Second Harmonic Blocking; Second Harmonic Blocking Implementation - GE MiCOM P40 Agile Technical Manual

P14D
15

SECOND HARMONIC BLOCKING

When a transformer is initially connected to a source of AC voltage, there may be a substantial surge of current
through the primary winding called inrush current.
Inrush current is a regularly occurring phenomenon and should not be considered a fault, as we do not wish the
protection device to issue a trip command whenever a transformer, or machine is switched on. This presents a
problem to the protection device, because it should always trip on an internal fault. The problem is that typical
internal transformer faults may produce overcurrents which are not necessarily greater than the inrush current.
Furthermore faults tend to manifest themselves on switch on, due to the high inrush currents. For this reason, we
need to find a mechanism that can distinguish between fault current and inrush current. Fortunately this is
possible due to the different natures of the respective currents. An inrush current waveform is rich in harmonics
(particularly the second), whereas an internal fault current consists only of the fundamental. We can thus develop
a restraining method based on the harmonic content of the inrush current. The mechanism by which this is
achieved is called second harmonic blocking.
15.1

SECOND HARMONIC BLOCKING IMPLEMENTATION

Second harmonic blocking can be applied to the following overcurrent protection types:
● Phase Overcurrent protection (POC)
Earth Fault protection (derived and measured) (EF1 and EF2)
●
Sensitive Earth Fault protection (SEF)
●
Negative Phase Sequence Overcurrent protection (NPSOC)
●
Second harmonic blocking is implemented in the SECURITY CONFIG column of the relevant setting group.
Second harmonic blocking is applicable to all stages of each of the elements. Each protection element has a
relevant blocking setting with which the type of blocking is defined.
For phase overcurrent, 2nd harmonic blocking can be applied to each phase individually (phase segregated), or to
all three phases at once (cross-block). This is determined by the I> Blocking setting.
P14D-TM-EN-8
Chapter 6 - Current Protection Functions
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