10-05-2013, 02:32 PM
New method of current control for LCL-interfaced grid-connected three phase voltage source inverter
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Abstract
This paper presents an approach for the connection of a photovoltaic
generator to the utility grid. A theoretical analysis, modelling, controlling and a
simulation of a grid connected photovoltaic system using an output LCL filter are
described in detail. In order to reduce the complexity of the system, a linear voltage and
current controllers have been developed for a three phase grid connected inverter. Those
controllers simultaneously regulate the dc link voltage (the photovoltaic power is
delivered to the grid) and the current injected to the grid (to operate at unity power
factor). The simulation results show the proposed methods work properly. Moreover they
show that the control performance and dynamic behaviour of the inverter-LCL-grid
combination. Simulation results show that the output LCL filter installed at the inverter
output offer high harmonic attenuation.
INTRODUCTION
Renewable energy sources such as solar energy make increasing contributions to
electric utility networks. Those sources are commonly coupled to the grid through a
pulse width modulation inverter and filter [1]. Different output filter topologies are
commonly used to interface inverter to the network, namely the L and the LCL filter.
The use of the filter coupling the inverter to the grid reduces the high frequency
pollution of the grid that can disturb loads [2].
They provide to the grid a nearly sinusoidal line current waveforms and a low line
current distortion [3-5]. Of all filters used in the field of power electronic applications,
the LCL filter is currently the most frequently used topologies [6].
DESCRIPTION OF THE SYSTEM
A three phase voltage source inverter and its controller are shown in Fig. The
system is composed of three phase voltage inverter connected to the grid through an
output LCL filter.
The input to the inverter is mimicked by a current source which is represented by a
photovoltaic generator (PVG) and a dc − dc converter associated with the maximal
power point tracking (MPPT) command [9].
Here, the control of dc − dc converter is not covered. The inverter which is the key
element of the system is connected to the ac mains through L-C-L combination.
Inductor L2 in Fig. 1 represents the inductance from both the output filter and the
distribution line.
Three-phase inverter
An average model for the three phase voltage inverter is presented in [10]. This
model describes the relation between dc side and ac side including the connexion
functions ( f11 , f12 , f13 ). This model is adapted to the application studied here. Then,
the inverter is considered as a double modulator of variable states. Voltage u12 and
u13 are the modulations of dc link voltage ( Vdc ) and the input inverter current ie is a
modulation of the output currents i11 and i12 . They can be described as follow [10].
CONTROL OF THE SYSTEM
The overall control structure consists of a DC link voltage controller and a line
current controller, refer to figure 1. To supply a line current with low distortion the
connection to the grid is made via an ac filter witch consists of L-C-L combination. The
line current controller consists of a model based cascade controller.
The cascade controller is composed of an outer current controller for the network
line current, i.e. current i2j , an intermediate voltage controller for the filter capacitor
voltage and an inner current control loop for the inverter current i1j . The cascade
configuration enables the use of proportional or proportional-integral controllers.
DC link voltage control
The purpose of the DC link voltage controller is to preserve the DC-link voltage at
its reference value ( vdc * ) and to provide the reference current ( ie * ). This controller
achieved the transfer of the active power flow, drawn from the photovoltaic generator,
to the utility network.
CONCLUSION
In the paper both the modelling and control of a three phase inverter employing an
LCL filter used to reduce the switching frequency ripple injected to the grid is
investigated.
The goal of the paper is to provide a control procedure for the inverter-LCL-grid
combination and study the performance and the dynamic of linear controllers calculated
to control the dc link voltage and the output inverter current.
The modelling and control have been tested with simulation. Stability and high
dynamic are obtained, and moreover all results have been obtained using a simple
control method but more sensors.