44x/EN AP/Hb6
-16 MiCOM P40 Agile
Phase selection
As the superimposed values no longer include the load currents, it is possible to make high-
speed phase selection.
2.2.1 Fault Modelling
Consider a stable network status-the steady-state load flow prior to any start. When a fault
occurs, a new network is established. If there is no other modification, the differences
between the two networks (before and after the fault) are caused by the fault. The network
after the fault is equivalent to the sum of the values of the status before the fault and the
values characteristic of the fault. The fault acts as a source for the latter, and the sources act
as passive impedance in this case.
Relay
Relay
Relay
Relay
R F
R F
R F
Relay
Relay
R F R F
R F
Z
S
Z
L
Z
R
Unfaulted Network (steady state prefault conditions)
V
R
I
R
Z
L
Z
S
Z
L
Z
R
V
R
I
R
Fault Inception
P3033ENa
Z
L
V
F (prefault voltage)
-V
F
Z
L
R
F
R
F
Z
S
Z
L
Z
R
V
R
'
I
R
'
Faulted Network (steady state)
V
R
I
R
=
Voltage at Relay Location
Current at Relay Location
Voltage at Relay Location
Current at Relay Location
Voltage at Relay Location
Current at Relay Location
=
=
V
R
'
=
=
=
I
R
'
V
R
R
I
V
R
I
R
V
R
'
I
R
'
V
R
I
R
Figure 4: Prefault and fault inception value
Network Status Monitoring
The network status is monitored continuously to determine whether the "Deltas" algorithms
can be used. The network must be "healthy". This is characterised by the following:
• The circuit breaker(s) should be closed just prior to fault inception (2 cycles of healthy
pre-fault data should be stored) – the line is energised from one or both ends,
• The source characteristics should not change noticeably (there is no power swing or
out-of-step detected).
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