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Power Electronics for Renewable Energy Systems

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Page 1: Renewable Energy Swansea

Power Electronics for Renewable Energy Systems

Page 2: Renewable Energy Swansea

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.

Page 3: Renewable Energy Swansea

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.

Page 4: Renewable Energy Swansea

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

Page 5: Renewable Energy Swansea

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.

Page 6: Renewable Energy Swansea

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.

Page 7: Renewable Energy Swansea

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.

Page 8: Renewable Energy Swansea

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.

Page 9: Renewable Energy Swansea

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.

Page 10: Renewable Energy Swansea

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.

Page 11: Renewable Energy Swansea

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.

Page 12: Renewable Energy Swansea

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.

Page 13: Renewable Energy Swansea

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

Page 14: Renewable Energy Swansea

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.

Page 15: Renewable Energy Swansea

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.

Page 16: Renewable Energy Swansea

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.

Page 17: Renewable Energy Swansea

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.

Page 18: Renewable Energy Swansea

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.

Page 19: Renewable Energy Swansea

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.

Page 20: Renewable Energy Swansea

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.

Page 21: Renewable Energy Swansea

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.

Page 22: Renewable Energy Swansea

Current predictions place smart metering into a much wider-ranging power management network.

This is potentially a huge growth area.

Page 23: Renewable Energy Swansea

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.

Page 24: Renewable Energy Swansea

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.

Page 25: Renewable Energy Swansea

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.

Page 26: Renewable Energy Swansea

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.

Page 27: Renewable Energy Swansea

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.

Page 28: Renewable Energy Swansea

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.

Page 29: Renewable Energy Swansea

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.

Page 30: Renewable Energy Swansea

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

Page 31: Renewable Energy Swansea

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.

Page 32: Renewable Energy Swansea

• 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

Page 33: Renewable Energy Swansea

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.