Stator Winding Insulation Check Through Polarisation Index; Removal Of Moisture From Windings - MarelliGenerators MJB 160 Instrucciones Y Advertencias De Seguridad

4.8 Stator winding insulation check through Polarisation Index

Qualitative insulation resistance versus time curves:
It is possible to check the generator insulation condition by measuring the polarization index, according to IEEE 43.
Execute the insulation resistance measure and insulation resistance registration at ambient temperature and in different times: T1', T2',
..., T10'. Space the measures of a conventional time (one minute for example).
Insulation resistances have to be measured leaving the 500V DC of "Megger" instrument applied for the full duration of 10 minutes.
The comparison between 10 minutes insulation resistance (R
evaluate the condition of the machine winding insulation.
The ratio between those insulation resistances is called polarisation index (PI):
POLARISATION INDEX
=
PI
The slope in insulation resistance versus time curve indicates the dryness and cleanliness of a winding.
Winding insulation could be considered GOOD if the diagram obtained is similar to the curve A.
Winding insulation could be considered UNSATISFACTORY if the diagram obtained is similar to the curve B. In that case insulation is
affected by moisture or dirt and should be dried-out and cleaned.

4.9 Removal of moisture from windings

An increase of insulation resistance between phase and earth is normally obtained by removing the moisture.
Several methods can be followed for this scope:
•
Stator winding drying by internal heat source.
Heaters have to be distributed below the generator main winding stator.
•
Stator winding drying-out by self heating method.
The stator can be heated with the circulation of low voltage DC current (i.e. obtained by an industrial welding set) through the windings.
A current of about 25% of the full load current, as marked on the generator rating plate, should be used.
If both phase terminals are available, generator winding can be re-connected to adjust its internal resistance , in order to suit the direct
current supply available. A thermometer should be placed inside the stator windings.
Temperature should not be allowed to exceed 80° C.
Could be useful to cover the machine to conserve the heath.
In case it is possible have to be unclosed all the openings on frame, if available. Those openings if positioned on the top of the
generator (i.e. removing terminal box cover or removing end shields for vertical constructions) can improve moisture escape.
•
Drying of stator with oven heating
You brings the oven to 110 – 150° C maximum, the dry ing of winding for generators MJB 160 – 200 – 225 could continue for 2 – 4 hours
depending on the starting condition insulation resistance.
If the insulation resistance doesn't reach at least the recommended value, it's possible that the cause is a solid contamination.
It will be in this case necessary to clear the winding once more and then repeat the drying process.
SIN.UM.017.11
R
°
isol 20 c T10'
R
°
isol 20 c T1 '
) and 1 minute insulation resistance (R
isol20° C T10'
PI ≤ ≤ ≤ ≤ 1
Bad
PI <1,5 Dangerous
1,5 < PI < 2 Uncertain
2 < PI < 3 Good
PI > 3 Very good
24
) may be used to
isol20° C T1'
INSULATION LEVEL
ENGLISH