imperial college london - power electronics · 2019-06-19 · size: 165m x 95m efficiency: 99% ac...

Underpinning Research

HVDC activity at Imperial college

Dr Michael M.C. Merlin 28th July 2015

Underpinning Research Future Transmission Systems

• High-Fidelity System Models • Energy System (esp Electricity System) driven

by extremes of the range

• Average energy flows indicate little: will system

work on coldest, stillest winter evening and

sunniest summer day?

• What balance of network, demand-action,

storage optimises cost/benefit case

Iberia

France

UK & Ireland

Nordic

Benelux & Germany

Italy & Malta

South East Europe

Central Europe

Poland & Baltic

4GW

21GW

41GW

5GW

10GW

4GW

19GW

10GW

10GW

3GW

2GW

3GW

4GW

Netherlands Offshore wind

Norway

Belgium Shore Line

Netherlands Shore Line

Sco

tlan

d S

ho

re L

ine

(5 G

W)

Engl

and

Sh

ore

Lin

e (2

8 G

W)

BritNed

Nemo N

orN

ed 3

or

No

- B

e

No

rNed

2

Dogger Bank

Hornsea

Norway Offshore wind

Belgium 4 GW

Scotland 9 GW

England Rounds 1 & 2

(7 G

W In

terf

ace

Cap

acit

y)

Germany Offshore

Wind

Source: National Grid

• Continental-Scale Energy

Systems • How would a new trans-

continental layer be designed

• What technology and operation

issues arise

• Is mixed use (collection and

interconnection) sensible

• Should this be planned or should

it evolve

2

Underpinning Research HVDC Systems

Advantages:

• Interconnect asynchronous networks or at different frequencies

• Theoretically no upper limit on transmission length

• Ability to control the power flows on the HVDC network

• Could improve AC system stability

• Two lines (DC) instead of three (AC) per circuit

• More power pushed through the lines at higher efficiency

• No reactive power compensation required

3

Underpinning Research HVDC Systems

Disadvantages:

• Higher station cost

• Large converter losses

• Shorter equipment lifetime

Long distance is often a decisive aspect in

favour of HVDC

4


HVDC Converters

5

Underpinning Research Semiconductor devices

0

1000

2000

3000

4000

5000

6000

7000

0 2000 4000 6000 8000 10000

Voltage Blocking

Maximum Current

IBGT GTO IGCT Thyristor

Thyristor (Press-Pack)

IGBT (Hi-Pack)

IGBT (Press-Pack)

HVDC >300kV

6

Underpinning Research CSC Project

Grita Project

COOLING FAN

VA

LV

E

HA

LL

TECHNICAL ROOMS

HV

DC

T

RA

NS

FO

RM

ER

S

AC BUS BARS AND PROTECTIONS

AC FILTERS

Power rating: 500MW DC voltage: ±200kV

Cable length: 43km (U) + 160km (S) OHL length: 110km Station Size: 225m x 120m

Station Cost: 40M€ Cable Cost: 350M€ Total Cost: 500M€ Efficiency: 99.5%

From: R.L. Sellick, M. Akerberg, “Comparison between HVDC Light (VSC) and HVDC Classic (LCC) Site Aspects, for a 500MW 400kV HVDC Transmission Scheme”, IET ACDC 2012, November 2012 7

LCC

Underpinning Research Voltage Source Converter

Rectifier Capacitive

8

•First VSC HVDC in 1997 – HÄLLSJÖN (3 MW)

•Uses self-commutated IGBT switches

•Independent control of active and reactive power

•Less (zero?) filtering requirement

•Lower footprint compared to LCC

•No dependence on AC system strength

•No voltage reversal – stronger and lighter cables, meshing

Underpinning Research VSC Project

East-West Interconnector (EWIC)


