Infinite Impulse Response Filters - Digital Signal Processing in Power System Protection and Control - page 53

Chapter 5
Infinite Impulse Response Filters
5.1 IIR Filter Fundamentals
Digital filters allow to pass assumed spectrum part or particular component of the
signal and prevent from passing the other signal components, sometimes called
noise. Similarly to analog filters one can distinguish: low pass, high pass, band
pass and band rejection digital filters [ 1 - 6 ]. The frequency response of these types
of filters are shown in Fig. 5.1 . In most cases it is required that the filters should
have flat frequency response in pass band, steep transition region(-s) and close to
zero rejection band. Phase frequency response is also important, especially useful
are filters having this response linear. This is also valid in protection systems,
where extracting orthogonal components is of importance.
Fundamental criteria describing requirements and performance of digital filters
applied in protections are following [ 5 ]:
• accuracy in transmission of useful signal components and efficient rejection of
noise,
• fast stabilizing of output signal after step change of input signal,
• linear phase shift frequency response,
• minimum burden during digital realization of given filter.
Some of the requirements are quite contradictory and that is why design of a
digital filter is a matter of compromise.
All analog filters have infinite impulse response, i.e. their response is such at
least theoretically. Digital filters, on the other hand, can be divided for:
• filters similar to analog ones having infinite impulse response called shortly IIR
filters, (sometimes also called recursive filters)—described in detail in this
chapter,