10-05-2014, 04:55 PM
Electrical Design Lab II Progress Report
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Abstract
This report outlines the team’s progress of our project. The objective of the project is to design
a quazi impedance source inverter (qZSI) that operates on DC voltage from photovoltaic cells
to supply a power up to 3 kW. Initially, our team conducted literature review on the topic of
photovoltaic power generation and inverter topologies in order to understand the various
topologies of inverters. The team studied the customer needs in Qatar, conducted an
ethnographic study, and performed benchmarking analysis. After that, the functions of the
inverter were determined and different concepts were generated; eventually, one concept was
selected, based on some design criteria, to implement the qZSI circuit. At the same time,
simulations of a qZSI circuit were done using MATLAB Simulink. Also, the control circuit
which generates signals to control the switches of the inverter was designed and simulated.
The simulation results will be compared with the built qZSI circuit and discussed in the final
report.
Introduction
Recently, the booming economy in Qatar has made it one of the largest producers of
greenhouse gas emissions and carbon dioxide worldwide. Therefore, it was found that
photovoltaic power generation is one of the most promising technologies to be implemented in
Qatar, especially for its high annual solar irradiation. Photovoltaic (PV) power generation can
be used for multiple purposes in Qatar such as traffic signals, communication towers and
camps. Also, it can be used in household appliances and industrial applications. [1-2]
Photovoltaic cells generate electrical energy by absorbing the sunlight and converting it to DC
voltage. The solar inverter is an essential component in photovoltaic power generation system.
It converts the DC voltage from the photovoltaic cells into AC voltage that can be connected
to electrical grid, electrical networks or directly to devices. One of the important design
considerations that should be taken into account when designing the solar inverter is the
variability of the DC voltage supplied by the PV cells. This variability is caused by changes in
the weather conditions and the amount of dust accumulating on the cells. Hence, taking such
changes into considerations is essential in order to develop a reliable source of energy.
The traditional inverters which are the voltage source inverter (VSI) and the current source
inverter (ISI) have some limitations. First, they operate only as buck or boost inverters; this
proposes the need to add an additional dc-dc buck or boost inverter according to the output
voltage need. In contrast, the qZSI can operate as a buck-boost inverter. Second, traditional
inverters do not allow for the shoot-through
1
state; if two switches in the same leg are ON
simultaneously, a short circuit occurs in the VSI, which might damage the inverter. Similarly,
at least one of the upper and one of the lower switches of one leg in the ISI has to be gated on
at the same time, otherwise it will result in an open circuit. That’s why an advantage of the
qZSI is that allows for the shoot-through states unlike the traditional inverters. In addition to
that. these limitations of the VSI and ISI require complex designs of the used inverters; hence,
they increase cost and lower the efficiency. [3-4]
The designed qZSI is composed of several components as shown in Figure 1. These
components are: Digital Signal Processor (DSP), opto-coupler, qZSI network, IPM, and LC
filter. Each component will be explained, in details, later in this report.
DSP Programming
Programming the DSP includes providing a programming code to the DSP circuit which controls the
opening and closing of the switches, depending on the calculated duty ratio. The program is obtained
using MATLAB Simulink and is downloaded to the DSP using Code Composer 3.1. The team started
studying the schematics of the DSP. After that, we started reading, downloading and running some
codes on the DSP before programming it using our code.
The control circuit performs the following functions:
- A PWM technique is used to control the duty ratio.
- This PWM should account for both shoot through and non-shoot through states.
- A closed-loop control system is used to adjust the duty ratio according to the supplied
input voltage that is fed to the inverter’s circuit from the PV cells.
Initially, the team displayed an output square signal on one of the GPIOs of the DSP using MATLAB
Simulink in order to be familiar with the software. Then we tried using the control circuit in Figure 6
above to display the results of the six switches of the inverter on six GPIOs of the DSP. The team then
faced a challenge because the triangular signal that the MATLAB generates contains a built in
continuous clock under the mask. The code could not be downloaded to the DSP because of its digital
nature.
After that, the team decided to build another alternative which is shown in Figure 11 below. In these
schematics an ePWM block was used and was given an input of the triangular signal and the three
shifted sine waves [10]. The control signals of the switches for the non- shoot through states were
generated as an output of these schematics. Three of these signals are shown in Figure 12 below. The
next step would be modifying the schematics to include the shoot through states.
Conclusion
To conclude, working on the course assignments this semester helped us gain better
understanding of our project requirements. More specifically, we could come up with the
specifications and functions that will be implemented in the inverter. The team shall continue
working on the project according to the timeline presented in this report. Our team conducted
regular meetings in which to discuss the progress of the project with our mentor, Dr. Haitham
Abu Rub. His feedback and recommendations has helped guide us on what should be done
next. There were several difficulties we faced when understanding what our team will design
exactly, however, at this stage all team members know what they are responsible for.