Interpretation of Voltage and Current Data
Voltage and current data in the event report are determined using the following steps. The
process uses secondary quantities presented to the rear panel of the relay.
1.
Input analog signals are filtered by two pole, low pass filters with cutoff frequencies of
approximately 85 Hz.
2.
Filtered analog signals are sampled four times per power system cycle and converted 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:
The filter has the property of a double differentiator smoother.
4.
The latest four samples are processed through the digital filter every quarter-cycle.
Successive outputs of the filter arrive every 90°. 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°.
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.
1, -1, -1, 1
The following example may clarify why we refer to the older data as 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° 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° are 1 and 0; its phasor
representation is (0,l). The phasor (0,1) leads the phasor (1,0) by 90°. This coincides with
a 90° 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°. 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 the process of converting the rectangular format voltages and
currents displayed in the event report to polar format.
4-6
Event Reporting
SEL-221F, -1 Instruction Manual
Date Code 920423
Summary of Contents for SEL-221F
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Page 110: ...Example Event Report 2 4 12 Event Reporting SEL 221F 1 Instruction Manual Date Code 930830 ...
Page 158: ...SETTINGS SHEET FOR SEL 221F RELAY PAGE 2 OF 6 DATE Date Code 921110 ...
Page 160: ...SETTINGS SHEET PAGE 4 OF 6 FOR SEL 221F RELAY DATE Date Code 930601 ...
Page 161: ...SETTINGS SHEET PAGE 5 OF 6 FOR SEL 221F RELAY DATE Date Code 961208 ...
Page 229: ...SEL 200 Series Relay Main Board Troubleshooting Test Points and Jumper Locations ...