1. Describe, with the aid of a diagram, the process involved in tap changing using a centre-tapped reactor.
2. (a) Describe, with the aid of a diagram, the method of operation of a Buchholz relay.
(b) State which faults can be detected by this relay and in what way they activate the relay.
(c) What is the major advantage of this relay over other forms of transformer protection?
3. An oil-filled transformer has a c.m.r. of 500 kVA which allows it to run continuously in an ambient of 35°C with an oil temperature rise of50°C. The transformer has a time constant of 1.5. hours and the ratio of copper losses to iron losses at c.m.r. is 1.5:1.
After switch-on, the transformer supplies a load of 600 kVA for one hour and the load then rises to 750 kVA. Calculate the period of time that the transformer can supply this load without exceeding its rated maximum temperature. Sketch the heating curves for the two loads.
