High Efficiency Modes of Operation: Path Toward Highest Energy Efficiency Without Load Availability Trade-Off

Overview

The paper compares three modes: VFI double conversion, VI (line-interactive style) and VFD eco/bypass. Legacy double-conversion UPS units are about 93-94% efficient and modern ones about 97%, while high-efficiency modes reach 98-99.5%. The loss gap is large: in a 1,000 kW example losses are 63.8 kW at 94%, 30.9 kW at 97% and 10.1 kW at 99%. A point of efficiency looks small but is significant in kW.

Efficiency is paid for in protection level. Legacy UPS units need up to about 10 ms worst-case transfer from eco mode, while VI mode reacts in under 2 ms on mains failure. An IGBT-rectifier input holds power factor near 0.99 and THDi under 5% down to 20% load. In 2N systems, however, server power supplies typically run at 10-40% of capacity, so power factor and THDi are poor at low load.

The paper's argument is that a dynamic online mode can approach eco-mode efficiency while keeping the inverter in control. Its blended case assumes 25% of time in VFI, 60% in VI and 15% in VFD, giving 98.5% average efficiency, about 1.7 points better than 96.8% for pure VFI. Example: for 1,200 kVA at 70% load that gain is 14.3 kW and 125,093 kWh per year. The paper also gives payback tables per UPS rating, but the numbers rest on the vendor's own product family and the stated mode mix. Because eco and VFD modes mean a lower protection class under IEC 62040-3, selecting them for critical loads trades protection for efficiency and that should not be overlooked.

Key points

1. Legacy double-conversion UPS ~93-94% efficient; modern ~97%; high-efficiency modes 98-99.5%
2. Example 1,000 kW: losses 63.8 kW at 94%, 30.9 kW at 97%, 10.1 kW at 99%
3. Legacy UPS need up to ~10 ms worst-case transfer from eco mode; VI mode <2 ms on mains failure
4. IGBT rectifier input PF ~0.99 down to 20% load, THDi <5%
5. Server PSUs typically operate at 10-40% of capacity in 2N, giving poor PF/THDi at low load
6. Blended 98.5% efficiency assumes 25% time VFI, 60% VI, 15% VFD; saves ~1.7 points vs 96.8% VFI
7. Savings example: 1,200 kVA at 70% load, 1.7% gain = 14.3 kW, 125,093 kWh/yr

Using it in design

• Add to the UPS catalog an efficiency curve per mode (VFI/VI/VFD) at 25/50/75/100% load and a transferTimeMs per mode.
• Compute annual UPS loss as IT_kW x (1/eff - 1) x 8760 at actual per-module load; 2N systems run at 50% or below.
• When eco/VFD mode is selected, warn that protection drops to the IEC 62040-3 VFD class and show the efficiency gain side by side.
• Evaluate efficiency at the real partial-load point in 2N, not at nameplate load.

Who should read it

Electrical designers choosing UPS topology and operating mode, and decision makers managing operating energy cost.

Related standards

• IEC 62040-3
• EN 50600-2-2
• EU Code of Conduct for Data Centres

Publication details

Authors

Vertiv

Publisher

Vertiv

Year

2022

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation