28-07-2011, 11:20 AM
Abstract
This paper deals with a new isolated wind–hydro
hybrid generation system employing one squirrel-cage induction
generator (SCIG) driven by a variable-speed wind turbine
and another SCIG driven by a constant-power hydro turbine
feeding three-phase four-wire local loads. The proposed system
utilizes two back-to-back-connected pulsewidth modulationcontrolled
insulated-gate-bipolar-transistor-based voltage-source
converters (VSCs) with a battery energy storage system at their
dc link. The main objectives of the control algorithm for the VSCs
are to achieve maximum power tracking (MPT) through rotor
speed control of a wind-turbine-driven SCIG under varying wind
speeds and control of the magnitude and the frequency of the
load voltage. The proposed wind-hydro hybrid system has a capability
of bidirectional active- and reactive-power flow, by which
it controls the magnitude and the frequency of the load voltage.
The proposed electromechanical system using SCIGs, an MPT
controller, and a voltage and frequency controller are modeled
and simulated in MATLAB using Simulink and Sim Power System
set toolboxes, and different aspects of the proposed system are
studied for various types of linear, nonlinear, and dynamic loads,
and under varying wind-speed conditions. The performance of the
proposed system is presented to demonstrate its capability ofMPT,
voltage and frequency control (VFC), harmonic elimination, and
load balancing.
Index Terms—Battery energy storage system (BESS), small
hydro, squirrel-cage induction generator (SCIG), wind-energyconversion
system (WECS).
I. INTRODUCTION
RENEWABLE energy sources have attracted attention
worldwide due to soaring prices of fossil fuels. Renewable
energy sources are considered to be important in improving
the security of energy supplies by decreasing the dependence
on fossil fuels and in reducing the emissions of greenhouse
gases. The viability of isolated systems using renewable energy
sources depends largely on regulations and stimulation
measures. Renewable energy sources are the natural energy
resources that are inexhaustible, for example, wind, solar,
geothermal, biomass, and small hydro generation [1]. Among
the renewable energy sources, small hydro and wind energy
have the ability to complement each other [2]. For power
generation by small or microhydro as well as wind systems,
the use of squirrel-cage induction generators (SCIGs) has been
reported in literature [3]–[18].
Although the potential for small hydroelectric systems depends
on the availability of suitable water flow, where the
resource exists, it can provide cheap clean reliable electricity.
Hydroelectric plants convert the kinetic energy of a waterfall
into electric energy. The power available in a flow of water
depends on the vertical distance the water falls (i.e., head)
and the volume of flow of water in unit time (i.e., discharge).
The water powers a turbine, and its rotational movement is
transferred through a shaft to an electric generator [1]. When
SCIG is used for small or microhydro applications, its reactivepower
requirement is met by a capacitor bank at its stator
terminals. The SCIG has advantages like being simple, low
cost, rugged, maintenance free, absence of dc, brushless, etc.,
as compared with the conventional synchronous generator for
hydro applications [3], [4].
As regards wind-turbine generators, these can be built either
as constant-speed machines, which rotate at a fixed speed
regardless of wind speed, or as variable-speed machines in
which rotational speed varies in accordance with wind speed.
For fixed-speed wind turbines, energy-conversion efficiency is
very low for widely varying wind speeds. In recent years, windturbine
technology has switched from fixed speed to variable
speed. The variable-speed machines have several advantages.
They reduce mechanical stresses, dynamically compensate
for torque and power pulsations, and improve power quality
and system efficiency [12]. The grid-connected variable-speed
wind-energy-conversion system (WECS) based on SCIG use
back-to-back connected power converters [13], [15]. In such
systems, the power converter decouples the SCIG from the grid,
resulting in an improved reliability.
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