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The University of Nottingham
Power Electronic Converters for HVDC Applications
Prof Pat Wheeler
Email [email protected]
Power Electronics, Machines and Control (PEMC) Group UNIVERSITY OF NOTTINGHAM, UK
The presenter would like to acknowledge support from the Universidad Tecnica Federico Santa Maria and CONICYT through project MEC 80130065,
"Estructuras de Avanzadas de Convertidores de Potencia para Conexion a Red".
Introduction
• Power Electronics for HVDC Power System Applications
– Applications in Renewable Energy
• Offshore wind power
• European DC Grid
– Power Converter topology options
• MMC
• Series Hybrid – Alternate Arm Converter
• Parallel Hybrid
HVDC Applications
AC or DC Transmission
Cost
DC Convertor Stations
AC Stations
DC
AC
Break Even Distance (illustrative)
800km Overhead Line
50km Submarine Cable
Transmission Distance
Lines & Stations
• Cost of construction
– Components, space, transportation,…
• Efficiency of operation
– Losses, downtime,…
• Environmental impact
– Location, visual impact, resources,…
Traditional overhead line with AC
Overhead AC line with FACTS
HVDC overhead line
• HVDC Applications
– Offshore wind farms
– Remote generation
• Hydro, wind, wave, solar….
– Country interconnections
• Example: UK-France
AC or DC Transmission
Cost
DC
AC
Break Even Distance (illustrative)
800km Overhead Line
50km Submarine Cable
Transmission Distance
Lines & Stations
Traditional overhead line with AC
Overhead AC line with FACTS
HVDC overhead line
• Cost of construction
– Components, space, transportation,…
• Efficiency of operation
– Losses, downtime,…
• Environmental impact
– Location, visual impact, resources,…
AC or DC Transmission
Traditional overhead line with AC
Overhead AC line with FACTS
HVDC overhead line
• Cost of construction
– Components, space, transportation,…
• Efficiency of operation
– Losses, downtime,…
• Environmental impact
– Location, visual impact, resources,…
Multiport HVDC Networks
Offshore Wind
Country A
Offshore Wind
Country C
T&D System
Country A
T&D System
Country B
T&D System
Country C
HVAC to
HVDC
HVAC to
HVDC
HVAC to
HVDC
HVAC to
HVDC
HVAC to
HVDC
Multi-port HVDC
Transmission System
A Simplified Multiport HVDC system
3.3kV AC, 5kV DC, 5MW Circulating Power Research on energy management, protection, power electronic converters
M V3000Full Power Wind Converters
Low Voltage
M V3000Full Power Wind Converters
Low Voltage
M V3000Full Power Wind Converters
Low Voltage
Multiport HVDC Test Facility at University of Nottingham
Multiport HVDC Networks
• UK investing 10GW of offshore wind capacity with HVDC links to shore
• STATCOM-less systems, grid control, ride through and protection
• rugged, cheap, proven, but traditionally large foot print
• Voltage Source IGBT Converters (VSC - HVDC Plus 500MW)
• full controllability, small foot print, black start
• VSC HVDC topologies for high efficiencies, fault performance
Offshore Wind Farm
F
F
F
F
F
HF
50-150km DC cable Land Station
0
0.5
1
Wind farm active (1) and reactive (2) powers, pu
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1
0
1STATCOM active (1) and reactive (2) powers, pu
0
1
2
3
HVDC DC-link current, kA
80
100
120
140
160Offshore grid voltage, kV
1
2
1
2
Time, s
HVDC Transmission
HVDC Converter Topologies
• Tried and tested technology (50 year history)
• State of the art 132kV, 6400MW, 2000km
• Problems:
• Converter station footprint (filters approx 50%)
• Inability to work on “dead” networks - voltage needed for commutation of thyristors
• Limited scope for reactive power control
• Power reversal requires DC voltage reversal - Limits usage to certain cable types
• No scope for multi-terminal DC operation
Conventional HVDC Transmission
Based on line commutated converter (LCC) thyristor technology
Filter
AC System DC System
ConverterTransformer
Upper Valve Group
Lower Valve Group
Ground
+VDC
• Voltage source converter HVDC is growing rapidly (800kV 10GW)
• Many advantages over conventional HVDC eg:
• “Black start” – can feed a dead network
• Active and reactive power control
• Much smaller filters/no compensation (station footprint much smaller)
• “Constant” voltage DC link multi-terminal applications
• No voltage reversal for power reversal
• Some key problems for
VSC converters
• Efficiency (switching
converter losses)
• Operation with faulted AC or
DC side
• Scalability (modularity)
Voltage Source Converter (VSC) HVDC transmission
Two-level inverter based on series connected devices
DC supply
(100+kV) A
One
switch
One phase of 3 shown
• Simple concept
• Requires series devices
• 2-level operation – needs
relatively high PWM frequency
