me2010_avravi comanda invertor

Control Power Supply Architecture For

Ride Through In Power Converters Using

Ultracapacitors

A Project Report

Submitted in Partial Fulfilment of

Requirements for the Degree of

Master of Engineering

in

Electrical Engineering

By

Anand Vivek Ravi

Department of Electrical Engineering

Indian Institute of Science

Bangalore - 560 012

India

June 2010

Acknowledgements

I extend a great thanks to Prof. Vinod John for giving me an opportunity to work in the

Power Electronics Group.I would like to express my admiration for him on the various in-

teresting ideas suggested during the course of project. Work under him had always been a

great pleasure and I take this opportunity to acknowledge all the timely help rendered by

him, right from awarding me a very interesting project till the final completion of it. I would

like to acknowledge the confidence and the encouragement provide by him during the final

bottleneck situations, which was a great motivation, helping me complete the work.

I express my humble gratitude to Prof. V. Ramanarayanan, who has been a pillar of support

to all the students in the group. I,having cherished attending his lectures and seminars,

would like to thank him for teaching us to progress on becoming good power electronics

design engineers.

I would like to thank Prof. V. T. Ranganathan for his patient lectures and in making a not so

conspicuous topic into an easily comprehensive one. I would like to thank Prof. G. Narayanan

for providing insights on the various PWM techniques through his course.

I thank my close frinds Venkat, Manoj, Shan, Arun Karuppaswamy,Raju and Tarak for

being supportive and critical of me and for the interesting discussions had with them. I

would also like to thank Vishnu, Prakash and the other students of the batch for their sup-

port. I thank all PhD students of the PEG group Kamalesh Hatua, Amit Jain, Shivaprasad,

Anirban, Dipankar De, Soumitra Das and Binoj Kumar for their help. I would like to thank

Srinath in helping me out on various aspects of the project.

I thank Ms. Silvi Jose, Mr. Paul, Mr. Ravi and members of the department workshop for

their cordial demeanour. I also extend my thanks to Mr D. M. Channegowda and his team

at the department office for their good administrative activities.

I would like to thank Shankar, Anil Adapa and Mr. Krishna of M/s CHIPKRAFT TECH-

i

ii Acknowledgements

NOLOGIES for helping at a very difficult situation, the result of which the project got

completed.

Finally, I would like to thank my parents who have put up with me inspite of my maverick

behaviour and for being by my side in all my failures and success. I would like to thank

my adoring grandparents and my guru shri Anandha Bharathi Ayya for providing me the

mental strength during bleak periods and the God Almighty for having been my best buddy,

for giving me the opportunity to study at this prestigious institute, to work under such an

understanding and a patient guide and finally for providing me a career through this project

along with the completion of the project.

Abstract

High power converters are used in variable speed induction motor drive applications. These

power converters consist of active front end rectifier, a DC link and an inverter feeding the

motor. The power devices employed in this converter are high voltage IGBTs.

The following figure shows a typical block diagram of the inverter fed induction motor drive

system

Figure 0.1: A typical induction motor drive system

The switching action of these devices depends on the gating pulses generated by the gate

drive cards. The voltage and currents on the ac side are sensed through voltage and current

sensing cards respectively to monitor faults and for closed loop control. The power supply

failure of these control cards results in shut down of the entire system.

This project addresses the failure of the control power supply due to voltage sags or blackouts

by providing backup using ultracapacitors for a period of 10secs. The project involved design

and testing of a bi-directional buck-boost converter, design of a stack of ultracapacitor cells

and characterization of the individual cells, design and implementation of the controllers for

closed loop control using Microchips dsPIC30F2023.

iii

Contents

Acknowledgements i

Abstract iii

List of Tables vii

List of Figures viii

Nomenclature x

1 Introduction 1

1.1 Outline of the Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 The Ride-Through System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.1 Need for a power converter . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Organisation of the Project Report . . . . . . . . . . . . . . . . . . . . . . . 4

1.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Ultracapacitors - Modelling and Sizing 6

2.1 Construction of Ultracapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Comparison of Various Energy Sources . . . . . . . . . . . . . . . . . . . . . 8

2.3 Electrical characteristics of Ultracaps . . . . . . . . . . . . . . . . . . . . . . 9

2.4 Design of the Ultracapacitor Stack for the Ride Through System . . . . . . . 9

2.4.1 Number of Ultracapacitor cells . . . . . . . . . . . . . . . . . . . . . . 11

2.4.2 Ultracapacitor voltage management . . . . . . . . . . . . . . . . . . . 12

2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

iv

Contents v

3 Hardware Design 14

3.1 Power Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.1.1 Power Devices Selection . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.1.2 Gate Drive Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.1.3 Filter Inductor and Capacitor Selection . . . . . . . . . . . . . . . . . 15

3.1.4 Losses in the switches . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.1.5 Power Circuit Schematic . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.2 Theory of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.3 Controller Platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.4 Current and Voltage Sensing Circuit . . . . . . . . . . . . . . . . . . . . . . 18

3.4.1 Voltage Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.4.2 Current Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4 Controller Design 21

