Underpinning Research
HVDC activity at Imperial college
Dr Michael M.C. Merlin
28th July 2015
Future Transmission Systems
Underpinning Research
• High-Fidelity System Models
• Continental-Scale Energy
Systems
•
•
•
•
How would a new transcontinental layer be designed
What technology and operation
issues arise
Is mixed use (collection and
interconnection) sensible
Should this be planned or should
it evolve
UK &
Ireland
Benelux &
Germany
19GW
France
10GW
41GW
2GW
4GW
21GW
3GW
Poland
& Baltic
10GW
4GW
Central Europe
4GW
10GW
South East
Europe
Italy &
Malta
Iberia
Norway
Scotland
9 GW
Norway
Offshore wind
Dogger Bank
Hornsea
BritNed
England
Rounds
1&2
NorNed 2
•
5GW
3GW
NorNed 3 or No - Be
•
Energy System (esp Electricity System) driven
by extremes of the range
Average energy flows indicate little: will system
work on coldest, stillest winter evening and
sunniest summer day?
What balance of network, demand-action,
storage optimises cost/benefit case
England Shore Line
(28 GW)
7 GW Interface Capacity)
Scotland Shore Line
(5 GW)
•
Nordic
Germany
Offshore
Wind
Netherlands
Offshore wind
Nemo
Belgium
4 GW
Netherlands Shore Line
Belgium Shore Line
2
HVDC Systems
Underpinning Research
Advantages:
• Interconnect asynchronous networks or at different frequencies
• Theoretically no upper limit on transmission length
• Ability to control the power flows on the HVDC network
• Could improve AC system stability
• Two lines (DC) instead of three (AC) per circuit
• More power pushed through the lines at higher efficiency
• No reactive power compensation required
3
HVDC Systems
Underpinning Research
Disadvantages:
• Higher station cost
• Large converter losses
• Shorter equipment lifetime
Long distance is often
a decisive aspect in
favour of HVDC
4
Underpinning Research
HVDC Converters
5
Semiconductor devices
Underpinning Research
Maximum Current
7000
6000
Thyristor (Press-Pack)
5000
4000
3000
IGBT (Hi-Pack)
2000
HVDC
>300kV
1000
0
0
2000
4000
6000
8000
10000
Voltage Blocking
IGBT (Press-Pack)
IBGT
GTO
IGCT
Thyristor
6
CSC Project
Underpinning Research
Grita Project
Power rating: 500MW
DC voltage: ±200kV
Cable length: 43km (U) + 160km (S)
OHL length: 110km
Station Size: 225m x 120m
Station Cost: 40M€
Cable Cost: 350M€
Total Cost: 500M€
Efficiency: 99.5%
LCC
HALL
VALVE
HVDC
AC BUS BARS AND PROTECTIONS
TRANSFORMERS
AC FILTERS
COOLING
TECHNICAL
FAN
ROOMS
From: R.L. Sellick, M. Akerberg, “Comparison between HVDC Light (VSC) and HVDC Classic (LCC) Site Aspects, for a 500MW
400kV HVDC Transmission Scheme”, IET ACDC 2012, November 2012
7
Voltage Source Converter
Underpinning Research
Rectifier
Capacitive
•First VSC HVDC in 1997 – HÄLLSJÖN (3 MW)
•Uses self-commutated IGBT switches
•Independent control of active and reactive
power
•Less (zero?) filtering requirement
•Lower footprint compared to LCC
•No dependence on AC system strength
•No voltage reversal – stronger and lighter
cables, meshing
8
VSC Project
Underpinning Research
East-West Interconnector (EWIC)
AND PROTECTIONS
AC FILTERS
AC BUS BARS
HVDC
FAN
COOLING
HALL
VALVE
TRANSFORMERS
Power rating: 500MW
DC voltage: ±200kV
Cable length: 75km (U) + 186km (S)
Station Size: 180m x 115m
Station Cost: 51M€
Cable Cost: 420M€
Total Cost: 600M€
Total Efficiency: 98%
TECHNICAL
ROOMS
VSC
LCC
From: R.L. Sellick, M. Akerberg, “Comparison between HVDC Light (VSC) and HVDC Classic (LCC) Site Aspects, for a 500MW
400kV HVDC Transmission Scheme”, IET ACDC 2012, November 2012
9
Underpinning Research
Modular Multilevel Converters
10
H-Bridge Sub-Modules
Full H-Bridge SM
Underpinning Research
Half H-Bridge SM
Modular Multi-level Converter
Underpinning Research
Stack of SMs
AC transformer
Phase reactor
DC Capacitor
Arm inductor
Modular Multi-Level Converter
Underpinning Research
• Staircase waveform
• As many steps as SMs
• Sum of arm voltages always
equals to the DC bus voltage
• Redundant switching combinations
• Voltage steps provided by
cell capacitors
•AC current splits equally
between top and bottom arms
• DC current runs through
both arms
i  12 iB  12 iC  12 Iˆcost   cost  23   cost  23   0
1
2 A
Graphics from: http://en.wikipedia.org/wiki/HVDC_converter
13
Power Efficiency of the MMC
Underpinning Research
Jacobsson, B., Karlsson, P., Asplund, G., Harnefors, L., Jonsson, T., VSC - HVDC transmission with cascaded two-level
converters, CIGRÉ session, Paris, 2010, paper reference B4-110.
