Control and Application of Modular for HVDC Transmission ...

Design, Control and Application ofModular Multilevel Converters for

HVDC Transmission SystemsChapter 1: Introduction to Modular Multilevel Converters

by Kamran Sharifabadi, Lennart Harnefors, Hans‐Peter Nee, Staffan Norrga, Remus Teodorescu

ISBN‐10: 1118851560Copyright Wiley 2016

13

Chapter 1: Introduction to Modular Multilevel Converters

Outline

1.Two‐level voltage source converters2. Why multilevel converters? 3. Diode Clamped Multilevel Converters4. Flying Capacitor Multilevel Converters5. Cascaded Multilevel Converters6. Submodules and Submodule Strings7. MMC for AC/DC conversion8. Other cascaded converters

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Two‐level VSCTopology

2dV

2dV

• Converts a DC voltage into an AC voltage by switching between two voltage levels.

• Two parts:- Unidirectional semiconductor

valves- One single capacitive energy

storage• Used in most low voltage

applications

15

Two‐level VSCVariants

Single-phase, one phase leg Single-phase, full-bridge

Three-phase

16

Two‐level VSCBasic operation

17

THD not altered by 2‐level modulation!

Two‐level VSC2‐level modulation ‐‐ Impact on harmonic properties

2 2

2,1

11 1 1rms d

rms d

V VTHDV mV m

21 t T

rmst

V v dtT

2

2

18

…but harmonics are shifted to higher frequency!

Two‐level VSC2‐level modulation ‐‐ Impact on harmonic properties

-1

0

1p=

9

Time domain

0

0.5

1Frequency domain

-1

0

1

p= 1

5

0

0.5

1

-1

0

1

p= 2

1

0

0.5

1

-1

0

1

p= 2

7

t [rad] 0 50 100 1500

0.5

1

Harmonic order

19

Two‐level VSCSteady‐state operation

1 cos( )2a dv mV t

ˆcosai I t

1 ˆ cos cos 24a a dap v i mV I t

2 / 3 2 / 3cbap t p t p t

Second harmonic power fluctuation cancels outbetween phase legs

Only HF harmonics in DC capacitors

Onephase

21


Outline

1.Two‐level voltage source converters2.Why multilevel converters?3.Diode Clamped Multilevel Converters4.Flying Capacitor Multilevel Converters5.Cascaded Multilevel Converters6.Submodules and Submodule Strings7. MMC for AC/DC conversion8. Other cascaded converters

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Phase voltages are multi‐level (>2).

Pulse number and switching frequency are decoupled.

The output voltage swing is reduced – less insulation stress

Series‐connected semiconductors can be avoided for high voltage applications

More complicated converter topologies are required

Typical applications: high‐power converters operating at medium or high voltage.

Why multilevel converters?Impact in time and frequency domain

0 1 2 3 4 5 6-1

0

12 levels

0 1 2 3 4 5 6-1

0

13 levels

0 1 2 3 4 5 6-1

0

15 levels

0 1 2 3 4 5 6-1

0

17 levels

(Constant switching frequency)

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0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

22 levels

WTHD0: 4.1%

Line-line voltage spectra (pu Ud), plev=11, ma=0.9

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

2 3 levels

WTHD0: 1.7%

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

25 levels

WTHD0: 0.51%

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

27 levels

WTHD0: 0.2%

Harmonic order

(Constant switching frequency)

Why multilevel converters?Impact in time and frequency domain

Phase voltages are multi‐level (>2).

Pulse number and switching frequency are decoupled.

The output voltage swing is reduced – less insulation stress

Series‐connected semiconductors can be avoided for high voltage applications

More complicated converter topologies are required

Typical applications: high‐power converters operating at medium or high voltage.

24

One phase leg, or equivalent, shown in each case

Neutral point clamped (NPC) topologies

Flying capacitor topologies

Cascaded topologiesHalf-bridge and full-bridge variants

2dV

2dV

2dV

2dV

2dV

Multilevel converter topologies

25


Outline


26

• Prof. Nabae 1981• Bus‐splitting common dc

capacitor + diodes for clamping

• Any number of levels, but the number of diodes increases

• Also, the mechanics become complex with many interconnects

• Widespread use in MV drives and STATCOMS(mainly 3‐level NPC)

Diode‐clamped convertersTopologies

Three levels Neutral‐point clamped

(NPC) converter

Four levels

27

Diode‐clamped convertersOperation

31

• Gives alternative ways of implenting the zero‐voltage state

• Allows for more even distribution of losses

• Used in at least two VSC HVDC projects

Diode‐clamped convertersANPC Active Neutral‐point Clamped

B Bijlenga, US Patent 6480403, “HVDC Device for Converting Between Alternating Voltages and Direct Current Voltages”, filed 1998

