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Views: 0 Author: Site Editor Publish Time: 2024-10-21 Origin: Site
Visible faults are those with clear and easily observable characteristics. This article analyzes the causes and symptoms of visible faults in large transformers based on field experience and provides common on-site solutions. These methods are relatively simple yet effective.
When a transformer encounters anomalies or faults during operation, these often present external symptoms. By observing these simple external phenomena, one can detect potential defects and conduct a qualitative analysis of the fault, helping to develop further diagnostic or repair plans. For more complex faults, this also helps determine repair and testing strategies. The following are the common approaches for analysis and solutions:
When a transformer cannot breathe properly, the air above the oil conservator's diaphragm expands rapidly as the temperature rises, increasing pressure. If the diaphragm breaks, a large amount of transformer oil may spray out. The main causes and measures are:
1. Clogged Breather: Caused by excess silica gel or oil seals, or foreign objects in the pipes. The silica gel should occupy two-thirds of the breather, and one-third of the oil seal should contain oil. Nitrogen can be used to check the pipeline.
2. Tightened Diaphragm Bolts or Deformed Oil Conservator Flanges: This can occur during installation or maintenance. Replace the damaged diaphragm or oil conservator.
3. Internal Transformer Short Circuit: This generates large amounts of gas, usually accompanied by gas relay activation. Extract gas samples from the gas relay and test for flammability. If flammable, further chromatographic oil analysis and electrical testing are required. The transformer should not be re-energized until the fault is identified and resolved.
4. Stuck Expansion Mechanism in Pressure Relief Valve: Replace or have the manufacturer address this issue.
5. Improper Oil Filling During Maintenance: When filling the oil conservator with oil, if the air in the upper part is not fully evacuated, issues may arise. Shut down the transformer, fill the oil conservator completely, and then adjust the oil level to the appropriate height.
6. Blocked Oil Conservator Diaphragm Structure: If the diaphragm is blocked due to a low oil level, install a support at the pipe connection to prevent blockage.
Bushing flashover can cause the bushings to overheat and age, potentially leading to short-circuit faults at the transformer terminals. This is particularly severe for low-voltage bushings. The main causes and solutions are:
1. Surface Contamination: In rainy or humid weather, surface contamination can increase conductivity, leading to flashovers. De-energize the transformer, clean the bushings, and apply RTV silicone coating for long-term flashover protection.
2. Manufacturing or Installation Defects: If insulation parameters, such as the dielectric loss angle, exceed limits, or porcelain components are incomplete, replace the bushing.
3. Inadequate Creepage Distance: If the transformer operates in an area with high pollution levels, replace with bushings that have larger creepage distances or silicone rubber external insulation, or apply silicone rubber skirts for enhanced flashover protection.
4. Overvoltage (Internal or Atmospheric): Conduct testing and inspection on the bushings and transformer. Ensure all tests pass before re-energizing.
5. Foreign Objects on Bushing Surface: If debris short-circuits the porcelain skirts, use an insulated rod to remove the debris.
6. Ice or Snow: During icy or snowy weather, the transformer bushings are prone to icing. De-energize the unit.
Oil leakage is a common issue with transformers, as any seal point or even iron plates can develop pinhole leaks. While oil leakage does not immediately stop transformer operation, if left untreated, it can severely affect the transformer's safety. Since transformers have numerous sealing points, each with different structures, the symptoms of oil leakage can vary widely. Common causes and measures include:
1. Aging Sealing Gaskets: Poor quality or overdue replacement of gaskets can lead to aging and cracking. Replace during maintenance.
2. Improper Tightening at Sealing Points: Loose sealing points accelerate gasket aging, allowing air to enter the transformer. Tighten as soon as leaks are detected.
3. Faulty Valves: Poor manufacturing quality may result in valves that do not close properly. Drain the oil and replace the valve.
4. Weld Defects or Pinholes: Pinholes in iron plates or poor welding quality can be addressed by surface welding a larger area or replacing affected plates. Perform oil-side welding with strict fire protection measures in place.
5. Oil Pump Leaks: Leaks may occur only when the pump is stopped, due to air entering the transformer, which can trigger gas relays. Inspect each pump individually.
6. Deformed Gaskets: Overstressed gaskets or improper sealing structures during manufacturing or installation can also cause leaks. Address the specific cause based on inspection.
Many transformer faults are accompanied by overheating, which can cause changes in color, temperature, or emit unusual odors from the affected parts.
Loose bolts or screws, oxidation, or insufficient contact area at connection points can cause overheating, indicated by discoloration or loss of gloss at the hotspot. Surface temperature may exceed 70°C, and temperature-sensitive strips may change color or turn black. During maintenance, measure the DC resistance of transformer windings, including the clamps. If suspicious, measure the contact resistance of the clamp (generally less than 500 µΩ). Sand down the contact surface and tighten connections as needed.
The silica gel in the breather absorbs moisture entering the transformer’s oil conservator. When dry, the gel is light blue, but when saturated, it turns pink. Normally, silica gel should be replaced annually. Causes of faster-than-normal color change and corrective measures include:
1. Poor Seal in the Breather: Check for gasket aging, loose screws, or cracked glass. Replace the gasket, tighten screws, or replace the glass cover as needed.
2. Oil Seal Issues: Low oil levels or water contamination in the oil seal allow moisture to bypass the seal. Add oil to the appropriate level.
3. High Humidity: Rainy or humid conditions can speed up the color change. Conversely, if the silica gel changes color too slowly, check for blockages in the breather system.
A transformer in normal operation emits a continuous, uniform hum due to alternating magnetic fields. Abnormal sounds may indicate:
1. Overvoltage: Check the system for overvoltage and adjust accordingly.
2. Overloading: Verify loading against transformer nameplate specifications.
3. Loose Core Fastening: Loose core clamps or vibrating silicon steel sheets cause noise and require maintenance.
4. Poor Tap Changer Contact: Static discharge from ungrounded metal components or poor tap changer contact can cause crackling sounds.
5. Loose External Components: Items such as nameplates or fan motors may produce abnormal noise if not securely fastened.
Transformer faults are often accompanied by temperature changes. Per regulations, the upper oil temperature of a transformer should not exceed 95°C, with a maximum temperature rise of 55°C. Causes and solutions include:
1. Internal Faults: Multiple grounding points on the core or overheated bare metal parts require investigation and resolution.
2. Cooling Issues: After installation or major repairs, check that radiators are fully open and that the cooling system is functioning properly.
3. Breather Blockage or Severe Oil Leakage: Address breather blockages or significant leaks that can impair cooling.
4. Design Flaws: Localized overheating due to stray magnetic flux may be mitigated by adding magnetic shielding materials.
Visible faults in transformers, such as abnormal oil levels, load fluctuations, or accessory malfunctions, are identified through external observations. Proper assessment of these faults helps determine whether the transformer can continue operating or requires further testing. Since the causes of the same visible fault can vary, it is crucial to understand the history of each transformer's installation, maintenance, and operation, as well as its specific structure and performance characteristics.
