Voltage-Source Converters for HVDC Systems - CT...
-
Upload
vuongthuan -
Category
Documents
-
view
223 -
download
1
Transcript of Voltage-Source Converters for HVDC Systems - CT...
1
Voltage-Source Converters for HVDC Systems
Josep Pou(1)(2)
(1) The University of New South WalesAustralian Energy Research Institute (AERI),School of Electrical Engineering and Telecommunications, Australia
(2) Technical University of Catalonia, Barcelona TechTerrassa Industrial Electronics Group (TIEG),Department of Electronic Engineering, Catalonia, Spain
(3) TECNALIAEnergy Unit, Basque Country, Spain
Co-authors: Salvador Ceballos(3), Georgios Konstantinou(1),Vassilios G. Agelidis(1) and Jordi Zaragoza(2)
Technical University of Catalonia, Barcelona TechTerrassa Industrial Electronics Group (TIEG),
Department of Electronic Engineering, Catalonia, Spain
Technical University of CataloniaCampus of Terrassa
2
The University of New South WalesAustralian Energy Research Institute (AERI),
School of Electrical Eng. and Telecommunications, Australia
The University of New South WalesAustralian Energy Research
Institute (AERI)
Outline
Page 4 COBEP 2013, Gramado, Brazil
I. HVDC Systems
II. Voltage-Source Converter (VSC)
III. Neutral-Point-Clamped (NPC) Converter
IV. Modular-Multilevel Converter (MMC)
3
Page 5 COBEP 2013, Gramado, Brazil
I. HVDC Systems
II. Voltage-Source Converter (VSC)
III. Neutral-Point-Clamped (NPC) Converter
IV. Modular-Multilevel Converter (MMC)
Outline
AC transmission is impractical over long distances, while for DC transmission distance is not a significant restriction.
Although rectifier/inverter stations costs and losses are higher, from long distances HVDC transmission results in lower costs and losses than AC transmission.
HVDC can interconnect asynchronous systems and systems with different frequencies.
HVDC can be controlled faster so that the AC system stability can be improved.
I. HVDC Systems
Reasons for High-Voltage Direct Current (HVDC)
I. HVDC Systems
Page 6 COBEP 2013, Gramado, Brazil
4
HVDC Applications
Long distance transmission from remote energy sources.
Interconnection between power systems.
High power underground (submarine) distribution system feeders.
Reconfiguration of old AC lines to DC to increase capacity, e.g. a double circuit 230 kV AC line, now limited to 400 MW transmission capacity can be used for a DC line with capacity of about 1500 MW.
I. HVDC Systems
Page 7 COBEP 2013, Gramado, Brazil
First HVDC Transmission Line
1954: Between Sweden and the island of Gotland in the Baltic sea, 20 MW @ 100 kV using mercury arc valves and control equipment was based on vacuum tubes (ASEA).
I. HVDC Systems
Page 8 COBEP 2013, Gramado, Brazil
5
Conventional HVDC System
Transformer
AC System2
AC System1
AC Filters AC Filters
Transformer
Converter 1Rectifier
Converter 2Inverter
DC Line
TimeTimeTime
Current Current Current
Sending End Receiving End
I. HVDC Systems
Page 9 COBEP 2013, Gramado, Brazil
Based on thyristors - Line-commutated converters (LCCs)
I. HVDC Systems
Page 10 COBEP 2013, Gramado, Brazil
Typical Cost of an HVDC Project
Converter Transformer
16%
AC filters10%
Control7%
Other10%
Erection -Commisioning
8%
Engineering10%
Freight, Insurance
5%
Civil Works, Building
14%
Valves20%
6
Investment Cost vs. Distance
I. HVDC Systems
Page 11 COBEP 2013, Gramado, Brazil
HVDC Schemes
Depending upon the function and location of the converter stations, various schemes and configurations of HVDC systems exist as follows:
Back-to-Back
Monopolar
Bipolar
Multiterminal
I. HVDC Systems
Page 12 COBEP 2013, Gramado, Brazil
7
Back-to-Back HVDC Scheme
AC 1 AC 2
DCFilters
ACFilters
ACFilters
I. HVDC Systems
Page 13 COBEP 2013, Gramado, Brazil
Monopolar HVDC Scheme
I. HVDC Systems
Page 14 COBEP 2013, Gramado, Brazil
AC 1 AC 2
DCFilters
ACFilters
ACFilters
DC Line
8
Bipolar HVDC Scheme
DCFilters
DC Line
AC 1
DCFilters
ACFilters
DC Line
AC 2
ACFilters
I. HVDC Systems
Page 15 COBEP 2013, Gramado, Brazil
Multiterminal HVDC Scheme
AC 1
ACFilters
AC 2
ACFilters
AC 3
ACFilters
Converter1
Converter2
Converter3
I. HVDC Systems
Page 16 COBEP 2013, Gramado, Brazil
9
Bipolar with 24-Pulse Converter HVDC
DCFilters
DC Line
AC 1
DCFilters
ACFilters
DC Line
AC 2
ACFilters
I. HVDC Systems
Page 17 COBEP 2013, Gramado, Brazil
Old Thyristor Valve 50 kV, 200 A
Suspended Thyristor Valve 150 kV, 914 A
Co
urt
esy
of
AB
BI. HVDC Systems
Page 18 COBEP 2013, Gramado, Brazil
10
Light triggerable thyristors (LTT) with integrated protection functions and blocking voltages between 5.2 kV to 8 kV
2” and 5” LTT with Light Guide
Surge current capabilities up to 100 kAfor single sine half waves
I. HVDC Systems
Page 19 COBEP 2013, Gramado, Brazil
I. HVDC Systems
Page 20 COBEP 2013, Gramado, Brazil
New Zealand Inter-Island Link (ABB)
Connection between the south and the north islands.
