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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 21, NO. 3, SEPTEMBER 2006
Application of Voltage- and Current-Controlled
Voltage Source Inverters for Distributed
Generation Systems
Sung-Hun Ko, Seong R. Lee, Hooman Dehbonei, Member, IEEE, and Chemmangot V. Nayar, Senior Member, IEEE
Abstract—Voltage source inverters (VSI) have been widely used
in uninterruptible power supplies, unified power flow controllers
or unified power quality conditioners, and distributed generation
systems (DGS). VSIs are inherently efficient, compact, and economical devices used to control power flow and provide quality
supply. VSIs can be classified as voltage-controlled VSIs (VCVSIs)
and current-controlled VSIs (CCVSIs), depending on their control mechanism. In this paper, a detailed comparison of VCVSIs
and CCVSIs for DGS applications is presented. This paper examines the advantages and limitations of each control technique in
a single-phase DGS, without incorporating additional hardware
and/or extra complex control techniques. Discussions on the concepts, hypotheses, and computer simulations of different VSIs in
the presence of different loads and conditions are presented. The
experimental results confirm the validity of the analysis and simulations outlined. The paper provides design recommendations for
the use of VCVSIs and CCVSIs in various applications.
Index Terms—DC–AC power conversion, energy conversion,
power conditioner, power electronics.
I. INTRODUCTION
UE 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. Renewable energy sources (RES)
(e.g., solar, wind, biomass, wave, hydropower, etc.) can play a
major role in the preservation of our underground resources and
the reduction of air pollutants. DGS based on RES have been
known to be one of the most cost-effective, reliable, and durable
power systems to provide energy saving and noninterrupted
power with high power quality [1]–[5]. DGS can be classified further into stand-alone and grid connected systems (series
and parallel processing), according to the output of the voltage
source inverters (VSIs) and connection to other ac sources and
loads [6]. Typical examples of other ac sources are any available grid (strong, weak, or diesel grids) or other DGS sources.
VSIs are inherently efficient, compact, and economical and offer
D
Manuscript received October 25, 2005; revised February 7, 2006. This work
was supported in part by the Australian Research Council under Grant LP
0348994 and in part by the Postdoctoral Fellowship Program of the Ministry of
Commerce, Industry & Energy (MOCIE). Paper no. TEC-00360-2005.
S.-H. Ko and S. R. Lee are with the School of Electronic & Information
Engineering, Kunsan National University, Kunsan 573-701, Korea.
H. Dehbonei and C. V. Nayar are with the Department of Electrical and
Computer Engineering, Curtin University of Technology, Perth 6854, Australia
(e-mail: [email protected]).
Digital Object Identifier 10.1109/TEC.2006.877371
numerous functions that require a minimum number of power
conversions [7]–[11].
The parallel processing DGS controls power flow and quality by controlling the power conversion between the dc bus
of bi-directional VSIs and the available grid [12], [13]. The
bi-directional VSIs can be further classified into VCVSIs and
CCVSIs, depending on their control mechanism [14]. In DGS,
VCVSIs use the amplitude and phase of an inverter output voltage relative to the grid voltage to control the power
flow [15]. In VCVSIs, the desired current flow is generated
by controlling the voltage across the decoupling inductor. The
CCVSI uses switching instants to generate the desired current
flow in the VSI’s inductor filter, using instantaneous current
feedback [16].
There are advantages and limitations associated with each
control mechanism. For instance, VCVSIs provide voltage support to the load (the VSI operating as a voltage source), while
CCVSIs provide current support (the VSI operating as a current source). The CCVSI is faster in response compared to the
VCVSI, as its power flow is controlled by the switching instant,
whereas in the VCVSI, the power flow is controlled by adjusting
the voltage across the decoupling inductor. Active and reactive
power is controlled independently in the CCVSI, but are coupled
in the VCVSI. Generally, the advantages of one type of VSIs
are considered as a limitation of the other type. In this paper,
a detailed comparison of VCVSIs and CCVSIs is investigated
for DGS under various conditions. The experimental results on
a scaled-down version (1 kVA) of DGS, confirm the validity of
theoretical and simulation studies. The design consideration and
summary of different VSI controls is presented in Section V.
