An Electrically Isolated UPS System with Surge Protection

1
An Electrically Isolated UPS System with Surge
Protection
Department of Electrical and Computer
Engineering Part IV Project

• Presented by Thusitha Mabotuwana
• Duleepa Thrimawithana
• Supervisors Mr. Nihal Kularatna
• Dr. Patrick Hu

2
Presentation Outline

• 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

• Power stage design and control
• Results
• Future developments
• Conclusions

3
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.
  

4
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.

5
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)

6
What is Transient Protection?

• Protection of user devices from whatever that
  happens at the primary power sources or in the
  environment.

7
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
8
Our Tasks and Specifications

• Investigate possibilities of using
  supercapacitors for power transfer while
  maintaining complete isolation.
• Design a UPS with the following specifications
• 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.
9
System Overview

10
Why Supercapacitors?

• Properties of supercapacitors
• 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
11
New Concept for Surge Minimisation
12
New Concept for Surge Minimisation
(cntd..)
Energy Pump
Inverter and Load
Charge Transfer Unit
13
Our System
14
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

15
Energy Pump (cntd)
16
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
  charging-standby-discharging cycles.

17
Charge Transfer Unit (cntd)
3rd bank (Discharging)
2nd bank (Standby)
1st bank (Charging)
18
Charge Transfer Unit (cntd)

• Charge transfer unit output waveforms

Output waveform
Charging logic
Discharging logic
1V ripple
2V ripple
19
Charge Transfer Unit (cntd)

• Load regulation characteristics when tested with
  the commercial inverter confirmed
  supercapacitors capability to transfer energy.

20
Charge Transfer Unit (cntd)

• Discharge time for a supercapacitor bank of 0.2F
  based on load variations

21
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 push-pull
  scheme.

22
Inverter (cntd)
23
Inverter (cntd)

• Inverter output characteristics with a 25W load

24
Inverter (cntd)

• Load regulation characteristics

25
System Cost
Component Cost (NZ) Per unit price Cost (NZ) Per 10000 units price
Transformers 120.00 40.00
Supercapacitors 120.00 30.00
Microcontroller 20.00 5.00
Other components (FETs, Opto-couplers etc) 120.00 55.00
Total Cost (approximately) 380.00 130.00
26
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.

27
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.

28
Questions?