Manifestation Characteristics of Transformer Core Grounding Fault

The causes and on-site manifestations of transformer core grounding faults are varied, but their fault characteristics often follow common patterns. Understanding the structural characteristics of the transformer and identifying parts prone to multi-point grounding is crucial. A comprehensive analysis of various test data helps accurately locate redundant grounding points. When conducting such analysis, focus on the following manifestation characteristics of core grounding faults:

Oil Chromatography Analysis Characteristics

• Normal Circulation: Circulation between core grounding points leads to core overheating, accelerating the cracking and decomposition of transformer oil, producing flammable characteristic gases. The initial temperature ranges from 300 to 700 degrees Celsius, typically with a fault code of 0 2 1, resulting in abnormal chromatographic analysis.
• Total Hydrocarbon Increase: Total hydrocarbon content increases, often exceeding 150 µL/L, with ethylene and methane as main components, and trace or absent acetylene. Long-term operation without faults generally keeps acetylene below 5 µL/L.
• Serious Faults: Overheating without high-energy discharge produces a fault code of 0 2 2 or 0 0 2 (high-temperature thermal fault above 700 degrees Celsius). Rapid gas production exceeding guideline values results in significant acetylene presence.
• Involvement of Insulating Materials: Some faults involve insulating materials, causing increased CO and CO₂ levels.
• Intermittent Grounding Faults: Produce acetylene due to discharge sparks, with C₂H₂ exceeding 5 µL/L.

Electrical Measurement Characteristics

• Increased Grounding Current: Multiple grounding points form circulation channels, leading to significantly increased grounding current, often exceeding the 0.1A limit set by “Preventive Test Regulations for Power Equipment”. Insulation resistance testing during power off often shows values below 5 MΩ.
• Measurement Techniques: To avoid misjudgment, use a clamp meter placed horizontally at half the oil tank height to measure grounding current. For accuracy, connect a reliable short-circuit line parallel to the grounding lead and measure with an AC ammeter. Repeated observations are necessary for intermittent multi-point grounding, where current values fluctuate.
• Challenges in Electrical Measurement: Single-point grounding faults are hard to determine with electrical measurements alone. Chromatographic analysis, power-off checks, and visual inspections are necessary for accurate fault identification.

Hanging Cover Inspection Characteristics

• Visual Inspection: Look for discharge, burns, and overheating marks on the core. Check for metal objects like transportation nails, core clamps, welding slag, copper wire, metal chips, or debris touching the core.
• AC Voltage Application: Applying AC voltage to the core lead may produce discharge sounds or smoke, indicating redundant grounding points.
• DC Voltage Measurement: Disconnect the core from the clamp, apply DC voltage on both sides of the iron yoke, and measure voltage across core laminations. Zero voltage indicates a redundant grounding point.

By understanding and analyzing these characteristics, transformer core grounding faults can be effectively identified and addressed to ensure safe and reliable operation.