florkowska2009

The voltage stresses of insulation systems under
PWM inverter supplies
B. Florkowska(1), M. Florkowski(2), J. Furgaá(1), J. Roehrich(1), P. Zydron(1),
1University of Science and Technology, Department of Electrical Engineering and Electrical Power,
Al. Mickiewicza 30, 30-059 Krakow, Poland
2ABB Corporate Research , ul.StarowiĞlna 13A, 31-038 Kraków, Poland
Abstract- Exploitation stresses are causing degradation of high
voltage insulation systems. The assessment of intensity and
dynamics of these processes, being a consequence of local,
working electric field strength is considered mainly at the
sinusoidal voltage. However in applications where power
electronics converters are used the voltage stress has usually a
form of fast switching pulses composed of repetitive sequences.
Such pulse trains pose usually a modulated width and fast rise-
and fall-times. Such conditions have essential influence on
inception and development of partial discharges in insulating
systems subjected to non-sinusoidal stimulus. The insulation
degradation mechanism is especially important for cables and
electrical machines subjected to non-sinusoidal waveforms. A
novel time-resolved partial discharge (PD) surge pulse
acquisition has been described. The method is based on very fast
PD registration during repetition of HV surge pulses on
insulating material and visualization if form of PRPD like
pattern. Comparison between PD patterns obtained at surge
pulse and subjected to power frequency sine and trapezoidal is
shown.
I. INTRODUCTION
Voltage source pulse with modulated (PWM) inverters are
the most common type of drives, which are currently in use to
supply electric machines. PWM uses a square wave whose
pulse width is modulated, which allows for variation and
control of the average value of the waveform. PWM based
supply voltage results in a sine-like current in a magnetic
circuit of an electric motor. The smoothness of the waveform
can be adjusted by the width and number of modulated pulses.
In the past special attention was paid to voltage distortion
caused by harmonics, whereas last decade the effect of step
waveform introduced by converters with fast switches like
IGBT (Insulated Gate Bipolar Transistor) started to be treated
as even more dangerous. In those drives the control unit is
based on pulse-width modulated train of fast pulses with slew
rate up to 100 kVμs-1 and frequency repetition up to 100 kHz. In
consequence large spikes, overshoots and oscillations are visible
on motor terminals. The electrical stress caused by such
distorted supply voltage may lead to the insulation system
degradation and breakdown of electrical machines and cables
[e.g. 1, 2, 3]. The problem may not be meaningful under
sinusoidal supply at power frequency (50/60Hz), as the turn-
to-turn voltage is relatively small. However in case of PWM
inverter based supply also the steepness of the applied voltage
are much higher then at the sinusoidal voltage. Thus the
voltage distribution within the coils and windings is highly
non-linear and may cause large voltage stresses between two
consecutive turns [4, 5].
Hence, testing and diagnosing methods of insulation system
integrity subjected to high frequency stresses pose additional
requirements. Those needs refer to the development of
improved, unambiguous methods, which might be applied to
both manufacturing and on-site diagnostics.
II. PULSE BASED ASSESSMENT METHOD
Assessment of insulation systems designed for operation
under PWM like stress creates new challenges as conventional
methods are not appropriate. In case of partial discharge (PD)
based assessment the main problem is related with detection of
PD pulses and simultaneous influence of the supply voltage
[e.g. 6].
0246810
0.0
0.5
1.0
1.5
2.0
a)
u
, pu
t,
μ
s
1
2
Fig. 1. Overvoltages at the motor terminal and
at the inverter output for different rise time
(dotted red – approximation by a surge pulse):
1 - rise time 300ns, 2 - rise time 1 μs
overvoltage at the motor terminal,
voltage at the inverter output
In frequency domain, both the spectrum of the PWM power
supply with fast rise time and PD spectrum are overlapping
preventing application of easy filtering and separation of both
components. In consequence, the PD detection during fast
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978-1-4244-4559-2/09/$25.00 © 2009 IEEE
2009 Annual Report Conference on Electrical Insulation and Dielectric Phenomena

slopes of the stimulus is much more difficult, due to the
significant content of that signal at the receiver input.
Additional difficulty in phase/time-resolved registration is the
repetition rate of the PWM voltage, which can reach up to
100kHz.
