Characteristics of Measurement of Criterion Values and Adaptive - Digital Signal Processing in Power System Protection and Control - page 190

I, P, R, f I

1

f

0.8

0.6

R

0.4

0.2

P

0

100 125 150 175 200

t [ms]

Fig. 9.13 Waveshapes of selected criteria values measured during sudden change of frequency at
t = 100 ms

9.4 Adaptive Frequency Insensitive Estimators

Measurements of criterion values of power system protections use the signals that
in most cases have constant frequency. However, there are some situations, for
instance during great power imbalance, when frequency changes by as much as a
few Hz. Then some problems arise concerning accuracy of applied measurement
algorithms since the filters are tuned and matched to constant nominal frequency
(50 or 60 Hz). Protection relay are also used in reversible generation units where
the range of frequency changes is wide and under such conditions the protection
criteria values should be measured. This is why one must also consider designing
protection systems, which either can operate during wide range of frequency
changes or which are insensitive to small frequency deviations, depending on
application.
It can be proved that deviations of signal frequency cause errors of measured
criteria values. In some cases these errors depend on signals phase shifts (mag-
nitude and active power). When a frequency deviation occurs, second harmonic of
the signals as well as constant value proportional to measured quantity may
appear. This is illustrated in Fig. 9.13 . After step change of frequency from 50 to
40 Hz (this is simulation only, such change must not appear in practice) measured
current magnitude, power and resistance (standard algorithms applied) exhibit
unsymmetrical oscillations around accurate values. The frequency of oscillations
is two times higher (2*40 Hz) than signal frequency. The errors are too big to
accept them in practice.
However, beside standard algorithms one has on disposal many others that are
less or even much less sensitive to frequency deviations. Responses of two types of
algorithms of active power and resistance measurement are presented in Fig. 9.14