2010 6TH INTERNATIONAL CONFERENCE ON ELECTRICAL AND POWER ENGINEERING

EPE
28-30 October 2010 - Iaşi, Romania

RENEWABLE SOURCES GRID CONNECTED CONVERTERS

Ahmad El-Ayoubi and Nazih Moubayed
Lebanese University, Faculty of Engineering 1, Tripoli, Lebanon

Abstract – The use of distributed energy resources is

increasingly being pursued as a supplement and an alternative 2. WIND ENERGY
to large conventional central power stations. The specification
of a power electronic interface is subject to requirements The energy of the wind helps the nacelles to rotate.
related not only to the renewable energy source itself but also Thus, this mechanical energy is transformed into
to its effects on the power-system operation, especially where electrical one. In some cases, the speed of the nacelles
the intermittent energy source constitutes a significant part of should be increased, to approximately 1500tr/min, by
the total system capacity. In this paper, a review on power using speed multiplier. After that, a static converter is
electronic systems for the grid integration of different
needed to fix the frequency of the generated current at
renewable energy sources is discussed. An exemple of power
control for solarenergy generation is presented. 50Hz, independently on the speed of the wind [9]. In
Keywords – Renewable energy, power converters, control fact, the voltage produced by the generator varied from
one wind turbine to another, it depends on the generator
1. INTRODUCTION power. For example, the voltage value given by some
generators lies between 600 V to 1000 V. This value is
Because of the gradual depletion of fossil energy not sufficient to import the current into the grid. In this
resources and the increasingly serious issue of case, a transformer is used to increase the voltage to 20
environmental pollution, the scale of renewable energy kV or 30 kV [10].
has been increasing, which draws more researchers’ In addition, several wind spins can be collected into
attention to the quality and efficiency of renewable something called « Wind Spins Park ». In this case, the
energy and its effects on the power grid [1],[2],[3]. The power domain is concerned. The parks can produce
increasing number of renewable energy sources and variable electrical power that varies in function of the
distributed generators requires new strategies for the length of this park. Wind plants can range in size from a
operation and management of the electricity grid in order few megawatts to hundreds of megawatts in capacity.
to maintain or to improve the power-supply reliability There are various methods to introduce the voltage to the
and quality. In addition, liberalization of the grids leads grid. The two basic methods are [11]:
to new management structures, in which trading of  The Full-Scale Power Electronics (PE) (Fig. 1),
energy and power is becoming increasingly important  The partial rated generator (Fig. 2).
[4]. Power electronics are an integral part of these
energy systems being able to convert generated These two methods are used to control the currents in
electricity into consumer usable and utility compatible such way to obtain the best performance. Furthermore,
forms [5]. But the addition of power electronics adds they are used to control active and active power which
costs with some reliability issues. Different power gives a better grid performance, and let the wind turbine
electronics topologies are recently reviewed that are to act as a more dynamic power source to the grid. In
typically used with distributed energy systems [6]. The many countries, energy planning is going on with a high
Integrated Power Electronics Module (IPEM) based penetration of wind energy, which will be covered by
back-to-back converter topologies are found to be most large offshore wind farms. These wind farms may, in the
suitable interface that can operate with different future, present a significant power contribution to the
distributed energy systems [7],[8]. national grid, and therefore, play an important role on
In this paper, a review on renewable sources grid the power quality and the control of power systems.
connected power converters is presented. The paper is Consequently, very high technical demands are expected
organized as follows. Section 2 is devoted to present the to be met by these generation units, such as to perform
power electronic systems used to connect the wind frequency and voltage control, regulation of active and
energy to the grid. Then, the grid connected converters reactive power, quick responses under power system
to solar panels is discussed in section 3. Section 4 transient and dynamic situations. For long distance
studies the case of the hydroelectric energy connection transmission of power from off-shore wind farm, High
to the grid. Power electronic systems for the grid Voltage Direct Current (HVDC) may be an interesting
integration of biomass energy and geothermal energy are option. Thus, in an HVDC transmission, the low or
respectively presented in section 5 and section 6. An medium AC voltage at the wind farm is converted into a
exemple in using power converters in order to high dc voltage on the transmission side and the dc
maximizing the delivered power from solar panels is power is transferred to the onshore system where the dc
treated in section 7. Finally, conclusions are given in voltage is converted back into ac voltage (Fig. 3).
section 8.
Another possible dc transmission system configuration is power electronic converter, so it is possible to operate
shown in Fig. 4, where each wind turbine has its own each wind turbine at an individual optimal speed [12].

Fig. 1 - Full-Scale PE [11]

Fig. 2 - Partially rated PE [11]

Fig. 3 - Common power electronic converter for wind farms [12]

Fig. 4 - Separate power electronic converters for wind farms [12]

The solar energy received in one day is equivalent to

3. SOLAR ENERGY more than ten times the annual energy consumption of
everyone on earth. The grid-connected Photo- Voltaic In parallel of the developing wind farms, comes the
(PV) system consists of two parts: PV arrays that development of solar farms that returns the same
convert irradiation to electrical energy, and the converter benefits on the national grid, and may help the wind
that feeds the energy into the grid [13]. To keep overall farms in fulfilling the needs of the country in electricity.
output power per unit cost low, the grid-connected Consequently, there exist many ways to obtain the best
converters should be able to extract the maximum quality on the grid. The first method is the « Centralized
available power from the PV arrays [14]. And the other PV » (Fig. 6). This method is used for an average of
role of the converter is to adjust the frequency at 50Hz power about 10 kW-250 kW, and it has a high
(Fig. 5). efficiency, low cost, low reliability, and it’s very used
for power plants.