This paper presents novel method based on time resolved
PD acquisition under repetitive surge stimulus. In drive fed
motors, both insulation systems of a motor and cables are
subjected to overvoltages, whose crest value depend on the
configuration, length of cables, impedance matching etc.. The
exemplary overvoltages at the motor terminal and at the
inverter output are shown in Figure 1 for two different rise
time values of supply voltage: 300 ns and 1 μs. Such
additional stress can be approximated by a surge pulse with a
fast rise time and relatively slow decay time (dotted red line in
Fig.1). Applying the repetitive train of such pulses to the
insulation system, the partial discharges are registered in time-
resolved mode. The surge pulse can be treated to some extent
as an analog of a lightning/switching pulse test performed on
MV/HV insulation systems. The following parameters of the
surge pulse can be adjusted with relation to the PWM supply
voltage: rise time, decay time, repetition rate of the train of
pulses and amplitude.
The block diagram of the measuring system is shown in
Fig.2. The test object is subjected to repetitive train of pulses
from the high voltage surge generator with controlled
amplitude and rise/decay time.
Fig. 2. Measuring setup of the time-resolved
surge acquisition
The rise time was selected in the range between 50ns and
1000ns and fall time between 20-1000μs. The partial
discharge signal can be obtained from 3 sources: ultra wide
band current transformer, coupling impedance in form of high
pass filter or antenna. Fast PD acquisition was performed with
a sampling rate 2GS/s. The PDs have been recorder during
surge pulse exposition. The conversion of the PD record to the
time-resolved pattern has been done by the dedicated software.
The communication to the host computer is provided by GPIB
interface. Typical measurement consists of certain number of
surge impacts, typically 100 to 10000. In order to obtain
consistent pattern, the acquisition of all surges has been
synchronized by a trigger point on the surge pulse wavefront.
The repetition rate has been set between ms to 5 seconds.
Fig. 3. Exemplary time-resolved PD pattern obtained
for repetitive train of fast surge stimulus
III. TEST SAMPLES
The investigations have been performed on the stator-bar
samples of the motor insulation, which have been subjected to
surge pulses with steep front as well as to slow sine and
trapezoidal stimulus at power frequency. The stator bar
samples represent the form-wound type of insulation. The
main insulation is based on mica type insulation. All
measurements have been performed at room temperature. The
test voltage level for sine and trapezoidal case has been
adjusted to 1.25 inception voltage for sine supply. The ground
electrode has been formed from the aluminum foil.
IV. RESULTS
In order to compare the impact of PWM based inverter
supply on insulation system, the measurement of partial
discharges at three different waveforms has been curried out.
The PD have been registered at sine (50Hz), trapezoidal (rise
time 140μs, 50Hz) and surge voltage (rise time 100ns, decay
time 100μs repetition in the range from ms to seconds). The
comparison of the steepness dU/dt and the rise time for these
waveforms has been presented in Table I. The rise time (tr) has
been measured between the ground level and the crest value of
the corresponding waveform.
TABLE I
Waveform dU/dt [kV/
μ
s] Rise time t
r
[
μ
s]
0.0019 5000
0.044 140
90 0.1
The PD patterns obtained at sine and trapezoidal voltage are
shown in Fig 4. The voltage in both cases is 1.25Uinc, where
Uinc is the inception voltage at sine. In case of sine pattern, the
structure of discharges reminds typical void distribution in the
stator bar insulation with many sources of discharges.
Asymmetry of the phase-resolved images within a period of
No of
repetitions
surge decay time [
μ
s]
373

the test voltage indicate that some of the sources adhere to the
conductor.
a)
b)
Fig. 4. The PD patterns at: a) sinusoidal voltage b) trapezoidal voltage,
tr=140μs, 1.25Uinc_sin=6.25kV, freq=50Hz
The PD pattern obtained for trapezoidal waveform
consists of two groups of PD pulses in each half of the period:
the first group represents discharges appearing on the rising /
falling slope of the voltage and the second one corresponds to
the flat part of the waveform [7, 8]. During flat part of the
trapezoidal voltage the rare activity of discharges results from
stable dU/dt inside the discharge source. PD activity
concentrated on the rising and falling slope comprises narrow
phase range (ca. 12 deg), while at sine wave it stretches to
approximately 90 deg. PD patterns show that the maximal
value of PD magnitude is larger at trapezoidal wave. This
escalation may results from remaining and not neutralized
space charge on the surface of discharge source [9, 10].
