User manual
PQM-702, PQM-703 Operating Manual
82
The analyzer is capable of measuring the voltage and current unbalance with a symmetrical
components method. This method is based on the assumption that each set of three unbalanced
vectors can be resolved to three groups of vectors: positive sequence, negative sequence and zero
sequence.
Fig. 34. Example of determining positive sequence component.
Presented example shows the method for calculating voltage positive sequence component.
By definition:
where: U
+
is a vector of positive sequence,
where U
1A
, U
1B
, U
1C
are vectors of fundamental components of phase voltages U
A
, U
B
, U
C
Fig. 34 shows graphical method of determining this component. As we can see from the defini-
tion, the vector of positive-sequence component equals one third of the sum of the components:
U
1A
, aU
1B
, a
2
U
1C
. Operator a and a
2
are unit vectors with angles of 120 and 240. The procedure
is as follow: turn voltage vector U
1B
by 120 counter-clockwise (multiply by a) and add to vector U
1A
.
Then, turn the vector U
1C
by 240 and add to the previous sum of vectors. The result is vector 3U
+
.
Vector U
+
is the desired symmetrical positive sequence component. Note that in case of perfect
symmetry (equal voltages and angles) the positive sequence component is equal to the value of the
phase-to-neutral voltages.
The positive sequence component is a measure of similarity of the tested set of three-phase
vectors to the symmetrical set of positive sequence vectors.
Similarly, the negative sequence component is a measure of similarity to the symmetrical set of
negative sequence vectors.
The zero sequence component exists in the systems in which the sum of three voltages (or
currents) is not equal to zero.
A measure of the system unbalance which is widely used in the power generation is the negative
sequence and zero sequence unbalance (formulas are for voltage):