Dynamic Cold Load Pickup Function; Inrush Restraint - Siemens SIPROTEC 4 7SJ61 Manual

User-defined Characteristics
When user-defined characteristic are used, the tripping curve may be defined point by point. Up to 20 value
pairs (current, time) may be entered. The device then approximates the characteristic, using linear interpola-
tion.
The dropout curve may be user-defined as well. See dropout behavior for ANSI and IEC curves in the function
description. If no user-defined dropout curve is required, the element drops out as soon as the respective
current falls below approx. 95% of the set pickup value. When a new pickup is evoked, the timer starts at zero
again.

Dynamic Cold Load Pickup Function

2.2.5
It may be necessary to dynamically increase the pickup thresholds of the overcurrent protection if certain
system components exhibit an increased power consumption when they are switched on after a long period
of zero voltage (e.g. air-conditioning systems, heating installations, motors). Thus, a general increase of
pickup thresholds can be avoided taking into consideration such starting conditions.
This dynamic pickup value changeover fuction is common to all overcurrent elements and is described in
Section 2.3 Dynamic Cold Load
of the time overcurrent protection.

Inrush Restraint

2.2.6
When the multi-functional protective relay with local control 7SJ61 is installed, for instance, to protect a
power transformer, large magnetizing inrush currents will flow when the transformer is energized. These
inrush currents may be several times the nominal transformer current, and, depending on the transformer size
and design, may last from several tens of milliseconds to several seconds.
Although pickup of the relay elements is based only on the fundamental harmonic component of the meas-
ured currents, false device pickup due to inrush is still a potential problem since, depending on the trans-
former size and design, the inrush current also comprises a large component of the fundamental.
The 7SJ61 features an integrated inrush restraint function. It prevents the "normal" pickup of 50-1 or 51 relay
elements (not 50-2 and 50-3) in the phases and the ground path of all directional and non-directional overcur-
rent relay 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 (
but the overcurrent tripping is blocked (see also logic diagrams of time overcurrent elements,
Figure 2-12).
Inrush current contains a relatively large second harmonic component (twice the nominal frequency) which is
nearly absent during a fault current. The inrush restraint is based on the evaluation of the 2nd harmonic
present in the inrush current. For frequency analysis, digital filters are used to conduct a Fourier analysis of all
three phase currents and the ground current.
Inrush current is recognized if the following conditions are fulfilled at the same time:
•
The harmonic content is larger than the setting value 2202 2nd HARMONIC (minimum 0.125 * Ι
•
the currents do not exceed an upper limit value 2205 I Max;
•
an exceeding of a threshold value via an inrush restraint of the blocked element takes place.
In this case an inrush in the affected phase is recognized (annunciations 1840 to 1842 and 7558
Gnd Det „InRush Gnd Det", see
Since quantitative analysis of the harmonic components cannot be completed until a full line period has been
measured, pickup will generally be blocked by then. Therefore, assuming the inrush restraint feature is
enabled, a pickup message will be delayed by a full line period if no closing process is present. On the other
hand, trip delay times of the time overcurrent protection feature are started immediately even with the inrush
restraint being enabled. Time delays continue running with inrush currents present. If inrush blocking drops
out after the time delay has elapsed, tripping will occur immediately. Therefore, utilization of the inrush
SIPROTEC 4, 7SJ61, Manual
C53000-G1140-C210-6, Edition 05.2016
Pickup. The alternative pickup values can be set individually for each element
Figure 2-13) and its blocking being carried out.
2.2 Overcurrent Protection 50, 51, 50N, 51N
„....Timeout." ) is output,
Figure 2-7
InRush
Functions
to
);
Nom,sec
55