4.1 Principle of Closed Loop Control . . . . . . . . . . . . . . . . . . . . . . . . 21

4.1.1 Control Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.2 Buck Converter Control with Resistive Load . . . . . . . . . . . . . . . . . . 24

4.2.1 Inner current loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.2.2 Outer Voltage Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.3 Buck Converter Controller for Charging Ultracapacitors . . . . . . . . . . . . 28

4.3.1 Inner Current Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.3.2 Outer Voltage Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.4 Boost Converter Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5 Experimental Results 33

5.1 Ultracapacitor Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.2 Ultracapacitor Voltage Monitoring Circuit . . . . . . . . . . . . . . . . . . . 33

5.3 Closed Loop Control Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.3.1 Buck Converter - Ultracapacitor Charging Test Results . . . . . . . . 35

5.3.1.1 Charging at 1A . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.3.1.2 Charging at 2A . . . . . . . . . . . . . . . . . . . . . . . . . 36

5.3.2 Boost Converter Test Results . . . . . . . . . . . . . . . . . . . . . . 38

5.3.3 Boost Converter test results with ultracapacitor input . . . . . . . . . 42

vi Contents

5.3.3.1 Boost Converter Operation with 25W(23V, 20) load . . . . 42

5.3.3.2 Boost Converter Operation with 50W(23V, 10) load . . . . 42

5.3.4 Integrated Operation of Buck and Boost converter . . . . . . . . . . . 43

5.4 Efficiency of the Power Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.5 Thermal Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

6 Conclusions 48

A Schematics and Experimental Setup 50

A.1 Power Circuit Schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

A.2 Controller Schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

A.3 Ultracapacitor board - Top View . . . . . . . . . . . . . . . . . . . . . . . . 52

A.4 Ultracapacitor board - Side View . . . . . . . . . . . . . . . . . . . . . . . . 53

A.5 Power Circuit Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

A.6 Voltage and Current Sensing Circuit Hardware . . . . . . . . . . . . . . . . . 55

A.7 Controller Circuit Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

References 57

List of Tables

1.1 System Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1 Comparison between batteries and ultracapacitors . . . . . . . . . . . . . . . 9

2.2 Power consumed by the inverter control cards . . . . . . . . . . . . . . . . . 10

3.1 Inductor Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

5.1 Ultracapacitors Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.2 Efficiency of Power Circuit in Buck mode . . . . . . . . . . . . . . . . . . . . 45

6.1 Ride Through System Results . . . . . . . . . . . . . . . . . . . . . . . . . . 49

vii

List of Figures

0.1 A typical induction motor drive system . . . . . . . . . . . . . . . . . . . . . iii

1.1 Control Power Flow in a Motor Drive . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Block Diagram of the Ride Through System . . . . . . . . . . . . . . . . . . 3

1.3 Need for a Bi-directional Converter . . . . . . . . . . . . . . . . . . . . . . . 4

2.1 construction of double layer capacitor . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Ragones Plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.4 Discharge Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.5 cell voltage management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1 Power circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.2 Buck Mode of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.3 Boost Mode of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.4 Voltage Sensing Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.5 Current sensing Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.1 Closed Loop Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.2 Closed Control Loop - Outer Voltage Loop . . . . . . . . . . . . . . . . . . . 22

4.3 Current to Control transfer function i(s)/d(s) . . . . . . . . . . . . . . . . . 25

4.4 inner current loop - Loop gain GH transfer function i(s)/ iref (s) . . . . . . . 25

4.5 Voltage to Current transfer function V(s)/i(s) . . . . . . . . . . . . . . . . . 27

4.6 Outer Voltage Loop - Loop gain GH transfer function V(s)/Vref (s) . . . . . . 27

4.7 Current to Control transfer function - ultracap charging i(s)/d(s) . . . . . . 29

4.8 Current Loop gain GH - ultracap charging i(s)/iref . . . . . . . . . . . . . . 29

viii

List of Figures ix

4.9 Voltage to Control transfer function - ultracap discharging v(s)/d(s) . . . . . 31

4.10 Voltage Loop gain GH - ultracap Discharging V(s)/Vref (s) . . . . . . . . . . 32

5.1 Ultracapacitors Charging Plots . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.2 Ultracapacitors Voltage Management Circuit . . . . . . . . . . . . . . . . . . 35

5.3 Charging of Ultracaps - 1A . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

5.4 Charging of Ultracaps - 2A . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

5.5 Boost Converter Constant Voltage Control - 4.7V input, 6V output, 20ohm

load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.6 Boost Converter Constant Voltage Control - Step Increase in Input . . . . . 39

5.7 Boost Converter Constant Voltage Control - Step Decrease in Input . . . . . 39

5.8 Boost Converter Constant Voltage Control - 15V input, 25V output, 20ohm

load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.9 Boost Converter Constant Voltage Control - Step Increase in Voltage . . . . 40

