1. Uninterruptible Power Systems
A study and comparison of topologies

2. UPS; An introduction
An uninterruptible power source is an electromechanical/electrochemical system that provides backup or emergency power to a load when the power supply from the input becomes absent (or becomes unusable due to fluctuations in voltage signal ).
The main difference of an UPS from a Auxiliary Power Supply system like generators is that it provides instantaneous or near-instantaneous protection from input power interruptions.

3. UPS; An introduction
Power is stored in an UPS in batteries or flywheels, with flywheels being very uncommon and mostly used for extremely specific purposes.
Uninterruptible Power Systems can broadly classified into two categories based on the network topology : -
On-Line UPS
Off-Line UPS

4. Basic Components
A battery based UPS system consists of three main components : -
Rectifier
Battery (storage)
Inverter
These components are connected in various topologies, along with transformers, to cater to different application needs.

5. Basic Components

6. Interruptions in supply
Blackout

7. Interruptions in supply
Brownout

8. Interruptions in supply
Surge

9. Interruptions in supply
Spike

10. Interruptions in supply
Frequency Drift

11. Specifications Hold-up time
Time for which the load can continue operating after input voltage has vanished (10~30ms)
Transfer Time
Time taken by UPS to switch over from mains supply to storage supply
Backup Time
Time for which storage can provide supply at rated voltage and power

12. Conventional topologies
1. Off-line / VFD / passive
2. On-line / VFI / double conversion
3. Line-Interactive / VI

13. Off-line UPS

14. Off-line UPS
An off line UPS, during normal operation, feeds raw mains to the critical load through a bypass line and static switch.
If the mains transgresses preset limits, the static switch transfers the load to the UPS’s inverter and battery
Transfer-time ~ 10ms

15. On-Line UPS

16. On-line UPS
In this configuration, the load is always fed from the backup source, and never fed directly supply power. There is no transfer switch involved and hence it has no transient characteristics.
Transfer Time = 0

17. Line-Interactive UPS

18. Line-interactive UPS
These use off line topology, but address the voltage transgression problems by providing voltage regulation in the raw mains line which supplies the load during normal operation.
Transfer Time ~10ms

19. Standardisation
The need to standardise UPS topologies has arisen due to following reasons :
Frequently used misnomers like “in-line” and “parallel” and “independent”
Duping of general public by UPS manufacturers by clever renaming of topologies
The term “line” can be referred to as the the supply line or the conversion circuit, resulting in advent of pseudo on-line UPS models

20. Standardisation As per the standards IEC 62040-3 (USA) and ENV
(Europe), broad UPS topologies are :
passive-standby (formerly off-line); where load is supplied by utility power, which may or may not be filtered.
line-interactive ; The inverter is connected in parallel and acts to backup utility power and charges the battery through it reversible characteristics
double-conversion (formerly on-line) ; The load is continuously supplied via the rectifier/charger - inverter combination which carries out a double conversion AC-DC- AC

21. Comparison of Topologies
We shall compare the following properties between the three topologies
1. Transfer time characteristics
2. Voltage and Frequency dependence
3. Scope of operation
4. Energy Efficiency
5. Reliability

22. Transfer time characteristics

23. Voltage & Frequency dependence
Online UPS is also known as VFI (Voltage and Frequency Independent) UPS because output waveform has no dependence on input waveform.
Offline UPS is also known as VFD (Voltage and Frequency Dependent) UPS because output waveform can get affected by disruptions in both Voltage as well as frequency.
Line-interactive UPS is known as VI (Voltage Independent) UPS because output waveform is shielded against Voltage fluctuations by the AVR but it still is not independent of input waveform's frequency.

24. Scope of Operation

25. Energy Efficiency On-line UPS : 85~92 %
Due to double conversion, rectifier and inverter loss always present.
Higher cost of ownership
Off-line/Line-interactive : 96~98 %
Only AVR in circuit during normal mode
AVR is a Transformer, thus efficiency >95%
Lower ownership cost, suitable for domestic usage

26. Reliability Off-line/Line-interactive UPS
This topology is generally more reliable because :
Lesser no. of parts
Converter and inverter not always in use
Lower operating temperature
On-line UPS
Reliability of on-line ups in long term is low as :
conversion circuits always in use
higher operating temperatures and higher heat dissipation

27. Present situation
On-line, or double-conversion model is currently considered superior because : -
continuous conversion of the power supplied at the inverter output for the load
isolation of the load from the upstream distribution system
very wide input-voltage tolerances and precise regulation of the output voltage
precise regulation of the output frequency, and possibility of the UPS operation as a frequency converter
much superior performance levels under steady-state and transient conditions
Power factor correction

28. Present situation
Circumstances where an off-line or line-interactive UPS is not possible to use :
Load has very low or zero hold up time
Very high voltage loads where fast switching can cause extreme waveform distortion
Load needs tight frequency regulation and supply has frequency drifts very often
Other than mentioned circumstances, the use of off-line systems are still more widespread than on-line systems due to the economics of it.

29. Present Situation

30. Alternate models and future scope
Modular UPS
UPS is modularised in form of racks.
Adding modules increases capacity, thus making the model scalable
Can be implemented in both off-line and on-line topologies
Easy to achieve N+1 configuration and very cost effective
Replaceability of modules makes maintenance easier
Suitable for IT hubs and Data Centers

31. Alternate models and future scope
Hybrid/Double-conversion on-demand
New model being conceived by HewlettPackard and Gammatronics
Works as off-line UPS as long as supply signal is within certain tolerance window.
Can switch over to on-line mode when supply signal distortions are outside tolerance ranges
One of highest efficiency models conceptualised till now

32. Alternate models and future scope
Rotary UPS (High Speed flywheel)
Energy stored as rotational kinetic energy in a flywheel which rotates at a very high speed.
Due to it’s nature, flywheels are inherently on-line UPS
Backup times are very low, so used in conjunction with an auxiliary power supply
Can be used in conjunction with an off-line UPS to make it appear as an online UPS

33. Conclusion
As we move towards more and more components, including electrical and mechanical ones having computer-based control interfaces, need of UPS systems will keep increasing.
Improvement in reliability and efficiency is needed as often momentary loss of power can lead to loss of lives and/or huge damage to property.
Market continues to be dominated by traditional on-line and line-interactive models.

34. Conclusion We can expect in future :
Better standardisation of UPS topologies and proper classification of even hybrid modes
Increased efficiency in on-line models
Reduction in transfer times of static transfer switches used in off-line UPS
Improvements in hybrid models to make use of advantages of both models
Development of alternate topologies like on-demand VFI and flywheel-augmented UPS