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Note: the calculation was carried out with absolute values, which is sufficiently precise for overhead lines. If the
angles of the source impedance and the line impedance vary considerably, a complex calculation must be
carried out.
A similar calculation must be carried out for earth faults, with the maximum earth current occurring at the line
end during a short-circuit being decisive.
The set time delays are pure additional delays, which do not include the operating time (measuring time).
The parameter I>> Telep/BI (address 2614) defines whether the time delays T Iph>> (address 2611) and
T 3I0>> (address 2613) can be bypassed by the binary input „>O/C InstTRIP" (No 7110) or by the oper-
ational automatic reclosure function. The binary input (if allocated) is applied to all stages of the time overcur-
rent protection. With I>> Telep/BI = YES you define that the I>> stages trip without delay after pickup if the
binary input was activated. For I>> Telep/BI = NO the set delays are always active.
If the I>> stage, when switching the line on to a fault, is to trip without delay or with a short delay, SOTF Time
DELAY (address 2680, see above under margin heading „General"), the parameter I>> SOTF (address 2615)
is set to YES. Any other stage can be selected as well for this instantaneous tripping.
Overcurrent StagesI
>, 3I
ph
For the setting of the current pickup value, Iph> (address 2620), the maximum operating current is most de-
cisive. Pickup due to overload should never occur, since the device in this operating mode operates as fault
protection with correspondingly short tripping times and not as overload protection. For this reason, a pickup
value of about 10 % above the expected peak load is recommended for line protection, and a setting of about
20 % above the expected peak load is recommended for transformers and motors.
When using a personal computer and DIGSI to apply the settings, these can be optionally entered as primary
or secondary values. For settings with secondary values the currents will be converted for the secondary side
of the current transformers.
Calculation Example:
110 kV overhead line 150 mm
maximum transmittable power
P
max
corresponding to
I
max
Current Transformer
Safety factor
With settings in primary quantities the following setting value is calculated:
Set value I> = 1.1 · 630 A = 693 A
With settings in secondary quantities the following setting value is calculated:
The earth current stage 3I0> (address 2622), shall detect even the smallest earth fault current to be expected.
For very small earth currents the earth fault protection is best suited.
The settable time delay T Iph> (address 2621) results from the grading coordination chart defined for the
system.
The time T 3I0> (address 2623) can normally be set shorter, according to a separate time grading schedule
for earth currents.
SIPROTEC, 7VK61, Manual
C53000-G1176-C159-3, Release date 05.2009
> for definite time overcurrent protection
0
2
= 120 MVA
= 630 A
600 A / 5 A
1.1
Functions
2.3 Overcurrent protection (optional)
75
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