II. PARALLEL PROCESSING DISTRIBUTED GENERATION
SYSTEMS
A typical configuration of the parallel processing DGS using
a VSI is shown in Fig. 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. It is generally expected that
the VSI performs the following functions in DGS [7], [17]–[19]:
1) Load voltage stabilization (±5% voltage regulation) in
both parallel processing and stand-alone modes;
2) Uninterruptible power supply (UPS);
3) Reactive power support—grid power conditioning including power factor correction (>0.9) and harmonics mitigation (THD<5%) (only in parallel processing mode) as
per IEEE standard 1159 [17];
0885-8969/$20.00 © 2006 IEEE
KO et al.: APPLICATION OF VOLTAGE- AND CCVSIs FOR DISTRIBUTED GENERATION SYSTEMS
Fig. 1.
783
Schematic diagram of a parallel processing DGS.
Fig. 3. Phasor diagram of a VCVSI-based DGS with resistive load and grid
is responsible for supplying the active power.
Fig. 2.
The equivalent circuit diagram of a VCVSI-based DGS.
4) Active power support—load power conditioning including demand side management (DSM). In this mode of
operation, a bi-directional VSI is responsible for controlling the active power flow between the dc bus and the ac
grid.
A. Voltage-Controlled VSIs in DGS
Fig. 2 shows the simplified/equivalent schematic diagram of a
VCVSI. For the following analysis it is assumed that the output
low pass filters (Lf and Cf ) of VSIs will filter out high-order
harmonics generated by pulse width modulations (PWMs). The
decoupling inductor (Xm ) is an essential part of any VCVSI as
it makes the power flow control possible. In a VCVSI, the power
flow of the DGS is controlled by adjusting the amplitude and
the phase [power angle (δ)] of the inverter output voltage with
respect to the grid voltage. Hence, it is important to consider
the proper sizing of the decoupling inductor and the maximum
power angle to provide the required power flow when designing
VCVSIs.
Assuming the maximum permissible voltage fluctuation in
the grid voltage (Vg ) is ±20% and the grid has to supply the
active power demanded by a resistive load, the phasor diagram
of the VCVSI-based DGS is shown in Fig. 3. In this figure, it is
assumed that the voltage of the inverter has to be kept constant
(Vc1 = Vc2 = Vc3 , load voltage stabilization). Fig. 3 shows that
as the VCVSI voltage retains constant, any changes in the grid
voltage to control the desired power flow, the power angle has
to change in proportion. The power angle could be both lagging
or leading, providing either the active power flow from the grid
to the VCVSI or vice versa. Fig. 3 shows that the lagging power
angles result in active power from the grid towards the inverter,
regardless of the grid voltage’s amplitude and minimum power
angle obtains when the grid and the VCVSI voltage are identical. This figure shows that reactive power always flows from
the higher voltage source to the lower voltage source. Hence,
the higher voltage source has to supply all the reactive power
demanded by the decoupling inductor as well as load. In weak
grid applications, when the grid voltage drops considerably, the
VCVSI has to supply both the rated active power and full reactive power, resulting in over sizing of the inverter (>100%
of the rated power). Unity power factor operation (Ig p 1 = Ig 1 )
is only possible if the grid voltage is reduced to Vg 1 and at the
specific power flow corresponds to Vx1 . This is a special case,
which depends on the size of the decoupling inductor, the load
and maximum permissible power angle. Therefore, power factor correction is not possible using VCVSIs in DGS. This is one
of the main drawbacks of VCVSI-based DGS.
Using Fig. 2 the fundamental grid current can be expressed
as (1)
Ig =
Vg 0 − Vc δ
Vc sin δ
Vg − Vc cos δ
=−
−j
jXm
Xm
Xm
(1)
where Vg and Vc are, respectively, the grid and the
VCVSI’s fundamental voltages, and Xm is the decoupling inductor impedance. Using per unit values (Sbase =
2
Vbase
/Zbase , Vbase = Vc and Zbase = Xm ) where Vbase , Zbase ,
and Sbase are the base voltage, impedance, and complex power
values, respectively, the grid apparent power can be expressed
as (2):
2
− Vgpu cos δ .
(2)
Sgpu = −Vgpu sin δ + j Vgpu
Using per unit values, the complex power of the VCVSI and
decoupling inductor are
Scpu = −Vgpu sin δ + j[Vgpu cos δ − 1]
2
Sxpu = j Vgpu
− 2Vgpu cos δ + 1
(3)
(4)
where Sgpu , Scpu , and Sxpu are per unit values of the grid,
VCVSI and decoupling inductor apparent power respectively,
and Vgpu is the per unit value of the grid voltage.
