How does a UPS Systems Work
How does a UPS Systems Work
Critical Power Supplies has pleasure in bringing you this guide on how UPS Systems work
An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically the utility mains, fails.
A UPS differs from an auxiliary or emergency power system or standby generator in that it will provide instantaneous or near-instantaneous protection from input power interruptions by means of one or more attached batteries and associated electronic circuitry for low power users, and or by means of generators and flywheels for high power users. The on-battery runtime of most uninterruptible power sources is relatively short—5–15 minutes being typical for smaller units—but sufficient to allow time to bring an auxiliary power source online, or to properly shut down the protected equipment.
While not limited to protecting any particular type of equipment, a UPS is typically used to protect computers, data centres, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption and/or data loss. UPS units range in size from units designed to protect a single computer without a video monitor (around 200 VA rating) to large units powering entire data centres (>1MVA), buildings (>300kVA), or manufacturing processes.
Different types of UPS design
There are three general categories of modern UPS systems are on-line, line interactive or standby/offline.
An standby/offline UPS system the load is powered directly by the input power and the backup power circuitry is only invoked when the utility power fails. Most UPS below 1 kVA are of the line-interactive or standby variety which are usually less expensive.
An 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.
An on-line 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 120V/240V AC for powering the protected equipment.
For large power units, dynamic uninterruptible power supplies are sometimes used. A synchronous motor/alternator is connected on the mains via a choke. Energy is stored in a flywheel. When the mains power fails, an Eddy-current regulation maintains the power on the load. DUPS are sometimes combined or integrated with a diesel-generator[clarification needed], forming a diesel rotary uninterruptible power supply, or DRUPS.
A fuel cell UPS has been developed in recent years using hydrogen and a fuel cell as a power source, potentially providing long run times in a small space.
Standby/Offline UPS design
The Standby/Offline UPS system (SPS) offers only the most basic features, providing surge protection and battery backup. With this type of UPS, a user’s equipment is normally connected directly to incoming utility power with the same voltage transient clamping devices used in a common surge protected plug strip connected across the power line. When the incoming utility voltage falls below a predetermined level the SPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The SPS then mechanically switches the connected equipment on to its DC-AC inverter output. The switchover time can be as long as 25 milliseconds depending on the amount of time it takes the Standby UPS to detect the lost utility voltage. Generally speaking, dependent on the size of UPS connected load and the sensitivity of the connected equipment to voltage variation, the UPS will be designed and/or offered (specification wise) to cover certain ranges of equipment, i.e. Personal Computer, without any obvious dip or brownout to that device.
Line-interactive UPS design
This type of UPS is able to tolerate continuous under voltage brownouts and overvoltage surges without consuming the limited reserve battery power. It instead compensates by auto-selecting different power taps on the autotransformer. Changing the autotransformer tap can cause a very brief output power disruption,[citation needed] so the UPS may chirp for a moment, as it briefly switches to the battery before changing the selected power tap.
This has become popular even in the cheapest UPS because it takes advantage of components already included. The main 50/60 Hz transformer used to convert between line voltage and battery voltage needs to provide two slightly different turns ratios: one to convert the battery output voltage (typically a multiple of 12 V) to line voltage, and a second one to convert the line voltage to a slightly higher battery charging voltage (such as a multiple of 14 V). Further, it is easier to do the switching on the line-voltage side of the transformer because of the lower currents on that side.
To gain the buck/boost feature, all that is required is two separate switches so that the AC input can be connected to one of the two primary taps, while the load is connected to the other, thus using the main transformer’s primary windings as an autotransformer. Note that the battery can still be charged while “bucking” an overvoltage, but while “boosting” an under voltage, the transformer output is too low to charge the batteries.
Autotransformers can be engineered to cover a wide range of varying input voltages, but this requires more taps and increases complexity, and expense of the UPS. It is common for the autotransformer to only cover a range from about 90 V to 140 V for 120 V power, and then switch to battery if the voltage goes much higher or lower than that range.
In low-voltage conditions, the UPS will use more current than normal so it may need a higher current circuit than a normal device. For example to power a 1000 watt device at 120 volts, the UPS will draw 8.32 amps. If a brownout occurs and the voltage drops to 100 volts, the UPS will draw 10 amps to compensate. This also works in reverse, so that in an overvoltage condition, the UPS will need fewer amps of current.
On-Line UPS Design
The online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations. Although once previously reserved for very large installations of 10 kW or more, advances in technology have permitted it to now be available as a common consumer device, supplying 500 watts or less. The online UPS is generally more expensive but may be necessary when the power environment is “noisy” such as in industrial settings, for larger equipment loads like data centres, or when operation from an extended-run backup generator is necessary.
The basic technology of the online UPS is the same as in a standby or Line-Interactive UPS. However, it typically costs much more, due to it having a much greater current AC-to-DC battery-charger/rectifier, and with the rectifier and inverter designed to run continuously with improved cooling systems. It is called a Double-Conversion UPS due to the rectifier directly driving the inverter, even when powered from normal AC current.
In an online UPS, the batteries are always connected to the inverter, so that no power transfer switches are necessary. When power loss occurs, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When power is restored, the rectifier resumes carrying most of the load and begins 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.
