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.vsd
Power System
IN>
IN>
IEC05000136 V1 EN-US
Figure 275: 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. 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.
UseStartValue: 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 Off/On, the parallel transformer logic.
8.4.3.5
Switch onto fault logic
M15282-106 v3
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.
The function is divided into two parts. The SOTF function will give operation from step 2 or 3
during a set time after change in the position of the circuit breaker. The SOTF function has a
set time delay. The Under Time function, which has 2
nd
harmonic restrain blocking, will give
operation from step 4. The 2nd harmonic restrain will prevent unwanted function in case of
transformer inrush current. The Under Time function has a set time delay.
Below the settings for switch on to fault logics are described.
SOTF operation mode: This parameter can be set: Off/SOTF/Under Time/SOTF+Under Time.
1MRK 505 343-UEN B
Section 8
Current protection
423
Application manual
Содержание Relion 670 series
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