Post on 01-Jun-2020
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Tutorial—Principles and Applications of Modular Multilevel Converters
Introduction of Modular Multilevel Converter
Mr. Shaoze Zhou
2019-5-16 Lund
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Tutorial—Principles and Applications of Modular Multilevel Converters
Part I- Backgrounds
(15mins)
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Tutorial—Principles and Applications of Modular Multilevel Converters
Global Warming
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Tutorial—Principles and Applications of Modular Multilevel Converters
Source: Key World Energy Statistics from the IEA , 2015
Energy Consumption
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Tutorial—Principles and Applications of Modular Multilevel Converters
Source: REN21, Renewables 2016 Global Status Report
Renewable Energy Alternatives
63
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Tutorial—Principles and Applications of Modular Multilevel Converters
Common issue:
Located at remote places or off-shore, posing challenges for power
transmission techniques.
Renewable Energy Alternatives
Source: ABB
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Tutorial—Principles and Applications of Modular Multilevel Converters
With the increase of power transmission rating, DC outperforms AC.
HVDC versus HVAC: lower active power loss, no reactive power loss
(especially cable used), more economical when distance is long enough,
capable of asynchronous connections, less visual pollution, …
Power Transmission Techniques
Source: ABB
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Tutorial—Principles and Applications of Modular Multilevel Converters
HVDC Power Transmission
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Tutorial—Principles and Applications of Modular Multilevel Converters
Traditional LCC-HVDC: Current Source Converter Technology
Low station loss(0.75%), low cost, large capacity: up to 7200MW/±800kV
Thyristor-based, causes significant harmonics, requires bulky filters, large footprint
Requires a strong AC grid
Requires 50% rating reactive compensation
Specialized transformers
Slow control response
Difficulty with power reversal
Multi-terminals or DC grids are unavailable
HVDC Power Transmission
Source: https://en.wikipedia.org/wiki/HVDC_converter
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Tutorial—Principles and Applications of Modular Multilevel Converters
Offshore HVDC Power Transmission
Submarine AC Cables Overhead Lines
Offshore Wind Farms Urban Consumers
Submarine DC Cables
Voltage Sourced Converters
Voltage Sourced Converters (VSC) based HVDC is more suitable for Offshore
Wind Farms over LCC because:
• Lower filtering requirements, much lighter weight, saving the platform footprint;
• Do not rely on strong AC grid, and can provide reactive power support to wind farms;
• Easy power reversal, Black starting of the offshore wind farms.
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Tutorial—Principles and Applications of Modular Multilevel Converters
L
Udc
ua
ub
uc
Traditional VSC Topologies
IGBT series is required in high voltage (hundreds in HVDC) Two level waveform
Basic Two-level Converter
High voltage capacitors
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Tutorial—Principles and Applications of Modular Multilevel Converters
L
Udc
ua
ub
uc
Traditional VSC Topologies
Three level waveform
Three-level Converters---Neutral Point Clamped(NPC)
High voltage capacitors
IGBT and diode series
Difficult to extend to more than five voltage levels, due to circuit complexity and component counts
Flying capacitor (FC) has the same problem
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Tutorial—Principles and Applications of Modular Multilevel Converters
L
Udc
ua
ub
uc
L
Udc
ua
ub
uc
Main Limitations:
Difficulty in dynamic voltage sharing of series-connected IGBTs and diodes (in ns)
