H.3
Date Code 20081022
Instruction Manual
SEL-787 Relay
Synchrophasors
Synchrophasor Measurement
Figure H.2
Waveform at Relay Terminals May Have a Phase Shift
Equation H.1
If the time shift on the pt measurement path
Δ
t
pt
= 0.784 ms and the nominal
frequency, freq
nominal
to obtain the correction
angle:
Equation H.2
Figure H.3
Correction of Measured Phase Angle
The phasors are rms values scaled in primary units, as determined by Group
setting PTR, CTR1, and CTR2 (for the IW1 and IW2 current sources).
Because the sampling reference is based on the GPS clock (IRIG-B signal)
and not synchronized to the power system, an examination of successive
synchrophasor data sets will almost always show some angular change
between samples of the same signal. This is not a malfunction of the relay or
the power system, but is merely a result of viewing data from one system with
an instrument with an independent time base. In other words, a power system
has a nominal frequency of either 50 or 60 Hz, but on closer examination, it is
usually running a little faster or slower than nominal.
94.851
–94.851
Δ
t
pt
0
t
V
A
(t)
Actual
Waveform
Measured
Waveform
Compensation Angle
Δ
t
pt
1
freq
n omina l
----------------------------
⎝
⎠
⎛
⎞
----------------------------------
360
°
•
=
Δ
t
pt
freq
no mina l
360
°
• •
=
0.784
10
3
–
s
60s
1
–
360
°
• • •
16.934
°
=
Imaginary
Real
M
M
V
A measured
V
A corrected
Compensation Angle
Summary of Contents for SEL-787
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