The minimum time to saturate (T
m
) in TR3PTDF is 10 ms.
Two typical cases considered for the determination of the sufficient
accuracy limit factor (F
a
) are a fault occurring at the substation bus and
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
kmax
is typically 10 pu for a substation's
main transformer. At a short circuit fault close to the supply transformer, the
DC time constant (T
dc
) of the fault current is almost the same as that of the
transformer, the typical value being 100 ms.
F > K I (T (1 - e ) + 1) 40
a r kmax dc
-
Tm
⋅ ⋅ ⋅ ⋅ ≈
ω
Tdc
(Equation 175)
I
kmax
10 (pu)
T
dc
100 (ms)
ω 100π (Hz)
T
m
10 (ms)
K
r
1
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
dc
) of the fault current is now smaller, here
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 0.9 (T (1 - e ) + 1) 40
a r kmax dc
-
Tm
⋅ ⋅ ⋅ ⋅ ⋅ ≈
ω
Tdc
(Equation 176)
I
kmax
0.7 · 10 = 7 (pu)
T
dc
50 (ms)
ω 100π (Hz)
T
m
10 (ms)
K
r
1/(1-0.4) = 1.6667
1MRS759142 F Protection functions
REX640
Technical Manual
741
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