Criteria which define optimal moment for insulation revitalozation

Revitalization at appropriate time insures maximum power transformer lifetime increase and its reliability on the network. Since cellulose depolymerization is irreversible process, late revitalization has limited results. Depending on the moment when revitalization is done, power transformer lifetime could be prolonged from 10 to more 30 years.

Based on our experience and according to IEC, IEEE and other international regulations and specifications, in order to obtain maximal transformer lifetime extension and increase of reliability operation, revitalization (complete or partial) should be performed when any of the following insulation characteristics border values are reached:

Characteristics

Unit

Voltage kV

≤72.5

72.5-170

170-400

≥400

Water content in solid insulation (20°C)

%

3

2,5

2

1,5

Water Content in Oil (20°C)

ppm

20

15

15

10

Dielectric Strength

kV

42

45

50

55

Acid (neutralization) number

MgKOH/g

0,15

0,1

0,1

0,1

Interfacial Tension (IT)

MN/m

25

25

30

32

Dielectric Dissipation Number tg δ (90°C)

/

0,1

0,1

0,06

0,06

IT/ Acid Number

/

166

250

300

320

Degree of Cellulose Polimerization (DP)

/

600

600

600

600

The above stated limits are more stringent than technical recommendations defined by international and national specifications (IEC, IEEE, Doble ....).

The reasons are numerous. Current recommendations represent partially a compromise made between manufacturers (of transformers, solid insulation, oil), owners of the transformers and the maintenance companies.

After all, the owners of power transformers and insurance companies are those who benefit the most from clearly defined criteria.

A compromise has been made due to the limited possibilities, the invasiveness and complexity of conventional insulation treatment technologies, and because of the high cost of these treatments.

Beginning of the implementation of synthetic adsorbents technology in On-Load mode in the last decade, fully justifies significantly more stringent criteria for the treatment of insulation.

This technology has been accepted and recognized as a non-invasive, inexpensive and the one that guarantees maximum life extension of transformers.

Taking into account all of the above, it was possible to define criteria exclusively related to the maximum extension of insulation lifetime, and by implementation of VIMAP R2000 technology they can be effectively and easily fulfilled.