Impedance Protection (ANSI 21)
133
7UM62 Manual
C53000-G1176-C149-3
The following relation allows to estimate the rate of change:
Meaning:
X
Reactance between the sources of the power swing
f
p
Power swing frequency
δ
Power swing angle
Figure 2-64 shows an example of how the rate of change evolves as a function of the
power swing angle. The rate of change is smallest at an angle of 180°, but increases
proportionally with the angle considered.
Figure 2-64 Evolution of the Rate of Change (f
p
= 1 Hz; X = 10
Ω
)
For this reason, the setting value dZ/dt must also be coordinated with the impedance
jump occurring at the start of a short-circuit.
To do so, you determine the minimum operating impedance (Z
L, min
), form the differ-
ence to the setting of the impedance zone (e.g. Z1) and calculate the impedance gra-
dient, taking into account the one-cycle measuring interval.
Example:
U
min
= 0.9 U
N
, I
max
= 1.1 I
N
, u
K
= 10 %,
∆
t = 20 ms
U
N
= 100 V, I
N
= 1 A
dZ t
( )
dt
--------------
dR t
( )
dt
--------------
≈
X
π
f
p
2sin
2
π
f
p
t
(
)
-----------------------------
X
π
f
p
2sin
2
δ
2
---
è ø
æ ö
-----------------------
in
Ω
s
----
=
=
50
0
100
150
200
250
300
350
400
50
100
150
200
250
300
δ
Degrees
dZ(
δ
)
Ω
/s
Z1
0 7
,
u
⋅
K
U
N
⋅
100 %
3 I
N
⋅
⋅
---------------------------------------
4 04
Ω
,
=
=
