Improved tracking algorithm for rooftop pv systems employing multi-input DC-DC converter

Abstract

The energy need of humankind has been increasing rapidly with the population and consumption increment. Various energy production methods have been investigated to meet this need since the beginning of the 20th century. After half of that century, solar energy has become one of the most studied concepts of energy production methods. With the help of economy-politics crises upon oil or natural gas, investment in non-dependent energy types has increased. Solar energy has become one of the invested areas. Here wishful thoughts may be such that the environmental risks of using fossil fuels are also one of the reasons for this tending, but it is not. The solar energy concept consists of three following main parts. Photovoltaic (PV) panels for transforming solar photon energy into DC electrical energy. Power electronics devices for MPPT implementation and manipulating the electrical power according to the load side. Lastly, the load part of the systems can be a DC load, AC load, or the utility grid directly. In this thesis, a study about the power electronics part of the concept has been completed. At the power electronics aspect, the system may have a single DC/AC converter or two-stage with a DC/DC and a DC/AC converter. As known, PV panel characteristics are not linear; therefore, a maximum power point tracking (MPPT) algorithm should be designed to extract the maximum available power from the panel. Also, to transform the PV panel's DC power into AC power, some electronic manipulation should be configured with switching mode power supplies. These main requirements can be provided within a converter that forms the single-stage PV power system or can be divided into two converters to build a two-stage PV power system. Both systems have their benefits and drawbacks. This study's content is designing a DC/DC converter of the two-stage PV power system. The main targets of the converter are implementing the MPPT algorithm and boosting the low DC voltage level of the PV panel up to 400V DC level for being transformed into AC voltage for utility grid injection. Additionally, the designed converter accepts four PV panels as its input and applies the MPPT algorithm to each one independently. The converter is named as Collector module. As a result, the Collector module consists of four small power electronics topologies whose outputs are connected in parallel to form the single high 400V DC voltage output. The input of the system (PV panels) can have various parameters between 25V to 50V voltage and up to 400W power. Thus, the total nominal output of the module is 1600W. The reason for this individual MPPT configuration is to eliminate the problems with the string-connected PV panel systems. As known, a PV panel has I-V and P-V curves due to PV cell configuration and environmental aspects such as irradiance strength and temperature. The MPPT algorithm aims to carry the PV panel operation point through these curves and locates the maximum power point. When the PV panels are serial or parallel connected to increase the system's power, these curves change according to connection configuration. However, the system performance degrades significantly if some shading effect or other problem occurs on even a single PV panel. Because in this case, the problematic PV panel is not just a lack of contribution to the total system but also has adverse effects on the power produced by other PV panels. In literature, many MPPT algorithms have been theoretically and practically examined and applied to PV system converters. They have advantages and disadvantages regarding implementation easiness, accuracy, stability, or settling speed towards ambient changes. These aspects can be calculated and predicted with theoretical methods. However, another phenomenon is named "power traps" above the I-V and P-V curves of the system. This phenomenon is caused by the interaction of PV panels and power electronics circuits. The outcome of this phenomenon is a disordered structure of a non-linear I-V curve. Such as, even though the ideal theoretical curve is not linear, its fundamental concept is that when PV voltage decreases, PV current should increase at the same irradiance strength as an inverse relationship. However, with the result of the panel and circuit integration, the resultant curve does not follow this fundamental, especially around the DCM-CCM limit. Consequently, when a regular MPPT algorithm is applied to the system, it is observed that the steady-state operation point is far away from the actual maximum power point. As a result, an improved version of the Incremental Conductance (InC) algorithm has been developed and applied to each circuit independently by a single microcontroller. This thesis mainly focuses on the system's software structure, such as designing the novel MPPT algorithm and time-shaping between the moments of required measurement occurrences for four circuits and the MPPT calculations. Lastly, these four circuits are driven with the interleaved technique by having a 45° phase shift between the consecutive circuit's PWM signals. Last, the collector module's hardware structure has been designed for this study. Push-pull topology has been used for four power circuits. The designed module has been tested in various ways. Firstly, individual power circuits were connected to a PV simulator device separately to check the MPPT accuracy. A PV simulator is an analog device whose output characteristic coincides with actual PV panels. With this device, a controllable imitation of a PV panel has been used; hence the circuits in the collector module could be tested under various input powers. According to the results, the MPPT efficiencies of all circuits are above 99%. This verifies that the designed MPPT algorithm has successfully tracked the maximum power point. On the other hand, power transfer efficiency is around 92%-93% for each circuit. Then, all inputs of the collector module were loaded at the same time to verify simultaneous power transfer. Firstly, 4 PV panels are used as inputs. Secondly, 3 PV panels and the PV simulator are used as inputs. In both cases, both MPPT and power transfer efficiency ended up with similar values to the individual test results. Consequently, simultaneous MPPT operation and power transfer are verified with these tests, as well as the availability of using different PV sources simultaneously.