Measurement Algorithms for Digital Protection - Digital Signal Processing in Power System Protection and Control - page 110

Fig. 8.1 Basic scheme of
signal processing in a digital
protection relay

u ( n )
i ( n )

Digital
signal
conditioning

Measurement
of criterion
values

Decision
making

protection signals have been developed, some of them are described in next sub-
section.
Then a number of measurement algorithms belonging to several families are
described. The averaging methods are presented first, then, more deeply the
methods of measurement of criterion values emerging from digital technique are
described, with special attention paid to the ones applying orthogonal components
of input signals.

8.1.1 Digital Signal Conditioning

Digital signal conditioning is aimed at realizing several important functions. The
first, perhaps the most important one, is extraction of useful information from
the signals on disposal. This is done by either dumping or rejection of noise. The
second task is producing such signal components which make calculation of
protection criterion values possible in the simplest way. It should be mentioned
that all the above is to be realized within the shortest possible period of time. It is
easy to notice that the tasks are in most cases contradictory according to the known
speed-accuracy dilemma and thus proposed solutions are always a compromise.
Since particular requirements are different depending on protected or controlled
element of the power system, then sometimes the most important is the operation
time and sometimes measurement accuracy or noise reduction. According to given
requirements many methods are applied in signal conditioning and further signal
processing. They can be divided for those designed and optimized in either fre-
quency or time domain. An example of the first one could be digital filter synthesis
starting from required frequency response ( Sect 6.3 ). An example of the second
group could be synthesis of Kalman filter on basis of mean square error and the
fastest possible time response.
The most important methods of digital signal conditioning are following:

• Fast Fourier transform ( Chap. 4 ),
• digital filters (described in Chaps. 5 and 6 ),
• separate methods of orthogonalization,
• digital methods basing on LSE,
• correlation, etc.

The processes of filtering/extracting of useful signal components and their
orthogonalization can be joint (using appropriate FIR filter pairs) or separate (i.e.
without filtering that should be implemented separately, if needed). Since the first
approach has already been presented in Chap. 6 , the second approach with its