Pole 2 and 3 combined can transfer up to 1200 MW.
610 km between Canterbury (south island) and Lower Hutt (north island).
11
New Zealand Inter-Island Link (ABB)
Valve Hall
I. HVDC Systems
Page 21 COBEP 2013, Gramado, Brazil
I. HVDC Systems
Page 22 COBEP 2013, Gramado, Brazil
±500kV Longquan-Zhengping HVDC Project, China (ABB)
940 kilometers Commercial operation in 2004 Power rating 3000 MW
12
Chandrapur Back-to-Back HVDC Link, India (Alstom)
Two independent poles, each with a nominal power transmission rating of 500 MW.
One quadrivalve is approximately 3.8 x 3.8 x 6.2 m and weighs 14 tonnes.
I. HVDC Systems
Page 23 COBEP 2013, Gramado, Brazil
HVDC Itaipu, Brasil (ABB)
Transmit power generated at 50 Hz from the Paraguay side of the ItaipuDam to São Paulo (aprox. 800 km).
When completed in 1985, it became the world's largest HVDC system.
±600 kV bipoles, each with a rated power of 3150 MW.
I. HVDC Systems
Page 24 COBEP 2013, Gramado, Brazil
13
Foz do Iguaçu converter station
HVDC Itaipu, Brasil (ABB)
I. HVDC Systems
Page 25 COBEP 2013, Gramado, Brazil
HVDC Transformer
I. HVDC Systems
Page 26 COBEP 2013, Gramado, Brazil
14
Discussion on Using Thyristors (LCC Technology)
I. HVDC Systems
Page 27 COBEP 2013, Gramado, Brazil
LCC technology requires existing AC grids for proper commutation of the thyristors. Auxiliary diesel generators may be needed at the generation side of the transmission line.
Reactive power compensation should be provided.
The size of the thyristor valve hall and auxiliary filters, and the need of auxiliary generation systems make HVDC LCC stations bulky and difficult to install in offshore platforms.
Reversing the flux of power requires changing the DC polarity.
Outline
I. HVDC Systems
II. Voltage-Source Converter (VSC)
III. Neutral-Point-Clamped (NPC) Converter
IV. Modular-Multilevel Converter (MMC)
Page 28 COBEP 2013, Gramado, Brazil
15
II. Voltage-Source Converter (VSC)
II. Voltage-Source Converter (VSC)
Page 29 COBEP 2013, Gramado, Brazil
Three-phase two-level voltage source converter (VSC)
Vdc
bc
a
+
-
S1 S2 S3
S1 S2 S3
O
Vdc
2
Vdc
2 -
+
+
-
Vdcbc
a
+
-
S1
S2
S3
1
0 1
01
0
O
Vdc
2
Vdc
2 -
+
+
-
2L VSCSwitches
S1 S2 S3
Sw
itchi
ng S
tate
s
ST0 0 0 0
ST1 1 0 0
ST2 1 1 0
ST3 0 1 0
ST4 0 1 1
ST5 0 0 1
ST6 1 0 1
ST7 1 1 1
Three-Phase Two-Level VSC
II. Voltage-Source Converter (VSC)
Page 30 COBEP 2013, Gramado, Brazil
16
2L VSCSwitches
S1 S2 S3
Sw
itchi
ng S
tate
s
ST0 0 0 0
ST1 1 0 0
ST2 1 1 0
ST3 0 1 0
ST4 0 1 1
ST5 0 0 1
ST6 1 0 1
ST7 1 1 1
Vdcbc
a
+
-
S1
S2
S3
1
0 1
0 1
0
O
Vdc
2
Vdc
2 -
+
+
-
Three-Phase Two-Level VSC
II. Voltage-Source Converter (VSC)
Page 31 COBEP 2013, Gramado, Brazil
II. Voltage-Source Converter (VSC)
Page 32 COBEP 2013, Gramado, Brazil
0 st
Vdc/2
-Vdc/2
VSM
VTM
vs modulation signal vT carrier
0 st
v (output voltage)
Modulation signal:
vs(t)=VSMsinst
Output voltage:
If vsvT sa1 ON sa2 OFFv=va0=Vdc/2
If vs<vTsa1 OFF sa2 ONv=va0=Vdc/2
Modulation signal
+-
+-
2dcv
)0(dcv
C2
C2
a1s
a2s
2dcv
Ci
0avv )(a
Pulse-Width Modulation (PWM)
17
Series-Connected Semiconductors
Typical IGBT ratings
II. Voltage-Source Converter (VSC)
Page 33 COBEP 2013, Gramado, Brazil
Operation of the converter beyond the voltage rating of existing semiconductor devices requires additional devices to be connected in series.
