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A Series Active Power Filter Based on a Sinusoidal Current-Controlled Voltage-Source
#1

A Series Active Power Filter Based on a Sinusoidal Current-Controlled Voltage-Source Inverter

.pdf   A Series Active Power Filter Based on a Sinusoidal.pdf (Size: 256.25 KB / Downloads: 76)
Abstract
A series active power filter working as a sinusoidal
current source, in phase with the mains voltage, has been developed
and tested. The amplitude of the fundamental current in
the series filter is controlled through the error signal generated
between the load voltage and a preestablished reference. The
control allows an effective correction of power factor, harmonic
distortion, and load voltage regulation. Compared with previous
methods of control developed for series active filters, this method
is simpler to implement, because it is only required to generate a
sinusoidal current, in phase with the mains voltage, the amplitude
of which is controlled through the error in the load voltage. The
proposed system has been studied analytically and tested using
computer simulations and experiments. In the experiments, it
has been verified that the filter keeps the line current almost
sinusoidal and in phase with the line voltage supply. It also
responds very fast under sudden changes in the load conditions,
reaching its steady state in about two cycles of the fundamental.
Index Terms—Active filters, current control, power electronics,
power filters, pulsewidth-modulated power converters.
I. INTRODUCTION
HARMONIC contamination, due to the increment of nonlinear
loads, such as large thyristor power converters,
rectifiers, and arc furnaces, has become a serious problem
in power systems. These problems are partially solved with
the help of LC passive filters. However, this kind of filter
cannot solve random variations in the load current waveform.
They also can produce series and parallel resonance with
source impedance. To solve these problems, shunt active
power filters have been developed [1], [2], which are widely
investigated today. These filters work as current sources,
connected in parallel with the nonlinear load, generating the
harmonic currents the load requires. In this form, the mains
only need to supply the fundamental, avoiding contamination
problems along the transmission lines. With an appropriated
control strategy, it is also possible to correct power factor and
unbalanced loads [3] .
However, the cost of shunt active filters is high, and they
are difficult to implement in large scale. Additionally, they also
present lower efficiency than shunt passive filters. For these
Manuscript received April 15, 1996; revised April 7, 1997. This work was
supported by Conicyt under Proyecto Fondecyt 1940997 and 1960572.
J. W. Dixon is with the Department of Electrical Engineering, Pontificia
Universidad Cat´olica de Chile, Santiago, Chile (e-mail: jdixon[at]ing.puc.cl).
G. Venegas was with the Department of Electrical Engineering, Pontificia
Universidad Cat´olica de Chile, Santiago, Chile. He is now with Pangue S.A.,
Santiago, Chile.
L. A. Mor´an is with the Department of Electrical Engineering, Universidad
de Concepci´on, Concepci´on, Chile (e-mail: lmoran[at]renoir.die.udec.cl).
Publisher Item Identifier S 0278-0046(97)06534-9.
reasons, different solutions are being proposed to improve the
practical utilization of active filters. One of them is the use of
a combined system of shunt passive filters and series active
filters. This solution allows one to design the active filter for
only a fraction of the total load power, reducing costs and
increasing overall system efficiency [4].
Series active filters work as isolators, instead of generators
of harmonics and, hence, they use different control strategies.
Until now, series active filters working as controllable voltage
sources have been proposed [5]. With this approach, the
evaluation of the reference voltage for the series filter is
required. This is normally quite complicated, because the
reference voltage is basically composed by harmonics, and
it then has to be evaluated through precise measurements of
voltages and/or current waveforms. Another way to get the
reference voltage for the series filter is through the “ –
theory” [6]. However, this solution has the drawback of
requiring a very complicated control circuit (several analog
multipliers, dividers, and operational amplifiers).
To simplify the control strategy for series active filters, a
different approach is presented in this paper, i.e., the series
filter is controlled as a sinusoidal current source, instead of a
harmonic voltage source. This approach presents the following
advantages.
1) The control system is simpler, because only a sinusoidal
waveform has to be generated.
2) This sinusoidal waveform to control the current can be
generated in phase with the main supply, allowing unity
power-factor operation.
3) It controls the voltage at the load node, allowing excellent
regulation characteristics.
II. GENERAL DESCRIPTION OF THE SYSTEM
The circuits of Fig. 1(a) and (b) show the block diagram and
the main components, respectively, of the proposed system: the
shunt passive filter, the series active filter, the current transformers
(CT’s), a low-power pulsewidth modulation (PWM)
converter, and the control block to generate the sinusoidal
template for the series active filter. The shunt passive
filter, connected in parallel with the load, is tuned to eliminate
the fifth and seventh harmonics and presents a low-impedance
path for the other load current harmonics. It also helps to
partially correct the power factor. The series active filter,
working as a sinusoidal current source in phase with the line
voltage supply , keeps “unity power factor,” and presents a
very high impedance for current harmonics. The CT’s allow
0278–0046/97$10.00 ã 1997 IEEE
DIXON et al.: SERIES ACTIVE POWER FILTER BASED ON VOLTAGE-SOURCE INVERTER 613
(a)
(b)
Fig. 1. Main components of the series active filter. (a) Block diagram. (b)
Components diagram.
for the isolation of the series filter from the mains and the
matching of the voltage and current rating of the filter with
that of the power system. In Fig. 1, represents the load
current,, the current passing through the shunt passive filter,
and the source current. The source current is forced to
be sinusoidal because of the PWM of the series active filter,
which is controlled by . The sinusoidal waveform of
comes from the line voltage , which is filtered and kept in
phase with the help of the PLL block [Fig. 1(b)].
By keeping the load voltage constant, and with the
same magnitude of the nominal line voltage , a “zeroregulation”
characteristic at the load node is obtained. This
is accomplished by controlling the magnitude of through
the error signal between the load voltage and a reference
voltage . This error signal goes through a PI controller,
represented by the block . is adjusted to be equal
to the nominal line voltage .
The two aforementioned characteristics of operation (“unity
power factor” and “zero regulation”), produce an automatic
phase shift between and , without changing their magnitudes.
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#2
A series active power filter has been developed and tested which functions as a sinusoidal current source, in phase with the mains voltage. The amplitude of the fundamental current in the serial filter is controlled by the error signal generated between the load voltage and a preset reference. The control allows an effective correction of the power factor, harmonic distortion and regulation of the load voltage. Compared with previous control methods developed for serial active filters, this method is simpler to implement because it is only necessary to generate a sinusoidal current, in phase with the mains voltage, whose amplitude is controlled by the error in the voltage of load. The proposed system has been studied analytically and tested using simulations and computer experiments. In the experiments, it has been found that the filter maintains the line current almost sinusoidal and in phase with the line voltage supply. It also responds very quickly under sudden changes in the load conditions, reaching its steady state in approximately two cycles of the fundamental.
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