Electrical Power Engineering 3

Power Electronics

Dr Ewen MacphersonDr Sasa Djokic

Dr Markus Mueller

Electrical Power Engineering 3

• Power Systems– 14 lectures

• Power Electronics– 10 lectures

• Powerway– lab exercise, 2 mornings

• Castaway– power system design exercise, weeks 6-11

Electrical Power Engineering 3

Assessment:• 60% Exam (Power Systems, Power Electronics)

• 15% Powerway

• 25% Castaway

Electrical Power Engineering 3

Powerway:• Starts week 2

• 2 mornings total

• Groups of 8-12

• Work in pairs

Check when you are due to attend!

Prepare Beforehand

Electrical Power Engineering 3

Castaway:• Design power system expansion in Sri Lanka

• Starts week 6

• Groups of 5 (approx.)

• Power System simulation (“Powerworld”)

• Costings required

• Presentation week 11

Power ElectronicsDr Ewen Macpherson

• Lectures– Monday 2-3 (every week)

• Examples Class– Friday 2-3 (every 3rd week)

• Tutorials– Wednesday 12-1 (weeks 4/5 and 9/10)

Power Electronics

Recommended Text:

Power Electronics: Converters, Applications & Design

Mohan, Undeland & Robbins (Wiley)

(also used for Power Electronics 4)

Power Electronics uses Semiconductor Devices to process power (1W to

10,000MW)

Devices

Diode p-n 8,000A, 6,500V

Bipolar transistor n-p-n (rarely used)

Thyristor p-n-p-n 7,000A, 12,000V

Triac 150A, 1,500V

Power MOSFET 100A, 1,200V

IGBT (insulated gate bipolar transistor)

2,400A, 6,500V

GTO (gate turn off thyristor) 4,000A, 6,500V

Circuits

• Rectifier converts ac → dc

• Inverter converts dc → ac

• (dc) chopper converts dc → dc

• ac controller converts ac → ac

General term for all: Converter

Applications

Motor Speed Control:

Rectifier

Control Circuit

E

IaRa If

Vdc

3-phase

ac

Speed controlled by varying Vdc

Applications

Uninterruptible Power Supplies (UPS):

For critical loads where a mains failure is unacceptable

• Computers• Safety systems (oil rigs)• Emergency lighting (hospitals, cinemas)

Applications

Uninterruptible Power Supplies:

Rectifier Inverter Loadacdcac

Applications

High Voltage DC Power Transmission (HVDC):

• AC normally used, except:– bulk power transmission, long distances

– submarine cables

• 2000 MW cross-channel link, France-UK• 500 MW Scotland-Ireland link

Applications

High Voltage DC Power Transmission (HVDC):

Rectifier

Inverter+

-

3 phaseac

3 phaseac

UK

FRANCE

Applications

Electronic Power Supplies:

All electronic equipment needs a power supply

Power Supply

+24 V dc+12 V dc+5 V dc0- 12 V dc

230 V50 Hz

Worldwide market: $20 billion?

ThyristorSCR – Silicon Controlled Rectifier

• 4 layer (p-n-p-n) device• Switched on by applying gate pulse• Stays on when gate signal removed, if current

flowing through it• Like a diode, current only flows in one direction

Thyristor

ReverseBreakdown

Voltage

ForwardBreakover

Voltage

ReverseLeakageCurrent

LatchingCurrent

HoldingCurrent

ON

OFF

I

• Reverse bias– blocking

– reverse breakdown voltage exceeded - dies!

Thyristor

ReverseBreakdown

Voltage

ForwardBreakover

Voltage

ReverseLeakageCurrent

LatchingCurrent

HoldingCurrent

ON

OFF

I

• Forward bias– blocking

– forward breakover voltage exceeded – turns on

Thyristor

ReverseBreakdown

Voltage

ForwardBreakover

Voltage

ReverseLeakageCurrent

LatchingCurrent

HoldingCurrent

ON

OFF

I

• Forward bias – gate pulse applied– turns on

– stays on

Thyristor – Example Circuit

ControlCircuit

V

I

IG

R

A

CG

• Short gate pulse (3V, 20us, 50mA to 1A)– turns on

• Gate pulse removed– stays on if current > holding current

Thyristor

ControlCircuit

V

I

IG

R

A

CG

Serious problem:• Not easily switched off• Only switches off when current through it goes to

zero (or drops below holding current)

Power MOSFET

• Voltage controlled device• Remove gate drive to turn off• Very low gate current• Very high speed (up to 10MHz)• Up to 100A, 1,200V

Gate Turn Off Thyristor (GTO)

• Current controlled device• Turned off by negative gate pulse• Very high gain at turn-on• Low gain at turn-off (typically 4)• Low frequency (up to 3 kHz)• Up to 6.5kV, 4kA

Insulated Gate Bipolar Transistor (IGBT)

• Combines high gate impedance of MOSFET with high current capability of BJT

• Medium frequency (up to 100 kHz)• Up to 2.4 kA, 6.5 kV