Henry du Preez is an independent consultant with over 52
years experience since graduating. Specialising in electri-
cal machines, both ac and dc, and transformers, he has had
extensive experience in mining and industry and offers a
range of specialised training courses in association with
‘Specialized Knowledge and ABB (Transformers)’.
Enquiries: Email
peak to peak voltage at which PDs occur with a repetition rate of
one or more PD pulses every two voltage impulses as the mean
value for a specified test, in the test arrangement where the voltage
applied to the test object is gradually increased from zero volts.
• PD Extinction Voltage (PDEV): voltage at which PDs are extin-
guished in the test arrangement when the voltage applied to the
test object is gradually decreased from a higher value at which
such discharges are observed
Note: With sinusoidal applied voltage, the PDIV/PDEV/RPDIV is defined
as the rms value of the voltage. With impulse voltages, the PDIV/PDEV/
RPDIV is defined as the peak to peak voltage.
Measurement of PDs
The electrical measurement of PD both off-line and on-line requires
special instruments and, in the case of off-linemeasurement, a suitable
power supply. PD may be measured in volts (mV) or pico-Coulombs,
but for comparison purposes the same measuring system and unit
should be used and conversion from one to the other is not recom-
mended. (1 volt applied to a capacitor of 1 pico-Farad results in a
pico-Coulomb charge). Currently there are a number of suppliers of
PDmeasuring equipment using UHF antennas and capacitive couplers
which is connected to a suitablemonitoring systemwith indications of
themagnitude and intensity of the PD including PDIV/PDEV values, fre-
quency and polarity of the discharges. MVmachines are usually tested
at line frequency using some type of capacitor coupler; however, HF
current transformers and other coupling systems can be used. Various
manufacturers of test equipment have their own preferences and these
should be evaluatedwith respect to the application. IEC 60034-18-41 [1]
and 42 [2] require an insulation quality qualification test on sample
systems for the particular application. If these tests prove that the PDIV
is above the operating voltage for the selected stress category volt-
age multiplied by the enhancement factor (EF), with suitable thermal
ageing, the samples will have met the acceptable pass criteria. If tests
are performed under impulse or power frequency according to the IEC
specification, and the design qualification testing has been performed
successfully on the complete winding, the insulation system has ef-
fectively passed its type test and no additional type test is required.
Conclusion
IEC specifications [1, 2] explain and state criteria for testing and evalu-
ating insulation systems in machines for medium power frequency
and inverter-fed machines. Test equipment for all tests is available
from a number of suppliers with options for particular applications.
Vicki Warren is quoted in a recent webinar [5] as stating that the
expected life of insulation system that is subjected to PD could be
on 11 to 13,8 kV - five years, 6 to 8 kV - two to three years and on LV
machines - a matter of months. The time period relates to the thick-
ness of the insulation system. LV systems have a very thin insulation
system, and if conductors are not contained in a parallel group, this
insulation is soon punctured by the PD, resulting in failure.
The design and manufacture of the insulation system and wind-
ing has a major effect on the elimination and reduction of PD. MV
machine coils, besides being correctly insulated, should be designed
and formed to have a reasonable gap between coils in the overhang.
DRIVES, MOTORS + SWITCHGEAR
Random wound machines should ensure that the coil’s conductors
(wires) run parallel to one another and do not cross or run close to
other coils or phases. PD measurement and analysis, in conjunc-
tion with Tan Delta tests, could assist in defining the quality of an
insulation system in a machine – Tan Delta being an off-line test that
should be done at the manufacture or rewind stage with capacitance
measurement, followed by PD measurement prior to installation,
giving a finger print for future comparisons after and during service.
PD can be measured during operation, continuously or periodically,
if suitable couplers are fitted.
New equipment is available for PD and Tan Delta measurement at
an extremely low frequency of 0,1 Hz. This equipment is comparatively
smaller and lighter, enabling easier handling for on-sitemeasurement.
Themeasurements obtained using low frequencies indicate PD and Tan
Delta values that can be interpreted to indicate possible problemareas
in the insulation system. The values are not comparable with operating
frequency (50 or 60 Hz) values. Tan Delta testing can only be done on
stationary machines during a shutdown. In a factory you would have
to use the same type of measuring frequency to do comparisons. PD
can be monitored during operation which would always be at operat-
ing frequency (50 or 60 Hz.) and therefore for comparative purposes it
would be best to measure at this frequency, even in the static mode.
References
[1] IEC/TS 60034-27. 2006. Rotating electrical machines–Part 27:
Off-line partial discharge measurements on the stator winding
insulation of rotating electrical machines.
[2] IEC 60034-18-41. 2014. Rotating electrical machines–Part 18–41:
Partial discharge free electrical insulation systems (Type 1 and
Type 11 insulation systems) used in rotating electrical machines
fed fromvoltage converters–Qualification and quality control tests.
[3] IEC 60034-18-42. 2008. Rotating electrical machines. Qualification
and acceptance tests for partial discharge resistant electrical
insulation systems (Type II) used in rotating electrical machines
fed from voltage converters.
[4] Fornasari L, Caprara A and Montanari GC. Partial discharge
measurements in electrical machines controlled by variable speed
drives: from design validation to permanent PD Monitoring.
[5] Warren V. Iris Power/Qualitrol Webinar June 2014.
Bibliography
Bogh D, Coffee J, Stone G, Custodio J. Partial Discharge Inception
Testing on low voltage motors. IEEE PCIC-2004-27.
Suppliers’ literature.
take note
• PD activity can lead to insulation failure.
• Regular assessment of motor insulation must be done.
• PD measurements can be made on line.
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July ‘14
Electricity+Control
