Table of Contents
Advertisement
2 CALCULATION OF SETTINGS
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, then 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-O system is supervised by a fault detector (FD) that uses the following operating quantity:
I ∆
FD
=
+
I
+
I
op
1
2
0
Tripping will not be permitted unless there is a concurrent output from the fault detector, resulting in an
increase of security. The LPS-O system 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 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.
601: 50 - Phase Instantaneous Overcurrent
The 50 function can be used in many applications to provide high speed, direct tripping for heavy close-in
faults. To use the phase instantaneous overcurrent trip function, set 50 = YES, otherwise set 50 = NO. See
Protection Settings 602 and 603 for further discussion on use of 50 function. Also, Protection Setting 201:
Z2PHASE must be set to YES.
GE Power Management
· I
1
n
Xc
Zs
Xc > Zs
V
V
Figure 2–6: NT/NB FUNCTIONS
), or
0
LPS-O Line Protection System
2.3 PROTECTION SETTINGS
ZL
F
Vf
2
2.3.7 OVERCURRNT
2-
15
Advertisement
Table of Contents
