Power Electronics Architecture for Renewable Energy Generation

1

• Power Electronics Architecture for Renewable
  Energy Generation
• System Strategies

2
Structure

• Introduction
• Power System Topologies
• Grid-Connected Stand-Alone
• Series Parallel topologies
• System Components
• Power Converters (Inverters)
• Inverters for PV Systems
• Energy Storage (Batteries)
• Advanced Metering Systems (Smart Meters)

3
The Role of Power Electronics in Renewable Energy
Systems
Biofuel Hydrogen
Utility Distribution
Biomass
Hydrogen
Fuel Cell
Combustion
Tidal
Power Inverter
Local Consumer
Wind
Electric Generator
Smart Metering
Solar
Energy Store
Power Electronics
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Grid-Connected System
Inverter
Energy Harvester
Smart Metering
Utility Distribution
Energy Consumer
Energy Store
Grid-connected systems do not necessarily need
backup energy storage, but it is often used to
balance fluctuations in generated energy.
Stand-Alone System
Inverter
Stand-alone systems are generally only used
where there is no utility grid available.
Energy Harvester
Energy Consumer
Energy Store
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• System Topologies

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Series Configuration
PV Controller
Battery Charger
PV Array
Back-up Generator / Utility Grid
Wind Charger
Power Inverter
Wind Generator
AC Load
DC BUS
Battery Bank
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Series Configuration
Disadvantages
Advantages

• The inverter has to handle all the output power
  and so must be sized to cope with the peak load.
• The system relies heavily on the battery bank.
• Power cannot be supplied directly from the
  grid/back-up source.
• Inverter failure results in complete power loss
  to the load.

• The battery bank, inverter and back-up generator
  can be optimally matched .
• The DC bus is easy to interface to and expand.
• No interruption or disturbance of supply to load
  when changing from renewable power source to
  grid/back-up power source.

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Switched Configuration
PV Controller
Battery Charger
PV Array
Back-up Generator / Utility Grid
Wind Charger
Change-over switch
Power Inverter
AC Load
DC BUS
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Switched Configuration
Disadvantages
Advantages

• The inverter has to handle all the output power
  and so must be sized to cope with the peak load.
• The system relies heavily on the battery bank.
• AC power disturbance when switching from
  renewable to grid/generator supply

• The battery bank, inverter and back-up generator
  can be optimally matched .
• The DC bus is easy to interface to and expand.
• System can supply AC load if inverter fails
• AC load can be supplied directly from back-up
  source.

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Parallel Configuration
PV Controller
PV Array
Utility Grid
Wind Charger
Bi-directional Power Converter
Wind Generator
AC Load
AC BUS
DC BUS
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Parallel Configuration
Disadvantages
Advantages

• The converter complexity is increased.
• DC integration of renewable sources results in
  custom system.

• The power converter can be sized to match all or
  just some of the peak load.
• The DC bus is easy to interface to and expand.
• Grid can supply AC load directly if converter
  fails
• No AC power disturbance when switching from
  renewable to grid supply.
• System is less reliant on battery bank.

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AC Connected Parallel Configuration
PV Inverter
PV Array
Utility Grid
Wind Inverter
Wind Generator
Bi-directional Power Converter
AC Load
AC BUS
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AC Connected Parallel Configuration
Disadvantages
Advantages

• Inverters are integrated on the AC bus and must
  be synchronised to the grid supply.

• Inverters can be sized to match power sources.
• The AC bus is standardised.
• Grid can supply AC load directly if converter
  fails
• No AC power disturbance when switching from
  renewable to grid supply.
• System is less reliant on battery bank.

