Power Converters And Control Renewable Energy Systems.pdf
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Power Converters and Control of Renewable Energy Systems
Frede Blaabjerg, Remus Teodorescu, Zhe Chen
Aalborg University, Institute of Energy Technology,
Denmark
fbl@iet.aau.dk, ret@iet.aau.dk, zch@iet.aau.dk
Marco Liserre
Politecnico di Bari, DEE
Italy
liserre@poliba.it
II.
R
ENEWABLE
E
NERGY
S
OURCES
Abstract
— The global electrical energy consumption is steadily
rising and therefore a continous demand to increase the power
generation capacity. A significant percentage of the required
capacity increase can be based on renewable energy sources.
Wind turbine technology, as the most cost effective renewable
energy conversion system, will play an important part in our
future energy supply. But other sources like microturbines,
photovoltaics and fuel cell systems may also be serious
contributors to the power supply. Characteristically, power
electronics will be an efficient and important interface to the
grid for the renewables and this paper will first briefly discuss
three different alternative/renewable energy sources. Next,
various configurations of small and medium power conversion
topologies are presented including their control (mainly for
PV-systems). Finally wind turbine configuration and their
control are described.
Three different renewable energy sources are briefly
described. They are wind power, fuel cell and photovoltaic.
A.
Wind power conversion
The function of a wind turbine is to convert the motion of
the wind into rotational energy that can be used to drive a
generator, as illustrated in Fig. 1. Wind turbines capture the
power from the wind by means of aerodynamically designed
blades and convert it into rotating mechanical power. At
present, the most popular wind turbine is the Horizontal
Axis Wind Turbine (HAWTs) where the number of blades is
typically three.
Wind turbine blades use airfoils to develop mechanical
power. The cross-sections of wind turbine blades have the
shape of airfoils as the one shown in Fig. 2.
Airflow over an airfoil produces a distribution of forces
along the airfoil surface. The resultant of all these pressure
and friction forces is usually resolved into two forces and a
moment, lift force, drag force and pitching moment, as
shown in Fig. 2.
The aerodynamic power,
P
, of a wind turbine is given by:
I.
I
NTRODUCTION
The energy consumption is steadily increasing and the
deregulation of electricity has caused that the amount of
installed production capacity of classical large power
stations cannot follow the demand. A method to fill out the
gap is to make incentives to invest in alternative energy
sources like wind turbines, photovoltaic systems,
microturbines and also fuel cell systems. Two renewable
energy systems are the most dominant so far which are the
wind turbines and the photovoltaic systems. The wind
turbine technology is one of the most promising alternative
energy technology [1]-[3]. The modern development started
in the 1980’s with sites of a few tens of kW to Multi-MW
range wind turbines today. E.g. Denmark has a high
penetration (> 20%) of wind energy in major areas of the
country and in 2003 15% of the whole electrical energy
consumption was covered by wind energy. A higher
penetration level will even be seen in the near future. As the
power range of the wind turbines increases the key
parameters like control of active and reactive power become
more and more important. The power electronics is the key-
technology [4]-[5]to change the basic characteristic of the
wind turbine from being an energy source to be an active
power source [6]-[36]. The power electronic possibilities are
also used to interface other renewable energy sources [37]-
[46].
This paper will first explain the basic principles of wind
power conversion, fuel cells and photovoltaic. Next different
PV configurations are explained as well as power converters
and their control. The three-phase inter-connection is also
discussed including control. Different wind turbine
configurations are finally reviewed together with their
control methods.
1
ρπ
2
3
P
=
R
v
C
(1)
p
2
where ρ is the air density,
R
is the turbine radius,
v
is the
wind speed and
C
P
is the turbine power coefficient which
represents the power conversion efficiency of a wind turbine.
C
P
is a function of the tip-speed ratio (λ), as well as the
blade pitch angle (β) in a pitch controlled wind turbine. λ is
defined as the ratio of the tip speed of the turbine blades to
wind speed, and given by:
R
Ω
⋅
(2)
λ
v
where Ω is the rotational speed of the wind turbine.
The Betz limit,
C
P
,max (theoretical)
=16/27, is the maximum
theoretically possible rotor power coefficient. In practice
three effects lead to a decrease in the maximum achievable
power coefficient [1]:
•
Rotation of the wake behind the rotor
•
Finite number of blades and associated tip losses
•
Non-zero aerodynamic drag
Electrical Power
ind power
W
Generator
Power converter
(optional)
Rotor
Gearbox (optional)
Supply grid
Consumer
Power conversion &
power control
Power conversion &
wer contro
Power transmission
Power conversion
Power transmission
po
l
Fig. 1. Conversion from wind power to electrical power in a
wind turbin
Fig. 1. Conversion from wind power to electrical power in a wind turbine [11].
e [11].
