Q1.

(a) What is a Vector grouping?

(b) How does the knowledge of vector grouping assist in connecting transformers to operate in parallel?

(c) What additional information is needed to ensure that transformers are capable of parallel operation?

(d) What are the only phase displacements in degrees with which it is possible to parallel the secondaries of two transformers?

(e) Draw the phase relationships and connections for (i) Dd0 connection (ii) Yy6 connection.

 

Q2.

(a) Name the types of on-load tap changer.

(b) Which is the most commonly used and why?

 

Q3.

(a) What is the advantage of monitoring winding temperature as a method of overload protection?

(b) Name two other types of overcurrent protection.

 

Q4.

(a) What is a conservator?

(b) How does this device protect against pollution of the transformer oil?

 

Q5. List the symbols, and their meaning, used to classify the methods of cooling a transformer and state the types of cooling of an ONAF transformer.

 

Q6. Describe, with the aid of a diagram, the process involved in tap changing using a centre-tapped reactor.

Q7.

(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?

 

Q8. 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 of 50°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.

 

Electrical Engineering ; Transformers