Hence, PD activity depends on voltage steepness, this problem
has been investigated applying fast surge impulses with a rise
time 100 ns and a decay time 100 μs. For PD visualization, the
PRPD like, time-resolved acquisition was used.
In case of the fast surge stimulus (tr=100ns), the quasi PD
inception (Uinc_surge) was defined as a presence of discharges
on the tail part of the waveform. The potential discharges
occurring on the fast rising slope were not acquired. PD
inception level defined in such a way for surge voltage
stimulus has been observed at lower voltage level comparing
to sine/triangle case. Surge patterns obtained for motor
insulation at voltage range Uinc_surge - 1.5⋅Uinc_surge are shown in
Fig. 5.
Fig. 5. Surge (PRPD like) patterns obtained for motor insulation
up to 1.5⋅Uinc_surge (surge pulse rise time 100ns, decay 100μs),
Y-axis amplitude on the plots is in arbitrary units
While increasing the crest value of the surge, the PD group
shows the tendency to appear earlier. One can also observe
increase in the PD magnitude and number of pulses. Surge
voltage with very short rise time reflects the stress voltage
generated at PWM conditions including the effect of
overvoltages.
PHASE [deg]
PHASE [deg]
Amp.
[au]
Amp.
[au]
time =70
μ
s
Uinc_surge
1.25
⋅
Uinc_surge
1.5
⋅
Uinc_surge
374

Thus a surge pulse may be treated as an approximation of the
square wave with superimposed transients. The surge patterns
illustrate the accumulated in a time-resolved mode set of PD
pulses, which reflect the condition on an insulating system
subjected to high du/dt voltage stress.
V. PD AT VARIOUS dU/dt STEEPNESS
The comparison of PD dynamics on form-wound motor
insulation subjected to the following three cases of voltage
stimulus has been performed: sine at power frequency,
trapezoidal waveform with the rise time 140μs and surge pulse
with a steepness 90kV/μs. Relationship of the PD inception
voltage versus wave-front voltage steepness in stator bar
motor insulation has been illustrated in Fig. 6. The transition
from sine to trapezoidal waveform indicated the decrease of
the inception voltage. Further, this trend is also confirmed for
very fast surge. In sine/trapezoidal cases the PD inception
voltage refers to the crest value of the waveform. Whereas for
surge pulse the inception voltage has been defined as
appearance of the stable PD group on the decaying part of the
surge impulse. The time proceeding the PD set on the wave-
tail is related to the formation of the PD channels in voids and
to the time constant of the voltage build-up on the voltage
source in response to fast du/dt. Increasing the surge voltage
one can notice higher steepness on the wave-tail, which is
influencing also the PD dynamics.
Fig. 6. Relationship of the PD inception voltage vs. wave-front
voltage steepness in stator bar motor insulation
VI. CONCLUSIONS
Paper presents the results of investigations of voltage
steepness influence on the PD mechanism in order to verify
the impact of the waveforms used is PWM based inverters.
The insulation assessment approach based on repetitive train
of fast surges has been shown. The individual surge is
approximating the typical PWM based inverter supply with
overvoltages. The method of time-resolved PD acquisition
resulting in PRPD like patterns has been described. The
accumulated surge patterns reflect the superposition of partial
discharges occurring in the consecutive periods of the PWM
waveform. It has been noticed the influence of the wave-front
rise time on the partial discharges activity occurring on the
surge impulse voltage wave-tail. Employing of the surge pulse
with decaying part instead of square waveform results in more
dynamic PD occurrence in the wave-tail due to changing
dU/dt. The application of the surge mimics the partial
discharges appearing on the flat part of the square waveform
in real inverter-motor system, in case when overvoltage on
motor terminal are incorporated. The presented results
demonstrate the coherent PD pattern obtained for surge pulse.
The comparison of results for the stator-bar insulation of form-
wound motor subjected to sine, trapezoidal and surge stress
reveal the decrease of PD inception voltage while increasing
the wave-front steepness.
Presented approach may be applied for test and
diagnostics of insulation system integrity subjected to steep-
front voltage.
ACKNOWLEDGMENT
The work described in the paper was partly carried out in
project NR 01 0019 04 sponsored by the Polish Ministry of
Science and Higher Education.
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