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SIF5600 - Manual - 03 - 2008
If several protective devices are connected to the block input circuit of the same upstream protec-
tive device, then it is appropriate that the output of control impulses be enabled on a single protective
device (in the example shown, the enabling of Pulse Blout must be set on one of the two protective
devices B or
C).
Operation of the accelerated logic systems and the consequent selective elimination of faults as-
sume the correct opening of the automatic breakers associated with the digital protection relays as
well as the integrity of the pilot wires connecting said relays.
T
he SIF5600 relay has several functions for correcting the anomalies which might arise in accelerat-
ed logic systems; the complete description, including the terminology and time diagrams, is reported
in the paragraph pertaining to accelerated logic.
Breaker failure
In the event of a breaker failure , in the absence of any suitable measures, traditional interlock sys-
tems bring about the blockage of all the digital protection relays located upstream of the breaker, and
consequently, the fault would not be eliminated.
By using the SIF5600 relay in an accelerated logic system, the problem of a breaker failure may be
resolved through the use of a tF block output signal backup timer. In order to avoid all downstream
relays being persistently blocked by the blocking signals originating from the upstream relays (the
only relay free to intervene acts on the faulty breaker), a timer on the (tF) block output circuit has
been
envisaged. This timer also has the role of keeping the upstream protective devices blocked,
even after the fault has been eliminated by the correct opening of the breaker, so as to ensure that
the downstream protective devices do not intervene inappropriately due to a greater reset time with
respect to that of the upstream protective devices which must eliminate the fault (whereby loss of
selectivity would be obtained).
C
ounting of the tF time starts from the instant when the protective device using the output block
function stars, and continues for the entire duration, the output block signal is in any case kept active
(hence allowing the upstream protective devices to be kept in the blocked state until they are reset);
upon reaching the end of the tF time, the output block signal is in any case deactivated (hence allow-
ing the elimination of the fault by the upstream protective devices if there is a breaker failure).
T
he tF time of any given relay must be set according to the following criterion:
tF = t + T
AP
+ t
rip
+ e
t
+ e
s
wherein t represents the greater between the protective function intervention times in relation to
the relay under consideration (both against short-circuiting between the phases and against earth
faults), TAP
is the time required for opening and arc quenching for the breaker associated with the
relay under consideration, trip is the greater between the reset times for all the accelerated logic
protective devices, e
t
is a possible margin of chronometric selectivity with respect to the tF time of
the relay downstream to the one under consideration, es is a safety margin for including the errors
due to the tolerances on the times under consideration.
T
he chronometric selectivity applied between the tF times of the accelerated logic relays allows, for
a breaker failure, the opening of just the breaker immediately upstream (thus avoiding the opening
of more upstream breakers).
T
he delays applied in order to allow opening of the upstream breakers must in any case ensure
selectivity with possible protective devices upstream of those in relation to the accelerated logic
system.
Short circuiting of pilot wires
Short circuiting of a pilot wire causes the blocking of the protective device receiving the signal,
whereby any potential faults (contemporary or subsequent) present in the protected section of the
system, may not be eliminated since the protective device is blocked.
In order to prevent malfunctioning, a block circuit activation timer tB is envisaged.
At the end of the count of the set time tB, (initiated by the incoming block signal), the SIF5600 relay
ignores the block signal, enabling intervention of the protection functions, causing the de-energisa-
tion of any output relay intended for the Self-test function.
For any given relay using the input block function, the time tB must hence be adjusted in accordance
with the following criterion:
tB = t
Fv
+ e
t
+ e
s
wherein tFv is the value of the block output signal backup timer tF in relation to the relay downstream
of the one under consideration, et is a chronometric selectivity margin to be applied with respect to
time tB in relation to the relay downstream of the one under consideration (not to be taken into con-
sideration in the event that said margin has been considered for the adjustment of time tF as previ-
ously indicated), es
is a safety margin. The chronometric selectivity applied between the times tB of
the accelerated logic relay allows avoiding the simultaneous opening of several breakers following
the elimination of a fault in a section of the system happening at the same time as the short-circuiting
of the pilot wire pertaining to the same section.
In any case, the delays applied in order to allow opening of the upstream breakers must ensure
selectivity with possible protective devices upstream of those in relation to the accelerated logic
system.
Pilot wire breaking
Pilot wire breakage causes loss of protection system selectivity.
Protective systems with block inputs connected to broken twisted pair cables, since not being able
to receive the block signals from downstream protective devices, may intervene inappropriately on
faults which should be detected by relays positioned downstream.
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