High switching frequency (1~2kHz)
High losses(2~3%)
Severe dv/dt and di/dt
EMI problems
Needs harmonic filters
Needs high voltage capacitors
…
2% power loss means: 0.0943($/kWh)×500(MW)×2%×30(years) ≈ $248million
Traditional VSC Topologies
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Tutorial—Principles and Applications of Modular Multilevel Converters
SMA1SMA2SMAN
SMB1SMB2SMBN
SMC1SMC2SMCN
L
ua
ub
uc
SMA1SMA2SMAN
SMB1SMB2SMBN
SMC1SMC2SMCN
L
ua
ub
uc
Traditional VSC Topologies
Cascaded H-Bridge Converter (CHB)
S2
S1
CSM
Full-bridge SM
S4
S3
Advantages:
‐ Modular structure for cost reduction
‐ Use low-voltage (LV) IGBTs, capacitors
‐ Nearly sinusoidal outputs
‐ Modularity and scalability
Disadvantages:
‐ No dc link available ‐ No active power transfer capability----only
viable for STATCOM applications
S2
S1
CSM
Full-bridge SM
S4
S3
Rectifier
- Add Diode rectifiers in SMs
- Disadvantages caused by phase-shifting transformer :
Add Phase-shifting transformer:
- High-cost, high-complexity
- Large number of cables on transformer secondary side
- Limited power rating
- Not scalable any more
- Only viable for MV Drive applications
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Tutorial—Principles and Applications of Modular Multilevel Converters
SMAl1
SMAlN
SMAl2
SMBl1
SMBlN
SMBl2
SMCl1
SMClN
SMCl2
SMAu1
SMAuN
SMAu2
SMBu1
SMBuN
SMBu2
SMCu1
SMCuN
SMCu2
ua
ub
uc
Udc
Circuit Structure: CHB
Modular Multilevel Converter (MMC)
+ CHB = MMC
Multi-level waveform
Features:
- With DC link terminals, DC/AC or AC/DC
- Use low-voltage (LV) IGBTs, capacitors
- Modular structure, scalable and easy
maintenance
- Low switching frequency (150Hz)
- Low losses (1%)
- Very low di/dt, dv/dt, and EMIs
- Nearly Ideal sinusoidal waveform
- No filters
- Series connection of IGBT is
avoided, capacitor voltage
balancing is much easier (in ms)
S2
S1
CSM
Half-bridge SM
by Prof. Marquardt
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Tutorial—Principles and Applications of Modular Multilevel Converters
Trans bay cable, First MMC-HVDC project, 2010
Very rapid VSC-HVDC development since 2010, more than 40 projects are (or to be)
commissioned.
Most of them are based on MMC
Modular Multilevel Converter (MMC)
Source: www.iea-isgan.org
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Tutorial—Principles and Applications of Modular Multilevel Converters
Part II- Principles and Characteristics of MMC
(20mins)
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Tutorial—Principles and Applications of Modular Multilevel Converters
─ Basic Operating Principle
─ Circuit Analysis
─ Components Dimensioning
Principles and Characteristics of MMC
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Tutorial—Principles and Applications of Modular Multilevel Converters
SM1
SM2
SMN
SM1
SM2
SMN
SM1
SM2
SMN
SM1
SM2
SMN
SM1
SM2
SMN
SM1
SM2
SMN
ua
ub
uc
Udc
L L L
L L L
Basic Operating Principle
Circuit Structure
Common DC link
Three phases
Six arms (upper and lower arms)
Six arm inductors
Each arm consists of N series
connected SMs
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Tutorial—Principles and Applications of Modular Multilevel Converters
SM Type States Terminal voltage
S1 on, S2 off usm=UC
S1 off, S2 on usm=0
S1 on, S2 off
S3 off, S4 on usm=UC
S1 on, S2 off
S3 on, S4 off usm=0
S1 off, S2 on
S3 off, S4 on usm=0
S1 off, S2 on
S3 on, S4 off usm=-UC
SMu1
SMu2
SMuN
ioL
L
SMl1
SMl2
SMlN
Udc
Udc
1
2
Fictitious mid-point RLoad LLoad
uo
iu
il
1
2 uu
ul
S2
S1
CSM
Half-bridge SM
UC
uSM
S2
S1
CSM
Full-bridge SM
S4
S3
UC
uSM
Basic Operating Principle
Sub-Module (SM) Operation Principle
Fig. One phase of MMC
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Tutorial—Principles and Applications of Modular Multilevel Converters
[1] A. Nami, “Modular multilevel converters for HVDC applications: review on converter cells and functionalities,” IEEE Trans. Power Electron., vol. 30, no. 1, pp. 18–36, Jan. 2015.
