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Functions
2.15 Sensitive Earth Flt.(comp/ isol. starp.)
In resonant-earthed networks directional determination is made more difficult since a much larger reactive
current (capacitive or inductive) is superimposed on the critical wattmetric (active) current. Therefore, depend-
ing on the system configuration and the position of the arc-suppression coil, the total earth current supplied to
the device may vary considerably in its values concerning magnitude and phase angle. The relay, however,
must evaluate only the active component of the earth fault current, the earth fault residual current, that is
I
·cosϕ. This demands extremely high accuracy, particularly with regard to phase angle measurement of all the
E
instrument transformers. Furthermore, the device must not be set to operate too sensitive. When applying this
function in resonant-earthed systems, a reliable direction determination can only be achieved when connecting
cable core balance current transformers. Here, too, the following rule of thumb applies: set the value to half the
expected measured current, whereby only the residual wattmetric current is applicable. Residual wattmetric
current predominantly derives from losses of the arc-suppression coil.
Example: The same network, as in the previous example, is considered to be compensated by a arc-suppres-
sion coil. The coil is matched to the total network. The compensation current is thus 62.5 A. The losses should
be 4 %. For earth fault directional determination, core balance current transformers 60 A/1 A are fitted.
Since the residual wattmetric current is derived principally from the coil losses, it is, independent of earth fault
location, approximately the same:
4 % of 62.5 A = 2.5 A or secondary
2.5 A/60 A = 0.042 A.
As setting value 3I0> = 0.020A is selected.
If the earth fault protection is also to trip (address 3001 Sens. Earth Flt = ON: with Trip), set in address
3008 TRIP Direction, if for earth faults the signal is tripped Forward (normally in line direction), Reverse
(normally in direction of busbar) or Non-Directional. This parameter can only be altered in DIGSI at
Display Additional Settings.
Angle Error Compensation
The high reactive current component in resonant-earthed networks and the unavoidable air gap of the core
balance type current transformers require a phase angle compensation of the current transformer. This can be
done at addresses 3010 to 3013. For the actually connected burden you enter the maximum angle phase dis-
placement CT Err. F1 (address 3011) and the associated secondary current CT Err. I1 (address 3010)
and an additional operating point CT Err. F2/CT Err. I2 (address 3013 and 3012), above which the angle
displacement remains practically constant (see Figure 2-152). The device thus approximates the transforma-
tion curve of the transformer with considerable accuracy. In isolated systems angle compensation is not re-
quired.
Figure 2-152
300
Parameters for the phase angle correction
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
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