報告_OCP2026_Siemens_800VDC兩線與三線接地_20261015

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原始內容

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2-Wire vs. 3-Wire at 800VDC: Grounding, Fault Behavior, and Protection for AI Data Centers AI OPEN DATA CENTER: Power Distribution Guang Yang, Director of R&D, Siemens Lizhi Wang, Research Scientist, Siemens Acknowledgement: We would like to thank our former Siemens colleague Gopal Mondal for his contributions to this study.

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800 VDC in Data Centers Presents New Challenges AI demand is rewriting power economics, and OCP is active in development • Both architectures can maintain safe and resilient operations—at what cost? • Which practices are critical in both architectures? Where 2-wire and 3-wire differ: • Build cost and hardware count • Power density and usable capacity • Protection cost and downtime exposure • … OCP Diablo 400 refers to ±400VDC OCP SST Specs refer to 800VDC

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Two DC Architectures, Two Grounding Options 3-wire: 400 V across P–M and M–N Floating / IT Floating / IT Solid / TN-S Solid / TN-S 35k VAC 4.6MW SST Source 1 MW 800 VDC Rack AC DC 35k VAC 4.6MW SST Source 1 MW 800 VDC Rack AC DC 1 MW 800 VDC Rack 4.6MW SST Source 35k VAC AC DC 1 MW 800 VDC Rack 4.6MW SST Source 35k VAC AC DC Both architectures evaluated with floating and solid grounding! 2-wire: 800 V across P–N

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04 Trade-offs in One View 2-Wire 3-Wire System implementation System complexity Low High Load Voltage 800 VDC ±400 VDC Balancing components No Yes 35k VAC 4.6MW SST Source 1 MW 800 VDC Rack AC DC 35k VAC 4.6MW SST Source 1 MW 800 VDC Rack AC DC Power delivery Power density w/ same conductor size High Low Load flexibility Low High Protection implications Fault energy High Low

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Local Storage: Power Quality v.s. Fault current Local storage sizing for ride-through during voltage dips Local storage discharge current Increasing storage extends ride-through Increasing storage effectively remove high frequency components Local storage sizing for dynamic loads Increasing storage can contribute to large fault current 1 MW 800 VDC Rack 4.6MW SST Source 35k VAC AC DC Storage Storage has value—but oversized storage without fast protection becomes a fault liability.

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35k VAC 4.6MW SST Source 1 MW 800 VDC Rack AC DC Parasitic Capacitance Needs to Be Considered for Safety Parasitic capacitance discharge through human body in ungrounded system Human Body Current vs. Parasitic Capacitance Higher capacitance → longer exposure • At the first ground fault, parasitic capacitance discharges through the body. • Higher capacitance prolongs exposure: 0.3 ms at 0.01 µF vs. 151 ms at 5 µF. • Both architectures behave similarly; limiting parasitic capacitance improves personnel safety. Parasitic Capacitance (µF) IEC 60479-2 effect zone UL 1400-1 Current- Duration Assessment <0.5 No fibrillation risk Compliant 1 No fibrillation risk Exceeds threshold 5 Low to average risk of fibrillation Exceeds threshold

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Half-Isolation Is Not Isolation IT P–P Fault: Half-Bus Capacitor Voltage Reversal Opening a single pole is not enough to take a rack safely offline. Opening one pole can reroute current and redistribute voltage—it does not guarantee that the rack is de-energized. 3-Wire System: Effects of Uncoordinated Pole Clearing Positive pole sees negative voltage if negative pole is not interrupted • 3-wire o IT P–P Fault: partial clearing can reverse the voltage on half the bus. o TN-S P–G Fault: current still flows after one pole opens through the other pole—the rack is not isolated. • 2-wire o IT Double ground fault: capacitors in faulty racks continue to be charged by DC bus and stay at full voltage！

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Half-Isolation Is Not Isolation – Not Safe for Maintenance • Current NEC does not require grounded pole to open • Potentially hazardous conditions during maintenance if a ground fault happens elsewhere DC unique fault scenario as large DC link capacitor keeps the voltage up during discharge Possible personnel-current path Human body current (A) Opening a single pole is not enough to take a rack safely offline. 2-Wire grounded System

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Protection device: Faster Protection Reduces Total Cost of Ownership (TCO) • Faster interruption significantly improves power quality during interruption • Partial-isolation risk: Uncoordinated pole opening shifts the DC bus relative to earth. Interruption performance Efforts to achieve selectivity Protection against capacitor discharge Realistic fault protection Disconnect switch needed? Storage needed for operation stabilities Redundancy design flexibility Heat dissipation Component cost Fuse Ok High Inconsistent Inconsistent Yes Large Low Med Low SSCB Excellent Low Excellent Excellent No Small High Med High • Faster interruption significantly reduces local storage needed for power quality • Fuse: 300mF for 373V • SSCB: 10mF for ~400V Example: DC bus voltage during double ground fault in floating (IT) systems DC Bus Pole-to-Ground Voltages in a 2-Wire System with IT Grounding

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Protection Against Capacitor Discharge • Performance of both MCCBs and fuses are sensitive to circuit conditions, e.g. cable length. Restrikes possible, which increases interruption time • SSCBs are not affected by cable length 800VDC 20mF discharge over 6-ft of cable 800VDC 20mF discharge over 10-ft of cable MCCB restriked Fuse NOT fully-blown, likely to restrike if left longer

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Consider Realistic Faults At 65kA, breaker significantly limits current to 4.8kA At 1.1kA, peak current is ~1.5kA. Breaker relies on zero-crossing AC fault interruption with a circuit breaker DC fault interruption MCCB AC/DC converter interrupted. No selectivity Fuse(Low current is easier to cause restrike) • In AC, bolted-short circuit faults are worst- case scenarios • Zero-crossings improves interruption performances at lower fault current levels • In DC, bolted-short circuit faults are NOT worst-case scenarios • Need to consider realistic faults: • Failed capacitors • Failed insulations • Magnetizing current that drives high primary side current • Clamping components failures • Power electronics failures • … • SSCBs are not affected by fault current levels

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What to Do Next Actions for your DC distribution decision 1 Plan for all fault scenarios Assess realistic fault scenarios—capacitor discharges, realistic fault currents, full range coordination and so on 2 Recognize the unique aspects of DC distribution and the unique aspect of data center applications 3 Choose architecture, grounding, redundancy and protection together for safe, stable and serviceable operations Protections for Data Centers Powered by Direct Current Comparison of 2-Wire and 3-Wire DC Power Architectures for Hyperscale Data Centers Based on SST-Interfaced Distribution Systems 4 Workforce training to raise awareness, proper operating procedures and PPE

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Thank You!