UPS | Uninterruptible Power Supply | Types of UPS | Application of UPS

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In this chapter we are going to learn about the UPS (Uninterruptible Power Supply), the Types of UPS, the working of UPS, and its application of UPS in a very detailed manner. So before starting to know about UPS I would like to tell you what the UPS system, why we need it.

What is the need of UPS?

Certain application areas such as personal computers, computer workstations, medical equipment, and intensive care units (ICU) need a continuous supply of high-quality sinusoidal voltage.

For such loads, the user cannot depend solely on the sinusoidal voltage available at the main supply. This is due to frequent outages, poor quality of voltage waveform, fluctuations in mains voltage, etc.

Hence an alternative system to mass supply has been developed called Uninterruptible Power supply (UPS).

The UPS systems have become an integral part of our day-to-day life. We see these systems along with computers, or with expensive instruments in hospitals, etc.

The UPS is used for supplying power to such critical loads. The UPS system not only supplies uninterrupted power to these critical loads but it takes care of the following things as well:

  1. It regulates the load voltage when the mains voltage fluctuates.
  2. It suppresses the line transient (spikes) that otherwise would have been easily coupled to the load.
  3. It supplies a pure, harmonics-free signal to the load.

What are the types of UPS?

Depending on the arrangement of the basic blocks of the UPS system, they can be classified into the following types:

  1. On-Line or Inverter preferred UPS system
  2. offline or Line preferred UPS system 
  3. Line Interactive UPS

1. On-Line UPS ( Inverter Preferred ) or (Continuous) UPS System

The configuration of an Online UPS system is shown in the figure below:

Online UPS system

Operation of Online UPS:

In the online UPS system, the load is always connected to the inverter through the UPS static switch.

The UPS static switch in the above figure is a “normally ON” switch” It turns off only when the UPS system fails.

In that case, the “mains static switch” is turned on to connect the ac mains directly to the load.

This switch is a “Normally OFF” switch and is used only when UPS is to be bypassed.

A static switch consists of a pair of antiparallel SCRs. Therefore conduction through a static switch is bidirectional.

Static switches are preferred over conventional mechanical switches because they can be activated electrically. Earthing and sparking do not take place while operating the static switches.

The operation is divided into two modes:

1. Mode I (Mains ON):

When the ac mains are on, the rectifier circuit will supply the power to the inverter as well as to the battery. So it acts as a rectifier cum charger. The inverter output is connected to the load via a UPS static switch. The battery in this mode.

The equivalent circuit for this mode is shown in the figure below:

online ups system mode-I

2. Mode II (Mains OFF):

Under the condition of a mains outage, the rectifier output is zero. So the battery bank now supplies power to the inverter.

The inverter then drives the load through the UPS static switch.

This transfer from the rectifier to the battery bank is instantaneous and during the transfer period, there s no power interruption.

After the restoration of the mains, the rectifier again starts supplying power to the inverter and will also charge the battery bank.

The equivalent circuit and power flow in this mode is shown in the figure below:

online ups system mode-II

3. operation when UPS fails:

Due to some reason if UPS fails then the normally-off mains static switch is turned on to connect the mains to the load bypassing the UPS system.

The UPS static switch is automatically turned off and disconnects the load from the UPS.

The transfer to load from UPS to mains takes place within a 1/4 cycle period with no phase change.

The equivalent circuit and power flow in mode III is shown in the figure below:

online ups system mode-III

Specification of Online UPS:

When a UPS system is being purchased, the following specifications should be carefully studied.

Sr. NoParameterExpected value
1.Power rating1 kVA, 2 kVA, 5 kVA, etc.
2.Output Voltage230V ± 0.5%
3.Output frequency50 Hz ± 2 Hz
4.Input Voltage190 to 260 volts
5.Nature of output voltage waveformSinusoidal
6.Power factor> 0.8 lagging
7.Back uptime30 minutes to 4 hours
8.Total harmonic distortion< 5%
9.The efficiency of the inverter> 85%
10.Protection circuits(a) Over voltage and under voltage cutout(b) Overcurrent protection

Advantages of Online UPS:

The online UPS system is the most popular UPS configuration of all. The online UPS provides isolation between the load and the ac supply. This is its advantage over the rest of the UPS configuration.

Some other important features are as follows:

  • It provides isolation between ac source and load.
  • Due to the continuous use of inverters, the quality of load voltage is better than that of mains.
  • All voltage and current spikes are suppressed.
  • Better regulation

2. Off-Line UPS ( Line Preferred ) or Transfer UPS System

The block diagram of an offline UPS system is as shown in the figure below:

Offline UPS system

Operation of Online UPS:

If you observe carefully of above diagram then it is the same as OnlineUPS. The only difference is that the “Mains static switch” here is a “normally on” switch.

It connects the ac mains directly to the load when the mains are on. The battery is charged through the charger.

The other static switch i.e. “UPS switch” is a “normally off” switch. It is closed only when the mains fail.

Thus in the offline UPS, the inverter comes into the circuit only when the mains fail.

The rectifier/charger has only one function to perform i.e to charge the battery bank. Therefore its size and power rating are lower than that of an online UPS system charger.

There are two modes of operation of offline UPS systems:

1. Mode I (Mains ON):

When the mains are ON, the power is supplied to the load via the normally On mains static switch.

The battery is charged through the charger.

The equivalent circuit and power flow for this mode is shown in the figure below:

Offline UPS system mode I

2. Mode II (Mains OFF):

When the mains are off, the mains static switch gets open-circuited and the UPS static switch gets closed automatically. 

