A. Baseline Parameters (Operational Model)

• Fleet Size: 50 × 12m Electric Buses

• Daily Energy Demand: ≈ 12.5 MWh/day (avg. 250 kWh per vehicle)

• Charging Window: 8 hours (Overnight)

• Charging Infrastructure: 120 kW DC charging stations OR 44 kW AC charging points

• Power Management: Dynamic load management to prevent grid connection overload

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1) Depot Zoning (funkcional units)

Zone Z1 – Parking & Charging

• 50 designated parking bays equipped with charging points.

• Cabling infrastructure, cable trays, and mechanical impact protection.

• Charging status signaling and emergency shutdown protocols within the zone.

Zone Z2 – Maintenance & Workshop

• Service bays with integrated diagnostic tools.

• Independent power branch to isolate electrical interference.

• Power provisions for lifting equipment, compressors, and workshop machinery.

Zone Z3 – Bus Wash & Cleaning

• Technological equipment with dedicated circuit protection and metering.

• Priority: Electrical isolation from the charging zone to ensure the bus wash does not "drain" power during peak charging hours.

Zone Z4 – Administration / IT / Dispatch

• Server rooms, communication hubs, and security systems.

• Recommended UPS (Uninterruptible Power Supply) for critical IT and control systems.

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2) Grid Connection & Substations (MV/LV)

Recommended Configuration (for 50 units with DC charging): MV Connection + Private Substation.

• Medium Voltage (MV) Connection: Typically 22 kV (subject to local distribution network).

• Substation: 2 × Transformers (Redundancy / N+1 Philosophy).

  • Example: 2 × 2.5 MVA (one transformer covers base load; two cover full operation + reserve).

• LV Distribution: 0.4 kV Main Switchboard (MSB) with dedicated feeders for each zone.

• Why Dual Transformers? Enables maintenance without downtime and provides scalability for future depot expansion.

 

3) Power Sizing (Example: 50 Units – 120 kW DC)

• P_peak ≈ 50 × 0.7 × 120 kW = 4.2 MW

 

Recommended Connection/Transformer Capacity (including reserve + auxiliary tech):

• ≈ 5.0 MW (Approximate)

 

LV Power Distribution Breakdown (Example):

• Charging (Z1): 4.2 MW

• Workshop/Service (Z2): 0.2–0.4 MW

• Bus Wash (Z3): 0.1–0.3 MW

• Admin/IT (Z4): 0.05–0.1 MW

• Expansion Buffer: 10–20%

 

4) Charging Topology (Electrical Architecture)

Recommended DC Depot Charging Scheme:

• Main Switchboard (MSB) → Feeders to Sub-Switchboards (SSB) for designated parking sectors.

• Partition the parking lot into sectors (e.g., 5 sectors of 10 bays each).

• Per Sector:

  • SSB (circuit breakers, protection, metering).

  • Power supply for DC stations (or DC Power Cabinets + Satellites).

Benefits of Sectoring:

• Shortened cable runs.

• Simplified maintenance (only one sector needs to be powered down).

• Clear protection selectivity.

 

5) Protection, Safety & Power Quality

• Protection Selectivity: Coordinated hierarchy from Main Breaker → Sector → Individual Station (time/current curve grading).

• Residual Current Protection: RCD/Leakage protection based on charger type (AC typically Type B or equivalent).

• Surge Protection (SPD): Installed at LV entry, sector switchboards, and sensitive IT nodes.

• Power Factor Correction & Harmonics: * Assessment of Total Harmonic Distortion (THD) and the need for filters is highly recommended for high-power DC installations.

• Grounding & Bonding: Rigorous implementation throughout Zone Z1 (metal structures, chargers, bays).

 

6) Metering & Data Layer (EMS)

Electrical Metering:

• Billing Meter: Primary utility-grade meter at the entry point.

• Sub-metering:

  • By Sector (Z1-S1 through S5).

  • By Function (Workshop, Wash, Admin).

  • Real-time monitoring of peak loads and load profiles.

Energy Management System (EMS) / Load Management:

• Centralized charging control based on:

  • Scheduled departure times.

  • Required State of Charge (SOC).

  • Time-of-Use (ToU) tariffs.

  • Grid connection power limits.

• Prioritization: Morning routes > Backup vehicles > Low priority units.

Communication:

• Ethernet/Fiber optics to all sectors (highly recommended).

• Isolated VLANs for charging infrastructure vs. administrative traffic.

• Cybersecurity: Network segmentation and robust access policies.

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7) Backup Power Systems (example for 50 pieces)

Battery Energy Storage Systems (BESS) – Two Primary Objectives:

1. Peak Shaving: Reducing reserved power capacity.

   • Power: 1–3 MW / Capacity: 2.5–5 MWh.

2. Operational Continuity: Control systems + limited emergency charging.

   • Power: 0.5–1.5 MW / Capacity: 0.5–2 MWh.

• UPS for Dispatch/IT: 10–30 kVA (depending on scope).

• Note: Full-scale charging backup for long durations requires massive capacity. In practice, backup is defined as maintaining control continuity + partial charging for high-priority vehicles.

Scaling Guidelines (Quick Reference)

• 20 Units: DC Peak (70%): ≈ 1.68 MW → ~2.0 MW Connection. (Typically 1 × Transformer, e.g., 2 MVA).

• 100 Units: DC Peak (70%): ≈ 8.4 MW → ~10 MW Connection. (Recommended: 2–3 Transformers and 10-sector partitioning).