ABB RELION REX640 Technical Manual page 741

1MRS759142 F
The minimum time to saturate (T
Two typical cases considered for the determination of the sufficient
accuracy limit factor (F
re-energizing against a fault occurring further down in the network.
A fault occurring at the substation bus
The protection must be stable when a fault occurs during a normal operating
situation. Re-energizing the transformer against a bus fault leads to very high fault
currents and thermal stress. Therefore, re-energizing is not preferred in this case.
With this assumption, the remanence can be neglected.
The maximum through-going fault current I
main transformer. At a short circuit fault close to the supply transformer, the
DC time constant (T
transformer, the typical value being 100 ms.
F > K I
a
I
kmax
T
dc
ω
T
m
K
r
Re-energizing against a fault occurring further down in the network
The protection must be stable during the re-energization against a fault on the line.
In this case, the existence of remanence is very probable. In this example, it is 40
percent.
The fault current is now smaller and since the ratio of the resistance to the
reactance is greater in this location, having a full DC offset is not possible.
Furthermore, the DC time constant (T
50 ms.
Assuming a maximum fault current is 30 percent lower than in the bus fault and a
DC offset 90 percent of the maximum.
F > K I
a
I
kmax
T
dc
ω
T
m
K
r
REX640
Technical Manual
) are a fault occurring at the substation bus and
a
) of the fault current is almost the same as that of the
dc
Tm
-
Tdc
(T (1 - e ) + 1) 40
ω
⋅ ⋅ ⋅ ⋅
r kmax dc
10 (pu)
100 (ms)
100π (Hz)
10 (ms)
1
0.9 (T (1 - e
ω
⋅ ⋅ ⋅ ⋅ ⋅
r kmax dc
0.7 · 10 = 7 (pu)
50 (ms)
100π (Hz)
10 (ms)
1/(1-0.4) = 1.6667
) in TR3PTDF is 10 ms.
m
is typically 10 pu for a substation's
kmax
≈
) of the fault current is now smaller, here
dc
Tm
-
Tdc
) + 1) 40
≈
Protection functions
(Equation 175)
(Equation 176)
741