Battery charging will stop and the battery starts supplying power through the inverter as shown in the figure below:

The total time taken to sense the power failure and make a changeover from mains to UPS is about 5msec.

Offline UPS system mode II

Specification of Offline UPS:

Sr. NoParameterExpected value
1.Output voltage1. Mains On : 230 ± 5% Volts

2. UPS On : 230 ± 0.5% Volts

2.Output voltage waveformSinusoidal
3.Output frequency1. Mains On : 50Hz ± 3Hz

2. UPS On : 50Hz ± 2 Hz

4.Input voltage24V, 48V, 72V, etc. with AH capacity
5.Power rating1 kVA, 2 kVA, etc.
6.Time is taken by the battery to charge3 to 4 hours
7.Back Uptime30 minutes or more
8.Transfer timeLess than 5 msec
9.Efficiency> 85%
10.Protection circuits1. Mains overvoltage

2. Output over current3. Low battery

Disadvantages of Offline UPS:

The offline UPS system uses the UPS sparingly only when the mains is OFF, therefore it is more economical as compared to the online UPS system.

However, as the UPS is not being used continuously all the advantages of using the inverter are lost.

This system is, therefore, less reliable than the online UPS.


3. Line Interactive UPS System

The block diagram of a line-interactive type UPS system is shown in the figure below:

interactive UPS system

Operation of Line Interactive UPS:

The operation of the system can be divided into two modes:

1. Mode I (Mains ON):

The static switch is closed and the load gets connected directly to the ac mains through the inductance L.

The inverter/charger block operates as a charger and charges the battery bank. the equivalent circuit for this mode is shown in the figure:

interactive UPS system mode I

2. Mode II (Mains OFF):

As soon as the mains fail, the static switch is turned off and the inverter/charger block operates as an inverter, and the battery supplies power to the load through the inverter.

The equivalent circuit for this mode is shown in the figure below:

interactive UPS system mode II

The total time taken for sensing and changeover after the failure of mains is less than 5 msec.

This configuration does not provide any isolation between the load and the ac mains, however, it is possible to give a pure, clean voltage to the load with good voltage regulation by using power conditioners like an ac stabilizer or a constant voltage transformer ( CVT).

Another problem with this system is that if the charger/inverter block fails, then no power will be supplied to the load.

This system is however more economical than the online UPS system.


Comparison of UPS Systems

Sr. NoParameterOnline UPSOffline UPSLine Interactive UPS
1.Output voltage waveformSinusoidalQuasi squareQuasi square
2.Total harmonic distortionLowHighHigh
3.Transfer Time0< 5 ms< 5ms
4.EfficiencyLowHighHigh
5.CostCostliestLess costlyLess costly
6.ApplicationsCritical loadsGeneral-purpose loadsGeneral-purpose loads
7.Isolation between the load and mainsFull isolationNo isolationNo isolation
8.size and weightHighestLowLow
9.Rectifier/Charger power ratinghighestLowestHigh
10.OperationContinuousOnly in the absence of mainsOnly in the absence of mains
11.ReliabilityVery reliableNot so reliableNot so reliable

Application of UPS Systems

Nowadays UPS is being used in almost all fields. Some of the important applications areas are as follows:

  • As a power supply for computers
  • For supplying continuous power to medical equipment
  • In intensive care units
  • for EPABX
  • Flight Simulation
  • Uninterruptible Power Supply (UPS) systems are critical components in various industries, ensuring continuous and reliable power supply to sensitive electronic devices. A UPS system acts as a backup power source to prevent data loss, system crashes, and equipment damage during power outages or fluctuations. Let’s explore the applications of UPS systems in different sectors.
  1. Data Centers:
  • Data centers require a continuous and reliable power supply to maintain operations. A power outage can lead to data loss and damage to hardware, which can result in significant financial losses. UPS systems ensure that data centers remain operational during power outages, providing critical time for backup generators to come online. Data centers often have redundant UPS systems to ensure maximum uptime.
  1. Medical Facilities:
  • In the healthcare industry, an uninterrupted power supply(UPS) is critical to avoid any damage to medical equipment, prevent data loss, and ensure patient safety. Medical facilities, including hospitals, clinics, and laboratories, use UPS systems to provide continuous power to medical equipment such as X-ray machines, MRIs, and life support systems. UPS systems also help protect critical data such as patient records and test results.
  1. Manufacturing Industries:
  • Manufacturing industries rely on electrical systems to operate production lines and control systems. A power outage or voltage sag can cause equipment to malfunction, leading to production delays and financial losses. UPS systems help maintain power quality and continuity in manufacturing plants, reducing downtime, and increasing productivity.
  1. Financial Institutions:
  • Financial institutions, including banks and trading companies, depend on continuous power to operate trading platforms and ATMs. A power outage or voltage fluctuation can lead to significant financial losses. UPS systems help protect sensitive financial data, prevent equipment damage, and keep trading platforms and ATMs operational during power outages.
  1. Telecommunications:
  • Telecommunications companies require continuous power to operate their networks and maintain connectivity. A power outage can lead to dropped calls and network disruptions, which can result in significant financial losses. UPS systems help maintain the power supply to telecommunication equipment, ensuring that communication systems remain operational during power outages.
  • In conclusion, UPS systems play a crucial role in various industries, providing uninterrupted power supply to sensitive electronic devices. By ensuring a continuous and reliable power supply, UPS systems help reduce downtime, prevent equipment damage, and protect critical data, resulting in increased productivity and financial stability.

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