BE1-951
Protection and Control
4-17
Figure 4-10. 51P Pickup Level
Compensation
S<g>-x51 Command Examples
1.
Read the 51 setting data for the phase element in setting group 0.
>S0-51P
10,2.4,S1R
(The data returned indicates a pickup setting of 10 amperes, a time dial setting of 2.4,
and an S1 time dial curve with integrating reset.)
2.
Change the 51 phase element of setting group 0 so that the time dial setting is 3.0.
>S0-51P=,3.0
3.
Set the 51 negative sequence element of setting group 0 for a pickup of 0.6 A, a time delay of 30
seconds. Use the 46 curve as the time dial curve.
>S0-51Q=0.6,30,46
Voltage Restraint/Control for Phase Time-Overcurrent Function
The 51P protection function can be set for voltage control or voltage restraint mode of operation (51V). This
feature is used to allow increased overcurrent sensitivity while providing security from operation due to load
current. This feature is also often used for generator backup protection to ensure delayed tripping during a
short circuit where the fault current contribution from the generator falls to a value close to the full load rating
of the generator.
When set for
Control mode of operation, the phase overcurrent element is disabled until the measured
voltage drops below the threshold. Thus, as long as the
voltage on the appropriate phase is above the 27R threshold
setting, the overcurrent element will be blocked. When set for
this mode of operation, the 51P pickup setting is typically set
near or below load current levels.
When set for
Restraint mode of operation, the pickup of the
phase overcurrent element is adjusted based upon the
magnitude of the measured voltage. Figure 4-10 shows how
the overcurrent pickup threshold setting is adjusted in
response to the measured voltage level. Equation 4-1
determines the pickup level for the 51P elements when the
measured voltage is between 25% and 100% of the 27R
threshold setting. Below 25%, the pickup level stays at 25%.
Above 100%, the pickup level stays at 100%. For example, if
the 27R threshold is set for 120V and the measured voltage
on the appropriate phase is 100V, (83% of the 27R threshold
setting), the overcurrent pickup level for that phase will be reduced to 83% of its setting. When set for this
mode of operation, the 51P pickup setting is typically set above worst case, load current levels.
The 51/27R function can be set to monitor either Vpp or Vpn depending upon the VTP connection settings.
See Section 3,
Input and Output Functions, Power System Inputs for more detail on how to set the VTP
Connections. Table 4-9 shows which voltage measurements are used by each phase overcurrent element
for each possible VTP connection and 51/27 voltage monitoring mode setting.
Table 4-9. VTP Connection Cross Reference
VTP Connection
51/27 Mode
51A
51B
51C
4W
Vpp
Vab
Vbc
Vca
4W
Vpn
Van
Vbn
Vcn
3W
Vpp
Vab
Vbc
Vca
AN
Vpn
Van
NA
NA
BN
Vpn
NA
Vbn
NA
CN
Vpn
NA
NA
Vcn
AB
Vpp
Vab
NA
NA
BC
Vpp
NA
Vbc
NA
CA
Vpp
NA
NA
Vca
