Transformer Inrush - ABB REX 521 Technical Reference Manual, Standard Configurations

REX 521
9.3.3.2.
130
Protection Relay
Technical Reference Manual, Standard Configurations
The high-set overcurrent stage 3I>> is configured to operate in case of a short-circuit
in the poles of the LV side. The stage serves also as back-up at short-circuits in the
MV busbar system. The protection method for the stage is based on blocking and the
blocking signal is delivered from the high-set stage 3I>> of the relay on the MV side.
The low-set overcurrent stage 3I> of the relay is employed as definite time
overcurrent back-up protection for the MV feeders. The stage is time-graded with
the down-stream relay. In this case where this application example is used together
with the second application example (chapter 9.2.), the operate time will be the
operate time of the low-set stage 3I> in the second example plus time grading. The
start current setting and the inrush current and its duration may have to be
considered.
The settings for the overcurrent protections can be found in
Main menu\Protection\3I>\ ... \3I>>>\Setting group 1\ where
the parameters "Operation mode", "Start current" and "Operate time" can be set for
the function. In Main menu\Protection\3I>\Control setting\ the
"Drop-off time" and the length of the trip signal, "Trip pulse", for example, can be
set.
The data of the three last operations, start or trip, are recorded and the values of the
most recent operation can be found in
Main menu\Protection\3I>\Recorded data1\...3\.

Transformer inrush

The Inrush 3I2f> function is used for doubling the set start-current of overcurrent
protection in a transformer magnetizing inrush situation or at motor start-up. The
operation of the overcurrent stages 3I>> and 3I>>> can be inhibited by the inrush
function.
Transformer magnetizing inrush currents occur when energizing the transformer.
The inrush current may be many times the rated current and the halving time may be
up to several seconds. The inrush current that may arise, would cause the relay to
start practically always when the transformer is connected to the network. Typically,
the inrush current contains a large amount of second harmonics. Doubling the
operation of the overcurrent stage of the relay at magnetizing inrush current is based
on the ratio of the amplitudes of the second harmonic digitally filtered from the
current and the fundamental frequency.
The overcurrent stage is doubled once the numerically derived ratio I2f/I1f between
the amplitudes of the second harmonic and the fundamental frequency current in one
phase exceeds the value set for the "Ratio I2f/I1f>" parameter. Thus a set start
current value below the connection inrush current level may be selected for the
overcurrent stages. The doubling can be activated only if the amplitude of the
fundamental frequency current of the corresponding phase is above 2.0% In. A
control parameter is used for setting the minimum pulse width of the doubling
signal. However, the signal remains active until the ratio I2f/I1f drops below the
value set for the "Ratio I2f/I1f>" parameter in all phases, i.e. until the inrush
situation is over, even if the pulse counter has elapsed earlier.
The settings for the inrush protection can be found in
Main menu\Protection\3I2f>\.
1MRS 751802-MUM