BECKWITH ELECTRIC M-0420, Instruction Book

Page: 36 / 174

2–18
M-0420 Instruction Book
51V Inverse Time Overcurrent, Three-Phase
with Voltage Control or Voltage Restraint
Time-overcurrent relays, one per phase, are basic
to any protection scheme. This is the main device
used to trip circuits selectively and to time-
coordinate them with other up- or downstream
devices. For this function, four complete series
of inverse time tripping characteristics are included,
based on the ABB CO and COV curves. This is
accomplished in a way that allows the protection
engineer to use the same descriptions and
nomenclature which are traditionally used with
electromechanical relays. Thus, the four curve
families to be chosen are definite time, inverse,
very inverse or extremely inverse, which relate
to the degree of slope of the basic characteristic
curves. In the menu, these are abbreviated as
DEF, INV, VINV, and EINV. Within each family,
the operator selects time dial setting and tap setting
through the menu, just as one would through
electromechanical means.
The curves available for use are shown in Figures
2-10 through 2-13. They cover a range of from
1.5 to 20 times the tap setting. An additional one-
cycle time delay should be added to these curves
in order to obtain the relay operating time. For
currents beyond 20 times the tap setting, the relay
operating time will be the same as the time for
20 times the tap setting; i.e., the relay operates
at definite minimum time. 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.
A unique feature of this relay concerns the
calculation of the current used for tripping. The
current is derived from an rms calculation, but,
by user selection, will be computed based on either
inclusion or exclusion of the contribution of
harmonics to the value. Since it is not well
established by the industry whether the calculation
should be based simply upon the fundamental
frequency component or a broader frequency
range, a simple DIP switch setting (No. 8) allows
the operator to select either one.
The inverse time overcurrent function can be
voltage controlled (VC) or voltage restrained (VR).
When voltage restraint is selected, the tap setting
of the 51VR is modified continuously according
to the voltage inputs as shown in Figure 2-14.
The relay continues to operate independently of
current decrement in the machine. The voltage
restraint function is well-suited to small generators
with relatively short time constants. (Voltage
restraint is disabled as shipped from the factory.)
When the generator is connected to the system
through a delta/wye transformer, proper voltages
(equivalent to the high-side of the transformer)
should be used for the 51VR or 51VC element.
The relay can internally determine the equivalent
high-side voltages of the delta/wye unit transformer,
saving auxiliary instrument transformers. The
voltage-current pairs used are shown in Table
2-6.
▲
CAUTION: When 69.3 V is chosen for the
VT secondary voltage, 69.3 V is internally con-
verted to 120 V (1 pu) for all calculations and for
setting and display purposes. For example, if a
pickup of 73 V (1.1 pu) is desired when 69.3 V
has been chosen for the VT secondary voltage,
this will be displayed and entered as 126.4 V (1.1
pu on 120 V base).
100
50
50 25 75 100
Input Voltage (% of rated voltage)
Tap Setting as %
of Tap Setting at
Rated Voltage
25
75
0
0
Figure 2-10 Voltage Restraint (51VR)
Characteristic