ABB REX 521 Technical Reference Manual, Standard Configurations Download

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1MRS751802-MUM

REX 521

159

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 Section 9.2. Incoming feeder, Basic
B01), 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\

9.3.3.2.

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 the second harmonic. 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, that is, 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>\