Application Guide - NP900 Series
345 (504
)
A996A
Now this result is still very low considering of the magnetizing inrush current magnitudes
but still the differential relay would definitely trip in this case if it would not be prevent from
operating by 2
nd
harmonic blocking. Situation is the same with all of the setting variations
calculated.
In following figure is presented principle operation of the harmonic blocking in the
transformer differential. When the transformer is energized both, the fundamental
frequency component as well as the 2
nd
harmonic component will increase significantly. In
this example the harmonic blocking limit was set to 15% (ratio of 2
nd
harmonic/fundamental per each phase), which seems more than sufficient for this
transformer and the pick-up in the example is set to 30%. Now when the flux in the
transformer core starts to catch up, saturation of the core reduces and the current for
magnetizing reduces as well, the blocking shall be active until the setting is reached which
after the blocking shall release per each phase separately. For this transformer the
harmonic blocking limit could have been set to 30% and the energizing would have been
successful still since the 2
nd
harmonic is heavily still present in time the fundamental
currents reduce below the pick-up limit of the differential stage.
Содержание NP900 Series
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Страница 49: ...Application Guide NP900 Series 49 504 A996A Figure 3 2 1 3 25 Definite time operating characteristics...
Страница 50: ...Application Guide NP900 Series 50 504 A996A Figure 3 2 1 3 26 IEC predefined characteristics NI VI LTI and EI...
Страница 52: ...Application Guide NP900 Series 52 504 A996A Figure 3 2 1 3 28 IEEE predefined characteristics EI MI and VI...
Страница 243: ...Application Guide NP900 Series 243 504 A996A Figure 3 3 2 1 122 NPS biased thermal trip curves with kNPS values 7 and 10...
Страница 491: ...Application Guide NP900 Series 491 504 A996A 3 7 5 6 SETTING EXAMPLE...
Страница 504: ...Application Guide NP900 Series 504 504 A996A A996A...