Page 65 - Practical Power System and Protective Relays Commissioning

62 Practical Power System and Protective Relays Commissioning

connect the voltmeter before the ammeter. A heavy current of 100 A and
sensitive meters should give good results.
During the test we connect the voltmeter after the current reaches the sat-
uration point and we disconnect voltmeter before disconnecting the ammeter.
To reduce the time needed for the current to reach saturation condition, we
connect a variable resistance in series with the circuit.
The resistance reading at site should be compared to the resistance tem-
perature reading at factory by using this equation:

ð T 2 1 235Þ 3 R 1
R 2 5
ðT 1 1 235Þ
where R 1 is the Resistance reading at factory, T 1 is the Temperature
which reading is taken at factory, R 2 is the Resistance reading at site, T 2 is
the Temperature which reading is taken at site.
The measurement difference, D, calculated by
D 5 R 2 R 1 Þ=R 1 3 100
ð
should not exceed 2%.
If the resistance measurement is very low, there is a possibility of short
circuits in the transformer windings.
If the resistance measurement is low, then there is a possibility of bad
contacts between the winding and insulators, or bad contact in the tap
changer, or the winding may be damaged.

5.6.5 Measurement of No-Load Current and No-Load Circuit
Losses Test
Under open-circuited conditions on a transformer’s high voltage side, the
transformer no-load current (magnetizing current) is less than 4% of full
load current. Any power losses in this condition is the loss of iron which
includes hysteresis losses and eddy current losses. The test is done as shown
in Fig. 5.17.
P 0 5 Measured no-load power losses
No-load current losses I 0 5 A—ammeter reading in each phase.
These losses affect the power transformer efficiency.

5.6.6 Measurement of Load Losses: Current and Impedance
Voltage Test
Short-circuit (load) losses and short-circuit impedance voltage are guaranteed
and reported values by the manufacturer to customers. Short-circuit imped-
ance voltage is an important parameter especially for the parallel operation
of the transformers, whereas short-circuit losses are important from economi-
cal point of view as they effect the transformer efficiency.