HD MMC for platform-less HVDC offshore wind power collection system

Dr Chong Ng

Knowledge Area Lead, Electrical Infrastructure

20 Jan 2016

• ORE Catapult introduction

• Platform-less Offshore HVDC System

• Research History

• Current Status

• HD MMC

• State of the art

• HD Proposed Solution

Content

Catapults: A long-term vision for innovation & growth

• Established and overseen by the

Technology Strategy Board

• Bridging the gap between business,

academia, research and government to

create new products and services

• Open up global opportunities and generate

sustained economic growth for the future

• Delivering the ‘know-how’ economy

3

7Catapults

£1.4bnprivate and

public sector

investment

Offshore Renewable Energy (ORE) Catapult

Existing1. 50m blade test 2. Still water tank3. Wave flume4. Simulated seabed5. Wind turbine training tower6. Electrical and materials laboratories

New7. 3MW tidal turbine drive train 8. 100m Blade Test Facility9. Wind Turbine Nacelle Test Facility-201310. Offshore anemometry hub11. 7MW Wind Turbine

A Controlled and Independent Development Platform

1

2

3

4

6

7

89

5

10

11

Electrical (HV & LV) Test Lab – Brief

• HV development laboratories – 600kVac, 1MVdc, 8kA, Rain drop simulator, Material lab

• Live environmental chamber – HV and current into chamber

• Flexible three phase LV network – generators & converters array (up to 100kW)

• Grid conformance testing – G59 test equipment in the facility

• 11kV 50Hz network available

• Vibration test rig – loads up to 500kg for endurance and accelerated ageing programmes

Project Example

HD MMC for Platform-less Offshore HVDC System

Project Example

Research History in here

Platform-less Offshore HVDC System

Features:

• HVDC power transmission from the very beginning• Reduce losses and components

• Decentralised multi-terminal HVDC system

• Increase availability – Offers flexibility and redundancy

• Reduce cost – Removal/minimise offshore substation

• Increase MMC voltage level without additional hardware

Objective: Develop a dedicated high fault tolerances, flexible and cost effective power collection technology for offshore wind industry

Converter topology analysis

Analysed across frequency range; 100Hz to 2kHz,

single phase

Findings:

Transformer core loss ≤ converter loss

Transformer core loss ≤ copper loss

HB-HB configuration:

+ Lower component count

+ Lower converter loss

- Less stable (power control) due to higher voltage

gradient

- Higher transformer loss (i.e. 1% higher than MMC)

MMC-MMC configuration:

+ Better control stability (<500Hz)

+ Lower transformer core loss

- Higher component count

- Higher converter loss

Results

• Optimum configuration found to

be HB-MMC at 1.4 kHz

• Majority of losses attributed to

converter conduction losses

• This can be decreased by

moving to 3-phase

• Different converter configurations modelled in Simulink

• HB-HB

• HB-MMC

• MMC-MMC

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

HB HB-MMC 7-11L MMC 11-11L MMC 11-25L MMC

50.5%55.1% 52.5% 50.4% 50.1%

26.4%23.1%

21.5%20.7% 20.8%

20.7% 19.8%23.9% 26.6% 26.8%

1.5% 1.4% 1.8% 1.9% 1.9%

0.8% 0.7% 0.3% 0.3% 0.3%

PL

oss

Core Loss

Secondary Winding Loss

Primary Winding Loss

Conveter Switching Loss

Converter Conduction Loss

• Based on input waveforms, the transformer

specifications are optimised and losses calculated

• Repeated for frequencies between 500 – 2,000 Hz

The Modular Multilevel Converter

• Its modular design make it ideal for

scaling up.

- Now used in a variety of

applications, including HVDC

• Very attractive for offshore wind

- Low THD on AC terminal

therefore no bulky filters

- High Efficiency

- High degree of controllability

L0 R0

iΔ_j Udc/2

Arm jp

Arm jn

ipj

ij

uarm_jn

Rdc idc

L0 R0

Rdc idc

inj

uarm_jp

Mpj1

Mpj1

Mpjm

Mnj1

Mnj1

Mnjm

Sb1

Sb2

SbnSwn

Sw1

Sw2 ucap

Vs

Vp Cmod

L0 R0

iΔ_j Udc/2

Arm jp

Arm jn

ipj

ij

uarm_jn

Rdc idc

L0 R0

Rdc idc

inj

uarm_jp

The Modular Multilevel Converter (MMC) Limitations

Mpj27

Mpj26

Mpj28

Mpj24

Mpj23

Mpj25

Mpj21

Mpj20

Mpj22

Mpj18

Mpj17

Mpj19

Mpj15

Mpj14

Mpj16

Mpj13

Mpj12

Mpj10

Mpj9

Mpj11

Mpj8

Mpj7

Mpj5

Mpj4

Mpj6

Mpj2

Mpj1

Mpj3

Mnj27

Mnj26

Mnj28

Mnj24

Mnj23

Mnj25

Mnj21

Mnj20

Mnj22

Mnj18

Mnj17

Mnj19

Mnj15

Mnj14

Mnj16

Mnj13

Mnj12

Mnj10

Mnj9

Mnj11

Mnj8

Mnj7

Mnj5

Mnj4

Mnj6

Mnj2

Mnj1

Mnj3

• Each module can only create 1 AC voltage

level Lconv:

𝐿𝑐𝑜𝑛𝑣 = 𝑛𝑚𝑜𝑑 + 1

• Therefore many modules required to reduce

THD < 3 % (≈ 30)

• Each module requires 2 valves and a large

capacitor

- Capacitor contributes to roughly 50 % of the

module volume

• Therefore low THD increases converter losses

but crucially converter size and weight

• A LARGE offshore platform required to support

it.

• Platform accounts for ≈ 70 % of substation

cost therefore significant savings possible by

reducing size

L0 R0

iΔ_j Udc/2

Arm jp

Arm jn

ipj

ij

uarm_jn

Rdc idc

L0 R0

Rdc idc

inj

uarm_jp

The High Definition Modular Multilevel Converter

• By using the novel HD-MMC control

algorithm 1 module corresponds to

multiple AC voltage levels

• Using the HD-MMC algorithm only 12

modules are required to create 29 L

• This is achieved by grouping modules into

sets, controlling each to provide additional

voltage levels such that LHD is given by:

• Therefore fewer capacitors and fewer

valves

• This results in a more compact converter

reducing platform size and cost

Mpj27

Mpj26

Mpj28

Mpj24

Mpj23

Mpj25

Mpj21

Mpj20

Mpj22

Mpj18

Mpj17

Mpj19

Mpj15

Mpj14

Mpj16

Mpj13

Mpj12

Mpj10

Mpj9

Mpj11

Mpj8

Mpj7

Mpj5

Mpj4

Mpj6

Mpj2

Mpj1

Mpj3

Mnj27

Mnj26

Mnj28

Mnj24

Mnj23

Mnj25

Mnj21

Mnj20

Mnj22

Mnj18

Mnj17

Mnj19

Mnj15

Mnj14

Mnj16

Mnj13

Mnj12

Mnj10

Mnj9

Mnj11

Mnj8

Mnj7

Mnj5

Mnj4

Mnj6

Mnj2

Mnj1

Mnj3

Reduce

modules

Reduce

modules

Group

into sets

Group

into sets

Proposed HD-MMC Control

• Non intrusive, the HD-MMC control algorithm (red) can be

inserted as an add on to the standard control methods

(blue) of the MMC.

• This simplifies implementation

HD-MMC Simulation Results

• On the left is a standard 28 level MMC and on the right is the HD-MMC concept.

• Capacitor voltages are maintained at set value throughout simulation

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