Second Harmonic Blocking; Second Harmonic Blocking Implementation - GE P642 Technical Manual

Chapter 9 - Current Protection Functions
7

SECOND HARMONIC BLOCKING

7.1

SECOND HARMONIC BLOCKING IMPLEMENTATION

A separate second harmonic blocking function is applied to the following overcurrent protection types:
Phase Overcurrent protection (Overcurrent 1, Overcurrent 2 and Overcurrent 3)
●
Earth Fault protection elements (Earth Fault 1, Earth Fault 2 and Earth Fault 3)
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● There is no second harmonic blocking for Negative Phase Sequence Overcurrent protection
Second harmonic blocking is applicable to all stages of each of the elements. For POC, 2nd harmonic blocking can
be applied to each phase individually (phase segregated), or to all three phases at once (cross-block).
The function works by identifying and measuring the inrush currents present at switch on. It does this by
comparing the value of the second harmonic current components to the value of the fundamental component. If
this ratio exceeds the set thresholds, then the blocking signal is generated. The threshold is defined by the settings
IH2 I> Set and IH2 IN> Set for Phase Overcurrent protection and Earth Fault protection respectively.
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 IH2 IN> Unblock for Phase Overcurrent protection and IH2 I> Unblock for Earth
fault protection, which when exceeded, stops the 2nd harmonic blocking function.
Each overcurrent protection element has an I>Blocking setting with which the stages to be blocked can be
selected.
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