5.10 Boost Converter Constant Voltage Control - Step Decrease in Voltage . . . . 41

5.11 Boost Converter Operation with 25W load . . . . . . . . . . . . . . . . . . . 42

5.12 Boost Converter Operation with 50W load . . . . . . . . . . . . . . . . . . . 43

5.13 Integrated operation - 75W(23V,7) . . . . . . . . . . . . . . . . . . . . . . 43

5.14 Integrated operation - 75W(zoomed view) . . . . . . . . . . . . . . . . . . . 44

5.15 Integrated operation 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.16 Integrated operation 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.17 Thermal Test - Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.18 Thermal Test - Processor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

5.19 Thermal Test - IR2110 and HCPL 3101 . . . . . . . . . . . . . . . . . . . . . 47

A.1 Power Circuit Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

A.2 dsPIC Controller Board Schematic . . . . . . . . . . . . . . . . . . . . . . . 51

A.3 Ultracapacitor Hardware - Top View . . . . . . . . . . . . . . . . . . . . . . 52

A.4 Ultracapacitor Hardware - Side View . . . . . . . . . . . . . . . . . . . . . . 53

A.5 Power Circuit Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

A.6 Sensing Circuit Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

A.7 Controller Circuit Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Nomenclature

Symbols : Definitions

P : Maximum load to which the ultracaps are subjected to

Vmax : Maximum Voltage across the Ultracapacitor stack

Vmin : Minimum Voltage across the Ultracapacitor stack

Vnominal : Nominal Voltage across the Ultracapacitor stack

Vcell : Rated Voltage of each Ultracapacitor

Imax : Maximum Current from the supercapacitor stack

Ct : Capacitance of the ultracapacitor stack

: Time constant of the ultacapacitors capacitance and ESR

L : Filter inductor in the power converter

C : Output filter capacitor in the power converter

Irms : RMS current through the switching device

RDSon : On state drain-source resistance of MOSFET

Vav : Average voltage across the body diode of MOSFET

Iav : Average current through the body diode of MOSFET

tr, tf : Rise and fall times of the switching device

Pswon, Pswoff : Turn on and turn off switching power loss

Vg : Input voltage variable used in controller design

v(s) : Very Small perturbation in Output voltage

i(s) : Very Small perturbation in Output current

ev : Error voltage corresponding to difference between reference and fedback voltage

ei : Error voltage corresponding to difference between reference and fedback current

error[n] : Error at nth sampling instant in digital implentation

kp[n] : Proportional controller output at nth sampling instant

x

Nomenclature xi

ki[n] : Integral controller output at nth sampling instant

D : Steady State Duty ratio

d(s) : Small variation perturbation in the duty cycle

Chapter 1

Introduction

The power line disturbances like voltage sags and blackouts affect the continuous process

industries to a great extent, the severity being dependent on the magnitude and duration

of the sag. Faults lasting even less than 0.5s can have dentrimental effects on the produc-

tion. Each industrial equipment reacts to voltage sag in different ways [2]. Voltage sags are

defined as a momentary dip in voltage - below 10 % lasting from a few cycles(10ms) to 150

cycles(0.3s) [16].

Voltage sags cause diruption of service but do not cause damage to sensitive loads. On the

Figure 1.1: Control Power Flow in a Motor Drive

other hand, the voltage swells, which may exist cause damage but doesnt dirupt sensitive

1

2 Chapter 1. Introduction

loads. More than 62% of the disturbances are due to voltage sags with duration less thanhalf

a second [16]. A severe voltage is one where voltage falls below 85 %. Voltage sags are caused

by faults within the system, starting currents of motor.

In the induction motor drive, severe voltage sags on the control power supply feeding the

DSP, Gate drive cards, Current and Voltage sensing cards might cause the entire motor drive

system to shutdown. The control flow diagram is shown in figure 1.1. So a ride-through sys-

tem consisting of DC-DC converter, an energy source has been designed to overcome the

effect of these voltage sags and momentary black-outs.

1.1 Outline of the Project

The project involves

1. Design of The Energy Storage System Designing a stack of Ultracapacitors for provid-

ing fault ride through for a maximum load of 85W for 10s. This involves choice of

the capacitors based on the required capacitance and design of the capacitor voltage

balancing circuit.

2. Design of The Power Processing System- Designing a bi-directional buck-boost con-

verter. This involved selection of the power devices, Gate drive circuit, Heat sink,

design of the dsPIC30F2023 digital controller board and the design of feedback signals

sensing circuit.

3. Characterisation of the ultracapacitors assuming a simple first order R-C circuit model.

This involved testing the capacitors for their leakage resistance and capacitance.

4. Testing the converter in buck mode of operation and boost mode of operation in closed

loop fashion and developing the controllers and implementing them in dsPIC30F2023

controller.

5. Testing of the ride-through system with the converter.

1.2. The Ride-Through System 3

1.2 The Ride-Through System

The project aims at developing a fault ride through system using Ultracapacitors as the

source of energy. The ride through system consists of a bank of ultracapacitor charging and

discharging through a bi-directional buck-boost converter. The system has been designed to

provide back-up over a period of 10s. The ride through system is shown in figure 1.2. The

main power to the control cards is fed through the PFC boost converter circuit, consisting

of a transformer-rectifier system in cascade with boost converter to make the input current

continuous, feeding the bi-directional converter and the sensing cards in parallel.