Sub-Module (SM) Operation Principle
Many other SM structures or hybrids
Half-bridge SM is the most efficient and commonly adopted structure.
Basic Operating Principle
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Tutorial—Principles and Applications of Modular Multilevel Converters
_ cosu ref ou D K t
_ cosl ref ou D K t
Upper and lower arm voltages contain a same dc component and an opposite ac component;
DC bias D, AC amplitude K, K ≤ min[D, 1-D];
Arm Voltage References
SMu1
SMu2
SMuN
ioL
L
SMl1
SMl2
SMlN
Udc
Udc
1
2
Fictitious mid-point RLoad LLoad
uo
iu
il
1
2 uu
ul
Basic Operating Principle
1
0
D
K
Upper-arm Reference
1
0
D
KLower-arm Reference
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Tutorial—Principles and Applications of Modular Multilevel Converters
ic is the dc loop current;
NUC is the aggregated capacitor voltage;
The ac voltages between the upper and lower arms are counteracted; the lumped voltage 2NUC .
DC Loop Analysis
SMu1
SMu2
SMuN
ioL
L
SMl1
SMl2
SMlN
Udc
Udc
1
2
Fictitious mid-point RLoad LLoad
uo
iu
il
1
2 uu
ul
S2
S1
L
L
Udc
NUC
S2
S1
ic
NUC
Duty cycle D
S2
S1
2L
ic
2NUC
Boost Converter
Udc
2dc CU DNU
DC-loop Relationship:
Circuit Analysis
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Tutorial—Principles and Applications of Modular Multilevel Converters
RLoad
LLoad
uo
KNUCcos(ωt)
io
L L
KNUCcos(ωt)
Fictitious mid-point
RLoad
LLoad
uoS2
S1S2
S1
NUC NUC
Udc
1
2Udc
1
2
The dc voltages are counteracted; the aggregated sinusoidal arm voltage is KNUCcosωt .
io is split equally between the upper and lower arms;
AC Loop Analysis
SMu1
SMu2
SMuN
ioL
L
SMl1
SMl2
SMlN
Udc
Udc
1
2
Fictitious mid-point RLoad LLoad
uo
iu
il
1
2 uu
ul
ˆo CU KNU
AC-loop Relationship:
io1/2io1/2
Circuit Analysis
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Tutorial—Principles and Applications of Modular Multilevel Converters
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.5
1.0
Available Gain region
DC bias, D
Vo
lta
ge
Ga
in, G
2dc CU DNU
ˆo CU KNU
Optimal D=0.5
ˆ
2
o
dc
U KG
U D K ≤ min[D, 1-D]
Voltage Gain Analysis
AC phase voltage amplitude is less than half of the DC-link voltage
D should be within [0, 0.5] to ensure voltage gain;
As UC=Udc/2DN, The larger D, the smaller UC, thus the optimal value D is 0.5.
