Mikro DPM680 - B.2 Total Harmonic Distortion Calculation; B.3 Sequence Components Calculation

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B.2 TOTAL HARMONIC DISTORTION CALCULATION
Total Harmonic Distortion (THD) is a power quality indicator used to show the extent of voltage and
current waveform distortion by the load. The THD is calculated by the following equation:
THD={h
2
+h
3
+h
4
+........+h
32
}/ h
1
x100%
where h
n
represents the n
th
harmonic component of the quantity, resolved using Fourier’s Transform.
B.3 SEQUENCE COMPONENTS CALCULATION
Sequence components are abstract quantities resolved from the phase voltages and currents.
They describe the degree and nature of imbalance and phase reversal. Any 3-phase system can
be resolved into 3 balanced 3-phase components: positive, negative and zero sequences using the
following equations:
I
zero
=
1
⁄
3
{ I
1
+ I
2
+ I
3
}
I
pos
=
1
⁄
3
{ I
1
+ aI
2
+ a
2
I
3
}
I
neg
=
1
⁄
3
{ I
1
+ a
2
I
2
+ aI
3
}
V
zero
=
1
⁄
3
{ V
1
+ V
2
+ V
3
}
V
pos
=
1
⁄
3
{ V
1
+ aV
2
+ a
2
V
3
}
V
neg
=
1
⁄
3
{ V
1
+ a
2
V
2
+ aV
3
}
These values are useful for identifying sources of imbalance and for troubleshooting protective relay
settings and wiring faults such as phase reversals.
where a = 120˚ unit vector phase shifter
Appendix B.2 B.3