Siemens siprotec 7SA6 User Manual page 192

Functions
Rated Frequency
System Starpoint
6-10
Ratio of earth current transformer
⁄
----------------------------------------------------------------------------------------------------- -
I I =
4 ph CT
Ratio of phase current transformers
This is independent on whether the device has a normal measured current input for
I or a sensitive measured current input for I
4
non-earthed systems)
Example:
Phase current transformers 500 A/5 A
Cable core balance current transformer 60 A/1 A
60 1 ⁄
⁄
---------------- -
I I =
500 5 ⁄
4 ph CT
• Connection of the I
4
compensation of the distance protection and/or fault location function, refer to
Appendix A, Figure A-22):
Address  is then set to: , WUDQVIRUPHU = ,Q SDUDO OLQH and
usually address  is set to ,,SK &7 = 1.
If the set of current transformers on the parallel line however has a different ratio to
those on the protected line, this must be taken into account in address :
Address  is then set so that: , WUDQVIRUPHU = ,Q SDUDO OLQH and
Address  so that ,,SK &7 = I
Example:
Current transformers on protected feeder 1200 A
Current transformers on parallel feeder 1500 A
1500
⁄
------------ - 1.250
I I = =
4 ph
1200
• Connection of the I
4
connection can be used for the polarisation of the directional earth fault protection
(refer to Appendix A, Figure A-23):
Address  is then set to: , WUDQVIRUPHU = ,< VWDUSRLQW, and
address  ,,SK &7 according to the ratio of the transformation ratios of the
current transformer in the transformer neutral and the set of current transformers on
the protected feeder.
• If the I
input is not required, the following settings are applied:
4
Address  , WUDQVIRUPHU = 1RW FRQQHFWHG,
Address  ,,SK &7 is then irrelevant.
In this case the zero sequence current for the protection functions is computed by
means of the sum of the phase currents.
Address  5DWHG )UHTXHQF\ corresponds to the frequency at which the power
system operates. The preset value is dependent on the model number of the relay
purchased, and must be in accordance with the nominal frequency of the power
system.
The system starpoint must be observed for the correct processing of earth faults and
double earth faults. Therefore address  6\VWHP6WDUSRLQW 6ROLG (DUWKHG
or 3HWHUVRQ&RLO HDUWKHG or ,VRODWHG must be set correspondingly. For low-
resistant earthed systems set HDUWKHG
0.600
=
input to the earth current of the parallel line (for parallel line
N paral. line
input to the neutral current of a power transformer; this
(for sensitive earth fault detection in
4
/I .
N prot. line
C53000-G1176-C133-1
7SA6 Manual