Circuit Analysis
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Tutorial—Principles and Applications of Modular Multilevel Converters
SMu1
SMu2
SMuN
ioL
L
SMl1
SMl2
SMlN
Udc
Udc
1
2
Fictitious mid-point RLoad LLoad
uo
iu
il
1
2 uu
ul
Arm Voltages
2dc CU DNU
D=0.5
dcC
UU
NCapacitor voltage:
Arm voltages:
1 ˆ cos2
1 ˆ cos2
u dc o o
l dc o o
u U U t
u U U t
Define modulation index M (0<M<1):
12
ˆ ˆ2o o
dc dc
U UM
U U
11 cos
2
11 cos
2
u o dc
l o dc
u M t U
u M t U
Circuit Analysis
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Tutorial—Principles and Applications of Modular Multilevel Converters
ic
SMu1
SMu2
SMuN
ioL
L
SMl1
SMl2
SMlN
Udc
Udc
1
2
Fictitious mid-point RLoad LLoad
uo
iu
il
1
2 uu
ul
Arm Currents
Arm currents:
where ic is phase dc current : 1
2c u li i i
1
2
1
2
u c o
l c o
i i i
i i i
Since AC current io is split equally between the upper and lower arms:
Ideally, ˆ coso o oi I t
1
3c dci I
1 1 ˆ cos3 2
1 1 ˆ cos3 2
u dc o o
l dc o o
i I I t
i I I t
Circuit Analysis
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Tutorial—Principles and Applications of Modular Multilevel Converters
Circuit Analysis
Define current ratio H: 1
2
13
ˆ ˆ3
2 o o
dc dc
I IH
I I
SMu1
SMu2
SMuN
ioL
L
SMl1
SMl2
SMlN
Udc
Udc
1
2
Fictitious mid-point RLoad LLoad
uo
iu
il
1
2 uu
ul
Arm Currents
According to power balance between DC and AC sides, (assuming no loss in MMC):
3 ˆ ˆ cos2
dc dc dc ac o oP U I P U I
2
cosH
M
3 cos ˆ4
dc o
MI I
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Tutorial—Principles and Applications of Modular Multilevel Converters
1
1
u u u u
l l l l
u N N N i dtC
u N N N i dtC
2 u l u u u l l l
Nu u N N i dt N N i dt
C
• Arm voltages:
2 2 0 c
u l c
diu u Ri L
dt
Circuit Analysis
Circulating Current Analysis
[1] K. Ilves, “Steady-state analysis of interaction between harmonic components of arm and line quantities of modular multilevel converters,” IEEE Trans. Power Electron., vol. 27, no. 1, pp. 57–68, Jan. 2012.
• Circulating currents are generated due to inner voltage unbalance among each phase of MMC and circulate within the three phases units without affecting the dc and ac side voltages and currents.
• In the following equivalent circuit, each arm of MMC is represented by a voltage source.
L
RRR
LL
RRR
LLLicB icC
ulCulBulA
uuCuuBuuA
• KVL:
1
2c u li i i
11 cos( )
2
11 cos( )
2
u o
l o
N M t
N M t
*Insertion indices:
*where:
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Tutorial—Principles and Applications of Modular Multilevel Converters
• There are only even-order harmonics in the circulating current, no odd harmonics in the arm currents as the solution to (m2) are zero.
• The dominated circulating current harmonic is second–order:
2
4
6
1
3
5
22 2
4 4 4 4
6 6 66
11 1
3 3 3 3
5 5 55
ˆ
ˆ 0
0ˆ
ˆ 0
ˆ 0
0ˆ
j
j
j
j
j
j
i ev z r
x v z i e
x v z i e
i ev z
x v z i e
x v z i e
(m1)
(m2) 2
o2
2 2
3ˆ( )8 6
2 6 4( 4 2 )
12
o
j dc
j to o
oo
M IMj I e
i Re eC M
j L R jN
Circuit Analysis
Circulating Current Analysis
• Solution to the equation:
• These circulating current harmonics will increase the arm current amplitude and cause higher power losses. Therefore, methods must be taken to suppress these harmonics (will be presented in Part III).
2 2 2 2 2
2
2( 1) 2, ( 2 2 ),
4( 1) 4( 1)2 ( 1)n n n
o o
M n n M C Mx j v j jn L R z j
n N nn n
*
23ˆ( )8 6
j dco
o
M IMr j I e
1
c n
n
i i
[1] K. Ilves, “Steady-state analysis of interaction between harmonic components of arm and line quantities of modular multilevel converters,” IEEE Trans. Power Electron., vol. 27, no. 1, pp. 57–68, Jan. 2012.
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Tutorial—Principles and Applications of Modular Multilevel Converters
In essence, the average arm voltage and arm current of two-level VSC is basically similar to MMC.
Average arm current in traditional two-level VSC also contains 2nd order harmonic.
