Loading issue that seriously affects transformer operation

Why Loading is Often a Problem?

Before its on-site installation and commissioning, a transformer has been designed and tested to a particular specification or standard. In real life, a transformer must adapt to a number of variations from the standard operational criteria set down in these documents. In order to achieve the greatest possible benefit from a transformer, a good understanding of operating variations is required.

This technical article highlights loading as a number one issue that affect transformer operation. With the knowledge from this technical article, it is possible to understand and better manage the long-term operation of the transformers, thus offering a longer, optimized life-cycle.

The rating of a transformer is derived from the steady-state load (apparent power) it may carry without exceedingthe guaranteed temperature rises. At a normalized ambient temperature (within the guaranteed limits), the insulation is assumed to degrade (or age) at a normal rate when the transformer is operated at constant rated load.

In practice neither the load nor the ambient temperature is constant; the instantaneous load follows the demands of industrial and domestic consumersand the ambient varies with the time of day and with each season.

Thermal aging of paper is important because it reduces the mechanical strength making it more susceptible to tearing which could lead to dielectric failure.

Table of Contents:

1. Variation of Transformer Aging with Load
2. Loading Transformer Beyond Nameplate Rating:
   1. Regular Cyclic Loading
   2. Long Time Emergency Loading
   3. Short Time Emergency Loading
3. Problems in Generator Transformer Application
4. Effect of Loss of Transformer Cooling:
   1. Coolers employing Plate-type Radiators:
      1. Loss of Fans
      2. Loss of Pumps
      3. Multi-mode Cooling
   2. Coolers Employing Single-mode, Compact Heat-exchangers:
      1. Loss of Fans
      2. Loss of Pumps
      3. Cooling Plant Redundancy
5. Attachment 🔗 Download ‘Transformer Condition Control: Advanced vs Traditional Technologies’

1. Variation of Transformer Aging with Load

Cellulose insulation degrades faster at higher temperatures. Thus, it is usually the thermal degradation of cellulose insulation (at the hottest spot) which imposes the limit. In addition, at a certain temperature, oil starts to undergo a chemical change and may give off bubbles of gas. This must also be taken into account when considering the safe operation of the transformer.

The actual temperature dependencies of oil and cellulose depend on a number of variables including moisture content, acidity, oxygen availability and the presence of catalysts. Montsingee showed the gradual reduction of tensile strength over time, with an accelerating rate of reduction of strength as the temperature increased. Later investigators showed similar results for other cellulose products.

The basic principle of thermal aging of insulation as a function of temperature and time followed the Arrhenius relationship. The rate of deterioration of mechanical properties doubles for each 5-10°C increase in temperature, but is not constant at all temperatures. Equations have been formulated that allow an approximate estimate of the loss-of-life relative to “normal” expected life for known operating conditions to be calculated.

Such equations can be found in the IEC 60076-7 and IEEE C57.915 loading guides.

The hot spot temperature which gives rise to an ageing rate of unity is one of the most important design criteria for a transformer. Although the rate of aging is known to depend on a number of factors, the normal method of estimating it is to consider only the hot spot temperature.

For instance, the IEC loading guide considers that a transformer will age at a rate of unity when operating continuously at a hot spot temperature of 98°C.

Thus the increased use of life when operating with an ambient temperature above 20°C will be offset by the reduced use of life when it is below 20°C. The recommended practice according to IEC 60076-7 is to use the yearly weighted ambient temperature for the aging calculation because it gives a more accurate result than using the arithmetic mean value.

Based on the weighted ambient temperature, the normal ageing of the insulation occurs when the operating duration with elevated hot spot temperature is balanced by an appropriate period of the operation at a reduced hot spot temperature.

Whereas the yearly weighted ambient temperature is to be used for thermal aging calculations, IEC recommends that the monthly average temperature of the hottest month is used for design and test considerations of the maximum hot spot temperature.

Watch Lesson – Relation between temperature rise and transformer loading

For dynamic considerations, such as monitoring or short-time emergency loading, the actual temperature profile should be used directly. Ambient temperature is of course not the only variable encountered during operation.

The majority of transformers do not operate at constant rated load. Under normal operating conditions pertaining to the majority of transformers, there is a potential margin in the life expectancy.

In the IEC loading guide, the evaluation of the relative aging of normal paper, related to the winding hot spot, is given by the following formula:

• V = Aging rate at θ_h / Aging rate at 98°C, or
• V = 2^{(θ_h−98)/6}

Where:

• V is the relative aging,
• Q is the hot spot temperature.

By monitoring the load and ambient temperature and deriving the hot spot temperatures, it is thus possible to obtain a coarse estimate of the average aging of the transformer for the year. However to assess the true aging state of an operating transformer requires a much more accurate estimation that takes into account the effect of any harmonics, hot spot location, and a reliable methodology for loss of life prediction.

In addition, the water content, in both cellulose and oil, is among the critical parameters affecting the thermal life of the transformer.

Suggested Guide – Power transformer testing procedures and schemes

Power transformer testing procedures and schemes

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2. Loading Transformer Beyond Nameplate Rating

Loading the transformer above its nameplate rating or operating at rated power at higher than the “rated” ambient temperature, requires careful consideration of the effects of aging of the transformer and also the risk of premature failure that is associated with the increase of temperature.

Increasing the load current also increases the stray flux which causes greater losses (and increasing temperature) in the conductors and constructional parts.

Eventually the thermal limit of the insulation with which they are in contact is reached. Further limitations in the increase of current are imposed by the switching capacity of the tap-changer, by other equipment with insulation components such as the tap-changer and bushings, and by the maximum volume of the oil expansion vessel.

In addition, the cumulative thermal degradation effect with time of application will progressively age the insulation. The standard loading guides give recommendations on permissible overloads with regards to the maximum hot spot temperature and transformer aging rate for certain loading regimes.

The more data that is available, the more accurate and detailed the assessment can be.

Watch Lesson – Effect of loading on transformer

For a new transformer it would normally be expected that thermal calculations and factory temperature rise test information would be available. These would permit calculation of the thermal time constant and a more rigorous assessment of the hot spot temperature and its location.

However, for an existing transformer, such data may not exist, although the manufacturer may be able to provide limited design or factory test information.