The strive for efficient and cost-effective photovoltaic systems motivated the power electronic design developed here. The work resulted in a DC-DC converter for module integration and distributed maximum power point tracking (MPPT) with a novel adaptive control scheme. The latter is essential for the combined features of high energy efficiency and high power quality over a wide range of operating conditions. The switching frequency is optimally modulated as a function of solar irradiance for power conversion efficiency maximization. With the rise of irradiance, the frequency is reduced to reach the conversion efficiency target. A search algorithm is developed to determine the optimal switching frequency step. Reducing the switching frequency may, however, compromise MPPT efficiency. Furthermore, it leads to increased ripple content. Therefore, to achieve a uniform high power quality at all conditions, interleaved converter cells are adaptively activated. The overall cost is kept low by selecting components that allow for implementing the functions at low cost. Simulation results show the high value of the module integrated converter for DC standalone and microgrid applications. A 400 W prototype was implemented at 0.14 Euro/W. Testing showed efficiencies above 95% taking into account all losses from power conversion, MPPT, and measurement and control circuitry.
- Boost converter
- Distributed maximum power point tracking (DMPPT)
- Module integrated converter (MIC)
- Photovoltaics (PV)
- Power optimizer
- Power quality
- Switching frequency modulation (SFM).
Fig. 1. PV module integrated converter in DC microgrid application.
EXPECTED SIMULATION RESULTS:
Fig. 2. Standalone case simulated irradiance, switching frequency, and power: (a) at step changing irradiance; (b) at continuously changing irradiance.
Fig. 3. Standalone case experimental step changing irradiance: (a) output power and the switching frequency; (b) output voltage and its ripple content.
Fig. 4 DC microgrid case experimental step changing irradiance: (a) MIC output power and the DC bus voltage; (b) load and DER power; (c) MIC output current and its ripple content.
Fig. 5 MIC EMI spectrum at 1 kW=m2 and fs=20 kHz with and without the interleaved MIC cell under identical operating conditions.
A novel PV module integrated converter (MIC) suitable for boosting voltages for DC standalone and DC microgrid applications was designed, implemented, and tested. The proposed switching frequency modulation (SFM) selects an irradianceadapted switching frequency that is always high enough to avoid operation in discontinuous conduction mode. At a high irradiance, the switching frequency modulation sets a lower value for the frequency, guided by the strive for high efficiency through low switching losses. The proposed automated procedure has shown to be effective in searching for the optimal number and values of switching frequencies. Furthermore, an interleaved boost cell is activated at high irradiance to retain a high level of power quality. Hysteresis functions support the transitions between different discrete switching frequencies as the irradiance changes.
The adaptive MIC control scheme is complemented by an MPPT designed for fast tracking. Thus, by combining the SFM with the adaptive usage of the boost converter interleaved cells and a fast MPPT, targets of efficiency and power quality are reached. The efficiency for the entire MIC including all power conversion and control functions was measured at around 95% or higher for irradiance levels ranging from 0.3 kW=m2 to 1.0 kW=m2. The voltage ripple remained below 0.7% during testing. The prototype was rated at 400 W to make the design well suited for integrating photovoltaics in DC microgrids or solar homes. Distributed maximum power point tracking is implicitly supported through the module integration. The prototype’s cost of parts amounted to 0.14 Euro/W when ordering parts individually in the year 2015. Scale effects will allow for further cost reductions. Together with the convincing technical performance, the cost effectiveness makes this MIC design a compelling candidate for renewable solutions of DC microgrids, DC buses, and solar home applications.
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