2Nd Harmonic Blocking Logic; 5Th Harmonic Blocking Implementation; Figure 48: 2Nd Harmonic Blocking Logic - GE P642 Technical Manual

Chapter 6 - Transformer Differential Protection
4.2

2ND HARMONIC BLOCKING LOGIC

I fundamental
diffa
Low current (hard-coded )
I 2 harmonic
nd
diffa
I2H Diff Set
I fundamental
diffb
Low current (hard-coded )
I 2 harmonic
nd
diffb
I2H Diff Set
I fundamental
diffc
Low current (hard-coded )
I 2 harmonic
nd
diffc
I2H Diff Set
V00706

Figure 48: 2nd harmonic blocking logic

4.3

5TH HARMONIC BLOCKING IMPLEMENTATION

The IED filters the differential current to determine the fundamental Idiff(fn) and the fifth harmonic component
Idiff(5fn). The device uses these quantities to produce a blocking signal, which will block the protection in the event
that the fifth harmonic component exceeds a certain level.
5th Harmonic blocking can be used to prevent unwanted operation of the low set differential element under
transient overfluxing conditions.
The 5th harmonic blocking threshold is adjustable between 0 - 100% differential current. The threshold should be
adjusted so that blocking will be effective when the magnetizing current rises above the chosen threshold setting
of the low-set differential protection.
Fifth harmonic blocking is phase segregated. If the ratio Idiff(5-fn)/Idiff(fn) exceeds an adjustable threshold (set by
I5H diff Set in two consecutive calculations, and if the differential current is larger than 0.1 pu (the minimum
setting of Is1), then the fifth harmonic blocking signal is issued for the relevant phase. The DDB signals are:
IA5H Diff Start (blocking signal for phase A)
IB5H Diff Start (blocking signal for phase B)
IC5H Diff Start (blocking signal for phase C)
No cross blocking is available for 5th harmonic blocking.
No blocking signal is asserted if the differential current exceeds the set thresholds Is-HS1 or Is-HS2.
The following flow diagram summarises the 5th harmonic blocking procedure:
118
&
I 2 nd harm / I fund
diffa diffa
&
nd
I 2 harm / I fund
diffb diffb
&
I 2 harm / I fund
nd
diffc diffc
P64x
IA 2H Diff Start
IB 2H Diff Start
IC2H Diff Start
P64x-TM-EN-1.3