Diagnosing Transformers Getting to the Heart of the Matter
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FEATURE
Diagnosing Transformers Getting to the Heart of the Matter Albert M. Rose
Submitted: 21 April 2010 / Published online: 11 May 2010 Ó ASM International 2010
Abstract From drying your hair in the morning to using your computer at work to watching a DVD on your big screen TV at night, transformers are involved in the many things we do every day. As the name implies, transformers transform electricity from one voltage to another. In the early days of electricity, it was costly to move electricity from Point A to Point B. As a result, people who wanted electricity had to live very close to a power plant. At the 1893 World’s Fair, George Westinghouse and Nikola Tesla introduced alternating current, or AC power, to the world. Up until this time, Thomas Edison’s direct current, or DC power, was the only form of electricity in the United States. Among the many positive attributes of AC power, engineers discovered that if the power could be converted (or transformed) to use higher voltage and lower current, then it would be cost-effective to transport the electricity over longer distances from a power plant to a home or business. This discovery and the invention of the transformer led to the power distribution system as we know it today. Transformers are the backbone of our electrical transmission and distribution system. Of these, high- and mediumvoltage transformers are the most complex and the most expensive pieces of equipment in this system. Consequently, it is important that transformers be properly constructed and maintained to ensure a long service life. Keywords Transformers Windings Oil tests Electrical tests
A. M. Rose (&) Engineering Design & Testing Corp., Asheville, NC, USA e-mail: [email protected]
What’s Inside the Box? To the casual observer, a transformer looks like a big, metal box. Within this box (referred to in transformer language as a ‘‘tank’’) are three major components: the core, the windings, and the insulating fluid. The core is the heart of a transformer. It is the core that allows the transformer to convert the voltage either up or down. The design of the core has not changed much since the beginning of AC power: thin, flat layers of silicone steel. These laminations are normally around .30 mm (0.012 in.) thick and are stacked to a width and height determined by the transformer’s size. After the core is assembled, the laminations are clamped tightly together. The windings (also called ‘‘coils’’) are assembled around the core and are made from either copper or aluminum. Copper has the advantage of having a greater mechanical strength and better electrical conductivity, a necessity for larger power transformers. Aluminum is lighter, costs less, and is better at heat dissipation, which works well for smaller transformers such as the one on the utility pole supplying electricity to your home. Figure 1 is an example of a simplified sketch of a transformer core and windings. It is the ratio of winding turns that determines how much the voltage ‘‘steps’’ up or down. In the sketch
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