Vdc
+
-
il
Three-Phase Two-Level VSC
The voltages across the power devices have to be limited.
The quality of the waveform is poor (only two levels).
II. Voltage-Source Converter (VSC)
Page 34 COBEP 2013, Gramado, Brazil
18
Series Connected Semiconductors
Vdc bc
a
+
-
S1 S2 S3
S1 S2 S3
O
II. Voltage-Source Converter (VSC)
Page 35 COBEP 2013, Gramado, Brazil
Challenge: Voltage distribution across the series-connected semiconductors.
Vdc
+
-
RC
il Vdc
+
-
C
il
R
R
R
R
Passive snubbers Regenerative snubbers
II. Voltage-Source Converter (VSC)
Page 36 COBEP 2013, Gramado, Brazil
Series Connected Semiconductors
19
An active overvoltage clamping scheme can be implemented to limit the collector–emitter voltage during switching transients.
It provides protection of the device from overvoltage at the expense of device switching losses.
Active Clamping
Rg
C
E
Control Circuit
Rg
C
E
ilVmax
Vmax
II. Voltage-Source Converter (VSC)
Page 37 COBEP 2013, Gramado, Brazil
II. Voltage-Source Converter (VSC)
Page 38 COBEP 2013, Gramado, Brazil
VSC-Based HVDC Transmission
Real Power
Reactive Power
AC 2Reactive Power
AC 1
20
Active-Reactive (PQ) Locus Diagram of Power Transmission System
VSC (Transistors)LCC (Thyristors)
II. Voltage-Source Converter (VSC)
Page 39 COBEP 2013, Gramado, Brazil
VSC HVDC Systems - Advantages
Can operate when the grid voltages are reduced or distorted.
No need for generators, which makes it suitable for weak networks and long distances.
Low order harmonics are greatly reduced and harmonic filters can be smaller.
No reactive power compensation is required. Real and reactive power independently controlled.
Faster response owing to the increased switching frequency of the PWM.
Reversing of power is achieved by changing the direction of the DC current (while keeping the voltage polarity).
II. Voltage-Source Converter (VSC)
Page 40 COBEP 2013, Gramado, Brazil
21
Four-terminal PWM VSC-based HVDC system forwind turbines/wind parks
II. Voltage-Source Converter (VSC)
Page 41 COBEP 2013, Gramado, Brazil
Multi-Terminal HVDC Systems
Five-Terminal VSC-HVDC System
II. Voltage-Source Converter (VSC)
Page 42 COBEP 2013, Gramado, Brazil
22
Triple extruded polymer insulation system (XLPE)
Same polarity (can be used in VSC-based HVDC systems)
DC Cable Technology
II. Voltage-Source Converter (VSC)
Page 43 COBEP 2013, Gramado, Brazil
Directlink - A Power Trading Machine, Australia
Technical DataCommissioning year: 2000Power rating: 180 MW (3 x 60)AC Voltage: 132/110 kVDC Voltage: ± 80 kVDC current 342 ALength of DC cable: 6 x 59 km
Main reasons for choosing HVDC system: Controlled asynchronous connection for trading. Easy to get permission for underground cables.
II. Voltage-Source Converter (VSC)
Page 44 COBEP 2013, Gramado, Brazil
23
Directlink
Bungalora converter station
II. Voltage-Source Converter (VSC)
Page 45 COBEP 2013, Gramado, Brazil
Troll A HVDC Light® link
The HVDC Light® installation between the Norwegian main land and the Troll A oil platform consists of two circuits, each feeding a 40 MW compressor motor
Technical DataCommissioning year:2004/2005Power rating: 2 x 42 MWAC Voltage: 132 kV/56 kV DC Voltage: ± 60 kVDC current 350 ALength of DC cable: 4 x 70 km
Main reasons for choosing HVDC Light® system:Environmental improvement by elimination of gas turbines on platform. Low weight and small space on platform. Ability to feed and black-start motors, without local generation.
