26-07-2012, 12:53 PM
Single-Phase to Three-Phase Drive System Using Two Parallel Single-Phase Rectifiers
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Abstract
This paper proposes a single-phase to three-phase
drive system composed of two parallel single-phase rectifiers, a
three-phase inverter, and an induction motor. The proposed topology
permits to reduce the rectifier switch currents, the harmonic
distortion at the input converter side, and presents improvements
on the fault tolerance characteristics. Even with the increase in the
number of switches, the total energy loss of the proposed system
may be lower than that of a conventional one. The model of the
system is derived, and it is shown that the reduction of circulating
current is an important objective in the system design. A suitable
control strategy, including the pulsewidth modulation technique
(PWM), is developed. Experimental results are presented as well.
Index Terms—Ac-dc-ac power converter, drive system, parallel
converter.
I. INTRODUCTION
SEVERAL solutions have been proposed when the objective
is to supply a three-phase motors from a single-phase
ac mains [1]–[9]. It is quite common to have only a singlephase
power grid in residential, commercial, manufacturing,
and mainly in rural areas, while the adjustable speed drives may
request a three-phase power grid.
Single-phase to three-phase ac–dc–ac conversion usually
employs a full-bridge topology, which implies in ten power
switches, as shown in Fig. 1. This converter is denoted here as
conventional topology.
Parallel converters have been used to improve the power capability,
reliability, efficiency, and redundancy. Parallel converter
techniques can be employed to improve the performance of
active power filters [10]–[13], uninterruptible power supplies
(UPS) [14]–[16], fault tolerance of doubly fed induction generators
[17], and three-phase drives [18], [19]. Usually the operation
of converters in parallel requires a transformer for isolation.
However, weight, size, and cost associated with the transformer
may make such a solution undesirable [20]. When an isolation
transformer is not used, the reduction of circulating currents
among different converter stages is an important objective in
the system design [21]–[26].
In this paper, a single-phase to three-phase drive system composed
of two parallel single-phase rectifiers and a three-phase
Manuscript received June 8, 2009; revisedNovember 2, 2009. Current version
published May 7, 2010. This work was supported by the National Council for
Scientific and Technological Development (CNPq), by the Coordination for the
Improvement of Higher Education Personnel (CAPES), and by the Foundation
for Research Support of the State of Para´ıba (FAPESQ). Recommended for
publication by Associate Editor F. Blaabjerg.
The authors are with the Electrical Engineering Department, Federal University
of CampinaGrande, 58109-970 CampinaGrande,Brazil (e-mail: jacobina@
dee.ufcg.edu.br; euzeli[at]dee.ufcg.edu.br; nadyrocha[at]gmail.com; edgardluiz@
gmail.com).
Digital Object Identifier 10.1109/TPEL.2009.2037420
Fig. 1. Conventional single-phase to three-phase drive system.
Fig. 2. Proposed single-phase to three-phase drive system.
inverter is proposed, as shown in Fig. 2. The proposed system
is conceived to operate where the single-phase utility grid is the
unique option available. Compared to the conventional topology,
the proposed system permits: to reduce the rectifier switch
currents; the total harmonic distortion (THD) of the grid current
with same switching frequency or the switching frequency
with same THD of the grid current; and to increase the fault
tolerance characteristics. In addition, the losses of the proposed
system may be lower than that of the conventional counterpart.
The aforementioned benefits justify the initial investment of the
proposed system, due to the increase of number of switches.
VI. RATINGS OF SWITCHES
Assuming same rms voltages at both grid and machine sides,
a machine power factor of 0.85 and neglecting the converter
losses, currents of the rectifier switches normalized in terms
of currents of the inverter switches are 2.55 and 1.27 for the
conventional and the proposed single-phase to three-phase converter,
respectively. Fig. 7(a) and (b) shows the flow of active
power in the conventional and in the proposed single-phase to
three-phase converter, respectively. For balanced system (L
g =
La = L
a = Lb = L
b ), voltage vo is close to zero, so that the
dc-link voltage is equal to that required by the conventional system.
Since the parallel connection scheme permits to reduce the
switch currents and preserve the dc-link voltage, the rating of
each power switch in the rectifier side is reduced.
VII. DC-LINK CAPACITOR
The dc-link capacitor current behavior is examined in this
section. Fig. 8 illustrates the harmonic spectrums of the dc-link
capacitor current for the conventional converter (μ = 0.5) [see
Fig. 8(a)] and for the proposed converter using single-carrier
with μ = 0.5 [see Fig. 8(b)], double-carrier with μ = 0.5 [see
Fig. 8©] and double-carrier with μ = 0 [see Fig. 8(d)]. The
proposed converter using double-carrier with μ = 0 provides
the best reduction of the high frequency harmonics. Table I
(obtained from Fig. 8) presents the THD of the dc-link capacitor
current of the proposed converter (THDp) referred to the THD
of the conventional converter (THDc). The highest reduction
of THD is obtained for the converter using double-carrier with
μ = 0. The THD obtained for μ = 1 is equal to that for μ = 0.
Fig. 8. Harmonic spectrum of the dc-link capacitor current. (a) Conventional
converter (μ = 0.5). (b) Proposed converter with single-carrier (μ = 0.5).
© Proposed converter with double-carrier (μ = 0.5). (d) Proposed converter
with double-carrier (μ = 0).
TABLE I
NORMALIZED THD OF DC-LINK CURRENT OF THE PROPOSED CONVERTER
It is possible to reduce the second order harmonic introduced
by single-phase operation, but this is not of interest because
it requires unbalancing and increasing rectifier currents ia
and ib .
VIII. INPUT INDUCTORS
The PWMwith double-carrier strategy reduces the WTHD of
the resultant rectifier voltage vab , as observed in Fig. 4. When
the input inductors of the proposed topology (L
g ) are equal to
that of the conventional topology (Lg ), the reduction of the THD
of the grid current is directly indicated in Fig. 4.
Fig. 9 depicts the THD of the grid current as a function of
μ for different values of ln [the inductances of rectifiers A and
B (l
g ) referred to that of the conventional configuration (lg ),
i.e., ln = l
g /lg ]. For ln > 0.4 (l
g > 0.4lg) the THD of the grid
current of the proposed topology is smaller than that of the
conventional topology.
The harmonic distortion of the rectifier currents (ia , i
a , ib, i
b ,
and io ) is higher than that of the grid current ig . The adequate
choice of the PWM strategy permits to operate with minimum
harmonic distortion. We have considered the losses as the main
Authorized licensed use limited to: Guru Anandan Saminathan. Downloaded on June 14,2010 at 07:54:00 UTC from IEEE Xplore. Restrictions apply.
1290 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 5, MAY 2010
Fig. 9. Inductor specification in terms of THD of ig and μ.
concern to define the maximum acceptable harmonic distortion
of the rectifier currents (see Section X).
In any case, the use of additional common-mode inductors
is a very efficient manner of reduce the harmonic distortion of
these currents [12]. This approach may be also employed in
the present case to reduce the total inductance required for an
adequate operation of the system.
The design of inductors may follows the guide lines presented
in [12] for an active power filter system.