24-08-2013, 04:54 PM
Simulation and Analysis of a Multilevel Converter Topology for Solar PV Based Grid Connected
Inverter
Simulation and Analysis .pdf (Size: 1.99 MB / Downloads: 31)
ABSTRACT
There has been a noticeable increase in use of Solar PV based systems for power generation, given its renewable na-
ture. A solar PV based grid tie inverters are used for dc-ac conversion. The conventional line commutated ac-to-dc in-
verters have square-shaped line current which contains higher-order harmonics. The line current with the high har-
monic contents generates EMI and moreover it causes more heating of the core of distribution/power transformers.
Alternatively, PWM based inverters using MOSFET/IGBT switches are also used for the same purpose. However, apart
from higher switching losses, the power handling capability and reliability of these devices are quite low in comparison
to thyristors/SCR. Nevertheless, the conventional thyristor based forced commutated inverters are not suitable for PWM
applications due to the problems of commutation circuits. A pure sinusoidal line current or waveform with low har-
monic contents is the most desirable. In the present work, a multilevel line commutated inverter topology has been pro-
posed and analyzed which improves the wave shape and hence reduces the total harmonic distortion (THD) of the line
current in a grid tie line commutated inverter. The scheme has successfully been implemented and tested. Moreover, the
performance of the proposed topology is far better than the conventional line-commutated inverter. It reduces THD,
losses, switching stress and EMI.
Introduction
Solar PV based systems are being seen as a major con-
tributor to the present power generation technology. One
of the important applications of the solar PV based power
generation is to feed the generated power (dc) into grid
(ac). For this purpose, normally, PWM inverters are used
which use gate commutated devices (IGBT, MOSFET,
GTO etc.). However, apart from higher switching losses,
the power handling capability and reliability of these
devices are quite low in comparison to thyristors/SCR.
Moreover, the conventional line commutated ac-to-dc in-
verters have square-shaped line current which contains
higher-order harmonics. The line current with the high
harmonic contents generates EMI and moreover it causes
more heating of the core of distribution/power transfor-
mers.
Proposed Scheme
A multilevel converter circuit with RLE load works in
two modes of operation i.e. rectification mode and in-
verter mode. It works in inversion mode when the swit-
ching angle of each of the converter is greater than 90 ̊.
Here, a new multilevel circuit topology has been develo-
ped as shown in Figure 1. The dc load side has been iso-
lated from the grid via multi winding transformer.
The circuit has been analyzed and implemented for
three level of line current and can be extended to higher
levels for better performance. But the increase of level
adds to the cost of converter and more number of secon-
dary windings. So, a suitable compromise has to be made
between the THD of the line current and cost of addi-
tional hardware. When the circuit works in inverter mode,
the dc source transfers power to the main (ac source). The
major advantage of the proposed configuration is that in
continuous current mode of operation, the waveform re-
sembles a stepped sinusoidal wave and with suitable se-
lection of switching angles the harmonic contents can be
reduced drastically.
Harmonics and Power Transfer Analysis
The THD and Power flow analysis is done for various
configuration of switching angles. The effect of variation
of dc voltage on the THD and the power transfer is also
analyzed. Though a battery has been considered as a dc
source for simulation as well as experimental analysis,
the result obtained can be utilized when using solar PV
panel. Since the dc voltage of a solar PV panel is not
constant (vary with insolation, temperature and load), the
effect of varying dc voltage on THD and power flow will
help in developing a control strategy for operating the
system in optimum condition i.e. changing the switching
angle combination at different voltage level of PV panel.
Variation of THD and power with switching angle com-
bination for a three level inverter is shown in Figure 6
and Figure 7.
Conclusions
Multilevel inverter for three levels has successfully been
implemented. A thorough study has been made to obtain
optimal performance with battery as dc source and can be
extended for solar PV modules. With reduced THD and
features of multilevel inverter.