Download Importance of DC-DC Transformation in Grids of the Future

• Session 1B ‐ Monday, Oct. 11, 2015
Importance of DC-DC Transformation in
Grids of the Future
L. Barthold, M. Salimi, D. Woodford
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The Expanding Role of DC
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The Expanding Role of DC
Europe
1. The prospect of HVDC overlays to AC
systems Europe and North America
DC Grid Benefits:
•
•
•
•
North America
Time Diversity
Best use of efficient sources
Easier Frequency regulation
Improved load limits on underlying AC
DC:DC Transformation Needs/Challenges
•
•
•
•
•
Flow regulation within a grid system
Interchange with existing HVDC lines
Fault Isolation
Voltage boost on long HVDC lines
Coupling of dc systems having differing grounding and/or
commutation system.
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The Expanding Role of DC
2. Growth in DC generation
- Solar
Synchronous AC
Asynchronous AC
HVAC
DC
- Wind (?)
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The Expanding Role of DC
Proposed Atlantic Wind Connection
2. Growth in DC generation
DC:DC Transformer Benefits:
• Reduced nacelle weight, size, cost
32 kV DC
•
•
•
•
320 kV DC
Lower Maintenance
internal transformer redundancy
Reduced cable costs with dc
On-shore reactive power support
HV AC
Shore
Grid
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The Expanding Role of DC
3. Energy Storage
• Batteries now dominate
• Ratings…now up to 30 MW
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The Expanding Role of DC
4. MicroGrids … DC now favored
1. DC growing rapidly as % of
ultimate load
2. DC grows as % of Local
Generation
3. Local Storage will be DC
4. Flexibility in degree of
dependence on overlying ac
system.
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DC:DC Transformer
Requirements
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DC:DC TRANSFORMER REQUIREMENTS
1. High MW ratings(> 1,000 MW)
2. Efficiency comparable to VSC bridges
3. Power flow proportional to ∆(V1/V2) without need for a power control signal
4. Ability to control flow as an AC transformer does through tap changes
5. Ability for bidirectional flow
6. Produce relatively smooth input and output current with a small filtering
burden
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DC:DC TRANSFORMER REQUIREMENTS
7. Modular in structure to reduce cost, increase design carryover
8. Interruption-free redundancy in the event of component failures
9. Equal voltage division among modules to minimize switching and insulation costs
7. Isolation of primary or secondary faults
8. Use existing components to provide reliability carry-over
9. Transform between systems differing in grounding and/or commutation systems
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A Multi-Module DC Transformer (MMDCT)*
* US & International Patents Pending
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Simplified Principle of Operation
Step1: Receive energy from
one bus
Step2: Deliver the energy to the
other bus
Various partial by-pass techniques achieve step-up or step-down operation
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Step 2
Step 1
Simplified Principle of Operation
Input Current
Output Current
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Energy Exchange in DC Resonance Circuit
Capacitor Voltage
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V2
Vdc
DC Source
V1  VC (0)
Vdc
V1
Current
Vdc  V1
0
T
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Confidential
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Multi-Module DC Transformer
(MMDCT)
Bus 1
Bus 2
V2
V1
Three Parallel Modules:
• Smooth input and output current waveforms
• MW rating triples
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Comparison of MMDC with an AC Transformer
Characteristic
AC
DCT
Magnetic
Capacitive
MW Range
High
High
Responds to
∆θ
∆V
Controller?
No
No
High
High
Variable
Variable
Bi-directional
Bi-directional
Modular?
No
Yes
Internal Redundancy
No
Yes
Primary-Secondary
Fault Isolation?
No
Yes
Medium
Efficiency
Voltage Ratio
Power Flow
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Simulation Example
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CIGRE B4 DC Grid Test System
DCS1
Ba-A0
Bo-C1
200
200
50
Bb-A1
200
Bo-C2
300
500
400
DCS3
Bo-D1
200
200
Ba-B0
200
200
DC Sym.
Monopole
DC Bipole
AC Onshore
AC Offshore
Cable
Overhead line
300
200
Bo-E1
200
200
200
DCS2
AC-DC Converter
Station
DC-DC Converter
Station
200
Bm-B2
100
200
Ba-B3
100
Bo-F1
CIGRE B4 DC Grid Test System Comparison
With idealized DC transformer
Identical PSCAD model with MMDCT
Input and output DC Voltages
Input and output DC Voltages
Input and output DC Currents
Input and output DC Currents
Transformation Between
Dissimilar HVDC systems
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MMDCT Coupling two grounded-bipole systems
+
+
S2
S3
S4
S6
S5
~=
~=
S1
~=
~=
S6
S5
S4
S3
S1
-
S2
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+
+
S1
S2
S3
S4
S5
S6
~=
~=
S6
S5
S4
S3
S1
-
S2
-
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+
Half Bridge Sub-Modules
+
S1
S2
S3
S4
S6
S5
~=
Symmetrical
Monopole
System
Grounded
Bipole System
~=
~=
S6
S5
S4
S3
S1
-
S2
-
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+
+
S1
S2
S3
S4
S6
S5
~=
Symmetrical
Monopole
System
Grounded
Bipole System
~=
~=
S6
S5
S4
S3
S1
-
Full Bridge Sub-Modules
S2
-
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Conclusions
1. DC’s role in the electricity supply game will increase steadily
2. That change will demand an efficient DC:DC transformer the
performs, within a dc context, in a manner analogous to an ac
transformer in an ac context.
3. Among several approaches being proposed, the MMDCT
appears best at satisfying all performance requirements.
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Questions?
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