08-05-2013, 01:01 PM
A Comparative Study of the Voltage Controlled and Current Controlled Voltage Source Inverter for the Distributed Generation System
A Comparative Study.pdf (Size: 1,020.7 KB / Downloads: 30)
ABSTRACT
Voltage source inverters (VSI) have been widely used in
Uninterruptible Power Supplies (UPS), Unified Power
Quality Conditioners (UPQC) and Distributed
Generation Systems (DGS). VSIs are inherently efficient,
compact and economical devices to control power flow
and provide quality supply. VSIs can be classified as
voltage controlled VSIs (VCVSI) and current controlled
VSIs (CCVSI) depending on their control mechanism. In
this paper, a detailed comparison of VCVSI and CCVSI
for DGS applications is presented. This paper examines
the advantages and limitations of each control technique
for different conditions and loads. The design
consideration, theoretical analysis, computer simulation
and experimental results are presented.
INTRODUCTION
Due to improvement in technologies, electrical power
can be generated more efficiently and closer to the point
of consumption. Additionally, Distributed Generation
Systems (DGS) enable Alternative Energy Sources
(AES) to easily utilize and supplement fossil fuels. One
group of AES is Renewable Energy Sources (RES) (eg,
Solar, Wind, Biomass, Wave, Hydro power, etc.).These
can play a major role in the preservation of our
underground resources and the reduction of air
pollutants. Distributed generation systems using RES
have been known to be one of the most cost-effective,
reliable and durable power systems to provide energy
saving and non-interrupted power with high power
quality [1, 2]. DGS can be classified into stand-alone and
grid connected systems (series and parallel processing),
according to their output and connection to other AC
sources and loads. Typical examples of other AC sources
may be the available grid (strong, weak or diesel grids)
or other DGS sources. Recently, the parallel processing
system using power conditioning units (PCU) has
become more popular. As it is inherently efficient,
compact and economical, it offers numerous functions
requiring a minimum number of power conversions [3-
7].
PARALLEL PROCESSING DISTRIBUTED
GENERATION SYSTEMS
The typical configuration of the parallel processing DGS
using VSI is shown in Figure 1. This system consists of
a VSI, which is connected in parallel to the grid for a
CCVSI and through a decoupling inductor for a VCVSI.
The PCU generally connects to batteries and / or RES
such as solar cells and fuel cells. The parallel processing
DGS has two operating modes, parallel processing (gridtied)
and stand-alone.
CURRENT CONTROLLED VSI IN DGS
The CCVSI is used in the parallel processing DGS as
shown in Figure 1. Figure 6 shows the equivalent
schematic diagram of a CCVSI. As a CCVSI controls
the current flow using the VSI switching instants, it is
modelled as a current source. It is important to note that
in this case, as the CCVSI controls the current there is no
need for decoupling inductor. As the output voltage of
the CCVSI is filtered, this current can be assumed to
have only a fundamental frequency (50/60Hz)
(sinusoidal). As the current generated from the CCVSI
can be controlled independently from the voltage, the
active and reactive power controls are decoupled. Hence
unity power factor operation for the whole range of the
load is possible. This is one of the main advantages of
CCVSIs.
1) Load voltage stabilization
It is shown that the VCVSI can regulate the load voltage
within ±5% as per standard. In contrast, as a CCVSI is
connected directly to the grid it cannot compensate the
grid voltage fluctuations without additional hardware
and control feedback algorithm. A decoupling inductor
is essential to decouple the effect of grid voltage
fluctuation, which can be achieved by using standard
VCVSIs.
2) Uninterruptible power supply
As a VCVSI is by nature the same as a voltage source, it
can maintain voltage support for the load in the absence
of a grid (stand-alone operation). It is shown that the
VCVSI cannot provide a pure sinusoidal waveform in
the presence of a nonlinear load, without extra control.
However, as is shown [4] wave shaping of the VCVSI is
possible with extra feedback and hence the sinusoidal
output voltage is guaranteed even in the presence of
nonlinear loads. The CCVSI cannot provide proper
voltage support as by nature it is a current source.
CONCLUSIONS
Voltage source inverters have been widely used in
Uninterruptible Power Supplies and Distributed
Generation Systems. Inherently VSIs are efficient,
compact and economical devices to control power flow
and quality of supply. VSIs can be further classified into
voltage controlled VSIs and current controlled VSIs
depending on their control mechanism. In this paper, a
detailed comparison of VCVSIs and CCVSIs for DGS
applications is presented. It is shown that none of the
VCVSIs or CCVSIs can offer all the functions required
in DGS. Hence, the most appropriate PCU can be chosen
based on its application. This paper examines the
advantages and disadvantages of control techniques for
different loads. The experimental results verify the
design consideration, theoretical analysis and computer
simulation.