Page 70 - Practical Power System and Protective Relays Commissioning

Power Transformers Theory Testing and Commissioning Chapter | 5 67

Determine the existence of a definite partial discharge in the tested object
at a predetermined voltage. This is done by increasing the applied voltage
where the partial discharging begins (partial discharge inception voltage)
and by decreasing the applied voltage where the partial discharging extin-
guishes (partial-discharge extinction voltage).
To determine the magnitude of the partial discharge at a predetermined
voltage.
The partial discharges (which do not cause flashover between the electro-
des) are the discharges in a certain area of the insulation between the con-
ductors of the test object. These discharges may occur in the gaps of the
insulating environment, in the gaps of the solid-materials or in the contact
surfaces of two different insulations. This discharge can be captured as a sin-
gle current impulse in the outer region. Although these discharges do not
cause permanent deterioration in the insulating media as their energy is rela-
tively small, the thermal energy of the discharges shall cause depreciation,
aging, and deterioration in the insulating media. The electrical discharge
magnitude at the partial discharge point is not a direct measurement for dete-
rioration of the insulating material in this region. Besides the numerical
value, the intensity and the waveform of the impulse, regional discharge con-
centration, the manufacturing and the placing of the insulation also affects
the situation.

5.7 COMMISSIONING TESTS FOR POWER TRANSFORMERS
AT SITE
5.7.1 General
Testing of a complete power transformer includes:

Winding insulation resistance, winding resistance, vector group, tan delta,
no-load and power losses test, short-circuit test and copper losses, tem-
perature rise test and transformer oil breakdown test.
Control and relay panels testing: junction boxes (local control panels) and
marshaling kiosks (including transformer and fan control box and tap
changer box).
Transformer fans and pumps.
Check the operation of the tap changer with a voltage regulator relay dur-
ing secondary tests.

5.7.2 Primary Tests

Stability test of transformer differential relays using 380 V (explained
elsewhere in this book).