II. Voltage-Source Converter (VSC)
Page 46 COBEP 2013, Gramado, Brazil
24
MotorFormer4-pole, 40MW0 - 65 Hz
56kV
Troll A
SM
70 km+/- 60kV
HVDC Light
138kV
Kollsnes
~
=
=
~
Troll A HVDC Light® link
II. Voltage-Source Converter (VSC)
Page 47 COBEP 2013, Gramado, Brazil
Troll A, Converters on the Platform
• The converters are housed in a pre-fabricated module, shipped and lifted onto the platform.
II. Voltage-Source Converter (VSC)
Page 48 COBEP 2013, Gramado, Brazil
25
Outline
Page 49 COBEP 2013, Gramado, Brazil
I. HVDC Systems
II. Voltage-Source Converter (VSC)
III. Neutral-Point-Clamped (NPC) Converter
IV. Modular-Multilevel Converters (MMCs)
IV. Neutral-Point-Clamped (NPC) Converter
III. NPC Converter
Page 50 COBEP 2013, Gramado, Brazil
Split DC-link with capacitors and clamping diodes to generate three voltage levels.
Voltage across the capacitors should equal half the DC-link voltage.
Vdc/2
Vdc/2
Vdc
+
-
Oio
bc
a
+
-
-
+
26
The NPC can provide three voltage levels at the outputs.
Higher quality of the output voltages compared to the two-level case imply:
(1) smaller reactive components needed to filter, and
(2) lower switching frequency and power losses.
Still series connection of power devices is needed for HVDC applications.
The NPC Converter
III. NPC Converter
Page 51 COBEP 2013, Gramado, Brazil
Vdc/2
0
-Vdc/2
a
Vdc/2
Vdc/2
Vdc
+
-
S1
S2
S3
S4
Pos.
Neg.
0
Three-Level Voltage Waveforms
S1 S2 S3 S4 VaO
Pos. 1 1 0 0 +Vdc/2
0 0 1 1 0 0
Neg. 0 0 1 1 -Vdc/2
III. NPC Converter
Page 52 COBEP 2013, Gramado, Brazil
27
Pulse-Width Modulation (PWM) Strategies
temps/T 0 0.005 0.01 0.015 0.02
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
va0
vb0
vc0
-1-11
-101
-11-1
-110
-111
0-11
01-1
1-1-1
1-10
1-11
10-1
11-1
11000-1
100 0-1-1
011-100
001 -1-10
1010-10
111 000
-1-1-1
010 -10-1
Region 1
Region 2
Region 3
Region 4
Sector 1
Sector 2
Sector 3
Sector 4
Sector 5
Sector 6
REFV
Sector 1
Sector 2
Sector 3
Sector 4
Sector 5
Sector 6
Carrier-based PWM (CB-PWM) Space-vector PWM (SV-PWM)
III. NPC Converter
Page 53 COBEP 2013, Gramado, Brazil
Pulse-Width Modulation (PWM)
The line-to-line voltages can produce up to five levels.
III. NPC Converter
Page 54 COBEP 2013, Gramado, Brazil
Line-to-line output voltage (vab), output currents (ia, ib and ic), and capacitor voltages (vC1 and vC2)
28
Vd/4
Vd
Vd/4
Vd/4
Vd/4
O a
S1
S2
S3
S4
S1 S2 S3 S4 VaO
State 1 1 1 1 1 +Vdc/2
State 2 0 1 1 1 +Vdc/4
State 3 0 0 1 1 0
State 4 0 0 0 1 -Vdc/4
State 5 0 0 0 0 -Vdc/2
Extension of the NPC Concept
Extension to higher number of levels. Multiple capacitors in the DC-link provide multiple “neutral points”.
Five-level NPC converter
III. NPC Converter
Page 55 COBEP 2013, Gramado, Brazil
Vdc
a b c
Generalized n-Level NPC Converter Model
III. NPC Converter
Page 56 COBEP 2013, Gramado, Brazil
29
Voltage balancing cannot be always achieved and depends on the operating point and load connected to the converter.
The clamping diodes on the different NPs support different reverse voltages.
The central switches of the topology conduct current longer than the outer switches resulting in unbalanced distribution of the losses.
Issues with High Order (n>3) NPC Converters
III. NPC Converter
Page 57 COBEP 2013, Gramado, Brazil
0 10 20 30 40 50 60-400
-200
0
200
400
600
800 vC1
vC2
vC3
ia ib ic
Time (ms)
vab /3
0 10 20 30 40 50 60 -400
-200
0
200
400
600
800
Time (ms)
vC1
vC2
vC3
ia ib ic
vab /3
m=0.4, Unity Power factor m=0.6, Unity Power Factor
Voltage balancing might not be always achieved for large modulation indices and high power factors.