Cable length: 75km (U) + 186km (S) Station Size: 180m x 115m

AC BUS BARS AND PROTECTIONS

CO

OLIN

G

FA

N

VA

LV

E

HA

LL

TECHNICAL ROOMS

HVDC TRANSFORMERS

AC

FILTER

S

Station Cost: 51M€ Cable Cost: 420M€ Total Cost: 600M€ Total Efficiency: 98%


LCC

VSC


Modular Multilevel Converters

10

Underpinning Research H-Bridge Sub-Modules

Full H-Bridge SM Half H-Bridge SM

Underpinning Research Modular Multi-level Converter

Stack of SMs

Arm inductor

Phase reactor

DC C

apacito

r

AC transformer

Underpinning Research Modular Multi-Level Converter

• Staircase waveform

• As many steps as SMs

• Sum of arm voltages always

equals to the DC bus voltage

• Redundant switching combinations

• Voltage steps provided by

cell capacitors

•AC current splits equally

between top and bottom arms

• DC current runs through

both arms

Graphics from: http://en.wikipedia.org/wiki/HVDC_converter

0coscoscosˆ3

23

221

21

21

21 tttIiii CBA

13

http://en.wikipedia.org/wiki/HVDC_converter

Underpinning Research Power Efficiency of the MMC

Jacobsson, B., Karlsson, P., Asplund, G., Harnefors, L., Jonsson, T., VSC - HVDC transmission with cascaded two-level converters, CIGRÉ session, Paris, 2010, paper reference B4-110. 14

http://en.wikipedia.org/wiki/International_Council_on_Large_Electric_Systems

http://www.e-cigre.org/bib/B4_110_2010.pdf



Underpinning Research MMC Project

Cascaded Two-Level VSC (Suggested Layout)


Size: 165m x 95m Efficiency: 99%

AC BUS BARS AND PROTECTIONS C

OO

LIN

G

FA

N

VA

LV

E

HA

LL

TE

CH

NIC

AL

RO

OM

S

HV

DC

T

RA

NS

FO

RM

ER

S


LCC

VSC

MMC

Underpinning Research Siemens: HVDC Plus®


Cable length: 85km (S) Size: 165m x 95m Efficiency: 97%

TransBay Project

INELFE interconnector, Siemens publication. 16

http://www.energy.siemens.com/us/en/power-transmission/hvdc/hvdc-plus/references.htm#content=2014 INELFE, France-Spain

http://en.wikipedia.org/wiki/Siemens

Underpinning Research ABB: HVDC Light®

Cascaded 2-level Converter

Jacobsson, B., Karlsson, P., Asplund, G., Harnefors, L., Jonsson, T., VSC - HVDC transmission with cascaded two-level converters, CIGRÉ session, Paris, 2010, paper reference B4-110. 17

http://en.wikipedia.org/wiki/International_Council_on_Large_Electric_Systems




Underpinning Research Alstom Grid: MaxSine®

18

TenneT awards offshore grid connection project DolWin3 to Alstom with a capacity of 900 MW with new DC technology over a distance of 162 km (26/04/2013)


PowerEfficiency

CostEffective

Volume /Weight

Performance

Reliability

HVDC converter

HVDC Converters

19

PowerEfficiency

CostEffective

Volume /Weight

Performance

Reliability

LCC

LCC

+ Mature Technology

+ Large power ratings

+ DC-side fault blocking

- Large footprint

- Requires strong AC grid

VSC (MMC+)

+ Full quadrant operation

+ Power weak AC grid

+ Smaller footprint

(+) DC-side fault blocking

- Higher complexity

- Limited power ratings

Offshore Technology

PowerEfficiency

CostEffective

Volume /Weight

Performance

Reliability

LCC Classic VSC

PowerEfficiency

CostEffective

Volume /Weight

Performance

Reliability

LCC MMC Classic VSC

PowerEfficiency

CostEffective

Volume /Weight

Performance

Reliability

LCC Hybrid VSC MMC Classic VSC


Hybrid Multilevel Converters

20


Hybrid Multilevel Converter

Technologies

21

Full H-Bridge SM Alternate Arm Converter

Stack of Submodules

Arm inductor

Phase reactor

DC B

us

AC transformer

Director Switches

Underpinning Research Alternate Arm Converter (AAC)

Advantages:

• Similar advantages as the MMC

• VSC

• no AC filter

• Modular design

• Smaller valve hall

• DC fault tolerant

Disadvantages:

• Non smooth DC current

• Difficult control

22

Underpinning Research HVDC Converter – DC Fault

23

- DC fault blocking capability

- STATCOM mode for grid support

Underpinning Research HVDC Converter - Sizing

24

- Number of devices - Stack voltage submodule count - Converter voltage director switch

- Voltage and current ratings - Submodule capacitors

- Intra-cycle voltage deviation - SM Rotation heuristics

- Inductor sizing - Topology dependent - Fault limiting factors

Underpinning Research HVDC Converter – Control Systems

25

- Energy Management - Total energy storage - Horizontal balancing - Vertical balancing

