Introduction of Modular Multilevel Converter of MMC.pdf · -6- Tutorial—Principles and Applications of Modular Multilevel Converters Common issue: Located at remote places or off-shore,

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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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Part I- Backgrounds

(15mins)

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Global Warming

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Source: Key World Energy Statistics from the IEA , 2015

Energy Consumption

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Source: REN21, Renewables 2016 Global Status Report

Renewable Energy Alternatives

63

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Common issue:

Located at remote places or off-shore, posing challenges for power

transmission techniques.

Renewable Energy Alternatives

Source: ABB

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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

-8-


HVDC Power Transmission

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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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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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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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L

Udc

ua

ub

uc


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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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


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SMA1SMA2SMAN

SMB1SMB2SMBN

SMC1SMC2SMCN

L

ua

ub

uc

SMA1SMA2SMAN

SMB1SMB2SMBN

SMC1SMC2SMCN

L

ua

ub

uc


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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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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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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Part II- Principles and Characteristics of MMC

(20mins)

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─ Basic Operating Principle

─ Circuit Analysis

─ Components Dimensioning

Principles and Characteristics of MMC

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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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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


Sub-Module (SM) Operation Principle

Fig. One phase of MMC

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[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.


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_ 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


uo

iu

il

1

2 uu

ul


1

0

D

K

Upper-arm Reference

1

0

D

KLower-arm Reference

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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


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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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


uo

iu

il

1

2 uu

ul

ˆo CU KNU

AC-loop Relationship:

io1/2io1/2

Circuit Analysis

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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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SMu1

SMu2

SMuN

ioL

L

SMl1

SMl2

SMlN

Udc

Udc

1

2


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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ic

SMu1

SMu2

SMuN

ioL

L

SMl1

SMl2

SMlN

Udc

Udc

1

2


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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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


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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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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• 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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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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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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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


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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Phase-B Phase-C Phase-A Contactors & Inductors

Controller


MMC laboratory prototype example, 3kV/1MW, 36 SMs in total

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Phase-B Phase-C Phase-A Controller Contactors & Inductors


MMC laboratory prototype example, 3kV/1MW, 36 SMs in total

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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


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Exterior of a sub-module


Submodule Structure：

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Circuits in a sub-module


Submodule Structure：

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DC-busbar

Storage capacitors

Auxiliary power supply IGBT modules

Bleeding resistor

Heat sink

Unit board


Submodule Components：

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Thank you for your attention!

Documents

Introduction of Modular Multilevel Converter of MMC.pdf · -6- Tutorial—Principles and Applications of Modular Multilevel Converters Common issue: Located at remote places or off-shore,