Siemens SIPROTEC 4 7VK61 Manual page 77

[formel-ueberstromzeit-hochstrom-3-oz-010802, 1, en_GB]
If short-circuit currents exceed 2365 A (primary) or 19.7 A (secondary), there is a short circuit on the line to be
protected. This fault can immediately be cleared by the time overcurrent protection.
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>> InstTrip BI (address 2614) determines, whether the time delays
(No 7110) or the automatic reclosure in ready state can be bypassed by the binary input T Iph>> (address
2611) and T 3I0>> (address2613) is possible. The binary input (if allocated) is applied to all stages of the
time-overcurrent protection. With I>> InstTrip BI = YES you define that the Ι>> stages trip without delay
after pickup if the binary input was activated. For I>> InstTrip BI = NO the set delays are always active
If the Ι>>-stage, when switching the line onto a fault, is to trip without delay or with a short delay, SOTF
Time DELAY (address 2680, see above under margin heading "General") set the parameter I>> SOTF
(address 2615) to YES. Any other stage can be selected as well for this instantaneous tripping.
Overcurrent Stages Ιph>, 3Ι0> in Definite-time Overcurrent Protection
For the setting of the current pickup value, Iph> (address 2620), the maximum operating current is most
decisive. 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
Ι
max
Current Transformer 600 A/5 A
Safety factor
With settings in primary quantities the following setting value is calculated:
Set value Ι> = 1.1 · 630 A = 693 A
With settings in secondary quantities the following setting value is calculated:
[formel-ueberstromzeit-ueberstrom-oz-310702, 1, en_GB]
The earth current stage 3I0> (address 2622), should be set to detect the smallest earth fault current to be
expected.
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 4, 7VK61, Manual
C53000-G1176-C159-5, Edition 05.2018
2
= 120 MVA
= 630 A
1.1
Functions
2.3 Overcurrent protection (optional)
>O/C InstTRIP
77