ISSN 2319-8885
Vol.04,Issue.36,
September-2015,
Pages:7877-7882
www.ijsetr.com
Simulation of a Synchronous Reference Frame Voltage Control for a
Z-Source Single Phase Inverter with Renewable Energy Sources
GUNDETI RAVALI1, K. VAMSEE KRISHNA2
1
2
PG Scholar, Dept of EEE, VBIT Engineering College, Ghatkesar, R.R. (Dt), TS, India, E-mail: ravali31@gmail.com.
Asst Prof, Dept of EEE, VBIT Engineering College, Ghatkesar, R.R. (Dt), TS, India, E-mail: kvamsee11@hotmail.com.
Abstract: This paper presents the control of single phase power converters with Z-source network for renewable energy sources.
Deals with the design of an SRF multiloop control strategy for Z-source single-phase inverter-based islanded distributed
generation networks. Proposed controller uses an SRF proportional–integral controller to regulate the instantaneous output
voltage and a capacitor current shaping loop in the stationary reference frame to provide active damping, improve both transient
and steady-state performances, a voltage decoupling feed forward to improve the system robustness, and Z-source is a multi
resonant harmonic compensator to prevent load current harmonics to distort the inverter outputs and decreases the shoot through
problem for a single phase inverter. Since the voltage loop works in the SRF, it is not straight forward to fine tune the control
parameters and evaluate the stability of the whole closed-loop network. To overcome this problem the stationary reference frame
equivalent of the voltage loop is derived. This method of approach can be used in high power applications to produce high voltage
gain when compared to the conventional converter. Simulations using MATLAB/SIMULINK are carried out to verify the
performance of the proposed system.
Keywords: DC-AC Converter, Z-Source Network.
I. INTRODUCTION
Distributed generation (DG), mainly from renewable
energy sources, has increased during recent years. Smallscale electricity generation units, such as microturbines, roofmounted photovoltaic and wind generation systems, and
commercially available fuel cells, are being widely utilized at
the distribution level. Almost all these systems utilize some
kind of power electronic converters to provide a controlled
and high-quality power exchange with the single-phase grid
or local loads. A voltage source inverter (VSI) is the most
common topology which can operate either in grid-connected
or standalone mode. In stand-alone or island operation mode,
i.e., when the grid is not present, the local loads should be
supplied by the DG system, which now acts as a controlled
voltage source. DC-link voltage ripple of Z-Source inverter is
one of major concerns since it is directly related to inverter
output power quality. The essential requirement is to control
the system voltage parameters such as amplitude and
frequency with fast dynamic response and zero steady-state
error. Different control techniques for single-phase VSIs in
standalone mode have been presented in the literature. Owing
to availability and low cost of advanced digital signal
processors and digital control strategies based on repetitive
control, dead-beat control and discrete-time sliding-mode
control have been proposed.
Digital repetitive control is proposed to reduce harmonic
distortions of the output voltage produced by nonlinear load,
with its excellent ability in eliminating periodic disturbances.
However, in practical applications, slow dynamics, poor
tracking accuracy, a large memory requirement, and poor
performance to non periodic disturbances are the main
limitations of this technique. The deadbeat and sliding-mode
controllers exhibit excellent dynamic performance in direct
control of the instantaneous inverter output. Unique feature is
that even with their fast response, they prevent overshoot.
These techniques suffer from some drawbacks such as
complexity, sensitivity to parameter variations and loading
conditions and steady-state errors. Proportional–resonant
(PR) control has shown superiority in eliminating the steadystate error associated to the tracking problem of ac
generations. The proposed technique has also attracted
increasing interests in instantaneous voltage control of singlephase VSIs. The PR controller has certain disadvantages, the
mains being exponentially decaying response to step changes
and great sensitivity. The possibility of instability to the
phase shift of sensed signals and the synchronous reference
frame (SRF) proportional–integral (SRFPI) controller is
widely used for three-phase converter systems to obtain a
zero steady-state error.
In the SRFPI control, the electrical signals are all
transformed to the synchronous reference frame, where
quantities are dc and as a consequence the zero steady-state
error is ensured by using conventional PI regulators. This
transformation requires at least two orthogonal signals; thus,
a fictitious second phase must be generated to allow
emulation of two-phase systems. In this paper, SRFPI
Copyright @ 2015 IJSETR. All rights reserved.
