ABB REB670 - Parallel Transformer Inrush Current Logic

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
nd
harmonic restrain can prevent unwanted operation as well.
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
M15282-97 v6
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
nd
harmonic of the inrush currents of the two transformers will be in phase opposition.
The summation of the two currents will thus give a small 2
nd
harmonic current. The
residual fundamental current will however be 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 high 2
nd
harmonic
current initially. After a short period this current will however be small and the normal
2
nd
harmonic blocking will reset.
en05000136_ansi.vsd
Power System
51N
51N
ANSI05000136 V1 EN-US
Figure 96: Application for parallel transformer inrush current logic
If the BlkParTransf function is activated, the 2
nd
harmonic restrain signal will latch as
long as 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 (51N_67N). 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.
Section 7 1MRK 505 370-UUS A
Current protection
200 Busbar protection REB670 2.2 ANSI
Application manual