14
MMC Project
Underpinning Research
Cascaded Two-Level VSC (Suggested Layout)
From: R.L. Sellick, M. Akerberg, “Comparison between HVDC Light (VSC) and HVDC Classic (LCC) Site Aspects, for a 500MW
400kV HVDC Transmission Scheme”, IET ACDC 2012, November 2012
FAN
COOLING
ROOMS
TECHNICAL
AND PROTECTIONS
HVDC
AC BUS BARS
TRANSFORMERS
LCC
HALL
VSC
VALVE
MMC
Power rating: 500MW
DC voltage: ±200kV
Size: 165m x 95m
Efficiency: 99%
15
Siemens: HVDC Plus®
Underpinning Research
TransBay Project
Power rating: 400MW
DC voltage: ±200kV
Cable length: 85km (S)
Size: 165m x 95m
Efficiency: 97%
INELFE interconnector, Siemens publication.
16
ABB: HVDC Light®
Underpinning Research
Cascaded 2-level Converter
Jacobsson, B., Karlsson, P., Asplund, G., Harnefors, L., Jonsson, T., VSC - HVDC transmission with cascaded two-level
converters, CIGRÉ session, Paris, 2010, paper reference B4-110.
17
Alstom Grid: MaxSine®
Underpinning Research
TenneT awards offshore grid
connection project DolWin3 to
Alstom with a capacity of 900 MW
with new DC technology over a
distance of 162 km (26/04/2013)
18
HVDC Converters
LCC
Underpinning Research
LCC
Hybrid
LCC
HVDC
VSC
MMCLCC
Classic
converter
MMC
Classic
VSCVSC
Classic VSC
LCC
Power
Efficiency
+ Mature Technology
+ Large power ratings
+ DC-side fault blocking
- Large footprint
Cost
Effective
Reliability
- Requires strong AC grid
VSC (MMC+)
+ Full quadrant operation
+ Power weak AC grid
+ Smaller footprint
(+) DC-side fault blocking
Performan
ce
Volume /
Weight
- Higher complexity
- Limited power ratings
Offshore Technology
19
Underpinning Research
Hybrid Multilevel Converters
20
Hybrid Multilevel Converter
Technologies
Underpinning Research
Alternate Arm Converter
Full H-Bridge SM
Stack of
Submodules
Director
Switches
Arm inductor
DC Bus
AC transformer
Phase
reactor
21
Alternate Arm Converter (AAC)
Underpinning Research
Advantages:
• Similar advantages as the MMC
• VSC
• no AC filter
• Modular design
• Smaller valve hall
• DC fault tolerant
Disadvantages:
• Non smooth DC current
• Difficult control
22
HVDC Converter – DC Fault
Underpinning Research
- DC fault blocking
capability
- STATCOM mode
for grid support
23
HVDC Converter - Sizing
Underpinning Research
- Number of devices
- Stack voltage  submodule count
- Converter voltage  director switch
- Voltage and current ratings
- Submodule capacitors
- Intra-cycle voltage deviation
- SM Rotation heuristics
- Inductor sizing
- Topology dependent
- Fault limiting factors
24
HVDC Converter – Control Systems
Underpinning Research
- Energy Management
-
Total energy storage
Horizontal balancing
Vertical balancing
- Current Control
- Low-level Control
- Computing System
EUA
ELA
EUB
ELB
EUC
ELC
EUA
EUB
EUC
Average
Average
Energy
Energy
ELA
ELB
ELC
25
Alternate Arm Converter (AAC)
Underpinning Research
26
Alstom Press Release on the AAC
Underpinning Research
http://think-grid.org/fault-blocking-converters-dc-networks1?utm_source=newsletter&utm_medium=email&utm_content=faultblocking-converters-dc-networks&utm_campaign=newsletter-thinkgrid
27
Underpinning Research
Lab Experiments
28
Configured Experimental Setup
Underpinning Research
Tests at 1500V successful (full-bridge)
Can be reconfigured as half-bridge
MMC (also tested)
Extending the test rig to emulate more
AC and DC conditions using
Triphase converters.
