ABSTRACT:
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.
KEYWORDS:
1.
Boost
converter
2.
Distributed
maximum power point tracking (DMPPT)
3.
Microgrid
4.
Module
integrated converter (MIC)
5.
Photovoltaics
(PV)
6.
Power
optimizer
7.
Power quality
8.
Solar
irradiance
9.
Switching
frequency modulation (SFM)
SOFTWARE:
MATLAB/SIMULINK
CONCLUSION:
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 irradiance
adapted 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.
REFERENCES:
[1] REN21. 2016, “Renewables 2016 Global
Status Report,” Renewable Energy Policy Network for the 21st Century, Paris,
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[3]
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2016.
[4]
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