Example: four-level NPC converter
Capacitors Voltage Balance
III. NPC Converter
Page 58 COBEP 2013, Gramado, Brazil
30
The NPC converter produces unequal distribution of losses among the semiconductors.
A solution to overcome these problems is derived from the concept of active neutral-point clamping.
Clamping diodes on the neutral-point are replaced with active switches.
Zero voltage states and commutations can be selected actively and are not defined by the direction of the phase current.
Active NPC (ANPC) Converter
III. NPC Converter
Page 59 COBEP 2013, Gramado, Brazil
Vdc/2
Vdc/2
Vdc
+
-
Oio
bc
aActive Switches
Three-Level ANPC Converter
III. NPC Converter
Page 60 COBEP 2013, Gramado, Brazil
31
S1 S2 S3 S4 S5 S6
Pos. 1 1 0 0 0 1 +Vdc/2
0 0 1 0 0 1 0 0
0 0 1 0 1 1 0 0
0 1 0 1 0 0 1 0
0 0 0 1 0 0 1 0
Neg. 0 0 1 1 1 0 -Vdc/2a
Vdc/2
Vdc/2
Vdc
+
-
S1
S2
S3
S4
S5
S6
Three-Level ANPC Converter
In the case of NP connection, the state of the switches S5 and S6 will define the path of the current.
III. NPC Converter
Page 61 COBEP 2013, Gramado, Brazil
The commutations to or from the zero states determine the distribution of the switching losses. All commutations take place between one active switch and one diode.
Even if more than two devices turn on or off, only one active switch and one diode essentially experience switching losses.
Three-Level ANPC Converter
III. NPC Converter
Page 62 COBEP 2013, Gramado, Brazil
32
Losses distribution
T1
T2
D1
D2
D3
Uneven loss distribution between the switching devices. The converter needs to be overrated.
NPC Converter with Series-Connected IGBTs
III. NPC Converter
Page 63 COBEP 2013, Gramado, Brazil
T1
T2
D1
D2D3
T3
ANPC Converter with Series-Connected IGBTs
Losses distribution
III. NPC Converter
Page 64 COBEP 2013, Gramado, Brazil
The ANPC introduces additional switching states that can be used to balance the power losses between semiconductor devices. Better converter use is achieved.
33
ABB adopted the ANPC converter technology for HVDC applications but came back to the two-level converter (less expensive).
Two and three level converters can facilitate AC/DC conversion in HVDC applications. However:
High switching frequency is mandatory due to the reduced number of levels.
Poor efficiency.
Direct connection of switching devices required.
Not easy scalable to higher power/voltage levels.
Modular Multilevel Converters (MMCs)
III. NPC Converter
Page 65 COBEP 2013, Gramado, Brazil
Remarks
Page 66 COBEP 2013, Gramado, Brazil
Outline
I. HVDC Systems
II. Voltage-Source Converter (VSC)
III. Neutral-Point-Clamped (NPC) Converter
IV. Modular-Multilevel Converters (MMCs)
34
Cascaded connection of sub-modules (SMs) or cells.
The SMs are connected in series to create arms.
A phase-leg comprises two arms (upper and lower).
It is structurally scalable and can theoretically meet any voltage level requirements.
(c)
SM 1
SM 2
SM N
SM 1
SM 2
iu L
Lil
Vdc
+
-
a
SM N
Vc-
+
IV. Modular Multilevel Converters (MMCs)
Page 67 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
Operation of switches within a SM is complementary.
Half-bridge SM
SM States s1 s1 vx
Activated 1 0 vc
Deactivated 0 1 0
C
S1
S1Vx
Vc-
+
Half-Bridge Sub-Modules (SMs)
Page 68 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
35
Number of SMs per arm: N
Reference voltage for the SM capacitors:
Reactors L are inserted in the circuit to control the circulating currents and to limit the fault currents.
(c)
SM 1
SM 2
SM N
SM 1
SM 2
iu L
Lil
Vdc
+
-
a
SM N
Vc-
+
MMC Arms
Page 69 COBEP 2013, Gramado, Brazil
NV
V dcC *
IV. Modular Multilevel Converters (MMCs)
C
S1
S1Vx
Vc-
+
S1
S1
S2
S2
C
SM Configurations
Half-bridge: vx={0, vC} Full-bridge: vx={vC, 0, -vC}
IV. Modular Multilevel Converters (MMCs)
Page 70 COBEP 2013, Gramado, Brazil
36
SM Configurations
Page 71 COBEP 2013, Gramado, Brazil
Modular multilevel converters: Manufacturers’ implementation
Siemens, Alstom
IV. Modular Multilevel Converters (MMCs)
ALSTOM ABB
IV. Modular Multilevel Converters (MMCs)
Modular multilevel converters: Manufacturers implementations
Page 72 COBEP 2013, Gramado, Brazil
37
HVDC-VSC technology overview
IGBT1 ON Capacitor ON IGBT2 ON Capacitor OFF
DC short-circuit T1 ONSubmodule failure SW1 ON
IV. Modular Multilevel Converters (MMCs)
Switching States
Page 73 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
Alstom Implementation
Page 74 COBEP 2013, Gramado, Brazil
38
The MMC offers salient features such as:
It is structurally scalable and can theoreticallymeet any voltage level requirements.
