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

The Expanding Role of DC

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

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

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

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

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.

DC:DC Transformer Requirements

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  

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

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

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

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

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

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

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?

Simulation Example

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

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

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 - -

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

~ ~ ~ = = = - - 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 - -

~ ~ ~ = = = - - 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 - -

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.

Questions?