Digital Signal Processing Reference
In-Depth Information
(a)
Digital
filters
Signal
averaging
u
(
n
)
i
(
n
)
(b)
u
(
n
)
i
(
n
)
Digital
filters
Orthogonal-
ization
Measurement
algorithms
(c)
Orthogonal
digital
filters
u
(
n
)
i
(
n
)
Measurement
algorithms
(d)
Other
methods of
orthogonal-
ization
Measurement
algorithms
u
(
n
)
i
(
n
)
Fig. 3.12
Various ways of protection criteria measurement
least significantly suppressed. Then the criteria values can be calculated either by
signal averaging or by using of measurement algorithms based on orthogonal
components of the signals [
15
].
The orthogonalization process is meant as calculation/extraction of two
mutually orthogonal components of the sinusoidal signal. It can be performed by
special methods of orthogonalization (e.g. by time delay) or by taking two
orthogonal
filters.
The
correlation
method,
discrete
Fourier
transformation,
Kalman filtering, and others can also be used for the purpose.
The filter types, their design and analysis are thoroughly described in
Chaps. 5
and
6
. The details related to measurement procedures and algorithms can be found
in
Chaps. 8
and
9
.
Measured values are further compared with operational limits/protection
thresholds or characteristics to reach the final protection decision, which is
described in
Chap. 10
.
3.3 Hierarchical Structure of Protection and Control
Contemporary protection relays are multifunctional universal devices, realizing a
lot of additional functions (metering, event/fault recording…), offered as standard
even with smallest relays. Power quality monitoring is only partly covered by
protection relays (registration of voltage dips longer than 10 ms and harmonics up
to the 5th or 10th order—at least; more PQ functions expected to arrive in coming
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