Figure 1.2: Block Diagram of the Ride Through System

1.2.1 Need for a power converter

The block diagram of the ride through system consists of a power converter interfacing the

ultracapacitor and the loads. The converter is used to take control of the voltage levels

enabling the ultracapacitor to be charged to a voltage higher than the dc bus voltage or less

than that, in other words to shape the voltage profile of capacitor during charging and dis-

charging. Without the converter, the capacitor will get charged to the dc bus voltage without

any control over the charging current. The ultracapcitor is charged through the converter

at constant current when the main power is available, and during surges/blackouts the ca-

4 Chapter 1. Introduction

Figure 1.3: Need for a Bi-directional Converter

pacitor bank discharges through the boost converter at constant voltage. The specifications

of the ride through system are as given in table 1.1:

1.3 Organisation of the Project Report

The project project has been organised as follows elaborating the various points mentioned

in the outline

1 The characteristics of Ultracapacitors is explained in the second chapter. The con-

structional features of ultracapacitors along with their structural differences from the

electrolytic capacitors is discussed. With the construction, the applications of the ultra-

capacitors as back-up energy storage devices are put forth. Finally for the specifications

mentioned in table 1.1, the ultracapacitor bank design and cell voltage monitoring are

explained.

2 The third chapter discusses the the design of the experimental set up which consists

of:

i The bidirectional buck-boost converter which in turn involves:

- Selection of the Switching devices along with their gate drive circuit.

- Filter inductor and capacitor design.

1.4. Conclusion 5

Table 1.1: System Specifications

Parameter Value

VA Rating 240 VA

Maximum Load 85 W

Back up Capacity 10s

Maximum Voltage of the Ultracapacitor bank 30V

Nominal Voltage of the Ultracapacitor bank 24V

Switching Frequency 100kHz

ii The Digital controller dsPIC30F2023 board for the generation of PWM signals

iii The Feedback signals sensing circuit for sensing the currents and voltages for the

purpose of closed loop control.

iv Power factor correction circuit which becomes the primary source of power for

the loads.

3 The fourth chapter discusses the results. First the ultracapacitor characterisation test

results are presented. Then the closed loop control test results of the converter in buck

and boost mode of operation is discussed along with the bode plots for the design of

controllers . The fifth chapter gives conclusion of the designs done and the experiments

conducted.

1.4 Conclusion

This chapter gave an outline of the project. It discussed the voltage sag effects and reason

behind the project. The next chapter describes in detail the characteristics, construction and

applications of the ultracapacitors along with their sizing for the above mentioned system

specifications.

Chapter 2

Ultracapacitors - Modelling and

Sizing

This chapter describes the constructional features of ultracapacitor, applications of ultraca-

pacitor and sizinf of ultracapacitor. Ultracapacitors store energy in electrostatic form with

the constructional features similar to that of a battery. A simple parallel plate capacitor

stores energy in the electric field between the plates, the capacitance given by

C = Ad

(2.1)

Where A is the area of the parallel plates and d is the distance between the two parallel plates

and e is the permittivity of the medium. The electrolytic capacitor differs in construction

from the ordinary capacitor. An electrolytic capacitor has a metal electrode over which

there is a metal foil contact with the external circuit. There is a thin layer of metal oxide

formed on the electrode surface by the process of electrolysis i.e. passing current through a

electrolyte placed in contact with the electrode and the quantity of material deposited on

the electrode is given by the Faradays law of electrolysis

m = Z I t (2.2)

where m is the mass of material deposited and z is the electrochemical equivalent and I*t is

the charge applied.

The electrolytic capacitor consists of the electrolyte which acts as the cathode and the

electrode acts as anode. There is a separator which is used to prevent contact of the anode

with the wall of the container. The breakdown strength of the oxide layer is of the order of

several MV/cm which is the reason for very high capacitance of electrolytic capacitor. The

6

2.1. Construction of Ultracapacitor 7

typical metals used are valve metals like Aluminium, Titanium, Niobium and Tantalum,

Aluminium being more preferred because of its low cost. The electrolyte will vaporize at

high temperatures and crystallize at low temperatures making the ESR of the capacitor

requiring attention.

2.1 Construction of Ultracapacitor

Ultracapacitors are Electrochemical double layer capacitors (EDLC). A double layer is

formed at the interface between two different materials or phases. One phase is the metal

electrode and the other phase is the electrolyte. The particles at the interface represent a

double layer capacitor. figure 5.11 shows construction of a typical double layer capacitor.

Figure 2.1: construction of double layer capacitor

[13]

An ultracapacitor consists of two carbon electrodes (active excited porous carbon electrodes).

The active excitation increases the diameter of pores on the surface of carbon electrode which

helps in absorbing more ions and hence very high capacitance per unit volume. Electrolyte

acts as the ionic conductor flowing between the plates. The very high capacitance is due

to the small thickness of the layer separating the charges. The electrolyte type and the

number of pores determine the typical voltage withstand ability of a single ultracapacitor

cell. The decomposition voltage of sulphuric acid is about 1.2V. Typical capacitance of an

8 Chapter 2. Ultracapacitors - Modelling and Sizing

ultracapacitor varies from a few to 3000 Farads.