GUNDETI RAVALI, K. VAMSEE KRISHNA
controller is proposed to regulate the instantaneous output
voltage. While the use of SRFPI controller in three-phase
(3)
systems is a mature topic, in single-phase systems, it has not
Where,
been yet properly investigated. Proposed multiloop structure
I and V - cell output current and voltage;
employs a simple inner capacitor current shaping loop to
Io - cell reverse saturation current;
provide active damping and improve both transient and
T - Cell temperature in Celsius;
steady-state performance. Also, voltage decoupling feed
K - Boltzmann’s constant;
forward is utilized to improve the system robustness and at
q - Electronic charge;
the same time simplify the system modeling and controller
Ki- short circuit current/temperature coefficient;
design. A multiresonant harmonic compensator (HC) is
G - Solar radiation in W/m2;
added to the suggested scheme which prevents low-order
Gn- nominal solar radiation in W/m2;
load current harmonics to distort the inverter output voltage,
Eg - energy gap of silicon;
particularly under distorted and nonlinear loads. Combining
Io,n - nominal saturation current;
the multiloop control, harmonic resonators, and the voltage
Rs - Series resistance;
feed forward with the SRFPI in single-phase systems has not
Rsh - shunt resistance;
been yet explored.
The I-V characteristic of a PV module is highly nonlinear in nature. This characteristics drastically changes with
respect to changes in the solar radiation and cell
temperature..Whereas the solar radiation mainly affects the
output current, the temperature affects the terminal voltage.
Fig.2 shows the I-V characteristic of the PV module under
varying solar radiations at constant cell temperature (T = 25
ºC).
Fig.1. Power stage of a single-phase VSI.
SRFPI control algorithm involves several reference frame
transformations; therefore, the classical control techniques
cannot be simply applied to evaluate the performance of the
closed-loop system. Thus, single-phase equivalent of the
SRFPI regulator is obtained and which significantly
simplifies the controller design and stability analysis.
Detailed design procedure with consideration of the practical
implementation issues, such as the effect of loading
conditions and the control delay is proposed.
II. OVERVIEW OF A PHOTOVOLTAIC (PV)
MODULE
To understand the PV module characteristics it is
necessary to study about PV cell at first. A PV cell is the
basic structural unit of the PV module that generates current
carriers when sunlight falls on it. The power generated by
these PV cell is very small. To increase the output power the
PV cells are connected in series or parallel to form PV
module. The electrical equivalent circuit of the PV cell is
shown in Fig.2
Fig.3. Current versus voltage at constant cell temperature
T = 25 ºC.
Fig.3 shows the I-V characteristics of the PV module
under varying cell temperature at constant solar radiation
(1000 W/m2). Fig.4 shows Current versus voltage at constant
solar radiation G = 1000 W/m.
Fig.2. Electrical equivalent circuit diagram of PV cell.
The main characteristics equation of the PV module is
given by
(1)
Fig.4. Current versus voltage at constant solar radiation
G = 1000 W/m.
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.36, September-2015, Pages: 7877--7882
(2)
Simulation of a Synchronous Reference Frame Voltage Control for a Z-Source Single Phase Inverter with Renewable
Energy Sources
current-source converter and provides a novel power
III. PROPOSED SYSTEM
The power stage of a single-phase VSI, consisting of an
conversion concept. The Z-source concept can be applied to
insulated-gate bipolar transistor (IGBT) full-bridge
all AC-to-DC, DC-to-AC, AC-to-AC, DC-to-DC power
configuration followed by an LC filter, is illustrated in Fig. 1.
conversion. To describe the operating principle and control,
Throughout this paper, the dc-link voltage is assumed to be
this paper focuses on an example: a Z-source inverter for
constant. This assumption can be simply realized by using a
DC-AC power conversion needed in fuel cell applications.
sufficiently large capacitance at the dc link. Fig 5.shows the
proposed system of single phase inverter with Z-source
network by using SRF voltage control.
Fig.6. Z-source inverter circuit diagram.
Fig.5. Single phase inverter with Z-source.
IV. Z-SOURCE CONVERTER
Z source network is a one type of DC-DC converter
which is used to control the shoot through problem and also
used to reduce the harmonics, electromagnetic interference
and acts as a Buck-boost converter operation. Z source
inverters are recent inverter topologies that can perform both
buck and boost functions as a single unit as shown in Fig.6.
A unique feature of Z source inverter is the shoot through
state, by which two semiconductor switches of the same
phase leg can be turned ON simultaneously. Therefore, no
dead time is needed and output distortion is greatly reduced
and thus reliability is greatly improved. Feature is not
available in the traditional voltage source and current source
inverters. The proposed Z source inverters are mainly applied
for loads that demand a high voltage gain such as motor
drives and as a power conditioning unit for renewable energy
sources like fuel cells, solar, etc to match the input source
voltage differences. The development in Z source inverter
topologies provides a consecutive enhancement in voltage
gain and output waveforms. A tradeoff between the boosting
capability and component count is always a major concern to
keep the cost stable. It is to be noted that increase in the
passive components with suitable modifications can improve
the performance of these types of inverters. The topological
growth has been in terms of addition or reduction of passive
component, inclusion of extra semiconductor switches,
alteration or inclusion of dc sources and also changes of
modulation schemes etc.
