Schweitzer Engineering Laboratories SEL-749M, Instruction Manual

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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
: Equation 9.2
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 =
Courtesy of NationalSwitchgear.com