Dynamic Cold Load Pickup Function; Inrush Restraint - Siemens siprotec SJ62 User Manual

2 Functions
User Defined
Curves
2.2.5

Dynamic Cold Load Pickup Function

2.2.6

Inrush Restraint

69
Disk emulation offers advantages when the overcurrent relay elements must be coor-
dinated with conventional electromechanical overcurrent relays located toward the
source.
When user defined characteristics are utilized, the tripping curve may be defined point
by point. Up to 20 pairs of values (current, time) may be entered. The device then ap-
proximates the characteristic, using linear interpolation.
The dropout curve may be user-defined as well. See dropout 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 again at zero.
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 condi-
tions 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.
When the multi-functional protective relay with local control 7SJ62/63/64 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 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 poten-
tial problem since, depending on the transformer size and design, the inrush current
also comprises a large component of the fundamental.
The 7SJ62/63/64 features an integrated inrush restraint function. It prevents the "nor-
mal" pickup of the 50-1 or 51 elements (not 50-2) in the phases and the ground path
of the non-directional and directional time-overcurrent protection. The same is true for
the alternative pickup thresholds of the dynamic cold load pickup function. After detec-
tion 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 corre-
sponding message („....Timeout.") is output, but the overcurrent tripping is
blocked (see also logic diagrams of time overcurrent elements, Figures 2-5 to 2-10).
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 fre-
quency analysis, digital filters are used to conduct a Fourier analysis of all three phase
currents and the ground current.
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015