Hydropower station DC panel UPS integration
Last updated 2026-11-07
A station's DC panel protects the protection relays, but it cannot feed the AC loads that sit beside them. This project paired an industrial-frequency UPS for the AC board with a DC-fed inverter that reuses the panel's existing 220 V battery bank — no second battery string, no change to the protection architecture.
Project background
A hydropower station in Yunnan operates a DC operating power system that supplies protection relays, control circuits and signalling. The DC panel already has a battery bank, so the DC side is covered.
Two gaps remained on the AC side:
- Station-service transfer. When station service switches between sources, AC loads — measurement and control devices, communications equipment — see a brief interruption.
- AC-only loads. Some equipment is AC by design and cannot be fed from the DC panel at all.
What made this site difficult
The load profile at a hydro station is not the load profile of a computer room.
Impulsive loads. Breaker operating mechanisms, oil pumps and valve motors draw several times their rated current at start. A UPS sized on steady-state load will transfer to bypass every time a breaker operates — and a UPS on bypass is not protecting anything.
Grid behaviour. Station-internal voltage moves with unit start and stop. The input window has to be wide enough that the UPS does not drop to battery on every unit transition.
Monitoring integration. The equipment had to report into the station's existing supervisory system over MODBUS RTU on RS485, with a complete point table supplied in advance.
Configuration
Primary supply — industrial-frequency UPS. An industrial-frequency three-phase unit was selected for the AC board, chosen for three reasons: an output isolation transformer that separates the station's AC distribution from the UPS output, an overload window of 125% continuous, 150% for one minute, which covers breaker operation, and an input voltage window of 380 V ±25%, wide enough for station-internal variation.
Secondary supply — DC-fed inverter. A rack inverter was added to take 220 V DC from the existing DC panel and produce 220 V AC for the remaining AC loads. This is the part that changed the economics: instead of a second battery bank for the AC side, the AC loads share the protection system's battery. For a station where the DC bank is already sized and maintained, that removes both a capital cost and a maintenance item.
Monitoring. Both units report over MODBUS RTU to the station system, with the point table agreed before delivery so that commissioning did not stall on protocol mapping.
Results
- Protection devices saw no interruption during station-service transfer.
- Breaker operations produced no abnormal UPS transfer events — the overload window absorbed them as designed.
- The DC-fed inverter removed the need for a dedicated AC battery bank, reducing both first cost and the number of battery strings under maintenance.
- Both units reported into the existing supervisory system from day one.
What transfers to other power-sector projects
Design around the existing DC system, not beside it. In substations and power plants the battery bank is already there, already maintained, already in the protection scheme. A DC-fed inverter exploits that investment; a second UPS duplicates it.
Size on the impulsive load. Breaker mechanisms, pump starts and valve motors set the UPS rating far more often than the steady-state load does. Ask for the operating mechanism's current curve, not just the kW.
Agree the point table early. Protocol compatibility is usually fine; point naming and scaling are where commissioning loses days.
Confirm the input window against the real grid. A station-internal bus that swings with unit dispatch will push a narrow-window UPS to battery repeatedly, which quietly consumes battery cycles and alarms.
Scope boundary
This configuration is for stations where the DC panel exists and the AC load is modest relative to it. Where the AC load approaches or exceeds the DC system's capacity, the design needs a dedicated AC UPS with its own battery — sharing the bank would compromise protection autonomy, which is not a trade to make on a protection system.
Product references: GX series industrial-frequency UPS 6–600 kVA, H6-10K with external isolation transformer, industrial power-frequency range, and line-frequency vs high-frequency UPS.