UPS Topologies Explained: Offline, Line-Interactive and Online Double-Conversion

An uninterruptible power supply (UPS) is often specified simply by its kVA rating and battery autonomy. Yet two UPS units with the same rating can protect a load very differently, depending on how they are built and how they react when the mains supply is disturbed. That behaviour is set mainly by the topology: offline (standby), line-interactive or online double-conversion. IEC 62040-3 gives a way to describe the result that does not depend on marketing names.

This article answers the question: How do offline, line-interactive and online double-conversion UPS topologies differ, and how should a project choose and specify between them?

Why topology matters

A UPS sits between a supply that is sometimes disturbed and a load that may be sensitive to those disturbances. Disturbances range from complete loss of supply to sags and swells, sustained under- or over-voltage, frequency variation when running on a generator, transients and harmonic distortion. Each topology deals with a different subset of these, with different transfer times, efficiency and cost.

How the UPS fits with generators, transfer switches and distribution is covered in Critical Power Systems: Generators, UPS and ATS. This article focuses on the UPS itself.

The IEC 62040-3 classification

IEC 62040-3 sets out a method of specifying UPS performance and its test requirements. It describes performance with a code in three parts.

Output dependency (first part):

• VFD (voltage and frequency dependent). The output depends on the mains voltage and frequency. The UPS protects the load against loss of the AC input.
• VI (voltage independent). The output also stays within limits during sustained under- and over-voltage of the input, through voltage-correction devices and without discharging the battery within the declared input range. Frequency still follows the mains.
• VFI (voltage and frequency independent). The output is independent of supply voltage and frequency variations within declared limits, without discharging the energy storage.

Output voltage waveform (second part): for normal mode and for stored-energy mode respectively, S means sinusoidal, X means sinusoidal with higher distortion under non-linear load, and Y means non-sinusoidal.

Dynamic output performance (third part): classes 1, 2 and 3 describe how far, and for how long, the output voltage deviates during a change of operating mode and during a linear step load. Class 1 is the tightest.

The classification is performance-based. IEC 62040-3 does not reserve a classification for a particular topology, and its informative annex shows topologies only as examples. In practice, the topologies below are usually associated with the classifications shown, but the declared code on the datasheet is what should be compared. The 2021 edition removed the non-linear step-load type test, which shortened the code from eight to seven characters. Datasheets that show three dynamic-performance digits, for example VFI-SS-111, use the 2011 edition’s format, so check which edition a declaration refers to.

Offline (standby) UPS

How it works. In normal operation the load is fed directly from the mains, usually through basic filtering. A charger keeps the battery charged, and the inverter is idle or on standby. When the mains fails or goes outside limits, a transfer switch moves the load to the inverter, which runs from the battery.

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Characteristics:

• The output follows the mains in normal mode, so it is typically VFD.
• There is a short transfer interruption when switching to battery.
• The inverter output may be non-sinusoidal on simpler designs.
• Losses in normal mode are low because the power path is direct.

Where it fits. Small, single-phase loads such as individual workstations, small network devices and point-of-use equipment that tolerate a brief transfer and a less-than-perfect waveform during battery operation.

Line-interactive UPS

How it works. The inverter is connected in parallel with the load and stays active. In many designs the same converter charges the battery in normal mode. An automatic voltage regulator, typically a tapped transformer, boosts or bucks the mains voltage to keep the output within limits without using the battery. When the mains fails, the inverter supplies the load from the battery.

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Characteristics:

• Voltage correction without battery discharge gives VI behaviour.
• Output frequency follows the mains in normal mode.
• Change to battery operation still involves a mode change, so the declared dynamic performance during that change matters.
• Normal-mode losses are low.

Where it fits. Small to medium loads in areas where sustained voltage variation is common, such as small server rooms, network cabinets and commercial equipment, where the load does not need isolation from frequency variation.

A related approach, the ferroresonant UPS, uses a special transformer to regulate voltage and filter disturbances. It is less common today but is still found in some industrial applications.

Online double-conversion UPS

How it works. A rectifier converts the incoming AC to DC, which feeds an inverter that produces the output AC. The battery connects to the DC link, so loss of mains causes no transfer at the output: the inverter simply continues from the battery. A static bypass allows the load to transfer to the mains if the inverter is overloaded or faulty; the transfer is without interruption only when the bypass supply is present and within tolerance and the inverter is synchronised with it, otherwise it is inhibited or made with a short break, depending on the design. A maintenance bypass allows the UPS to be isolated for service.

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Characteristics:

• The output is regenerated continuously, so voltage and frequency are independent of the input: typically VFI.
• There is no transfer interruption on mains failure.
• The UPS can act as a frequency converter, for example supplying 60 Hz loads from a 50 Hz supply, although then the static bypass cannot be used for transfers.
• Modern rectifiers can draw near-sinusoidal input current at high power factor, which helps when a generator supplies the UPS.
• Because power passes through two conversion stages, losses in normal mode are higher than in offline or line-interactive designs. This means more heat to remove.

