Page 265 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

To conduct this test portable test kit is available consisting of:

1 Two polythene bottles containing 100ml each of ethyl alcohol and sodium carbonate solution of 0.0085N
concentration.
2 An indicator bottle containing universal indicator.

3 Four clean glass test tube.
4 Three graduated droppers, which serves as pipettes.
5 Colour chart with acidity range.
6 Instruction booklet.
PROCEDURE

The test is conducted by taking 1.1 ml of insulating oil (to be tested) in test tube, 8 ml oil 1 ml of rectified spirit
is added and mixture is to be gently shakened. Further 1 ml of solution of 0.008 5 N sodium carbonate added.
After shaking the test tube once again 5 drops of universal indicator is added. The resulting mixture develops a
colour depending on the acidity value of the mixture.
The approximate colour range will be as follows:

Total acidity value in No. Colour
0.00 Black
0.2 Green
0.5 Yellow
1.0 Orange

Any how the colour chart will be provided with the test kit to indicate exact value.
Three phase transformer
Uses of three phase transformers
• Power generated about 11kV
• Transmission voltages of 110, 132, 275, 400 and 750 kV

• Step up and step down by three phase transformers
• Previously interconnection of 3 single phase transformers used
• Single unit 3ftx. Costs 15% less and need less space

Star star (YY) connection (Fig 5)
This connection is most economical for small, high-voltage transformers.Numberof turns/phase and the amount
of insulation required is minimum (as phase voltage is only 1/3 of line voltage. Phase shift of 30° between the
phase voltages and line voltages both on the primary and secondary sides.Line voltages on both sides as well
as primary voltages are respectively in phase with each other. Works satisfactorily only if the load is balanced.
Delta delta (DD) connection (Fig 6)

This type connection is economical for large, low-voltage. No internal phase shift between phase and line voltages
on either side as was the case in Y Y.This connection can be used for unbalanced load. Better sinusoidal voltage
output. If one transformer becomes disabled, the system can continue to operate in open-delta or in V V although
with reduced available capacity. The reduced capacity is 58% and not 66.7% of the normal value

252
CITS : Power - Electrician & Wireman - Lesson 41-49