Dynamic Cold Load Pickup Function; Inrush Restraint - Siemens SIPROTEC 7SK80 Manual
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Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
2.2.5
Dynamic Cold Load Pickup Function
It may be necessary to dynamically increase the pickup values if, during 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 increase of pickup thresholds can be avoided taking such start-
ing conditions into consideration.
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 time overcurrent protection.
2.2.6
Inrush Restraint
When the multi-functional protective relay with local control 7SK80 is installed, for instance, to protect a power
transformer, large magnetizing inrush currents will flow when the transformer is energized. These inrush cur-
rents 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 measured
currents, false device pickup due to inrush is still a potential problem since, depending on the transformer size
and design, the inrush current also comprises a large component of the fundamental.
The 7SK80 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 func-
tion. 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 (see also logic diagrams of time overcurrent elements, Figures 2-8 to 2-13).
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 0.025 * I
• 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 „InRush Gnd
Det", see Figure 2-14) and its blocking being carried out.
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 en-
abled, 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 restraint feature will
not result in any additional tripping delays. If a relay element drops out during inrush blocking, the associated
time delay will reset.
64
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
);
Nom,sec
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