A typical
C
P
-λ
curve for a fixed pitch angle β
is shown in
Fig. 3. It can be seen that there is a practical maximum
power coefficient,
C
P
,max
. Normally, a variable speed wind
turbine follows the
C
P
,max
to capture the maximum power up
to the rated speed by varying the rotor speed to keep the
system at the optimum tip-speed ratio, λ
opt
.
As the blade tip-speed typically should be lower than half
the speed of sound the rotational speed will decrease as the
radius of the blade increases. For MW wind turbines the
rotational speed will be 10-15 rpm. A common way to
convert the low-speed, high-torque power to electrical
power is to use a gear-box and a normal speed generator as
illustrated in Fig. 1. The gear-box is optional as multi-pole
generator systems are alternative solutions.
The development in the wind turbine systems has been
steady for the last 25 years and four to five generations of
wind turbines exist. It is now a proven technology.
It is important to be able to control and limit the power at
higher wind speeds, as the power in the wind is a cube of the
wind speed.
Wind turbines have to be cut out at a high wind speed to
avoid damage. A turbine could be designed in such a way
that it converts as much power as possible in all wind speeds,
but then it would have to be too heavy. The high costs of
such a design would not be compensated by the extra
production at high winds, since such winds are rare.
Therefore, turbines usually reach maximum power at a
much lower wind speed, the rated wind speed (9-12 m/s).
The power limitation may be done by one of the
aerodynamic mechanisms: stall control (the blade position is
fixed but stall of the wind appears along the blade at higher
wind speed), active stall (the blade angle is adjusted in order
to create stall along the blades) or pitch control (the blades
are turned out of the wind at higher wind speed).
B. Fuel Cell power conversion
The fuel cell is a chemical device, which produces
electricity directly without any intermediate stage and has
recently received much attention [7]. The most significant
advantages are low emission of green house gases and high
power density. For example, a zero emission can be
achieved with hydrogen fuel. The emission consists of only
harmless gases and water. The noise emission is also low.
The energy density of a typical fuel cell is 200 Wh/l, which
is nearly ten times of a battery. Various fuel cells are
available for industrial use or currently being investigated
for use in industry, including
•
Proton Exchange Membrane
•
Solid Oxide
•
Molten Carbonate
•
Phosphoric Acid
•
Aqueous Alkaline
The efficiency of the fuel cell is quite high (40%-60%). Also
the waste heat generated by the fuel cell can usually be used
for cogeneration such as steam, air-conditioning, hot air and
heating, then the overall efficiency of such a system can be
as high as 80%.
Lift force
Pitching moment
Drag force
β
Trailing edge
φ
α
Angle of attack:
α
Leading edge
Pitch angle:
β
wind
Fig. 2. A simple airfoil used in wind turbines.
Fig. 3. Typical Cp-λ curve for a wind turbine for a fixed angle β.
i
PV
i
SC
i
d
u
PV
(a)
Fig. 4. V-I characteristics of a fuel cell [12].
A typical curve of the cell electrical voltage against current
density is shown in Fig. 4. It can be seen that there exists a
region where the voltage drop is linearly related with the
current density due to the Ohmic contact.
Beyond this region the change in output voltage varies
rapidly. At very high current density, the voltage drops
significantly because of the gas exchange efficiency. At low
current level, the Ohmic loss becomes less significant, the
increase in output voltage is mainly due to the activity of the
chemicals. Although the voltage of a fuel cell is usually
small, with a theoretical maximum being around 1.2 V, fuel
cells may be connected in parallel and/or in series to obtain
the required power and voltage.
The power conditioning systems, including inverters and
DC/DC converters, are often required in order to supply
normal customer load demand or send electricity into the
grid.
C. The photovoltaic cell
Photovoltaic (PV) power supplied to the utility grid is
gaining more and more visibility due to many national
incentives [7]. With a continuous reduction in system cost
(PV modules, DC/AC inverters, cables, fittings and man-
power), the PV technology has the potential to become one
of the main renewable energy sources for the future
electricity supply.