As addressed above, since the load voltage must remain constant (load voltage stabilization), the only controllable parameter
in the VCVSI is the power angle (δ). Hence, the power angle is
used in a VCVSI for DSM operation. For DSM operation, it is
important to extract the maximum power from RES and supply
this power to the load or DGS. Assuming that both RES and the
grid are supplying the demanded active power by the load, the
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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 21, NO. 3, SEPTEMBER 2006
Fig. 6.
Fig. 4. Control block diagram of the VCVSI-based DGS used in the simulations and experiments.
Fig. 5.
The equivalent circuit of a CCVSI-based DGS.
power angle can be calculated from (5)
Pg = PL − PRES =
Vg Vc
sin δ.
Xm
Thus, the power angle (δ) is
(PL − PRES )Xm
δ = sin−1
Vg Vc
(5)
(6)
where PL and PRES are the load and RES active power, respectively. Equation (6) explains that as the available RES energy is
increased, the power angle is reduced. This means that the RES’
penetration will increase. Fig. 4 shows the block diagram of a
VCVSI control system based DGS used in both the simulations
and experiments. This control block diagram includes the DSM
function (6). If the RES available energy is more than the load
consumption, then the power angle can be leading to export this
extra active power to the DGS.
In Fig. 4,the phase locked loop (PLL) is responsible for synchronizing the inverter output voltage with the grid voltage. The
sampling from the load current, RES voltage and current is also
∗
) (for DSM opused to generate the required power angle (δref
eration). After comparing the required/reference values and the
actual variables, an error signal is generated to feed a PI controller. After generating the desired reference signal, it is given
to the PWM generator block to generate the required switching
signals.
B. Current-Controlled VSIs in DGS
Fig. 5 shows the equivalent schematic diagram of a CCVSI.
As CCVSI controls the current flow using the VSI switching
instants, it can be modeled as a current source and there is no
need for a decoupling inductor (Fig. 5). As the current gener-
Phasor diagram of a CCVSI-based DGS with inductive load.
ated from the CCVSI can be controlled independently from the
ac voltage, the active and reactive power controls are decoupled. Hence, unity power factor operation is possible for the
whole range of the load. This is one of the main advantages
of CCVSIs.
As the CCVSI connects in parallel to the DGS, it follows
the grid voltage. Fig. 6 shows the phasor diagram of a CCVSIbased DGS in the presence of an inductive load (considering the
same assumption as VCVSI section). Fig. 6 shows that when
the grid voltage increases, the load’s active power consumption,
which supplied by the grid increases and the CCVSI compensates the increase in the load reactive power demand. In this
case, the CCVSI maintains grid supply at unity power factor,
keeping the current phase delay with respect to the grid voltage
at a fixed value (Θ). Therefore, the CCVSI cannot maintain the
load voltage in the presence of a DGS without utilizing extra
hardware and control mechanisms. This limitation on load voltage stabilization is one of the main drawbacks of CCVSI-based
DGS.
Assuming the load active current demand is supplied by the
grid (reactive power support function), the required grid current
can be rewritten as follows:
SL
∗
(7)
Ig = Re[IL ] = Re
Vg
where SL is the demanded load apparent power. For grid power
conditioning, it is preferred that the load operate at unity power
factor. Therefore, the CCVSI must provide the remainder of the
required current (8)
Ic = IL − Ig∗ .
(8)
In DSM, it is desirable to supply the active power by the RES,
where excess energy from the RES is injected into the DGS. The
remaining load reactive power will be supplied by the CCVSI.
Hence, (8) can be rewritten as (9)
SL − PRES
∗
Ig = Re[IL ] − Re[Ic ] = Re
(9)
Vg
Equations (8) and (9) show that in the worst case, the CCVSI
has to supply both the active and reactive power demanded by
the load. This means that the CCVSI sizing can be rated at
full load without the need to oversize. This is an advantage of
CCVSI-based DGS compared to the VCVSI. The control block
diagram of the CCVSI-based DGS used in the simulation and
experiment using (9) and (10) is shown in Fig. 7.
