Page 61 - Practical Power System and Protective Relays Commissioning

58 Practical Power System and Protective Relays Commissioning

Overfluxing causes core saturation and this causes heating. Over heating
of the transformer winding can cause gases which affect the operation of the
Buchholz Relay.
During saturation of the transformer core, a rise in the primary magnetiz-
ing current can cause operation of differential relay. This can be detected by
fifth harmonic blocking of the differential relay.
Overexcitation can be determined by using the V=F ratio. When this ratio
exceeds 1.4 then the overexcitation relay trips the transformer instantly.

5.5 POWER TRANSFORMER INSTALLATION

5.5.1 Transportation
Transportation of a power transformer depends on its weight. The trans-
former is filled with oil above the winding by 15 20 cm, the oil transported
with nitrogen with a pressure of 0.1 bar. The transformer bushing, as well as
the current transformers, are also filled with oil.

5.5.2 Site Inspections

Once on site, the following checks of the transformer should be made:
Check the tightness between the transformer and the base which was
transported with it.
Inspect the transformer tank, pumps, oil seals, and valves to see if they
are correct and there is no oil leakage.
Inspect the condition of transformer bushing and current transformers.
Inspect the cooling system of the transformer.
An oil sample should be taken from the bottom of the transformer and
sent to a laboratory for sample chemical analysis.
The transformer silica gel should be blue in color. If there are a lot of
brown particles, then the transformer should be dried and tested.

5.5.3 Transformer Storage

A transformer should be filled with oil maximum of one month from the
date of it reaching the site, if it did not contain oil during transportation.
The transformer silica gel should be checked and replaced if it is not of
optimal condition.
Current transformers should be stored in oil with minimum breakdown
voltage of 30 kV.
The transformer bushing should be filled with oil during storage.