Download photo voltaic microinverter control for grid-connected

PHOTO VOLTAIC MICROINVERTER CONTROL FOR GRIDCONNECTED APPLICATIONS
1
N.VASEEM RAJA
2
S.GURU PRASAD
1
M.Tech Student GCET,KADAPA,JNTU Ananthapur, AP-India,[email protected]
2
Assistant Professor, EEE Dept, GCET,KADAPA, JNTU Ananthapur, AP-India
applied to both the dc–dc converter and the inverter
Abstract-- This project work presents a gridconnected photovoltaic (PV) micro-inverter
system and its control implementations. A half
bridge dc-dc converter is used to interface the
low-voltage PV module and grid under light and
heavyloaded conditions . A full-bridge pulse
width-modulated inverter is cascaded which
injects synchronized sinusoidal current to the
grid.
A plug-in repetitive current controller
accommodated with IIR filter is proposed to
regulate the grid current and also to eliminate
periodic harmonics,to achieve high power factor
and low harmonic distortion under varying loads
The model of the proposed scheme of PV microinverter
control
has
been
built
using
MATLAB/Simulink.
,here a constant voltage dc link decouples the power
Index Terms—Micro inverter ,boost half-bridge
converter, repetitive current control ,IIR filter
distortions (THDs), power factor, and dynamic
flow in the two stages. By contrast, the second
configuration
utilizes
a
quasi-sinusoidal
PWM
method to control the dc–dc converter in order to
generate a rectified sinusoidal current (or voltage) at
the inverter dc link. For best MPPT performance and
output current quality, the first method of PV micro
inverter is adopted in this paper.
A full-bridge PWM inverter with an output LCL
filter is incorporated to inject synchronized sinusoidal
current to the grid. In general, its performance is
evaluated by the output current total harmonic
response. Repetitive control (RC) is known as an
effective
solution
for
elimination
of
periodic
harmonic errors. An IIR filter is incorporated in RC
I.INTRODUCTION
In single phase grid connected photovoltaic power
to obtain very high system open-loop gains at a large
systems, the concept of Micro inverter has become a
number of harmonic frequencies such that the
future trend for its removal of energy yield
harmonic rejection capability is greatly enhanced. In
mismatches among PV modules, A boost-half-bridge
this work, a plug-in repetitive current controller is
DC-DC converter cascaded by an inverter is the most
proposed. It is composed of a proportional part and
popular
topology, in which a HF
step up
an RC part, to which the IIR filter is accommodated.
transformer is often implemented within the DC-DC
conversion stage.
Pulse width modulation (PWM) techniques employed
in the PV microinverter system, where two major
methods are mostly used. In the first, PWM control is
The
proposed
current
controller
following superior features:
1) high power factor is obtained;
exhibits
the
2) current harmonic distortions (up to the 13th-order)
III. DIGITAL DESCRIPTION OF PV MICRO
caused by the grid voltage non ideality are
INVERTER SYSTEM CONTROL
minimized;
A simple dual power processing PV inverter
3) better current regulation is guaranteed.
employing in the paper is shown in Fig. 2.
4) fast dynamic response is achieved during the
transient loads.
II. MICROINVERTER EMPLOYING BOOST
HALF-BRIDGE CONVERTER
The boost is a popular non-isolated power stage
Fig.2 Dual power processing inverter.
topology, sometimes called a step-up power stage.
Power supply designers choose the boost power stage
A digital approach is adopted for the control of the
because the required output is always higher than the
PV micro-inverter system, as shown in Fig. 5. The
input voltage. The input current for a boost power
PV voltage VPV and current IPV are both sensed for
stage is continuous, or non-pulsating, because the
calculation of the instantaneous PV power PPV. At
output diode conducts only during a portion of the
the inverter side, the grid voltage Vg is sensed to
switching cycle. The output capacitor supplies the
extract the instantaneous sinusoidal angle θg, which
entire load current for the rest of the switchingcycle.
is commonly known as the phase lock loop. The
inverter output current Iinv is prefiltered by a firstorder low-pass filter on the sensing circuitry to
eliminate the HF noises. The filter output Iinv is then
fed back to the plug-in repetitive controller for the
inner loop regulation. Vdc can be sensed which is
Fig.1 basic schematic diagram of a boost converter.
used as a reference voltage for tracking of the grid
voltage. The grid current and the dc voltage
The
boost-half-bridge
microinverter
for
grid
connected PV systems is composed of two decoupled
power processing stages. In the front-end dc–dc
converter, a conventional boost converter is modified
by splitting the output dc capacitor into two separate
ones.. The center taps of the two MOSFETs (S1 and
S2 ) and the two output capacitors (C1 andC2 ) are
connected to the primary terminals of the transformer
Tr , just similar to a half bridge. The boost-halfbridge converter is controlled by S1 and S2 with
complementary duty cycles.
