ABB RED650 - Page 206

1. The fundamental frequency component of the current > 1% of IBase
2. The second harmonic component > 1% of IBase
3. The ratio of the 2nd harmonic component in relation to the fundamental
frequency component in the residual current exceeds the preset level defined
by the parameter 2ndHarmStab setting
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
88.
Section 8 1MRK 505 394-UEN A
Current protection
200 Line differential protection RED650 2.2 IEC
Technical manual