32


Outline


33

• Prof. Meynard, CNRS Toulouse, 1988

• Common dc capacitor + flying capacitors

• Any number of levels, but the number of capacitors increases

• Also, the mechanics become complex with many interconnects

Flying capacitor convertersTopology

34

Flying capacitor convertersOperation

Switching states of one phase leg of a three‐level flying capacitor converter

35


Outline


36

• Two‐level and diode‐clamped topologies aresuitable up to medium voltage (HV with series connection)

• But:– Redundancy difficult to achieve– Scale poorly to many levels– Trade‐off between switching losses and harmonicperformance becomes critical for MV and HV converters

Cascaded multilevel converters

37

• Cascaded converters(= modular multilevel converters= chain‐circuit converters)are based on series‐connectionof converter cells(= submodules = chain links)

• This gives:– Modularity– Scalability– Excellent harmonic properties– Redundancy can be implemented

Cascaded multilevel converters

38

• Two‐level converters are most competitive for low voltage applications (up to few kV)

• Multilevel converters offer several important benefits – Decoupling of fsw and pulse frequency– More levels in phase voltages– Can avoid direct series connection of semiconductors

• Diode‐clamped converters are competitive for MV applications (several kV) but do not scale well to many levels

• Modular multilevel converters offer scalability, reduce harmonics and avoid direct series connection

Summary

39


Outline


40

• Based on two‐level phase legs.

• Act as independent voltage sources

• Capacitor voltage must be balanced over time!

Submodules and Submodule stringsBasics

41

• Gives scalability in terms of voltage

• More cellsMore levels Higher voltageMore redundancy

• Power balance:

Submodules and submodule stringsSubmodule strings

0t T

t

v t i t dt

1

1 ˆ ˆ cos 02d d k k k

kV I V I

TD

FD

42

• Commonly encountered in modular multilevel converters

• ≠ ± /2 → AC/DC conversion at the terminals

Submodules and submodule stringsSine + dc operation

ˆ ˆ cos 02 ad adV I V I

ˆ cos( )d av V V t

ˆ cosd ai I I t

Power balance

43

• Same total capacitor voltage Vc assumed

• No ac without dc with half‐bridge string• Significant widening of the operating region with full

bridges 43

Submodules and submodule stringsSine + dc operation ‐ limits

half‐bridge string full‐bridge string

44


Outline


45

• Prof. Marquardt, 2002• Voltage source converter – towards both ac and dc sides

• Overall structure similar to two‐level converter

• Inductors in phase arms

MMC for AC/DC conversionGeneral

46

• Sinusoidal ac‐side emfs vs provided

• Ac and dc voltages maintained simultaneously

• Submodule strings produce AC and DC voltage

MMC for AC/DC conversionBasic operation ‐ voltage

47

• No zero‐sequence currents on ac side

• No common‐mode currents on dc side

• Submodule strings have ac and dc current componentssimultaneously

MMC for AC/DC conversionBasic operation ‐ currents

48

Linear transformation:

MMC for AC/DC conversionEquivalent schematic

2

2

2

li uisi

ui lici

si ui li

ui lici

v vv

v vv

i i ii ii

49

Decoupling enabled by linear transformation on previous slide

MMC for AC/DC conversionDecoupling of circuit equations

S. Norrga et al. "Decoupled steady-state model of the modular multilevel converter with half-bridge cells”, IET PEMD 2012 proc

50

0

1

c

c

t

c c

v sv

i si

v i dtC

Time domain

,,

1

c

c

c hc h

V S V

I S I

IV

hC

The symbol * represents convolution of Fourier coefficients

Frequency domain

MMC for AC/DC conversionNon‐linear behavior of submodules/strings

Norrga, S.; Ängquist, L.; Ilves, K.; Harnefors, L.; Nee, H., "Frequency-domain modeling of modular multilevel converters," IEEE IECON 2012 proc

51

51

• For a half‐bridge MMC the ac magnitude can never exceed the dc level

• A full‐bridge MMC can do AC/AC conversion

• Capacitor voltage fluctuation not considered

MMC for AC/DC conversionMMC voltage capability

Same total submodule string capacitor voltage assumed

52

power balance for three‐phase converter:

