HVDC Transmission Systems Based on Modular Multilevel
Converters
PSERC WebinarFebruary 3, 2015
Maryam SaeedifardGeorgia Institute of Technology
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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