Impedance Protection (ANSI 21)
129
7UM62 Manual
C53000-G1176-C149-3
with its operating time or with a slight time delay (undelayed tripping). A 0.1 s time
delay is preferred.
For
the reach could be set to about 100 % of the transformer reactance, or
in addition to a network impedance. The corresponding
time stage is to be
selected in a way that it overreaches the network protective relays of the following
lines. The
time is the last back-up time.
The following formula is generally valid for the primary impedance (with limiting to the
unit transformer):
with
k
R
Zone reach of zone Z1 in [%]
u
sc
Relative transformer short-circuit voltage [%]
S
N
Rated transformer power [MVA]
U
N
Machine-side rated transformer voltage [kV]
The values determined such must be converted for the secondary side of current and
voltage transformers. In general:
The nominal current of the protective relay (= secondary nominal current of the current
transformer) is automatically considered by the relay. You have already
communicated the transformation ratios of the current and voltage transformers to the
relay by entering the nominal transformer quantities (see Section 2.3).
Example:
Transformer data:
u
sc
= 7 %
S
N
= 5.3 MVA
U
N
= 6.3 kV
Transformation ratios:
Current transformer transformation ratio = 500 A/1 A
This results to a 70 % reach for zone 1:
Z
prim
k
R
100
----------
u
sc
100
----------
U
N
2
S
N
--------
⋅
⋅
=
Z
secondary
CT transformation ratio
VT transformation ratio
-------------------------------------------------------------
Z
primary
⋅
=
6.3 kV
3
-----------------
100 V
3
---------------
⁄
=
VT transformation ratio
Z1
prim
70
100
----------
7
100
----------
6.3
2
5.3
-----------
⋅
⋅
0.3669
Ω
=
=
