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Power Transformers Theory Testing and Commissioning Chapter | 5 59

5.6 POWER TRANSFORMER TESTING AND
COMMISSIONING

5.6.1 Measurement of Winding Insulation Resistance
In this test we use a Megger tester for insulation measurements 5 kV for
1 minute; if the reading is not stable then we should extend the test for
5 minutes.
As a reference guide, when testing a 66/11 kV star/delta power trans-
former the following results were derived:

3 phases 66 kV winding to earth is 5000 MΩ.
3 phases 11 kV winding to earth is 1000 MΩ.
3 phases 66 kV winding to 11 kV winding is 1500 MΩ.

Refer to Fig. 5.14.
The insulation resistance measured for 1 minute is called R 60 . It is also
measured for 15 seconds at the site temperature. These two calculations
should be compared to the factory temperature as shown in Table 5.2.
The calculation should be:
R 60 measured at factory 5 K 3 R 60 measured at site
R 60 measured 2 R 60 at factory
Insulation difference 5
R 60 at factory
3 100 should be less 30%
This percentage for overhaul maintenance should not exceed 15%.

FIGURE 5.14 Measurement of power transformer winding insulation resistance.