How UPS online works?

Across today’s highly connected and data-driven business landscape, the need for continuous, clean power cannot be overstated. Even the briefest amount of downtime can be devastating to an organization, regardless of its size or vertical. Not only is the current price tag for downtime estimated at $5,600 per minute, but unplanned incidents can result in equipment damage, data loss, missed opportunities and tarnished reputation.

Thankfully, an uninterruptible power system (UPS) is one of the most simple, cost-effective solutions to help companies avoid the unwelcome consequences of downtime. But with several different types of systems available, the challenge is selecting the one that best matches your needs and budget.

What Is a UPS?

A UPS is a backup system that provides power in the event of a utility failure. By supplying an adequate window to safely shut down sensitive equipment, a UPS helps prevent data loss and minimizes the stress of a hard shutdown on electronics. But protecting equipment against a complete power loss isn’t the only reason you need a UPS. Depending on the UPS model, these systems also shield connected devices from common power problems and unsafe output voltage fluctuations that can damage electronics, reduce lifespan and affect performance.

How Does a UPS Work?

Essentially a battery in a box, a UPS powers the devices that are plugged in to its AC outlets when the flow of electricity drops to an inadequate voltage, or if a complete outage occurs. In case of a blackout, the UPS immediately switches to battery to provide a continuous power source for the length of the battery, which varies by system for periods of time ranging from minutes to hours.

Depending on the size and technology of the unit, a UPS is capable of protecting a single computer up to an entire data center. Essential for mission-critical environments, a UPS keeps computer systems and IT equipment safe and operational during a power loss until generators can be activated or protected devices such as servers and network components can be properly shut down, preventing loss of data and work-in-progress. In addition to delivering backup power during a utility failure, UPSs also afford varying degrees of protection against other damaging power problems, including voltage sags, surges, brownouts, line noise, frequency variations, overvoltage conditions, and switching transients and harmonic distortion.  

Types of UPS Systems

It’s important to understand that all UPS systems are not created equal. Rather, there are three primary topologies: standby, line-interactive and online. Each type of UPS topology not only delivers varying levels of protection against power problems but achieves the result in a separate manner while placing differing frequency demands on the battery.

Standby UPS

Also referred to as an offline UPS or passive UPS, standby technology offers the most basic type of protection, allowing equipment to run off of incoming utility power until it detects a problem. In a standby UPS model, connected devices receive utility power through a direct AC connection, with the unit essentially remaining on hold until backup power is needed. A standby UPS will switch to battery to safeguard connected equipment when power fails, as well as adjust for routine sags and surges. The transfer time generally occurs in milliseconds after a power loss, and although the switch is not instantaneous, in most cases it will not interrupt the flow of power to equipment. If a lengthy outage is expected, the battery backup power of the UPS will allow for safe shutdowns so equipment and data remain protected.

Because standby topology does not buffer equipment against other common power anomalies, it is best suited for non-critical and less demanding home network and office environments that are not subjected to frequent disruptions. While standby UPSs are the least expensive type of UPS, the downside to this topology is that it frequently resorts to the battery, which can reduce runtime and service life.

Line-Interactive UPS

A UPS with line-interactive topology is designed to shield connected devices from power failures, sags and surges like a standby model, while also providing protection against voltage spikes and voltage drops. Typically used to safeguard enterprise network and IT applications, the line-interactive UPS is controlled by a microprocessor that monitors the quality of incoming power and reacts to fluctuations. In addition to providing more protection than standby UPSs, line-interactive units offer better power conditioning and regulation, which helps prolong battery life.

One of the biggest advantages of line-interactive topology is that it compensates for undervoltage and overvoltage circumstances without using the batteries. Under normal power conditions, line-interactive UPSs pass input power to connected equipment through a surge/noise filter and voltage regulator, while an inverter charges the battery for emergency use during an outage. However, if the voltage fluctuates outside of a safe window, the UPS relies on built-in automatic voltage regulation (AVR) to increase or decrease the output voltage to an acceptable level. As long as the input voltage remains within a safe range, line-interactive UPS systems regulate the output voltage without using battery power. This advantage prevents frequent switching to battery, which can drain reserve power intended for an outage and reduce battery lifespan. During an outage, the line-interactive UPS converts the stored battery power into regulated AC output power to support the connected equipment load.

Online UPS

While standby and line-interactive UPS models provide varying degrees of power conditioning, an online or double-conversion UPS is designed to deliver continuous protection against all nine of the most common power problems, supplying a consistent clean power regardless of any incoming instabilities. In order to create a power supply without any electrical interference, the output voltage of an online UPS is entirely regenerated by a sequence of AC to DC conversion, followed by DC to AC conversion. During erratic power or fleeting disturbances when AC input power falls outside of preset tolerances for line-interactive mode, the online UPS switches to online double-conversion mode, completely isolating equipment from incoming power. If power fails altogether, or the input power exceeds the tolerances of the rectifier, the UPS will rely on the battery to keep loads operating, then convert back to high-efficiency mode when it is safe.

