The Implications of varios PWM shaped contributions Superimposed on DC Bias Voltages
Paper in proceedings, 2019

Speed drives fed by pulse width modulated (PWM)
inverters allow an efficient use of electric energy. The drawback
is however that the stress imposed on insulation systems
increases due to a higher frequency harmonic content compared
to usage of classical 50 Hz sinusoidal voltages. Similarly, on a
much larger energy scale, similar frequency contributions have
been observed in high voltage DC (HVDC) applications. This
increases electrical stress and degradation. In particular, partial
discharges (PDs) are considered one major contributor to the
reduction in the insulation life time. It has been shown that the
usage of electrical filters to increase the rise time of the applied
voltage decreases electrical stress imposed on the insulation.
Another option is to use multi level inverters. For the latter case,
it remains to explain how the size of these voltage steps as well as
polarity influences the level of PDs. Additionally to explore
similarities between PD characteristics in bipolar PWM and DC
voltages with superimposed high frequency contributions. Here
as an example, twisted pair test objects are exposed to PWM
inverter waveforms at various rise times. Both bipolar and
unipolar waveforms as well as different step sizes and rise times
of each voltage flank are employed to present a gradual change
between bipolar PWM to a more constant DC voltage magnitude
with superimposed voltage ripple to elucidate the influence on
PD characteristics. To compare the performances, measurements
of both PD inception and extinction voltages were conducted for
the different cases. Additionally, the total number of PDs, their
average maximum amplitude per cycle and pulse repetitive PD
pattern (PRPD) are presented. The decreased step size to
resemble more HVDC waveform and the resulting change in PD
characteristics is discussed. Important factors are the time until
PDs are observed (time lag) together with observed polarity
dependences. Additionally, three level PWM shaped voltage
waveforms superimposed on DC voltages are employed to
evaluate influence of frequencies and material properties aiming
to compare behavior of PDs at various waveform and detect
similarities to facilitate the development of future insulation

Twisted pair cables.

Power Frequency

Partial discharges

Pulse width modulation

Machine windings

Pulse generation



Thomas Hammarström

Chalmers, Electrical Engineering, Electric Power Engineering, Power grids and Components

2019 IEEE Electrical Insulation Conference
Calgary, ,

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