Digital Signal Processing Reference
In-Depth Information
Chapter 9
Characteristics of Measurement
of Criterion Values and Adaptive
Algorithms
Standard control systems are designed and evaluated taking into account dynamics
and required accuracy for various input signals. Digital protection systems should
also be evaluated in a similar way, however, the fundamental parameter is time
needed to take decision concerning the state of protected element of the power
system. Sometimes it is as short as a fraction of period of the fundamental fre-
quency component. This decision time, different for various power system ele-
ments, is usually fixed to avoid possible extended damage of protected plants. For
some criterion values and some decision-making methods the final decision is to
be done even during transient states of measurement when one can still expect
substantial errors criteria values determination. This is why while designing a
protection system one must take dynamic features of measurement algorithms into
consideration and perform optimization of its elements and the entire system as a
decision-making device.
9.1 Dynamics of the Measurement Process
Processes of measurement and decision-making in protection devices are realized
under heavy dynamic conditions [
9
]. Signals, usually voltages and currents,
change rapidly during faults, causing also changes of the measured criterion val-
ues. In order to reach adequately fast decision concerning the state of protected
element it is necessary to have properly short transient period between normal and
faulty steady states of measurement.
Applied measurement algorithms can be divided for either operating continu-
ously or starting its operation when fault appears. In many protection systems both
types are applied, however, general principle is that in the fastest protection sys-
tems the second ones must be used. Examples could be systems using Kalman
filters or mean square error (MSE) methods. But even in such a case it is necessary
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