The PV cell is an all-electrical device, which produces
electrical power when exposed to sunlight and connected to
a suitable load. Without any moving parts inside the PV
module, the tear-and-wear is very low. Thus, lifetimes of
more than 25 years for modules are easily reached. However,
the power generation capability may be reduced to 75% ~
80% of nominal value due to ageing. A typical PV module is
made up around 36 or 72 cells connected in series,
encapsulated in a structure made of e.g. aluminum and tedlar.
An electrical model of the PV cell is depicted in Fig. 5.
(
u
MPP
, i
MPP
)
I
PV
i
SC
p
MPP
P
PV
U
PV
u
OC
(b)
Fig. 5. Model and characteristics of a PhotoVoltaic (PV) cell.
(a)
Electrical model with current and voltages defined.
(b)
Electrical characteristic of the PV cell, exposed to a given amount
of sunlight at a given temperature.
Several types of proven PV technologies exist, where the
crystalline (PV module light-to-electricity efficiency: η =
10% - 15%) and multi-crystalline (η = 9% - 12%) silicon
cells are based on standard microelectronic manufacturing
processes. Other types are: thin-film amorphous silicon (η =
10%), thin-film copper indium diselenide (η = 12%), and
thin-film cadmium telluride (η = 9%). Novel technologies
such as the thin-layer silicon (η = 8%) and the dye-sensitised
nano-structured materials (η = 9%) are in their early
development. The reason to maintain a high level of
research and development within these technologies is to
decrease the cost of the PV-cells, perhaps on the expense of
a somewhat lower efficiency. This is mainly due to the fact
that cells based on today’s microelectronic processes are
rather costly, when compared to other renewable energy
sources.
The series connection of the cells benefit from a high
voltage (around 25 V ~ 45 V) across the terminals, but the
weakest cell determines the current seen at the terminals.
6
1000 W/
2
15
o
C
40
o
C
75
o
C
Power flow
4
600 W/m
2
Loads
Appliance
Industry
Communication
Power converter
2
2-3
2-3
Load /
generator
200 W/m
2
Generators
Wind
Photo-voltaic
Fuel cell
Other sources
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Cell voltage [V]
(a)
Control
2.5
15
o
C
40
o
C
75
o
C
Reference (local/centralized)
2
Fig. 7. Power electronic system with the grid, load/source, power
converter and control.
III.
S
INGLE
-
PHASE
PV-
INVERTERS
The first systems to be discussed will be single-phase
connected PV inverters. The general block diagram of a
single-phase grid connected photovoltaic systems is shown
in Fig. 8a. It consists of a PV array, a PV inverter with a
filter, a controller and the grid.
1.5
1
0.5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Cell voltage [V]
(b)
Fig. 6. Characteristics of a PV cell. Model based on the British Petroleum
BP5170 crystalline silicon PV module. Power at standard test condition
(1000 W/m
2
irradiation, and a cell temperature of 25 °C): 170 W @ 36.0 V.
Legend: solid at 15
o
C, dotted at 40
o
C, and dashdot at 75
o
C [7].
This causes reduction in the available power, which to
some extent can be mitigated by the use of bypass diodes, in
parallel with the cells. The parallel connection of the cells
solves the ‘weakest-link’ problem, but the voltage seen at
the terminals is rather low. Typical curves of a PV cell
current-voltage and power-voltage characteristics are plotted
in Fig. 6a and Fig. 6b respectively, with insolation and cell
temperature as parameters. The graph reveals that the
captured power is determined by the loading conditions
(terminal voltage and current). This leads to a few basic
requirements for the power electronics used to interface the
PV module(s) to the utility grid.
The job for the power electronics in renewable energy
systems is to convert the energy from one stage into another
stage to the grid (alternative voltage) with the highest
possible efficiency, the lowest cost and to keep a superior
performance. The basic interfacing is shown in Fig. 7.
Usually the power converter interfacing a dc source to the
load and/or to the grid consists of a two stage converter: a
standard buck inverter and an ac/ac voltage amplifier or a dc
boost converter [7]. The use of current source inverters is
quite limited because they require several devices producing
a large amount of conduction losses, sluggish transient
response and high cost [66]. An interesting alternative
solution could be the use of a step-up inverter made by the
connection of two [67] or three [68] dc/dc boost converters
in order for the inverter and boost the voltage in only one
stage.
This power electronic system can be used with many
different loads and generators. In this case focus will be on
PV and wind turbines.
PV
Array
PV Inverter
& Filter
Grid
Control
reference
a)
(b) (c) (d)
Fig 8. General schema for single-phase grid connected photovoltaic
systems. a) Block diagramof PV inverter;
b) Central inverter; c) String
inverter; d) Module integrated inverter
The PV array can be a single panel, a string of PV panels
or a multitude of parallel strings of PV panels. Centralized
or decentralized PV systems can be used as depicted in Fig.
