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2.3 PROTECTION SETTINGS
The IPBKFACTOR controls the amount of positive sequence current restraint that is used in the IPB function.
IPBKFACTOR can be set to 0 or 0.066. Please refer to the description for Protection Setting 504:
IPB_PICKUP to determine the IPBKFACTOR setting to be used.
506: NT_OFFSET: NT/NB Offset
The NT/NB functions use an offset to provide a reliable polarizing quantity for the conditions shown in Figure
2–9: NT/NB FUNCTIONS.
2
In this system, the voltage ( V ) at the relay is in the proper direction to provide correct directional sensing, but,
depending on the source to line impedance ration (Zs/ZL), the magnitude of the voltage may not be sufficient to
provide a reliable polarizing quantity. The problem is most onerous on a long transmission line with a strong
source impedance behind the relay (Zs/ZL is quite small). The effect of the offset in this case is to add a com-
ponent to the actual voltage so as to replicate the voltage at a point some distance down the transmission line.
NT_OFFSET can be determined as follows:
NT_OFFSET = (0.005)·( Z
where Z
= positive sequence impedance of protected line (in secondary ohms)
1
I
= Magnetics Module CT rating (1A or 5A)
n
If the calculated value is greater than 0.05, set NT_OFFSET = 0.05. Otherwise, use the calculated value and
make the next lowest available setting. For example, if the calculated setting is 0.027, set NT_OFFSET = 0.02.
507: UNBALALARM - Unbalanced Current Alarm
The LPS-D is supervised by a fault detector (FD) that uses the following operating quantity:
I ∆
FD
=
I
+
+
op
1
2
Tripping will not be permitted unless there is a concurrent output from the fault detector, therefore an increase
in security is obtained. The LPS-D monitors the output of the fault detector – if it produces an output that lasts
for more than 60 seconds, then it is an indication that:
1. there is an unbalance in the current on the system as indicated by the presence of negative-sequence cur-
rent ( I
) and/or zero-sequence current ( I
2
2. the fault detector has failed (unlikely).
If either of these conditions is detected, then an UNBALARM signal is produced. This signal can be routed via
the Xpression Builder program to drive one of the programmable outputs, or it may be used as an input to one
of the programmable outputs that may already have been selected for alarm duty.
2-
26
)·( I
)
1
n
Zs
Xc > Zs
V
V
Figure 2–9: NT/NB FUNCTIONS
I
0
), or
0
LPS-D Line Protection System
Xc
ZL
F
Vf
2 CALCULATION OF SETTINGS
GE Power Management
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