ECE 4600 GROUP DESIGN PROJECT PROGRESS REPORT GROUP 03 Implementation of a Grid Connected Solar Inverter with Maximum Power Point Tracking Authors Radeon Shamilov Kresta Zumel Valeria Pevtsov Reza Fazel-Darbandi Ian Swintak Supervisor Athula Rajapakse, Ph.D, P.Eng Progress coverage dates: September 10, 2013 January 10, 2014 Submission date: January 13, 2014
Contents Glossary... iii 1 Introduction... 1 2 Progress Summary... 1 2.1 Simulation... 1 2.2 Power Electronics Circuit... 1 2.2.1 DC/DC Converter Hardware (Reza Fazel-Darbandi)... 2 2.2.2 DC/AC Inverter (Kresta Zumel and Valeria Pevtsov)... 2 2.3 Gate Driver (Ian Swintak)... 3 2.4 Microcontroller Software (Radeon Shamilov)... 3 2.4.1 General Software... 3 2.4.2 DC/DC and DC/AC Software... 4 2.5 Revised Specifications... 4 2.6 Completed Work Summary... 5 3 Future Work... 5 4 Conclusion... 5 Appendix A... 6 Appendix B... 8 ii
Glossary AC ADC CMP DAC DC IC IGBT LED MPPT PI PLL PSCAD PV PWM THD Alternating current Analog to Digital Converter Comparator Digital to Analog Converter Direct Current Integrated Circuit Insulated-gate Bipolar Transistor Light Emitting Diode Maximum Power Point Tracking Proportional-Integral Phase-Locked Loop Power Systems CAD Photovoltaic Pulse-Width Modulation Total Harmonic Distortion iii
1 Introduction Photovoltaic technologies offer an environmentally friendly solution to the rapidly growing energy needs of modern society. The objective of this project is to design and implement a microcontroller based photovoltaic grid-tie power inverter system for residential applications. The system will harvest the solar energy and convert it to electrical power synchronized with the utility grid. As the work progressed, the project specifications mentioned in the proposal were changed. As a result, the simulation stage of project took longer than we expected which put us two weeks behind the initial schedule. We revised our Gantt chart accordingly to make up for the delays and we expect to complete the project by March 10, 2014. Our next immediate step is to interface the hardware and software. Further details of the project progress will be discussed in the following section. This will be followed by future plans and appendices sections. 2 Progress Summary The overall project workload was divided into four parts: the overall system simulations, power electronics circuitry (DC/DC converter and DC/AC inverter), gate driver circuits, and the microcontroller software. As the project progressed, we changed our proposed system specifications to accommodate a larger range of solar panel ratings which resulted in design delays. Currently, design and simulation of each part have been completed. The prototypes for the inverter and gate driver have been built and all necessary microcontroller code has been developed. 2.1 Simulation (Valeria Pevtsov and Kresta Zumel) A PSCAD case was used to simulate the entire hardware and microcontroller section of the system. Initially, only the inverter section was simulated, and satisfactory results were obtained. Next, to make our simulation case more representative of our project, the DC/DC converter section, a solar panel model, MPPT, PI controllers and PLL were added. We had difficulty in making the simulation work due to initial unfamiliarity with the software. Therefore, our supervisor assisted us with the proper configuration and setup of the PSCAD case. The values for components in the simulation, such as DC/DC converter parts and the output filter, were set based on our calculations. Since the simulation results met the project specifications outlined in Table 1, we moved on to the prototyping stage. 2.2 Power Electronics Circuit This section discusses the power electronics circuit which includes the DC/DC converter and the DC/AC inverter hardware. The DC/DC converter design was done by Reza and the DC/AC inverter design was done by Valeria and Kresta. At this point, we started integrating the two hardware circuits together. 1
2.2.1 DC/DC Converter Hardware (Reza Fazel-Darbandi) Calculations for the converter and the sensing circuitry have been performed in order to ensure the system operates within the product specifications. A schematic that includes the DC/DC converter, a halleffect current sensor, the voltage sensing circuitry, and the solar panel was developed. The duty cycle of the converter is set based on the current and voltage measurements from the solar panels. Initially, the measurement system was designed to include a differential amplifier and a current-sense resistor. However, further analysis and discussion with Mr. Dirks revealed that the setup will have isolation and grounding issues. Therefore, the design was changed to use an opto-isolating IC for voltage measurement as well as a Hall Effect IC for current measurement. The DC/DC converter design experienced minor delays due to changes to the original product specifications in our proposal. Currently, the converter accommodates up to 1kW input power instead of only 300W as originally specified. The parts for the converter were ordered and received. The circuit was recently assembled. The next step is to test its functionality and integrate it with the software and other hardware components. The design of the converter was done in 6 weeks and it went through revisions for an additional 2 weeks. 