Eaton Power Xpert PXM 4000, Руководство пользователя и инструкция по установке

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Page 12
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1 Introduction
SYMMETRICAL COMPONENTS
Symmetrical components are commonly used for analysis of three-phase electrical power
systems. Phase vectors are expressed as sets of phasors, either as real or quadrature or
magnitude and phase. For example, a vector for three-phase voltages could be written as
V
abc
.
.
and the three symmetrical component phasors arranged into a vector as V
012.
:
where V
o
is the zero-sequence of component (the portion of voltage with no rotation)
where V
1
is the positive-seqience component (the portion of voltage with abc rotation)
where V
2
is the negative-sequence component (the portion of voltage with abc rotation).
A phase rotation operator ...... is defined to rotate a phasor vector forward by 120 de -
grees. As a result ...... 2 is a phase rotation of 240 degrees.
A materix “A” can be defined using this operator to transform the phase vector into sym -
metrical components.
The phase voltages are generated by the sequence equation: V
abc
= A • V
12
.
Conversely, the sequence components are generated from the analysis equations V
012
=
A-1 • V
abc
.
While the analysis of symmetrical components is primarily focused on voltage, the analy-
sis of current is equally valid.
VOLTAGE AND CURRENT UNBALANCE
When individual phases of a three-phase system diffeer, an unbalance results. Further-
more, harmonic distortion can also result in unbalance. The measure of unbalance is the
ratio of the negative-sequence to the positive-sequence components and is represented
as a percentage.