1. 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

2. The Expanding Role of DC

3. The Expanding Role of DC
1. The prospect of HVDC overlays to AC systems Europe and North America
Europe
DC Grid Benefits:
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.
North America

4. The Expanding Role of DC
2. Growth in DC generation
- Solar
Asynchronous AC
Synchronous AC
HVAC DC
- Wind (?)

5. The Expanding Role of DC
Proposed Atlantic Wind Connection
2. Growth in DC generation
DC:DC Transformer Benefits:
Reduced nacelle weight, size, cost
Lower Maintenance
internal transformer redundancy
Reduced cable costs with dc
On-shore reactive power support
32 kV DC
320 kV DC
HV AC Shore Grid

6. The Expanding Role of DC
3. Energy Storage
Batteries now dominate
Ratings…now up to 30 MW

7. The Expanding Role of DC
4. MicroGrids … DC now favored
DC growing rapidly as % of ultimate load
DC grows as % of Local Generation
Local Storage will be DC
Flexibility in degree of dependence on overlying ac system.

8. DC:DC Transformer Requirements

9. DC:DC TRANSFORMER REQUIREMENTS
High MW ratings(> 1,000 MW)
Efficiency comparable to VSC bridges
Power flow proportional to ∆(V1/V2) without need for a power control signal
Ability to control flow as an AC transformer does through tap changes
Ability for bidirectional flow
Produce relatively smooth input and output current with a small filtering burden

10. DC:DC TRANSFORMER REQUIREMENTS
Modular in structure to reduce cost, increase design carryover
Interruption-free redundancy in the event of component failures
Equal voltage division among modules to minimize switching and insulation costs
Isolation of primary or secondary faults
Use existing components to provide reliability carry-over
Transform between systems differing in grounding and/or commutation systems

11. A Multi-Module DC Transformer (MMDCT)*
* US & International Patents Pending

12. 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

13. Simplified Principle of Operation
Input Current
Output Current
Step 1
Step 2

14. Energy Exchange in DC Resonance Circuit
Confidential
Energy Exchange in DC Resonance Circuit
Capacitor Voltage
Current
DC Source

15. Multi-Module DC Transformer (MMDCT)
Bus 1
Bus 2 V1 V2
Three Parallel Modules:
Smooth input and output current waveforms
MW rating triples

16. Comparison of MMDC with an AC Transformer
Characteristic AC
DCT
Medium
Magnetic
Capacitive
MW Range
High
Responds to ∆θ ∆V
Controller? No
Efficiency
Voltage Ratio
Variable
Power Flow
Bi-directional
Modular?
Yes
Internal Redundancy
Primary-Secondary Fault Isolation?

17. Simulation Example

18. CIGRE B4 DC Grid Test System
Ba-A0
Ba-B0
DCS1
200 50
300
400
500
100
DCS2
DCS3
Bb-A1
Bo-C2
Bo-C1
Bm-B2
Bo-D1
Bo-E1
Bo-F1
Ba-B3
DC Sym. Monopole
DC Bipole
AC Onshore
AC Offshore
Cable
Overhead line AC-DC Converter Station DC-DC Converter Station

19. 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

20. Transformation Between Dissimilar HVDC systems

21. MMDCT Coupling two grounded-bipole systems
150
MMDCT Coupling two grounded-bipole systems + + S1 S2 S3 S4 S5 S6 ~ = ~ = ~ = = ~ S5 S6 S3 S4 S1 S2 - -

22. 150 + + S1 S2 S3 S4 S5 S6 ~ = = ~ S5 S6 S3 S4 S1 S2 - -

23. ~ ~ ~ = = = - - Half Bridge Sub-Modules + +
150
Half Bridge Sub-Modules + + S1 S2 S3 S4
Symmetrical Monopole System
Grounded Bipole System S5 S6 ~ = ~ = = ~ S5 S6 S3 S4 S1 S2 - -

24. ~ ~ ~ = = = - - Full Bridge Sub-Modules + +
150 + + S1 S2 S3 S4
Symmetrical Monopole System
Grounded Bipole System S5 S6 ~ = ~ =
Full Bridge Sub-Modules = ~ S5 S6 S3 S4 S1 S2 - -

25. Conclusions
DC’s role in the electricity supply game will increase steadily
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.
Among several approaches being proposed, the MMDCT appears best at satisfying all performance requirements.

26. Questions?