30-07-2012, 11:09 AM
[attachment=29458]An Overview of Some Important Issues Related to Wind Energy Conversion System (WECS)
Abstract
Wind power capacity has experienced tremendous
growth in the past decade. There are many loads (such as
remote villages, islands, ships etc) that are away from the main
grid. They require stand-alone generator system (which can
provide constant nominal voltage and frequency) to provide for
their local electrification. This requirement has lead to
widespread research on development of new technologies for
stand-alone generators. Initially an overview of different
existing generator technologies for grid connected operation is
given. This paper presents the recent developments in wind
energy conversion systems, their classifications, choice of
generators and their social and environmental benefits , a
review of the interconnection issues of distributed resources
including wind power with electric power systems, hybrid
power system and reports the developments of interconnection
standards in Canada and IEEE.
Index Terms—wind energy conversion system,
interconnection, power quality, renewable energy, wind
turbine, Hybrid Power Systems.
I. INTRODUCTION
The major components of a typical wind energy
conversion system include a wind turbine, generator,
interconnection apparatus and control systems. Wind
turbines can be classified into the vertical axis type and the
horizontal axis type. Most modern wind turbines use a
horizontal axis configuration with two or three blades,
operating either down-wind or up-wind. A wind turbine can
be designed for a constant speed or variable speed operation.
Variable speed wind turbines can produce 8% to 15% more
energy output as compared to their constant speed
counterparts, however, they necessitate power electronic
converters to provide a fixed frequency and fixed voltage
power to their loads. Most turbine manufacturers have opted
for reduction gears between the low speed turbine rotor and
the high speed three-phase generators. Direct drive
configuration, where a generator is coupled to the rotor of a
wind turbine directly, offers high reliability, low
maintenance, and possibly low cost for certain turbines.
Several manufacturers have opted for the direct drive
configuration in the recent turbine designs. At the present
time and in the near future, generators for wind turbines will
be synchronous generators, permanent magnet synchronous
generators, and induction generators, including the squirrel
cage type and wound rotor type.
Rajveer Mittal is with the Department of Electrical and Electronics
Engineering, Maharaja Agrasen Institute of Technology, Rohini,Delhi ,
India (e-mail : rajveermittal[at]yahoo.com, : rajveermittal[at]hotmail.com )
K.S Sandhu is with the Department of Electrical Engineering, N.I.T,
Kurukshetra, Haryana, India. (e-mail l:kjssandhu[at]yahoo.com.).
D. K. Jain is with the Guru Prem Sukh Memorial College of engineering
under GGSIP University , Delhi, India. (e-mail: jaindk66[at]gmail.com).
For small to medium power wind turbines, permanent
magnet generators and squirrel cage induction generators
are often used because of their reliability and cost
advantages. Induction generators, permanent magnet
synchronous generators and wound field synchronous
generators are currently used in various high power wind
turbines. Interconnection apparatuses are devices to achieve
power control, soft start and interconnection functions. Very
often power electronic converters are used as such devices.
Most modern turbine inverters are forced commutated
PWM inverters to provide a fixed voltage and fixed
frequency output with a high power quality. Both voltage
source voltage controlled inverters and voltage source
current controlled inverters have been applied in wind
turbines. For certain high power wind turbines, effective
power control can be achieved with double PWM (pulse
width modulation) converters which provide a bi-directional
power flow between the turbine generator and the utility
grid.
Capacity factor-Since wind speed is not constant so
annual energy production is never as much as the sum of the
generator nameplate ratings multiplied by the total hours in
a year. The ratio of actual productivity in a year to this
theoretical maximum is called the capacity factor. Typical
capacity factors are 20-40%, with values at the upper end of
the range in particularly favorable sites.[1,2] For example, a
1 megawatt turbine with a capacity factor of 35% will not
produce 8,760 megawatt-hours in a year (1x24x365), but
only 0.35x24x365 = 3,066 MWh, averaging to 0.35 MW.
Online data is available for some locations and the capacity
factor can be calculated from the yearly output [3,4]
Unlike fueled generating plants, the capacity factor is
limited by the inherent properties of wind. Capacity factors
of other types of power plant are based mostly on fuel cost,
with a small amount of downtime for maintenance. Nuclear
plants have low incremental fuel cost, and so are run at full
output and achieve a 90% capacity factor.[5] Plants with
higher fuel cost are throttled back to follow load. Gas
turbine plants using natural gas as fuel may be very
expensive to operate and may be run only to meet peak
power demand. A gas turbine plant may have an annual
capacity factor of 5-25% due to relatively high energy
production cost. According to a 2007 Stanford University
study published in the Journal of Applied Meteorology and
Climatology, interconnecting ten or more wind farms allows
33 to 47% of the total energy produced to be used as
reliable, base load electric power, as long as minimum
criteria are met for wind speed and turbine height.[6,7]
An Overview of Some Important Issues Related
to Wind Energy Conversion System (WECS)
Rajveer Mittal, K.S.Sandhu and D.K.Jain
International Journal of Environmental Science and Development, Vol. 1, No. 4, October 2010
ISSN: 2010-0264
352
II. CLASSIFICATION OF WIND ENERGY CONVERSION
SYSTEMS
There are number of ways to classifying the WECs.