2.2.2 DC/AC Inverter (Kresta Zumel and Valeria Pevtsov) The development of the inverter began with a paper design based on the desired performance specifications of the system. The paper design consisted of calculation of the output inductance and modulation index values based on the system real and reactive power specifications. In addition, the filter components were chosen based on the total harmonic distortion requirement of the system. Moreover, an H-bridge topology was chosen as opposed to a half-bridge to further reduce harmonic contents of the output waveforms. It took two months to complete the research and initial design. Two three phase IGBT modules were ordered and acquired. Only four switches are required out of the 6 IGBT switches on each module; the extra switches will be used for possible testing failures. A heat sink was attached to one of the modules to dissipate heat while operating. Currently, the IGBT modules are ready to be integrated into the system and tested with the software. In addition to the H-bridge of the inverter, we designed voltage and current measuring circuits for the grid signals. Since the low and the high power circuitry require isolation, we decided to use current and potential transformers to step down the grid current and voltage. On the secondary side of these transformers, dc offset circuits were designed, built and tested in order to produce a signal that can be fed to the microcontroller. We spent 2 weeks in total on the measurement circuitry and it was completed in the last week of November. In our proposal, we mentioned about using 1:1 transformer at the output of the system. However, as we progressed and with the advice of Mr. Erwin Dirks, we decided to use a 1:2 transformer instead. The reason for the modification was that we were able to order lower power rating parts that were cheaper. Hence, to accommodate this change, we recalculated the values of the output filter components. At this point, the inverter is ready to be integrated with the software. 2
2.3 Gate Driver (Ian Swintak) The gate driver circuit has had a few iterations of design throughout the project. The first design iteration was going to be built from scratch, but this idea was abandoned because of time constraints and the availability of driver ICs. This initial design was worked on for approximately 1 month. The second design iteration included a single gate driver IC for the DC/AC inverter and one for the DC/DC converter. The reason these gate drivers were selected was that they could be driven with low power and could handle low and high side driving. The gate driver for the DC/AC inverter could also drive up to 6 IGBTs so it could work with the H-bridge design. The second iteration was worked on for another month. A problem discovered while testing the selected driver circuit was the missing isolation between the microcontroller and power electronics. To solve this problem, we included an optocoupler and a buffer in the third design iteration. The optocoupler isolates the microcontroller from the power electronics as well as provides a separate ground for both. The buffer is needed to increase the amplitude of the signal from the microcontroller to turn on the LED in the optocoupler. The final iteration has been worked on for 1 month. The gate drivers have been assembled and tested with a function generator and are now ready to be integrated with the DC/DC converter and the DC/AC inverter circuits. 2.4 Microcontroller Software (Radeon Shamilov) The software development approach that was chosen for this project is the divide and conquer method. The main benefit of this approach is the ability to separately develop and test smaller software modules without the need to wait for the entire program to function. An additional benefit of the divide and conquer method with regards to the software development is the ability to test hardware subcircuits as soon as they are available. Therefore, the first development stage that has been completed is a detailed software design report. Moreover, the report is a living document that is subject to change as new tests and problems are unveiled. The first version of the report was completed by November 1 st. The intention of this report is to be a reference guide for the actual software implementation and it includes verbal and visual descriptions of the software. The software design was divided into three submodules: General Software, DC/DC Software, and DC/AC Software. The following subsections describe the progress made in each software submodule. 2.4.1 General Software The general software had been developed and tested by the middle of December. This software pertains to internal components of the microcontroller that are not directly linked to either the DC/DC software or the DC/AC software. The main responsibility of this submodule is to initialize the microcontroller peripherals: analog to digital converters (ADC), digital to analog converters (DAC), timers, interrupts, comparator (CMP), and the pulse width modulator (PWM). All of these peripherals were initialized and tested with satisfactory results. In particular, the ADC was utilized to generate a sinusoidal 3
