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Table 2-10
Transient Machine Reactances (Referred to Secondary Side)
Generator Type
X
Non-salient
0.13..0.35
pole rotor
Salient-pole rotor 0.20..0.45
Table 2-11
Per Unit Impedance Voltages and Impedances of Transformers
Transformer Type
u
SC
Unit transformer 8 %...13 %
General
3 %...16 %
7UM62 Manual
C53000-G1176-C149-3
X
' can be calculated from the per unit reactance x
d
U
N, Gen
---------------------------- - x
X
'
=
d
⋅
3 I
N, Gen
where X
'
– Transient reactance of the machine
d
x
'
– Transient per unit reactance
d
U
– Primary rated machine voltage
N, Gen
I
– Primary rated machine current
N, Gen
ü
– Current transformer ratio
ct
ü
– Voltage transformer ratio
vt
Dependent on the type of generator and on the secondary current, a secondary
voltage U
= 100 V or 120 V leads to approximately the reactance ranges listed in
N
Table 2-10.
X
d
d
U
= 100 V/ I
= 1 A
N
N
7.5 Ω...20.2 Ω
11.5 Ω...26.0 Ω
As it is presupposed that the generator is connected with the network via a unit
transformer, the setting in the network direction is chosen such that the reactance
reach of characteristic 1 is approximately 70 % to 90 % of the transformer impedance,
and the reach of characteristic 2 is into the network. Thus, Zc at address 3506 is set
to 70 % to 90 % of the transformer impedance X
portion of the transformer impedance is set at 3507 Zd - Zc, if necessary
complemented by the impedance of the additional line section to be monitored.
Table 2-11 shows typical values of the impedances of unit transformers X
secondary rated currents I
according to the following equation:
U
SC
X
---------------- -
=
SC prim
⋅
3 I
X
X
=
SC
SC prim
.
X
SC
U
= 100 V/ I
= 1 A
N
N
4.6 Ω...7.5 Ω
1.7 Ω...9.2 Ω
ü
ct
⋅
⋅
------ -
'
d
ü
vt
X
d
U
= 120 V/ I
= 1 A
U
N
N
9.4 Ω..24.3 Ω
13.9 Ω...31.2 Ω
= 1 A and I
= 5 A, the relationship of the values is
N
N
⋅
u
U
u
SC
N
SC
-------------------------------
------------------------- -
=
=
⋅
⋅
100 S
100
3 I
N
N
ü
ct
⋅
------ -
ü
vt
X
SC
U
= 120 V/ I
= 1 A
U
N
N
5.5 Ω...9.0 Ω
2.1 Ω...11.1 Ω
Out-of-Step Protection (ANSI 78)
' as follows:
d
X
d
= 100 V/ I
= 5 A
U
= 120 V/ I
N
N
N
1.5 Ω...4.0 Ω
1.9 Ω...4.9 Ω
2.3 Ω...5.2 Ω
2.8 Ω...6.2 Ω
. For characteristic 2, the remaining
SC
2
⋅
U
N
⋅
N
X
SC
= 100 V/ I
= 5 A
U
= 120 V/ I
N
N
N
0.9 Ω...1.5 Ω
1.1 Ω...1.8 Ω
0.3 Ω...1.8 Ω
0.4 Ω...2.2 Ω
X
d
= 5 A
N
for
SC
X
SC
= 5 A
N
143
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