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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 ZONE Z2 the reach could be set to about 100 % of the transformer reactance, or
in addition to a network impedance. The corresponding ZONE2 T2 time stage is to be
selected in a way that it overreaches the network protective relays of the following
lines. The T END time is the last back-up time.
The following formula is generally valid for the primary impedance (with limiting to the
unit transformer):
Z
=
prim
with
k
Zone reach of zone Z1 in [%]
R
u
Relative transformer short-circuit voltage [%]
sc
S
Rated transformer power [MVA]
N
U
Machine-side rated transformer voltage [kV]
N
The values determined such must be converted for the secondary side of current and
voltage transformers. In general:
Z
secondary
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
= 7 %
sc
S
= 5.3 MVA
N
U
= 6.3 kV
N
Transformation ratios:
Current transformer transformation ratio = 500 A/1 A
VT transformation ratio
This results to a 70 % reach for zone 1:
Z1
prim
2
k
u
U
R
sc
N
⋅
⋅
--------- -
--------- -
------- -
100
100
S
N
CT transformation ratio
------------------------------------------------------------ - Z
=
VT transformation ratio
6.3 kV
⁄
---------------- -
=
3
2
70
7
6.3
⋅
⋅
0.3669 Ω
--------- -
--------- -
---------- -
=
=
100
100
5.3
Impedance Protection (ANSI 21)
⋅
primary
100 V
-------------- -
3
129
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