Converter Build
Underpinning Research
.
Full-scale
DC bus
1,500V
AC current
7-12A
AC voltage
1070/918V
SMs per stacks
10
SM voltage
106/150V
1P.Clemow
and al. “Lab-scale Experimental Multilevel Modular HVDC
Converter with Temperature Controlled Cells” EPE ECCE 2014
30
Full-scale Dry Converter
Underpinning Research
31
Full-scale Dry Converter
Underpinning Research
32
Hardware Tests on MMC
Underpinning Research
33
MMC experimental results
Underpinning Research
34
AAC experimental results
Underpinning Research
35
DC Fault
Underpinning Research
36
Underpinning Research
Organisation Management and
Control
PE Centre – WP 4.1
37
State of Health
Underpinning Research
HVDC Converter uses thousand of semiconductor devices to operate.
The State of Health (SOH) can be affected by a number of aspects
(Temperature, lightning, dust, aging…)
SOH can be estimated through different means but is essential to be
monitored to anticipate faults
- Ambient
Temperature
- Lightning
- Current
Waveforms
- Aging
- Si
Temperature
- Gate voltage
- Model
SM Control
Underpinning Research
Some degrees of freedom in each individual SM
Full H-Bridge can alternate their zero-voltage state
Effect of using more the Upper
zero-state combination to
compensate temperature
imbalance between IGBT
modules
Judge, P. D., et al. "Power loss and thermal characterization of IGBT
modules in the Alternate Arm converter." (ECCE), 2013 IEEE
Stack Control
Underpinning Research
Another way to affect the utilization of the SM is by acting on the voltage and
current waveforms of the stacks
Adding DC offset to the AC voltage shift the distribution of power losses between
the top and bottom IGBT modules
No DC Offset
4.2%
4.2%
Power Losses Distribution
between IGBT modules
4.2%
5% DC Offset
3.0%
3.0%
3.0%
12.5% 12.5% 12.5%
12.4% 12.4% 12.4%
4.2%
5.6%
4.2%
4.2%
12.5% 12.5% 12.5%
5.6%
5.6%
12.3% 12.3% 12.3%
Underpinning Research
Additional Research topics on
HVDC in the CAP group
41
Xin Xiang
The Case for Using Low-Frequency AC
(LFAC v. HVAC v. HVDC) Underpinning Research
HVAC transmission system (50/60Hz)
50/60Hz
Grid
Generator
Step-up
Transformer
Step-down
Transformer
HVDC transmission system (0Hz)
0Hz
Generator
Step-up
Transformer
AC/DC
DC/AC
Converter
Converter
Grid
Step-down
Transformer
Low-Frequency AC transmission system (16.7/20Hz)
600
16.7/20Hz
480
AC/AC
LF Step-up
Transformer
Converter
Cost (M£)
Generator
Grid
Step-down
360
Transformer
CPLC
SPLC
240
Costs of converters, cables, transformers,
platforms and power losses assessed for
each configuration as a function of
distance and power capacity
CC
QC
OPC
OPPC
120
0
CPLC
SPLC
CC
QC
OPC
OPPC
HVAC
3.134
10.88
54.25
4.926
3.204
32
LFAC
35km
2.43
18.795
49
1.525
35.4
59
HVDC
HVAC
2.302
31.855
29.925
0
48.6
91
18.708
10.787
213.964
13.532
3.204
32
LFAC
105km
7.291
18.752
147
4.574
35.4
59
HVDC
HVAC
6.906
31.814
89.775
0
48.6
91
25.478
10.747
446.25
24.108
3.204
32
LFAC
175km
12.152
18.71
245
7.624
35.4
59
HVDC
11.51
31.772
149.625
0
48.6
91
Cost Comparison for
0.6GW Offshore Wind Farm
Xin Xiang
Transmission Power =0.6 GW
1250
Underpinning Research
LFAC
HVAC
VSC-HVDC
1000
325
Transmission Power =0.6 GW
LFAC
HVAC
VSC-HVDC
750
Cost (M£)
305
285
Cost (M£)
500
265
250
245
0
40
160
120
80
Transmission Distance l (km)
240
200
225
76
80 81
87
86
91
96
Transmission Distance l (km)
98
101
Well-known HVAC v. HVDC comparison: costs of HVAC are approximately
quadratic and exceed cost of HVDC at about 80 km
LFAC has lower unit distance cost than HVAC but suffers high terminal costs and
so has little or no range over which it is preferred
Conclusions broadly similar for range of power and for overhead lines
Frequency Services to AC networks from
offshore DC interconnections
Yousef Pipelzadeh et al.