The CCVSI control scheme samples the DGS voltage (Vg )
for synchronization using a PLL. The samples of load current
(ILoad ), RES current (IRES ), and voltage (VRES ) are used to
KO et al.: APPLICATION OF VOLTAGE- AND CCVSIs FOR DISTRIBUTED GENERATION SYSTEMS
785
Fig. 7. Control block diagram of the CCVSI-based DGS used in the simulations and experiments.
TABLE I
SIMULATION CONDITIONS AND SELECTED PARAMETERS WHERE L m IS THE
DECOUPLING INDUCTOR, L f IS THE FILTER INDUCTOR, AND C f IS THE FILTER
CAPACITOR
generate the desired inverter current amplitude (Id∗ ) (reference
current) using (9) and (10). After generating the reference current signal (Ic-ref )), this current is compared to the instantaneous CCVSI current in order to generate the error current
(Ierr ). This error current is then given to the current regulator
block to generate the desired instantaneous switching PWMs.
III. SIMULATION RESULTS
To compare the performance of the parallel processing DGS
using a VCVSI and CCVSI, a 1kVA system including linear
and nonlinear loads was simulated using PSim software. Table I
shows the different parameters and selected values identical
with the experimental hardware used in simulation, to provide
a foundation to compare results.
A. Power Conditioning in DGS
This simulation was conducted to evaluate the performance
of the different VSIs in the presence of different loads, where
Vg and Vc are the voltage waveforms of the grid and inverter,
and Ig , Ic , and Iload are current waveforms of the grid, inverter
and load, respectively. Fig. 8 shows the power conditioning of
a DGS in the presence of an inductive load (Z = 40 36.7◦ [Ω]),
using different VSIs. Fig. 8(a) shows that the grid can supply
the load’s active power when the grid voltage is almost the
same as the VCVSI voltage at a low power angle. In this case,
the required reactive power (600 var) supplied by the VCVSI,
the power factor is good as the inverter size and hence the
decoupling inductor is relatively small. Fig. 8(b) shows that the
CCVSI can supply the reactive power required by the load while
Fig. 8. Waveform results of power conditioning of a DGS in the presence of
an inductive load (z = 40 36.7◦ [Ω]). (a) VCVSI. (b) CCVSI.
the active power is supplied fully by the grid in the same way
as for VCVSI.
Fig. 9(a) shows the VCVSI as a power conditioner for a
DGS in the presence of a nonlinear load (a RLC diode bridge
rectifier). The VCVSI cannot maintain pure sinusoidal voltage
across the nonlinear load (Vc ). Hence, a portion of low-order
current harmonics will be injected into the grid (Fig. 10(b),
ITHD = 10.9%). Fig. 9(b) shows that the CCVSI can provide
all the reactive power demanded by the nonlinear load and hence
the grid supplies only the remaining active power (unity power
factor operation). In this case, the CCVSI prevents any low-order
harmonics from being injected into the grid (active filtering)
(Fig. 10(c), ITHD = 1.1%).
Fig. 10(a) shows the DGS in the absence of VSIs. In this
case, all the reactive power associated with low-order harmonics
from the nonlinear load must be supplied by the grid (ITHD =
60.8%). This figure also signifies that a VCVSI cannot meet the
IEEE standard 1159 (less than 5% of THD) when a nonlinear
load is presented, while a CCVSI can achieve unity PF and
satisfies THD requirements of voltage and current for the full
range of the load, without the need for an additional controller
(assuming that the grid voltage is sinusoidal).
B. UPS Function in DGS
To study the performance of each VSI controller in UPS
mode, the system was simulated in the presence of nonlinear
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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 21, NO. 3, SEPTEMBER 2006
Fig. 9. Waveform results of power conditioning of a DGS in the presence of
a nonlinear load (a RLC diode bridge rectifier). (a) VCVSI. (b) CCVSI.
loads (Fig. 11). It was assumed that at 30 ms the grid fails and
both VSIs had to supply the load. As it is shown, before grid
failure the VCVSI supplied the reactive power demanded by
the nonlinear load and the rest of the reactive power demanded
by the decoupling inductor supplied by the grid (when both the
grid and the VCVSI voltages were identical). This figure shows
that the VCVSI picked up the load rapidly after the grid failed
[Fig. 11(a)]. Fig. 11(b) shows that the CCVSI provides/absorbs
almost all the nonlinear current to/from the load before grid
failure and full load current afterwards to supply the load. It is
shown in the presence of a nonlinear load that a CCVSI cannot
provide and maintain a sinusoidal voltage waveform even with
extra voltage feedback [Fig. 11(b)]. However, the VCVSI can
provide the required voltage support and UPS function, even in
the presence of a nonlinear load and without the need for extra
feedback or load estimation control algorithms.