Following are the main types of classifications of WECs:
A. According to size of Electrical Power Output. [19]
(i) Small size (up to 2kW): These may be used for remote
applications,or at places requiring relatively low power.
(ii) Medium Size (2-100 kW): These turbines may be
used to supply less than 100 kW rated capacity, to several
residences or local use.
(iii)Large Size (100 kW and up): They are used to
generate power for distribution in central power grids.
B. According to type of electrical power output ;
There are mainly following three classes of
generators:[16]
1) D.C. generators
D.C. generators are relatively unusual in wind/microhydro
turbine applications because they are expensive and
require regular maintenance. Nowadays for most of d.c.
applications, for example, it is more common to employ an
a.c. generator to generate a.c., which is then converted to d.c.
with simple solid-state rectifiers.
2) Synchronous generators
The major advantage of synchronous generator is that its
reactive power characteristic can be controlled and therefore
such machines can be used to supply reactive power to other
items of power systems, which require the reactive power. It
is normal for a stand-alone wind-diesel system to have a
synchronous generator, usually connected to the diesel.
Synchronous generators when fitted to a wind turbine must
be controlled carefully to prevent the rotor speed
accelerating through synchronous speed especially during
turbulent winds. Moreover it requires flexible coupling in
the drive train, or to mount the gearbox assembly on springs
or dampers to absorb turbulence. Synchronous generators
are more costly than induction generators, particularly in
smaller size ranges. Synchronous generators are more prone
to failures.
3) Induction generators
Induction generator offers many advantages over a
conventional synchronous generator as a source of isolated
power supply. Reduced unit cost, ruggedness, brush less (in
squirrel cage construction), reduced size, absence of
separate DC source and ease of maintenance, self-protection
against severe overloads and short circuits, are the main
advantages Further induction generators are loosely coupled
devices, i.e. they are heavily damped and therefore have the
ability to absorb slight change in rotor speed and drive train
transient to some extent can therefore be absorbed. Whereas
synchronous generators are closely coupled devices and
when they are used in wind turbines which is subjected to
turbulence and requires additional damping devices such as
flexible couplings in the derive train or to mount gearbox
assembly on springs and dampers. Reactive power
consumption and poor voltage regulation under varying
speed are the major drawback of the induction generators,
but the development of static power converters has
facilitated the control of induction generator, regarding
output voltage and frequency.
C. According to Rotational Speed of Aeroturbines:
Several kinds of generator technologies have been
developed and are in use today. In this section a short
overview of these different generator topologies is presented.
Each of them is discussed with its advantages and
drawbacks.
1) fixed speed system
Fixed speed systems are the simplest and most widely
used arrangement. They operate at constant (or nearly
constant) speed [also called constant speed constant
frequency (CSCF) mode of operation]. This implies that
regardless of the prime mover speed, the angular speed of
the rotor is fixed and determined by the frequency of supply
grid and gear ratio This arrangement, in general, has simple
and reliable construction of the electrical part while the
mechanical parts are subject to higher stresses and
additional safety factors need to be incorporated in the
mechanical design. This arrangement can use induction
generator (IG) and the wound rotor synchronous generator
(SG) as the electric machine. But the squirrel cage induction
generator has been the prevalent choice. The reasons for this
popularity are mainly due to its simplicity, high efficiency
and low maintenance requirements. To compensate for the
reactive power consumption of the induction generator, a
capacitor bank (normally stepwise controlled) is inserted in
parallel with the generator in order to obtain about unity
power factor. Further, to reduce the mechanical stress and to
reduce the interaction between supply grid and turbine
during connection and start-up of the turbine, a soft starter is
used. The main advantage of this system is that it is a simple
and reliable arrangement. However, capacitors need to be
cutin or cutoff regularly to maintain power factor. This
random switching gives rise to undesirable transients in the
line currents and voltages. The fluctuations in prime mover
speed are converted to torque pulsations, which cause
mechanical stress. This causes breakdown of drive train and
gear box. The power generated from this arrangement is
sensitive to fluctuations in prime mover speed. To avoid this
pitch control of rotor blades is required.
