The iron core of an oil-immersed transformer is a core component that serves as both a magnetic conduction path and structural skeleton. It must be single-point grounded to ensure safe operation—multi-point grounding will trigger circulation, overheating, and even major accidents. Below is a detailed breakdown of the core’s functions, grounding principles, and main grounding fault causes.
1. Core Functions of the Oil-Immersed Transformer Iron Core
The iron core’s role is critical to the transformer’s magnetoelectric conversion efficiency, focusing on two key aspects:
- Magnetic Conduction Path: It provides a closed, low-reluctance path for magnetic flux generated by the winding’s alternating current. This enhances mutual induction between the primary and secondary windings, enabling efficient voltage/current conversion.
- Structural Skeleton: It acts as the fixed support for windings and other components, ensuring stable assembly of internal parts and maintaining mechanical strength during operation.
As the electromagnetic field around the windings changes, the core’s magnetic susceptibility and magnetic induction also fluctuate—its presence is essential for realizing effective mutual induction between windings.
2. Grounding Principle: Why Single-Point Grounding Is Mandatory
The core and other metal parts must follow strict grounding rules to avoid insulation damage and discharge hazards:
(1) Necessity of Grounding
During normal operation, parasitic capacitance exists between high-voltage windings and low-voltage windings, low-voltage windings and the core, and the core and the tank. These capacitances cause the charged windings to induce a floating potential on the core.
- Due to unequal distances between the core and other metal parts, potential differences arise. When the potential difference exceeds the insulation withstand capacity, spark discharge occurs, which degrades insulating oil and solid insulation over time.
- Single-point grounding equalizes the core’s potential with the tank (ground), eliminating floating potential and preventing discharge.
(2) Hazards of Multi-Point Grounding
Multi-point grounding (two or more grounding points) forms a closed loop on the core. Under the action of the alternating magnetic field:
- Circulation current flows through the closed loop, causing local overheating of the core.
- Overheating leads to insulating oil decomposition, reduced insulation performance, and even breakdown. In severe cases, silicon steel sheets of the core may be burned or broken, triggering major transformer failures.
Thus, the core must be grounded at only one designated point—any additional grounding points must be eliminated.
3. Main Causes of Oil-Immersed Transformer Core Grounding Faults
Core grounding faults (mostly multi-point grounding) are mainly caused by construction, design, and environmental factors, including:
- Construction and Design Defects: Poor construction processes (e.g., incomplete removal of burrs on silicon steel sheets) or unreasonable grounding plate design (e.g., loose grounding connections) lead to unintended grounding points.
- Accessory and External Factors: Multi-point grounding caused by external accessories (e.g., loose clamping parts, damaged insulation pads between the core and tank) or external environmental influences (e.g., moisture-induced insulation degradation).
- Internal Foreign Object Contamination: Metal foreign objects (e.g., welding slag, rust, metal shavings) left inside the transformer during manufacturing or maintenance form conductive paths, causing multi-point grounding.
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