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Power Electronics for Renewable Energy Systems
Power Electronics systems work in conjunction with renewable energy generation technologies to
convert harvested energy into useable electrical power.
The Power Electronics system is designed to convert this harvested energy at the
maximum efficiency.
A Power Electronics system is made up of a number of sub-systems. The major components will be the
Inverter (to deliver the required AC output), Charge Controllers (associated with the methods
of energy harvesting used), Energy Storage (generally in the form of a battery bank), Back-up
Supply (generator or grid supply) and a Controller to co-ordinate these sub-systems.
System Architecture
describes how the various elements
of a Power Electronics system are configured.
PV Controller
Wind Charger
Battery Charger
Power Inverter
PV Array
Wind Generator
Battery Bank
Back-up Generator
AC Load
DC BUS
In this series configuration, generated energy is stored in a battery bank via a common DC bus system. This same DC
bus delivers power to the inverter which provides the AC output required
PV Controller
Wind Charger
Battery Charger
Power Inverter
PV Array
Wind Generator
Battery Bank
Back-up Generator
AC Load
DC BUS
Change-over switch
In the switched configuration, the AC output can be supplied from either the
renewable energy source, via the DC bus, or directly from the back-up power source.
PV Controller
Wind Charger
Bi-directional
Power Inverter
PV Array
Wind Generator
Battery Bank
Back-up Generator
AC Load
DC BUS AC BUS
This parallel configuration, requires no switching of the AC load supply while
maintaining flexibility of energy source. The downside is that the power inverter
complexity is increased.
PV Inverter
Wind Inverter
PV Array
Wind Generator
Battery Bank
Back-up Generator
AC Load
AC BUS
Bi-directional
Power Inverter
An alternative parallel configuration eliminates the DC bus completely, combining all power
sources and the power output on just an AC bus. In this configuration, each energy source
requires its own power inverter.
The term Micro-Generation is used to describe small-scale
renewable energy harvesting by individual houses or small
groups of houses. Typically, the capacity of these systems will be from a few KW up to a few 10’s
of KW.
Micro-Generation systems can be either
grid-connected or off-grid.
In an off-grid system, there is no utility network to provide back-up power. The system relies
entirely on its energy generating capability and its back-up energy storage (battery bank/generator).
The off-grid configuration is most commonly used in remote locations where no access to the utility
network is possible.
In a grid-connected system, energy generated is used locally. Surplus generated energy is sold
back to the utility service.
If no energy is generated, eg at night, power is taken from the grid in the conventional way. The grid is used as a back-up to the
micro-generation system and the electronics is required to perform all the control and
switching functions.
Inverters
At the centre of the power electronics
system is the inverter which takes harvested DC electrical energy and converts it into
240V 50Hz AC suitable for consumer
use.Inverters may be sized
from a few KW for micro-generation
systems to 100’s of KW for large-scale
installations.
Modern power conversion techniques are based on the high speed switching of power
semiconductor transistors.The full bridge converter is a
common topology for generating a single phase AC
output. Four semiconductor switches are
utilised and are switched in
diagonal pairs to direct the output
current in alternate
directions at high frequency.
High frequency switching strategies use
pulse-width modulation (PWM)
techniques to generate a pseudo-AC output which
is reconstructed and smoothed by filtering.
Typical PWM switching frequencies are in the range 20 to 100 Khz
DC-DC Conversion
Vin Vout
It is often necessary to convert DC power from one voltage to another, eg to match the DC bus level. There are several topologies commonly
used to do this. DC-DC converters utilise semiconductor switches
operating at high speed to charge inductor currents.
The inductor energy is then transferred into a storage
capacitor, where it is available to be output. The switching frequency of DC-
DC converters may be MHz.
0V
GateDriver
Cbs
M2
High sideDriver
Vaux
D2Low sideDriver
DbsVs (High Voltage Rail)
PWMLevelshifter
D1
M1
The high speed switching techniques employed in power conversion create the need for semiconductor switches and drive circuits capable of operating at
high frequency. For topologies where the semiconductor switches are ‘stacked’, level shifting
techniques are used to drive the high-side switch.MOSFET’s and
IGBT’s are commonly used as
the switching elements. The
drive circuit must charge and
discharge the gate capacitance
fast enough to maintain the
switching frequency.
MPP trackingTo extract the
maximum energy from harvesting
technologies such as PV, the output has to
be carefully controlled. There is
an optimum operating point on PV
characteristics for maximum power
output.
Maximum Power Point (MPP) tracking maintains this optimum as the PV output characteristics change with temperature and light intensity. MPP tracking
ensures that the maximum power is delivered by the energy
harvester.