The Fixed Speed Induction Generators (FSIG) wind
turbine is a simple squirrel cage induction generator, which
can be directly coupled to the electricity supply network.
The frequency of the network determines the rotational
speed of the stator’s magnetic field, while the generator’s
rotor speed changes as its electrical output changes.
However, because of the well known steep torque- Slip
characteristic of the induction machine, the operating range
of the generator is very limited. The wind turbine is
therefore effectively fixed speed. FSIGs do not have the
capability of independent control of active and reactive
power, which is their main disadvantage. Their great
advantage is their simple and robust construction, which
leads to lower capital cost. In contrast to other generator
topologies, FSIGs offer no inherent means of torque
oscillation damping which places greater burden and cost on
their gearbox. The wind energy system and power quality
aspects are discussed in detail in the literature
The Doubly Fed Induction Generators (DFIG) Wind
International Journal of Environmental Science and Development, Vol. 1, No. 4, October 2010
ISSN: 2010-0264
353
Turbines is a wound rotor induction generator whose rotor
is fed via slip rings by a frequency converter. The stator is
directly coupled to the electrical power supply network. As
a result of the use of the frequency converter, the network
frequency is decoupled from the mechanical speed of the
machine and variable speed operation is possible, permitting
maximum absorption of wind power. Since power ratings
are a function of slip, DFIGs operate over a range of speeds
between about 0.75 and 1.25 pu of synchronous frequency,
which requires converter power ratings of approximately
25%. A great advantage of the DFIG wind turbine is that it
has the capability to independently control active and
reactive power. Moreover, the mechanical stresses on a
DFIG wind turbine are reduced in comparison to a FSIG.
Due to the decoupling between mechanical speed and
electrical frequency that results from DFIG operation, the
rotor can act as an energy storage system, absorbing torque
pulsations caused by wind gusts . Other advantages of the
DFIG include reduced flicker and acoustic noise in
comparison to FSIGs. The main disadvantages of DFIG
wind turbines in comparison to FSIGs are their increased
capital cost and the need for periodic slip ring maintenance.
2) Fully Variable Speed System
With the increase in the size of turbine, the inherent
problems of the constant speed systems become more and
more pronounced, especially in areas with relatively weak
grids. To overcome these problems, the trend in modern
generator technology is toward variable-speed concepts. A
variable-speed system keeps the generator torque constant
and it is the generator speed which changes. Variations in
the incoming power are absorbed by rotor speed changes.
The variable-speed system therefore incorporates a
generator control system that can operate with variable
speed. In this arrangement the variable-voltage variable
frequency (VVVF) power generated by the machine is
converter to fixed-frequency fixed voltage power by the use
of back to back power converters. The arrangement can
have either induction generator or synchronous generator as
the electric machine. The machine side converter supplies
the lagging excitation to the machine while the line side
converter maintains unity power factor at grid interface and
also regulates the dc link voltage constant. The synchronous
machine offers the least possible configuration for a
variable-speed sys- tem. It can operate without gear box,
with a good multi-pole design. This is an important
objective since gear box is a component that has a tendency
to fail. The advantages of this scheme are that mechanical
oscillations in the drive train are absent as it is in fixed
speed systems. The torque is under control if Direct Torque
Control or Field Oriented Control techniques are used. This
does not allow the generator to be overloaded. Gear box is
not required with a multi-pole synchronous machine.
However, converters have to manage entire generated
power. Therefore they have to be rated equal to machine
rating. Inverter output filters and EMI output filters are
rated for 1 p.u ( with respect to output power) making filter
design difficult and expensive. Converter efficiency plays
an important factor in total system efficiency over the entire
operating range. It cannot be operated above synchronous
speed with full torque.
a) Direct Drive Synchronous Generator Wind Turbine
An alternative to the much-used induction machine
generator is the use of a multipole synchronous generator,
fed through a power electronic AC/DC/AC stage. The
excitation of the synchronous generator can be given either
by an electrical excitation system or by permanent magnets.
The AC/DC/AC converter acts as a frequency converter and
decouples the generator from the Grid. It consists of two
back-to-back voltage source converters, usually with IGBT
switches, which can independently control the active power
transfer through the DC link and the reactive power output
at each converter terminal .The speed range is generally
similar to that of DFIGs. The multipole construction of the
synchronous generator leads to a low mechanical rotational
speed of the generator rotor and can permit direct coupling
to the wind turbine. The possibility of reducing the number
of stages in the gearbox or eliminating it completely is often
quoted as an advantage of direct drive synchronous
generator wind turbines. However set against this is the
greater VA rating of the power electronic converter
compared with DFIGs and the larger physical generator size.
