Measurement Algorithms for Digital Protection - Digital Signal Processing in Power System Protection and Control - page 167

Nevertheless, when higher accuracy is required, other methods, including
those based on orthogonal components, are recommended.
• Impedance estimation based on fault circuit model
This approach is characterized by full independence of the decaying DC
component (this feature is inherited in the algorithm, being taken into account
in the circuit model). One of the disadvantages in certain susceptibility to
higher frequency oscillating components. This shortcoming can be limited by
applying appropriate input filters, what on the other hand increases the total
measurement time.
• Filtration of symmetrical components
This operation is most frequently realized by applying phase signals delaying
or by use of their orthogonal components. The first method is very simple, but
requires sampling frequencies dividable by 3 (e.g., 1200 Hz) and phase sig-
nals devoid of distortions. The second one is more complex, but thanks to
signal filtration one gets elimination of distortions and orthogonal components
that can also be used for measurement of other criteria signals. Thus, usage of
this method seems justified to be recommended for practical applications.

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