Date Code 20010521
Event Reports and Sequential Events Reporting
9-21
SEL-387E Instruction Manual
Figure 9.6: Derivation of Event Report Current Values and RMS Current Values From
Sampled Current Waveform
In Figure 9.6 note that any two rows of current data from the event report that are one-quarter
cycle apart can be used to calculate RMS current values. One-quarter cycle represents 90
electrical degrees; therefore, the two samples are effectively perpendicular to each other and can
be treated as rectangular components of the phasor quantity. By using the normal method of
taking the square root of the sum of the squares of the samples, the magnitude of the phasor can
be extracted. Because the actual sample values have been divided by the square root of two
(multiplied by 1/sqrt(2) in the drawing) before being entered into the report column, no further
adjustment is needed after doing the magnitude calculation. In the example in Figure 9.6
successive pairs of samples result in magnitude calculations very close to the true value of
2748 A, RMS. The true RMS value is shown as IA
peak
times 1/sqrt(2) = 3887 • 0.707 = 2748 A.
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