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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.
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.
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.
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.
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.
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.
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.
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 ✔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.
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.
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.
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.
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.
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 HighWhen 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.
During power surges and failures, Uninterruptible Power Supply (UPS) devices keep computer systems and IT equipment safe and operational. A UPS provides battery backup power when the flow of electricity drops to an inadequate voltage, or if it stops. An uninterrupted power source is vital for the mission-critical environment. Depending on the size and technology of the UPS unit, backup power is provided for a specific period of time, until generators can be activated or network components can be properly shut down. When electricity flows adequately, computers and accessories are protected from damage. A UPS unit can help to shield a single device or an entire data centre effectively. See our review of best UPS devices.
There are three main types of UPS systems available to accommodate a complete range of applications, meeting enterprise and consumer needs. A Standby UPS is an offline unit that can detect an electrical failure and switch to battery power automatically. The two other UPS categories are Line Interactive and Online Devices, with online being the more expensive option. Each type of UPS keeps network devices operational when power is unavailable. Features, such as energy metering, vary by model.
The basic Standby UPS is an uninterruptible power source that delivers short-term, battery-sourced power during outages. With this category of UPS, hardware receives utility power, under normal conditions, through a direct AC connection. The standby unit and its inverter are essentially on hold until backup power is needed. Depending upon the model, a standby UPS device can also safeguard data and sensitive equipment from surges, spikes, and dips. Compact devices are available for home network protection. A Standby UPS is commonly used to protect computers, modems, VoIP equipment, and other hardware. This category of UPS is the least expensive of the three UPS types.
Battery backup power is initiated with the Standby UPS, which is also referred to as a Backup UPS. Following a blackout, a Backup UPS delivers power for short periods of time. When it detects losses, the transfer switch initiates backup processes. Switchover times occur in milliseconds after failure, with response times varying by a standby UPS unit. The switch time is not instantaneous but should not normally 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 are protected.
Offline UPS Provides Basic Power for Home and Office
A standby UPS may also be referred to as an Offline UPS, a term that differentiates it from the fully uninterruptible, Online UPS. Though fundamental in design, an offline uninterrupted power source delivers backup runtimes for less demanding home and professional environments. Comms Express representatives have detailed information regarding Offline, Line Interactive and Online UPS models for network, server and desktop UPS applications.
An Online UPS is a type of uninterruptible power supply that utilises either a double or delta conversion technology. With double conversion, network equipment does not receive electricity directly from the AC outlet. Instead, AC power travels to a rectifier, where it becomes DC power. Next, it travels to the battery, and then an inverter. After inversion back to AC, the power is delivered to equipment. With this online UPS device process, computing equipment receives continually clean power. With delta conversion, a certain amount of power is sent to run computers, routers, and other equipment directly. This creates an energy-efficient online UPS system where some of the power skips the processing steps.
If an electrical failure occurs, an Online UPS system maintains consistent current flow for the protection of network equipment. Upon fluctuation or failure, the rectifier within the UPS is automatically bypassed, and power is sourced from the battery until restoration occurs. The online UPS circuity is seamless. This is why Online UPS systems cost more than units in the offline or line-interactive UPS categories.
Double Conversion UPS Technology
Sensitive equipment can be protected with double-conversion UPS technology. Online UPS units are available for a wide range of users. With double-conversion UPS processes, network servers, data centres, and a full scope of environments can be protected, and there is no transfer time in the transition to battery power. Intelligent online UPS systems are available to support demanding loads and run times. More affordable double-conversion UPS units are available to smaller offices. Normally, the larger the unit, the longer equipment can be kept running.
A Line-interactive UPS is one type of uninterruptible power source that can regulate voltage automatically. The line-interactive technology responds to high and low voltage conditions. Units also support systems during outages without battery drainage. With a line-interactive UPS, the electrical source is the first line of power; however, an inverter/converter technology allows for the unit’s battery to be charged throughout normal operations. During an outage, this UPS converts battery power to AC flow for device delivery.
Line-interactive UPS Systems for IT Applications
Within the category of power protection products, line-interactive UPS systems will safeguard sensitive equipment during brownouts and blackouts. Units in the line-interactive UPS classification are more expensive than standby models but more affordable than an on-line UPS unit. A line-interactive UPS will maintain performance during low voltage situations and short-term power failures. If a lengthy outage is anticipated, its battery power allows devices to be shut down safely. Some live interactive units also offer filtering features.
Before buying a power protection unit, you may want to compare on-line versus line-interactive UPS technology. A line-interactive UPS adds an autotransformer to the basic standby design. It has the capability to increase or decrease voltage output. This transformer responds to changes in the AC power and can automatically regulate issues. In comparison, online UPS models use an inverter to transmit all or some of the power, at all times. This means an online UPS can live up to the term “uninterruptible” with a zero transfer time. It takes an interactive UPS a matter of milliseconds to switch into battery backup mode when a failure occurs.
Reliable power management systems help to ensure that electrical power to valuable equipment is never interrupted. When buying a UPS unit, the volt-ampere (VA) rating of the device should be compatible with the total load that must be protected. This load will include all equipment and network accessories. An advanced online UPS system uses double conversion technology for power management. It delivers consistently clean power, even during the brownout or blackout, for server-grade and data centre systems. Contact Comms Express for questions about VA and wattage ratings on UPS devices.
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