1.235 1.24 1.245 1.25 1.255 1.26 1.265 1.27
Time
0
5
10104
Arm Current of MMC with 100 Sub-Modules
Volt
age /
(V
)C
urr
en
t /
(kA
)
-4
-2
0
2
4 Arm Current
Average
1.165 1.17 1.175 1.18 1.185 1.19 1.195 1.2
Time
0
5
10
104
Arm Voltage
Average
Arm Voltage of Conventional VSC
Arm Current of Conventional VSC
Volt
age /
(V
)C
urr
en
t /
(kA
)
-4
-2
0
2
4
Arm Voltage of MMC with 100 Sub-Modules
Comparison between Two-level VSC and MMC
Circuit Analysis
ˆ cos cos u oi I t M t
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Tutorial—Principles and Applications of Modular Multilevel Converters
Limit rate of rise of the arm current when DC side short-circuit:
Components Dimensioning
Device Voltage:
The SM capacitor voltage Udc/N, with additional margin for voltage ripple of capacitors, and di/dt during switching.
Device Current:
The arm current amplitude is: 1 1 ˆ3 2
peak dc oI I I
The rms arm current is: 2 21 1 ˆ
9 4 rms dc oI I I
Arm inductors:
[1] C. Oates, “Modular multilevel converter design for VSC HVDC applications,” IEEE J. Emerging Sel. Topics Power Electron., vol. 3, no. 2, pp. 505–515, 2015.
0 max 0
ˆ
( )
ac
ac
UL L
I I
• Lδ is the transformer leakage inductance;
• Iac-max is the maximum allowed fault current;
• I0 is the current flowing through the inductors at the instant the fault occurs.
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Tutorial—Principles and Applications of Modular Multilevel Converters
2( )
Δ
2 Z
SM
C avg
WC
U
1 1( ) ( ) ( ) [1- cos( )] [1 cos( )]
2 3u u u dc o dc op t u t i t U M t I H t
1
1( , ) arccos( )x K
K
2
1( , ) 2 arccos( )x K
K
2 3/22 cos[1 ( ) ]
3 cos 2Zo
S MW
MN
2
1
( ) Z
x
uxW p t dt
pu(t) has two zero points (x1 and x2) in a cycle, so maximum ΔWZ can be derived as:
uu(t) iu(t)
pu(t)
ωot x1 x2ZW
ZW
Components Dimensioning
Required SM capacitance:
Capacitance is dimensioned to keep the capacitor voltage fluctuation within reasonable limits.
SM capacitance:
The energy variation on capacitors can be calculated by integration of power in each arm:
ε represents the voltage ripple percentage.
o
3 ˆ ˆ=2
oS U I*
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Tutorial—Principles and Applications of Modular Multilevel Converters
Phase-B Phase-C Phase-A Contactors & Inductors
Controller
Components Dimensioning
MMC laboratory prototype example, 3kV/1MW, 36 SMs in total
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Tutorial—Principles and Applications of Modular Multilevel Converters
Phase-B Phase-C Phase-A Controller Contactors & Inductors
Components Dimensioning
MMC laboratory prototype example, 3kV/1MW, 36 SMs in total
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Tutorial—Principles and Applications of Modular Multilevel Converters
Structure of the control system
Control and Communication:
• DSP (TMS320F28335) for reference generation; FPGA (EP3C25Q240C8) for
pulse-width modulation and communication with the submodules by optic-
fibers.
Picture of the control board
Components Dimensioning
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Tutorial—Principles and Applications of Modular Multilevel Converters
Exterior of a sub-module
Components Dimensioning
Submodule Structure:
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Tutorial—Principles and Applications of Modular Multilevel Converters
Circuits in a sub-module
Components Dimensioning
Submodule Structure:
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Tutorial—Principles and Applications of Modular Multilevel Converters
DC-busbar
Storage capacitors
Auxiliary power supply IGBT modules
Bleeding resistor
Heat sink
Unit board
Components Dimensioning
Submodule Components:
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Tutorial—Principles and Applications of Modular Multilevel Converters
Thank you for your attention!