It is shown that in the presence of a nonlinear load, the VCVSI
can maintain the load voltage VTHD at 11.2% while the load
current ITHD is 50.7%. These values can be read as high as
36.3% load voltage VTHD and as low as 0.7% load current
ITHD in CCVSI and stand-alone operations. The significance
of this data is that both the VCVSI and the CCVSI cannot
compensate low-order harmonics of the load voltage in order
to meet IEEE standards (eg., 1159 and 944) in the presence of
Fig. 10. The harmonic spectrum of the grid current in the absence and presence
of VSIs supplying nonlinear load. (a) Without power conditioning. (b) VCVSI.
(c) CCVSI.
nonlinear loads, without extra feedback and complex control
algorithms [8].
C. DSM Function in DGS
In RES-based DGS, it is required to give priority of supply
to the RES and reduce the share of the grid to supply the load.
If the RES is not enough, then both the RES and the grid will
supply the load demand. Fig. 12 illustrates the case that the grid
is the only available source to supply the required active power
demanded by the linear load (100% of 1 kw) and suddenly,
when the RES becomes available (50%), allows the RES to
take part and supply 50% of the load demand. Fig. 12(a) shows
that the VCVSI maintains load voltage while the RES begins to
supply 50% of the load demand. In this case, the grid current is
reduced to 50% while the load current is maintained at 100%.
Hence, the load does not detect any abnormality in the supply.
Fig. 12(b) shows that a CCVSI can perform DSM and supply
the load (at 50%) in the same way as a VCVSI. The delay in the
power waveforms in Fig. 12 are due to the existence of a low
pass filter in the power meter used for power measurements in
KO et al.: APPLICATION OF VOLTAGE- AND CCVSIs FOR DISTRIBUTED GENERATION SYSTEMS
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Fig. 11. UPS mode waveform results of a DGS in the presence of nonlinear
load. (a) VCVSI. (b) CCVSI.
Fig. 12. Waveform results of demand side management in DGS (grid supplies
linear load from 100% to 50%). (a) VCVSI. (b) CCVSI.
PSim software. Both power diagrams in Fig. 12 show the DSM
capability of CCVSIs and VCVSIs.
D. Voltage Regulation in DGS
In this simulation, the grid voltage was changed from its
nominal value (here 200 V) to 160 V. It was assumed that the
grid has to supply the load active power. Fig. 13(a) shows the
voltage stabilization for the load when the grid voltage fluctuated
in the presence of a VCVSI. After a step down in the DGS
voltage from 200 to 160 V, the grid can still supply the active
power while the VCVSI maintains the load voltage. In this case,
the VCVSI has to supply the extra reactive power demanded
by the decoupling inductor, which is dependent on the DGS
voltage and the inductor’s size. In this case, the grid current
increases due to an increase in reactive current flow from the
VCVSI to the grid. Fig. 13(b) shows the voltage stabilization
for the load when the grid voltage fluctuated in the presence
of a CCVSI. Due to the direct connection of the CCVSI to the
DGS, it cannot compensate the grid voltage fluctuations without
additional hardware and control feedback algorithm [20], [21].
Hence, the load voltage cannot be maintained and the load will
suffer from grid voltage fluctuations. As it is assumed in this case
that the DGS must supply the active power demanded by load,
the CCVSI current remains at zero even after the DGS voltage
drops. In this case, the load demand was reduced in proportion
to the decreases in the DGS voltage, and hence resulted in less
active power support by the DGS.
IV. EXPERIMENTAL RESULTS
Fig. 14 shows a photograph of a scale down version of a
DGS that was prototyped to examine the analytical and simulation analysis. The experimental setup consists of a computer to
monitor and program the desired control techniques (Figs. 4 and
7) into a digital signal processor (DSP), to provide a switching
signal to a control board and a VSI. A variac is used to simulate
a weak grid, while different loads are connected to the output of
the VSIs. The scope and power analyzers were used to record the
information for further evaluative comparisons of the analytical
and simulation results.
System specifications are given in Table II. As a battery bank
of 180 V was used in this test, a low frequency transformer was
utilized to step-up the output voltage of the H-bridge inverter to
the required value (200 V).
