A transformer being taken out of service does not always mean that a sudden or major failure has occurred. In many cases, the problem begins with an indication that can be observed during operation or routine maintenance and may then develop into a condition requiring the transformer to be taken out of service to prevent further damage. Based on Triple M’s maintenance team records and field inspection and maintenance activities, certain causes and indicators are repeatedly observed in cases where transformers are taken out of service. These cases can be associated with the following causes:
1. Electrical Short Circuit at the Tap Changer Terminals
The Tap Changer is one of the components exposed to repeated electrical and mechanical stresses. Insulation damage, contact problems, or connection faults may lead to a short circuit, causing the protection system to trip and disconnect the transformer.
2. Low Oil Level
Transformer oil does not only provide insulation; it also helps transfer heat away from the active parts. Therefore, a low oil level caused by leakage or another issue may affect both insulation and cooling, making it necessary to identify the cause before returning the transformer to service.
3. Gas Formation Inside the Main Tank
The presence of gases or an increase in gas generation inside the transformer may indicate internal thermal or electrical stress. This is why DGA – Dissolved Gas Analysis is used to analyze dissolved gases in transformer oil and relate them to potential fault conditions. IEEE C57.104 provides guidance on using dissolved gas analysis to assess the condition of oil-immersed transformers and help diagnose possible faults.
4. High Temperature Under Load and Poor Cooling Performance
Higher loads increase transformer losses and operating temperature. If the cooling system is not functioning efficiently, temperatures can rise to unsafe levels, particularly under prolonged loading.
Therefore, monitoring the load alone is not sufficient. Transformer temperature, cooling-system performance, and the condition of fans or pumps—depending on the transformer design—should also be monitored.
5. Dust Accumulation and Increased Humidity in Ring Main Units
In RMUs, dust and moisture can affect insulation performance and increase the possibility of problems in connections and insulating components, particularly in industrial environments or challenging climatic conditions.
RMU maintenance practices include visual inspection, cleaning, checking connections and earthing, and inspecting protection and insulation-related components.
6. Damaged Fuses
A damaged or blown fuse should not simply be replaced without investigation. It may be the result of an overcurrent condition or a fault in the circuit it protects. The cause of the fuse operation should therefore be identified before the equipment is returned to service, along with checking the connections, load, and associated protection system.
7. Weak or Missing Earthing
Earthing is an essential part of the protection system. It provides an appropriate path for earth-fault currents and helps protection devices operate as designed.
Therefore, any issue with the earthing of the transformer or distribution equipment can directly affect safety and the effectiveness of the protection system.
8. Electrical Short Circuit
A Short Circuit is one of the conditions that can cause the protection system to operate and disconnect the transformer or the affected part of the network. It may result from insulation failure, component breakdown, wiring errors, or other operating conditions.
The important point is that transformer disconnection should not be viewed as the problem itself. In many cases, it is the protection system’s response to an underlying fault.
Identifying the cause of the trip through fault records, visual inspection, electrical testing, and oil condition analysis when required helps determine whether and how the transformer can be safely returned to service.
What Should Be Checked Before Returning the Transformer to Service?
Returning a transformer to service should not simply mean reconnecting the power supply. The required checks depend on the nature of the fault, but may include:
- Reviewing the cause of the trip and protection records.
- Checking the oil level and any signs of leakage.
- Checking operating temperatures and the cooling system.
- Inspecting connections and the tap changer.
- Verifying the earthing condition.
- Performing the appropriate electrical tests.
- Analyzing transformer oil when indicated.
In maintenance, effective troubleshooting starts with understanding the cause of the fault before addressing its symptoms. This is the approach followed by Triple M teams when dealing with transformer outages: first diagnosing the condition, then determining the appropriate tests and corrective action based on inspection results rather than relying solely on the visible symptom.
المصادر
- IEEE C57.104 – Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers.
- Egyptian Electricity Holding Company (EEHC) – المواصفات الفنية لوحدات الربط الحلقي RMU.
- إرشادات وممارسات صيانة وحدات RMU المتعلقة بالنظافة، التوصيلات، العزل والحماية.