Transformer overloading - A guide

The scenarios where you expect or operate a transformer above and beyond its nameplate ratings are termed overload capacity or overloading a transformer. Every transformer manufacturer, therefore, cautiously comprehends the buyer’s power requirements. Furthermore, design and constructs the transformer with a higher rating to prevent frequent overloading of the transformer.

Every transformer installed in the electrical setup runs on a load capacity lower than the nameplate rating. Utilities maintain lower loads as it improves the life expectancy of a transformer. However, the utilities can overload the transformers in both the long-term and short term.

Transformer overloading is gaining popularity for a simple reason - saving on the capital expenditure of installing a new transformer to meet the requirement of short term/ any contingency. The general reasons for overloading a transformer include meeting increased power requirements, possibilities of contingencies in transmission lines, faults or failures in power setups/ substations, or economic considerations.

However, the overloading results in the augmentation of the operating temperature of the transformer. A primary reason why the transformer owners do not prefer consistent overloading of a transformer.

Let’s understand the general types of transformer loading for a clear understanding -

1. Normal Life Expectancy loading - the loading capacity at which the transformer operates for the long run, resulting in a hot-spot temperature of 110°C. Considering the volatility of loads in real life, it is reasonable if transformers operate above 110°C in the short run and at temperatures lower than 110°C in the long run. It results in standard life expectancy as thermal aging is an accumulated process.
2. Planned overloading -  A type of overloading where the operator preplans the overlading of the transformer, typically for a specific period. The hot-spot temperatures may rise to 120°C - 130°C. Planned continual loads, shorter life expectancy, and no-system outages are the attributes of such loading. For such loadings, the experts can calculate the time frame in which the admissible loss of life can be achieved.
3. Long-term emergency overloading - The utilities plan to overload for a specific period as a result of a prolonged outage of any system component or issues on a transmission line. In such circumstances, the utility operates the transformer above the nameplate rating for a lengthier period. It may last up to several days, weeks, or even months. The hot-spot temperature in such can rise to 120°C - 140°C. Moreover, the risk of failure is higher, and it may even lead to a rise in top-oil temperature over the permissible limits. The occurrence frequency of such events can be 3-4 times over the entire life expectancy of the equipment. The technicians can readily estimate the time frame within which the acceptable insulation life loss can be obtained.
4. Short-time overloading - It refers to the heavy loading of a transformer during a short time, leading to a temperature hike above the nameplate ratings. The hot-spot temperatures can rise to 180°C for a short period with an intense loss of insulation life. The attributes of such overloading are -
   1. Highly unfavorable operating conditions on a transmission system.
   2. Expected to occur once or twice in the entire transformer lifespan and usually last for a shorter period (typically less than a half-hour).
   3. Greater risks of equipment failure than that in long-term overloading. The reason is gas bubble formation in the oil.

Many transformer users rely on short-term overloading in case of serious emergencies. It might even require the use of up to 50% or even higher of the corresponding lifespan for preventing severe shutdown.

Risks of overloading a transformer -

Certain risks are associated with overloading the transformers, particularly for short-term overloading. The quantum of risk depends on multiple factors like - the amount of free gas, voltage, and content of moisture in oil. Some contingent situations/risks are -

• Gas bubbling
• Temporary deterioration of mechanical attributes
• Augmented resistance in the connections of tap changers
• Dielectric or mechanical breakdowns
• Bushing might experience increased pressure
• Oil expansion in the tank
• Higher voltage regulation by the transformer.

The transformer users can overload the equipment within the prescribed limits as per the nameplate ratings. According to the circumstances and appropriate estimation of loss of insulation and the life. Certain factors holding significant importance are - hot-spot temperature, top-oil temperature, aging of the transformer, ambient temperature, etc. Therefore, every user must be highly cautious and plan enough scope for transformer overloading in both planned or contingent situations.