As a result of the increased mechanical stresses experienced
by FSIG wind turbines at present there is a practical limit to
the rating of commercial models of this technology. All
present commercial models for multi-MW wind turbines in
the range above 3MW are either DFIGs, or synchronous
generators coupled to the network through back-to-back
converters.
A comparison between the variable speed wind turbine
and the constant speed wind turbine shows that variable
speed reduce mechanical stresses: gusts of wind can be
absorbed, dynamically compensate for torque and power
pulsations caused by back pressure of the tower. This
backpressure causes noticeable torque pulsations at a rate
equal to the turbine rotor speed times the number of rotor
blades. The used of a doubly fed induction generator in
WECS with the rotor connected to the electric grid through
an AC-AC converter offers the following advantages:
• Only the electric power injected by the rotor needs
to be handled by the convert , implying a less cost
AC-AC converter.
• Improved system efficiency and power factor
control can be implemented at lower cost the
converter has to provide only excitation energy.
Hence, taking advantage of power electronic advances in
recent years, WECS equipped with doubly fed induction
generator systems for variable speed wind turbine are one of
the most efficient configurations for wind energy
International Journal of Environmental Science and Development, Vol. 1, No. 4, October 2010
ISSN: 2010-0264
354
conversion
b) PM synchronous generator
The scheme of a grid-connected PMSG for direct-drive
wind turbines is shown in Fig. 2. The advantages of PM
machines over electrically excited machines can be
summarized as follows according to literatures:
• Higher efficiency and energy yield.
• No additional power supply for the magnet field
excitation,
• Improvement in the thermal characteristics of the PM
machine due to the absence of the field losses,
• Higher reliability due to the absence of mechanical
components such as slip rings,
• Lighter and therefore higher power to weight ratio.
However, PM machines have some disadvantages, which
can be summarized as follows:
• high cost of PM material,
• difficulties to handle in manufacture,
• Demagnetization of PM at high temperature.
In recent years, the use of PMs is more attractive than
before, because the performance of PMs is improving and
the cost of PM is decreasing. The trends make PM machines
with a full-scale power converter more attractive for directdrive
wind turbines. Considering the performance of PMs is
improving and the cost of PM is decreasing in recent years,
in addition to that the cost of power electronics is
decreasing, variable speed direct-drive PM machines with a
full-scale power converter become more attractive for
offshore wind powers. On the other hand, variable speed
concepts with a full-scale power converter and a single- or
multiple-stage gearbox drive train may be interesting
solutions not only in respect to the annual energy yield per
cost but also in respect to the total weight. For example, the
market interest of PMSG system with a multiple-stage
gearbox or a single-stage gearbox is increasing [38].
Fig. 2. General arrangement of the full variable-speed system.
3) Limited Variable-speed systems
Compared to the squirrel-cage induction generator, the
main difference that the doubly- fed induction generator
configuration provides is the access to the rotor windings,
thereby giving the possibility of impressing the rotor voltage.
With this arrangement, power can be extracted from or fed
to the rotor circuit and the generator can be magnetized
from either the stator circuit or the rotor circuit. Basically
two methods of speed control can be applied to the
induction generator, namely rotor resistance control and
back to back converter control. The effective scheme for
limited variable speed system is back to back converter used
doubly-fed configuration. Fig. 4 shows this topology, the
stator is directly connected to the grid, while the rotor is
connected via slip rings to the converter. The gear ratio is
set so that the nominal speed of the induction generator
corresponds to the middle value of the rotor-speed range of
the turbine. This is done to minimize the size of the inverter,
which will vary with rotor-speed range. A step up
transformer is required between the line side converter and
utility, to match the voltage ratio between the stator and
rotor in the machine. This [38] configuration with two
converters offers many advantages. The main features of
this configuration are listed below:
Fig. 3. General arrangement of limited variable-speed system with Doublyfed
configuration.
1. Reduced converter cost, as they have to be rated for
slip power only (typically about 0.25 pu).
2. Converter on the rotor side enables both positive and
negative slip power control through control of rotor current
in phase magnitude and frequency. This allows both sub
synchronous and super-synchronous operation.
3. DC link capacitor acts as a source of reactive power,
which in a way can control power factor on the stator side.
4. Line side converter has ability to work as active filter,
apart from maintaining unity power factor operation and
regulating dc bus voltage.
5. Reduced cost and weight of inverter filter and EMI
filters (to about 0.25pu of total system power). Inverter
harmonics represent a fraction of total system harmonics.
6. System efficiency is better, due to reduced losses in
the converters.
Important developments in the technology of doubly-fed
system occurred in last two decades. With the advances in
power electronic devices and digital signal processors, it is
now feasible to implement complex algorithms such as field
oriented control etc easily. This had lead to new
technologies or grid connected generators using doubly-fed
configuration.