Schweitzer Engineering Laboratories SEL-749M - Figure 9.5 Derivation of Phasor RMS Current Values from Event Report Current Values

9.13
Date Code 20080918 Instruction Manual SEL-749M Relay
Analyzing Events
Event Reporting
Figure 9.5 Derivation of Phasor RMS Current Values From Event Report
Current Values
In Figure 9.5, note that two rows of current data from the event report in
Figure 9.3, 1/4 cycle apart, can be used to calculate phasor rms current values.
In Figure 9.5, at the present sample, the phasor rms current value is:
Equation 9.1
The present sample (IA = 1135 A) is a real rms current value that relates to the
phasor rms current value
:Equation9.2
IA
t
IA
970
–1135
–971
1135
Event Report Column
X = Present Sample
Magnitude =
X = 1135
Y = –971
Angle = Arctan
= Arctan
Y
X
–971
1135
=–40.5˚
Real Axis
Axis Imaginary
1/4 cyc.
(–971)
2
+ (–1135)
2
= 1493
Y = Previous
1/4 Cycle
NOTE: The arctan function of many
calculators and computing programs
does not return the correct angle for
the second and third quadrants (when
X is negative). When in doubt, graph
the X and Y quantities to confirm that
the angle that your calculator reports
is correct.
IA 1493 A 40.5°–∠=
1493 A cos 40.5°–()• 1135 A=
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