METREL MI 2893 - Under-Deviation and Over-Deviation

MI 2893 / MI 2892 / MI 2885 Measurement methods
194
The supply voltage unbalance is evaluated using the method of symmetrical components. In addition to
the positive sequence component U
+
, under unbalanced conditions there also exists negative sequence
component U
-
and zero sequence component U
0
. These quantities are calculated according to the
following equations:
)(
3
1
3
2
21
UaUaUU



)(
3
1
3210
UUUU


,
)(
3
1
32
2
1
UaUaUU



,
(66)
where
0
120
13
2
1
2
1
j
eja 
.
For unbalance calculus, instrument use the fundamental component of the voltage input signals (U
1
, U
2
,
U
3
), measured over a 10/12-cycle time interval.
The negative sequence ratio u
-
, expressed as a percentage, is evaluated by:
100(%) 



U
U
u
(67)
The zero-sequence ratio u
0
, expressed as a percentage, is evaluated by:
100(%)
0
0


U
U
u
(68)
Note: In 3W systems zero sequence components U
0
and I
0
are by definition zero.
The supply current unbalance is evaluated in same fashion.
5.1.12 Under-deviation and over-deviation
Voltage Under-deviation (U
Under
) and Over-deviation (U
Over
) measurement method: Standard compliance:
IEC 61000-4-30 Class A (Section 5.12)
Basic measurement for the Under-deviation and Over-deviation is RMS voltage magnitude measured
over a 10/12-cycle time interval. Each RMS voltage magnitude (i) obtained through recording campaign
is compared to nominal voltage U
Nom
from which we express two vectors according to the formulas
below:






NomRMSNom
NomRMSiRMS
iUnder
UUU
UUU
U
)12/10(
)12/10(),12/10(
,
if
if
(69)






NomRMSNom
NomRMSiRMS
iOver
UUU
UUU
U
)12/10(
)12/10(),12/10(
,
if
if
(70)
Aggregation is performed on the end of recording interval as:
 
%
1
2
,
Nom
n
i
iUnder
Nom
Under
U
n
U
U
U




(71)