High-Voltage, High-Current DCDC Converters
Applications and Topologies
Converters Theme
Underpinning Research
DC Power Networks
• DC power can reduce losses and allow better
utilisation of conductor ratings.
• Power Networks must accommodate growing levels of
power electronic connected generation and loads:
• Most power electronic converters operate from DC.
• Elimination of intermediate AC interconnection
appears to reduce the number of conversion
stages required.
Underpinning Research
Advantages of DC Power
Networks
Potential Benefits
• Elimination of charging currents and
• Reduced conductor losses
• Reduced size and weight ?
Underpinning Research
Ac vs. dc distribution for off-shore power delivery Wang, F.; Yunqing Pei; Boroyevich,
D.; Burgos, R.; Khai Ngo Industrial Electronics, 2008. IECON 2008. 34th Annual
Conference of IEEE
Reduced Conversion Stages
dc/AC
AC/dc
G
G
ac/dc
dc/ac
dc/dc
dc/AC
ac/DC
DC/ac
DC/dc
G
Loads
ac/dc
Loads
G
ac/DC
Underpinning Research
Offshore Wind Connection
MVAC/
HVDC
DC/DC converters required to interface
Generation- Collection Network
Collection Network-Transmission
Underpinning Research
DC-DC converters must be rated for the
region
≈3kV: 50kV Collection
≈50kV: 800kV Collection
MVDC/
HVDC
DC/DC Converters
• Different DC voltages may be preferable at generation,
collection/distribution and transmission stages.
• DC/DC converters will be required to interface these stages.
• HVDC networks may require DC/DC interface to connect different
transmission (legacy) voltages.
• Meshed DC networks may require DC/DC converters to provide
auxiliary functions.
• Balancing power flows in meshed DC networks
• Controlling DC fault currents stages required.
Underpinning Research
DC-DC converter options
Vin
Switching
stage with
DC-offset
DC Power Transfer
Filter
Vout
• Single switching stage required.
• Poor device utilisation (Switches must be rated for highest voltage and current )
• High switching frequency required to minimise filter
Vin
Switching
stage with
AC output
Passive
Network
And/Or
Transformer
Rectification
stage
Switching
Frequency Power
Transfer
Vout
• Two switching stages required.
• Transformer may be used to improve device utilisation
• Resonant passive network may be used to reduce switching loss
Underpinning Research
Challenges for high capacity DCDC converters
• Voltage: Voltages exceed the capabilities of single switching
devices.
• Series connection
• Multi-level/modular techniques
• Efficiency: The use of DC/DC for network applications must not
compromise gains in system losses.
• Size: Raised switching frequencies necessary in order to reduce
size of passive components and coupling transformers.
• Maintain device switching speeds
• Soft switched circuits
• Isolation: Galvanic Isolation
• Transformer coupled circuits
Underpinning Research
DC-DC Transformer: Requirements
•
•
•
•
•
•
•
•
•
•
•
Voltage step up/down using an AC transformer.
Device Utilisation and galvanic isolation.
Lower medium frequency range (< 1 kHz).
Device constraints and switching losses.
Not resonant.
Avoiding high internal voltage stresses and tuning problems.
Modularity.
Facilitating manufacturing and installation.
DC fault blocking.
Protection and reliability of supply.
Bidirectional.
Underpinning Research
Series Connection
• Dynamic voltage sharing may limit the achievable switching speed of individual
devices.
• HVDC links have shown that the use of series connection of IGBTs is feasible
to the region of ±200kV
• Power frequencies up to 1-2kHz may be achievable to reduce the size and
weight of magnetic components. This will result in overall semiconductor
losses of a similar level to that of a3 two-level HVDC converter.
• Switching large voltage steps, such as 400kV or higher, at 1 or 2kHz impresses
extremely high dv/dt upon passive components and interfacing transformers.
Underpinning Research
DC-DC Transformer:
Dual Active Bridge, Series Devices
Underpinning
Research
AC transformer
Modularity
Not resonant
dv/dt stress
Bidirectional
Voltage sharing
DC fault blocking
High frequency
Multi-Level Techniques
• A number of multilevel DC/AC topologies exist which could be applied
to transformer coupled DC-DC converters
• Multi-level can decouple switching frequency from output power quality
in DC-AC converters working at power frequencies of 50-60Hz resulting
in significant efficiency improvements.. However:
• Advantages for DC-DC conversion are not clear since efficiency will
reduce if raised power frequency employed to reduce transformer size.
• Capacitor energy storage requirement and resulting volume can be
high. (Reduction in capacitor requirement with raised power frequency.)
Underpinning Research
DC-DC Transformer:
DAB with multi-level converter
Front-to-front MMC connection
Underpinning
Research
AC transformer
MMC half-bridge cells
Not resonant
Acceptable (Quasi-two level)
Bidirectional
Equal static and dynamic sharing
DC fault blocking
Low frequency (250-500Hz)
DC-DC Transformer: MMC Topology
Quasi Square-Wave Switching
Front-to-front MMC connection
•
Very small cell capacitance (small cell volume).
•
Very small arm inductance.
•
Device voltage limited by cell capacitance.
•
Device switching speed does not compromised for voltage sharing.
•
Controlled dv/dt
•
The arm current contains no common-mode component, except during staircase
switching periods.
DC-DC Transformer: Simulation
Single Q2L converter
N=10
10+1 level output
Underpinning
Research
DC-DC Transformer: Simulation
Single Q2L converter
Ccell=50μF
Ripple = 3%
Underpinning
Research
DC-DC Transformer: Simulation
Single Q2L converter
2% ripple
Transformer
Array
i0utput Converter Array
Input Converter Array
Modular Transformer Coupled
DC-DC
The DC-DC converter consists of an array of transformer coupled DC-DC converters each of
which operates at a voltage compatible with a single power semiconductor device.
DC-DC converter modules can be sized to optimise size and efficiency independent of system
voltage requirement.
Modular Transformer Coupled
DC-DC
• Reduced current stress:
Input parallel connection reduces switching device current stress
• Reduced voltage stress:
All modules operate at a voltage compatible with a single power semiconductor rating.
• Size and Weight:
Switching frequency similar to established DC/DC converters. Significant reduction in the size and
weight of the magnetic component possible.
• Redundancy:
High availability could be achieved by introducing the desired level of redundant cells
• Modular Structure:
Standardised components reduces cost of converter
• Disadvantages:
Multiple transformers, isolation, complex control.
Modular Transformer Coupled
DC-DC Building Blocks
Front-end Inverter
Unit
M1
Transformer
Unit
M2
Output Rectifier/Inverter
Unit
M11
M22
85mH
Lout1
10mH
Vin = 1kVdc
Inductor
Unit
Lk
Cin
M4
M3
Cout
T1
1:2
Lout2
85mH
M44
M33
Vout = 1kVdc – 1.5kVdc
Iout = 10A
Active bridge can be
replaced by rectifier
for unidirectional
power flow
Building blocks may be standard DC-DC converter modules sized to optimise size
and efficiency independent of system voltage requirement.
e.g. Single Dual active bridge may be used for bidirectional power flow.
Underpinning
Research
Modular Transformer Coupled
DC-DC
Input Parallel +Series
-Output Series
Input Parallel -Output Series
Module 1

