Inverters gained their popularity when semiconductors became accessible to common population and when the number of alternative energy sources increased. One of the best-known and also one of the most popular types of alternative sources of energy is solar power. Solar energy is converted into electrical energy using photovoltaic (solar) cells. The voltage, generated using solar cells, is DC, so inverters are needed to connect solar cells to the grid. An inverter converts DC voltage into AC voltage with desired amplitude and frequency. There are many types of inverters, and everyone of them has their pros and cons. In this thesis I present some upgrades I did on the cascaded H-bridge inverter. The main advantage of this type of the inverter is capability of reaching relatively high output voltage with low input voltage. That is done with series connection of multiple H-bridge circuits. The supply voltages of the circuits are electrically isolated.
The main goal of this thesis was to upgrade the program code of the inverter. Until now, the output of the inverter was controlled voltage. I upgraded the program code so the output of the inverter is now regulated current. I wrote the program code for overvoltage and overcurrent to prevent damages or destruction of the inverter. Also the program code for switching the transistors was upgraded. There were four previously written algorithms (named ALL, ONE, X2, X3) for controlling the output voltage. When using algorithms ALL and ONE, all three supply voltages had to be symmetrical, while using algorithms X2 and X3, the supply voltages had to be asymmetrical. The transistors were switching proportionally to the ratio of the supply and reference output voltage, but the voltage was not regulated. I upgraded the algorithms and named them ALL_tok, ONE_tok, X2_tok, X3_tok. The new algorithms ensure that the switching depends on the error, which is the difference between the reference and actual output current. I used the PIR controller made of three parallel parts – proportional, integral and resonant. The PI controller can’t achieve zero error when the reference output current is not constant. The resonant part is added parallel to the PI controller to minimize the error. Testing the new algorithms was done with the inductive-resistive load and was separated into two parts. In the first part, all four algorithms were tested with the reference amplitude of the output current equal to 5 A. The comparison between RMS (root mean square) value of the output current and RMS value of reference current gives us results between 98,9% and 102,9%, which means deviation less than 3% between mentioned RMS values. This calculation does not provide any information of possible higher harmonics in the output current, so I performed a harmonic analysis of output current. I calculated the THD (Total Harmonic Distortion) factor from the first 2000 higher harmonics - up to 100 kHz. It turned out that the current with the lowest harmonic distortion is a result of using algorithms ONE_tok and X2_tok. Nevertheless, we decided to continue with testing algorithms ALL_tok and ONE_tok. The first reason for choosing these two algorithms is that they enable transmitting the highest possible power due to symmetrically loaded isolating transformers. The second reason is the fact that these two algorithms are the most suitable for general usage, because they need symmetrical voltage supply. In the second part of the testing, I tested the two chosen algorithms with higher power transmission, with reference output current amplitude equal to 10 A and 15 A. Again, the THD factor of the output voltage and current was calculated and from the results we can conclude that the best algorithm for usage is algorithm ONE_tok. Compared to the algorithm ALL_tok, ONE_tok can produce a higher number of levels of the output voltage which reflects on the lower harmonic distortion. Also, the problem with the second harmonic is presented as the result of the unstable DC voltage on the capacitor package.
In the last part of this thesis I presented possible future work on that inverter. While working on the thesis, I came up with some ideas about the possible upgrades of this inverter. Remaking the program code and upgrading it from bipolar to unipolar modulation, upgrading protective functions of the inverter and power control from grid to capacitor package are only some minor upgrades towards the next milestone in the inverter functionality. Another interesting upgrade or the next logical step in the process of upgrading this inverter is connecting the inverter to the grid and using it for an actual power transfer. Even though the reactive power does not need to be controlled when a small power source is connected to the grid, I think this in addition to the active power control, would be an interesting challenge for future work.