4.109
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
to meet the “healthy voltage” criteria (settings 25VHI and 25VLO—see
Figure 4.67). 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 2.23 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 is 90 degrees for the ACB phase rotation.
Use the voltage ratio correction factor (setting 25RCF) to compensate magni-
tude of the phase voltage to match the synchronism voltage VS. Many appli-
cations require 25RCF := 1.00, however some applications may need a
different setting. For example, Figure 2.23 requires 25RCF := PTR /
(1.732*PTRS). This is 0.58 if the PTR and PTRS are equal.
See the Application Guide entitled Compensate for Constant Phase Angle Dif-
ference in Synchronism Check with the SEL-351 Relay Family (also applies to
SEL-751) for more information on setting SYNCPH with an angle setting.
Synchronism-Check Elements Voltage Inputs
The two synchronism-check elements are single-phase elements, for both of
which you would use single-phase voltage inputs VP and VS:
1. 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.)
2. VS Synchronism-check voltage, from SEL-751 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 2.23).
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 4.67 and Figure 4.68.
Voltage Window
Refer to Figure 4.67. Single-phase voltage inputs VP and VS are compared to a
voltage window, to verify that the voltages are “healthy” and lie within setta-
ble voltage limits 25VLO and 25VHI. If both voltages are within the voltage
window, the following Relay Word bits assert:
59VP indicates that voltage VP is within voltage window setting limits
25VLO and 25VHI
NOTE: Settings SYNCPH := 0 and
SYNCPH := VAB are effectively the
same (voltage VS is directly
synchronism checked with voltage
VAB; VS does not lag VAB). The relay
displays the setting entered
(SYNCPH := VAB or SYNCPH := 0).
To Next Page
Loading...
