EN
In the operation management of the distribution network, faults of distribution transformers are the main cause of power outages. Through the statistical analysis of a large amount of operational data, insulation damage and moisture ingress, winding short circuit deformation, as well as lightning-induced overvoltage have consistently ranked among the top three causes of faults. A thorough understanding of the causes and mechanisms of these three major faults is the foundation for formulating effective operation and maintenance strategies and enhancing power reliability.
- Insulation damage and moisture ingress: The most insidious "number one killer"
The decline in insulation performance is the most common and serious fault source for distribution transformers. It is highly concealed and usually only becomes apparent during sudden accidents.
- Long-term thermal aging: During the operation of the transformer, heat is generated. If it is overloaded for a long time, the insulation paper of the windings will accelerate decomposition under high temperatures, resulting in gradual loss of mechanical strength and electrical strength.
- Moisture intrusion: Oil-immersed transformers are prone to absorb moisture due to the "breathing effect". When the moisture content in the oil exceeds the standard or the oil level is too low, the withstand voltage level of the insulation materials will sharply decrease.
- Latent breakdown: Degraded insulation is highly likely to experience internal inter-turn or inter-phase breakdown when encountering operational overvoltage or system resonance, ultimately leading to short circuit and burnout.
- Winding short circuit and deformation: The most destructive "acute emergency"
Winding faults are mostly caused by external short circuits. The electric stress generated is extremely high, and it is the direct cause of severe damage to the transformer itself.
Short-circuit current impact: When a metallic short circuit occurs at the low-voltage side outlet or branch line, the short-circuit current can reach tens of times the rated current, generating extremely strong radial and axial electrodynamic forces.
- Mechanical deformation and cumulative effect: The huge electrodynamic force will cause the conductor to be compressed, twisted or displaced. Even if a short circuit does not directly burn out the component, the cumulative plastic deformation will gradually reduce the short-circuit resistance of the winding.
- Final burnout: The deformation leads to local insulation wear or changes in the winding gap distance. In the next current impact, this weak point will quickly heat up and develop into a winding short circuit, until the equipment is scrapped.
- Lightning strikes and overvoltage: Seasonally frequent "external threats"
Lightning-induced overvoltage is the primary external cause for the damage of distribution transformers during the rainy season, and it is particularly prominent in rural areas and open spaces.
- Lightning wave intrusion: After a lightning strike hits the overhead line, it penetrates the transformer along the conductor. The sharp wavefront and high-amplitude overvoltage directly impact the main insulation and longitudinal insulation of the windings.
- Protection failure superposition: If the lightning arrester on the high-voltage side is aged and fails, the grounding resistance exceeds the standard, or no lightning arrester is installed on the low-voltage side, the invading wave cannot be effectively discharged, which will directly cause flashover of the bushing or insulation breakdown.
- Low-voltage side counterattack: When the low-voltage line is subjected to induced lightning, the overvoltage will be transmitted to the high-voltage side through the low-voltage winding, forming a positive and negative alternating overvoltage, which exacerbates the risk of insulation damage.
Summary and Key Points for Operations and Maintenance
Insulation protection, short-circuit tolerance and lightning protection are the three main pillars for ensuring the reliable operation of distribution transformers. For the above three types of faults, it is recommended to focus on strengthening the following measures during operation and maintenance:
- Insulation monitoring: Conduct regular oil-based dissolved gas analysis (DGA) to monitor the moisture content and dielectric loss factor of the medium, and promptly identify latent insulation defects.
- Short-circuit tolerance verification: Reasonably configure protection settings to ensure rapid tripping in case of faults; promptly perform winding deformation tests on transformers that have experienced near-zone short circuits.
- Lightning protection management: Regularly test the performance of lightning arresters and grounding resistances, improve low-voltage side lightning protection, and ensure that the lightning energy has a smooth discharge path.
By targeting the control of these three core fault sources, the failure rate of distribution transformers can be significantly reduced, the service life of the equipment can be prolonged, and a solid guarantee can be provided for the safe and stable operation of the distribution network.
