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sequence) in the value field, both FDOG and RDOG
units will be polarized by zero sequence voltage 3V
However, both units will be polarized by either zero
sequence voltage 3V
and/or zero sequence current
0
I
(zero sequence current from a power transformer
P
neutral CT) if the DIRU is selected to the value of
DUAL. Both the FDOG and RDOG units will be oper-
ated by negative voltage/current quantities if NSEQ
(negative sequence) is selected in the value field.
6.4.1
Zero sequence polarizing ground direc-
tional FDOG/RDOG units
(a)
Voltage polarizing element
The voltage polarizing directional element is deter-
mined by the angular relationship of the 3I
phasors. Forward direction is identified if 3I
3V
by 30 -210 . F o r reverse direction 3I
o
to 150 or if 3I
leads 3V
o
o
The sensitivity of this element is 3I
3V
> 3.0 volts.
o
(b)
Current Polarizing
The current polarizing directional element is deter-
mined by the angular relationship of the I
phasors, where I
is the ground current from the
P
power transformer neutral CT. The forward direc-
tional unit will have maximum torque when I
are in-phase. The reverse directional unit will have
maximum torque when I
phase.
The sensitivity of this element is 3I
> 0.5 amp.
6.4.2
Negative sequence polarizing ground
directional FDOG/RDOG units
The negative sequence polarizing ground directional
units utilize the negative sequence voltage V
polarizing quantity and negative sequence current I
as the operating quantity. Its operation is determined
by the angular relationship of the V
Maximum sensitivity for the forward direction unit
occurs when V
lags with I
2
The sensitivity of this element is 3V
3I
> 0.5 amp.
2
6.5
FAULT DETECTION UNITS
MDAR (REL-300) detects faults by digital computa-
tion rather than analog.
and 3V
o
leads
o
lags 3V
o
o
by 0 -30 .
> 0.5 amp. and
o
and I
o
and I
o
and I
are 180 out of
o
P
> 0.5 amp. and I
o
as the
2
and I
phasors.
2
2
by 98 .
2
> 1.0 volts and
2
The relay normally operates in a "background mode"
.
where it looks for phase current or phase voltage dis-
0
turbance. Once a phase disturbance is detected, the
relay enters "fault mode" and start the Z-unit compu-
tations. During background mode, the four input cur-
rents (I
, I
, I
and I
A
B
C
and V
) are sampled, at a sampling rate of 8 times
C
per cycle, to test for line faults. The criteria for deter-
mining a disturbance in the MDAR (REL-300) design
is shown below, which comparing the instantaneous
samples taken one cycle apart:
(1) Each phase ∆I — if [I
and [I
- I
Kn
(K-1)n
o
(2) Each phase ∆V — if [V
and [V
- V
Kn
by 0
(3) ∆I
— if [(3I
o
where n = 1, 2, 3, 4, 5, 6, 7, 8
K = K
For every voltage and current disturbance the Gen-
P
eral Start (GS) relay will pickup for 3 cycle for starting
external (fault) recorder.
P
6.6
LOW VOLTAGE UNITS
There are three low voltage units (V
in MDAR (REL-300). Each unit senses the phase
voltage condition in the background mode. The unit
P
can be set from 40 to 60 volts in 1.0 volt steps. For
any phase voltage below its preset value, the LV logic
will produce a logic "1" output signal. The LV-units
are used in CIFT and weakfeed logic in the MDAR
(REL-300) system.
Note:
Three separate fixed low voltage
2
units at 7 volts are used for the
LOP logic.
6.7
BLINDER UNITS
Single blinder 21BI for 3Ø unit reach restriction on
heavy load. Double blinders, 21BI and 21BO, are
included in MDAR (REL-300) system for OS sensing.
The following quantities are used for the blinder
sensing:
) and the three voltages (V
o
- I
] > 1.0 amp.
Kn
(K-1)n
] / I
x 100% > 12.5%
(K-1)n
- V
] > 7.0 volts.
Kn
(K-1)n
] / V
x 100% > 12.5%
(K-1)n
(K-1)n
)
- (3I
)
] > 0.5 amp.
o
Kn
o
(K-1)n
th
. cycle
, V
AL
I.L. 40-385.5
, V
A
B
and V
)
BL
CL
6-3
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