Ian (DOC)

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I would like to have some discussion on the following two tables with
respect to hydro generator field windings:

     Table 1—Guidelines for dc voltages to be applied during insulation resistance test

     Winding rated              Insulation resistance test
      voltage (V)a                  direct voltage (V)

<1000                           500

1000–2500                       500–1000

2501–5000                       1000–2500

5001–12 000                     2500–5000
>12 000                         5000–10 000
 Rated line-to-line voltage for three-phase ac machines,
line-to-ground voltage for single-phase machines, and
rated direct voltage for dc machines or field windings.

Table 3—Recommended minimum insulation resistance values at 40 °C (all values in MΩ)

Minimum insulation
                                                             Test specimen
resistance (megohms)

IR1 min = kV + 1                For most windings made before about 1970, all field windings, and
                                others not described below

IR1 min = 100                   For most dc armature and ac windings built after about 1970 (form
                                wound coils)

IR1 min = 5                     For most machines with random-wound stator coils and form-
                                wound coils rated below 1 kV

NOTES 1—IR1 min is the recommended minimum insulation resistance, in megohms, at 40 °C of the entire machine
              2—kV is the rated machine terminal to terminal voltage, in rms kV

The discussion should centre around the value of direct voltage used to
test the winding and then the minimum value of Mohms to put it back in
service. The difference for the field winding is that it undergoes
forces the stator winding does not and how do we take that into account
when testing. Many times we will receive a field ground and test to
the IEEE 43 minimum which is 500V and cannot find the grounded pole and
we may get larger that kV+1. When we put this back in service the field
ground returns immediately (due to the movement of the field winding).
We have had to go to voltages approaching 2500V sometimes to find the
bad pole. So, should this type of situation be addressed in the
standard? Really, is 1.250Mohms really that great for a rotor winding?
I think not, this value as a minimum is not acceptable at OPG.   Anyway,
can you table this and we can chat at the meeting.

Let me know what you think.



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