Temperature Rise and Transformer Efficiency

This article is excerpted from "Premium-Efficiency Motors and Transformers", a CD-ROM available from CDA by going to the Publications List.

What Is Temperature Rise of a Transformer?

All devices that use electricity give off waste heat as a byproduct of their operation. Transformers are no exception. The heat generated in transformer operation causes temperature rise in the internal structures of the transformer. In general, more efficient transformers tend to have lower temperature rise, while less efficient units tend to have higher temperature rise.

Transformer temperature rise is defined as the average temperature rise of the windings above the ambient (surrounding) temperature, when the transformer is loaded at its nameplate rating.

Standard Ratings and Overload Capacity

Dry-type transformers are available in three standard temperature rises: 80C, 115C, or 150C. Liquid-filled transformers come in standard rises of 55C and 65C. These values are based on a maximum ambient temperature of 40C. That means, for example, that an 80C rise dry transformer will operate at an average winding temperature of 120C when at full-rated load, in a 40C ambient environment. (So-called hot spots within the transformer may be at a higher temperature than average.) Since most dry transformers use the same insulation on their windings (typically rated at 220C), irrespective of the design temperature rise, the 80C rise unit has more room for an occasional overload than a 150C rise unit, without damaging the insulation or affecting transformer life.

Transformer Efficiency and Temperature Rise

It is best to obtain the actual load and no-load losses in watts from the transformer manufacturer, but sometimes those data are not available. In that case, temperature rise is a rough indicator of transformer efficiency. For example, a transformer with an 80C temperature rise uses 13-21% less operating energy than a 150C rise unit.

A more efficient transformer generates less waste heat in the first place, but transformer temperature rise results from not only how much heat is generated but also how much heat is removed. Be careful that a unit carrying a low temperature rise figure is not also inefficient, using fans to remove the excess heat.

The examples of 1,500 kVA and 75 kVA transformers in the table below are of high-efficiency, copper-wound transformers designed to achieve an 80C rise and high efficiency. These are compared to standard-efficiency aluminum-wound units, that are designed for a 150C rise. As can be seen from this table, the higher-efficiency 80C rise transformers have a first-cost premium, but a shorter payback than the less-efficient 150C rise transformers. Not only will a lower-temperature-rise transformer have fewer losses, but also it will have a longer life expectancy.

How Does Temperature Affect the Life of a Transformer?

Temperature is one of the prime factors that affect a transformer's life. In fact, increased temperature is the major cause of reduced transformer life. Further, the cause of most transformer failures is a breakdown of the insulation system, so anything that adversely affects the insulating properties inside the transformer reduces transformer life. Such things as overloading the transformer, moisture in the transformer, poor quality oil or insulating paper, and extreme temperatures affect the insulating properties of the transformer. Most transformers are designed to operate for a minimum of 20-30 years at the nameplate load, if properly sized, installed and maintained. Transformers loaded above the nameplate rating over an extended period of time may have reduced life expectancy.

Lower Temperature Rise Means Increased Overload Capability

A lower-temperature-rise transformer results in a transformer with higher overload capability. For example, an 80C rise dry-type unit using 220C insulation has 70C reserve capacity compared to a 150C unit. This allows the 80C unit to operate with an overload capability of 15-30% without affecting the transformer life expectancy. Also, a cooler running transformer means a more reliable unit and more up-time.

Designing a Transformer with Lower Temperature Rise

Transformers with lower temperature rise often use windings with lower resistance. The low resistance per unit length of copper allows lower temperature rise transformers to be built without unnecessarily building a bigger transformer. For example, an aluminum-wound transformer coil requires conductors with approximately 66 per cent more cross-sectional area than a copper-wound transformer coil to obtain the same currentcarrying capacity.

High Efficiency and Conditioned Spaces

High-efficiency, low-temperature-rise (80C rise dry-type or 55 C liquid-filled) transformers are frequently found in confined spaces, like inside electrical rooms, underground vaults, and air-conditioned spaces in buildings. High efficiency means less waste heat generated, thus lower ventilation and airconditioning requirements. Selecting such a transformer, properly sized to the load requirements, assures greatest efficiency, longer life and increased overload capability.