[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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FIGURE 3.8.30 DC content of the differential current for Case 3.<br />
TABLE 3.8.3 Inrush Detection Methods Performance during Inrush Conditions<br />
Method Case 1 Case 2 Case 3<br />
<br />
<br />
Low even-harmonic Second-harmonic setting<br />
content<br />
6%<br />
8%<br />
Second- and fourthharmonic<br />
blocking<br />
All-harmonic restraint<br />
Low-current detection<br />
Even-harmonic restraint<br />
DC-ratio blocking<br />
4. An improved approach for transformer protection that combines harmonic-restraint and -blocking<br />
methods with a waveshape-recognition technique provides added security to the independent<br />
harmonic-restraint element without sacrificing dependability. This new method uses even harmonics<br />
for restraint, plus dc component and fifth harmonic for blocking.<br />
5. Using even-harmonic restraint ensures security for inrush currents with low second-harmonic<br />
content and maintains dependability for internal faults with CT saturation. The use of fifthharmonic<br />
blocking guarantees an invariant relay response to overexcitation. Using dc-offset blocking<br />
ensures security for inrush conditions with very low total harmonic distortion.<br />
References<br />
Percentage<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
0 1 2 3 4 5 6 7<br />
Cycles<br />
<br />
High third-harmonic<br />
content<br />
<br />
Low-current interval = 1 / 4<br />
cycle<br />
<br />
Low even-harmonic<br />
content<br />
<br />
DC ratio = zero<br />
Note: = Correct operation; = Misoperation.<br />
<br />
Even-harmonic content<br />
<br />
Low-current interval > 1 / 4<br />
cycle<br />
<br />
Even-harmonic content<br />
<br />
DC ratio > 0.1 after 1 cycle<br />
<br />
Second-harmonic setting<br />
<br />
Harmonic content<br />
<br />
Low-current interval < 1 / 4<br />
cycle<br />
<br />
Even-harmonic content<br />
<br />
DC ratio = zero<br />
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© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC