“Thermal expansion is another important issue. The thermal expansion of aluminum is much larger than copper’s from normal ambient temperatures down to –20°F. That’s important because substation transformers are built to a specific design coil height, and coil windings are clamped to keep that height fixed. As our load fluctuates, transformer temperatures rise and fall. The coil wants to compress and expand, but it can’t because it’s clamped. Instead, it compresses the paper insulation between turns (Figure 3). The effect is especially pronounced when a fault occurs, but it affects aluminum windings much more so than it does copper due to the difference in thermal expansion.

“Later, when the temperature returns to normal, the insulation in an aluminum transformer might not be as tight as it was before the overload. That allows the windings to loosen slightly, a condition that might lead to loose connections at some point in time.”

Leighty believes copper also helps reduce lifetime ownership costs. “Again,” he says, “it’s due to copper’s higher strength. Life-cycle cost is not just a matter of efficiency (even though that’s important), it’s also a function of reliability.

“For example, we expect a 40- to 50-year life cycle in our transformers. We monitor their condition with periodic oil analyses. When an analysis shows that gas and moisture are increasing, it’s a sign that the oil is breaking down and that there’s probably overheating going on at a connection inside the transformer. If you have aluminum-wound coils, the internal connections can loosen (creep) over time, just like connections in aluminum house wiring did 30 years ago. Loose connections cause the heating that breaks down the oil. We almost always identify the problem before it gets out of hand, but, when we don’t, the result can be catastrophic.