110kV substation DC 110V to AC 220V inverter case
Last updated 2026-11-13
A 110kV substation in Gansu, northwest China, needed its protection room, telecom rack and monitoring backend to ride through station-service outages. Instead of a second UPS with its own batteries, a 3kVA rack-mounted online inverter now draws directly from the substation's existing 110V DC bus — full isolation, zero-millisecond transfer, no new battery string.
Project background
A 110kV substation operated by a grid customer in Gansu province carries three groups of AC loads that must survive the loss of station service: the relay protection room, the telecom equipment rack, and the monitoring backend that reports the station's status upstream. The station already had what most substations have — a mature 110V DC battery system sized and maintained for the protection scheme.
The question was not whether backup power existed, but how to extend it to the AC loads without disturbing the DC system's operating arrangement. Station engineering staff were clear about the constraint: whatever was added could not alter how the existing DC batteries were charged, grouped or monitored.
Why a conventional UPS was the wrong answer
Three problems ruled out the standard response of installing a separate UPS:
- Duplicate battery investment. A conventional UPS means a second battery bank, a second charger and a second maintenance cycle. For three small AC load groups, that duplicates the most expensive and most failure-prone part of the power chain.
- Galvanic separation concerns. In a substation environment, the DC system is part of the protection scheme. A UPS whose DC side and AC side share reference potential risks coupling DC ground faults and ripple into the AC loads — or letting an AC-side fault propagate back toward the DC bus.
- Space and wiring. Protection rooms are compact. A tower UPS with its own battery cabinet competes for floor space that the existing 19-inch relay and communication racks already occupy efficiently.
What the customer asked for, in effect, was a device that behaves like the DC system's fourth load group on one side and like a clean AC source on the other.
The installed solution
The station installed a 3kVA rack-mounted online inverter from the H series 1–3kVA online DC-AC inverter line, taking the station's 110V DC bus as its primary energy source and delivering pure sine wave AC 220V to the monitoring host, telecom equipment and emergency lighting. Four characteristics drove the selection:
Online double-conversion topology. AC 220V station service supplies the load under normal conditions; when the AC input goes out of tolerance, the inverter draws from the 110V DC bus with zero transfer time. There is no relay-switching event for the loads to notice — the output stage never stops running.
Complete electrical isolation. Input and output, AC and DC sides are fully isolated from each other. A ground fault or ripple event on the DC side cannot reach the AC loads, and a fault on the AC side cannot feed back into the protection battery system. In a protection environment this is a hard requirement, not a preference.
Inverter-only design, no charger. The unit does not charge batteries. It simply parallels onto the existing 110V DC system and draws current. The station's existing chargers, battery management and DC distribution continue to operate exactly as designed — the constraint the operators set at the start.
19-inch rack format. The 3kVA unit mounts in the existing communication rack beside the equipment it feeds. Installation reduced to DC input cabling, AC output distribution and an input breaker — no floor space, no new cabinet.
Results
- Station-service interruptions since commissioning have produced no observable event on the loads: no monitoring host reboot, no telecom dropout, no alarm from the SCADA backend.
- Output waveform quality and noise levels satisfied the telecom equipment vendor's power requirements without conditioning.
- The station avoided purchasing a second battery bank — the DC bank already under maintenance covers these loads.
- Because the inverter draws from an existing, regularly tested battery system, the standby time for the AC loads equals the standby time of the protection scheme itself, already verified on the station's maintenance schedule.
What transfers to other substations
Start from the DC system, not from the load list. The economically correct question in a substation is rarely "which UPS?" but "which DC bus can this load group legally attach to?" A DC-fed inverter converts an existing asset into AC backup; a standalone UPS duplicates the asset.
Isolation is the specification that matters. Power ratings are easy to meet. What separates a power-sector inverter from a general-purpose one is documented input/output isolation and behavior under DC ground-fault conditions. Ask for the isolation diagram, not just the datasheet.
Confirm the pure-inverter configuration. A unit with a built-in charger changes the DC system's operating arrangement — exactly what substation operators usually prohibit. Specify zero-charger hardware deliberately, not as an option you remember to delete later.
Size above the inrush, not the nameplate. Monitoring hosts and telecom shelves with multiple internal supplies draw two to three times rated current at startup. The 3kVA rating here was selected against measured inrush with margin, not against the sum of nameplate watts.
FAQ
Why not feed substation AC loads from a normal commercial UPS? A conventional UPS requires its own battery bank and charger, duplicating infrastructure the substation already operates and maintains. It also introduces a second set of batteries into a protection environment where battery management is tightly controlled. A DC-fed inverter reuses the existing 110V or 220V protection battery and costs less upfront.
What happens to the AC loads when both station service and the DC battery are unavailable? They lose power, as any backup architecture would. The difference is that the DC battery system in a substation is sized, tested and maintained under the grid operator's protection regulations — typically far more rigorously than a standalone UPS battery. Attaching the AC loads to that system gives them the strongest available standby time without new hardware.Does connecting an inverter to the DC bus affect the protection system? Not when the inverter is specified with full input/output isolation and no charging function. It appears to the DC system as one more constant-power load, and its isolation prevents faults in either direction. This is the established approach for feeding communication and monitoring equipment in power plants and substations, and is accepted practice with grid operators.
How is the inverter capacity selected for a protection room? Add the steady-state load of the monitoring host, telecom equipment and lighting, then verify against measured inrush — switching power supplies commonly draw two to three times their rated current at startup. Select the next rating above that with margin; a 3kVA unit comfortably covers a typical single-room installation.
Product references: H series 1–3kVA online DC-AC inverter for 110V/220V DC bus applications, and the H series 6–20k online power inverter for larger control-room load groups.