Power Quality
L1
Load
C1
C2L2
Distortions in AC voltage or current
result in power losses. There are limits
defining the acceptable level of distortion due to consumer activity. A common type of
distortion is caused by harmonics (multiples of
the 50Hz fundamental frequency). The harmonic content can be reduced
by active or passive filtering.
Power Factor CorrectionPower factor is the
ratio of ‘real’ power to ‘reactive’ power. Only real power is useful,
reactive power is wasted. Capacitive and inductive loads cause poor power factor. Diode input circuits can
severely distort the AC current waveform resulting
in poor power factor.Power factor can be
corrected electronically to improve system efficiency.
Generators
Common types for renewable energy
systems are AC induction generators and DC permanent-magnet generators.
Generators convert kinetic energy (eg
wind, wave etc) into electrical energy.
Generators often have an increased number of magnetic poles to
provide an acceptable output at low
rotational speed.
DC Permanent-magnet generators are lighter and
more compact than AC induction generators, but
also more expensive.
Smart MeteringThere is a great deal of effort currently to
transform electricity metering from the simple KWh meter into a much more integrated part of
the power system.
Additional functionality may include – power quality assessment, variable tariffs (both for
usage and regeneration), remote interrogation and more detailed power monitoring – maybe
even down to individual appliances.
Current predictions place smart metering into a much wider-ranging power management network.
This is potentially a huge growth area.
Hybrid VehiclesPower electronics also finds applications in hybrid electric vehicles, where many of the
power conversion techniques can be applied.
Limitations in large-capacity, small-size
electrical energy storage prevent the commercialisation of an all-electric vehicle.
However, hybrid vehicles, which use electric motors in
tandem with combustion fuels like
petrol or even hydrogen, are
emerging.
Probably the best known of these hybrids is the
Toyota Prius. Its commercial success
demonstrates the market for such
vehicles.
The continued development of hybrid technology is another
potentially huge growth area.
Semiconductor and Circuit Modelling
Power circuit performance can be extensively analysed using circuit simulators. Simulator
accuracy and functionality continue to improve and circuit simulation is an increasingly important technique for performance evaluation and to
improve reliability.
Semiconductor device simulation models make it possible to analyse individual components within a
power circuit. Compact models provide rapid simulation times while maintaining accuracy of
simulation results.
Electro-thermal compact models simulate device temperatures in
parallel with electrical characteristics.
Elevated temperature is one of the major causes of device
failure, so this is a useful technique for
improving reliability.
Power Integration
S G P1
V =+5VC
Psub
P-epi
Pbody
P+
n+
D
V =0-(-3)VD
IC
CMOS Collector
P-epi P-epi
N-driftn+ n+
P+
n+
N-well N-well
P2NDMAAP
Advances in isolation structure technology are enabling more and more power circuitry to be
integrated with the control and processing functions on a single silicon chip. This is an
important growth area with potential benefits of reduced size and weight, reduced complexity,
increased functionality and improved reliability.
Reliability
Reliability is an important issue for all products and technologies, particularly for renewable energy systems which will be expected to perform faultlessly for many
years. The equipment
may be sited in a remote or
inaccessible location, making maintenance and repair difficult. Or it may be subject
to a harsh environment being
attacked by salt water spray or
high temperatures.
To maintain manufacturing
quality and ensure optimum reliability, monitoring systems based on feedback
loops are employed. There may be many
such loops throughout the manufacturing
process. Of course, the installer and operator must also ensure that the equipment is not subject to
environmental conditions or operating stresses which exceed the manufacturers specifications.
Power Electronics modules developed for the WEST project address the challenges raised by
renewable energy systems.
Module 1 Power Electronics Systems.
Module 2 Power Integrated Circuits
Module 3 Project
Module 1 Power Electronics Systems.
• System architecture.• Power conversion. ACDC DCDC DCAC.• Filters, harmonics & power factor correction.• Energy harvesting.• Power device drive techniques.• Thermal considerations.• Smart metering.• Reliability.
• Power device topologies.• Power device modelling strategies.• Electro-thermal compact models.• Power integration.• Isolation techniques.• Power IC applications.• Advanced devices and technologies.
Module 2 Power Integrated Circuits
Module 3 Project
A hands-on Power Electronics project designed to give an insight into the design of Power Electronics circuits. The project requires the student to :-
• Modify an existing design to meet a specific requirement. • Re-design the circuit and specify appropriate components for the new design. • Test and verify the circuit operation, measuring important circuit parameters. • Write a report detailing their new design.