2.2 Comparison of Various Energy Sources

The various energy storage devices like batteries, fuel cells, ultracapacitors are compared

using a log-log plot called Ragones plot. The Ragones plot is a plot of power density versus

energy density. figure 2.2 shows the Ragones plot from which the particular device can

Figure 2.2: Ragones Plot

[17]

be chosen depending on the application. It shows that Ultracapacitors have higher power

density but lower energy density indicating that ultracapacitors are suited for high bursts of

power whereas it cannot be used to supply power continuously i.e. not suitable as primary

source of energy,. Batteries have a very high energy density whereas very low power density

making it not suitable for delivering high bursts of power. Thus ultracapacitors are being

widely used where high power is required to be delivered or absorbed in a very short time as

is the case in regenerative braking of motor where very high energy is available for a short

duration, which can be trapped using ultracapacitor. Table 2.1 gives a comparison between

2.3. Electrical characteristics of Ultracaps 9

Table 2.1: Comparison between batteries and ultracapacitors

Feature Ultracapacitor Batteries

Energy Storage Electrostatic Electrochemical

Charge/discharge cycles greater than 1,00,000 cycles less than 1000cycles

Energy density less than one-tenth of battery high

Charge and discharge rates low high

Shelf life high low

Weight Very heavy for same capacity as batteries heavy

Maintenance Maintenance free Requires maintenance

ultracapacitors and batteries.

2.3 Electrical characteristics of Ultracaps

The Electrical characteristics of Ultracapacitors is similar to that of an electrolytic capacitor.

The Ultracapacitors have been assumed to have a simple first order R-C circuit as shown

in figure 2.3. The ESR represents the series resistance due to the lead resistances, Contact

resistance of the electrodes and the electrolyte resistance. Rleak represents the loss of charge

when the ultracapacitor is left charged to its rated voltage.

2.4 Design of the Ultracapacitor Stack for the Ride

Through System

The total power consumed by the control cards is given in table 2.2. The power requirement

of the Control cards of the inverter has been calculated to be 78W (considering a safety

factor of 2 for the total current drawn by the cards). For the above system specifications,

assuming an efficiency of 70% for the converter, the ultracapacitor is sized for 125W.


Figure 2.3: Electrical characteristics

Table 2.2: Power consumed by the inverter control cards

Inverter Card Currents drawn(A) Power consumed(W)

+15V -15V 5V

Annunciation card 0.052 0.005 1.71

Voltage sensing cards(AC + DC) 0.238 0.033 8.13

Current sensing cards(4) 0.736 11.04

PD card 0.122 0.066 0.504 10.64

DSP card 0.1 0.1 1.5 10

Gate Drive Cards(6) 3 45

Total power 78

2.4. Design of the Ultracapacitor Stack for the Ride Through System 11

2.4.1 Number of Ultracapacitor cells

Befor calculating the number of ultracapacitor cells required, it would be good to know the

discharge characteristics of ultracapacitors under constant power through the use of power

converter. The discharge characteristics of the ultracapacitor is given by figure 2.4. As per

Figure 2.4: Discharge Profile

the system specifications

Vmax = 30V, Vmin = 13.7V, Vnominal = 24V (2.3)

dV = Vnominal Vmin (2.4)

Imax =P

Vmin(2.5)

Imin =P

Vmax(2.6)

The maximum current is decided from the ultracapacitor datasheet, which fixes the minimum

voltage to which the ultracap can drop to.

dV =IavgCt

(t+ ) (2.7)

In equation 2.7, is the timeconstant of the ultracap which is taken normally as 1.1s and t

is the time for the ultracap to provide ride throuugh, in this case being 10s. From equations


2.3, 2.4, 2.5, 2.6 and 2.7 the required capacitance is calculated to be 11F.

The number of cells required is calculated by

Ct = Ccell parallelcellsseriescells

(2.8)

The number of series cells is determined by the voltage rating of the stack.

numberofcellsinseries =VmaxVcell

(2.9)

The number of cells in series and parallel are calculated to be 12 and 1 respectively. Hence

the total number of ultracapacitor cells was chosen to be 12. The ultracapacitors used were

Maxwell BCAP0150 capacitors of capacitance 150F.

2.4.2 Ultracapacitor voltage management

The maximum voltage withstood by each ultracapacitor is about 2.5V or 2.7V, limited by

the electrolyte decomposition voltage. The series connection of the ultracapacitor leads to

unbalance in the voltage across each cell because of the differences in self-discharge rates.

This might lead to excess voltage on one cell and a lesser voltage on some other cell. To

prevent overvoltage across a particular cell, voltage equalization is done.

The equalization method adopted here is connecting a series string of Diodes, LEDs and

Figure 2.5: cell voltage management

a resistor of appropriate value in parallel with each cell. When the voltage across each cell

2.5. Conclusion 13

is more than forward voltage drop of the diodes plus that of the LEDs, the LEDs starts

glowing thereby discharging the cell. A 1.5 resistor was chosen to be used along with one

diode (BA159) and one LED.

