Date Code 970103
Event Reporting
4-3
SEL-251, -2, -3 Instruction Manual
2. The relay samples filtered analog signals four times per power system cycle and converts
them to numerical values.
3. A digital filter processes the sampled data and removes dc and ramp components. The unit
sample response of this filter is:
1, -1, -1, 1
The filter has the property of a double differentiator smoother.
4. The digital filter processes the latest four samples every quarter-cycle. Successive filter
outputs arrive every 90
E
. With respect to the present value of the filter output, the previous
value was taken one quarter-cycle earlier and appears to be leading the present value by 90
E
.
Filter output values can be used to represent the signals as phasors:
The previous value of the output is the Y-component.
The present value of the output is the X-component.
The following example illustrates why the older data is the leading component of the phasor.
Consider a sinewave having zero-phase shift with respect to t = 0 and a peak amplitude of 1.
Now consider two samples, one taken at t = 0, the other taken 90
E
later. They have values 0 and
1, respectively. By the above rules, the phasor components are (X,Y) = (1,0).
Now consider a cosine function. Its samples taken at t = 0 and t+90
E
are 1 and 0; its phasor
representation is (0,1). The phasor (0,1) leads the phasor (1,0) by 90
E
. This coincides with a
90
E
lead of the cosine function over the sine function.
To construct a phasor diagram of voltages and currents, select a pair of adjacent rows from an
area of interest in the event report. On Cartesian coordinates, plot the lower row (newer data) as
the X-components and the upper row (older data) as the Y-components. Rotate the completed
diagram to any angle of reference. The magnitude of any phasor equals the square root of the
sum of its squares.
Note that moving forward one quarter-cycle rotates all phasors 90
E
. You can verify this by
plotting the phasor diagram with rows 1 and 2, then rows 2 and 3 of an event report. Example
Event Report 1 shows conversion of the rectangular format voltages and currents displayed in the
event report to polar format (see Example Event Reports).
R
ELAY
E
LEMENT
, O
UTPUT
C
ONTACT
, A
ND
I
NPUT
D
ATA
Relay elements, inputs, and output contact states appear in the right-hand columns of the event
report. The following information lists symbols corresponding to asserted combinations of
elements ("1" indicates element/output contact assertion or input energization).