Toshiba GRL150-100 Series Instruction Manual page 35

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2.6 Phase Undercurrent Protection
Phase undercurrent protection is used to detect a decrease in current caused by a loss of load. Two
independent stages UC1 and UC2 are provided, each with a programmable definite time delay.
The undercurrent element operates for current falling through the threshold level. The operation
can be blocked by UCDO element when the current falls below 4 % of CT secondary rating to
discriminate the loss of load from the feeder tripping by other protection. The UCDO element
output is input by PLC. Figure 2.6.1.1 shows the undercurrent element characteristic.
Setting value
Operating zone
0.04×In
0
0
I
I
In: rated current
(2) UCDO
(1) UC1, 2
Figure 2.6.1.1 Undercurrent Element Characteristic
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.
2.6.1 Scheme Logic
Figure 2.6.1.2 shows the scheme logic of the phase undercurrent protection.
The undercurrent elements UC1 and UC2 output UC1 TRIP and UC2 ALARM through delayed
pick-up timers TUC1 and TUC2.
This protection can be disabled by the scheme switch [UC1EN] and [UC2EN] or binary input
signals UC1 BLOCK and UC2 BLOCK.
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