Electrical Power and Energy Systems 78 (2016) 183–193
Contents lists available at ScienceDirect
Electrical Power and Energy Systems
journal homepage: www.elsevier.com/locate/ijepes
Performance and design of an open-delta connected grid tied
bidirectional PWM converter
Shilpi Bhattacharya a,⇑, Prabal Deb b, Sujit K. Biswas c, Suparna KarChowdhury c
a
Dept. of Electrical Engg., RCC Institute of Information Technology, Kolkata 700015, India
Dept. of Electrical Engg., Guru Nanak Institute of Technology, Kolkata 700114, India
c
Dept. of Electrical Engg., Jadavpur University, Kolkata 700032, India
b
a r t i c l e
i n f o
Article history:
Received 29 October 2014
Received in revised form 2 October 2015
Accepted 25 November 2015
Available online 17 December 2015
Keywords:
Open-delta connection
Bidirectional converter
Grid connected inverter
4-switch converter
a b s t r a c t
Bidirectional ac to dc converters connected to the ac supply lines is an important area of research in view
of the increasing requirements of grid tied converters for renewable energy systems, having nearsinusoidal ac line current and near-unity power factor. Total losses and hence the efficiency of these converters as well as its control complexity depend on the number of devices used in the particular structure
considered. In view of this, a four-switch bidirectional converter, connected in an Open Delta configuration to the three phase ac grid, is a cost effective and energy efficient solution compared to the traditional
six-switch topology. However, application of this configuration has been on a cautious pace in spite of the
number of positive traits, as the application details of this circuit for bidirectional grid connection is not
readily available in literature. This application oriented paper presents an in-depth study of this structure
with regard to its operating principle, its advantages & disadvantages, its start-up process, its system controller and performance in grid connected mode. The process of design of such converters is also highlighted. The discussions are supported by simulation and experimental results carried out on a
laboratory prototype, which are found to be in good agreement with the theoretical predictions.
Ó 2015 Elsevier Ltd. All rights reserved.
Introduction
Non-conventional energy sources are rapidly gaining popularity
for clean energy generation and require interfacing to the threephase power system grid [1]. An important issue in grid interfacing
is the optimal integration of the energy resources to the existing
electrical distribution systems, where efficiency is a primary
requirement [2]. Bidirectional converters are required for several
applications, including leveling control using Battery Energy Storage Systems (BESS) [3]. The inverter must also fulfill the required
grid codes [4] for all such grid tied operation without increasing
much complexity or sacrificing reliability.
To tackle these problems, Pulse Width Modulation (PWM)
based converters have been proposed that create a nearsinusoidal input current with near-unity Displacement Factor [5–
7]. Extensive research over the years has resulted in the adoption
of three-phase PWM Voltage Source Converter (VSC), involving
the use of six semiconductor switches in a three-phase bridge
⇑ Corresponding author.
E-mail addresses: shilpibhatt@yahoo.com (S. Bhattacharya), deb_prabal@yahoo.
com (P. Deb), sujit_biswas@hotmail.com (S.K. Biswas), suparna1964@yahoo.com
(S. KarChowdhury).
http://dx.doi.org/10.1016/j.ijepes.2015.11.079
0142-0615/Ó 2015 Elsevier Ltd. All rights reserved.
configuration, for grid tied systems [8–11]. The major advantage
of these topologies is that they permit bidirectional power flow
with near-sinusoidal current, which is at near-unity power factor
(or even leading pf), making them attractive for use as ac line connected front-end converters in a variety of applications ranging
from regenerative electric drives to grid connected energy sources.
As a result of the progress made in the field of power converter
topologies, various modified converter topologies have been developed. During a period, concentration was laid upon development of
economical low cost drive systems using lesser number of power
switches. Some component minimized converter topologies have
been reported for ac–ac power conversion such as in [12,13], an
induction motor drive system was assessed by using a fourswitch bridge inverter and a detailed comparative study with a
six-switch bridge inverter was carried out using specific control
strategies. Also single-phase to three-phase converters implemented for drive applications with input power factor and current
control were explored using a total of six power switches instead of
eight [14,15]. This structure essentially had single-phase ac–dc
conversion using two switches in one limb and two split capacitors
in the other limb forming a voltage-doubler type of arrangement.
