In addition to the PRPD analysis, it was observed that
it is of primary importance to understand the process of
signal propagation and attenuation at different frequencies
if ones wants to compare PD results with those of
others because the selection of the frequency range for
the measurement will have a major impact on the discharge
sources detected. The signal detected is greatly
affected by the bandwidth of the detection equipment of
which the coupler is one component. Detection at high
frequency will tend to favor external discharges, which are
not necessarily the most dangerous ones. At lower frequencies,
the detection may be affected by the presence
of noise which can in some cases be large enough to impede
detection of the PD signal. Although of importance,
the subject of noise discrimination exceeds the scope of
our investigation and the reader is referred to other publications
treating this problem in detail w10, 11, 20, 21x.
Usually, the choice of frequency does not alter very much
the shape of the PRPD pattern or of distinctive asymmetry,
but it will have a significant impact on the detected
amplitudes. Moreover, in some frequency ranges some
discharge sources may remain undetected. For instance,
internal discharges give no or very little signal at frequen-
In addition to the PRPD analysis, it was observed that
it is of primary importance to understand the process of
signal propagation and attenuation at different frequencies
if ones wants to compare PD results with those of
others because the selection of the frequency range for
the measurement will have a major impact on the discharge
sources detected. The signal detected is greatly
affected by the bandwidth of the detection equipment of
which the coupler is one component. Detection at high
frequency will tend to favor external discharges, which are
not necessarily the most dangerous ones. At lower frequencies,
the detection may be affected by the presence
of noise which can in some cases be large enough to impede
detection of the PD signal. Although of importance,
the subject of noise discrimination exceeds the scope of
our investigation and the reader is referred to other publications
treating this problem in detail w10, 11, 20, 21x.
Usually, the choice of frequency does not alter very much
the shape of the PRPD pattern or of distinctive asymmetry,
but it will have a significant impact on the detected
amplitudes. Moreover, in some frequency ranges some
discharge sources may remain undetected. For instance,
internal discharges give no or very little signal at frequen-
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