Westinghouse KRD-4, Installation, Operation, & Maintenance Instruction

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DUAL POLAR I Z E D GROUND DI R ECTI ONAL RELAY
unit, and does not p ermit the transistor 3Q1 to turn
on. For fault current in the trip direction, the direc­
tional unit op erates and removes this short circuit,
allowing the overcurrent unit to produce an output
simultaneously with the directional unit output.
C H A R A C T E R I S T I C S
Directional Unit
1 . As a voltage polarized directional unit, "m axi­
mum sensitivity" o ccurs with current l agging
voltage by 60° + 6°.
As a c urrent pol arized directional unit, "maxi­
mum s ensitivity " occurs with operating and
polarizing currents in p hase ± 6°.
Overcurrent Unit
2. The SRGU relays are available in the following
current r anges:
RANGE
SCALE MARKING
. 5- 2 AMPS. . 5 .
7
5
1 . 0 1 . 5 2.0
1 - 4 AMPS. 1 . 0 1 . 5 2 . 0 3 . 0 4.0
2 - 8 AMP S.
2 3 4 6
8
The setting is the minimum current required to
obtain a 20 volts de output. For pick up s ettings
between calibrated markings refer to the section
under "Setting " . The SRGU relay is designed
for dual polarizing and c an be polarized from a
zero s equence potential source, a zero sequenc e
current source or from both simultaneously.
Time Curves :
The time curves for SRGU relay are shown i n
Fig. 5, 6, and
7 .
Figure 5 includes directional unit
operating time for current, voltage or dual polarized.
Fig. 6 i s overcurrent unit op erating time, and
Fig.
7
is overcurrent unit reset time.
Operating Time:
The tuned circuits in the directional unit imp art
a slight inverse time characteristic at low energy
l evel s and cause a longer op erating time at the
minimum pick up .
Reset Time:
The tuned circuits in the directional unit also
del ays the re set time, and the amount of delay de-
p ends on the initial energi zation ( the max. reset
time is 30 ms).
Current Reversal Reset Ti me:
When a fault c urrent reversal occurs, the relay
is reversed and the directional unit resets in , 1 2 ms
or l ess.
Both the dire ctional unit and overcurrent unit
must operate before a trip output c an be obtained.
Hence, the unit which tak e s the longer time to op er­
ate determines w hen an output will occur. The over­
current unit c annot operate until the short circuit in
its input circuit is r emoved by the op eration of the
directional unit. Once the directional unit op erates,
the overcurrent unit will op erate instantaneously.
If the op erating and polarizing quantities are
more than the minimum required the directional unit
op erates, but because of the s etting, the o vercurrent
unit may not op erate and therefore the o verall outp ut
will be zero.
If under these conditions, a c urrent reversal
happ ens, the dire ctional unit reset s a maximum of
1 2 m s to short circuit the overcurrent unit, but the
overcurrent unit m ay op erat e in l ess than 1 2 ms, and
hence an ill egitimate 20 volts output will app ear
across the varicon terminals 5 and 3.
Reset Time:
Although the reset ( de-energization) of the direc­
tional unit is longer than overcurrent unit. The relay
reset time is completely independent of directional
unit reset time , and it only depends on overcurrent
unit reset time. Therefore, the curve s hown for ov er­
current unit reset time is the same for the relay reset
time.
Setting (Overcurrent Unit)
The operating l evel of the r elay is s el ected by
adj usting the rheostat 3S 1 in the front of the p anel .
Settings in between the scal e m arking can b e made
by applying the desired current and adjusting the
rheo stat until an output is obtained.
Di rectional Unit
No setting is required.
A D J U S T M E N T A N D M A I N T E N A N C E
The prop er adj ustments to insure correct opera­
tion of this rel ay have been m ade at the factory.
5
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