Page 10 - CE_Industral_Journal_Jan2008

A Novel Approach for Ground Fault Detection

The position argument keeps track of the temporal oped for the wavelet based system examines overlapping
change in current harmonics which is essential to the windows of the current at different scales and details
high impedance ground fault detection and the scale via a wavelet transform. Although proper detection can
change keeps track of bands of frequencies of the be accomplished using more than a single scale, ex-
current load. Both position and scale are continuous, perimental testing indicated that the energy component
therefore the above transform is not suited for compu- of the seventh detail signal carries the most significant in-
tation. A discrete version of the transform is needed formation that is more distinguishable from other normal
which is given by, arcing loads or normal non-linear loads. The additional
preprocessing needed is a FFT to render the current with
all its random delay components position
insensitive.
The seventh detail signal obtained via wavelet decom-
position corresponds to the frequency range between
where, k, m and n are all integers.
the second and fifth harmonic. The importance of the
proposed detection scheme is in considering the third,
The above transform is implemented by multi resolu- fourth, and fifth harmonic as well as the in-between
tion analysis where the signal is decomposed into a harmonics frequencies as a block of features for high
low pass and a high pass component via two sepa- impedance ground fault detection. Furthermore, the tem-
rate low pass and high pass filters known as wavelet poral change in high impedance ground fault currents
decomposition filters. After filtering, both low pass and is accounted for in the temporal change of the seventh
high pass signals are down sampled by a factor of 2. detail signal.
The high pass signal component corresponds to the
first detail look of the signal. The second detail look
can be obtained by further decomposition of the cur-
rent low pass signal into two new low pass and high
pass components. The third, fourth, etc. detail signals
can be obtained by further decomposition of subse-
quent low pass components.
The original signal can be reconstructed with minimal
error from its low pass and high pass components in a
reverse pyramidal manner. It is in these high pass com-
ponents where distinct features for high impedance
ground fault can be located and distinguished from
signatures of other non-linear loads of transient and
arcing in nature. The decomposition filters are associ-
ated with the type of mother wavelet used.

Figure 4. Wavelet based high
impedance fault detection
system.
Most of the exemplary tests of
this technique were conducted
using the Daubechies-4 wavelet
which is not a very smooth
wavelet but requires less
computation time. Use of other
wavelets or other Daubechies
wavelets did not show any no-
ticeable change in performance
nor in the threshold parameters
used.
The detection algorithm devel-
Industry Journal 9