Protection Functions
PRS-7367 145
3.27 Three phase voltage unbalance protection 59NT
3.27.1 59NT Overview
The three phase voltage unbalance protection for shunt capacitor banks function 59NT is used to
protect the Y-connected capacitor banks from internal faults. 59NT is suitable for the protection of
internally fused, externally fused and fuse less applications. 59NT can be enabled or disabled and
it starts when the measured unbalance current exceeds the current startup setting. The operation
time characteristics is definite time (DT).
3.27.1.1 Function Block
U3P
59NT
Blk
BlkStr
BlkOp
Str
Op
Alm
Figure 3.27-1 The function block
3.27.1.2 Signals
Table 3.27-1 Input Signals
The voltage magnitude in all the three phases
This signal blocks all the binary output signals of the function
This signal blocks all the start outputs of the function
This signal blocks all the trip signals of the function.
Table 3.27-2 Output Signals
This is the integrated start signal.
This is the integrated operation signal.
This is the integrated alarm signal.
3.27.2 59NT Protection Principle
A standard Y-connected shunt capacitor bank configuration is shown in following diagram. The
three-phase unbalance voltage is measured.
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