Underpinning Research
When wind displaces gas/coal-fired generators, there is less inertia in the system
System frequency is harder to control, especially in emergencies.
Need to exploit any source of stored energy to synthesise natural inertia.
2
“Fast” energy release
from HVDC converters
2
1
Unexpected generator
outage
3
“Slower” energy release
from Wind Turbines
Wind Turbine Kinetic Energy Release
Signal the WF to decelerate to release some kinetic energy. Energy has to be passed
through chain of dynamical systems (turbine, generator, AC/DC converter, DC link, DC/AC
converter). Turbine must be reaccelerated to regain optimal wind capture
HVDC link Capacitor Energy Release
Some discharge of capacitance in the DC/AC converter can be allowed and can be fast but
not long-lived Possibly storage in the converter could be enhanced but not an ideal
application for batteries
Yousef Pipelzadeh et al.
Blending energy storage from
Wind Farms and HVDC links
Underpinning Research
Four scenarios:
A: AC grid with 4 generators but no WF.
B: One WF with no emulated inertia.
C: One WF with emulated inertia.
D: One WF with emulated inertia but no
primary support.
The displacement of generation by wind causes
the RoCoF to be 33% faster, this is avoided by
enabling inertial response emulation.
The comparison confirms that primary
response has little effect during the initial
transient.
AC/DC Systems Dynamics: Disturbance
Propagation through VSC HVDC Links
Claudia Spallarossa
Underpinning Research
Example here is power export from GB to Scandinavia at 2GW
1
1
2
Great
Britain
3
2
Scandinavia
4
4
7
5
6
Area S1
Area GB1
9
10
13
12
12
HVDC Link
22
28
18
20
19
21
25
29
15
13
17
23
24
14
16
19
20
18
10
Scand
16
17
8
VSC
VSC
GB
14
9
7
11
15
11
5
6
Area S2
Area GB2
8
3
27
26
• In a simple case, the HVDC link acts as a firewall
(constant power regardless of system state).
• Adding supplementary frequency control helps
systems recover from a generation outage but
couples the dynamics of the two grids.
Claudia Spallarossa
by Supplementary Control of
Interconnector
Underpinning Research
2200.
8000.0
[MW]
1,800 MW loss of
generation applied in GB
and so frequency drops
In this case, reducing export
in response to the
frequency drop is helpful
locally, but passes some of
the problem to the remote
system
[MW]
[MW]
Loss of generation
7000.0
2040.
6000.0
1880.
5000.0
1720.
4000.0
1560.
(a)
without Droop
with Droop
(b)
3000.0
0
8.
16.
24.
1400.
0
32. [s] 40.
Frequency response in GB
[Hz]
50.0
B15 UK1 Frequency
without Droop
49.8
49.55Hz
with Droop
49.6
49.6
49.5 Hz
49.4
49.2
0
Frequency response in Scandinavia
[Hz]
50.0
(c)
32. [s] 40.
24.
16.
8.
50.2
50.2
49.8
When an increase of import
is need in link running at
capacity we have a difficulty
DC link power
without Droop
49.3Hz
8.
16.
24.
with droop
49.5 Hz
49.4
(d)
32. [s] 40.
with Droop
49.7Hz
49.2
0
without droop
8.
16.
24.
32. [s] 40.
Caitríona Sheridan et al.
Reduced Dynamic Models of
Multi-Level Converters
Underpinning Research
Full scale MMCs have over 4,000 IGBTs and 1,500 capacitors
Detailed models are not practical for large network simulation
Average Value Models (AVM) uses
controllable voltage source to
represent the converter. They are
known as Reduced Dynamic Models:
they retain the low frequency dynamics
but neglect the fast switching events
Reduces computation time
->Up 14 times faster
MMC Arm Representation
Caitríona Sheridan et al.
