Hardware and Functional Development of Protection Devices and Systems - Digital Signal Processing in Power System Protection and Control - page 18

Estimation
of phasors

u
i

PMU

Stability
margin
assessment

u
i

Phasor
evaluation

PMU

Emergency
measures

u
i

PMU

Load
control

Generator
control

Transformer
control

FACTS

Load
shedding

Generation
adjustment

Tap changer
blocking

Load flow
control

Fig. 3.5 Example of WAMPS functions for maintaining system integrity

frequency of the component important for protection operation) and to eliminate
frequencies higher than that. The cut-off frequency of the filter f C ought to meet the
following requirements [ 15 ]:

f k \f C f S f k
3

ð 3 : 1 Þ

;

where f S is a sampling frequency.
This would make all the components with frequencies lower than f k pass with
the minimal distortion, while the components with frequencies greater than that
would be suppressed. The high-side limitation for the cut-off frequency results
from the repercussions of the sampling process, discussed in the next section.
A general picture of the low-pass filter frequency response is presented in Fig. 3.6 .
The filter should have possibly flat characteristic in both pass and reject regions
(possibly small values of the errors e 1 and e 2 ). The analog input filters are in most cases
implemented in form of cascade RC circuits. Required slope of the transition part of
filter spectrum is obtained by choosing appropriate filter order, which means that
respective number of the first order RC four-poles are connected in series.
Considering that the relay operation is based on the level of the fundamental
frequency component (f k = 50 Hz) of the voltage or current signal and the sampling
frequency is f S = 400 Hz, the required filter cut-off frequency is f C = 117 Hz. If the
relay (e.g. transformer protection) has to evaluate the level of the fifth harmonic
(f k = 250 Hz), and the sampling frequency is f S = 1600 Hz (note that the sampling
rate resulting from ( 3.1 ) should be higher than 1000 Hz, plus some reasonable
margin), the filter cut-off frequency should be f C = 450 Hz.