Online UPSs are the optimal choice for mission-critical applications or those involving highly sensitive equipment, such as data centers, communications hubs and other installations where continuous, clean power is a business-critical requirement. They are also beneficial in environments that are particularly prone to dirty power.

Benefits of UPS

While there are numerous benefits to deploying a UPS, first and foremost, they protect against utility power failures, which are occurring on a dramatically increasing basis. While blackouts can be caused by a wide variety of issues ── including automobile accidents, animal interference and overtaxed power grids ── severe weather ranks as the No. 1 cause of power failures in the U.S. From hurricanes to heatwaves and flooding to freezing rain, weather is leaving organizations in the dark for longer than it has in the past, in large part due to climate change fueling more violent storms. In fact, in 2020, U.S. electricity customers experienced an average of 8 hours without power ── more than twice as long as the 2013 average.

In addition to keeping equipment operational during a power loss, a UPS also acts as a filter to shield devices against variances in the power supply, which can cause damage to sensitive equipment like computers and network gear. Depending on the topology, a UPS allows only a steady stream of clean power to reach attached devices, helping to safeguard against common power anomalies that can damage equipment. In addition, battery backup power prevents data loss and provides time to save work-in-progress during outages, surges or other power variances.

Finally, a UPS provides confidence that your organization will be able to avoid the financial cost, customer frustration and business disruption caused by network downtime ── and you can’t put a price tag on peace of mind. Whether protecting a network closet, a few server racks or an entire data center, it is imperative to safeguard against equipment damage, downtime and data loss.

UPS Systems from Unified Power

Never before has it been more critical for organizations of every size to protect their operations against the potentially disastrous impacts of downtime. While all three UPS topologies outlined above meet the input voltage requirements for IT equipment, there are significant differences in both performance and demands on the battery.

Determining the best topology for your organization depends on a number of factors, including the level of reliability and availability required, the type of equipment being protected and the overall application/environment. If you need assistance determining the optimal topology and the UPS model best suited for your environment, Unified Power can help you compare and identify systems.

An Uninterruptible Power Supply refers to a power system that provides emergency power to a load when the input power source or mains power fails, regarded as near-instantaneous protection from input power interruptions. The three general categories of modern UPS systems are Line-interactive UPS vs Online UPS vs Offline UPS, which will be illustrated exlaboratly in the following.

Line-interactive UPS vs Online UPS vs Offline UPS: Working Principles

Offline UPS—Entry-level Power Protection

In an off-line ("standby") UPS system, the load is powered directly by the input power, and the backup power circuitry will only be invoked when the utility power fails.

Specifically speaking, the load is fed directly from the raw mains power rather than the inverter output. The energy storage components—battery charger, battery, and inverter are off-line as far as the load is concerned, although the charger and battery still remain connected to the mains power in order to ensure the battery is always fully charged. When the mains power voltage fails or exceeds the limits, the switch will immediately connect the inverter output to the critical load.

Line-interactive UPS—Intermediate Level Power Protection

A line-interactive UPS maintains the inverter in line and redirects the battery's DC current path from the normal charging mode to supplying current when power is lost.

In this smart design, the battery-to-AC power inverter is always connected to the output of the UPS. When the input AC power is normal, the inverter of the UPS is in reverse operation and provides battery charging. Once the input power fails, the transfer switch will open and the power will flow from the battery to the UPS output. This design offers additional filtering and yields reduced switching transients since the inverter is always on and connected to the output.

Online UPS—The Ultimate Power Protection

An online UPS uses a "double conversion" method of accepting AC input, rectifying to DC for passing through the rechargeable battery (or battery strings), then inverting back to 120 V/230 V AC for powering the protected equipment.

In an online (aka double-conversion) UPS, the input AC is charging the backup battery source which provides power to the output inverter, so the failure of the input AC won't cause activation of the transfer switch. That is to say, if a power loss occurs, the rectifier will simply drop out of the circuit and the batteries will keep the power steady and unchanged. No transfer time during the failure. When power is restored, the rectifier will resume carrying most of the load and begin charging the batteries, though the charging current may be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte.

Line-interactive UPS vs Online UPS vs Offline UPS: Functions

All of the above-mentioned three categories are invented to protect hardware and electrical equipment where an unexpected power disruption may happen. However, influenced by various working principles, their inherent capabilities are different.

Surge/Noise Protection

All the three UPS systems possess surge suppression and line noise filtering functions to shield the equipment from damage caused by lightning, surges, and electromagnetic (EMI/RFI) line noise. Particularly, the online UPS system offers superior protection on account of the double-conversion operation that isolates equipment from problems on the AC line.

Transfer Time to Battery

When an outage occurs, the transfer time varies:

• A break in power to a load of typically 2 to 10 milliseconds is inevitable in offline/standby UPS systems.

