Table of Contents
Advertisement
Section 7
Current protection
IN>Dir : Operate residual current release level in % of IB for directional comparison scheme. The
setting is given in % of
directional measurement. The output signals, STFW and STRV can be used in a teleprotection
scheme. The appropriate signal should be configured to the communication scheme block.
7.3.2.2
2nd harmonic restrain
If a power transformer is energized there is a risk that the current transformer core will
saturate during part of the period, resulting in a transformer inrush current. This will give a
declining residual current in the network, as the inrush current is deviating between the
phases. There is a risk that the residual overcurrent function will give an unwanted trip. The
inrush current has a relatively large ratio of 2nd harmonic component. This component can be
used to create a restrain signal to prevent this unwanted function.
At current transformer saturation a false residual current can be measured by the protection.
Here the 2
2ndHarmStab : The rate of 2nd harmonic current content for activation of the 2nd harmonic
restrain signal. The setting is given in % of the fundamental frequency residual current.
7.3.2.3
Parallel transformer inrush current logic
In case of parallel transformers there is a risk of sympathetic inrush current. If one of the
transformers is in operation, and the parallel transformer is switched in, the asymmetric
inrush current of the switched-in transformer will cause partial saturation of the transformer
already in service. This is called transferred saturation. The 2
of the two transformers will be in phase opposition. The summation of the two currents will
thus give a small 2
significant. The inrush current of the transformer in service before the parallel transformer
energizing, will be a little delayed compared to the first transformer. Therefore, we will have
nd
high 2
the normal 2
Power System
IEC05000136 V1 EN-US
Figure 95: Application for parallel transformer inrush current logic
BlkParTransf function is activated, the 2
If the
the residual current measured by the relay is larger than a selected step current level. Assume
that step 4 is chosen to be the most sensitive step of the four step residual overcurrent
protection function EF4PTOC. The harmonic restrain blocking is enabled for this step. Also the
same current setting as this step is chosen for the blocking at parallel transformer energizing.
The settings for the parallel transformer logic are described below.
BlkParTransf : This is used to On blocking at energising of parallel transformers.
162
IB and must be set below the lowest INx> setting, set for the
nd
harmonic restrain can prevent unwanted operation as well.
nd
harmonic current. The residual fundamental current will however be
harmonic current initially. After a short period this current will however be small and
nd
harmonic blocking will reset.
IN>
IN>
nd
harmonic of the inrush currents
en05000136.vsd
nd
harmonic restrain signal will latch as long as
1MRK 505 370-UEN D
M15282-90 v6
M15282-97 v6
Busbar protection REB670
Application manual
Advertisement
Table of Contents