The second method is the « cascaded DC-DC and DC-

AC converters » (Fig. 7). It is used for an average of
power between 1.5-5 kW and it’s typically used for
residential applications. In addition, this method gives
each string his own inverter enabling better Maximum
Power Point Tracking (MPPT), and the orientation of
each of the series of PV can be controlled.

The third method is the « AC-module » (Fig. 8). This

method is used for an average of power between
50-180 W. Every panel has its own converter enabling a
Fig. 5 - Photovoltaic unit [15] high MPPT, but it has a lower efficiency and a difficult
maintenance and a higher cost. It’s used only for specific
utility.

Fig. 6 - Centralized PV [11]

Fig. 7 - Cascaded DC-DC and DC-AC converters [11]

 Fig. 8 - AC Module [16]

In fact, the AC current is rectified to DC current, and

4. HYDROELECTRIC POWER after a DC link (Fig. 9), it is converted again into AC
current and put it on the grid. This method is used with a
The hydroelectric power, is the power of the water fall DC link cable; it’s very used in under seas cables or
which is a kinetic energy transformed into an electrical transmission where high capacitance causes additional
one, by using a turbine linked to a generator. The current AC losses, and for power transmission and stabilization
at the output of the generator is an AC current. In order between unsynchronized AC distribution systems.
to increase the produced voltage, a transformer is used. The other method is the « HFAC-link power converter »
The power of the transformer varies in order of the (Fig. 10). This method is used at higher frequencies.
power of the generator. In fact, the majority of the used Power quality is improved because the harmonics are of
transformers amplify the voltage to 400 kV up to 800 higher orders and easily filtered out [20].
kV. This voltage can be putted on the grid to be The third method is the « Direct AC-AC converter ».
transmitted to another region. There are many ways to This method is used to control the voltage and the
link the produced power to the grid [18]. The first current at the output (Fig. 11).
method is called the « DC –link power converters ».

Fig. 9 - DC –link power converters [19]

Fig. 10 - HFAC link Power converter [11]

Fig. 11 - Direct AC-AC converter [11]

5. BIOMASS ENERGY
7.2 Output power characteristics
In the increasingly industrialized world, biomass has to In Tripoli, the second city of Lebanon, the temperature
be considered as a major sustainable energy resource for varied between 0C to 40C, and the sunshine changed
electricity production. Already, large quantities of from zero to 900 W/m2. For these extremities, the output
unused biomass material exist in the form of forestry power versus voltage for different values of temperature
waste and agricultural byproducts, and, the cultivation of is given in figure 13, and that versus voltage for different
arable energy crops is usually possible. The principle is values of shining is shown in figure 14. In these figures,
to generate fossil fuel and then by combustion obtain the the output power is not constant. To maximize this
green house gases. Those gases are used, due to their power and maintain it constant, it is necessary to use one
pressure, to rotate a flywheel motor or a generator using of the Maximum Power Point Tracking (MPPT)
its turbine. For this type of renewable source, two methods, and apply it to the controlled dc-dc converter
solutions are proposed. The first one is a DC-link power [23].
converters method based on a DC link between two
converters [21]. The use of a controlled rectifier allows
several advantages. The current may be controlled to be
sinusoidal, thus avoiding the above-mentioned problem.
The amplitude of the voltage may be controlled to match
the grid voltage, independent of shaft speed [22]. The
second solution is based on using high frequency link
[21]. In this case, two converters AC- AC are used, there
roles are to regulate the frequency obtained by the
generator. The frequency value is about 180 Hz. The
first converter’s role is to increase the frequency up to 20
KHz and the second’s role is to decrease it to the grid’s
frequency.
Fig. 13 - Output power at different temperature values.
6. GEOTHERMAL ENERGY

Thermal energy is the stored energy in the ground that is

available as either steam or hot water. Geothermal
energy is the conversion of this thermal energy into
electricity by various conversion technologies based on
steam turbines.
Conversion plants that use a hot water heat source must
expand the liquid to a high-pressure vapor/steam form
that is then used to mechanically turn a generator
turbine. Because geothermal plants use water heated
naturally in the earth to produce steam, it is not
necessary to burn any fossil fuels to generate electricity.
The solutions to distribution are mentioned above. Those Fig. 14 - Output power at different sunshine values.
solutions are those of a micro turbine with a generator.
7.3 Maximizing the output power
7. EXEMPLE IN USING POWER CONVERTERS The Perturb and observe method is the most one used to
maximize the output power. In fact, by applying a
7.1 System under consideration variation on the voltage (or on the current) toward the
Our system is achieved in the Faculty of Engineering of biggest or the smallest value, its influence appears on the
the Lebanese University in Lebanon. It is constituted of power value. If the power increases, one continues
one solar panel of 50 W. An accumulator is placed in varying the voltage (or the current) in the same direction,
order to stock the received solar energy (Fig. 12). In if not, one continues in the inverse direction (figure 15).
addition, our system contains a weather station in order
to manage better the produced power. The control of the
system is treated to maximize the produced power.

Fig. 12 - The solar conversion system. Fig 15 - Output power using Perturb and Observe
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