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

Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
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.
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 raise of pickup thresholds can be avoided taking such starting
conditions into consideration.
This dynamic pickup value changeover is common to all overcurrent elements and is described in Section 2.3.
The alternative pickup values can be set individually for each element of the time overcurrent protection.
2.2.6

Inrush Restraint

If the multi-functional protective relay 7SJ61 is installed, for instance, to protect a power transformer, large mag-
netizing 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 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.
7SJ61 features an integrated inrush restraint function. It prevents the „normal" pickup of 50-1 or 51 relay ele-
ments (not 50-2 and 50-3) in the phases and the ground path of the overcurrent relay elements. The same is
true for the alternative pickup thresholds of the dynamic cold load pickup function. After detection of inrush cur-
rents 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 2-6 to 2-11).
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, numerical 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;
• the current of the affected element has exceeded the pickup value.
In this case, an inrush in the affected phase is recognized (annunciations 1840 to 1842 and 7558 „InRush
Gnd Det", see figure 2-12) and its blocking being carried out.
58
SIPROTEC, 7SJ61, Manual
C53000-G1140-C210-1, Release date 02.2008
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Nom,sec