Consequently, energy savings and cost reduction were the key
issues. A three-phase to three-phase PWM VSC with four switches
184
S. Bhattacharya et al. / Electrical Power and Energy Systems 78 (2016) 183–193
switches of each limb is phase shifted by 60° from the other, such
that the resulting fundamental voltage at the output of limbs a & b
form an open-delta or V-connected source with respect to O. Thus,
fundamental voltages VaO and VOb are phase shifted by 120° from
each other as shown in Fig. 1b. The three phase ac output is taken
from the three points O, a & b corresponding to R, Y, & B, in a threewire open-delta-connection, using one line connection from the dc
mid-point O and the others from the outputs a & b of the two
limbs. This creates a three-level waveform between the two output
lines a & b due to the difference created between two two-level
waveforms in this case, while the waveforms between a & O or b
& O is still two-level. Thus, the behavior is not actually symmetric
between the lines, although the fundamental line voltages are
balanced.
was proposed in [16,17], with its advantages and restrictions also
given. This structure appeared to be quite promising and created
quite a lot of interest for the various advantages it offered with
reduced number of power devices. It is shown that a four-switch
inverter can produce balanced three-phase output voltage at
adjustable voltage and frequency. This topology can be controlled
to provide the positive features of sinusoidal input current, unity
or even leading power factor and bidirectional power flow as in a
six-switch structure. Compared to a six-switch PWM VSC, this
structure is more economical, provides more savings in energy
specifically due to reduction in conduction losses. However, apart
from its various advantages, this structure also suffers from some
demerits of increased voltage stress on the power devices,
increased switching losses in each device due to high dc bus voltage and fluctuation of voltage across individual split capacitors.
In spite of the existence of the four-switch inverter for quite
some time, its performance as a grid tied bidirectional converter
has not been evaluated so far. Neither has its design procedure
been highlighted. In order to promote the use of open-delta bidirectional converter, a study of its structure is made and its performance characteristics are investigated in this paper. Some essential
design guidelines of this converter are also presented. Finally, the
grid tied system is simulated and the simulation results are compared with actual test on a laboratory prototype.
System structure and operation
The structure of an open-delta connected or four-switch bidirectional Voltage Source Converter (VSC) is shown in Fig. 2. It has
two legs consisting of two switches each and the third leg consisting of two split capacitors across the dc bus. The power circuit thus
consists of four semiconductor power switches (IGBT’s/MOSFET)
with respective antiparallel diodes, two dc filter capacitors in series and three ac-side series reactors. The ac side inductors are
required as usual for several reasons, including forcing the currents
to track current references, providing the ability to boost the dc
voltage above the peak of the ac supply and supporting the instantaneous difference between the sinusoidal line voltage and the
PWM converter ac-side voltage. Although only two switching
limbs are used, three series inductors on the three ac lines are
needed to maintain balanced three-phase operation. The switches
operate in a PWM pattern to shape the input currents to follow the
created sinusoidal current references. In this structure currents iR
and iY are controlled while the current of the third phase
[iB = (iR + iY)] is automatically maintained nearly sinusoidal. In
this converter, the input current wave shaping is accomplished
Principle of the open-delta bidirectional converter
The Open-Delta PWM inverter topology using four power
switches in a bridge configuration is derived from the half-bridge
inverter as shown in Fig. 1, which generates a two-level PWM voltage at the mid-point of a limb comprising of two series connected
controlled switches (connected with anti-parallel diodes), with
respect to the mid-point O of the dc bus capacitors. This may be
considered as one phase of the converter and now two such limbs
are used to create an equivalent three-phase source comprising of
four switching devices, as shown in Fig. 1a. The control of the
+Vdc
+Vdc
b
VbO
a
O
b
O
a
O
VaO
0V
0V
+Vdc
R
VOb
a
O
Vba
b
B
Y
VaO
0V
(a)
(b)
Fig. 1. Open-delta connected three phase two-limb inverter system.
ID
398422
Title
Performanceanddesignofanopen-deltaconnectedgridtiedbidirectionalPWMconverter
http://fulltext.study/article/398422
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