Functions
2.3 Directional Overcurrent Protection 67, 67N
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
104
2.3.5
Interaction with Fuse Failure Monitor (FFM)
Spurious tripping can be caused by failure of a measuring voltage due to short-circuit, broken wire in the voltage
transformer's secondary system or pickup of the voltage transformer fuse. Failure of the measuring voltage in
one or two poles can be detected, and the directional time overcurrent elements (Dir Phase and Dir Ground)
can be blocked (see logic diagrams). Undervoltage protection, sensitive ground fault detection and synchroni-
zation are equally blocked in this case.
2.3.6
Dynamic Cold Load Pickup Function
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 pickup value changeover 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
The 7SJ62/64 features an integrated inrush restraint function. It prevents the "normal" pickup of all directional
and non-directional overcurrent relay elements in the phases and ground path, but not the high-set elements.
The same is true for the alternative pickup thresholds of the dynamic cold load pickup function. After detection
of inrush currents above a pickup value, special inrush signals are generated. These signals also initiate fault
annunciations 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 the overcurrent tripping is blocked
(for further information see "Inrush Restraint" in Section 2.2).
2.3.8
Determination of Direction
The determination of the fault direction for the phase directional element and the ground directional element is
effected independently.
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 phase directional element the fault current of the corresponding phase and the unfaulted phase-to-
phase voltage are used as reference voltage. The unfaulted voltage also allows for an unambiguous direction
determination even if the fault voltage has collapsed entirely (short- line fault). In phase-to-ground voltage con-
nections the phase-to-phase voltages are calculated. In a connection of two phase-to-phase voltages and V
N
,
the third phase-to-phase voltage is also calculated.
With three-phase short-line faults, stored voltage values are used to clearly determine the direction if the mea-
surement voltages are not sufficient. After expiry of the storage time period (2 s), the detected direction is
saved, as long as no sufficient measuring voltage is available. When closing onto a fault, if no stored voltage
values exist in the buffer, the relay element will trip. In all other cases the voltage magnitude will be sufficient
for determining the direction.
For each directional ground element there are two possibilities of direction determination:
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