- Current Control - Low-level Control - Computing System

Energy

Average

UAE

LAE LBE

UBE

LCE

UCE

Energy

Average

UAE

LAE LBE

UBE

LCE

UCE

Underpinning Research Alternate Arm Converter (AAC)

26

Underpinning Research Alstom Press Release on the AAC

27

http://think-grid.org/fault-blocking-converters-dc-networks-1?utm_source=newsletter&utm_medium=email&utm_content=fault-blocking-converters-dc-networks&utm_campaign=newsletter-thinkgrid



























Lab Experiments

28

Underpinning Research Configured Experimental Setup

Tests at 1500V successful (full-bridge)

Can be reconfigured as half-bridge

MMC (also tested)

Extending the test rig to emulate more

AC and DC conditions using

Triphase converters.

Underpinning Research Converter Build

.

Full-scale

DC bus 1,500V

AC current 7-12A

AC voltage 1070/918V

SMs per stacks 10

SM voltage 106/150V

1P.Clemow and al. “Lab-scale Experimental Multilevel Modular HVDC Converter with Temperature Controlled Cells” EPE ECCE 2014 30

Underpinning Research Full-scale Dry Converter

31

Underpinning Research Full-scale Dry Converter

32

Underpinning Research Hardware Tests on MMC

33

Underpinning Research MMC experimental results

34

Underpinning Research AAC experimental results

35

Underpinning Research DC Fault

36


Organisation Management and

Control

37

PE Centre – WP 4.1

Underpinning Research State of Health

HVDC Converter uses thousand of semiconductor devices to operate.

The State of Health (SOH) can be affected by a number of aspects

(Temperature, lightning, dust, aging…)

SOH can be estimated through different means but is essential to be

monitored to anticipate faults

- Ambient Temperature

- Lightning - Current

Waveforms - Aging

- Si Temperature

- Gate voltage - Model

Underpinning Research SM Control

Some degrees of freedom in each individual SM

Full H-Bridge can alternate their zero-voltage state

Judge, P. D., et al. "Power loss and thermal characterization of IGBT modules in the Alternate Arm converter." (ECCE), 2013 IEEE

Effect of using more the Upper zero-state combination to compensate temperature imbalance between IGBT modules

Underpinning Research Stack Control

Another way to affect the utilization of the SM is by acting on the voltage and

current waveforms of the stacks

Adding DC offset to the AC voltage shift the distribution of power losses between

the top and bottom IGBT modules

4.2% 4.2% 4.2%

12.5% 12.5% 12.5%

4.2% 4.2% 4.2%

12.5% 12.5% 12.5%

3.0% 3.0% 3.0%

12.4% 12.4% 12.4%

5.6% 5.6% 5.6%

12.3% 12.3% 12.3%

No DC Offset 5% DC Offset Power Losses Distribution between IGBT modules


Additional Research topics on

HVDC in the CAP group

41


HVAC LFAC HVDC HVAC LFAC HVDC HVAC LFAC HVDC

35km 105km 175km

CPLC 3.134 2.43 2.302 18.708 7.291 6.906 25.478 12.152 11.51

SPLC 10.88 18.795 31.855 10.787 18.752 31.814 10.747 18.71 31.772

CC 54.25 49 29.925 213.964 147 89.775 446.25 245 149.625

QC 4.926 1.525 0 13.532 4.574 0 24.108 7.624 0

OPC 3.204 35.4 48.6 3.204 35.4 48.6 3.204 35.4 48.6

OPPC 32 59 91 32 59 91 32 59 91

0

120

240

360

480

600

Cos

t (M

£)CPLC

SPLC

CC

QC

OPC

OPPC

HVAC transmission system (50/60Hz)

HVDC transmission system (0Hz)

The Case for Using Low-Frequency AC

(LFAC v. HVAC v. HVDC)

Step-up

Transformer

50/60Hz

GridGenerator

Step-down

Transformer

0Hz

Generator AC/DC

Step-up

Transformer

Grid

Step-down

Transformer

DC/AC

Converter Converter

Low-Frequency AC transmission system (16.7/20Hz)

LF Step-up

Transformer

16.7/20Hz

GridGenerator

Converter

Step-down

Transformer

AC/AC

Costs of converters, cables, transformers, platforms and power losses assessed for each configuration as a function of distance and power capacity

