28-09-2016, 09:16 AM
1456463190-173.ReconfigurableSolarConverterASingleStagePowerConversionPVBatterySystem.pdf (Size: 1.14 MB / Downloads: 57)
Abstract—This paper introduces a new converter called reconfigurable
solar converter (RSC) for photovoltaic (PV)-battery
application, particularly utility-scale PV-battery application. The
main concept of the new converter is to use a single-stage threephase
grid-tie solar PV converter to perform dc/ac and dc/dc operations.
This converter solution is appealing for PV-battery application,
because it minimizes the number of conversion stages, thereby
improving efficiency and reducing cost, weight, and volume. In this
paper, a combination of analysis and experimental tests is used to
demonstrate the attractive performance characteristics of the proposed
RSC.
INTRODUCTION
S OLAR photovoltaic (PV) electricity generation is not available
and sometimes less available depending on the time of
the day and the weather conditions. Solar PV electricity output
is also highly sensitive to shading. When even a small portion
of a cell, module, or array is shaded, while the remainder is in
sunlight, the output falls dramatically. Therefore, solar PV electricity
output significantly varies. From an energy source standpoint,
a stable energy source and an energy source that can be
dispatched at the request are desired. As a result, energy storage
such as batteries and fuel cells for solar PV systems has drawn
significant attention and the demand of energy storage for solar
PV systems has been dramatically increased, since, with energy
storage, a solar PV system becomes a stable energy source and
it can be dispatched at the request, which results in improving
the performance and the value of solar PV systems [1]–[3].
There are different options for integrating energy storage into
a utility-scale solar PV system. Specifically, energy storage can
be integrated into the either ac or dc side of the solar PV power
conversion systems which may consist of multiple conversion stages [4]–[33]. Every integration solution has its advantages
and disadvantages. Different integration solutions can be compared
with regard to the number of power stages, efficiency,
storage system flexibility, control complexity, etc.
This paper introduces a novel single-stage solar converter
called reconfigurable solar converter (RSC). The basic concept
of the RSC is to use a single power conversion system to perform
different operation modes such as PV to grid (dc to ac), PV to
battery (dc to dc), battery to grid (dc to ac), and battery/PV to
grid (dc to ac) for solar PV systems with energy storage. The
RSC concept arose from the fact that energy storage integration
for utility-scale solar PV systems makes sense if there is an
enough gap or a minimal overlap between the PV energy storage
and release time. Fig. 1 shows different scenarios for the PVgenerated
power time of use. In case (a), the PV energy is always
delivered to the grid and there is basically no need of energy
storage. However, for cases (b) and ©, the PV energy should
be first stored in the battery and then the battery or both battery
and PV supply the load. In cases (b) and ©, integration of the
battery has the highest value and the RSC provides significant
benefit over other integration options when there is the time gap
between generation and consumption of power.
Section II introduces the proposed RSC circuit, different
modes of operation, and system benefits. In Section III, control
of the RSC is introduced and necessary design considerations
and modifications to the conventional three-phase PV converter
are discussed. Section IV verifies the RSC with experimental results
that demonstrate the attractive performance characteristics.
Section V summarizes and concludes the paper.
Introduction
The schematic of the proposed RSC is presented in
Fig. 2. The RSC has some modifications to the conventional three-phase PV inverter system. These modifications allow the
RSC to include the charging function in the conventional threephase
PV inverter system. Assuming that the conventional
utility-scale PV inverter system consists of a three-phase voltage
source converter and its associated components, the RSC
requires additional cables and mechanical switches, as shown in
Fig. 2. Optional inductors are included if the ac filter inductance
is not enough for the charging purpose.
B. Operation Modes of the RSC
All possible operation modes for the RSC are presented in
Fig. 3. In Mode 1, the PV is directly connected to the grid
through a dc/ac operation of the converter with possibility of
maximum power point tracking (MPPT) control and the S1 and
S6 switches remain open. In Mode 2, the battery is charged with
the PV panels through the dc/dc operation of the converter by
closing the S6 switch and opening the S5 switch. In this mode,
the MPPT function is performed; therefore, maximum power is
generated from PV. There is another mode that both the PV and
battery provide the power to the grid by closing the S1 switch.
This operation is shown as Mode 3. In this mode, the dc-link
voltage that is the same as the PV voltage is enforced by the
battery voltage; therefore, MPPT control is not possible. Mode
4 represents an operation mode that the energy stored in the
battery is delivered to the grid. There is another mode, Mode 5
that the battery is charged from the grid. This mode is not shown
in Fig. 3.
System Benefits of Solar PV Power Plant
With the RSC Concept
The RSC concept provides significant benefits to system planning
of utility-scale solar PV power plants. The current stateof-the-art
technology is to integrate the energy storage into the
ac side of the solar PV system. An example of commercial energy
storage solutions is the ABB distributed energy storage
(DES) solution that is a complete package up to 4 MW, which is
connected to the grids directly and, with its communication capabilities,
can be utilized as a mean for peak shifting in solar PV
power plants [33]. The RSC concept allows not only the system
owners to possess an expandable asset that helps them to plan
and operate the power plant accordingly but also manufacturers
to offer a cost-competitive decentralized PV energy storage solution
with the RSC. Fig. 4 shows examples of the PV energy
storage solutions with the RSC and the current state-of-the-art
technology.
The technical and financial benefits that the RSC solution
is able to provide are more apparent in larger solar PV power
plants. Specifically, a large solar PV power plant using the RSCs
can be controlled more effectively and its power can be dispatched
more economically because of the flexibility of operation.
Developing a detailed operation characteristic of a solar
PV power plant with the RSC is beyond the scope of this paper.
However, different system controls as shown in Fig. 5 can be
proposed based on the requested power from the grid operator
Preq and available generated power form the plant Pgen . These
two values being results of an optimization problem (such as
unit commitment methods) serve as variables to control the solar
PV power plant accordingly. In other words, in response to the
request of the grid operator, different system control schemes
can be realized with the RSC-based solar PV power plant as
follows:
1) system control 1 for Pgen > Preq ;
2) system control 2 for Pgen < Preq ;
3) system control 3 for Pgen = Preq ;
4) system control 4 for charging from the grid (Operation
Mode 5).