Voltage buck inversion ability is also provided for those
applications that need low ac voltages. The Z-source
converter employs a unique impedance network to couple the
converter main circuit to the power source, thus providing
unique features that cannot be obtained in the traditional
voltage-source and current-source converters where a
capacitor and inductor are used, respectively. The Z-source
converter overcomes the conceptual and theoretical barriers
and limitations of the traditional voltage-source converter and
V. PROPOSED CONTROL SCHEME
The control of three-phase power converters in the DQ
rotating reference frame is now a mature and well-developed
research technology. However, for single-phase converters
and is not as well established as three-phase applications.
Main reason behind this lies partly in its more complex
structure than the conventional stationary reference frame
controller and also a secondary orthogonal signal that is
needed to implement a single-phase controller in the DQ
reference frame. Fig. 7 illustrates the suggested control
scheme, which includes an SRFPI controller to regulate the
instantaneous output voltage, an inner current shaping loop to
provide active damping and improve both transient and
steady-state performances, and a voltage-feed forward path to
improve the system robustness. The capacitor current is
selected as the feedback signal in the inner current loop, since
it brings better disturbance rejection capability than the
inductor current feedback. Indeed, because the capacitor
current is directly proportional to the time rate of change of
output voltage and gives some kind of prediction about
output voltage distortions caused by nonlinear load currents
and allows the inner control loop to compensate in advance.
On the other hand, it is simpler and definitely more cost
effective to sense the capacitor current instead of the higher
ampere inductor current. It should be noted here that a
practical difficulty in accurately measuring the filter
capacitor current, particularly for high capacitances, is that
the low-frequency current information is immersed by
switching frequency currents.
A low-pass filter in the current feedback loop may be
required. In practice and to reduce the filtering requirements
and the resultant phase delays, the LC filter capacitor should
be chosen as small as possible. It is also noteworthy that the
current ripple highly depends on the capacitor equivalent
series resistance (ESR). In practice, to reduce the ESR effect,
several low ESR capacitors are connected in parallel for the
LC filter.
Fig.7. Stationary (αβ) reference frame representation of
the SRFPI controller.
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.36, September-2015, Pages: 7877-7882
GUNDETI RAVALI, K. VAMSEE KRISHNA
Where Vll_pk is the peak value of the line-to-line voltage,
Vg is the DC link voltage. The performance of a modulation
scheme can be evaluated based on the following five aspects:
(1) distortion of the output voltage or current; (2) power
losses; (3) harmonic spectrum and EMI; (4) dynamic range;
and (5) complexity. It is always desirable to minimize the
distortion of the output voltage or current. It may change with
the modulation index in a nonlinear curve. The power losses
are related to the total number of switching actions in one
switching cycle, and the current level at switching. Therefore,
different modulation schemes may result in different
efficiencies. A PWM scheme with minimized switching
Fig.8. (a) Block diagram of the proposed control system
losses is desirable especially for high power applications.
and (b) its simplified representation.
Harmonic spectrum of the output voltage or current is related
to the EMI issue and acoustic noise. It is desirable to
The choice of the proportional gain of the PI
minimize the EMI and acoustic noise. Dynamic range refers
compensator is a tradeoff between the attainable voltage
to the maximum possible control level in steady state or
regulation bandwidth and the control loop stability as shown
during transient. It can also be interpreted as the ratio
in Fig.8. In this paper, Kp is chosen to provide a desired
between the maximum possible output and the input. It is
bandwidth of ωbv for v/v loop. A robust performance of the
desirable to have a higher ratio. For a voltage source inverter,
control system and a minimum steady-state error will be then
it means a better DC link voltage utilization, which is crucial
ensured by means of proper selection of the integral gain of
for high voltage applications. It is preferable to have a PWM
the compensator. For the sake of simplicity, the following
scheme that can be implemented easily, by either an analog
analytical analysis to determine Kp is based on the
means or digital means as shown in Fig.9.
assumption that the integral gain Ki has almost no effect on
the voltage regulation dynamics.
(4)
VI. PULSE WIDTH MODULATION TECHNIQUE
The pulse width modulation (PWM) concept is borrowed
from communication systems, wherever an indication is
modulated before its transmission, and so demodulated at the
receiving terminal to recover the initial signal. Constant idea
may be applied to an influence convertor. in an exceedingly
power convertor, the switch network has associate on/off
nonlinear nature. The desired continuous wave form is
modulated and reborn to digitized signals to management the
switch network. Then the modulated signals at the switch
network AC terminals area unit demodulated by the AC filter
to urge the specified continuous voltage or current wave
form. Normally a sinusoidal voltage or current is the control
target for a power converter. The first PWM scheme was the
sinusoidal PWM (SPWM) scheme and was proposed in 1964.
