R.7.8
SEL-421 Relay Reference Manual Date Code 20111215
Synchrophasors
Synchrophasor Measurement
Equation 7.3
With PTRY = 2000, and the signal in Figure 7.10 (with peak voltage
V
pk
= 94.851 V), use Equation 7.4 to obtain the magnitude, VALPMM:
Equation 7.4
Finally, the magnitude and angle pair for each synchrophasor is converted to a
real and imaginary pair using Equation 7.5 and Equation 7.6. For example,
analog quantities VALPMM and VALPMA are converted to VALPMR and
VALPMI. An example phasor with an angle measurement of 104.400° is
shown in Figure 7.11. Tab le 7. 13 lists all of the synchrophasor analog
quantities available in the SEL-421.
Figure 7.12 Example Calculation of Real and Imaginary Components of
Synchrophasor
Equation 7.5
Equation 7.6
Using the magnitude M from Equation 7.5, the real part is given in
Equation 7.7.
Equation 7.7
Similarly, the imaginary part is calculated in Equation 7.8
Equation 7.8
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.
Magnitude M
V
pk
2
---------
PTR
setting
• =
VALPMM
94.851
2
----------------
2000• =
134140 V=
134.140 kV=
VALPMI =
129.926 kV
VALPMA =
104.400˚
VALPMR = –33.359 kV
Real
VALPMM =
134.140 kV
Imaginary
Real part M cos (angle)• =
Imaginary part M sin (angle)• =
VALPMR 134.140 kV cos 104.400• =
-33.359 kV=
VALPMI 134.140 kV sin 104.400• =
129.926 kV=
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