Xin Xiang


Cost Comparison for 0.6GW Offshore Wind Farm

HVAC

VSC-HVDC

Transmission Distance l (km)

Cost

(M

£)

Transmission Power =0.6 GW

LFAC

0 40 80 120 160 200 240

250

500

750

1000

1250

HVAC

VSC-HVDC

Transmission Distance l (km)C

ost

(M

£)

Transmission Power =0.6 GW

LFAC

80

98

8776 81 86 91 96 101 106225

245

265

285

305

325

Well-known HVAC v. HVDC comparison: costs of HVAC are approximately quadratic and exceed cost of HVDC at about 80 km

LFAC has lower unit distance cost than HVAC but suffers high terminal costs and so has little or no range over which it is preferred

Conclusions broadly similar for range of power and for overhead lines

Xin Xiang


2

Frequency Services to AC networks from

offshore DC interconnections

Unexpected generator outage

1

“Fast” energy release from HVDC converters

3 “Slower” energy release from Wind Turbines

Wind Turbine Kinetic Energy Release Signal the WF to decelerate to release some kinetic energy. Energy has to be passed through chain of dynamical systems (turbine, generator, AC/DC converter, DC link, DC/AC converter). Turbine must be reaccelerated to regain optimal wind capture HVDC link Capacitor Energy Release Some discharge of capacitance in the DC/AC converter can be allowed and can be fast but not long-lived Possibly storage in the converter could be enhanced but not an ideal application for batteries

When wind displaces gas/coal-fired generators, there is less inertia in the system

System frequency is harder to control, especially in emergencies.

Need to exploit any source of stored energy to synthesise natural inertia.

Yousef Pipelzadeh et al.

2


Blending energy storage from

Wind Farms and HVDC links

Four scenarios: A: AC grid with 4 generators but no WF. B: One WF with no emulated inertia. C: One WF with emulated inertia. D: One WF with emulated inertia but no primary support.

The displacement of generation by wind causes

the RoCoF to be 33% faster, this is avoided by

enabling inertial response emulation.

The comparison confirms that primary

response has little effect during the initial

transient.

Yousef Pipelzadeh et al.


AC/DC Systems Dynamics: Disturbance

Propagation through VSC HVDC Links

1 2

3

4

5

6 8

7

9

15 10

12

14 13

11

16

17

18 21

19

22

20

23

24 25 26

28

29

Great

Britain Scandinavia

VSC Scand

VSC

GB

Area S1

Area S2 Area GB2

Area GB1

27

1

2 3

4 5

6

8

7

9

15

10

12 14

13

11

16

17 18

19 20

HVDC Link

Example here is power export from GB to Scandinavia at 2GW

• In a simple case, the HVDC link acts as a firewall

(constant power regardless of system state).

• Adding supplementary frequency control helps

systems recover from a generation outage but

couples the dynamics of the two grids.

Claudia Spallarossa


A Loss of Generation Event aided

by Supplementary Control of

Interconnector

1,800 MW loss of

generation applied in GB

and so frequency drops

In this case, reducing export

in response to the

frequency drop is helpful

locally, but passes some of

the problem to the remote

system

When an increase of import

is need in link running at

capacity we have a difficulty

[MW]

40.32.24.16.8.0 [s]

2200.

2040.

1880.

1720.

1560.

1400.

40.32.24.16.8.0 [s]

50.2

50.0

49.8

49.6

49.4

49.2

49.55Hz

49.3Hz

B15 UK1 Frequency

with droop

without droop

40.32.24.16.8.0 [s]

8000.0

7000.0

6000.0

5000.0

4000.0

3000.0

40.32.24.16.8.0 [s]

50.2

50.0

49.8

49.6

49.4

49.2

49.7Hz

Loss of generation [MW] DC link power

Frequency response in GB

without Droop

[MW]

with Droop

with Droop

without Droop

[Hz] Frequency response in Scandinavia

without Droop

with Droop

[Hz]

(a)

(c) (d)

(b)

49.5 Hz 49.5 Hz

DIg

SIL

EN

T

Claudia Spallarossa


Reduced Dynamic Models of

Multi-Level Converters

Full scale MMCs have over 4,000 IGBTs and 1,500 capacitors

Detailed models are not practical for large network simulation

Average Value Models (AVM) uses controllable voltage source to represent the converter. They are known as Reduced Dynamic Models: they retain the low frequency dynamics but neglect the fast switching events

MMC Arm Representation

Reduces computation time ->Up 14 times faster

Caitríona Sheridan et al.


