Transformers Questions and Answers

Answer: B

Correct answer: above 99 per cent

Large power transformers are among the most efficient machines built, commonly reaching 99.5 per cent or better at rated load, because they have no moving parts and hence no friction, windage or rotor loss. Even so, half a per cent of a 500 MVA unit is 2.5 MW of heat, which is why the cooling plant is substantial.

Answer: C

The core is a simple rectangle and the coils encircle its vertical limbs, which makes the winding accessible and the insulation straightforward. A shell type wraps the core around the coils instead and gives better mechanical bracing against short-circuit forces.

Answer: D

Correct answer: surrounding the windings on more than one side

In a shell type transformer the windings sit on a central limb and the core encloses them on both sides, so the magnetic circuit surrounds the coils. In a core type the coils surround the limbs instead. The shell arrangement gives better mechanical bracing against short circuit forces and a shorter mean flux path.

Answer: A

Thin sheets separated by insulation break up the paths available to circulating currents, and the loss falls with the square of the thickness. Hysteresis loss depends on the grade of steel rather than on the lamination.

Answer: B

Aligning the crystal grains with the direction the flux will take reduces the loss and raises the permeability along that axis. Mitred corner joints are used so that the flux stays as nearly as possible in the favourable direction.

Answer: C

Its dielectric strength lets the windings be spaced closely, and its circulation carries heat from the core and coils to the radiators. Moisture and particles degrade both functions, which is why oil is tested periodically.

Answer: D

Even a few parts per million of water lowers the breakdown voltage sharply, which is why the breather dries the incoming air and the oil is filtered and dried in service. Particles and acidity from ageing also contribute.

Answer: A

Different faults produce characteristic mixtures of hydrogen, methane, acetylene and the rest, so the ratios point to overheating, partial discharge or arcing. It is the principal condition-monitoring tool for a large transformer.

Answer: B

Correct answer: the winding and the top oil, each with its own limit

Two limits are specified because they measure different things: the top oil temperature governs the ageing of the oil, while the winding temperature, always the higher of the two, governs the ageing of the paper insulation. Insulation life falls roughly by half for each 6 to 8 degrees of sustained excess, which is why both are monitored.

Answer: C

The losses are unchanged, but a lower thermal resistance to ambient means the same temperature rise accommodates a larger loss and therefore a larger load. That is what the ONAN and ONAF ratings on a nameplate describe.