The capacitor voltage balancing task is relativelysimple and there is no requirement for isolated dcsources.
A DC-link capacitor is not required (only a smallone is connected to attenuate switching-frequency ripples).
MMC Features
Page 75 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
Operation of the MMC
On average, the number of SMs activated in a phase-leg is equal to N. Deviations from that number can be due to interleaving between the upper and lower arm carriers or due to external control actions.
Ideally, the DC voltage is distributed equally across the SMs: (c)
SM 1
SM 2
SM N
SM 1
SM 2
iu L
Lil
Vdc
+
-
a
SM N
Vc-
+
The output voltage level is defined by the number of SMs that are connected in the upper and lower arm of the converter.
Page 76 COBEP 2013, Gramado, Brazil
NV
V dcC
IV. Modular Multilevel Converters (MMCs)
39
States of the MMC. Example
Page 77 COBEP 2013, Gramado, Brazil
SM1
iout
Larm
Larm
Vdc
+
–
SM2
Rarm
Rarm
SM3
SM4
SM5
SM1
SM2
SM3
SM4
SM5
State 1
SM1
iout
Larm
Larm
Vdc
+
–
SM2
Rarm
Rarm
SM3
SM4
SM5
SM1
SM2
SM3
SM4
SM5
State 2
-3
-2
-1
+1
+2
+3
State 1
State 2
SM1
iout
Larm
Larm
Vdc
+
–
SM2
Rarm
Rarm
SM3
SM4
SM5
SM1
SM2
SM3
SM4
SM5
State 2
SM1
iout
Larm
Larm
Vdc
SM2
Rarm
Rarm
SM3
SM4
SM5
SM1
SM2
SM3
SM4
SM5
State 3
-3
-2
-1
+1
+2
+3
State 2
State 3
SM1
iout
Larm
Larm
Vdc
SM2
Rarm
Rarm
SM3
SM4
SM5
SM1
SM2
SM3
SM4
SM5
State 3
SM1
iout
Larm
Larm
Vdc
+
–
SM2
Rarm
Rarm
SM3
SM4
SM5
SM1
SM2
SM3
SM4
SM5
State 4
-3
-2
-1
+1
+2
+3
State 3
State 4
SM1
iout
Larm
Larm
Vdc
+
–
SM2
Rarm
Rarm
SM3
SM4
SM5
SM1
SM2
SM3
SM4
SM5
State 4
SM1
iout
Larm
Larm
Vdc
–
SM2
Rarm
Rarm
SM3
SM4
SM5
SM1
SM2
SM3
SM4
SM5
State 5
-3
-2
-1
+1
+2
+3
State 4
State 5
SM1
iout
Larm
Larm
Vdc
–
SM2
Rarm
Rarm
SM3
SM4
SM5
SM1
SM2
SM3
SM4
SM5
State 5
SM1
iout
Larm
Larm
Vdc
+
–
SM2
Rarm
Rarm
SM3
SM4
SM5
SM1
SM2
SM3
SM4
SM5
State 6
-3
-2
-1
+1
+2
+3
State 5
State 6
IV. Modular Multilevel Converters (MMCs)
Experimental Setup
Single-phase laboratory prototype a the Australian Energy Research Institute (AERI), the University of New South Wales (UNSW), Sydney, Australia
Number of SMs per arm: 5
SM1
SM2
SM3
SM4
SM5
Arm Inductor
Voltage Sensors
Communication Interface
Page 78 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
40
Output Voltage
Page 79 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
SM Capacitor Voltage Balancing
During the operation of the MMC, the arm current flows though the SM capacitors, which charge and discharge the capacitors.
In order to ensure the proper operation of the converter, the SM capacitor voltages have to be regulated to the reference value of
An active voltage balancing method is essential for the operation of the MMC.
Page 80 COBEP 2013, Gramado, Brazil
NV
V dcC *
IV. Modular Multilevel Converters (MMCs)
41
Page 81 COBEP 2013, Gramado, Brazil
SM Capacitor Voltage Balancing
The voltage balancing algorithm uses measurements from the SM capacitor voltages and arm currents to select the next SM that will be connected or bypassed.
If the arm current is in the charging direction:
and the PWM method requires the addition of one SM in the arm, the SM with the lowest voltage that is not connected to the arm will be selected and added to the arm.
and the PWM method requires the removal of one SM in the arm, the SM with the highest voltage that is connected to the arm will be selected and removed from the arm.
