Siemens siprotec 7SA6 User Manual page 297

Teleprotection with
Earth Fault
Protection
7SA6 Manual
C53000-G1176-C133-1
The position of the directional characteristic is determined with the setting parameters
'LU $/3+$ and 'LU %(7$ (addresses  und ). As these set values are
not critical, the pre-settings may be left unchanged. This setting can only be changed
®
with DIGSI 4 under "Additional Settings".
Finally, the threshold values of the polarizing signals must be set. 8! (address
) determines the minimum operating voltage for direction determination with U
If U is not used for the direction determination, this setting is of no consequence. The
E
set threshold should not be exceeded by unsymmetries in the operational measured
voltage. U is the sum of the phase voltages, i.o.w.
E
U = U + U + U
E L1 L2 L3
If the voltage dependent characteristic (U
reasonable for the minimum polarizing voltage to use a value that is equal to or below
the minimum voltage of the voltage-controlled characteristic (address ).
Only if the connection of the fourth current transformer , WUDQVIRUPHU (address
) = ,< VWDUSRLQW is registered in the 3RZHU 6\VWHP 'DWD  36\VWHP
'DWD  (see 6.1.1), the address  ,<! will appear. It is the lower threshold for
the current measured in the starpoint of a source transformer. A relatively sensitive
setting can be applied for this value, as the measurement of the starpoint current is
quite accurate by nature.
If the direction determination must be done with the negative sequence system
signals, the setting values 8! (address ) and ,! (address ) are
decisive for the lower limit of the direction determination. The setting values must in
this case also be selected such that operational unsymmetry in the system does not
lead to a pick-up.
The earth fault protection in the 7SA6 may be expanded to a directional comparison
protection using the integrated teleprotection logic. Additional information regarding
the available teleprotection schemes and their mode of operation may be obtained
from Section 6.6. If this is to be used, certain preconditions must already be observed
when setting the associated earth current stage.
Initially it must be determined which stage must operate in conjunction with the
teleprotection. This stage must be set directional in the forward direction. If for
>–stage should operate as directional comparison, the address 
example the 3I
0
2S PRGH ,! is set to )RUZDUG (refer to "Definite Time Stages" above, page 6-
110).
Furthermore, the device must be informed that the applicable stage has to function
together with the teleprotection to allow undelayed release of the tripping during
internal faults. For the 3I
set to <HV. The time delay set for this stage 7 ,! (address ) then functions
as a back-up stage, e.g. during failure of the signal transmission. For the remaining
stages the corresponding setting parameter is set to 1R, therefore, in this example:
address  ,!! 7HOHS%, for the 3I
7HOHS%, for the 3I >>>–stage, address  ,S 7HOHS%, for the 3I
0
(if this is used).
If the echo function is used in conjunction with the teleprotection scheme, or if the
weak-infeed tripping function should be used, the additional teleprotection stage
,R0LQ 7HOHSURW (address $) must be set to avoid non-selective tripping
during through-fault earth current measurement. This setting can only be changed
®
with DIGSI 4 under "Additional Settings". Further comments are given in Sub-
section 6.6.2 under the margin heading "Earth Fault Protection Prerequisites".
= 3·U
0
inverse) is used as directional stage, it is
0
>–stage this means that address  ,S 7HOHS%, is
0
>>–stage, address  ,!!!
0
Functions
.
E
–stage
0P
6-115