The ultracapacitor hardware is shown in the appendix figures A.3 and A.4

2.5 Conclusion

The chapter explained the construction of ultracaps, difference between the various energy

sources, sizing of ultracapacitors for the ride through system and the voltage management

circuit. The tests done to test the capacitance and leakage resistance of the ultracaps along

with the characterisation results is explained in later chapters.

Chapter 3

Hardware Design

In this chapter, the hardware, which includes the power circuit, controller board, current

and voltage sensing card, has been discussed.

3.1 Power Circuit

The power circuit used is a 240VA bi-directional buck-boost converter. It comprises the

choice of switching devices, design of the filter elements, Device gate drive circuit.

3.1.1 Power Devices Selection

The minimum voltage to which the stack of ultracapacitors is discharged is determined by

the maximum current rating of the ultracapacitors. The power devices must carry this

current. The switches selected for this topology were IRF540N, the blocking voltage and

current rating of which are 100V and 33A respectively. External anti-parallel diodes are not

used with the MOSFETs, the body diodes of these devices are made use of.

3.1.2 Gate Drive Circuit

The driver circuit for the MOSFET consists of an optocoupler IC HCPL3101 to isolate the

control circuit from the power circuit and IR2110 which is the driver with separate High

and Low side referenced output channels. The driver circuit is powered by the power circuit

itself with isolation through a flyback converter. The gate drive voltage obtained from the

IR2110 is in the range of 10-20V. The bootstrap capacitor is selected as 10F based on the

formula given in the IR2110 datasheet.

14

3.1. Power Circuit 15

3.1.3 Filter Inductor and Capacitor Selection

The switching frequency is chosen as 100kHz. The inductor is designed for a current ripple

of 0.2A as follows

V = L dIdt

(3.1)

dI is the current ripple of 0.2A. dt is the on/off time of the switch. The inductor details

are as specified in the table 3.1. The value of V is chosen such that the product V*d is

Table 3.1: Inductor DetailsParameter Value

Inductor 300H

Switching Frequency 100kHz

Peak Current 9.1 A

Current ripple (peak-peak) 0.2A

Core Ferrite, E65/32/27

SWG 13

Air gap 1.6 mm

maximum. This occurs at the minimum value of the ultracapacitor voltage. The required

inductance is calculated as 300H. The filter capacitor is chosen depending on the voltage

ripple which is chosen to be less than 0.1 %. The capacitor is calculated to be 5000F. Hence

five nos. of 1000F ,63V Electrolytic capacitors are chosen with a 1F ,63V high frequency

capacitor. The high frequency capacitor is used in parallel with electrolytic ones because at

high frequencies the electrolytic capacitor starts becoming resistive thereby exhibiting higher

voltage ripple at switching frequencies.

3.1.4 Losses in the switches

The various losses in the devices are calculated as follows:

1. Forward conduction loss The forward conduction loss in the active switch(Mosfet) is

calculated to be

I2rms RDSon = 3.2W (3.2)

16 Chapter 3. Hardware Design

2. Reverse conduction loss The reverse conduction loss in the passive switch(Body diode

of complementary Mosfet) is calculated to be

Vav Iav = 3.86W (3.3)

3. Switching loss

Pswon = 0.5 VDS ID trT 0.5W (3.4)

Pswoff = 0.5 VDS ID tfT 0.5W (3.5)

3.1.5 Power Circuit Schematic

The entire power circuit schematic is given in the appendix. The current in the circuit is

sensed through the wire wound current sense resistor of 0.1. LM 2576 is a 12V,3A Buck

converter IC from National Semiconductors. It eliminates the need for external power supply,

hence keeping the area of experimental set up small. It is used to power

1. Gate drive card

2. dsPIC30F2023 Controller board.

It gets power from the upstream mains when it is present and from the ultracapacitor fed

boost converter in case of outage. All the components used in the power circuit along with

their values are listed in appendix. The power circuit with the various constituents are shown

in the appendix figure A.5.

3.2 Theory of operation

The block diagram of the ride through system was shown in figure 1.2. The power circuit

used in the system is shown in figure 3.1. The switches Q1 and Q2 are MOSFETS with anti-

parallel diodes. The circuit represents one leg of an H-bridge circuit. The circuit operates in

buck mode to charge the ultracapacitor. The circuit diagram in buck mode is as shown in

3.2 The switch Q1 is turned on by Gate pulse and the diode D2 freewheels to conduct when

Q2 is turned off. The controllers are designed such that the current through the inductor

is constant and the Ultracapcitors are charged at constant current.When there is an outage

the ultracapacitor discharges through the active switch Q2 and the diode D1. Q2 is gated

3.2. Theory of operation 17

Figure 3.1: Power circuit

Figure 3.2: Buck Mode of Operation


and D1 freewheels. This is the boost mode of operation. The controllers are designed so

that Q2 switches accordingly by varying the duty ratio to the switch.

Figure 3.3: Boost Mode of Operation

3.3 Controller Platform

The controller is implemented digitally through dsPIC30F2023. The existing controller board

[3] was used without modifications. The schematic given in appendix is also referred from

[3]. The Controller board is shown in figure appendix A.7.

