ABB 615 Series ANSI, Техническое руководство, Страница: 371 / 713

Section 4
1MAC050144-MB C
Protection functions
364 615 series ANSI
Technical Manual
The parameter r is the maximum remanence flux density in the CT core in p.u. from
saturation flux density. The value of the parameter r depends on the magnetic material used
and on the construction of the CT. For instance, if the value of r = 0.4, the remanence flux
density can be 40 percent of the saturation flux density. The manufacturer of the CT has
to be contacted when an accurate value for the parameter r is needed. The value r = 0.4 is
recommended to be used when an accurate value is not available.
The required minimum time-to-saturate Tm in 87T is half fundamental cycle period (10
ms when fn = 50Hz).
Two typical cases are considered for the determination of the sufficient accuracy limit
factor (Fa):
1. A fault occurring at the substation bus:
The protection must be stable at a fault arising during a normal operating situation.
Re-energizing the transformer against a bus fault leads to very high fault currents
and thermal stress and therefore re-energizing is not preferred in this case. Thus, the
remanence can be neglected.
The maximum through-going fault current Ikmax is typically 10 IR for a substation
main transformer. At a short circuit fault close to the supply transformer, the DC
time constant (Tdc) of the fault current is almost the same as that of the transformer,
the typical value being 100 ms
Ik
max
10 IR
T
dc
100 ms
ω 100Ï € Hz
T
m
10 ms
K
r
1
When the values are substituted in
(Equation 45)
, the result is:
2. Re-energizing against a fault occurring further down in the network:
The protection must be stable also during re-energization 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 resis-
tance and reactance is greater in this location, having a full DC offset is not possible.
Furthermore, the DC time constant (Tdc) of the fault current is now smaller,
assumed to be 50 ms here.
Assuming a maximum fault current being 30 percent lower than in the bus fault and
a DC offset 90 percent of the maximum.
Ik
max
0.7* 10 = 7 (I
R
)
T
dc
50 ms
ω 100Ï € Hz
T
m
10 ms
K
r
1/(1-0.4) = 1.6667