BECKWITH ELECTRIC M-3425A, Instruction Book

Page: 212 / 482

4–81
System Setup and Setpoints – 4
51V is a true three-phase function, in that the relay
incorporates separate integrating timers on each
phase.
The inverse time overcurrent function can be voltage
controlled (VC), voltage restrained (VR), or neither.
For voltage-controlled operation, the function is
not active unless the voltage is below the voltage
control setpoint. This philosophy is used to confirm
that the overcurrent is due to system fault. When
applied, most users will set voltage control limits in
the range of 0.7 to 0.9 per unit RMS voltage. When
voltage restraint is selected (See Figure 4-60, Voltage
Restraint (51VR) Characteristic), the pickup setting is
continuously modified in proportion to the collapsing
terminal voltage. The voltage restraint function is
well-suited to small generators with relatively short
time constants.

NOTE
: The 51V function should be blocked by
fuse loss if in the voltage control mode
only. Fuse loss blocking is not desirable
for voltage restraint mode because the
pickup is automatically held at 100%
pickup during fuse loss conditions, and
operation will continue as normal.
The internally derived voltage used to realize the
voltage control or restraint feature depends on
the configured VT configuration and the Delta-Y
Transform setting (see Section 4.2 Setup System).
Table 4-7, Delta/Wye Transformer Voltage-Current
Pairs describes the calculation for the various
system VT configurations.
51V Inverse Time Phase Overcurrent with
Voltage Control/Restraint
Time-overcurrent relays, one per phase, are used
to trip circuits selectively and to time-coordinate with
other up- or downstream relays. For this function,
eight complete series of inverse time tripping
characteristics are included. The same descriptions
and nomenclature which are traditionally used with
electromechanical relays are used in the relay.
Thus, user may choose from four BECO curves
(BEDEF, BEINV, BEVINV, and BEEINV), four IEC
curves (IECI, IECVI, IECEI, and IECLT), and three
IEEE curves (MINV, VINV, EINV.) Within each
family, the operator selects time dial setting and
pickup (tap) setting, just as with electromechanical
relays. Ranges and increments are presented in
Figure 4-61.
The curves available for use are shown in Appendix
D,
Inverse Time Curves
. They cover a range from
1.5 to 20 times the pickup setting. An additional one
cycle time delay should be added to these curves
in order to obtain the relay operating time. Inverse
time curves saturate beyond 20 times pickup. For
currents in excess of 20 times pickup, operating
times are fixed at the 20 time pickup level. The
particular settings will be made by information from
short-circuit fault studies and knowledge of the
coordination requirements with other devices in the
system that respond to time overcurrent.
The pickup of the 51V is set in relay amps.
(Relay amps = primary amps ÷ CT ratio)
Selects one of the time curves as shown in Appendix D,
Inverse Time Curves . The appropriate curve in the selected
family of curves is designated here.
Disable if neither voltage control nor voltage restraint is
desired. If voltage restraint is designated, the tap setting
is modified as shown in Figure 4-60. If voltage control is
designated, the 51V will only operate when the voltage is less
than the 51V voltage control setting specified below. When
applied, the voltage control is usually set in the range of 70%
to 90% of the nominal voltage.
51V PICKUP
Amps
51V CURVE
bedef beinv bevinv

51V TIME DIAL
51V VOLTAGE CONTROL
disable V_CNTL v_rstrnt
51V VOLTAGE CONTROL
Volts