4-44
Protection and Control
BE1-951
The BE1-951 compares the VTP voltage magnitude and angle to the VTX voltage magnitude and angle to
determine synchronism. Therefore, proper connection of the VT inputs is vital to the correct operation of the
25 function.
The relay automatically compensates for phase angle differences associated with the phase and auxiliary
VT connections, including single-phase VTP connections. That is, for a VTP selection of phase to phase and
a VTX selection of phase to neutral, the relay will automatically compensate for the 30-degree angle
between the two voltage sources.
However, the relay does not scale for differences in magnitude
between the applied voltages.
For example, if VTP = 4W (L-N) and VTX=AB (L-L), the angle is
automatically compensated for. However, one of the input magnitudes needs to be scaled by SQRT (3) so
that the magnitude of the compared voltages are equivalent under sync conditions. Note: The sync-check
will not work if VTX connections are set for residual voltage input VTX=RG.
For clarification on single-phase VTP connections, refer to the interconnection diagrams shown in Figure
12-20 of this manual. Note, for single-phase sensing connections, terminals C13, C14, C15 (A, B, C) are
connected in parallel. The single-phase signal is connected between the paralleled group and C16 (N). This
ensures that the zero-crossing circuit is always connected to the sensed circuit.
Note that delta V is a single-phase test only. For 3W VTP connection, the relay uses A-B, for 4W VTP
connection it uses A-N, and for single-phase sensing the relay uses the applied phase. Also note that VM
performs three of three testing for all connections. For 3W and 4W, phases A, B, and C are actually tested.
For single-phase connections, the terminals are connected in parallel as described above and the single-
phase is tested three times. This is implemented this way for convenience, allowing the exact same code
for both conditions.
Measuring slip frequency directly allows the function to rapidly determine if systems are in sychronism and
requires no timer or inherent delay (as compared to systems that check only that phase angle is held within
a window for some stretch of time). The moment parameters a), b), and c) in the previous paragraph are
met, the systems may be considered in synchronism and the output becomes TRUE. Refer to Section 5,
Metering, Frequency for more information about slip frequency measurement.
BESTlogic Settings for the Sync-Check Element
Logic settings for the 25 function are made using the SL-25 (settings logic-Sync-Check) command. The
settings of this command determine whether the element is enabled or disabled. The SL command also
provides blocking control as determined by the logic expression assigned to the BLK input. Table 4-34 lists
the SL-25 command settings.
Table 4-34. Sync-Check Logic Settings
Function
Range/Purpose
Default
mode
0=Disable
1=enable
0
BLK logic
Logic expression that disables
function when true
0
SL-25 Command
Purpose:
Read or set logic for the Sync-Check element
Syntax:
SL-25[=<mode>,<BLK logic>]
Comments: Refer to Table 4-34 for settings definitions
SL-25 Command Example
Enable Sync-Check when breaker is Open as defined by 52b at IN1
SL-25=1,IN1
Operating Settings for the Sync-Check Element
Operating settings for the 25 function consist of the maximum voltage magnitude difference, maximum
phase angle difference, maximum slip frequency difference, and the generator frequency greater than bus
frequency setting. When selected for generator frequency greater than bus frequency mode, the sync rate