Where it fits. Data centres, hospitals, industrial process control, telecommunications and other loads where any interruption or variation is unacceptable, and essentially all three-phase UPS applications above small ratings.

High-efficiency and multi-mode operation

Many online UPS systems offer modes in which the load is supplied through the bypass, with the inverter ready to take over, or with the inverter providing active filtering only. These modes reduce losses but change the classification during that time, typically to VFD or VI, and introduce a transfer when a disturbance occurs. IEC 62040-3 allows a UPS to declare a classification for each mode.

If such modes are acceptable, the specification should state the conditions under which they may operate, for example only when a generator is not supplying the site, and which classification and transfer performance apply. If they are not acceptable, say so.

Other approaches

• Delta conversion and similar hybrid designs combine a series-connected converter with a main inverter. They should be assessed by their declared IEC 62040-3 classification, like any other design.
• Rotary UPS, including diesel rotary systems, use rotating machines and kinetic energy storage rather than static converters. They are covered by Part 11 of the ISO 8528 generating-set series, published as IEC 88528-11:2004, not by IEC 62040-3.

Comparison at a glance

Selection considerations

• Load sensitivity. Ask what the load can tolerate: interruption duration, voltage tolerance and waveform. Equipment with dual power supplies may tolerate more than single-corded equipment.
• Supply quality. Frequent sags, sustained voltage deviation or a weak supply favour VI or VFI designs.
• Generator supply. Generator frequency and voltage vary more than a grid supply, especially on load steps. An online UPS can ride through these variations, but its input current distortion, power walk-in and synchronisation window must suit the generator. Sizing a Diesel Generator: Loads, Starting Duty and Operating Conditions explains how UPS loads affect the generator.
• Efficiency and heat. Higher losses mean more cooling. Compare declared efficiency at the load levels the system will actually run at, and in each operating mode.
• Battery and autonomy. Battery chemistry, autonomy at the specified load, design life and the installation environment are as important as the converter. Battery installations follow safety guidance such as IEC 62485-2; lithium-ion systems also involve IEC 62619.
• Redundancy and maintainability. N+1 parallel or modular systems, bypass and maintenance arrangements depend on how much availability the load needs. IEC 62040-3 lists redundancy and availability as matters the purchaser should define.
• Isolation and earthing. State the system earthing on input and output, whether galvanic isolation is required, and whether the neutral is switched in the bypass.
• Site conditions. Ambient temperature affects battery life and converter rating. Document it as described in Defining Site and Service Conditions.

IEC and ANSI/IEEE practice; 50 and 60 Hz

The IEC 62040 series is used internationally, and its classification codes are widely quoted on datasheets. In North American practice, UPS equipment is assessed to different national standards, with different nominal voltages and 60 Hz supplies. The topology concepts are the same, but rated voltages, frequency and test references must match the destination. A UPS rated for one frequency should be confirmed for the other; frequency conversion between 50 Hz and 60 Hz is possible only with a VFI design and is usually declared as a separate rating.

Specification and procurement considerations

A UPS specification should state:

• the required IEC 62040-3 classification for each operating mode, for example VFI with sinusoidal output and the required dynamic class;
• rated output power in kVA and kW and the expected load power factor;
• input and output voltage, frequency, phases and system earthing;
• whether high-efficiency modes are permitted, and under what conditions;
• battery chemistry, autonomy at a stated load, design life and installation arrangement;
• redundancy configuration and future expansion;
• static and maintenance bypass arrangements and external maintenance bypass requirements;
• input current distortion and compatibility with any generator;
• communication and monitoring interfaces;
• site conditions and cooling;
• the safety standard (IEC 62040-1), EMC category (IEC 62040-2) and tests to be witnessed.

Preparing an RFQ for Electrical Power Equipment gives a general structure.

Common specification mistakes

• Specifying by topology name only. “Online” or “true online” means different things to different suppliers; specify the IEC 62040-3 classification.
• Ignoring the operating mode. An online UPS running in a high-efficiency mode may not be VFI at that time.
• Stating kVA without kW. Modern loads may have a power factor close to unity, so kW is often the limiting rating.
• Specifying autonomy without load. Autonomy depends on load, battery end-of-discharge voltage and temperature.
• Overlooking generator compatibility. A UPS that does not suit the generator’s behaviour can transfer to battery repeatedly.
• Forgetting bypass and maintenance arrangements. Without them, maintenance may need a shutdown.
• Assuming frequency conversion is standard. It must be specified.

Conclusion

The topology of a UPS decides what disturbances it can shield a load from and at what cost in efficiency and complexity. Offline and line-interactive designs suit small, tolerant loads; online double-conversion suits critical loads and larger systems. Specify the IEC 62040-3 classification for each mode, with the battery, redundancy and electrical environment, so that offers can be compared on performance rather than on names.

Related AdepaTrade solutions

AdepaTrade sources UPS systems, batteries, generators and associated transfer and distribution equipment. On request, AdepaTrade can arrange on-site engineering supervision and commissioning support under a separate scope. Critical & Backup Power · Low Voltage Distribution & Control

Relevant standards

IEC standards are the primary reference. Confirm the applicable edition for each project.