A low-cost MPPT multiple-input power converter for home applications in isolated areas
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
The focus of this research is to design and build a low-cost maximum power point tracking (MPPT) multiple-input-single-output power converter system for low power gridisolated applications. The design of this power converter concentrated on searching for a suitable topology that integrates multiple renewable power sources, each with their own MPPT requirements and with the lowest cost of components. With good power conversion efficiency, the converter provides power to a dc load output and is also able to appropriately charge an energy storage battery. In addition to the main functional blocks, all protections required are equipped with the converter, including under voltage lock-out (UVLO), over voltage protection (OVP), cycle-by-cycle current limit, and battery over charge and over discharge prevention.
The development and implementation of the converter was divided by different steps. The initial phase searched for, analyzed, and proposed the most suitable topology in terms of power delivery, cost, and feasibility. The non-isolated full-bridge was chosen for the power conversion topology for each channel with its own analogue controller. An interfacing circuit was designed to work with those full-bridge controllers for integrating MPPT control signals, constant output voltage control signals, and constant charging current control signals, from a microcontroller, a single output voltage feedback loop, and a single output current feedback loop, respectively. After a specification of the design had been selected, the detailed design and calculation of circuits was carried out. Simulations were also conducted to confirm the operation of the converter, including the start-up sequences, output load response, battery charging modes, and the transient between operation modes. The converter was then transferred to PCB design with two versions, a 2-layer and 4-layer board. A comparison was made for choosing the appropriate option among the designs regarding board size, cost, and performance. The final physical converter was formed by soldering components on the manufactured 170mm x 130mm 4-layer PCB, and programming of the microprocessor. The final step was the characterisation of the converter with standard power supplies and renewable energy source emulators, in the laboratory environment.
Results show that the converter functions as per the design specifications. With the input source of a solar photovoltaic panel, a micro wind/hydro turbine, or both, the converter can work with solely a dc load or with a dc-bus connected battery (where the converter provides a threestage charging profile). The smooth and fast transiting between operation modes of MPPT, constant output voltage, and constant charging current were recorded without any abnormal behavior. The MPPT functions separately with each input source with high accuracy and fast response to the input conditions. Depending on the state of the output, the converter automatically switches to either constant output voltage or constant charging current mode when the total available input power is higher than the output load demand. For each of these operation modes, the converter also achieves a fast response to the input sources voltage and output load variations.
With a designed capability of 2 kW maximum total power conversion, the converter is able to work with a wide range of input voltage (from 16 V to 60 V) for both input sources while its nominal output voltage is set at around 27 V for working with a nominal 24 V lead-acid battery. The peak power conversion efficiency of 95.3 % was recorded at 400 W of total input power when the turbine source voltage was 54 V (dc value after the rectifier) and the solar photovoltaic source voltage was 24 V. The operating temperature of the converter appeared to be higher than expected at some components, with a peak of 68.3 oC recorded on the gate driver chips. However, this issue could be mitigated by adding more heat sink components or modifying the design on a new revision.
With under $60 USD of total component cost (slightly above $55 for pre-manufactured components and materials, and about $3 for the PCB), the converter is shown to be a low-cost converter regarding its power capability while supporting multiple input sources which may have a wide range of nominal power outputs. This makes the converter more applicable in isolated areas of developing countries.