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.
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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- 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.
Difference Between Online UPS and OffLine UPS
What is an UPS? UPS, Uninterruptible Power Supply is a device that supplies power in case of power failures. A UPS can be either Online UPS or Offline UPS. Both types of UPS can be used as a backup power source but there are many differences between them.
Data Secure is become more important for whole world, UPS systems, Gives emergency power to a system if the grid is shut down, UPS are becoming an essential part of our lives. From the base structure, All UPS are categorized into two types: Online UPS and Offline UPS.
What online UPS? The Online UPS is a type of UPS that supplies power to the AC load through the Rectifier and Inverter Combo in normal operation and uses an inverter to supply AC power during a power failure. Therefore, the output power supply always stays ON and there is no need for switching. Hence, there is no time delay in switching between its sources. There is no interruption in the case of power failure even for a nanosecond.
What Offline UPS? The offline UPS is a type of UPS that directly supplies the power to the AC load from the AC mains in normal operation and uses an inverter to power the AC load from the DC battery. Since there are two separate lines of supplies, the output supply must be switched between the two sources.
The switching speed of an Offline UPS is around 5ms which is enough to keep a computer from shutting down. But not fast enough to keep a smooth operation of sensitive equipment in hospitals.
What is the difference between online UPS and offline UPS?
Here is is brief introduction to answer your questions about:
Operation difference:
The batteries of offline UPS are charged all the time. If the power fails, the load is powered by the inverter. Lower Cost offline UPS generally, when on inverter/battery backup, provide a square/pseudo sine wave output waveform.
Online UPS takes the incoming AC mains supply and converts it to DC which feeds the battery and the load via the inverter. If the mains supply fails, then the batteries feed the load via the inverter with no interruption to the output supply at all. It means our electronics get power from the online UPS, rather than directly from the AC mains. Therefore, even if the main AC power fails, the operation of our electronic equipment does not need to be stopped.
What&#;s more, The dual conversion design (AC-DC / DC- AC) of an Online system ensures a far higher degree of isolation of the load from the irregularities on the mains supply. That&#;s why the Online system delivers a fixed and stable output all the time.
Voltage distortion difference:
Online UPS: Voltage distortion does not reduce the performance.
Offline UPS: The more the voltage fluctuations, the more the offline UPS is used. A frequent switch could lead to switching delays or could ultimately reduce its performance.
Price difference&#;
Compared with online UPS price&#;offline UPS&#;s are cheaper.
ONLINE UPS
OFFLINE UPS
The UPS that supplies power from the AC mains to the load through the rectifier and inverter combination The UPS that supply the AC mains power directly to the load. It continuously draws current from the battery. It only draws current from the battery during primary source failure or power outage. It requires a large battery charger circuit because it carries the entire load current. It requires a small charger circuit because it only charges the battery. The input supply and the load is completely isolated. There is no isolation between the input supply and load. It has high-performance output because it protects the output load from input voltage spike and distortion. Its performance reduces with the input voltage distortion; which is directly connected to the load. There is not switching between the source, hence no time delay It uses a static transfer switch to switch between its source with a time delay of 5ms It stays ON almost at all times. It turns ON only at power failure. Due to its continuous ON condition, it is inefficient and unreliable. It is more efficient and reliable because it mostly stays off. It requires large heat sinks because of the high operating temperature It requires smaller heat sinks because of low operating temperature due to its off state mostly. The components used must have the tolerance to withstand high temperature There is no need for the components to have high-temperature tolerance. It is expensive than Offline UPS. It is cheaper than Online UPS. It is used for very sensitive electronics that cannot tolerate any gap between supplies such as in-hospital ICUs Offline UPS is used for electronic equipment that can tolerate a delay of 5ms such as Computer etcHow to Select: Inline UPS or Offline UPS?
In situations where a square/pseudo sine output waveform is not acceptable then true sine wave more expensive off-line or online UPS are recommended. But when you are much concerned about the cost and wish a lower operating temperature, offline UPS is better.
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