Power Electronics Research at the University of Nottingham

Professor Pat Wheeler

Email: pat.wheeler@nottingham.ac.uk

Power Electronics, Machines and Control (PEMC) Research Group

UNIVERSITY OF NOTTINGHAM, UK

The University of Nottingham

Professor Pat Wheeler

Email: pat.wheeler@nottingham.ac.uk

Introduction

• Introduction to the University of Nottingham

– Location in the UK

– University of Nottingham

– Power Electronics, Machines and Control Research Group

• Power Electronics for HVDC Applications

– HVDC Applications

– Power Converter topologies

– Topology comparisons

Nottingham

Manchester

Liverpool

LondonBristol

180km

City of Nottingham

Population 300,000

The city famous for the legend of

Robin Hood and Brian Clough

[one time manager of the

Nottingham Forest football team]

Nottingham

Nottingham

Manchester

Liverpool

LondonBristol

180km

City of Nottingham

Population 300,000

The city famous for the legend of

Robin Hood and Brian Clough

[one time manager of the

Nottingham Forest football team]

Nottingham

Robin Hood

Power Electronics, Machines and Control Research Group

Power Electronics, Machines & Control Group Te

Future Electricity Networks

Renewable Energy

Aerospace (More Electric Aircraft)

High-energy Physics applications

Automotive and Marine

Industrial Applications

Application Areas

Underlying scientific research

Power device packaging and cooling

New actuator topologies

Cooling methodologies & thermal integration

New modelling methods

Power Electronics for Aerospace and Energy

Advanced motor designs

Energy management system

High density power conversion

Electrical power systems (AC/DC/Hybrid)

Diagnostics and Prognostics

Electromagnetic compatibility and wireless

systems

Research Themes

1500m2 laboratories with a 2.5MVA power supply

Motor rigs 1kW to 750kW, voltage supplies to 13kV

Electrical Machine/Actuator manufacture

Machine and Power Systems testing to 800kW

Environmental testing chambers

Group Facilities

Power Electronics, Machines & Control Group

> 135 Researchers/Academics

17 Academic Staff [Faculty]

48 Contract Research Fellows

70 PhD students

4 Visiting Scholars

Division of

Electrical Systems

& optics

Division of Materials,

Mechanics and

Structures

Engineering Faculty

Applied

OpticsPEMC

Group

Institute

Electro-

magnetics

Heat

Transfer

Research

Research Council

29%

European Commission

33%

Knowledge Transfer Partnerships

15%

Industry

16%TSB/Industry

4%

Overseas Industry4%

Current Research Grants US$35M

Full Professors

Prof Jon Clare (Power Electronics)

Prof Mark Johnson (Power Device Technologies, Energy)

Prof Pat Wheeler (Power Electronics)

Prof Mark Sumner (Drives, Power Quality, Energy)

Prof Pericle Zanchetta (Control, Power Quality)

Prof Chris Gerada (Machines, Drives)

Associate Professors

Dr Christian Klumpner (Power Electronics)

Dr Alberto Castellazzi (Power Device Technologies)

Dr Sergei Bozhko [Aerospace Electrical Systems]

Lecturers

Dr Guarang Vakil (Electrical Machines)

Dr Alan Watson (High Power Electronics)

Dr Paul Evans (Power Device Technologies)

Dr Alessandro Costabeber (Power Converters and Control)

Dr Michael Galea (Machines and Drives)

Dr Tom Cox (Electrical Machines)

Dr Tao Yang (Aircraft Electrical Power Systems)

Dr Michele Degano (Electrical machines)

DC Transmission – Northern Europe

Multiport HVDC Networks

Offshore Wind

Country A

Offshore Wind

Country C

T&D System

Country A

T&D System

Country B

T&D System

Country C

HVAC to

HVDC

HVAC to

HVDC

HVAC to

HVDC

HVAC to

HVDC

HVAC to

HVDC

Multi-port HVDC

Transmission System

A Simplified Multiport HVDC system

• Interconnection of countries

• Sharing of resources

• Trading in electricity

• Generating in the least cost

areas

• Advantages when using

• Intermittent renewable

energy sources

• A European HVDC Super-grid by 2030?

• Politics?

• Cost?

• Brexit!!!

