Sel 751A - Figure H.2 Waveform at Relay Terminals May Have a Phase Shift; Figure H.3 Correction of Measured Phase Angle

H.3
Date Code 20100129 Instruction Manual SEL-751A 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 Δt
pt
= 0.784 ms and the nominal
frequency, freq
nominal
= 60Hz, use Equation H.2 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, PTRS (for sync check input), CTR, and CTRN.
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
Δt
pt
0
t
V
A
(t)
Actual
Waveform
Measured
Waveform
Compensation Angle
Δt
pt
1
freq
nominal
----------------------------
⎝⎠
⎛⎞
----------------------------------
360°• =
Δt
pt
freq
nominal
360°• • =
0.784 10
3–
s60s
1–
360°• • • 16.934°=
Imaginary
Real
M
M
V
A measured
V
A corrected
Compensation Angle