ABB REG316 4 Series - Page 188

ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-150
The impedance to be set on the protection in p.u. is:
Z - Setting =´´07.x
U
I
K
K
I
U
T
TN
TN
i
u
N
N
[1; 1; V; A]
or
Z - Setting =´´´07
1
1
.x
U
I
I
I
U
U
I
U
T
TN
TN
N
N2
N2
N
N
N
[1; 1; V; A]
For simple cases where U
TN
= U
N1
, I
TN
= I
N1
, U
N2
= U
N
and
I
N2
= I
N
:
Z-Setting = 0.7 x
T
[1; 1]
where:
z
1
impedance of the protected zone
x
T
short-circuit reactance of the step-up transformer
K
i
, K
u
ratios of c.t’s and v.t’s
I
N1
, I
N2
c.t. rated currents
U
N1
, U
N2
v.t. rated voltages
U
TN
, I
TN
rated voltage and current of the step-up transformer
U
N
, I
N
rated voltage and current of the underimpedance
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; 4.8 kA; x
T
= 0.1
C.t’s and v.t’s: 12000/100 V; 5000/5 A
Protection: 100 V; 5 A
K
I
I
i
N
N2
== =
1
5000
5
1000
K
U
U
u
N
N2
== =
1
12000
100
120