iin1
Tr1

Vcd 1
Vo1


iin 2
Module 2
Tr2

Vo 2

Vin
Module n
iinn
Trn

Von

Module n+1

iin ( n 1)
Tr(n+1)
Vcd 2

Vo
Vo ( n 1)


iin ( n 2)
Module n+2
Tr(n+2)

Vo ( n 2)

Module 2n
iin (2 n)
Tr2n

Vo (2 n )

System voltage and current requirements can be met through a combination of
parallel and series connection at both input and output terminals.
Cell Balancing Control
Control must control output
voltage and balance cell voltage.
Unbalanced cell parameters will
lead to unbalanced cell voltage
and poor dynamics
Common duty cycle may fail to respond to
parameter mismatch or cell un-balance
Underpinning
Research
Individual cell control with current demand
offset to correct cell unbalance
Cell Balancing Control
Module 1
Module 2
Module 3
Module 4
Test 1
Output
Capacitance
60μf
50μf
50μf
50μf
Test 2
Transformer Turns
Ratio
1:1.125
1:1
1:1
1:1
(a)
(b)
Response to step change in load voltage associated with mismatched
output capacitance (a) module output voltage (b) output voltage
mismatch between Module 1 and Modules 2, 3 and 4
Fault Ride-Through
+
Vo2
-
No.
1st
2nd
3rd
4th
5th
Fault Type
Short-circuit fault on the module end
side
Open circuit fault
Current transducer fault
Over-current fault
Master converter fault
Short circuit fault
Non-dedicated master control
scheme
Fault Ride-Through
+20% mismatch
Master
converter
fault at
t=50ms
+10% mismatch
System Specifications
output voltage
module output voltage
module inductor current
Conclusions
The lack effective DC/DC converters remains one of the
barriers to expansion of DC power networks.
High capacity DC/DC converters are feasible.
A wide range of topologies are under investigation but it is
unclear if any deliver the advances in efficiency and power
density necessary for utility scale DC applications.
Underpinning Research