The main advantage to the on-line UPS is its ability to provide an electrical firewall between the incoming utility power and sensitive electronic equipment. While the standby and Line-Interactive UPS merely filter the input utility power, the Double-Conversion UPS provides a layer of insulation from power quality problems. It allows control of output voltage and frequency regardless of input voltage and frequency.
A second major advantage is the inbuilt bypass circuit in on-line UPS which ensures should any problems occur with the UPS or batteries as long as there is mains power the load will be powered. The bypass is either automatic or a combination of manual and automatic depending on the model/manufacturer.
Difference Between Online UPS and Offline UPS – Which One is Better?
Main Difference between Online UPS and Offline UPS – Which One to Use?
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.
The major point that differentiates between the online UPS and offline UPS is that the online UPS supplies power from the AC mains to the Load through the rectifier and inverter combination while the Offline UPS directly supplies power from AC mains to the load.
Before going to the list of differences between Online UPS and Offline UPS, let’s discuss the basics of Online UPS and Offline UPS.
What is UPS (Uninterruptible Power Supply)?
UPS is an abbreviation for Uninterruptible Power Supply and the reason for its name is that it provides a constant supply of power without any interruption. In Normal operation, it draws current from the AC mains and during a power outage; it draws current from its backup source.
A UPS system utilizes a DC power stored in the battery as its backup source. The UPS charges the battery from the AC mains through a Rectifier and charge controller circuit. At the same time, it supplies power to the output AC load. In case of power failure, the UPS utilizes this charged DC power in the battery and converts it into AC using an inverter and supplies it to the AC load.
Now the switch over between the sources (AC mains and the battery in case of power failure) is instantaneous. But it does take some time. This time delay during the switching may affect some sensitive electronic equipment while other devices may tolerate it in a specific range. This is where Online UPS and Offline UPS comes into play.
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What is 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.
During normal operation, it converts the AC supply from the AC mains into DC using a rectifier and charges the battery using a charge controller circuit. Now at the same time, the DC power from the charged battery is being converted into AC using an inverter to supply it to the output AC load.
In case of power failure, the UPS will stop drawing current from the AC mains, and stops charging the battery. But the AC load will continuously draw current from the battery through Inverter.
Therefore, Online UPS has a zero time delay when switching its power source. It is also why it is known as Online UPS because it stays ON even during its normal operation.
Since the current drawn by the AC load is continuously flowing through its whole circuit, the temperature of the system increases. Therefore, it needs comparatively larger heat sinks and the components that can withstand high temperature and it can tolerate current flow for very long duration. Due to such a requirement, the cost of Online UPS increases significantly.
It continuously draws current from the battery which quickly reduces the lifetime of the battery compared to offline UPS. They are more inefficient because they continuously stay ON even in normal operation.
On the other hand, it does offer high-quality output because there is no gap during the switching between the source supplies. It completely isolates the output from the AC mains. Therefore, it helps in protection against the distortions or fluctuations in the AC mains.
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What is 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.
During normal operation, the power from AC mains is directly supplied to the output through a static transfer switch. At the same time, the AC is converted into DC using a rectifier to charge the battery using a charge controller. But unlike Online UPS, the inverter does not draw current from the battery until the primary supply (AC main) is interrupted.
When the primary supply breaks, the battery stops charging and the static switch transfer the output line to the inverter side very quickly. At the same time, the inverter starts drawing current from the battery which is then supplied to the AC load.
Since it stays off when it draws current from the AC line during normal operation, it is known as Offline UPS.
The Offline UPS mostly stays off in normal operation and because of that, the temperature of the components stays low. Therefore, it requires smaller heat sinks and the components with a lower tolerance can be used which also decreases the overall cost of the offline UPS.
Since it stays off in its normal operation and does not use the battery continuously, the efficiency of the system increases.
But the downside of offline UPS is that the directly supplied AC is not completely isolated from the load. Therefore, there is no protection between the load and source and the distortion or fluctuations in the Ac mains can damage the connected load.
Differences between Online UPS and Offline UPS
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 etc
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Which one to Use, Online or Offline UPS & Why?
Is online or offline UPS Better? and When to use it? it totally depends on multiple factors such as cost, usage and specific conditions etc. For example,
When to Use Online UPS:
When to operate the sensitive and important devices and equipment with continuous power supply without switching operations.
When the the main power supply fluctuates frequently or daily scheduled load shading and power outage
When there is no concerns about the operation, installation and maintenance cost as the online UPS system is costly than offline UPS system due to the additional parts in online uninterruptible power supply.
When you need the simultaneous operation of connected devices for longer time duration.
When to Use Offline UPS:
When cost is the major and considerable factor.
When switching is not a big issue i.e. you are OK to use it with auto or manual change-over or transfer switch between main power, portable generator or emergency generator set , solar or battery power.
The conclusion of this article showing the key difference between ONLINE & OFFLINE UPS systems is that the online UPS has zero time delay between the switching and they are expensive. while the Offline UPS has a time delay of 5ms but they are inexpensive. Based on efficiency, the offline UPS is far better than Online UPS and it should be used for domestic purposes. The Online UPS should be only used for very sensitive electronic equipment.
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