Second Harmonic Blocking Logic (Sef Input); Application Notes; Setting Guidelines; Figure 77: 2Nd Harmonic Blocking Logic (Sef Input) - GE MiCOM P40 Agile Technical Manual

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P14x
15.3

SECOND HARMONIC BLOCKING LOGIC (SEF INPUT)

ISEF fundamental
I>Lift 2H
Low current (hard-coded)
ISEF 2
harmonic
nd
ISEF>2nd Harm.
V00726

Figure 77: 2nd Harmonic Blocking Logic (SEF Input)

The function measures the current present at the SEF CT input and determines the ratio of the second harmonic
component to the fundamental. If this ratio exceeds the set threshold, then the SEF second harmonic blocking
signal is generated. The threshold is defined by the ISEF>2nd Harm. setting.
We only want the function to block the protection if the fundamental current component is within the normal
range. If this exceeds the normal range, then this is indicative of a fault, which must be protected. For this reason
there is another settable trigger I> lift 2H, which when exceeded, stops the 2nd harmonic blocking function.
The minimum detection level is 4 mA rms. The maximum detection level is 1.8 A rms in any sample over the
previous cycle. There is no delay on pick-up but there is a 1 cycle delay on drop-off.
15.4

APPLICATION NOTES

15.4.1

SETTING GUIDELINES

During the energization period, the second harmonic component of the inrush current may be as high as 70%. The
second harmonic level may be different for each phase, which is why phase segregated blocking is available.
If the setting is too low, the 2nd harmonic blocking may prevent tripping during some internal transformer faults. If
the setting is too high, the blocking may not operate for low levels of inrush current which could result in undesired
tripping of the overcurrent element during the energization period. In general, a setting of 15% to 20% is suitable.
P14xEd1-TM-EN-1
&
ISEF 2
harm /
nd
ISEF fund
Chapter 6 - Current Protection Functions
&
ISEF 2 H Start
151

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