Department of Electrical and Computer Engineering
Part IV Project
An Electrically Isolated UPS
System with Surge Protection
Presented by: Thusitha Mabotuwana
Duleepa Thrimawithana
Supervisors : Mr. Nihal Kularatna
Dr. Patrick Hu
Presentation Outline
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Project background
Transients and transient protection
Current protection mechanisms and drawbacks
A new transient minimisation scheme
Supercapacitors as energy storage devices
System we have implemented
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Power stage design and control
Results
Future developments
Conclusions
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Project Background
• Immense damage caused to electronic
equipment by heavy lightning.
• Current low cost UPS systems have
limited protection.
• Systems with good protection schemes
are very costly and bulky – not suitable for
domestic use.
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Project Goals
• Design and develop a new UPS topology
with complete isolation between supply
and load.
• Investigate possibilities of using
supercapacitors for energy storage in UPS.
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What are Transients?
• Forms of transients
- Spikes (in excess of 6000V in less than 200µs)
- Surges (about 20% over nominal line voltage.
Lasts for about 15-500ms)
- Sags (similar to surges. But under-voltage
condition)
- Electrical impulse noise (high frequency
interference)
- Blackouts and brownouts (total or short-duration
power loss)
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What is Transient Protection?
• Protection of user devices from whatever that
happens at the primary power sources or in
the environment.
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Current UPS Systems
Feature
Offline
Line-Interactive
Online
Surge
Protection
Poor
Poor
Good
Protection
Mechanism
Switches from main
supply to battery
during transients
Switches from main
supply to battery
during transients
Continuously
regenerates clean AC
using supply or battery
Weight
Low
Moderate
High
Size
Small
Moderate
Big
Cost
Low
Medium
Very high
Usage
Homes and small
office environment
Medium scale
operations
Power sensitive
equipment, network
protection systems
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Our Tasks and Specifications
• Investigate possibilities of using supercapacitors for
power transfer while maintaining complete isolation.
• Design a UPS with the following specifications:
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Input voltage – 230VAC at 50/60Hz
Output voltage – 230VAC at 50Hz
Output regulation – ±5%
Output power – 100W
Common and differential mode isolation
Common mode surge
Differential mode surge
Diagrams reproduced from Kularatna, N. (1998) Power Electronics Handbook. Boston, Newnes.
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System Overview
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Why Supercapacitors?
• Properties of supercapacitors
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Very high capacitance (even 1000F)
High power density
Virtually unlimited number of charge-discharge cycles
No toxic substances like in conventional batteries
Low energy density
High ESR
Extracted from Prophet, G. (2003). EDN. Supercaps for Supercaches, January, 53-58
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New Concept for Surge
Minimisation
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New Concept for Surge
Minimisation (cntd..)
Energy Pump
Inverter and Load
Charge Transfer Unit
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Our System
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Energy Pump
• Current controlled forward converter
topology was used.
- Simple and economical design
- Less number of exposed components to the
main supply
- Provide electrical isolation
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Energy Pump (cntd…)
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Charge Transfer Unit
• Transfers power to the inverter while maintaining
isolation.
• Banks are switched so that the discharging bank is
not connected to the input.
• Supercapacitor banks cycle through chargingstandby-discharging cycles.
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3rd bank (Discharging)
Charge Transfer Unit (cntd…)
2nd bank (Standby)
1st bank (Charging)
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Charge Transfer Unit (cntd…)
• Charge transfer unit output waveforms:
Output
waveform
Charging
logic
Discharging
logic
1V ripple
2V ripple
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Charge Transfer Unit (cntd…)
• Load regulation characteristics when tested with
the commercial inverter confirmed
supercapacitors’ capability to transfer energy.
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Charge Transfer Unit (cntd…)
• Discharge time for a supercapacitor bank
of 0.2F based on load variations:
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Inverter
• Needed a single stage sine wave inverter.
• Some techniques we looked at:
– PWM
– PAM
– Square wave
– Resonant
• Decided to implement a single stage PWM pushpull scheme.
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Inverter (cntd…)
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Inverter (cntd…)
• Inverter output characteristics with a 25W
load:
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Inverter (cntd…)
• Load regulation characteristics:
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System Cost
Cost
(NZ$)
Cost
(NZ$)
Per unit price
Per 10000 units price
Transformers
120.00
40.00
Supercapacitors
120.00
30.00
Microcontroller
20.00
5.00
120.00
55.00
380.00
130.00
Component
Other components
(FETs, Opto-couplers
etc)
Total Cost
(approximately)
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Future Developments
• Develop a commercial prototype
• Consider use of cheaper supercapacitors
with higher capacitance.
• Optimise inverter and energy pump
modules.
• Consider a compact FPGA or DSP
implementation strategy.
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Conclusions
• A method of energy transfer using
supercapacitors has successfully been
implemented.
• Complete supply-load isolation has been
achieved using three supercapacitor banks with
dynamic transfer.
• Sine wave inverter based on a 1kHz PWM has
been implemented.
• Charger has been implemented using a forward
converter with current mode control.
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Questions?
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