The Voltech (PM3000A) power meter measures a power factor of over 0.99 for a CCVSI and for a VCVSI, from 0.97 to
0.99, depending on the load and grid voltage fluctuation for
linear loads. This relatively good power factor for the VCVSI
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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 21, NO. 3, SEPTEMBER 2006
Fig. 13. Waveform results of voltage stabilization when the grid voltage
changes from 200 to 160 V in DGS in the presence of a linear load (R = 40 [Ω]).
(a) VCVSI. (b) CCVSI.
Fig. 15. Experimental waveform results of power conditioning of a DGS in
the presence of an inductive load (z = 40 36.7◦ [Ω]). (a) VCVSI. (b) CCVSI.
is due to the low power operation of the DGS (1 kVA). The
Tektronix (TDS3054B) digital scope was used to capture the
following results.
A. Power Conditioning in DGS
Fig. 14.
Photograph of the prototyped DGS.
TABLE II
THE SPECIFICATIONS OF THE PROTOTYPED DGS
Fig. 15 shows that, in the presence of an inductive load
(Z = 40 36.7◦ [Ω]), when the grid voltage is almost the same
as the VCVSI voltage, a slight adjustment to the power angle
will enable the grid to supply the load active power, and the required reactive power (600 var) can be supplied by the VCVSI.
Fig. 15(b) shows that at unity power factor, the grid supplies
the load active power and the CCVSI supplies the load reactive power (reactive power compensation). These results also
comply with the simulation results (Fig. 8).
Fig. 16 shows the experimental waveform results of the power
conditioning of a DGS in the presence of a diode bridge rectifier
with RLC (nonlinear load). It confirms that the CCVSI has
better performance in the presence of a nonlinear load for
low-order harmonic mitigation and provides unity power factor operation to the DGS [Fig. 16(b)], compared to the VCVSI
[Fig. 16(a)]. As was expected, the output voltage of the VCVSI
(Vc ) will be distorted in the presence of a nonlinear load
[Fig. 16(a)]. This deformation in the voltage waveform can be
KO et al.: APPLICATION OF VOLTAGE- AND CCVSIs FOR DISTRIBUTED GENERATION SYSTEMS
Fig. 16. Experimental waveform results of power conditioning of a DGS in
the presence of a nonlinear load (a diode bridge rectifier with RLC). (a) VCVSI.
(b) CCVSI.
corrected by adding a wave-shaping control algorithm and using
extra feedback signals [8].
B. UPS Function in DGS
Experiment results for the study of the UPS function of different VSIs are shown in Fig. 17. It observed that both the VCVSI
and CCVSI can supply and maintain the pure sinusoidal voltage
waveform when the grid fails, in the presence of linear loads.
However, in the presence of nonlinear loads, the CCVSI cannot provide the proper voltage without additional hardware and
control feedback algorithms [Fig. 17(b)]. Although the VCVSI
cannot mitigate low-order harmonics from the nonlinear loads,
it can maintain the load voltage after the grid fails [Fig. 17(a)].
These results verify the simulation results (Fig. 11). Fig. 17
illustrates that after grid failure, the CCVSI can provide sinusoidal current waveform to the load by adding predictive control
algorithms, however, this voltage will be distorted as it tries to
keep the load current sinusoidal.
C. DSM Function in DGS
Demand side management is an important function in any
DGS which defines a load sharing among the suppliers. In this
experiment, to study the DSM function of a VCVSI and CCVSI,
it was assumed that these VSIs would suddenly have to take 50%
of the load active power supply. Fig. 18(a) shows that in a DGS
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Fig. 17. Experimental waveform results of UPS function of a DGS using
different VSI in the presence of nonlinear load (a RLC diode bridge rectifier).
(a) VCVSI. (b) CCVSI.
system using a VCVSI, where the grid and the VCVSI voltages
are identical, grid supplies the reactive power demanded by decoupling the inductor, while the grid supplies the active power.
