so the transmitter s ends a blocking signal. Tripping
does not occur because ZP is open.
At
breaker A, ZP contact clos es, energizing CSP .
CSP contact energizes RRT; however, RRP contact
is held open by RRH current .
Internal Phase Fault
Operation is the same at both stations. Refer to
Table I. Z3 does not operate, since it is set to look
into the external system.
ZP contact close s , ener
gizing CSP . CSP contact closes to apply negative to
the stop lead, preventing blocking signal transmission.
Another CSP contact energizes RRT; since there is
no signal to produce RRH coil current, RRT coil
External Ground Fault Operation
Refer to Table I.
Since the ground overcurrent
unit s
los•
are not directional , they operate at both A&B
to open t heir contacts. At breaker A, 10s operation is
ineffective , since the stop lead is energized by CSG.
However, at breaker B, CSG and the stop lead are
not energized, so that the opening of the 10s contact
results in transmission of a blocking signal from B .
Tripping at breaker B does not occur b ecaus e D0 re
remains open.
At breaker A RRT coil is energized when D0
closes to operate CSG; however, tripping is prevented
by RRH coil current, which holds RRG open.
Internal Ground Fault Operation
Refer to Table I . Op eration i s the same a t both
station s .
D0 closes, energizing CSG.
CSG con
tacts stop blocking s ignal transmission and ener
gize the RRT coil . Since no blocking signal is
received.
RRH is not energize d ; therefore, RRG
closes , and the breaker trip coil is energized through
85 CO contact 3-4, D0 , 10 , RRG and 5 2a contacts .
0
peration
KS relay distance unit, Zo s• is s et to include the
ZP-3
¢
unit R-X diagram circle as shown in Figure 3.
A minimum separation of two secondary ohms is
recommended between the Z0s and ZP-3
¢
unit cir
cles.
This separation provides the means for dis
tinguishing b etween 3-phase faults and out-of-step
conditions.
When a fault occurs on t he protected
line, the impedance seen by the relays changes sud
denly from the prefault value, ZLoad• to the fault
value, represented by the line 0-F in Fig. 3. When
a swing or out-of-step condition occurs t he imped
ance s een by Z0s & ZP changes gradually , as the
voltage decreases and the current increases.
In
Fig. 3 the swing describes an arc which intersects
the Z0s circle at point Q and Z P-3
¢
circle , at point
P .
During an out-of-step condition the Z0 s contact
in figure 2 opens b efore ZP-3
¢
contact closes. OS
unit is energized, and after
4
cycles , an OS con
tact opens the ZP-3
¢
trip-circuit.
All this occurs
b efore the s wing reaches point , P , in Fig. 3.
During a fault, the ZP-3
¢
contact closes almost
at the same instant thaJ; the Z0s contact opens in
Fig. 2.
ZP-3
¢
contact short-circuits the OS coil
to preven
t
OS
from operating. Thus a fault condition
res ults in a nearly simultaneous operation of Z0 s and
ZP-3
¢
; while an out-of-step condition produces a
discrete difference in the operating time of these
two distance units.
In some cases it is preferable to block breaker
reclosing rather than blocking tripping, when an out
of-step condition occurs. A make contact of
OS
shown
in Figure 2, is available for this purpose . otherwise ,
this OS contact may be used for alarm purposes, as
shown by the dotted connection in Figure 2.
R ELAY S E T TINGS
It is essential t hat the local start units, which
initiate blocking signal transmission, operate for
any external fault for which the remote tripping units
also operate; otherwise undesired tripping of the
remote breaker will occur.
Phase
In Fig. 1, breaker A-ZS is s et to reach further
than the breaker B-ZP units by the distance, M-N .
It i s recommended that this distance be at least half
of N-P, as shown in Fig. 1.
Unless ZS is used for
tripping as well as for starting a blocking signal , and
unle s s an undesirably large tripping area would other-
wise result, it is recommended that the ZS setting
be made at least as large as the remote ZP setting.
Ground
Set the remote 10 unit for a pickup at least 25%
higher than the local 10s pickup to insure operation
of the 10 s for any fault which will also result in
10 operation.
W E S T I N G H O U S E E L E C T R I C C O R P O R A T I O N
R E LAY- I N STR U M E NT D IVI S I O N
N EWAR K, N . J.
l'rinted-in U.S.A.
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