MMC for AC/DC conversionSteady‐state operation ‐‐ voltages and currents

ˆ coss si I t ˆ cos( )s sv V t

3 ˆ ˆ cos2 s s d dV I V I

1 ˆ cos3 2d

u sIi I t

1 ˆ cos3 2d

l sIi I t

1 ˆ cos2u d sv V V t

1 ˆ cos2l d sv V V t

53

Fundamentalterm

2nd harmonicterm

MMC for AC/DC conversionSteady‐state operation capacitor power fluctuation

21 ˆ 2cos cos cos cos 28u d sp V I t m t m t

21 ˆ 2cos cos cos cos 28l d sp V I t m t m t

Ilves, K.; Norrga, S.; Harnefors, L.; Nee, H.-P., "On Energy Storage Requirements in Modular Multilevel Converters," IEEE Trans Power Electronics, vol.29, no.1, pp.77-88, 2014

54

-10123

pu

(c) Arm voltages

-10123

pu

(d) Arm currents

-2

0

2

pu

(e) Arm power exchange

-2

0

2

pu

(a) Terminal voltages

-2

0

2

pu

(b) Terminal currents

-0.2

0

0.2

pu

ω1t[rad]

(f) Stored arm energy exchange

0 2 3 4

-10123

pu

(c) Arm voltages

-10123

pu

(d) Arm currents

-2

0

2

pu

(e) Arm power exchange

-2

0

2

pu

(a) Terminal voltages

-2

0

2

pu

(b) Terminal currents

-0.2

0

0.2

pu

ω1t[rad]

(f) Stored arm energy exchange

0 2 3 4

MMC for AC/DC conversionSteady‐state operation ‐ waveforms

(Same power transferred)

M = √2 (requires full-bridges)M = 1.0

55

55

• Appears as symmetric short‐circuit from the AC side.

• System impact in HVDC (grid) applications

• Rating impact on diodes

MMC for AC/DC conversionImpact of dc‐side short‐circuit

56

AC grid short‐circuit currentmay be 10 X the converter ratedcurrent

MMC for AC/DC conversionImpact of dc‐side short‐circuit

57

Voltage• The capacitor DC voltage per arm is typically = the pole‐pole DC voltageTotal blocking capability twice of 2‐level converter

Current• Peak valve current = Peak arm current

Lower than for two‐level converter

MMC for AC/DC conversionComponent rating issues ‐‐ semiconductors

1 1ˆ ˆ cos4 2sI I M

58


Outline


59

• Prof. Peng• No DC terminal• Only reactive power

• In industrial use since 1990s

• Arm energy balancing critical with unbalanced loads

Other cascaded convertersCascaded full‐bridges STATCOM

60

• ZS current for balancing

Other cascaded convertersCascaded full‐bridges STATCOM

Whye and delta‐connected variants

• ZS voltage for balancing

61

• Internal ac current for balancing power

• Scalable w.r.t. voltage andcurrent

• Most beneficial at voltage ratios around 0.5.

Other cascaded convertersDC/DC MMCs – unisolated (example)

Norrga, S.; Ängquist, L.; Antonopoulos, A., "The polyphase cascaded-cell DC/DC converter", ECCE 2013 proc.

62

• MMCs connected by transformer on the ac side.

• Possible use in DC grids for voltage adaptation

• Transformer can operate at elevated frequency

Other cascaded convertersDC/DC MMCs – isolated (example)

C Oates, “A methodology for developing chainlink converters”, EPE 2009 proc

63

• Prof. R. Erickson 2001• Full‐bridge submodule strings in arms• Submodule strings see both frequencies• Possibly attractive for low‐speed drives

Other cascaded convertersMatrix MMCs for AC/AC conversion

Ilves, K.; Bessegato, L.; Norrga, S., "Comparison of cascaded multilevel converter topologies for AC/AC conversion," ECCE Asia 2014 Proc.0

Comparison of back-to-back MMC (blue) and Matrix MMC (red)

Cap

acito

r ene

rgy

rippl

e (J

/kVA

)Output frequency (Hz)

64

• Two‐level converters are most competitive for low voltage applications (up to few kV)

• Multilevel converters offer several important benefits – Decoupling of fsw and pulse frequency– More levels in phase voltages– Can avoid direct series connection of semiconductors

• Diode‐clamped converters are competitive for MV applications (several kV) but do not scale well to many levels

• Modular multilevel converters offer scalability, reduce harmonics and avoid direct series connection

Summary

65

• Submodule strings act as controllable voltage sources as long as power balance is maintained

• AC/DC MMCs are voltage source converters towards both acand dc sides

• MMCs require considerably larger capacitive energy storagethan two‐level converters

• For a half‐bridge MMC the ac voltage magnitide can never exceed the dc side voltage.

• Full‐bridges overcome this limitation but imply higher costand losses

Summary, cont’d

Control and Application of Modular for HVDC Transmission ...

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