For demand side management, the proportions of active power
to be supplied by the VCVSI and grid respectively can be controlled by changing the power angle (δ). In this experiment, the
power angle was modified in order that 50% of the active power
to be supplied by the VCVSI. In this situation, after the transient
it was observed that without changes in the load current the grid
current decreased while the VCVSI current increased. Fig. 18(b)
shows a DGS in the presence of a CCVSI. In this case, as the
grid was subject to supply the full resistive load there was no
reactive power compensation (no decoupling inductor), hence
the converter current was maintained at zero. After a command
from the control system to overtake 50% of the active power by
the CCVSI, it was observed that with a very smooth transient (no
change in the load current) the CCVSI supplied the remaining
50% of the load-demanded active power. These results support
the simulation results (Fig. 12).
D. Voltage Regulation in DGS
Voltage regulation is another important feature required
in most applications dealing with sensitive loads. Moreover,
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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 21, NO. 3, SEPTEMBER 2006
Fig. 18. Experimental waveform results of demand side management of a
DGS in the presence of a resistive load. (a) VCVSI. (b) CCVSI.
Fig. 19. Experimental waveform results of voltage stabilization in a DGS in
the presence of a resistive load. (a) VCVSI. (b) CCVSI.
A. Load Voltage Stabilization
voltage stabilization would be one of the most important requirements in weak grid applications. The following tests were
carried out in order to study the performance of the VCVSI and
CCVSI in stabilizing the voltage. In this experiment, the voltage of the grid was changed from 200 V to 160 V (Fig. 19).
Fig. 19(a) illustrates that the VCVSI shares in supplying the
reactive power demanded by the decoupling inductor with the
grid, when the grid voltage is the same as the VCVSI voltage.
This can be done by adjusting the power angle to allow the grid
to supply all the active power demanded by the load. This experiment confirms the findings from the simulation in Fig. 13,
namely that the VCVSI can maintain the load voltage regardless of changes in the grid voltage. However, due to the parallel
connection of the CCVSI to the DGS, the CCVSI follows the
grid voltage and hence cannot provide voltage support to the
load without extra hardware and complex control techniques
[Fig. 19(b)].
V. DESIGN CONSIDERATION AND COMPARISON OF VCVSIS
AND CCVSIS IN DISTRIBUTED GENERATION SYSTEMS
The comparison of the VCVSI and CCVSI-based DGS is
shown below.
It is shown that the VCVSI can regulate the load voltage
within ±5% as per IEEE standards (1159 and 944). In contrast,
as a CCVSI is connected directly to the grid it cannot compensate the grid voltage fluctuation. A decoupling inductor is
essential to decouple the effect of grid voltage fluctuation, which
can be achieved by using VCVSIs. Therefore it is suggested that
a VCVSI be used to provide the required voltage support to the
load in applications with a weak grid [22], [23].
B. Uninterruptible Power Supply
As a VCVSI by nature performs 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 mechanisms and feedback.
However, as is shown [8], 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 and in
stand-alone operations. On the other hand, the CCVSI cannot
provide proper voltage support as by nature it is a current source
and voltage follower. Therefore, a VCVSI is recommended for
those applications where a UPS function is of high priority [24].
KO et al.: APPLICATION OF VOLTAGE- AND CCVSIs FOR DISTRIBUTED GENERATION SYSTEMS
C. Reactive Power Support, Active Filtering and Power Factor
Correction
As the active and reactive powers are coupled in a VCVSI,
it generally offers poor power factor correction performance at
low load, or when the grid voltage is different from the voltage of the load/VCVSI. In contrast, a CCVSI provides good
reactive power support and decoupling from the active power.
This capability enables CCVSIs to perform at unity power factor and to mitigate low-order harmonics effectively. Therefore,
CCVSIs are recommended for those applications where reactive
power support, including unity power factor operation or active
filtering is the main goal, (i.e., active power line conditioners
(APLC) [25], [26]).
D. Active Power Support or DSM
Both the VCVSI and CCVSI offer effective bidirectional
power flow between their dc and ac bus. The power flow control
in a VCVSI is very sensitive, and depends not only on a limited
power angle, but also on the size of the decoupling inductor.
However, as the phase and amplitude are controlled separately
in a CCVSI, the power flow in a CCVSI is smoother due to the
decoupling of the active and reactive power in this control technique. Hence, for DSM operation when voltage support is not a
priority, CCVSIs are recommended (i.e., photovoltaic grid-tied
inverters).
E. Sizing the PCU
Due to the existence of a decoupling inductor, a VCVSI
must supply both the active and reactive power demanded from
the load as well as the reactive power required by the decoupling inductor. This means that the VCVSI has to be oversized
(>100%). This could be worsened by an increase in the voltage
of the grid and VCVSI. However, the CCVSI can be rated at
full load (100%), as there is no decoupling inductor and it only
needs to supply the active and reactive power required by the
load.
