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 online, line-interactive, and offline. This article will make an objective analysis of these three common types of topologies in views of their operating principles, functionalities, advantages & disadvantages, and applications.
ONE-SENTENCE EXPLANATION FROM WiKi
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.
ONE-SENTENCE EXPLANATION FROM WiKi
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.
ONE-SENTENCE EXPLANATION FROM WiKi
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.
During an outage, a break in power to a load of typically 2 to 10 milliseconds is inevitable in offline/standby UPS systems, while line-interactive UPS systems typically transfer from line power to battery-derived power within 2 to 4 milliseconds, faster enough to keep all but a small percentage of the most power-sensitive equipment operating without interruption. Surprisingly, the online UPS system does not have a transfer time, because the inverter is already supplying the connected equipment load when an outage occurs.
As is known, 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 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:
|Electrical Line Noise||✔|
When it comes to an apples-to-apples comparison, the analysis of their advantages and disadvantages can't be ignored, which may help to make a wise decision among the objects. In the following, the benefits and limitations of these three topologies of the UPS system will be presented clearly.
High-efficiency (The charger is not constantly on).
User-friendly to operate.
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 inverter when the mains power supply fails.
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.
Considering the previous benefits and limitations and combining the current situation with your expectations, it's time to deploy the three topologies into practical 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 change 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.
|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|
|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.