Multiphase Design and Control Techniques Applied to a Forward Micro-Inverter
TESIS DOCTORAL
Autor: David Meneses Herrera
Ingeniero Industrial por la Universidad Politécnica de Madrid
DEPARTAMENTO DE AUTOMÁTICA, INGENIERÍA ELECTRÓNICA E
INFORMATICA INDUSTRIAL
Universidad Politécnica de Madrid
Abstract
In the last decade the photovoltaic (PV) installed power increased with an
average growth of 49% per year and it is expected to cover the 16% of the global
electricity consumption by 2050. Most of the installed PV power corresponds to
grid-connected systems, with a significant percentage of residential installations.
In these PV systems, the inverter is essential since it is the responsible of
transferring into the grid the extracted power from the PV modules. Several
architectures have been proposed for grid-connected residential PV systems,
including the AC-module technology.
An AC-module consists of an inverter, also known as micro-inverter, which is
attached to a PV module. The AC-module technology offers modularity,
redundancy and individual MPPT of each module. In addition, the expansion of
this technology will enable the possibility of economies of scale of mass market
and “plug and play” for the user, thus reducing the overall cost of the
installation. However, the micro-inverter must be able to provide the required
voltage boost to interface a low voltage PV module to the grid while keeping an
acceptable efficiency in a wide power range. Furthermore, the quality standards
must be satisfied and size and lifetime of the solutions must be always
considered.
In this thesis a single-stage forward micro-inverter with boundary mode
operation is proposed to address the micro-inverter requirements. The
transformer in the proposed topology remains as in the classic forward converter
and bidirectional switches in the secondary side allows direct connection to the
grid. In addition the selected control strategy allows high power factor current
with a simple implementation. The operation of the topology is presented and
the main design issues are introduced. With the intention to propose a simple
and low-cost solution, an analog controller for a PFC operated in boundary mode
is utilized. The main necessary modifications are discussed, with the focus on the
zero current detection (ZCD) and the compatibility of the controller with a MPPT
algorithm. The experimental results show the limitations of the selected analog
controller implementation and the transformer is identified as a main losses
contributor.
The main objective of this thesis is to contribute in the application of control and
design multiphase techniques to the PV micro-inverters. Two different
multiphase configurations have been applied to the forward micro-inverter
proposed in this thesis. The first one consists of a parallel-series connected
variation which enables the use of low turns ratio, i.e. well coupled, transformers
to achieve a proper voltage boost with an improved performance. This
multiphase configuration implements BCM control at maximum load however.
With this control method the switching frequency increases significantly for light
load operation, thus jeopardizing the efficiency. Therefore, in order to keep
acceptable weighted efficiency levels, DCM operation is selected for low power
conditions.
The second multiphase variation considered in this thesis is the interleaved
configuration with two different phase shedding techniques: depending on the
DC power extracted from the PV panel, and depending on the demanded
instantaneous power. The application of interleaving techniques is interesting in
PV grid-connected inverters for the possibility of flat efficiency behavior in a
wide power range. The interleaved variations of the proposed forward micro-
inverter are operated in DCM to avoid the current loop, which is important when
the number of phases is large.
The adequate transformer cores for all the multiphase configurations are selected according to the area product parameter and a detailed design of each required transformer is developed. With this information and simulation results, the impact in size and efficiency of the number of transformer used can be assessed. The considered multiphase topologies are compared in this thesis according to the results of the introduced analysis.
LINK VIEW FULL TEXT:https://oa.upm.es/39988/1/DAVID_MENESES_HERRERA.pdf
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