In practice, it is possible to change the control algorithm in
VSIs with respect to the different functions required. However,
there are some prerequisites as well as pros and cons associated
with changing the control algorithm in VSIs which must be
considered.
VI. CONCLUSION
Voltage source inverters have been widely used in many applications, including distributed generation systems. VSIs are
inherently efficient, compact and economical devices, which
are used to control power flow and the quality of power supply.
In this paper, a detailed comparison of VCVSIs and CCVSIs
for DGS applications was presented. It was shown that neither
VCVSIs nor CCVSIs alone can offer all the functions required in
a DGS. Hence, the most appropriate VSI should be chosen based
on its application and priority. Alternatively, both types of VSIs
must be used for those applications where voltage stabilization,
unity power factor operation and active filtering are required.
This paper examined the advantages and disadvantages of each
791
VSI’s control techniques in the presence of different loads and
provides design recommendations for the use of VCVSIs and
CCVSIs in various applications. The experimental results verify
the theoretical analysis and computer simulations.
ACKNOWLEDGMENT
The authors are grateful to Curtin University of Technology
for providing opportunities to carry out this research. This work
was supported in part by the Australian Research Council under Grant LP 0348994 and partly by the Post-doctoral Fellowship Program of the Ministry of Commerce, Industry & Energy
(MOCIE).
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Sung-Hun Ko received the B.Sc. and M.Sc. degrees
from the Department of Control and Instrumentation
Engineering, Kunsan National University, Kunsan,
Korea in 1998 and 2000, respectively, and he is
currently pursuing the Ph.D. degree in the School
of Electronics and Information Engineering, Kunsan
National University, Kunsan, Korea.
From 2000 to 2001, he was with Research Laboratory, Seo-Young Electronics, Inc., Korea. Currently,
he is working as a Visiting Research Fellow with the
Department of Electrical and Computer Engineering
at Curtin University of Technology, Perth, Australia. His current research interests include renewable energy based distributed generation system, power factor
correction, inverter control, and neural network.
Seong R. Lee received the B.Sc. and M.Sc. degrees
in electrical engineering from Myong-Ji University,
Seoul, Korea in 1980 and 1982, respectively, and
the Ph.D. degree from Chonbuk National University,
Jeonju, Korea, in 1988.
From 1997 to 1998, he was the Visiting Professor
with the Department of Electrical and Computer Engineering at Virginia Tech, VA. From 2002 to 2004,
he was the Director of Engineering Research Institute
at Kunsan National University, Kunsan, Korea. Since
1990, he is a Professor with the School of Electronics and Information Engineering at Kunsan National University Currently, he is
working as a Visiting Professor with the Department of Electrical and Computer
Engineering at Curtin University of Technology, Perth, Australia. His current research interests include soft-switching inverter, power factor correction, switch
mode power supply, and renewable energy based distributed generation systems.
Hooman Dehbonei (S’01–M’03) received the B.Sc.
and M.Sc. degrees in electrical engineering from the
Iran University of Science and Technology, Tehran,
Iran in 1992 and 1997, respectively, and Ph.D. degree from Curtin University of Technology, Perth,
Australia, in 2003.
Presently, he is an Australian Research Council
Postdoctoral Fellow with the Department of Electrical and Computer Engineering at Curtin University
of Technology. He is a Chartered Professional Engineer and National Professional Engineers Register
with the Institute of Engineers, Australia. His current research interests include
power systems (design, analysis, quality, and control), power electronics (its
application in power systems and renewable energy), renewable energy, and
hybrid/distributed generation systems.
Chemmangot V. Nayar (M’86–SM’90) received the
B.Tech. degree in electrical engineering from the University of Kerala, India, in 1969, the M.Tech. degree
in electronics from the Indian Institute of Technology, Kanpur, in 1976, and the Ph.D. degree in electrical engineering, specializing in wind electrical power
generation, from the University of Western Australia,
Perth, Australia, in 1985.
He holds a Personal Chair in electrical engineering at Curtin University of Technology.
Prof. Nayar is a Chartered Engineer and Corporate
Member of IEE, and a Chartered Professional Engineer and Fellow of IEAust.