ABB Relion 670 series, Technical Reference Manual

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1. Current fundamental frequency component > IMinOpHarmBlk
2. Current second harmonic component > IMinOpHarmBlk
3. Ratio of the 2nd harmoinc component in relation to the fundamental frequency
component in the residual current exceeds the preset level defined parameter
2ndHarmStab setting
If all the above three conditions are fulfilled then 2NDHARMD function output signal
is set to logical value one and harmonic restraining feature to the function block is
applicable.
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 (51N67N) 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 pickup value for one of the four
residual overcurrent stages. By a parameter setting Use_PUValue it is possible to
select which one of the four pickup values that will be used ( Pickup1 or Pickup2
or Pickup3 or Pickup4 ).
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
Use_PUValue (see condition 3 above).
Simplified logic diagram for 2
nd
harmonic blocking feature is shown in figure 209 .
1MRK505222-UUS C
Section 7
Current protection
409
Technical reference manual