HVDC Transmission Systems Based on Modular Multilevel

Converters

PSERC WebinarFebruary 3, 2015

Maryam SaeedifardGeorgia Institute of Technology

(maryam@ece.gatech.edu)

Presentation Outline

• Introduction to HVDC Transmission Systems

• Converter Requirements for HVDC Transmission Systems

• The Modular Multilevel Converter (MMC)- Features- Operational Challenges- Solutions

• Future Work

2

Introduction: AC Corridor’sPower Flow Control

Boost or control ac voltage (V)

Reduce line reactance (X)

Regulate phase angle (δ)

Ref: ABB 3

Introduction: DC Corridor’s Power Flow Control

HVDC: High Voltage Direct Current Transmission

P =P = VDC. IDC

4

Introduction: AC vs DC Transmission

× Loading a function of Z

× Charging current a function of voltage level and cable capacitance

× Distance limitation

× 3 cables

Power flow controlled

No charging current effect or need for shunt compensation

No distance limitation

2 cables

AC Transmission DC Transmission

5

Introduction: AC vs DC Transmission

• Due to reactive power charging, AC transfer capacity is dramatically reduced with distance

• DC transfer capacity is almost independent of distance

Ref: ABB 6

Introduction: Overhead Line Transmission Investment vs Cost

Ref: ABB 7

Introduction: Types of HVDC Systems

• Point-to-Point Systems- Overhead lines- Subsea or underground cables

• Back-to-Back Systems- Interconnection of asynchronous AC grids

HVDC Converter Station HVDC Converter Station

8

Introduction: Basics of HVDC Systems

Ref: ABB 9

HVDC Technology: Converter Requirements

Shortcomings:× Harmonic distortion× Switching frequency and power

losses

Ref: ABB 10

HVDC Technology: Converter Requirements

Staircase voltage waveform ==> Reduced harmonic distortion and filtering size

Low switching frequency ==> High efficiency

AC grid

+

_

𝑉𝑉𝑑𝑑𝑑𝑑2

+

_

𝑉𝑉𝑑𝑑𝑑𝑑2

11

The MMC

Features: Modular and scalable design

Smooth and sinusoidal waveform Increased reliability and redundancy

Challenges:× SM capacitor voltage balancing

× Circulating currents

Small Number of Sub-Modules

Large Number of Sub-Modules

SubModule (SM)

12

Equivalent Circuit of an MMC

SM2

SMn

SM1

SM2

SMn

SM1

SM2

SMn

SM1

SM2

SMn

SM1

SM2

SMn

SM1

SM2

SMn

SM1

−

upai

lowai

oL

+

-

upav

lowav

+

-

+

dcv

oL

oL

oL

oL

oL

ai

bi

ci

avbvcv

dci

S1

S2

D1

D2

C

SM

+

−Cv+

−SMV

13

SM Capacitor Voltage Balancing

SM2

SMn

SM1

SM2

SMn

SM1

SM2

SMn

SM1

SM2

SMn

SM1

SM2

SMn

SM1

SM2

SMn

SM1

−

upai

lowai

oL

+

-

upav

lowav

+

-

+

dcv

oL

oL

oL

oL

oL

ai

bi

ci

avbvcv

dci

S1

S2

D1

D2

C

SM

+

−Cv+

−SMV

14

SM Capacitor Voltage Balancing

Example: Five-Level MMC

15

Circulating Current Control

High circulating current: Rating value/size of components SM capacitor voltage ripple Power losses

16

Circulating Current Control

• Circulating current – contains 2nd harmonic predominantly

• Controllers to eliminate circulating current:• Proportional Resonant (PR) Controller• Predictive Circulating Current Controller

17

Circulating Current Control: PR Controller

• Circulating current dynamics:

• PR Controller:

• ωn1 and ωn2 are tuned to 2nd and 4th harmonic.

