4.36
SEL-751A Relay Instruction Manual Date Code 20100129
Protection and Logic Functions
Voltage-Based Protection
Delta-Connected Voltages
NOTE: Settings SYNCPH := 0 and
SYNCPH := VAB are effectively the
same (voltage VS is directly
synchronism checked with voltage
VAB; VS does not l a g VA B) . The relay
will display the setting entered
(SYNCPH := VAB or SYNCPH := 0).
The angle setting choices (0, 30, …, 300, or 330 degrees) for setting SYNCPH
are referenced to VAB, and they indicate how many degrees VS constantly
lags VAB. In this application, voltage input VA-VB has to be connected and has
to meet the “healthy voltage” criteria (settings 25VHI and 25VLO—see
Figure 4.20). For situations where VS cannot be in phase with VAB, VBC, or
VCA, the angle setting choices (0, 30, …, 300, or 330 degrees) are referenced
to VAB.
Figure 2.20 shows a relay wired with delta-connected phase PTs, and a
C-phase-to-ground connected VS-NS input. With ABC rotation, the correct
SYNCPH setting for this example is 270 degrees, the amount that VC lags
VAB. However, the setting angle will be 150 degrees for the ACB phase
rotation.
Use the voltage ratio correction factor (setting 25RCF) to compensate
magnitude of the phase voltage to match the synch voltage VS. Many
applications will require 25RCF := 1.00, however some applications may need
a different setting. For example, Figure 2.20 requires 25RCF := PTR /
(1.732*PTRS). This will be 0.58 if the PTR and PTRS are equal.
See the Application Guide entitled Compensate for Constant Phase Angle
Difference in Synchronism Check with the SEL-351 Relay Family (also applies
to SEL-751A) for more information on setting SYNCPH with an angle
setting.
Synchronism-Check Elements Voltage Inputs
The two synchronism-check elements are single-phase elements, with single-
phase voltage inputs VP and VS used for both elements:
VP Phase input voltage (VA, VB, or VC *25RCF for Delta_Y := Wye; VAB,
VBC, or VCA*25RCF for Delta_Y := Delta), designated by setting SYNCPH
(If SYNCPH is set to one of the angle settings, then VP = VA*25RCF or
VAB*25RCF depending on the Delta_Y setting.)
VS Synchronism-check voltage, from SEL-751A rear-panel voltage
input VS
For example, if the rear-panel voltage input VS-NS is connected to B-phase (or
BC phase-to-phase for delta) then set SYNCPH := VB (or VBC for delta). The
voltage across terminals VB-N (or VB-VC for delta) is synchronism checked with
the voltage across terminals VS-NS (see Figure 2.20).
System Frequencies Determined from Voltages VA (or VAB for Delta)
and VS
To determine slip frequency, first determine the system frequencies on both
sides of the circuit breaker. Voltage VS determines the frequency on one side.
Voltage VP determines the frequency on the other side.
Synchronism-Check Elements Operation
Refer to Figure 4.20 and Figure 4.21.
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