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Power Converters
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Conversion Efficiency
Power conversion from one form of electrical
energy to another is generally quite
efficient. Both DC-DC and DC-AC converters have
typical efficiencies around 90.
For comparison, electric motors convert
electrical energy to kinetic energy with an
efficiency of 80 - 95, depending on load
conditions, while the internal combustion
(petrol) engine converts chemical energy to
kinetic energy at less than 40 efficiency.
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Inverters
Inverter Outputs
Each of the inverter outputs illustrated can be
called AC, but only the sine wave output is of
suitable quality. The stepped wave and square
wave outputs have an unacceptably high harmonic
content and will radiate (EMI) due to the sharp
edges of the waveform.
Sine wave
Stepped wave
Square wave
Many types of domestic appliances will only
operate satisfactorily with a sine wave supply.
With stepped or square wave supplies, some may
not operate at all, while others with motors
(washing machines, fridges etc) will take 20
more power. Fortunately, almost all modern
inverters will have sine wave output.
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Multiple Inverter Strategies
Redundancy
With the simple redundancy method shown, each
inverter must be capable of supporting the
maximum power. If one unit fails the other takes
over, allowing repair or replacement of the
faulty unit with no interruption to the supply of
power.
Power In
Power Out
Inverters in Parallel
The power is split between 2 or more inverter
units. The inverters are arranged as
master-slave. At low power, just one inverter
is operating. As the power increases to 90 of
the max inverter power, successive inverters will
become operational. In this way, optimum inverter
efficiency is maintained.
Power In
Power Out
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Inverter Efficiency
Typically, inverter efficiency is high across a
wide power range. (90) Efficiency at very low
power levels is poor. At power levels above the
maximum rated power, the inverter starts to shut
down to prevent damage from overheating
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n 1 Redundancy Scheme
A scheme for combining multiple inverters both
for load sharing and for redundancy.
Power Out
Power In
The maximum power required from the system is
divided between a number of paralleled inverters
and one extra inverter is added. The scheme
maintains optimum efficiency across a very wide
power range and is capable of supporting maximum
power while one faulty unit is replaced. The
n1 scheme dramatically improves system
efficiency and reliability.
n 1
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Inverters for PV systems
Single String
PV panels are connected together in STRINGS.
More Panels per string gives higher DC voltage to
the inverter.
Multiple Strings
Multiple strings can be connected to a single
inverter. The inverter size must be increased to
handle the total power of the system.
Multiple Inverters
Each string can have its own inverter. The
inverter outputs are combined to provide the full
power. The system uses more inverters, but each
inverter is smaller size.
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Maximising the PV Power Output
PV output characteristics change with solar
irradiation and panel temperature. The maximum
power point changes as the output characteristics
change. The power converter (inverter) must
ensure that PV panels are operating at maximum
power under all irradiation conditions and all
panel temperatures.
Using the inverter to control and optimise the
PV panel power output is termed Maximum Power
Point (MPP) Tracking.
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MPP Tracking Methods
Constant Voltage simplest control (cheapest).
Poor MPP tracking. 88 efficiency. Perturb
and observe (PAO) Good MPP tracking. Response
can be slow. 96.5 efficiency. Incremental
conductance technique (ICT) Good MPP tracking
and fast response. Greater complexity and
computation (more expensive). 98 efficiency.
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Choosing An Inverter
Maximum power input (DC) to inverter.
Defines the possible number of strings and the
number of PV panels per string.
Maximum power output (AC) from inverter.

Output power quality.
240V 50Hz grid connection.
Power conversion efficiency.
Technical Data for Sunny Boy 1100 PV Inverter
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System Sizing
Domestic electricity consumption measured at 1
minute intervals
Peak 7.18KW Mean 0.49KW Energy Consumption
11.8KWh
A suitable system might have capacity of 1KW
with the PV inverter split into 500W units (3 for
an n1 strategy).
The generation system supplies most of the daily
power requirements, with battery back-up taking
care of the short-duration peaks (kettle, toaster
etc). In the illustration, the system would
require grid back-up on 3 occasions during the
day, and during the night when generation stops.
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Siting Electrical Power Units

• Should be located in a clean, dry, well
  ventilated environment.
• Cabling carrying high current should be kept as
  short as possible.
• The location should have easy access for
  maintenance.
• Should be located so that any noise generated is
  not a nuisance.

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• Batteries for Renewable Energy Systems

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Energy Storage
Batteries provide energy storage and cater for
short-term breaks in renewable generation. In
off-grid systems the back-up battery supply may
be required to supply energy for many hours (or
even a few days).

• Ideal Requirements
• High energy storage capacity
• Deep discharge capability (70-80)
• Long life time
• Low maintenance
• Low cost

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Battery Types
Of all these battery technologies, only lead-acid
is currently practical for use as energy storage
in renewable systems. The other technologies fall
down on either cost or storage capacity.

• Lead acid
• Nickel-cadmium
• Lithium
• Zinc bromide
• Zinc chloride
• Sodium sulphur
• Nickel-hydrogen
• Redox
• Vanadium

The major limitations with lead acid technology
are Energy storage per unit volume (and
weight!) Limited lifetime (especially with deep
discharge cycles)
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Lead-Acid Battery Lifetime
The standard car battery is not designed for the
deep discharges needed for renewable energy
systems. Deep cycle batteries are available
which give high depth of discharge (DOD) while
maintaining an acceptable cycle lifetime.
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Smart Meters
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The case for upgrading meters

• The present humble KWh meter gives very little
  information to the consumer and not much to the
  utility supplier either.
• Meters only indicate past energy consumption, and
  then very poorly. Present consumption is almost
  impossible to determine.
• Meters have to be read visually.
• No provision for variable tariffs for different
  times of the day.
• Most meters cannot credit consumers for generated
  energy returned to the grid despite what some
  system suppliers will tell you!

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Smart Meter features
Minimum requirements
Other desirable features

• Immediate consumption display
• Consumer usage information (down to appliance
  level)
• Remote isolation
• Energy usage display in more prominent location
• Variable tariff profile
• Billing plan options

• Bi-directional power metering
• Remote communication
• Improved, easy to understand, consumer information

The precise functionality of future metering
systems relies on agreed standards and strategies
across the utility industry.
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Smart metering activity worldwide
Europe Italy 27 million smart meters deployed
between 2000-2005. Netherlands 7 million
households to have smart meters by 2013. Sweden
100 coverage for automatic remote meter reading
by 2009. North America Canada - 800,000 meters
installed by 2007. 100 coverage by 2010. US
State-by state, California pursuing same
technology as Canada. Australia
State-by-state. Victoria smart meters in small
businesses by 2013. In new residences since 2007.