22

22

21

21

1n

i

n

ip s

sKs

sKKωω +

++

+

dc,z,z,z,z

dtd

VmviRi

L abcabcabcoabc

o ≈=+

18

Circulating Current Control: PR Controller

ref,,z abci

SM1

21

PWM Generator-1

PWM Generator-2

21

Phase-a

Phase-b

Phase-c

edi ref,

edi

edv

eqi ref,

eqi

eqv

ede iL ˆ

eqω

qe abcm

eθ

sKK i

p +

sKK i

p +

edu

equ

dc

2V

eqm

edm

eqe iL ˆ

eqω

abcqd →0

00 =em

dc

2V

22

2i2

21

2i1

p1nn s

Ks

KKωω +

++

+abcpi ,

a b cni , 21 abci ,z

abci

abcpi ,

abcni ,

Hardware

210qdabc →

abcpi ,

a b cni ,

Software

eθ SM2

SMn

SM1

SM2

SMn

Ac-side current controller

Circulating current controller

SM Capacitor Voltage Balancing

abcpv ,1,,cabcpv ,2,,c

abcpv ,n,,c

SM Capacitor Voltage Balancing

abcpi ,

abcnv ,1,,c

abcnv ,n,,c

abcnv ,2,,c

abcni ,

dc

1V

abczv , abczm ,

Upper-arm switching

signals

Lower-arm switching

signals

Measured upper-arm

SM Capacitor Voltages

Measured lower-arm

SM Capacitor Voltages

Measured arm currents

SMi

i-th Sub-module

Si1

~Si1ivcC VS

19

Circulating Current Control: PR Controller

20

Circulating Current Control: Predictive Current Controller

From KVL:

,2

upadc aupa a sa

diV div l Ri L vdt dt

− = + + +

saa

alowa

lowadc v

dtdiLRi

dtdilvV

−−−=−2

Discrete model of the ac-side phase current:

++−

+−+=+ )(')1(

2)1()1(

'1)1( ki

TLkv

kvkvK

ki as

saupalowa

a

LlL +=2

' RTLK

s

+=''

Discrete model for capacitor voltages:

sl

cijcij TCkikVkV )()()1( +=+

( )( 1) ( 1) ( 1) ( )2

sz dc lowa upa z

Ti k V v k v k i kl

+ = − + − + +

Discrete model of the ac-side phase current:

Discrete model for circulating current and SM capacitor voltages:

21

Prediction based on cost function minimization:

Predictive Circulating Current Control of MMC

SM2

SMn

SM1

SM2

SMn

SM1

SM2

SMn

SM1

SM2

SMn

SM1

SM2

SMn

SM1

SM2

SMn

SM1

−

upai

lowai

oL

+

-

upav

lowav

+

-

+

dcv

oL

oL

oL

oL

oL

ai

bi

ci

avbvcv

dci

S1

S2

D1

D2

C

SM

+

−Cv+

−SMV

dccij z zj

i

VJ V in

λ λ = − +

∑

22

Closed-Loop Control of MMC-HVDC

MM

C-1

MM

C-2

Transmission Line

23

Predictive Control of MMC-HVDC

24

DC-Side Fault in MMC-HVDC Systems

25

SM Technologies: Normal Operation

Full-Bridge SM Clamp-Double SM

26

Full-Bridge SM

SM Technologies: DC-side Short-Circuit Fault Operation

Clamp-Double SM

27

DC-side Short-Circuit Fault Operation of Full-Bridge MMC

28

DC-Side Fault in MMC-HVDC Systems:Full-Bridge MMC Case

29

Power Losses for Various SM Circuits

Power Losses of Single SM-type MMCs Normalized

with Respect to Half-Bridge

MMC

30

Hybrid Design of MMC-HVDC Systems

31

DC-Side Fault in MMC-HVDC Systems:Hybrid MMC Case

32

Power Losses for Various Hybrid MMCs

Power Losses of Hybrid MMCs

Normalized with Respect to Half-Bridge

MMC

33

Future Work

• Control and protection of multi-terminal HVDC systems based on the MMC

• Accurate and efficient modeling and simulation tools for MMC-HVDC systems

• Operation of the MMC-HVDC systems under fault conditions

34

Acknowledgement

• This presentation contains data and graphs from ABB publications/presentations available on the public domain including:

• Mats Larsson, Corporate Research, ABB Switzerland Ltd, “HVDC and HVDC Light: An alternative power transmission system”,Symposium on Control & Modeling of Alternative Energy Systems, April 2, 2009.

• Gunnar Persson, Senior Project Manager,Power Systems - HVDC, ABB AB Sweden, “HVDC Converter Operations and Performance, Classic and VSC”, Dhaka, September 18, 2011.

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