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 90E. 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 90E.
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 90E 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+90E are 1 and 0; its phasor
representation is (0,1). The phasor (0,1) leads the phasor (1,0) by 90E. This coincides with a
90E 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 90E. 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).
RELAY ELEMENT, OUTPUT CONTACT, AND INPUT DATA
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).
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