Functions
2.8 Earth Fault Protection in Earthed Systems (optional)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
237
Inverse time stage with ANSI characteristic
If the fourth stage has been configured as an inverse time overcurrent stage with ANSI characteristic (address
131
Earth Fault O/C
=
TOC ANSI
), you first set the mode: Address
3140
Op. mode 3I0p
. This stage
can be set to operate
Forward
(usually towards line) or
Reverse
(usually towards busbar) or
Non-
Directional
(in both directions). If the stage is not required, set its mode to
Inactive
.
For the inverse time overcurrent stage 3
I
0P
it is possible to select different characteristics depending on order
variant and configuration (Section 2.1.1, address
131
). With ANSI characteristics (address
131
Earth Fault
O/C
=
TOC ANSI
) the following options are available in address
3152
ANSI Curve
:
Inverse
,
Short Inverse
,
Long Inverse
,
Moderately Inv.
,
Very Inverse
,
Extremely Inv.
,
Definite Inv.
.
The characteristics and equations they are based on are listed in the Technical Data.
The setting of the pickup threshold
3I0p PICKUP
(address
3141
) is similar to the setting of definite time stages
(see above). In this case it must be noted that a safety margin between the pickup threshold and the set value
has already been incorporated. Pickup only occurs at a current which is approximately 10 % above the set
value.
The time multiplier setting
3I0p Time Dial
(address
3144
) is derived from the grading coordination chart
which was set up for earth faults in the system.
In addition to the inverse time delay, a constant (fixed length) time delay can also be set if this is required. The
setting
Add.T-DELAY
(address
3147
) is added to the time of the set curve.
During the selection of the current and time settings, regard must be taken as to whether a stage should be
direction dependent and whether it uses teleprotection. Refer also to the margin headings „Determination of
Direction“ and „Teleprotection with Earth Fault Protection“.
Inverse time stage with logarithmic inverse characteristic
If you have configured the inverse time overcurrent stage with logarithmic inverse characteristic (address
131
Earth Fault O/C
=
TOC Logarithm.
), you set the operating mode first: Address
3140
Op. mode 3I0p
.
This stage can be set to operate
Forward
(usually towards line) or
Reverse
(usually towards busbar) or
Non-
Directional
(in both directions). If the stage is not required, set its mode to
Inactive
.
For the logarithmic inverse characteristic (address
131
Earth Fault O/C
=
TOC Logarithm.
) address
3153
LOG Curve
=
Log. inverse
.
The characteristic and the formula on which it is based can be found in the Technical Data.
Figure 2-115 illustrates the influence of the most important setting parameters on the curve.
3I0p PICKUP
(address
3141
) is the reference value for all current values, while
3I0p Startpoint
(address
3154
) deter-
mines the beginning of the curve, i.e. the lowest operating range on the current axis (referred to
3I0p
PICKUP
). The timer setting
3I0p MaxT-DELAY
(address
3146
) determines the starting point of the curve (for
3
I
0
=
3I0p PICKUP
). The time factor
3I0p Time Dial
(address
3145
) changes the slope of the curve. For
large currents,
3I0p MinT-DELAY
(address
3142
) determines the lower limit on the time axis. For currents
larger than 35 ·
3I0p PICKUP
the operating time no longer decreases.
Finally, at address
3147
Add.T-DELAY
a fixed time delay can be set as was done for the other curves.
