ABB RES670 Technical Manual page 214

Section 8
Current protection
208
2ndHarmStab), output signal 2NDHARMD is set to logical value one and the
harmonic restraining feature to the function block will be applicable.
Blocking from the 2nd harmonic element activates if all of three criteria are satisfied:
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
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.
nd
The 2 harmonic of the inrush currents of the two transformers is in phase opposition.
The summation of the two currents thus gives a small 2
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
component initially. After a short period this current is however small and the normal
nd
2 harmonic blocking resets. If the BlkParTransf function is activated, the 2
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
feature is activated when all of the following three conditions are simultaneously
fulfilled:
1. Feature is enabled by entering setting parameter BlkParTransf = On.
2. nd
Basic 2 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
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 62.
Phasor measurement unit RES670 2.2 ANSI
1MRK 511 408-UUS A
nd
harmonic blocking can
nd
harmonic current. The
nd
harmonic current
nd
nd
harmonic seal-in
nd
harmonic blocking
Technical manual