IV. Modular Multilevel Converters (MMCs)
Page 82 COBEP 2013, Gramado, Brazil
SM Capacitor Voltage Balancing
The voltage balancing algorithm uses measurements from the SM capacitor voltages and arm currents to select the next SM that will be connected or bypassed.
If the arm current is in the discharging direction:
and the PWM method requires the addition of one SM in the arm, the SM with the highest voltage that is not connected to the arm will be selected and added to the arm.
and the PWM method requires the removal of one SM in the arm, the SM with the lowest voltage that is connected to the arm will be selected and removed from the arm.
IV. Modular Multilevel Converters (MMCs)
42
Page 83 COBEP 2013, Gramado, Brazil
SM Capacitor Voltage Balancing
Harm
Sorting
SM voltage measurements
Number of SMs required in the arm from modulation stage
iarm
Polarity of the arm current
Selection of SMs
Switching pulses to the SMs of the arm
Sorted SMs
Ascending / Descending
(il or iu)
IV. Modular Multilevel Converters (MMCs)
Page 84 COBEP 2013, Gramado, Brazil
SM Capacitor voltages (Vdc= 300V, N = 5)
Experimental Results
IV. Modular Multilevel Converters (MMCs)
43
Page 85 COBEP 2013, Gramado, Brazil
From the equivalent circuit:
(a)
iu
il
iaL
L
(0)eazout
vu
vl
(a)
iu
il
iaL
L
(0)eazout
vu
vl
(a)
iu
il
iaL
L
vu
vl
Cu
Cl
Vdc/2
Vdc/2
(0)
Circulating Current Control
IV. Modular Multilevel Converters (MMCs)
Common and Differential Voltages
• Two distinct voltages in the phase-leg can be identified; the common voltage (vcomm) and the differential voltage (vdiff):
(a)
iu
il
iaL
L
(0)
eazout
vdiff
vdiffvcomm
Page 86 COBEP 2013, Gramado, Brazil
2
2
ludiff
lucomm
vvv
vvv
IV. Modular Multilevel Converters (MMCs)
44
Based on the superposition theorem, two circuits can be distinguished:
(a)ia
L
L
(0)
eazout
vcomm
icomm=ia/2
icomm=ia/2
(a)
ia=0L
L
(0)
zout
vdiff
vdiff
idiff
Common Mode Differential Mode
Common and Differential Circuits
22
2
alucomm
lucomm
iiii
vvv
Current
gCirculatin
2
2
ludiff
ludiff
iii
vvv
Page 87 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
Currents of the MMC
Arm currents
Output current (ia) and circulating current (idiff)
The circulating current contains a DC component that is essential to keep the phase-leg energized, i.e. maintain the capacitor voltages at the reference values.
It also contains some AC components (not essential).
Page 88 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
45
The common and differential circuits can be analyzed independently. The differential voltage defines the differential/circulating current:
t
diffdiffdiff IdtvL
i0
0
1
A differential voltage can be introduced to control the circulating current without affecting the output current.
Page 89 COBEP 2013, Gramado, Brazil
Control of the Circulating Current
IV. Modular Multilevel Converters (MMCs)
If the only reference for the circulating current is the DC component, the arm currents are minimized and this reduces power losses in the MMC.
A second order harmonic can be added to the DC component to reduce the capacitor voltage ripples.
Discussion on the Circulating Current Components
Page 90 COBEP 2013, Gramado, Brazil
DCdiff Ii
)2cos(ˆ22 tIIi DCdiff
IV. Modular Multilevel Converters (MMCs)
46
DCdiff Ii )2cos(ˆ22 tIIi DCdiff
Case 1: Case 2:
Normalized Capacitor Voltage Ripple Amplitudes
fCIvv
arms
NPNPn 22
Page 91 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
Circulating Current and Output Current
DCdiff Ii Case 1:
)2cos(ˆ22 tIIi DCdiffCase 2:
Page 92 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
47
Capacitor Voltages
DCdiff Ii Case 1:
)2cos(ˆ22 tIIi DCdiffCase 2:
Page 93 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
Page 94 COBEP 2013, Gramado, Brazil
x
x
x
11
2 N
x
12 N
x
11
2 N
x
N
N Level-shifted phase-disposition
PWM (PD-PWM) is normally applied to the MMC.
Other modulation strategies such as phase-shifted PWM (PS-PWM) can also be applied(*)
(*) M. Hagiwara and H. Akagi, “Control andexperiment of pulsewidth-modulatedmodular multilevel converters,” IEEETrans. Power Electron., vol. 24, no. 7, pp.1737-1746, Jul. 2009.
PD-PWM
IV. Modular Multilevel Converters (MMCs)
Modulation Techniques
48
A number N of carriers is needed for the modulation.
The reference signal is compared with the carriers.
