control and application of modular for hvdc transmission
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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
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Two‐level VSCBasic operation
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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
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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
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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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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.
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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
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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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• 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
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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
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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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• 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
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Flying capacitor convertersOperation
Switching states of one phase leg of a three‐level flying capacitor converter
35
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 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
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• 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
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• 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
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
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
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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• 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
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• 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
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• 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
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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
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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
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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
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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
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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
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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
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-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
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
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