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Philosophy of  quality systems and testing of  electrical machines 11/261

        100% of the rated voltage at intervals of 20%. The initial   Until a few years ago, there was no widely accepted
       value  of  tan  &b.zVr and  the  increment  Atan  6, i.e.   standard for  a voltage  endurance test  of  the  rotating
                   -
                             per
                    tan
        3 [tan 60.6~~ 60~2~~1 measuring step should   machines.  Different  agencies  had  adopted  different
       not exceed the  values indicated in Table  11.5 for the   practices on different assumptions, pending a final decision
       rated voltages up to 11 kV.                    by the IEC working committee TC-2 of ZEC  60034-15.
         It can be seen that up to  1.1 times the rated voltage   The  committee  submitted its  report  in  1988  and  the
       (VJ, the  tan  6 value remains almost constant, and  at   following test data, which are now universally adopted,
        1.2Vr it increases slightly. At higher voltages it increases   are based on this report.
       sharply, and may become too high to cause a discharge   The impulse test by means of a directly injected steep-
       sufficient to char the insulation if this voltage is allowed   fronted wave cannot be performed on a fully assembled
       to exist for a longer period.                  machine as the bulk  of  the voltage, if  the front of  the
                                                      wave is less than  Ips, will appear across the first few
        11.4.9  Impulse! voltage withstand test of the   turns only.  Also,  in the event of  a failure, the  whole
             insulation system for rated voltages     winding must be scrapped. The impulse test is therefore
                                                      performed on completed individual resin-rich formed coils
              2.4 kV and above for machines           only after insertion into the slots.
              wound with formed coils                   The impulse test is basically an in-house coil insulation
        As discussed in Section 17.5 a machine may be subjected   withstand test for surge voltages and forms a part of the
        to voltage surges due to external causes (lightning) or   test requirement for HT machines with resin-rich formed
        internal causes (switching). By the power frequency HV   coils of 2.4 kV and above. Once the machine is assembled,
        test  or  the  dissipation factor, as in  Section  11.4.8, or   such a test is unnecessary, as it may not be able to reveal
        insulation resistance  tests  as  Section 9.5.3, the  surge   deficiencies, if  any, in the insulation of  the coils deep
        withstand  level of  the  insulating  system  cannot  be   inside the slots. Moreover, if a failure is noticed on the
        determined. Hence, the need for an additional impulse   assembled machine, there is no option but to scrap the
        voltage withstand capability test of a coil for HV systems.   whole winding.
        The increasing application of vacuum- and gas-filled  (SF6)
        circuit  breakers  and  contactors  for  switching  of  HT   To assign the impulse level
        machines, has led to the need for a surge voltage withstand
        test on the multi-turn coils of a machine to account for   In Table 11.6 the values of column 2 relate to normal
        the surges generated by a re-striking pheno-menon, such   operating conditions. Abnormal conditions may prevail
        as that caused by the closing or interrupting of contacts.   when  the  machine  is  exposed to  overhead  lines,  the




        Type of  impulse   Winding switching impulse withstand level, when subjected  Winding lightning impulse  Rated power frequency
                                                               withstand level  1.2/50 ps
                      to switching impulses, with following wave fronts   kV (peak)   withstand voltage (rm.s.)
        'F                          3  Special design   value)   4                5
                                                               impulse, phase to ground
                                                                                  (as in Table 11.4 for
                      Between 0.2 and 0.4 p
                                                  0.2 p (average
                                                                                  stator windings) kV
                      Normal design
        Line voltage
                      level phase to
                                     level phase to
                                     ground
                      ground
                      kV (peak)
                                     kV  (peak)
                       2
                                                  3a
                                                                                   7.6
                                                               18.2
                                                  12
        11.0
                       8.1
                                    13.5
                       16.2         26.9          20           31.4               14.2
                       26.9         44.9          32           49.0               23.0
        13.8           33.8         56.3          39           60.2               28.6
        Design criteria  1   3 p.u. kV   5 p.u. kV   1  0.65 (4Vr + 5)" kV I  (4Vr + 5)b kV   1  (2V, + 1)'kV
        Note
         V, = line voltage in kV (r.m.s.)
                                                              vr  -
        p.u. = per unit voltage, which is the peak value of phase voltage in kV i.e.
                                                              43
        WC 60034-15 has prescribed an  average impulse voltage, considering the  average characteristics of  the  machine windings and  the
        switching conditions. One may therefore decide between columns 3 and 3a, depending upon the exposure of the machine to the internal
        switching surges and surge protection devices if provided with the motor.
        bIEC 60034-15 recommends these values to be rounded off to the nearest whole number. The values noted above are not rounded off for
        more clarity.
        'See  also Table  11.4.
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