• High switching losses (total
losses 1.7% per station)
• 2-level PWM generates
significant harmonics - filtering
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
Time /s
Am
plit
ude
Voltage Source Converter (VSC) HVDC transmission
Modular-Multilevel-Converter (M2LC)
A
2 phases of 3 shown
• Concept more complex
• Switching of cells controls BOTH the
DC side and AC side voltage
• No bulk DC capacitor
• No need for series devices
• Multi-level operation – low switching
frequency + good harmonic
performance
• Low switching losses (typical total
losses 1% per station)
• Twice the number of devices required
compared to 2-level approach
• Large number of capacitors of
significant size
• Voltage on individual capacitors must
be controlled
B
Edc
DC SIDE
CELL
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
Time /s
Am
plit
ude
Modular-Multilevel-Converter (M2LC)
A
2 phases of 3 shown
B
Edc
DC SIDE
CELL
Multi-level AC voltage
VAC and current IAC
Vdc/2 + VAC
Vdc/2 - VAC
Idc/3 + IAC/2
Idc/3 - IAC/2
Hybrid Topologies
Combination of: – Multi-level “wave shaping”
• Series bridges
• “Modest” switching frequency
• Fractional rating low loss
– Series device “wave steering” • Zero voltage switching
• Low loss
Combination gives: – High waveform fidelity
– Low loss
– Low device count • Semiconductors
• Capacitors
Scale demonstrator design
• 20MW, 20kV DC, 11kV AC
• 1200A, 3300kV IGBTs
• 1.5kV cell voltage
Parallel Hybrid Topology
H-bridge- soft Switched (low loss)
MMC type chain
Parallel Hybrid Topology
H-bridge- soft Switched (low loss)
MMC type chain
• Topology has a very low component count compared to many alternatives
• Chain-Link converters perform wave shaping function – Converters outside of main power path => low switching losses
– Mean Chain-link current typically <20% of DC current
• H-Bridge converters are zero voltage soft switched – Device switching frequency = fundamental frequency
• Research Topics – Theoretical work on energy control (unbalanced supply etc)
– Closed loop control and Energy management
Parallel Hybrid Topology
-0.02 -0.01 0 0.01 0.02
-150
-100
-50
0
50
100
150
Time [s]
Voltage [
V]
Line-Line voltages at the converter terminals
• Chain-link cell capacitors have a tendency to charge or discharge • Have to control the voltage of the individual converter chain-link capacitors
• Balancing can be achieved with a sorting algorithm based on the characteristics of the chain-link cells in a given chain-link position
-180-150
-120-90
-60 -30
0 30
60 90
120 150
180
12
34
56
-1
0
1
power facto
r
angle
Change in chainlink cell voltage with the 'virtual position' of the cell in the chainlink for different power factors
Chainlink
cell Position
V
c /
A
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
Parallel Hybrid Topology
Capacitor Voltage Control
one phase with set point = 50V
Series Hybrid Topology
Vcl1
Vcl2
0
S1
Vs1
Vs2
10kV
10kV
0 5 10 15 20-15
-10
-5
0
5
10
15
S1 ON
S2 ON
Condition for zero
energy exchange with
chain-links
VAC(peak) = 2EDC/
ZVS
VOFF < 20kV
Series Hybrid Topology - AAC
S1
ADVANTAGES
• Series IGBT switches commutate at near zero voltage • Reduce switching losses
• Improves converter efficiency
• Series H-bridges can support the AC voltage when there is a DC side fault • Actively control AC side current to zero
• No need to interrupt fault current with AC side breaker
• Actively control AC current to be reactive
• Gives option of STATCOM performance during DC side fault
Series Hybrid Topology - AAC
2-level converter
Half-link M2LC
Full-link M2MC
Series hybrid
Parallel hybrid
Total Semiconductor count (pu)
1 2 4 2.5 1.5
Total submodule DC capacitor rating (pu)
0 1 1 ~0.5 <0.25
Losses
AC Harmonic performance
Hard-switched IGBT valve needed?
Ability to suppress DC faults?
Topology Comparisons
Summary
• Power Converters will be an essential part of the future Electrical Energy Grid
• Renewable Energy Sources are not directly compatible with the grid – Requirement for Power Conversion for all power source
connections
• Challenges for Power Converter deployment in the Electricity Grid – Cost [both purchase cost and cost of losses] – Reliability/availability – Current regulations and legacy equipment
• Many other topologies exist for AC/AC and AC/HVDC – Newton-Picarte project between Universities of
Nottingham/Talca/Concepcion will look at some alternatives – Kick Off meeting was held in Talca a couple of weeks ago!
2015 Season eSuperBike
• 2015 electric bike will have 3 x Torque of the 2014 bike
• Custom designed and built frame
• Motor and Controller built by the team in Nottingham
• TT on Isle of Man in June 2015!
• > 280km/hr
2014 Bike
2015 Bike
