ABB RELION 620 Series Technical Manual page 454

Section 4
Protection functions
448
Fa > K I × (T × ×
ω
r kmax dc
GUID-1343CD99-BC2C-497C-A3C8-FCBB7DAE1231 V1 EN
I the maximum through-going fault current (in p.u.) at which the protection is not allowed to operate
kmax
T the primary DC time constant related to I
dc
ω the angular frequency, i.e. 2 x π x fn
T the time to saturate, that is, the duration of the saturation-free transformation
m
K the remanence factor, 1/(1-r).
r
The parameter r gives the maximum remanence flux density in the CT core. The value of
the parameter r depends on the magnetic material used and on the construction of the CT.
For example, the value r = "0.4" means that the remanence flux density may be 40 percent
of the saturation flux density. The manufacturer of the CT should 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 minimum time to saturate (T
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 occurring
a
further down in the network.
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. Therefore, re-energizing is not preferred in this case.
With this assumption the remanence can be neglected.
The maximum through-going fault current I
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 typical value
dc
being 100 ms.
-Tm / Tdc
(1 - e ) + 1)
kmax
) in 87T is 8.33 ms (when fn = 60 Hz).
m
is typically 10 p.u. for a substation main
kmax
1MAC504801-IB E
(Equation 50)
620 series ANSI
Technical Manual