wave and a triangle wave with an accuracy +/- 0.02 Hz. Additionally, the DAC was utilized to capture seven multiplexed analog signals with an accuracy of +/- 0.806 mv and a conversion time of 5.1 us per sample. Finally, an interrupt pin, a timer and the comparator were used to measure the frequency of a sinusoidal wave with an accuracy of +/- 0.02 Hz. 2.4.2 DC/DC and DC/AC Software Both the DC/DC and the DC/AC submodules use the general software in order to acquire the required input measurements and to generate the necessary outputs. The development of the DC/DC submodule was completed by January 1 st but not yet tested due to delays in hardware implementation. The DC/AC submodule is currently in progress. One of the main design changes in the DC/AC submodule is the method we use in order to generate the sinusoidal pulse width modulation (SPWM) signal required by the DC/AC hardware. The initial design made use of the comparator peripheral in order to compare the carrier (triangle) wave and the reference (sinusoidal) wave and output the SPWM signal. This design failed since the hardware comparator was not capable of operating at the required switching frequency of the SPWM. The current solution is to perform the comparison within the software rather than using a hardware comparator. We are currently performing additional analysis and tests to verify that the solution eliminates the comparator time response problem. 2.5 Revised Specifications The system specifications suggested in the proposal had to be revised as the project progressed. The revision was made to accommodate a wider range of solar panel ratings to be connected to the system. The following table shows the original and the revised project specifications. Table 1: System Specifications Parameter Value or Range Value or Range PV panel nominal power 150 W 1000 W* Max allowed DC bus voltage ripple 8.5 Vdc 10 Vdc* Inverter switching frequency 5kHz 1.98 khz* Inverter nominal output power 150 W 1000 W* Grid voltage 114-126 Vrms 114-126 Vrms Grid frequency 59.3-60.5 Hz 59.3-60.5 Hz Power factor 0.8 lagging - 0.8 leading 0.8 lagging - 0.8 leading MPPT system efficiency above 80 % above 80 % Boost switching frequency 3 khz - 300 khz 3 khz - 30 khz* *revised 4
2.6 Completed Work Summary At the start of the project, our team worked individually on doing research in regards to their assigned sections and met once a week to discuss progress within the team. As the project progressed and individual parts were completed, we started to work together to make sure that all system components were compatible with each other. The status of the completed work in our project is as follows: All system designs are 100% complete (2 months) The project proposal and progress reports are 100% complete (4 weeks) Individual subsystem testing and implementation are 75% complete (pending DC/DC converter testing and implementation) Overall, the project is 60% completed and the remaining work is discussed in the following section. 3 Future Work Integrating individual parts of the system is the final step in our project. In the next two months, we will be performing the following tasks to ensure the completion of the project by March 10, 2014: Build and test the voltage and current sensing circuitry for the DC/DC converter. Test the gate driver circuit with the microcontroller software. Build the DC/DC converter and test it with the microcontroller software. Test the DC/AC inverter with the microcontroller software. Combine all hardware components of the system and integrate them with the microcontroller software and the solar panels. Test the overall system functionality and adjust the system design until it satisfies the specifications. In conjunction with the above listed tasks, we will be writing the final report and creating the final presentation. In addition, we will design a Printed Circuit Board for our system if time and budget permit. 4 Conclusion Overall, our team has made significant progress since the start. The design and simulation of the entire system have been completed and all required parts have been acquired. We spent $473.39 out of our budget to date, which is within the limit of $500. Although we are behind schedule on the testing and implementation stage of our project by 2 weeks, our adjusted timeline shown in the revised Gantt chart in Appendix A will get us back on schedule. We have allocated 8 hours per week for the project and with Dr. Rajapakse s supervision for testing of our system, we will complete the project by March 10, 2014. 5
Appendix A The following Gantt Chart outlines the revised schedule for the project. 6
Revised Gantt Chart continued. 7
Appendix B The budget was revised to include spare components, additional parts and shipping costs. Total amount spent is $473.39 while $911.33 is contributed by the ECE department. The following table shows the revised budget. Table 2: Budget 8