8b - Fig. 8d.
Central inverters
In this topology the PV plant (typical > 10 kW) is arranged
in many parallel strings that are connected to a single central
inverter on the DC-side (Fig. 8b). These inverters are
characterized by high efficiency and low specific cost.
However, the energy yield of the PV plant decreases due to
module mismatching and potential partial shading
conditions. Also, the reliability of the plant may be limited
due to the dependence of power generation on a single
component: the failure of the central inverter results in that
the whole PV plant out of operation.
[38]. In the following, the different PV inverter power
configurations are described in more details.
on the LF side
with isolation
with DC-DC
converter
String inverter
Similar to the central inverter, the PV plant is divided into
several parallel strings. Each of the PV strings is assigned to
a designated inverter, the so-called "string inverter" (see Fig.
8c). String inverters have the capability of separate
Maximum Power Point (MPP) tracking of each PV string.
This increases the energy yield via the reduction of
mismatching and partial shading losses. These superior
technical characteristics lead increase the energy yield and
enhance the supply reliability. String inverters have evolved
as a standard in PV system technology for grid connected
PV plants.
An evolution of the string technology applicable for higher
power levels is the multi-string inverter [7]. It allows the
connection of several strings with separate MPP tracking
systems (via DC/DC converter) to a common DC/AC
inverter. Accordingly, a compact and cost-effective solution,
which combines the advantages of central and string
technologies, is achieved. This multi-string topology allows
the integration of PV strings of different technologies and of
various orientations (south, north, west and east). These
characteristics allow time-shifted solar power, which
optimizes the operation efficiencies of each string separately.
The application area of the multi-string inverter covers PV
plants of 3-10 kW.
on the HF side
without isolation
PV
Inverters
with isolation
without DC-DC
converter
without isolation
Fig. 9. Power configurations for PV inverters.
PV inverters with DC-DC converter and isolation
The isolation is typically acquired using a transformer that
can be placed on either the grid frequency side (LF) as
shown in Fig. 10a or on the high-frequency (HF) side in the
dc-dc converter as shown in Fig. 10b. The HF transformer
leads to more compact solutions but high care should be
taken in the transformer design in order to keep the losses
low.
DC
DC
PV
Array
Grid
DC
AC
(a)
DC
AC
DC
PV
Array
Grid
AC
DC
AC
(b)
Fig. 10. PV inverter system with DC-DC converter
and isolation transformer
a) on the Low Frequency (LF) side b) on the High Frequency (HF) side
In the Fig. 11 is presented a PV inverter with HF
transformer using an isolated push-pull boost converter [41]
Module integrated inverter
This system uses one inverter for each module (Fig. 8d).
This topology optimizes the adaptability of the inverter to
the PV characteristics, since each module has its own MPP
tracker. Although the module-integrated inverter optimizes
the energy yield, it has a lower efficiency than the string
inverter. Module integrated inverters are characterized by
more extended AC-side cabling, since each module of the
PV plant has to be connected to the available AC grid (e.g.
230 V/ 50 Hz). Also, the maintenance processes are quite
complicated, especially for facade-integrated PV systems.
This concept can be implemented for PV plants of about 50-
400 W peak.
PV inverter
The PV inverter technology has evolved quite a lot during
the last years towards maturity [42]. Still there are different
power configurations possible as shown in the Fig. 9.
The question of having a dc-dc converter or not is first of
all related to the PV string configuration. Having more
panels in series and lower grid voltage, like in US and Japan,
it is possible to avoid the boost function with a dc-dc
converter. Thus a single stage PV inverter can be used
leading to higher efficiency
.
The issue of isolation is mainly related to safety standards
and is for the moment only required in US. The drawback of
having so many panels in series is that MPPT is harder to
achieve especially during partial shading, as demonstrated in
Fig. 11. PV inverter with HF transformer in the dc-dc converter.
Also, the dc-ac inverter in this solution is a low cost
inverter switched at the line frequency. The new solutions
on the market are using PWM dc-ac inverters with IGBT’s
switched typically at 10-20 kHz leading to a better power
quality performance.
Other solutions for high frequency dc-dc converters with
isolations includes: full-bridge isolated converter, Single-
Inductor push-pull Converter (SIC) and Double-Inductor
Converter (DIC) as depicted in Fig. 12 [61].
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