31-01-2013, 04:50 PM
Control of Power Converters for Microgrids
1Control of Power.pdf (Size: 144.27 KB / Downloads: 56)
Abstract:
Nowadays, Distributed Generation and Microgrids (MGs) are becoming an important
research line because of their peculiar characteristics. MGs are composed of small power sources
which can be renewable, placed near customer sites. Moreover, they have the inherent property of
islanding: the disconnection of either the MG from the main grid or a part of a MG from the rest of
the MG. In this paper, the behaviour of a particular MG during grid connected and islanding
operation is investigated. The studded MG is based on different energy sources: a wind turbine, a PV
array, a backup diesel generator and a storage system. The renewable sources and the storage system
are connected on DC-bus which is interconnected with a main grid trough a Voltage Source Inverter
(VSI). The attention focuses on the control technique of the VSI during grid connected and islanding
operation of the MG. The evolution of the AC signals on the point of common coupling between the
MG and the main grid as well as the DC signals on the DC-bus on which are connected renewable
energy sources of the MG has been investigated on simulation using a MATLAB/Simulink model.
Introduction
In recent years, the development of Distributed
Generation Systems (DGS), and especially
Microgrids, has been increased considerably. A
DGS is composed by electrical sources placed
near local loads and it becomes more interesting
when those sources are of different kinds [1, 2],
such as wind turbines, photovoltaic (PV) panels,
fuel cells, etc. In that case, the DGS can be
considered a Hybrid Energy System (HES) and
it is very suitable to support the electrical
network in rural areas and remote sites. A MG
is an HES with an inherent characteristic: the
islanding capability. Islanding is the
disconnection of the MG from the main grid
without interruption of the energy generation
for the loads connected to the islanded part. It is
also possible to have an islanding of only one
part of the MG instead of all the MG. Moreover,
islanding can be either planned or unplanned.
The former corresponds to disconnections for
maintenance operations, but the latter is due to
faults or disturbances described in the standard
IEEE 1547 [3] which occur in the grid or in a
feeder of the MG.
Investigated microgrid
Many different architectures are presented in
the literature. A first example is the Microgrid
Central Controller (MGCC) which was firstly
developed in the EU Microgrids Project [5]. In
these MGs, the Central Controller, which is
installed on the side of the medium
voltage/low voltage (MV/LV) substation
connected to low voltage line (LV) is the most
important element. Its function is to carry out a
high level management of the MG operation
by means of technical and economic functions.
Another microgrid architecture example is
CERTS Microgrid (CM) [2]. This concept
proposes the unlimited installation of
distributed energy equipments in the electrical
grid. CM paradigm permits a smoothly
islanding, provides electricity to relatively
small installations (under 2 MW of peak) and
without complex and expensive control
systems. In other words, none of the devices is
essential by itself for the MG operation, giving
a high level of reliability.
Conclusion
A particular MG architecture has been modelled
in order to analyse its behaviour during grid
connected and islanding operation. The model is
based on a wind turbine, a PV panels array, a
backup DG and a VSI used for the
interconnection with the main grid. The VSI
plays an important role because it acts as
interface and fixes the AC voltage amplitude
and frequency of the signal into the MG. In
islanding, the reference given by the grid is lost
and a different control technique is required by
the VSI to create new references.