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Functions
2.3 Directional Overcurrent Protection Ground 67, 67N
2.3.6
Dynamic Cold Load Pickup
It may be necessary to dynamically increase the pickup values of the directional time overcurrent protection if,
at starting, certain elements of the system show an increased power consumption after a long period of zero
voltage (e.g. air-conditioning systems, heating installations, motors). Thus, a general raise of pickup thresholds
can be avoided taking into consideration such starting conditions.
This dynamic cold load pickup function is common to all overcurrent elements and is described in Section 2.4.
The alternative pickup values can be set individually for each element of the directional and non-directional time
overcurrent protection.
2.3.7
Inrush Restraint
7SK80 features an integrated inrush restraint function. It prevents the "normal" pickup of the 67N-1 or 67N-TOC
elements (not 67N-2 and 67N-3) in the ground path of the non-directional and directional overcurrent protec-
tion. The same is true for the alternative pickup thresholds of the dynamic cold load pickup function. If inrush
currents are detected, special inrush pickup signals are generated. These signals also initiate fault recording
and start the associated trip delay time. If inrush conditions are still present after the tripping time delay has
elapsed, a corresponding message ("....TimeOut ") is output, but tripping is blocked (for further information see
"Inrush Restraint" in Section 2.2).
2.3.8
Determination of Direction
Basically, the direction determination is performed by determining the phase angle between the fault current
and a reference voltage.
Method of Directional Measurement
For the directional ground fault element there are two ways of direction determination:
• Direction determination with zero-sequence or ground quantities
• Direction determination with negative-sequence quantities
Direction Determination with Zero-sequence or Ground Quantities
For the directional ground fault elements, the direction can be determined from the zero-sequence system
quantities. In the current path, the I
device. Otherwise, the device calculates the ground current from the sum of the three phase currents. In the
voltage path, the displacement voltage V
culates as reference voltage the zero-sequence voltage 3 · V
magnitude of V
ground element will not initiate a trip signal. If measurement from the zero-sequence system is not possible,
e.g. because only two current transformers are connected, the directional ground element will work neither.
Direction Determination with Negative-sequence Quantities
Here, the negative sequence current and the negative sequence voltage as reference voltage are used to de-
termine the direction. This is advantageous if the zero sequence is influenced via a parallel line or if the zero
voltage becomes very small due to unfavorable zero sequence impedances. The negative sequence system
is calculated from the individual voltages and currents. As with using the zero sequence quantities, the direction
is only determined once the values required for direction determination have exceeded a certain minimum
threshold. Otherwise, the direction will remain undetermined.
94
current is valid when the transformer neutral current is connected to the
N
N
or 3 · V
is not sufficient to determine direction, the direction is undefined. Then the directional
0
0
is used as reference voltage if connected. Otherwise, the device cal-
from the sum of the three phase voltages. If the
0
E50417-G1140-C344-A5, Release date 11.2012
SIPROTEC, 7SK80, Manual
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