The building has 18 floors of open-plan office space. The 14th floor was the test case: LED panels on the ceiling, IT racks against the wall, and a small commercial kitchen at the end of the corridor. The mechanical room has a 250 kW DC bus that supplies all three loads directly. No per-floor AC inverter. No per-room transformer.
The PV array lives on the roof, tied to the same 380 V DC bus through an MPPT stage. The battery rack charges from the same bus and discharges back into it when the office load rises above the PV output. The bus feeds the building’s normal AC distribution through a single bidirectional inverter at the basement level, so the grid is still there if everything else goes down.
Why DC coupling
Two reasons the consultant specced it. The first is efficiency: every AC-DC and DC-AC conversion costs about two percentage points. A typical hybrid PV system loses five or six points to chained conversions. The DC-coupled system runs at 96.5% measured round-trip at the bus. The second reason is simplicity: there are fewer boxes on the wall, fewer cards to fail, fewer fans to clean.
What the cabinet contains
- 250 kW MPPT stage, 4-string input, 380 V DC output
- 250 kW bidirectional PCS, 380 V DC bus / 380 V AC grid tie
- Local controller with BMS Modbus gateway and grid-tie protection relay
- Battery management closed loop, no manual cell-balancing visits
Commissioning
The cabinet was installed in two days. The PV cables from the roof came down the riser, hit the MPPT terminals on the top of the cabinet, and the AC tie-in landed on the bottom terminals. Battery commissioning took a third day. Acceptance test was run on a Saturday to keep the building fully powered through the work week.
Three months in, the system has run a peak summer week without any grid feed during the working hours. The owner asked whether the cabinet would run a second building. It would — the cabinet will parallel without modification. We have given them a quote.
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