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• Session 1B ‐ Monday, Oct. 11, 2015 Importance of DC-DC Transformation in Grids of the Future L. Barthold, M. Salimi, D. Woodford i MOD nc 1 The Expanding Role of DC i MOD nc 2 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. 3 The Expanding Role of DC 2. Growth in DC generation - Solar Synchronous AC Asynchronous AC HVAC DC - Wind (?) 4 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 5 The Expanding Role of DC 3. Energy Storage • Batteries now dominate • Ratings…now up to 30 MW 6 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. 7 DC:DC Transformer Requirements i MOD nc 8 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 9 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 10 A Multi-Module DC Transformer (MMDCT)* * US & International Patents Pending i MOD nc 11 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 12 Step 2 Step 1 Simplified Principle of Operation Input Current Output Current 13 i Energy Exchange in DC Resonance Circuit Capacitor Voltage i V2 Vdc DC Source V1 VC (0) Vdc V1 Current Vdc V1 0 T 2 Confidential 14 MOD nc Multi-Module DC Transformer (MMDCT) Bus 1 Bus 2 V2 V1 Three Parallel Modules: • Smooth input and output current waveforms • MW rating triples 15 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 16 Simulation Example i MOD nc 17 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 i MOD nc 20 MMDCT Coupling two grounded-bipole systems + + S2 S3 S4 S6 S5 ~= ~= S1 ~= ~= S6 S5 S4 S3 S1 - S2 21 + + S1 S2 S3 S4 S5 S6 ~= ~= S6 S5 S4 S3 S1 - S2 - 22 + Half Bridge Sub-Modules + S1 S2 S3 S4 S6 S5 ~= Symmetrical Monopole System Grounded Bipole System ~= ~= S6 S5 S4 S3 S1 - S2 - 23 + + S1 S2 S3 S4 S6 S5 ~= Symmetrical Monopole System Grounded Bipole System ~= ~= S6 S5 S4 S3 S1 - Full Bridge Sub-Modules S2 - 24 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. i MOD nc 25 Questions? i MOD nc 26