Parallel Transformer Inrush Current Logic; Switch Onto Fault Logic - ABB Relion 670 Series Applications Manual

1MRK 511 358-UUS A
7.4.3.4

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
transformers will be in phase opposition. The summation of the two currents will thus give a small
nd
2 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
initially. After a short period this current will however be small and the normal 2
blocking will reset.
Power System
ANSI05000136 V1 EN-US
Figure 65: Application for parallel transformer inrush current logic
BlkParTransf function is activated the 2
If 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.
Below the settings for the parallel transformer logic are described.
Use_PUValue : Gives which current level that should be used for activation of the blocking signal.
This is given as one of the settings of the steps: Step 1/2/3/4. Normally the step having the lowest
operation current level should be set.
BlkParTransf : This parameter can be set Disable / Enable , the parallel transformer logic.
7.4.3.5

Switch onto fault logic

In case of energizing a faulty object there is a risk of having a long fault clearance time, if the fault
current is too small to give fast operation of the protection. The switch on to fault function can be
activated from auxiliary signals from the circuit breaker, either the close command or the open/
close position (change of position).
This logic can be used to issue fast trip if one breaker pole does not close properly at a manual or
automatic closing.
SOTF and Under Time are similar functions to achieve fast clearance at asymmetrical closing
based on requirements from different utilities.
Application manual
51N 51N
en05000136_ansi.vsd
nd
harmonic of the inrush currents of the two
nd
harmonic restrain signal will latch as long as the
Section 7
Current protection
M15282-97 v5
nd
harmonic current
nd
harmonic
M15282-106 v3
143