ABB REG316 4 Series Operating Instructions Manual page 188

ABB Switzerland Ltd
The impedance to be set on the protection in p.u. is:
Z - Setting =
0 7 . x
or
Z - Setting =
0 7 . x
For simple cases where U
I = I :
N2 N
Z-Setting = 0.7 x
where:
z impedance of the protected zone
1
x short-circuit reactance of the step-up transformer
T
K , K ratios of c.t's and v.t's
i u
I , I c.t. rated currents
N1 N2
U , U v.t. rated voltages
N1 N2
U , I rated voltage and current of the step-up transformer
TN TN
U , I rated voltage and current of the underimpedance
N N
function
The factor of 0.7 avoids any risk of false tripping for a fault on the
HV system at the cost of a zone of protection that is shorter than
differential protection zone.
Example:
Transformer: 100 MVA; 12 kV;
C.t's and v.t's: 12000/100 V;
Protection: 100 V;
I
5000
N 1
K = = = 1000
i
I 5
N2
U 12000
N 1
= =
K
u
U 100
N2
3-150
REG 316*4 1MRB520049-Uen / Rev. F
U K I
TN i N
´ ´
T
I K U
TN u N
U I U
TN N 1 N2
´ ´
T
I I U
TN N2 N 1
= U , I
TN N1
T
4.8 kA;
5000/5 A
5 A
=
120
[1; 1; V; A]
I
N
´
[1; 1; V; A]
U
N
= I , U = U
TN N1 N2
[1; 1]
x = 0.1
T
and
N