The boost-half-bridge microinverter topology for
gridconnected PV systems is shown in Fig. 3.
references are represented by Iinv* and Vdc*,
respectively. In order to achieve fast dynamic
responses of the grid current as well as the dc
voltage, a current reference feed forward,Iinvff is
added in correspondence to the input PV power PPV.
The magnitude of the current feed forward is
expressed as follows:
𝐼𝑖𝑛𝑣𝑓𝑓=2𝑃𝑝𝑣\𝑉𝑔
(1)
where|Vg | is the magnitude of the grid voltage and
can be calculated by
𝑉𝑔 =1\2∫ 𝑉𝑔 𝑑𝜃𝑔
(2)
Fig.3.Boost half bridge PV micro inverter.
IV. DESIGN AND ANALYSIS OF PLUG-IN
REPETITIVE CURRENT CONTROLLER
Using an LCL filter in a grid-connected inverter
system has been recognized as an attractive solution
to reduce current harmonics around the switching
frequency, improve the system dynamic response,
and reduce the total size and cost. An undamped LCL
filter exhibits a sharp LC resonance peak, which
indicates a potential stability issue for the current
regulator design. Hence, either passive damping or
Fig.5 Proposed PV micro inverter control.
active damping techniques can be adopted to
attenuate the resonance peak below 0 dB. On the
The plug-in digital repetitive controller is designed,
other hand, a current regulator without introducing
as shown in Fig. 4 .The conventional proportional
any damping method can also be stabilized, as long
controller with a gain of Kp2 is used to obtain fast
as the LCL parameters and the current sensor
dynamics. The RC is then plugged in and operates in
location are properly selected. The current sensor is
parallel with the proportional controller. e(z) and d(z)
placed at the inverter side instead of the grid side.
represent the tracking error and the repetitive
disturbances, respectively.
In an ideal RC, a unity gain is along the positive
feedback path such that all the repetitive errors based
on the fundamental period are completely eliminated
when the system reaches equilibrium. However, in
Fig.4 proposed plug-in repetitive controller.
order to obtain a sufficient stability margin, a zerophase low-pass filter is often incorporated rather than
the unity gain. This can be realized by cascading a
linear-phase low pass filter Q (z) and a phase lead
power converters. If the converter dynamics are
compensator zk2.zk1 is another phase lead unit,
disregarded in the MPPT control, undesirable
which compensates the phase lag of inverter at HFs.
transient responses such as LC oscillation, inrush
current, and magnetic saturation may take place.
The proportional controller modifies the transient
Consequently,the conversion efficiency can be
response and steady state error. It produces an output
deteriorated or even malfunction of the converter
signal proportional to error signal, and amplifies the
may occur.
error signal to increases the loop gain of the system.
Table I summarizes the key parameters of the boost-
In this work, Kp2 = 50,is chosen to increase the loop
halfbridge dc–dc converter.
gain. It is noticeable that larger Kp2 will result in a
smaller tracking error during the transients.
In a fourth-order linear-phase IIR filter has been
synthesize for the repetitive voltage controller for
UPS systems. Compared with the conventional
linear-phase finite impulse response filters used for
RC, the linear-phase IIR filter exhibits a flat gain in
the pass band. Hence, it is a good candidate for the
repetitive current controller . In practice, Q(z) is
synthesized by cascading a second-order elliptic filter
Table.1. boost half bridge converter parameters.
.
Table II summarizes the key parameters of the full
bridge inverter.
Qe(z) and a second-order all-pass phase equalizer
Qa(z). Q(z), Qe(z), and Qa(z) are expressed by the
following equations
Q(z) = Qe(z)Qa(z)
(3)
0.1385+0.2564𝑍 −1 +0.1385𝑍−2
Qe(Z)=
1−0.7599𝑍−1 +0.2971𝑍 −2
0.1019−0.6151𝑍−1 +𝑍 −2
Qa(Z)=1−0.6151𝑍−1 +0.1019𝑍−2
(4)
(5)
Table.2. full bridge inverter parameters.