• Line-interactive UPS systems typically transfer from line power to battery-derived power within 2 to 4 milliseconds, faster enough to keep the most power-sensitive equipment operating without interruption.

• The online UPS system does not have a transfer time, because the inverter is already supplying the connected equipment load when an outage occurs.

Voltage Regulation

The voltage regulation is crucial, especially for low voltage conditions:

• Line-interactive UPS systems use automatic voltage regulation (AVR) to correct abnormal voltages without switching to the battery. When voltage crosses a preset low or high threshold value, this type of UPS will detect and use transformers to boost or lower the voltage by a set amount to return it to the acceptable range.

• Online UPS systems adopt a more precise method of voltage regulation: continuous "double-conversion" operation, isolating connected equipment from problems on the AC line, including blackouts, brownouts, overvoltages, harmonic distortion, electrical impulses, and frequency variations.

• When not operating from the battery, line-interactive UPS systems typically regulate output within ±8-15% of the nominal voltage (e.g. 120, 208, 230, or 240 volts), whereas online UPS systems typically regulate voltage within ±2-3%.

In fact, there are some common power issues that may occur in daily operations. The table below will present whether the above-mentioned UPS system will protect against the anomalies:

Offline Line-interactive Online Power Failure ✔ ✔ ✔ Power Sag ✔ ✔ ✔ Power Surge ✔ ✔ ✔ Under-Voltage ✔ ✔ Over-Voltage ✔ ✔ Electrical Line Noise ✔ Frequency Variation ✔ Switching Transient ✔ Harmonic Distortion ✔

Line-interactive UPS vs Online UPS vs Offline UPS: Pros & Cons

Offline UPS

Benefits Limitations

• High-efficiency (The charger is not constantly on).

• User-friendly to operate.

• Affordable price.

• Uses battery during brownouts, limited or no protection against power irregularities.

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  The load is continuously exposed to spikes, transients, and any other aberrations coming down the power line, resulting in the risk of loss or damage to sensitive equipment and data.

• Finite transfer time from mains power to the inverter when the mains power supply fails.

Line-interactive UPS

Benefits Limitations

• High-reliability.

• High-efficiency.

• Reasonable voltage conditioning.

• Lower electricity consumption.

• Lower component count.

• Lower operating temperatures.

• Impractical over 5kVA.

• Not protect against all forms of power irregularities.

• Do not provide power-factor correction or frequency regulation.

• Require frequent battery use in areas of extreme voltage distortion.

Online UPS

Benefits Limitations

• Better voltage regulation.

• Conversion time from DC to AC is negligible with no gaps in coverage.

• No fluctuations in the voltage, indicating stable voltage quality.

• The quality of the load voltage is free from distortion.

• Near ideal electrical output, highest protection against all power irregularities.

• Complex designs requiring a large heat sink.

• Higher power dissipation.

• The overall efficiency of UPS is reduced (The inverter is always on).

• The wattage of the rectifier is increased (It has to supply power to the inverter as well as charge the battery).

• Costlier than other UPS systems.

Line-interactive UPS vs Online UPS vs Offline UPS: Applications

The applications of these three topologies with different operating principles vary from small-size residential homes to large-scale data centers.

Due to its higher energy efficiency and economical nature, offline UPS is most commonly-used for households, small offices with low-budgets for power designs, and some fields that have relatively low requirements for power supply quality. Best value for personal computers, and also suitable for printers, scanners, emergency power supplies, and EPABX.

As one of the main choices, line-interactive UPS that meets the demands of high power reliability is ideal for departmental servers, homes, small businesses, and medium enterprises. In some infrastructure challenged areas where the AC line voltage is unstable, fluctuates wildly, or is highly distorted, a line-interactive UPS may need to charge the battery once or twice a day or even more frequently.

The most intelligent online UPS system, regarded as a default solution for providing back-up power and protection to mission-critical equipment and servers at data centers, can also be applied in fields like computer, transportation, banking, securities, communications, medical requiring a long-time power supply. Specifically, for some induction motor drives and similar other motor control applications, intensive care units, medical equipment, sensitive electrical appliances.

Line-interactive UPS vs Online UPS vs Offline UPS: Which to Choose?

The table below summarizes some of the key points discussed before among the three designs.

Offline Line-interactive Online Size Compact Typically large & heavy Typically small & light Practical Power Range (kVA) 0-0.5 0.5-5 5-5000 Voltage Conditioning Low Design Dependent High Cost per VA Low Medium Medium Efficiency High
(typically 95-98%) High
(typically 90-96%) Low-Medium
(typically 80-90%) Cost Low Medium High Typical Application Fields Homes; Small Offices Small and medium businesses Telecoms; Communications; Banking; Transportation; Industrial Environments

When considering a UPS solution, be aware of the UPS type and corresponding level of protection. The key difference among online vs offline vs line-interactive UPS lies in their working principles, which reflects in the diversities in their features, functionalities, benefits & limitations. Furthermore, the internal design of the topology of a UPS will further affect how it will operate in various application environments.