RDMs of Modular Multilevel
Converters
RDM created for two converter types: HBMMC and AAC
Verified against detailed model in point-topoint HVDC links
Maintained accuracy while improving
computation time
Line-to-Ground
DC Fault with
AAC
Underpinning Research
Model in System Studies
using PowerFactory
Claudia Spallarossa et al.
Underpinning Research
Development of MMC RDM in a system
oriented software platform allows:
• Analysis of dynamics of AC+DC+AC
systems
• Provision of frequency support via
HVDC converters (stack energy
storage, overload capability).
MMC Overload Capability to face
loss of in-feed event:
• It allows to transfer an extra
30% on top of the rated power
without damaging the converter
• The frequency nadir stays
within statutory limits (±0.5 Hz)
Paul Judge
•
Limiting Factors on P/Q Envelope:
Design for Overload
Underpinning Research
P/Q Envelope of MMC limited by
several factors
• Arm Current Limit
• Over-Modulation limit
• Arm Voltage Limit
• Peak Sub-Module Voltage Limit
•
To achieve overload expand P/Q envelope by
running controlled circulating current
• Design penalty small if reactive power
requirement during overload is decreased
• Causes increased losses – not attractive
during normal operation
Paul Judge
•
Junction Temperature
Limits
Device junction temperatures may
become an issue during overload
–Dynamic Rating
• Provide large amount of extra
power during start of system
events, reduce back down to a
steady-state overload rating
Underpinning Research
Phil Clemow
Power Transfer in a
Degraded State
Underpinning Research
•
•
Larges cables are now at ±500 kV and 2.5kA giving a link of 2.5 GW
We can not allow that to have a single-point failure
•
How much power can we transfer after various component failures?
• Cable faults; transformer faults; semiconductor faults etc.
•
Simulation studies of many scenarios underway and hardware verification now
beginning
Converter designs with fault-current limiting and ability to work in step-down mode
can transfer up to 50% of their rated power under a DC line to ground fault
Phil Clemow
Operation with a Pole-toGround Fault
Underpinning Research
Voltage collapse on one-pole;
avoided voltage-doubling on
healthy pole
Brief current spike caused by DC
bus capacitors discharging into the
fault.
Cell voltages and arm currents well
controlled
Converter can continue to operate
indefinitely at 50% power
Issues remain with cable return
path, grounding arrangements, DC
stress on transformer
Simulation of a line-to-ground fault on lab-scale full-bridge MMC
Geraint Chaffey
Reduced Breaker Requirement
in Meshed DC networks
DC circuit breakers are problematic: need to operate very
fast, they are large and there is no operational
experience in this context.
Size and complexity strongly influenced by peak current
requirement.
Converter that can control, limit or stop fault current would
reduce stress on breaker
This could be selectively applied to reduce peak currents in
some regions of a network – particularly helpful for
offshore platform.
Depending on levels of interconnection and inductances on
the network slower breaker topologies are applicable
Five Terminal Meshed Network
Underpinning Research
Example fault currents
Fault currents when implementing an
MMC (red) and an AAC (blue) at node E
Thomas Lüth
Modular DC/DC designs
Underpinning Research
DC/DC converters facilitate
AC
current
•
•
•
•
Connection of DC-links of different voltages
“Firewall” protection between sub networks
Step-up from wind farm collection networks
Step-down to small distribution networks
Modular designs easily scalable
Greater current control allowing for small/no
DC filters
High range of operable step-ratios & power
levels
DC
current
Resonant Modular DC/DC
Xiaotian Zhang
Modular converter of interest for creating large stepup/down ratios without using a transformer
Ratio set by number of modules
Resonant action used to raise operating frequency
without penalty on switching loss
Modular resonant converter has
Low switching loss but large
circulating current
High ripple frequency
Inherent balancing
Step ratio dependent on numbers
of modules
Ratio limited by module current
considerations
Underpinning Research
MMC Topology Optimization
Michaël M.C. Merlin
Underpinning Research
The MMC was a major step forward in VSC
technology and numerous optimization iterations
can still be accomplished
The injection of high harmonic circulating current
can help reduce the SM voltage fluctuation
Without Circulating Current
Circulating Current Waveforms
With Circulating Current
Underpinning Research
Thank you for your attention
59