Digital Signal Processing Reference
In-Depth Information
Chapter 8
Measurement Algorithms for Digital
Protection
8.1 Fundamentals of Digital Measurements
In case of protection devices of pre-digital era that were made in analogue tech-
nique the processes of measurement and decision-making were integrated. Mag-
nitude or phase comparators were applied and thus the measurement of criterion
values could not be realized directly. From the digital signal processing block
schemes presented in Fig.
3.12
it becomes evident that with the advent of digital
technology the principles of design of protection relays have been changed
substantially. In digital protection devices the measurement of criterion values and
decision-making were separated in principle from the very beginning of digital
era, thus allowing for optimization of each of the processes separately. The
measured criterion values could now be used for other purposes, moreover,
development of multi-criteria decision-making as well as application artificial
intelligence techniques was also possible.
Great number digital measurement algorithms of criterion values have appeared
in the past [
1
,
3
,
4
,
6
,
8
-
10
]. At the very beginning they were applying signals
averaging and some other methods of analogue technique origin. Very soon,
however, digital technique with its advantages, especially FIR digital filtering and
orthogonal signal components became a fundamental and exceptional solution.
Criterion values of digital protections are magnitudes of fundamental compo-
nents of currents and voltages, powers, impedance or admittance and their com-
ponents, frequency, phase shift, the values for symmetrical components and the
others. Very frequently many of them are used for given protection simultaneously
(multi-criteria solutions). Measurement algorithms for all of them are presented in
this chapter, whereas some special features of the algorithms including their
dynamics and frequency responses are described in
Chap. 9
.
Before the actual measurement of protection criteria can be performed the input
signals have to be pre-processed, which is referred to as digital signal conditioning,
Fig.
8.1
. Various methods allowing for extraction of noise-free information-rich
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