The output voltage level of the phase-leg, which ranges from 0 to N, is defined by the number of carriers that have an instantaneous value lower than the reference signal.
PD-PWM
Page 95 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
Interleaving between Upper and Lower Arms
Page 96 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
49
Page 97 COBEP 2013, Gramado, Brazil
Interleaving between Upper and Lower Arms
Arm currents
Output current and circulating current
IV. Modular Multilevel Converters (MMCs)
Page 98 COBEP 2013, Gramado, Brazil
Interleaving between Upper and Lower Arms
IV. Modular Multilevel Converters (MMCs)
50
When applying interleaving between the upper and lower arms the ripple in the arm currents increases.
In order to reduce the current ripples the value of the inductors should be increased.
The quality of the output voltage (common voltage) improves because of the increased number of levels provided by the MMC (from N+1 to 2N+1).
Effects of Interleaving
Page 99 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
MMC Legs Connected in Parallel
• Each leg is integrated by two arms (the upper and lower arms)
• P legs build up a phase of the MMC (e.g. Phase a)
• The output currents of the legs have to be balanced
(i1i2…iPia/P)
Vdc
s
sCv
L
L
l N
l
l
L
L
l N
l
l
L
L
a
lPN
lP
lP
iuP
ilP
ii
i
ia
iu
il
iu
il
Leg Leg LegP
a
u N
u
u
u N
u
u
uPN
uP
uP
Sub-Module
(SM)
Page 100 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
51
Equivalent Circuit
Page 101 COBEP 2013, Gramado, Brazil
Model of an MMC phase (Phase a) with P legs connected in parallel
IV. Modular Multilevel Converters (MMCs)
Control of the Currents
Each leg has its independent circulating current control.
The legs should share the output current evenly.
The common voltage of each leg will be modified to achieve balanced output currents.
Page 102 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
52
Current Balance Among the Legs (Phase a)
Page 103 COBEP 2013, Gramado, Brazil
i eout
v
commPv
1commv
1 Pcommv
i
i1
i 1
Control magnitude:
with:
Restriction to avoid distorting the output voltage (vcomm):
ps
comm iTL
vp
2
Pi
ii app
01
P
pcommp
v
IV. Modular Multilevel Converters (MMCs)
Modulation Scheme (Leg p)
Page 104 COBEP 2013, Gramado, Brazil
pcommvpdiffv
'pcommv
2dcV
vcommp is injected into the reference signal of each leg to achieve equal current sharing.
IV. Modular Multilevel Converters (MMCs)
53
Simulation Results
Page 105 COBEP 2013, Gramado, Brazil
SMu15
SMu14
u
i
dc
dc
SMu13
SMu12
SMu11
SMl15
SMl14
SMl13
SMl12
SMl11
l
SMu25
SMu24
u
i
SMu23
SMu22
SMu21
SMl25
SMl24
SMl23
SMl22
SMl21
l
oo
a
Single-phase MMC with two legs connected in parallel
Data:
IV. Modular Multilevel Converters (MMCs)
Simulation Results: Load 1
Page 106 COBEP 2013, Gramado, Brazil
IV. Modular Multilevel Converters (MMCs)
54
Page 107 COBEP 2013, Gramado, Brazil
Simulation Results: Load 2
IV. Modular Multilevel Converters (MMCs)
Discussion
Page 108 COBEP 2013, Gramado, Brazil
The rated current/power of an MMC can be increased by connecting legs in parallel.
The circulating current of each leg is controlled independently from the other legs.
The proposed current balancing strategy is able to achieve equal current sharing among the legs.
None of the current control actions applied affects the output voltages of the MMC.
The proposed strategy is general and applicable to MMCs with any number of voltage levels and legs connected in parallel.
IV. Modular Multilevel Converters (MMCs)
55
The MMC represents an important milestone in the evolution of the HVDC transmission links.
Research topics
New energy transmission layouts with MVDC collector systems.
Development of control algorithms for multiterminalHVDC links.
DC short-circuit handling.
IV. Modular Multilevel Converters (MMCs)
Future Research on HVDC Systems
Page 109 COBEP 2013, Gramado, Brazil
Trans Bay’s Underwater HVDC Plus (Siemens)
Courtesy of Siemens
IV. Modular Multilevel Converters (MMCs)
Page 110 COBEP 2013, Gramado, Brazil
56
IV. Modular Multilevel Converters (MMCs)
Page 111 COBEP 2013, Gramado, Brazil
Super Grids
Prof. Josep Pou ([email protected])
(1) The University of New South WalesAustralian Energy Research Institute (AERI),School of Electrical Engineering and Telecommunications, Australia
(2) Technical University of Catalonia, Barcelona TechTerrassa Industrial Electronics Group (TIEG),Department of Electronic Engineering, Catalonia, Spain
(3) TECNALIAEnergy Unit, Basque Country, Spain
Thank you!