage test, lightning impulse withstand voltage test, partial discharge measurement (ensuring the insulation integrity of the Porcelain Bushing and the fully assembled unit).
- Temperature Cycle Test: Simulates thermal shocks, checking the strength of the cemented joint between porcelain and metal flanges and the porcelain’s own thermal shock resistance.
- Seal Integrity Test: Long-duration pressure test to eliminate any risk of oil leakage.
5. Visual & Dimensional Inspection
- Surface Quality: The porcelain surface must be smooth, free from spots, glaze defects, or bubbles. For the Porcelain Shell, shed edges must be intact and unchipped.
- Metal Flange Assembly (Cemented Joint): The assembly between the porcelain and the upper/lower metal flanges (typically using high-strength cement) must be robust, dense, and crack-free. This is another critical point for mechanical strength and sealing.
3. Additional Advice for Selecting a Qualified Bushing Supplier
- Credentials & Track Record: Choose a manufacturer with full certifications, long-term specialization in high-voltage bushings, and a solid industry reputation. Changsha Rich Power has over 20 years on the production of porcelain bushing and porcelain shell in China.
- Technical Match: Provide accurate transformer parameters (voltage class, current, mounting angle, pollution level) for professional selection by the supplier.
- Total Lifecycle Cost: Look beyond the initial price. A bushing built with high-quality Porcelain Shell and Porcelain Bushing offers superior long-term reliability, lower maintenance costs, and extended service life, resulting in a lower total cost of ownership.
Conclusion
Selecting a qualified oil-immersed transformer bushing fundamentally means choosing excellence in its core insulating components: the Porcelain Shell and the Porcelain Bushing. By scrutinizing every aspect—from materials, processes, and design to testing and details—you ensure this vital “transformer gateway” operates flawlessly for decades, safeguarding the stability of the power grid.
Choose insulation security that stands the test of time for your critical assets.
About Us: We specialize in high-voltage electrical insulation solutions. Our oil-immersed transformer bushings feature high-strength Porcelain Bushings and Porcelain Shells manufactured using advanced dry isostatic pressing, ensuring every unit meets the highest international standards. For professional selection support and technical consultation, please contact our experts. Email: sales@asiapower.net, Whatsapp:008613786188769
