16-10-2012, 02:02 PM
Comparison of MLI and Z-Source Inverter for Transformerless Operation of Single-Phase Photovoltaic Systems
comparision of both inverters.pdf (Size: 353.36 KB / Downloads: 62)
Abstract
This paper presents a comparative analysis of single-phase transformer-less photovoltaic inverters and the topologies are compared in terms of efficiency, DC current injection and regulation. A model of cascaded multilevel inverter and Z-source inverter is built in MATLAB/SIMULINK and its performance is analyzed. Phase disposition control scheme is used for pulse generation for cascaded multilevel inverter and maximum boost control is used for Z-source inverter. The effects of intensity and modulation index on the output voltage are studied through simulation and prototypes of the inverters are implemented. The simulation, and the experimental results are presented in this paper.
Introduction
Nowadays, the high price of gasoline and increased demands on clean energy, renewable based generation are receiving more attention. Renewable energy sources (RES) include solar energy, wind energy, bio-fuel, geothermal energy, hydrogen and fuel cells. These sources are abundant and utilization of them creates zero emission. The power generated from the renewable energy systems are tied to the utility grid [1] and [2]. In remote places where there is less/no feasibility of utility grids, renewable energy systems provide electricity to the isolated region. These isolated renewable energy systems can be employed to power residential applications or low power industrial applications.
Z-Source Inverter
The Z-source inverter consists of a
Z-impedance network along with the
inverter circuit [21]. Fig. 2 shows the circuit
diagram of Z-source inverter. The Zimpedance
network consists of L and C
components connected in an X fashion. The
firing control of the Z-source inverter
includes the shoot through states. The Zsource
inverter advantageously utilizes the
shoot-through state to boost the DC bus
voltage by gating on both the upper and
lower switches of a phase leg. Fig. 3 shows
the Z-source network during the shootthrough
state. The inductor is energized and
the inductor voltage increases due to the
increasing current. The capacitor is
connected in parallel to the inductor and its
voltage is boosted during this state.
Simulation Results
To study the performance of the
cascaded multilevel inverter and Z source
inverter, simulations were performed in
MATLAB/SIMULINK with the configuretion
shown in Fig. 1 to Fig. 2 with the
following parameters: Modulation index=
0.3 to 1, switching frequency= 10kHz,
Intensity=40 to 100mW/cm2, output RMS
voltage =230V and the load is 0.9 PF RL
load. The simulation result for the Phase
disposition (PD) technique for eleven levels
is shown in Fig. 7 for an MI of 1. To
produce a firing pulse for an eleven level
inverter, ten carriers with a frequency of 10
KHz are compared with the sinusoidal
reference waveform of 50 Hz. In this
method, carriers are the same in frequency,
amplitude and phases, but they are just
different in DC offset to occupy continuous
bands. Output voltage and current of a
cascaded three level inverter is shown in
Fig. 8. At the minimum intensity of 40
mW/cm2, the rated output voltage of 230V
is achieved for an MI of 0.93. As the
intensity increases, the MI is decreased to
maintain the output voltage constant.
Conclusion
Based on the simulation and experimental results, it is found that the Z-source inverter achieves better performance with a single inverter and reduced the number of panels, when compared to the cascaded MLI. Since the Z-source inverter boost, and inverts the input DC in a single stage, its firing circuit is simple. Therefore, the Z-source inverter is identified to be the best topology for a PV inverter.