RDMs of Modular Multilevel

Converters

RDM created for two converter types: HB-

MMC and AAC

Verified against detailed model in point-to-

point HVDC links

Maintained accuracy while improving

computation time

Line-to-Ground DC Fault with

AAC

Caitríona Sheridan et al.


MMC Reduced Dynamic

Model in System Studies

using PowerFactory

Development of MMC RDM in a system

oriented software platform allows:

• Analysis of dynamics of AC+DC+AC

systems

• Provision of frequency support via

HVDC converters (stack energy

storage, overload capability).

MMC Overload Capability to face

loss of in-feed event:

• It allows to transfer an extra

30% on top of the rated power

without damaging the converter

• The frequency nadir stays

within statutory limits (±0.5 Hz)

Claudia Spallarossa et al.


Limiting Factors on P/Q Envelope:

Design for Overload

• P/Q Envelope of MMC limited by

several factors

• Arm Current Limit

• Over-Modulation limit

• Arm Voltage Limit

• Peak Sub-Module Voltage Limit

• To achieve overload expand P/Q envelope by

running controlled circulating current

• Design penalty small if reactive power

requirement during overload is decreased

• Causes increased losses – not attractive

during normal operation

Paul Judge


• Device junction temperatures may

become an issue during overload

–Dynamic Rating

• Provide large amount of extra

power during start of system

events, reduce back down to a

steady-state overload rating

Junction Temperature

Limits Paul Judge


Power Transfer in a

Degraded State

• Larges cables are now at ±500 kV and 2.5kA giving a link of 2.5 GW • We can not allow that to have a single-point failure

• How much power can we transfer after various component failures?

• Cable faults; transformer faults; semiconductor faults etc.

• Simulation studies of many scenarios underway and hardware verification now beginning

Converter designs with fault-current limiting and ability to work in step-down mode can transfer up to 50% of their rated power under a DC line to ground fault

Phil Clemow


Operation with a Pole-to-

Ground Fault

Voltage collapse on one-pole; avoided voltage-doubling on healthy pole Brief current spike caused by DC bus capacitors discharging into the fault. Cell voltages and arm currents well controlled Converter can continue to operate indefinitely at 50% power Issues remain with cable return path, grounding arrangements, DC stress on transformer

Simulation of a line-to-ground fault on lab-scale full-bridge MMC

Phil Clemow


Reduced Breaker Requirement

in Meshed DC networks DC circuit breakers are problematic: need to operate very

fast, they are large and there is no operational experience in this context.

Size and complexity strongly influenced by peak current requirement.

Converter that can control, limit or stop fault current would reduce stress on breaker

This could be selectively applied to reduce peak currents in some regions of a network – particularly helpful for offshore platform.

Depending on levels of interconnection and inductances on the network slower breaker topologies are applicable

Five Terminal Meshed Network Fault currents when implementing an

MMC (red) and an AAC (blue) at node E

Example fault currents

Geraint Chaffey

Underpinning Research Modular DC/DC designs

DC/DC converters facilitate • Connection of DC-links of different voltages

• “Firewall” protection between sub networks

• Step-up from wind farm collection networks

• Step-down to small distribution networks

Modular designs easily scalable

Greater current control allowing for small/no DC filters

High range of operable step-ratios & power levels

DC current

AC current

Thomas Lüth


Resonant Modular DC/DC

Modular resonant converter has

Low switching loss but large circulating current

High ripple frequency

Inherent balancing

Step ratio dependent on numbers of modules

Ratio limited by module current considerations

Modular converter of interest for creating large step-up/down ratios without using a transformer

Ratio set by number of modules

Resonant action used to raise operating frequency without penalty on switching loss

Xiaotian Zhang


MMC Topology Optimization

The MMC was a major step forward in VSC technology and numerous optimization iterations can still be accomplished The injection of high harmonic circulating current can help reduce the SM voltage fluctuation

Michaël M.C. Merlin

Without Circulating Current With Circulating Current

Circulating Current Waveforms


Thank you for your attention

59

imperial college london - power electronics · 2019-06-19 · size: 165m x 95m efficiency: 99% ac...

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