In addition to the basic functionality explained above, the 2
nd
harmonic blocking
can be set in such way to seal-in until residual current disappears. This feature
might be required to stabilize EF4PTOC during switching of parallel transformers
in the station. 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 is in phase opposition. The summation of the two currents thus gives a
small 2
nd
harmonic current. The residual fundamental current is however
significant. The inrush current of the transformer in service before the parallel
transformer energizing, is a little delayed compared to the first transformer.
Therefore, we have high 2
nd
harmonic current component initially. After a short
period this current is however small and the normal 2
nd
harmonic blocking resets.
If the BlkParTransf function is activated, the 2
nd
harmonic restrain signal is latched
as long as the residual current measured by the relay is larger than a selected step
current level by using setting UseStartValue.
This feature has been called Block for Parallel Transformers. This 2
nd
harmonic
seal-in feature is activated when all of the following three conditions are
simultaneously fulfilled:
1. Feature is enabled by entering setting parameter BlkParTransf = On.
2.
Basic 2
nd
harmonic restraint feature has been active for at least 70ms.
3. Residual current magnitude is higher than the set start value for one of the four
residual overcurrent stages. By a parameter setting UseStartValue it is possible
to select which one of the four start values that will be used (IN1> or IN2> or
IN3> or IN4>).
Once Block for Parallel Transformers is activated, the basic 2
nd
harmonic blocking
signal is sealed-in until the residual current magnitude falls below a value defined
by parameter setting UseStartValue (see condition 3 above).
Simplified logic diagram for 2
nd
harmonic blocking feature is shown in Figure
46.
Section 6 1MRK 505 384-UEN A
Current protection
122 Breaker protection REQ650 2.2 IEC
Technical manual
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