1.Objectives and Requirements
Design a 8-in-1-out AC combiner cabinet. As it is an off-grid project, it only requires combiner functionality.
According to the principle diagram above, the cabinet specifications are 3000kVA/800V. The total current entering the cabinet is: 350kW * 8 / 800V / 1.732 ≈ 2020A. The average current per branch is 253A.
2.Main Equipment Calculation and Design
Input Molded Case Circuit Breakers (MCCBs)
Quantity: 8 units, matching the "8-in" topology. One breaker per input branch for independent protection, isolating only the faulty branch without affecting others.
Current Rating: Total output current ~2020A. Divided equally among 8 branches gives a maximum of ~253A per branch. A 400A breaker is selected to prevent overload tripping during prolonged operation. (Formula: I_total = (P_total) / (√3 * V_LL) )
Poles: 3P (Three-phase). The core of the off-grid AC system is the three-phase power circuit. The neutral line is connected separately via terminals (not broken by the breaker), simplifying design and ensuring reliable protection for the main three-phase circuit.
Breaking Capacity: ≥50kA, capable of quickly interrupting estimated single-branch short-circuit current (~31.5kA) at 800V, preventing arc damage and ensuring reliable fault isolation.
Output Frame Circuit Breaker (FCB)
Quantity: 1 unit, matching the "1-out" topology. One main breaker controls the combined output from the 8 inputs.
Current Rating: Total current ~2020A. A 4000A frame breaker is selected (considering non-continuous full load and redundancy in busbars/cables), supporting short-time overload and meeting high-power output demands.
Undervoltage Delay Release: Off-grid systems lack grid support and are prone to voltage fluctuations. The delay function prevents nuisance tripping during transient voltage dips, ensuring power supply stability.
Poles: 3P, suitable for the main three-phase combined busbar. The neutral is wired independently, not passing through the main breaker, focusing protection (overload, short-circuit) on the three-phase main circuit.
Breaking Capacity: ≥63kA, effectively interrupting the estimated total bus short-circuit current (~40kA) with fast breaking and good arc extinguishing, preventing fault escalation.
Rotary isolator switch (isolating switch)
Current Rating: Matches the output side's maximum current of 2020A to prevent heating during isolation.
Function: For maintenance in off-grid systems, it provides a physical, arc-free break in the three-phase main circuit, offering more reliable isolation than a breaker for personnel safety.
Poles: 3P, simultaneously disconnecting all three phases of the main circuit, matching the breaker's configuration. The neutral does not require isolation (confirmed dead separately during maintenance), simplifying operation while meeting safety standards.
AC Surge Protective Device (SPD)
Voltage Rating: Matches the system's 800V AC rated voltage to prevent SPD failure.
Nominal Discharge Current (In): Off-grid scenarios lack coordinated grid lightning protection. It must independently withstand lightning surge voltages. A 100kA capacity covers requirements for moderate lightning areas, absorbing lightning energy.
Poles: 4P (Three-phase + Neutral). Although breakers are 3P, the neutral line still requires surge protection to prevent equipment damage via lightning overvoltage on the neutral, providing comprehensive circuit protection.
3. Equipment Selection and Bill of Materials (BOM)
Based on the system specifications (3000kVA/800Vac, 8-in-1-out, 253A per branch, 2020A total), select appropriate equipment.
Cabinet Enclosure
Dimensions: Width 1000mm, Depth 1000mm, Height 2200mm, Must accommodate 8 input breakers, 1 large output breaker, busbars, current transformers, etc. Width, depth, and height must allow for component layout, cooling space, and cable routing.
Material: Metal for high mechanical strength, resistance to outdoor environmental corrosion and impact, suitable for potential outdoor installation in off-grid scenarios.
Protection Rating: IP54 for dust and water protection, preventing ingress that could cause high-voltage shorts, ensuring outdoor operation safety.
Equipment Selection Summary (Table)
|
Item |
Specification |
Qty |
Selection Basis (Brief) |
Picture |
|
Input MCCB |
800V, 400A, 3P, Breaking Capacity ≥50kA |
8 |
Branch protection (253A load). 400A provides margin. |
|
|
Output FCB |
800V, 4000A, 3P, Breaking Capacity ≥63kA, with UV delay |
1 |
Main circuit protection & control (2020A total). Provides headroom and stability. |
|
|
Disconnect Switch |
800V, ≥3200A, 3P |
1 |
Safe, physical isolation for maintenance on main bus. |
|
|
AC SPD |
800V, In=100kA, 4P |
1 |
Comprehensive surge protection for phases and neutral in off-grid setting. |
|
|
Terminals (UT6-16) |
UT6-16 |
60 |
For reliable, low-resistance connection of high-current cables/busbars. |
|
|
Copper Busbar |
T2 Copper, 80*10mm, ~150kg |
- |
Carries 2020A + 20% margin. High conductivity, good heat dissipation. |
|
|
Power Cable |
0.6/1kV, 3*185mm², Copper, ~80kg |
- |
For branch and internal connections. Matches current rating and voltage. |
|
|
Cable Glands |
PG63, Nylon, IP54 |
13 |
Secure and sealed entry for 8 input + 1 output 3-core cables. |
|
|
Busbar Insulator |
For 6300A busbar |
4 sets |
Secures and insulates main busbars, prevents phase-to-phase faults. |
|
|
Accessories |
Lugs, ties, covers suitable for 800V/High Current |
1 set |
Ensures safe, reliable, and organized connections. |
|
|
Packaging |
Custom reinforced wooden crate |
- |
Protects cabinet during transportation to potentially remote site. |
4.Drawings
Wiring Block Diagram
Electrical Schematic Diagram
