BECKWITH ELECTRIC M-0420, Instruction Book

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M-0420 Instruction Book
function of the relay detects the value of the
instantaneously sampled wave, it can act in
situations where a relay reacting to rms voltage
would not. The setpoint range is from 1.05 to 1.50
per unit, with a time delay variable from 1 to 8160
cycles.
Utility-Side Ground Fault Protection and
Phase Voltage Unbalance Protection (27N,
59N)
The relay provides both rms undervoltage and
rms overvoltage functions for the neutral circuit
(27N and 59N). Used together with one VT con-
nected from any one phase to ground, the 27N
and 59N devices are an effective means of de-
tecting the most common line-to-ground fault. A
fault on the phase that includes the VT will pull
that phase voltage low and initiate operation of
the 27N function. A fault on either phase without
the VT will result in
S
3 x normal voltage appear-
ing at the VT, initiating operation of the 59N.
The 59N may also be used to detect system phase
voltage unbalance conditions in conjunction with
three VTs. To do so, the VT secondaries are con-
nected in broken delta and the 59N device in-
serted. In this case, voltage at the 59N will be
zero so long as the three-phase voltages are
balanced, but will rise above zero with any un-
balanced condition, as will occur with a utility line
or ground fault.
Implementation of the 27N and 59N functions pro-
vides two setpoints each with a range setting of
10 to 200 V and a time delay of 1 to 8160 cycles.
Phase Fault Protection (51V, 67)
Time overcurrent relays (51), one per phase, are
basic to any protection scheme. This is the main
device used to trip circuits selectively and time-
coordinate them with other up- or downstream
devices. Four complete series of inverse time trip-
ping characteristics from Definite Time to Extremely
Inverse (based on the ABB CO and COV curves)
are included in the relay.
For intertie protection applications, the optional
phase directional overcurrent function (67) allows
greater selectivity for utility system faults, since
the directional element can be set to look toward
the utility system. As a generator protection re-
lay, the 67 function, looking into the generator,
can provide protection against accidental energi-
zation of the generator on turning gear. When
line-side CT inputs are used, this function can
also provide high-speed protection against gen-
erator winding faults. However, with line-side CTs,
the protection against winding faults during off-
line operation (when the breaker is open) is not
available from the 51V, 50, 46, or 67 functions.
Calculation of the current used for tripping is a
unique feature of this relay. The current is derived
from an RMS calculation, but (by user selection)
can also be computed based on either including
or not including 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 switch
setting allows the operator to select either one.
Directional Power (32F, 32R)
Implementation of the directional power function
is straightforward, issuing a trip command when
the magnitude of the power flow (in either direc-
tion as selected) exceeds the setpoint for the pre-
scribed time. However, as opposed to reverse
power relays that rely on zero crossings for phase
angle information, the relay power calculation uses
the fundamental frequency phasor measurements
obtained from the DFT, and thus is immune to
harmonics in voltage and current signals. The
setpoints for either direction range from 0.02 to
3.0 pu, with time delays from 1 to 8160 cycles.
Anti-Motoring Protection (32R)
When the energy supply to the prime mover is
cut off while the generator is still on line, the gen-
erator will act as a synchronous motor driving
the prime mover. A reverse power relay is used
to detect this motoring. Because the power re-
quired to motor is a function of the type of prime
mover, the required sensitivity of the reverse power
relay can vary widely for different applications.
The reverse power function provides 2% sensi-
tivity, making it applicable to many anti-motoring
situations. A time delay of up to 8160 cycles can
be set to avoid tripping during power swings.
Ground Fault Protection (59N, 51N, 50N)
Detection of ground faults in a high-impedance-
grounded unit generator can be accomplished by
an overvoltage relay in the generator neutral. For
good fault sensitivity, the pickup of the relay should
be about 10 to 16 V. However, because the third
harmonics which normally flow in the system and