Minimum Insulation Resistance; Windings Evaluation And Preservation; Conversion Of The Measured Values - WEG G Serie Manual De Instalación, Operación Y Mantenimiento

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On machines that are already in operation, higher values
of insulation resistance, can be obtained compared to the
initial values for commissioning.
The comparison with values obtained in previous tests on
the same machine, in similar conditions of load,
temperature and humidity serves as a better indication of
the conditions of isolation than the value obtained in a
single test, and any sudden reduction is considered
suspicious.
4.5.5

Minimum insulation resistance

Table 4.2: Minimum insulation resistance
Stator voltage ≤ 1000 V
Stator voltage > 1000 V
Rotor and exciter
4.5.6

Windings evaluation and preservation

Table 4.3: Evaluation of the insulation resistance "I.R."
Insulation
Un R.I.
< 5 MΩ Critical*
≤1000V 5 a 100 MΩ Acceptable
> 100 MΩ Normal
< 100 MΩ Critical*
>1000V 100 a 500 MΩ Acceptable
> 500 MΩ Normal
* The alternator must not operate in this condition.
NOTE
The data in Table 4.3 serve as a reference. It is
recommended to record all insulation
resistance measurements performed on the
alternator and maintain a history of these
measurements.
Any sharp reduction in recorded amounts
should be investigated.
The insulation resistance is influenced by the presence of
moisture and dirt in the insulation material.
If the measured insulation resistance is lower than the
values reported in Table 4.2, before the alternator come
into operation, the windings must be carefully inspected,
cleaned and, if necessary, dried according to the
following procedure:

Disassemble the alternator by removing the rotor and
bearings;

Place the winding components, which have low
insulation resistance in an oven and heat them at a
temperature of 130°C, keeping this temperature for at
least 08 hours.

Check the insulation resistance is achieved within
acceptable values, as Table 4.3, otherwise refer to
WEG.
DANGER
Immediately after measuring the insulation
resistance, ground the winding to prevent
accidents.
24 l Installation, operation and maintenance manual – Synchronous Alternator – G line - Horizontal
Minimum R.I.
(referred to 40ºC)
5 MΩ
100 MΩ
5 MΩ
Procedure
Status
Cleaning and drying
Periodic monitoring
Normal operation
Cleaning and drying
Periodic monitoring
Normal operation
4.5.7

Conversion of the measured values

The insulation resistance should be referred to 40°C.
If the measurement is made at a different temperature,
the reading must be corrected to 40°C, using the
approximate correction provided by the curve of Figure
4.5, according to the IEEE43 standard.
The correction of the insulation resistance reading to
40ºC is done by the relation:
R = Kt . Rt
40
Where:
Rt = insulation resistance at temperature "t".
Kt = Correction factor of the insulation resistance as a
function of the winding temperature, as shown in Figure
4.5.
Figure 4.5: Coefficient of variation of insulation resistance with
temperature
The values used to generate the curve of Figure 4.5 are
shown in Table 4.4.
Table 4.4: Insulation resistance correction factor in with the
temperature
t (ºC)
10
20
30
40
50
60
70
80
Kt factor
0,7
0,8
0,9
1,0
1,5
2,3
3,3
4,6
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