Since the modulator has a great impact on voltage/current
distortions, switching losses, and EMI, it is of great interest
to the power electronics researcher. In the past there has been
intensive research on this topic and there is much literature
on it. All the proposed PWM schemes may be classified into
four categories, namely, (1) SPWM and its derivations; (2)
Optimal PWM (3) Space Vector Modulation (SVM); (4)
Hysteresis and Bang-Bang type modulation; and (5) Random
PWM. All the PWM schemes may be evaluated under a
certain switching frequency and the reference signal
frequency ratio, and the input and output voltage ratio, which
is also named as the modulation index M. The definition of
the modulation index M is given
Fig.9. PWM technique.
VII. SIMULATION RESULTS
The below figs.10 to 13 shows the simulation circuit
diagram of a proposed system and following shows the
waveforms getting from the simulation diagram.
(5)
Fig.10. Proposed simulation circuit diagram.
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.36, September-2015, Pages: 7877--7882
Simulation of a Synchronous Reference Frame Voltage Control for a Z-Source Single Phase Inverter with Renewable
Energy Sources
alone mode, which guarantees zero steady-state error at the
fundamental frequency and Z-source inverter is used to
decrease the shoot problem and increases the voltage gain.
Moreover, an inner capacitor current regulating loop brings
active damping and improves both transient and steady-state
performances. A voltage-feed forward path boosts the system
robustness. A multiresonant HC actively prevents the loworder harmonic currents to distort the inverter output voltage.
The single-phase equivalent of the SRFPI regulator was
provided, which significantly simplifies controller design and
stability analysis.
Fig.11. Solar Voltage.
Fig.12. Z-source output voltage.
Fig.13. Waveforms of Ic,Io,Vdc,Vo.
IX. REFERENCES
[1] Fang Zheng Peng, “Z - Source Inverter”, IEEE
Transaction on Industry Applications. 39: 2003,2.
Wuhan,China.
[2] G. Pandian and S. Rama Reddy, “Embedded Controlled
Z Source Inverter Fed Induction Motor Drive” IEEE
transaction on industrial application, vol.32, no.2, May/June
2010.
[3] K. Srinivasan and Dr. S. S. Das, “Performance Analysis
of a Reduced Switch Z -Source Inverter fed IM Drives”,
Journal of Power Electronics, Vol. 12, No. 2, May/June
2010.
[4] K.Niraimathy, S.Kr ithiga, “A New Adjustable - Speed
Drives (ASD) System Based On High - Performance Z Source Inverter”, 978 – 1 – 61284 – 379 - 7/11 2011 IEEE,
2011 1st International Conference on Electrical Energy
Systems
[5] Y. Y. Tzou, R. S. Ou, S. L. Jung, and M. Y. Chnag,
“High-performance programmable AC power source with
low harmonic distortion using DSP based repetitive control
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[6] K. Zhou, K. Low, D. Wang, F. Luo, B. Zhang, and Y.
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[7] K. Zhang, Y. Kang, J. Xiong, and J. Chen, “Direct
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VIII. CONCLUSION
This paper has proposed an SRFPI controller to regulate
the instantaneous output voltage of the single-phase inverter
with Z-source network for renewable energy source in standInternational Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.36, September-2015, Pages: 7877-7882
GUNDETI RAVALI, K. VAMSEE KRISHNA
Author’s Profile:
Gundeti Ravali, received the B.Tech
degree in electrical and electronics
engineering from Vidya bharathi institute
of technology,Warangal. She is pursuing
M.Tech degree in power electronics &
electrical drives from Vignana Bharathi
institute of technology, Hyderabad,
Telangana expected to receive in 2015.Her current research
interests include simulation of a synchronus reference frame
voltage control for a z-source single phase inverter with
renewable energy sources. Email id: ravali31@gmail.com.
K.Vamsee Krishna, Member of IEEE,
received the B.Tech. degree in Electrical
and Electronics Engineering from
Kakatiya University and the M.E degree
in Industrial Drives and Control,
Electrical Engineering from Osmania
University. He is currently an Assistant
Professor with the Department of Electrical and Electronics
Engineering, Vignana Bharathi Institute of Technology,
Hyderabad, Telangana. His research interests are Control
Systems, Embedded Controllers, and Power Electronics.
Email id: kvamsee11@hotmail.com.
International Journal of Scientific Engineering and Technology Research
Volume.04, IssueNo.36, September-2015, Pages: 7877--7882