3.4 Current and Voltage Sensing Circuit

To perform the controller action, all the voltages and currents should be converted into the

processors voltage range, here this being 5V. For this purpose, a LM-324 quad Op-amp

based cerrent and voltage sensing circuit was designed. The power for the LM-324 is fed

through the LM 2576 power supply IC. Hence the op-amps power supply terminals are

+12V and Ground. Negative potentials cannot be represented. The output of these cards

are fed to the ADC pins of the processor through an anti-aliasing filter circuit with a cut off

frequency less than 1kHz.

3.4. Current and Voltage Sensing Circuit 19

3.4.1 Voltage Sensing

Two voltages DC bus voltage and ultracapacitor voltage are sensed. The voltages are stepped

down by a magnitude of 10 using the non-inverting amplifier circuit. The circuit for voltage

sensing is shown in 3.4

Figure 3.4: Voltage Sensing Circuit

3.4.2 Current Sensing

The current is sensed through the current sense resistor of value 0.1. A capacitor is used

in parallel with the resistor, the resultant time constant being very less than switching time

of the converter. The current sensing part is used to produce a voltage as per the equation

VOadc = 2.5 + (0.167 I) (3.6)

Equation 3.6 is used to produce a output of 5V for a maximum current of 15A and 0V for

a maximum negative current of -15A. The circuit implementation and the hardware of the

sensing circuit are shown in fig. 3.5 and appendix figure A.6. This chapter explained the

design of entire hardware starting with power circuit to controller design and signals sensing

circuit. The next chapter explains the controller design.


Figure 3.5: Current sensing Circuit

Chapter 4

Controller Design

This chapter explains the design of controllers and their transformation to the z-domain and

finally implementation in dsPIC30F2023 through C programming.

4.1 Principle of Closed Loop Control

The closed loop control is done to prevent variations in the output voltage or output current,

due to any disturbances, by introducing a control variable in the form of duty ratio. This

control variable d, is used to turn on the active devices in the switching circuits for a duration

corresponding to its value. Closed loop control varies the duty ratio, in case of any deviation

in output parameter, from its present steady state value to a new steady state value thereby

correcting for the variation in the output.

4.1.1 Control Objective

The control objectives are as follows

1. Steady State Accuracy The steady state accuracy is required to be more than 99%.

The DC gain of the system is chosen to be very high.

2. Settling time in response to disturbances Any disturbance in the form of input variations

detours the output from its set value. The time to respond to this variation is kept as

small as possible. In other words, the bandwidth of the system is made high.

For the above objectives, first controller for the inner current loop is designed with a high

bandwidth and when controller for the outer voltage loop is designed, it is assumed the gain

21

22 Chapter 4. Controller Design

Figure 4.1: Closed Loop Control

Figure 4.2: Closed Control Loop - Outer Voltage Loop

4.1. Principle of Closed Loop Control 23

of inner current loop as unity. Then the controller is designed for the above objectives and

bandwidth of the outer voltage loop is kept atleast one-tenth the bandwidth of inner current

loop. This is because when the outer voltage loop is active, the actual current in the circuit

should have reached the set current reference. The outer voltage loop sets the inner current

reference. The references, output capacitor voltage and inductor current, are the dynamic

variables in the system.


4.2 Buck Converter Control with Resistive Load

The buck converter was first tested with a resistive load, before charging the ultracaps. The

control involved providing an inner current loop and an outer voltage loop. The bandwidth

of the inner current loop is kept as one-tenth the switching frequency. In the following

section, the controller equations, bode plot and their implementation in z-domain is given.

4.2.1 Inner current loop

The figure 4.1 shows the control structure adopted. The bandwidth of the voltage loop is

kept as one-tenth the bandwidth of the current loop. The small signal transfer function of

current to control is given by

g(s) =i(s)

d(s)=VgR

1 + sRC

LCs2 + sLR

+ 1

The bode plot of the transfer function with the experimental values of

Vg-19V,R-10, C=1000F, L=300 is shown in figure 4.3

g(s) =i(s)

d(s)= 1.9

1 + s100

1 + s33333

+ s1825

2 (4.1)

As per the control objective, a PI controller is designed to make

1. Steady State Gain very high

2. Crossover at unity gain

3. Bandwidth high. Here bandwidth of the inner current loop is kept at 10krad/sec.

The transfer function of the PI controller used is

h(s) =d(s)

ei(s)=

1 + s1825s

250

(4.2)

The bode plot of the loop gain GH is given in figure 4.4 The digital implementation of the

above PI controller is done using bilinear transformation. The controller implemented with

a sampling frequency of 20kHz is

kp[n] = (0.144 error[n]) (4.3)

ki[n] = ki[n 1] + (0.00625 error[n]) + (0.00625 error[n 1]) (4.4)The duty ratio is generated by the addition of equations 4.3 and 4.4

4.2. Buck Converter Control with Resistive Load 25

Figure 4.3: Current to Control transfer function i(s)/d(s)

Figure 4.4: inner current loop - Loop gain GH transfer function i(s)/ iref (s)


4.2.2 Outer Voltage Loop

The small signal transfer function of voltage to current is given by

g(s) =v(s)

i(s)=

R

1 + sRC(4.5)

g(s) =v(s)

i(s)=

10

1 + s100

The Voltage loop PI controller is chosen for a bandwidth of 1000 rad/sec. The transfer

function of the PI controller used is

h(s) =iref (s)

ev(s)=

1 + s100

s100

(4.6)

The above controller in digital domain is

kp[n] = (1 error[n]) (4.7)

ki[n] = ki[n 1] + (0.0025 error[n]) + (0.0025 error[n 1]) (4.8)iref [n] = kp[n] + ki[n] (4.9)

The reference current for the inner loop is set by 4.9. The bode plots for the outer voltage

loop are shown in figures 4.5 and ??

