17-09-2016, 11:25 AM
DC Load and Batteries Control Limitations for
Photovoltaic Systems. Experimental Validation
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Abstract—This study first presents an experimental control
strategy of photovoltaic (PV) system composed of: PV array, dc–dc
power converters, electrolytic storage, and programmable dc electronic
load. This control aims to extract maximum power from PV
array and manages the power transfer through the dc load, respecting
the available storage level. The designed system allows simultaneously
the supply of a dc load and the charge or the discharge of
the storage during the PV power production. The experimental results
obtained with a dSPACE 1103 controller board show that the
PV stand-alone system responds within certain limits that appear
as soon as one of the storage thresholds is reached: either loss of
energy produced, or insufficient energy toward the load. In urban
area, it is proposed to overcome these limitations by connecting the
utility grid with the PV system while maintaining the priority for
self-feeding. The experimental results of this PV semi-isolated system
are shown and discussed. For this first approach, the goal was
to verify the technical feasibility of the suggested system controls.
The final results are energetically relevant.
INTRODUCTION
P HOTOVOLTAIC (PV) systems convert directly solar energy
into electricity. Nowadays, the favorable politicoeconomic
context allows a significant development of small
means of decentralized PV power production, therefore, associated
or integrated to buildings. PV power purchase conditions
lead quite naturally these applications to grid-connected system
with a total and permanent energy injection. Thus, currently, the
PV grid-connected system is suggested in the most applications.
There are many studies performed on different problems of PV
grid-connected systems [1]–[5].
However, this development can lead to grid-connection incidents
that become true technical constraints (setting voltage and
frequency, islanding detection, etc.) [6].
In contrast, the other operating mode is the PV stand-alone
system whose applications context is specific to the countryside
or isolated locations. This system is seen as a substitute of
utility grid connection. Given the intermittent PV production,
the major problem associated with this system is the service
continuity, whence the importance of energy storage and the
addition of conventional sources, most often microturbines and
cogeneration plant. The studies in this axis concentrate more
on techno-economic feasibility conditions, optimized storage
sizing, and load management [7]–[11].
The energy produced by a PV stand-alone system is intended
for self-feeding, whence the isolated aspect. In order to obtain
a reliable power distribution, this system needs to be secured
by means of storage and/or conventional power production. Actually,
passive houses strongly insulated and fully electrified,
equipped with PV sources, could progressively become net-zero
energy with significantly reduced greenhouse gas emissions. PV
stand-alone system with adequate storage could cover the energy
needs. Regarding house electrical loads, in order to obtain
better energy efficiency, it is possible to supply electrical loads
directly with dc power, whose realization is not trivial but nowadays
technically feasible, and most often used in aerospace and
naval applications. Regarding the protection devices, a panel of
choice is possible, between the solid-state circuit breaker and the
hybrid breaker, and permit, following the place and the purpose
of the protection, to take the good switchgear in accordance with
the specification of breaking time, perturbation on the bus, etc.
These convergent factors have led to researches on dc power
supply for buildings [4], [12], [13].
Taking into account this context, this paper focuses on the
analysis of a PV stand-alone system control strategy and gives
the system limits, then exposes a possible extension of the PV
system. First, the PV system is presented: a PV array, dc–dc
converters, electrolytic storage cells, and a programmable dc
electronic load, which imposes currents in order to simulate the
functioning of a low-consumption house lighting. In order to
transfer to the load, at every moment, the maximum PV power
available, a maximum power point tracking (MPPT) strategy is
introduced. To feed correctly the dc load, the dc bus voltage is
secured thanks to the storage system within the respect of the
available storage level. The experimental results show that the
system responds within certain limits that appear as soon as one
of the storage voltage limits is reached. To overcome these limitations,
the solution could be to connect the utility grid to the
system, economically feasible in urban areas. Nevertheless, the
produced PV power remains priority for self-feeding. The experimental
results of this semi-isolated PV grid connected system
are energetically relevant and show the technical feasibility.
II. PV STAND-ALONE SYSTEM
PV systems operating in a stand-alone mode consist of a
PV source, storage means, ac or ac consumers and power conditioning devices. Per definition, a PV stand-alone system involves
no interaction with a utility grid. In almost all commercial
PV stand-alone systems the only existing control is the batteries
charge regulator. Compared to these systems, the suggested PV
stand-alone system has a control that allows simultaneously the
supply of a dc load and charge/discharge of the storage during
the PV power production, as presented in [14].
A. System Overview
The PV stand-alone system is suggested for local dc generation
that feed directly a dc load, in a safety configuration. This
system extracts maximum power from PV sources and manages
the power transfer through the dc load (house), respecting the
available storage level and taking into account the requested load
power, as illustrated in the Fig. 1. When the solar irradiation is
too weak to generate the entire necessary power to transfer to the
load, the dc load is supplied simultaneously by the PV system
and the storage. In contrast, when the generated PV power is
higher than the dc load requirements, and the storage has not a
full state of charge (SOC), the system sends power back to the
storage.
Aiming to optimize energy transfer, an experimental platform
has been installed in the Centre Pierre Guillaumat of our university,
whose images are given in Fig. 2. It refers mainly to 16 PV
panels [2 kWp, Fig. 2(a)], a weather station, a set of electrolytic
accumulators (Sonnenschein Solar S12/130 A, 12 V–130 Ah), a
dSPACE 1103 controller board, and power electronic necessary
devices (SEMIKRON SKM100GB063D, 600 V–100 A).
This system is associated with a programmable dc electronic
load (Chroma 63202, 2.6 kW 500 V–50 A) that has a role to absorb
and dissipate energy, allowing the simulation of the power
required by the virtual house lighting.
B. PV Array
The PV array (PVA)is composed of 16 PV panels Solar Fabrik
SF-130/2-125, whose electrical specifications are presented in
Table I. The electrical coupling is given in the Fig. 3 with D
reverse-current protection diodes and RLIG power line losses
resistances.