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Section 7. Protection
Phase-loss relay
The phase-loss relay function has the following
characteristics:
• Pickup is at 8–50% of the nominal voltage,
adjustable in increments of 1%
• Time delay is 0.5–600 seconds, adjustable in
increments of 0.5 seconds
• Pickup accuracy is ±4%
• Trip-time accuracy is ±0.1 seconds
• Drops out with the cooling circuit if the imbalance
drops below 97% of the setting
• May be blocked by enabling blocking voltage,
fixed at 5%
• May be disabled by Flex Logic controls
The phase-loss relay works by comparing the nomi-
nal voltage to the calculated negative-sequence
voltage for the fundamental voltage waveform. The
phase-loss relay function works for circuit breaker
with the voltage signal assigned to that circuit
breaker. The voltage input may change from
topology to topology as the PT assignment to buses
changes. The negative phase-sequence voltage is
used whether the PT configuration is delta or wye.
The negative-sequence voltage is only calculated for
the fundamental component. In addition, the relay
may have a blocking-voltage enabled. The blocking
voltage is fixed at 5%. If the negative-sequence
voltage falls below 5% with blocking enabled, the
relay stops timing and resets without cooling.
Negative phase-sequence voltage is calculated
as follows:
1
V
⁄
V
V
=
(
+ 1<240°
+ 1<240°
3
N
ab
bc
1
V
⁄
V
V
=
(
+ 1<240°
+ 1<240°
3
N
ab
ca
If the unbalance decreases to 97% or less of the
programmed threshold, the relay stops timing and
the accumulated count decrements according to
a cooling algorithm.
Underfrequency relay
The underfrequency relay function has the
following characteristics:
• Pickup (60 Hz systems) is at 50–60 Hz in 0.1 Hz
increments
• Pickup (50 Hz systems) is at 45–50 Hz in 0.1 Hz
increments
• Time delay is at 0.5–600 seconds, adjustable in
• Pickup accuracy is ±0.1 Hz
• Trip-time accuracy is ±0.1 seconds
• May be blocked by enabling the blocking voltage,
• May be disabled by Flex Logic controls
The underfrequency relay may be enabled for each
circuit breaker and works with the voltage signal
assigned to that circuit breaker for each different
topology. The underfrequency relay drops out of
pickup and stops timing if the frequency exceeds
103% of the pickup setting. A cooling algorithm
decrements the accumulator. If the sensed voltage
reaches the blocking level of 5% of nominal voltage,
the relay stops timing and is immediately reset.
Overfrequency relay
The overfrequency relay function has the
following characteristics:
• Pickup (60 Hz systems) is at 60–70 Hz in 0.1 Hz
• Pickup (50 Hz systems) is at 50–70 Hz in 0.1 Hz
• Time delay is 0.5–600 seconds, adjustable in
• Pickup accuracy is ±0.1 Hz
• Trip-time accuracy is ±0.1 seconds
• May be blocked by enabling the blocking voltage,
• May be disabled by Flex Logic controls
V
)
for ABC rotation
ca
V
The overfrequency relay may be enabled for each
)
for ACB rotation
bc
circuit breaker and works with the voltage signal
assigned to that circuit breaker for each different
topology. The relay drops out of pickup and stops
timing if the frequency drops below 97% of pickup.
A cooling algorithm decrements the accumulator.
If the sensed voltage reaches the blocking level of
5% of nominal voltage, the relay stops timing and
is immediately reset.
Synch check relay
The synch check relay function may provide up to
12 synchronism supervisory functions. Unlike the
other relays, which use voltage as an input, the
PTs assigned to the synch check relay do not
change with topology changes and thus the two
increments of 0.5 seconds
fixed at 5%. Blocking voltage accuracy varies
with frequency variation from ±2% at 0 Hz to
±15% at 10 Hz
increments
increments
increments of 0.5 seconds
which is fixed at 5% of the nominal voltage.
Blocking voltage accuracy varies with frequency
variation from ±2% at 0 HZ to ±15% at 10 Hz
39
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