Sel 411L - Figure 3.77 Fault Location on Three-Terminal Lines

P.3.128
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
Fault Location
However, when the quality of data alignment is poor, such as when the
channel-based 87L synchronization mode uses a slightly asymmetrical
channel, the differential current may be a worse source of polarization than the
local negative-sequence current. Therefore, the relay monitors the quality of
data synchronization and falls back to polarization with the local negative-
sequence current (i.e., reverts to the single-ended method) if either of the
following occur.
➤ The angular difference between the local aligned negative-
sequence current and the negative-sequence component in the
differential current is greater than 7 degrees, suggesting that
the data alignment quality may be sufficient for protection but
not for fault location.
➤ A high-accuracy IRIG-B timing signal is unavailable for the
external-time-based 87L synchronization mode.
Also, if the relay works in the outstation mode, it reverts to the single-ended
method; it cannot calculate the differential current and use it for polarization
of the fault locator.
The multi-ended method of Equation 3.68 delivers good results even in non-
homogeneous networks, as long as the quality of the 87L data synchronization
is high. Upon the fallback resulting from poor or suspected 87L
synchronization, the single-ended method of Equation 3.67 delivers good
results as long as the network is relatively homogeneous. Only in non-
homogeneous networks and under poor 87L synchronization would the relay
algorithm show increased fault location errors.
In three-terminal line applications, the fault location algorithms at each of the
relays assumes the fault to be located on the line section adjacent to its line
terminal. Refer to Figure 3.77. The T1 relay assumes the fault to be on the
T1-T section, the T2 relay assumes the fault to be on the T2-T section, and the
T3 relay assumes the fault to be on the T3-T section.
Figure 3.77 Fault Location on Three-Terminal Lines
All relays use the same differential current for polarization in Equation 3.68.
Each relay uses its own local voltage, local current, and the line section
impedance when running Equation 3.68. For example, the T1 relay runs the
following fault location equation for the case of Figure 3.77.
Equation 3.71
I
T3
I
T2
I
T1
Z
T3
V
T3
Z
T1
V
T1
Z
T2
V
T2
T2T1
T3
T
m
1
Im V
T1
I
DIF
*• 
Im I
T1
Z
T1
I
DIF
*• • 
--------------------------------------------------------=