• UK investing 10GW of offshore wind capacity with HVDC links to shore

• STATCOM-less systems, grid control, ride through and protection

• Rugged, cheap, proven, but traditionally large foot print

• Voltage Source IGBT Converters

• Full controllability, small foot print, black start

• VSC HVDC topologies for high efficiencies, fault performance

Offshore Wind Farm

F

F

F

F

F

HF

50-150km DC cable Land Station

0

0.5

1

Wind farm active (1) and reactive (2) powers, pu

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-1

0

1STATCOM active (1) and reactive (2) powers, pu

0

1

2

3

HVDC DC-link current, kA

80

100

120

140

160Offshore grid voltage, kV

1

2

1

2

Time, s

HVDC Transmission

HVDC to AC Converter

AC to HVDCConverter

AC Substations?

• Tried and tested technology

• 50+ year history

• Reliable and proven

• Problems:

• Converter station footprint (filters approx 50%)

• Inability to work on “dead” networks - voltage needed for commutation of thyristors

• Limited scope for reactive power control

• Power reversal requires DC voltage reversal - Limits usage to certain cable types

• No scope for multi-terminal DC operation

Conventional HVDC Transmission

Based on line commutated converter (LCC) thyristor technology

Filter

AC System DC System

ConverterTransformer

Upper Valve Group

Lower Valve Group

Ground

+VDC

• Tried and tested technology

• 50+ year history

• Reliable and proven

• Problems:

• Converter station footprint (filters approx 50%)

• Inability to work on “dead” networks - voltage needed for commutation of thyristors

• Limited scope for reactive power control

• Power reversal requires DC voltage reversal - Limits usage to certain cable types

• No scope for multi-terminal DC operation

Conventional HVDC Transmission

HVDC – Conventional HVDC - Future

• Voltage source converter HVDC is growing rapidly (800kV 10GW)

• Many advantages over conventional HVDC

• “Black start” – can feed a dead network

• Active and reactive power control

• Much smaller filters/no compensation (station footprint much smaller)

• “Constant” voltage DC link multi-terminal applications

• No voltage reversal for power reversal

• Some key problems for

VSC converters

• Efficiency (switching

converter losses)

• Operation with

faulted AC or

DC side

• Scalability and

modularity

Voltage Source Converter (VSC) HVDC transmission

AC GRID or LOAD

DC GRID or LOAD

12

3Modular multilevel or Chainlink convertersSM1

SM2

SM…

SMN

Equivalent N levels controllable voltage sources using a number of sub-modules (SM)

Multi-level ConvertersOptions

12

3Modular multilevel or Chainlink convertersSM1

SM2

SM…

SMN

Equivalent N levels controllable voltage sources using a number of sub-modules (SM)

Multi-level ConvertersOptions

0 0.005 0.01 0.015 0.02

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Time /s

Am

plit

ude

0 0.005 0.01 0.015 0.02

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Time /s

Am

plit

ude

0 0.005 0.01 0.015 0.02

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Time /s

Am

plit

ude

Two-level inverter based on series connected devices

DC supply

(100+kV)A

One

switch

One phase of 3 shown

• Simple concept

• Requires series devices

• 2-level operation – needs

relatively high PWM frequency

• High switching losses (total

losses 1.7% per station)

• 2-level PWM generates

significant harmonics - filtering

0 0.005 0.01 0.015 0.02

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Time /s

Am

plit

ude

Voltage Source Converters (VSC) Two Level Topologies

Chainlink converters – basic concepts 1

• Modular Multilevel are based on the concept of Chain-links • basic building block

• Consider a chain-link with N half-bridge submodules• assume that the capacitors are permanently charged all at the same voltage VSM

A chainlink behaves like an equivalent controllable N+1

level voltage source

+

-

CLV +

-

CLV

SMCL NVV 0

linkChainCL

HVDC Converter TopologiesChain-links

Modular-Multilevel-Converter (MMLC)

A

(2 phases of 3 shown)

• Concept more complex

• Switching of cells controls BOTH the

DC side and AC side voltage

• No bulk DC capacitor

• No need for series devices

• Multi-level operation – low switching

frequency + good harmonic

performance

• Low switching losses (typical total

losses 1% per station)

• Twice the number of devices required

compared to 2-level approach

• Large number of capacitors of

significant size

• Voltage on individual capacitors must

be controlled

B

Edc

DC SIDE

CELL

0 0.005 0.01 0.015 0.02

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Time /s

Am

plit

ude

Modular-Multilevel-Converter (M2LC)

A

2 phases of 3 shown

B

Edc

DC SIDE

CELL

Multi-level AC voltage

VAC and current IAC

Vdc/2 + VAC

Vdc/2 - VAC

Idc/3 + IAC/2

Idc/3 - IAC/2

Parallel Hybrid Topology

• Series connection of three single phase modular multilevel converters on the DC side

• Each leg made of a chainlink (CL) of half bridge submodules

• CLs act as controllable voltage sources

• CLs generate rectified sine waves

• Main H-bridge used to steer Waveform

Half-Bridge Submodule

IGBTs

Zero voltage switching!

