ABB REG316 4 Series Operating Instructions Manual page 169

REG 316*4 1MRB520049-Uen / Rev. F
When designing the scheme, the maximum current flowing
through the primaries of the v.t's during an E/F is determined
first.
Assuming that the permissible short-time primary current of the
v.t's is 5 A, then
I = 15 A
Emax
The corresponding minimum value for the grounding resistor is
æ
U
GN
³ ç
R
e min
è
3 I
max
where K is the influence of the v.t. reactance. A mean value of
0.7 may be assumed for v.t's.
To ensure that the E/F protection remains stable for an E/F on
the HV side of the step-up transformer, the resistor may not be
less than
0 05 . U
GN
£
R
e max
w
6 C U
12
The secondary current I
voltage calculated:
I
U = U E
2n 1n
3 I
e
The maximum voltage across R
U = R I
e e
R e
and the voltage across the broken delta windings 3U
voltage must be approximately 30 % higher than the voltage
R I across the resistor so that the design current I
e e
2
ö
3 U
2 n
÷ ´
K
ø
U
1 n
2
æ ö
3 U
2 n
ç ÷
è U ø
HV 1 n
is then chosen and the secondary rated
e
becomes
e
ABB Switzerland Ltd
. This
2n
can flow.
E
3-131