H.2
SEL-787 Relay Instruction Manual Date Code 20081022
Synchrophasors
Synchrophasor Measurement
The SEL-3306 Synchrophasor Processor is a PC-based communications
processor specifically designed to interface with PMUs. The SEL-3306 has
two primary functions. The first is to collect and correlate synchrophasor data
from multiple PMUs. The second is to then compact and transmit
synchrophasor data either to a data historian for post-analysis or to
visualization software for real-time viewing of a power system.
The SEL-787 supports the protocol portion of the IEEE C37.118, Standard for
Synchrophasors for Power Systems. In the SEL-787, this protocol is referred
to simply as C37.118. See Settings Affect Message Contents on page H.11.
Synchrophasor Measurement
The phasor measurement unit in the SEL-787 measures three voltages and six
currents on a constant-time basis. These samples are time-stamped with the
IRIG time source. The phase angle is measured relative to an absolute time
reference, which is represented by a cosine function in Figure H.1. The time-
of-day is shown for the two time marks. The reference is consistent with the
phase reference defined in the C37.118 standard. During steady-state
conditions, the SEL-787 synchrophasor values can be directly compared to
values from other phasor measurement units that conform to C37.118.
Synchrophasor values are available for the full frequency range of the
SEL-787.
Figure H.1 Phase Reference
The TSOK Relay Word bit asserts when the SEL-787 has determined that the
IRIG-B time source has sufficient accuracy and the synchrophasor data meets
the specified accuracy. Synchrophasors are still measured if the time source
threshold is not met, as indicated by Relay Word bit TSOK = logical 0. The
MET PM command is not available in this case.
The instrumentation transformers (PTs or CTs) and the interconnecting cables
may introduce a time shift in the measured signal. Global settings VCOMP,
IW1COMP, and IW2COMP, entered in degrees, are added to the measured
phasor angles to create the corrected phasor angles, as shown in Figure H.2,
Figure H.3, and Equation H.1. The VCOMP, IW1COMP, and IW2COMP
settings may be positive or negative values.
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