Vcl1

Vcl2

0

S1

Vs1

Vs2

10kV

10kV

0 5 10 15 20-15

-10

-5

0

5

10

15

S1 ON

S2 ON

Condition for zero energy exchange with chain-links

VAC(peak) = 2EDC/

ZVS

VOFF < 20kV S2

Series Hybrid Topology Alternate Arm Converter

● Positive arm active ● positive converter voltages are synthesised by modulating the positive arm’s sub-modules whilst the

positive arm’s director switch is closed

● Negative arm active ● negative converter voltages are synthesised by modulating the negative arm’s sub-modules whilst the

negative arm’s director switch is closed

● Overlap ● synthesise converter voltages and control the circulating current by modulating the positive and negative

arms’ sub-modules whilst the positive and negative arms’ director switches are closed

Series Hybrid Topology Alternate Arm Converter

Series Hybrid Topology Alternate Arm Converter

● Positive arm active ● positive converter voltages are synthesised by modulating the positive arm’s sub-modules whilst the

positive arm’s director switch is closed

● Negative arm active ● negative converter voltages are synthesised by modulating the negative arm’s sub-modules whilst the

negative arm’s director switch is closed

● Overlap ● synthesise converter voltages and control the circulating current by modulating the positive and negative

arms’ sub-modules whilst the positive and negative arms’ director switches are closed

-1006.5

-782.8333

-559.1667

-335.5

-111.8333

111.8333

335.5

559.1667

782.8333

1006.5

Convert

er V

olt

age

(V)

0.1 0.105 0.11 0.115 0.12 0.125 0.13 0.135 0.14-10

-5

0

5

10

Supply

Curr

ent

(A)

Time (s)

0.1 0.105 0.11 0.115 0.12 0.125 0.13 0.135 0.14-8

-6

-4

-2

0

2

4

6

8

Lim

b C

urr

ents

(V

)

-1006.5

-782.8333

-559.1667

-335.5

-111.8333

111.8333

335.5

559.1667

782.8333

1006.5

Convert

er V

olt

age

(V)

0.1 0.105 0.11 0.115 0.12 0.125 0.13 0.135 0.14-10

-5

0

5

10

Supply

Curr

ent

(A)

Time (s)

Converter voltage one phase

Limb Currents

Supply Current Circulating

One phase Current

• Most of the AC voltage is generated by the CLs, the SFBs provide the amplitude control required to generate reactive power

• The DC ripple (6n and 12n harmonics mainly) can be actively cancelled

• The primary of the transformer can be grounded

SFB

CL

Main H

BridgeSBC PHASE A

SBC PHASE B

SBC PHASE C

r:1

+

-

VDCC

IDC

r:1

r:1

+

+

+

AC NETWORK DC NETWORK

Series Bridge ConverterParallel/Series combination

SFB

CL

Main H

BridgeSBC PHASE A

SBC PHASE B

SBC PHASE C

r:1

+

-

VDCC

IDC

r:1

r:1

+

+

+

AC NETWORK DC NETWORK

+VDC=20kVVAC=11kVP=20MWQ=8.2MVARR=1.1

Full converter (average) waveforms for a 20MW demonstrator:

0 0.005 0.01 0.015 0.02-1000

-500

0

500SFB voltages [V]

0 0.005 0.01 0.015 0.020

0.5

1

1.5

2

2.5x 10

4Chainlink voltages [V]

Total DC

Grounded primary

0 0.005 0.01 0.015 0.020

5000

10000

15000voltages [V]

+

0 0.005 0.01 0.015 0.02-1

-0.5

0

0.5

1x 10

4Secondary voltages [V]

Pcc

Series Bridge ConverterParallel/Series combination

2-level converter

M2LC SBC Series hybrid

Parallel hybrid

Total Semiconductor count (pu)

1 2 3 2.5 1.5

Total submoduleDC capacitor rating (pu)

0 1 ~0.5 ~0.5 <0.25

Losses

AC Harmonic performance

Hard-switched IGBT?

Ability to suppress DC faults?

Topology Comparisons

UoN Superbike