ABB RELION REX640 Technical Manual page 659

1MRS759142 C
REX640
Technical Manual
Two typical cases considered for the determination of the sufficient accuracy limit
factor (F ) are a fault occurring at the substation bus and re-energizing against a fault
a
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 ) of the fault current is almost the same as that of the transformer, the
dc
typical value being 100 ms.
F > K I (T (1 - e
ω
⋅ ⋅ ⋅ ⋅
a r kmax dc
GUID-88F21523-2502-4500-BE4B-B469634E0A3A V1 EN
I 10 (pu)
kmax
T 100 (ms)
dc
ω 100π (Hz)
T 10 (ms)
m
K 1
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 ) of the fault current is now smaller, here 50 ms.
dc
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
ω
⋅ ⋅ ⋅ ⋅ ⋅
a r kmax dc
GUID-12B651A7-25F3-49A1-95AF-9D949EA951DA V1 EN
I 0.7 ⋅ 10 = 7 (pu)
kmax
T 50 (ms)
dc
ω 100π (Hz)
T 10 (ms)
m
K 1/(1-0.4) = 1.6667
r
is typically 10 pu for a substation's
kmax
Tm
-
Tdc
) + 1) 40
≈
Tm
-
(1 - e Tdc ) + 1) 40
≈
Section 4
Protection functions
(Equation 122)
(Equation 123)
653