ENGLISH
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4.6 Commissioning
Before putting into service it is necessary to check the insulation with a Megger at 500 Vdc after 1 minute of the application of the
test voltage.
For a new generator, the stator winding insulation resistance larger of 100 M
Ω
represent one of essential safety requirements.
ALREADY OPERATING GENERATORS OR AFTER PROLONGED PERIODS OF INACTIVITY THE MACHINE MUST
NOT BE OPERATED IF THE INSULATION RESISTANCE IS LESS THAN 30 M
Ω
AT THE TEMPERATURE OF 20°. In
this case, it is suggested to dry the winding previously to the generator star-up.
THE MACHINE MUST NOT BE OPERATED IF THE POLARISATION INDEX IS LESS THAN 1,5. (§ 4.7)
In order to prevent risks of electric shock, connect the windings briefly to the ground immediately after measurement.
BEFORE INITAL START – UP, MAKE THE FOLLOWING CHECKS:
Mechanical checks.
Check that:
- The bolts are suitably tight.
- Coupling is correct.
- The cooling air is sufficient and that no impurities are drawn in.
- The protection grills are in place.
- For single bearing generators, that the bolts of the disks are fixed with the correct torque.
Electrical checks.
Check that:
- The system is provided with suitable protection differential circuit breakers, according to current legislation.
- The connection to the terminal box terminals has been correctly executed (very tight terminals).
- There are no connection inversions or short-circuits between the generator and internal switches: it is good practice to
remember that there are normally no protection elements between the alternator and external switches.
In order to avoid any damage to current transformers and to the generator, all current transformers installed
on the generator have to be connected to proper loads: in case the current transformers were not used, they
must be shortcircuited.
4.7 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
isol20°C T10’
) and 1 minute insulation resistance (R
isol20°C T1’
) may be used to
evaluate the condition of the machine winding insulation.
The ratio between those insulation resistances is called polarisation index (PI):
POLARISATION INDEX
INSULATION LEVEL
'
T1
c
20
isol
T10'
c
20
isol
R
R
PI
°
°
=
PI
≤
1
Bad
PI <1,5
Dangerous
1,5 < PI < 2
Uncertain
2 < PI < 3
Good
PI > 3
Very good
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.
