Page 67 - Practical Power System and Protective Relays Commissioning

64 Practical Power System and Protective Relays Commissioning

5.6.7 Temperature Rise Test
The temperature rise test is undertaken using a back-to-back test of two par-
allel transformers. In most substations there are more than one transformer
of the same rating, so this test is possible. In a parallel operation the follow-
ing conditions should exist:
Polarity should be the same.
The voltage ratio should be the same.
The percentage impedance should be equal.
Phase rotation should be the same.
The two transformers should have the same vector group.
In back-to-back testing, the tap of one transformer should move in the
rise direction and the other in a lowering direction until the voltage differ-
ence between the two transformers are driving the nominal load current. A
graph is drawn to show the relation between temperature and time (24 hours)
for the oil transformer and the transformer winding temperature. The results
should be compared with the results from the manufacturer.

5.6.8 Transformer Oil Breakdown Test

In a new transformer, the oil should be a clear yellow color. A black or
dark-color oil means that the oil cannot be used. Impurities have a bad
effect on transformer oil and its insulating characteristics, which can lead
to flashover. Humidity also reduces oil insulation. A high flash point of

145 C is preferable. During this test, the oil humidity will be measured
and the sample is tested in standard cells of 80 mm 3 60 mm 3 100 m
(length 3 width 3 height). The poles are two yellow copper balls with a
distance between the balls of 4 6 0.02 mm. The sample of oil is taken
from the bottom of the transformer. Good oil will be workable up to
45 kV for 1 minute. Repeat the test at least five times to allow bubbles to
escape.

5.6.9 Measurement of Capacitance and Tan δ

All insulating materials used in practice have slightly small dielectric losses
at the rated voltage and at the rated frequency. These losses are fairly low
for good insulating materials. This loss changes proportionally with the
square of the applied voltage. Insulation, in terms of basic circuit elements,
is shown in Fig. 5.19.
As it can be seen from Fig. 5.19C the angle δ between the total current
“I” and capacitive current “I c ” is a basic value. Insulation angle is dependent
on the thickness, the surface, and the properties of the insulation material