Second Harmonic Blocking Logic (Poc Input); Application Notes; Setting Guidelines; Figure 63: 2Nd Harmonic Blocking Logic (Poc Input) - GE MiCOM P40 Agile Technical Manual

Chapter 6 - Current Protection Functions
15.2

SECOND HARMONIC BLOCKING LOGIC (POC INPUT)

IA fundamental
I>Lift 2H
Low current (hard-coded)
IA 2ndHarm
2 ndHarm Thresh
IA fundamental
I>Lift 2H
Low current (hard-coded)
IB 2 ndHarm
2 ndHarm Thresh
IC 2ndHarm
I>Lift 2H
Low current (hard-coded)
IC 2ndHarm
2 ndHarm Thresh
V00626

Figure 63: 2nd Harmonic Blocking Logic (POC Input)

The function works by identifying and measuring the inrush currents present in the phase currents 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 2ndHarm Thresh 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.
15.3

APPLICATION NOTES

15.3.1

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.
132
&
&
IA 2 harm / IA fund
nd
&
&
IB 2 harm / IB fund
nd
&
&
IC 2 nd harm / IC fund
&
&
I2H Any Start
1
IA2H Start
IB2H Start
IC2H Start
P14D
P14D-TM-EN-8