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Operation
Methods of Measurement/Formulas 3
3-23
Calculation of unbalance as per IEC 61000-4-30:
%100*
2
P
N
V
V
V
=
%100*
0
P
Z
V
V
V =
V
Z
: zero system
V
P
: positive system
V
N
: negative system
The calculation of V
0
, V
2
utilizes the above formulas for V
Z
, V
P
, V
N
or for a 3-wire
system the following formulas with phase-phase voltages (same results):
%100*
631
631
2
β
β
−+
−−
=
V
()
2
2
1,31
2
1,23
2
1,12
4
1,31
4
1,23
4
1,12
kkk
kkk
VVV
VVV
++
++
=
β
Note
For a 3-wire network the zero system component V
z
is 0 per definition.
The voltage values are averaged squared versus time, afterwards the
unbalance is calculated for the time interval.
For more information, see Table 3-2.
Current Unbalance
In addition to the system component parameters that are available for voltage
measurement (zero system, positive system, negative system) for current there is the
special current unbalance calculation.
In some applications when you want to check if the current load of the power system is
distributed equally over the three phases this simpler calculation of current unbalance is
preferred.
Calculation:
Calculate average current I
avg
= (I
1
+ I
2
+ I
3
)/3
Find the maximum deviation from this average current.
Maximum value of max(|I
1
– I
avg
|,|I
2
– I
avg
|,|I
3
– I
avg
|)
Current unbalance is 100 * max(|I
1
– I
avg
|,|I
2
– I
avg
|,|I
3
– I
avg
|) / I
avg
The purpose is to have a simple indicator of detecting an unbalanced current load in a 3-
phase system based on the RMS values only. The phase angles do not matter in this
calculation.
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