An Electrically Isolated UPS System with Surge Protection

Presented by: Thusitha Mabotuwana Duleepa Thrimawithana

Supervisors : Mr. Nihal Kularatna Dr. Patrick Hu

Department of Electrical and Computer EngineeringDepartment of Electrical and Computer Engineering

Part IV ProjectPart IV Project

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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

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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 SystemsFeature 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

UsageHomes 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:– 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 surgeDiagrams 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

- 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..)(cntd..)

Energy PumpInverter and Load

Charge Transfer Unit

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Our System

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• Current controlled forward converter topology was used.

- Simple and economical design- Less number of exposed components to the

main supply- Provide electrical isolation

Energy Pump

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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 charging-standby-discharging cycles.

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Charge Transfer Unit (cntd…)3rd bank (Discharging)

2nd bank (Standby)

1st bank (Charging)

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Charge Transfer Unit (cntd…)

• Charge transfer unit output waveforms:

2V ripple2V ripple1V ripple1V ripple

Charging logic

Discharging logic

Output waveform

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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 push-pull 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

ComponentCost (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

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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?