ABB RELION REX640 Technical Manual page 718

Section 4
Protection functions
712
The required minimum time-to-saturate T
period (10 ms when f = 50 Hz).
n
Two typical cases are considered for the determination of the sufficient actual
accuracy limit factor F :
a
1. A fault occurring at the substation bus.
The protection must be stable at a fault arising during a normal operating
situation. The reenergizing of the transformer against a bus fault leads to very
high fault currents and thermal stress. Therefore, reenergizing is not preferred in
this case. The remanence can be neglected.
The maximum through-going fault current Ik
a short circuit fault close to the supply transformer, the DC time constant T
the fault current is almost the same as that of the transformer, the typical value
being 100 ms.
Ik = 6 I
max R
T = 100 ms
dc
ω = 100π Hz
T = 10 ms
m
K = 1
r
Equation 162 with these values gives the result:
F K Ik ( T ω 1
> ⋅ ⋅ ⋅ ⋅ −
a r max dc
GUID-5FE0CBCD-06FF-4A0B-A610-EA17E9540F86 V1 EN
2. Reenergizing against a fault occurring further down in the network.
The protection must be stable also during reenergization against a fault on the
line. In this case, the existence of remanence is very probable. It is assumed to be
40 percent here.
On the other hand, the fault current is now smaller and since the ratio of the
resistance and reactance is greater in this location, having a full DC offset is not
possible. Furthermore, the DC time constant (T
smaller, assumed to be 50 ms here.
Assuming the maximum fault current is 30 percent lower than in the bus fault and
a DC offset 90 percent of the maximum.
Ik = 0.7 × 6 = 4.2 (I
max
T = 50 ms
dc
ω = 100π Hz
T = 10 ms
m
in MPDIF is half-fundamental cycle
m
T
−
m
T
( e ) 1 ) 24
dc
+ ≈
)
R
1MRS759142 C
is typically 6 I for a motor. At
max R
(Equation 163)
) of the fault current is now
dc
Technical Manual
of
dc
REX640