Testing Voltage

A third test consists of an iastantaneous appHcation of the full test voltage higher voltages ate impressed on the iasulation than ia previous tests. 

A d.c. insulation resistance test or polarization index reveals only the surface condition of the insulation and does not allow a realistic assessment of internal condition. Loss tangent values are true reflections of the insulation condition to detect moisture content, voids, cracks or general deterioration. The tan 5 versus test voltage curve may be drawn and compared with the original curve provided by the manufacturer, and inferences drawn regarding the condition of the insulation. The different starting tan lvalues will reveal the condition of the insulation in terms of amount of contamination, as noted in Table 10.4 (See lEE, Vol. 127, May 1980). 

Industrial Power Engineering and Applications Handbook Table 11.4 Test voltages for conducting dielectric tests... 

The HV test should be performed first by applying r.m.s. value (2 -i- I) kV for one minute, as shown in Table 11.4 between coil terminals and ground. Then the applied voltage should be increased at I kV/s up to 2(2 F,- + I) kV, and then reduced instantly at least at I kV/s to zero. If there is no failure, the test will be considered as successful, fulfilling the requirement of column 4, of Table 11.6. This procedure would give a test voltage equivalent to 2 l2 (2V, + I) kV, which is even more than the values of column 4, i.e. > (4V + 5) kV. 

The full test voltage of column 4 of Table 11.6, i.e. a standard lightning impulse of 1.2/50 p.s, should... 

As the windings are heated rapidly, the test voltage must be applied as quickly as possible. Care should be taken to ensure that the motor temperature does not... 

The test voltage must be as close to a sine wave as practicable and frequency as noted in column 3 of Tables 14.1 and 14.2, for series II and Table 13.2 for series I voltage systems, and applied for one minute as follows ... 

The test voltage at the moment of application should not exceed 50% of the rated test value and should be... 

Table 14.3 For series I voltage systems. Dielectric test voltages for control and auxiliary circuits and also LT power circuits...

For a switching device (which has not been previously tested for a short-circuit test). This should be closed and held in the normal service position. The test voltage (that would generate the required level of fault current) may be applied on one set of terminals, the other terminals being shorted. The test may be continued until the short-circuit device operates to clear the fault, but in no case for less than 10 cycles. In LT assemblies the point where the short-circuit is created should be 2 0.4 m from the nearest point of supply. 

When the required test voltage is not available at site, the reduced voltage power frequency withstand test may be carried out at still lower test voltages, depending upon the voltage availability at site. Then the duration of the test must be increased as shown in Table 14.8, and IS 10118-3 and BS 159. 

Table 14.8 Duration for dielectric test at reduced test voltages...

A repeal power frequency lest, if considered necessary, must be performed ai 80% of the prescribed test voltage. See also Section 14.5. 

Field tests Power frequency withstand test on the primary windings (for un-grounded VTs). The value of the test voltage and the test procedure, is almost the same as that for a switchgear assembly (Section 14.5). 

The test circuit may be connected to the test voltage and then switched to obtain the required oscillograms during switching operations and assess the following ... 

These capacitors differ from standard p.f, improvement capacitors, as they are designed to withstand higher test voltages and have a low internal inductance. They should preferably be non-inflammable, synthetic liquid impregnated and provided with a built-in discharge resistance. For specifications refer to VDE 0675 and VDE 0560 III,... 

The test procedure and test performance should be the same as noted in (3) above except the test voltages, which will be as specified in Table 26.3. 

The unit should be first charged by d.c. and then discharged through a gap situated nearby. The test voltages should be as follows. 

Star-connected units Test voltage 4 Vr/V3. The test should be canied out between any two terminals, leaving the third open. 

Delta-connected units Test voltage 2V. Any two terminals of the unit should be shorted and the test conducted between the third and the shorted terminal. 

The lest voltage should be 2.5 V, irrespective of star- or delta-connected units. Tlie capacitor unit should be charged with the test voltage and discharged five times within 10 minutes. Within 5 minutes after the test the unit may be subjected to voltage lest (.3) between the terminals. The... 

The procedure will be the same as that discussed for switchgear assemblies (Section 14.3.3). The test voltage may be applied for one minute as shown in Tables 32.1 (A) or (B) for series I and Table 32.2 for series II voltage systems. Any disruptive discharge or insulation breakdown during the application of high voltage will be considered to be dielectric failure. 

This will be carried out under similar parameters of room condition and the type of test voltage wave to those for a switchgear assembly (Section 14.3.5). The current in each phase should be within 2% of the specified test value (rated current). 

In the absence of a.c. test voltage at site, a d.c. test can also be conducted, the duration of which will be 15 minutes. 

For a power frequency voltage withstand test a d.c. test is generally not recommended on a.e. equipment, unless only d.c., test voltage is available at the plaee of testing. The d.e. test values, as above, are therefore for such eventualities only and are equivalent to power frequency a.c. voltage withstand test values. 

If il is not possible lo carry out the lest al 75%, 80% or 85% of the lest values, as prescribed above or for some reasons the lest duration may have to be increased by more than one minute, then it is permissible for the test voltage to be further reduced to suit the site and instead, the test duration increased The reduced test voltages for longer test durations are provided in Table 14.8. 

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