V. BOOST CONVERTER CONTROL
The PV voltage is regulated instantaneously to the
command generated by the MPPT function block.
Typically,
the
MPPT
function
block
in
pv
converter/inverter system periodically modifies the
tracking reference of the PV voltage, or the PV
current, or the modulation index, or the converter
duty
cycles.
In
most
cases,
these
periodic
perturbations yield step change dynamic responses in
VI. SIMULATION RESULTS
In the proposed Boost –Half-Bridge PV micro
inverter system diagram of PV system and MPPT and
Boost-Half-Bridge converter and full- Bridge Inverter
are designed in Matlab/Simulink. The control circuit
diagrams for Boost-Half-Bridge converter and fullBridge inverter designed in Matlab/Simulink are
shown in below figures.
(A). SYSTEM OUTPUT WAVE FORMS
The PV cell output current and output voltage and
output power wave forms for both light load and
heavy load condition are shown in below figures 8
and 9.
Fig.6 Simulation circuit diagram for Boost-HalfBridge converter control.
The subsystem for the MPPT algorithm for the Boost
Half bridge converter control is shown in the
following figure
Fig .8 PV cell output voltage
Fig.7 Simulation circuit diagram for MPPT
algorithm.
Fig .9 PV cell output power
The simulation circuit diagrams for Full-Bridge –
The proposed plug-in RC achieves a voltage THD as
Inverter control is shown in below figure.7.
low as 1.51% and current THD of 1.76% and 0.99
power factor. The proportional part in the plug-in RC
enables the controller to respond to the abrupt
reference change promptly. Meanwhile, RC part
cancels
the
harmonic
distortions
in
fundamental cycles.
Fig.6 Simulation circuit diagram for full bridge
inverter control.
Fig 10. grid current under loaded condition
several
power factor (>0.99) and low THD (0.9%–2.87%)
are obtained.
VII. REFERENCES
[1] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, “A review of
single-phase grid-connected inverters for photovoltaic modules,”
IEEE Trans. Ind.Appl., vol. 41, no. 5, pp. 1292–1306, Sep./Oct.
2005.
[2] Q. Li and P.Wolfs, “A review of the single phase photovoltaic
module integrated converter topologies with three different DC
link configurations,” IEEE Trans. Power Electron., vol. 23, no. 3,
Fig.11 output voltage and current under loaded
pp. 1320–1333, May 2008.
condition
[3] R. Wai and W. Wang, “Grid-connected photovoltaic
(B)FFT Analysis of total harmonic distortion
generation system,” IEEE Trans. Circuits Syst.-I, vol. 55, no. 3, pp.
953–963, Apr. 2008.
The performance is evaluated by the output current
[4] M. Andersen and B. Alvsten, “200W low cost module
integrated utility interface formodular photovoltaic energy
total harmonic distortions (THDs) by conducting the
systems,” in Proc. IEEEIECON, 1995, pp. 572–577.
FFT analysis for the output wave forms as shown in
the following figure
N. Vaseem Raja, Received B.Tech
Degree from NBKR Institute of Science
and Technology, SVU Under the
Department of Electrical and Electronics
in 2012. Currently he is working towards
his Masters Degree in the Department of
Power Systems from GCET ,KADAPA.
His interest includes High voltage
engineering, Power quality issues.
Fig .12. FFT Analysis for THD
VI. CONCLUSION
A boost-half-bridge PV microinverter system with its
advanced control implementations for grid-connected
PV applications has been presented. A plug-in
repetitive current controller was proposed and
illustrated. Simulation of Repetitive current controller
used as a control circuit for micro-inverter has been
carried out. Simulation of proposed system is done by
using MATLAB/Simulink. Performance metrics is
taken as THD and power factor. The current injected
to the grid is regulated precisely and stiffly. High
S.Guru Prasad, Received B.Tech
Degree from VAGDEVI institute of
Technology
and
science,
JNTU
Hyderabad in 2005 and master degree
from MAHALAKSHMI institute of
Technology
and
science,
JNTU
Hyderabad Under the Department of
Electrical and Electronics in 2008.
Currently he is working as assistant
professor in the Department of Electrical
Engineering. His interest includes
Electrical power systems and Electrical
machines.