What is a Vector grouping?

1. (a) What is a Vector grouping?

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

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

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

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

2. (a) Name the types of on-load tap changer.(b) Which is the most commonly used and why?

3. (a) What is the advantage of monitoring winding temperature as amethod of overload protection?(b)Name two other types of overcurrent protection.

4. (a) What is a conservator?(b) How does this device protect against pollution of the transformer oil?

5. List the symbols, and their meaning, used to classify the methodsof cooling a transformer and state the types of cooling of an ONAFtransformer.3Teesside University Open Learning(Engineering)© Teesside University 2011

6. Describe, with the aid of a diagram, the process involved in tapchanging using a centre-tapped reactor.

7. (a) Describe, with the aid of a diagram, the method of operation of aBuchholz relay.(b) State which faults can be detected by this relay and in what way theyactivate the relay.(c) What is the major advantage of this relay over other forms oftransformer protection?

8. An oil-filled transformer has a c.m.r. of 500 kVA which allows it to runcontinuously 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 ofcopper losses to iron losses at c.m.r. is 1.5:1.After switch-on, the transformer supplies a load of 600 kVA for onehour and the load then rises to 750 kVA. Calculate the period of timethat the transformer can supply this load without exceeding its ratedmaximum temperature. Sketch the heating curves for the two loads