ABB Relion 670 series - Page 493

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
257.
1MRK 502 066-UUS B Section 8
Current protection
487
Technical manual