Phase Undercurrent Protection; Undercurrent Protection Scheme Logic - Toshiba GRE110 Instruction Manual

2.4 Phase Undercurrent Protection

The phase undercurrent protection is used to detect a decrease in current caused by a loss of load,
typically motor load.
The undercurrent element operates for current falling through the threshold level. However,
operation is blocked when the current falls below 4 % of CT secondary rating to discriminate the
loss of load from the feeder tripping by other protection.
Each phase has two independent undercurrent elements for tripping and alarming. The elements are
programmable for instantaneous or definite time delayed operation.
The undercurrent element operates on per phase basis, although tripping and alarming is three-
phase only.
The tripping and alarming outputs can be blocked by scheme switches or a binary input signal.
Scheme Logic
Figure 2.4.1 shows the scheme logic of the phase undercurrent protection.
Two undercurrent elements UC1 and UC2 output trip and alarm signals UC1 TRIP and UC2
ALARM through delayed pick-up timers TUC1 and TUC2.
Those protections can be disabled by the scheme switches [UC1EN] and [UC2EN] or binary input
signal UC BLOCK.
71
A
72
B
UC1
73
C
[UC1EN]
+
"ON"
74
A
75
B
UC2
76
C
[UC2EN]
+
"ON"
UC BLOCK
Settings
The table below shows the setting elements necessary for the undercurrent protection and their
setting ranges.
&
&
&
&
&
&
1
Figure 2.4.1 Undercurrent Protection Scheme Logic
 34 
TUC1
t 0
t 0
0
t
0.00 - 300.00s
TUC2
0
t
0
t
0
t
0.00 - 300.00s
6 F 2 T 0 1 7 2
127
UC1-A TRIP
128
UC1-B TRIP
129
UC1-C TRIP
126
≥1
UC1 TRIP
131
UC2-A ALARM
132
UC2-B ALARM
133
UC2-C ALARM
130
≥1
UC2 ALARM