4.2. Buck Converter Control with Resistive Load 27

-20

-10

0

10

20

30

40

50

Magni

tude (d

B)

100 101 102 103 104-90

-45

0

Phase

(deg)

Frequency (rad/sec)

Figure 4.5: Voltage to Current transfer function V(s)/i(s)

Figure 4.6: Outer Voltage Loop - Loop gain GH transfer function V(s)/Vref (s)


4.3 Buck Converter Controller for Charging Ultraca-

pacitors

The control structure adopted for charging ultracapacitors is the same as shown in figure

4.1. The controller gains are different.

4.3.1 Inner Current Loop

g(s) =i(s)

d(s)=

VgCs

LCs2 + sLR

+ 1

Here the charging was done at

Vg=10V,C=12.5F,L=300H, R-0.2. R is the total ESR of the Ultracapacitor stack and

was measured per cell using Network analyser.

g(s) =i(s)

d(s)=

125s

1 + s667

+ s16

2 (4.10)

The PI controller used to achieve the control objective is

h(s) =d(s)

ei(s)=

1 + s1000s

253

(4.11)

and in digital domain, the PI controller is

kp[n] = (0.253 error[n]) (4.12)

ki[n] = ki[n 1] + (0.006325 error[n]) + (0.006325 error[n 1]) (4.13)d[n] = kp[n] + ki[n] (4.14)

The duty ratio is given by equation 4.14. During charging of the ultracapacitors, the duty

ratio varies from zero to one, with the ultracapacitor stack charging slightly over the set

reference and finally the duty ratio settles at zero at this overcharged value. The bode plots

of the current to control transfer function and current loop gain are shown in figures 4.7 and

4.8

4.3. Buck Converter Controller for Charging Ultracapacitors 29

Figure 4.7: Current to Control transfer function - ultracap charging i(s)/d(s)

Figure 4.8: Current Loop gain GH - ultracap charging i(s)/iref


4.3.2 Outer Voltage Loop

In this case the outer voltage loop controller can be just a proportional controller. The small

signal transfer function of voltage to current is given by

g(s) =v(s)

i(s)=

1

Cs(4.15)

g(s) =v(s)

i(s)=

1

12.5s

The Voltage loop PI controller is chosen for a bandwidth of 1000 rad/sec. The transfer

function of the PI controller used is

h(s) =iref (s)

ev(s)= 12500 (4.16)

The above controller in digital domain is

kp[n] = (12500 error[n]) (4.17)

iref [n] = kp[n] (4.18)

The reference current for the inner loop is set by 4.18.

4.4. Boost Converter Controller 31

4.4 Boost Converter Controller

The boost converter control structure implemented is a voltage loop control. Here, in ad-

dition to a PI controller, a lead compensator is used to improve phase margin at the gain

cross over frequency. The voltage to control transfer function is given by

g(s) =v(s)

d(s)=

VgR(1 D)2

1 s LR((1D)2

1 + s LR((1D)2 +

LC(1D)2 s

2

In the experiment done, the values used were C=1000F, Vg=10V, L=300H, R=20,

D=0.375 The transfer function turns out to be

g(s) =v(s)

d(s)= 38.4

1 s26042

1 + s26042

+ s510

2 (4.19)

h(s) =d(s)

ev(s)=

1 + s510

s208

1 + s510

1 + s10000

(4.20)

error(s) in equation 4.20 is the voltage error which is given by the difference in the voltage

between the set reference value through the processor and the sensed value from the system.

Bode plots of the voltage to control transfer function and the loop gain transfer function are

shown in figures 4.9 and 4.10 respectively.

-100

-80

-60

-40

-20

0

20

40

60

80

100

Magni

tude (d

B)

102 103 104 105 106-270

-180

-90

0

Phase

(deg)

Frequency (rad/sec)

Figure 4.9: Voltage to Control transfer function - ultracap discharging v(s)/d(s)


-60

-40

-20

0

20

40

60

80

Magni

tude (d

B)

101 102 103 104 105 106-270

-225

-180

-135

-90

-45

0

Phase

(deg)

Frequency (rad/sec)

Figure 4.10: Voltage Loop gain GH - ultracap Discharging V(s)/Vref (s)

Chapter 5

Experimental Results

In this Chapter, the results obtained as a result of experiments conducted are presented.

5.1 Ultracapacitor Characterisation

The first in the series of experiments was to test the Ultracapcitors individually for their

Capacitance and leakage resistance. For this, the ultracaps were charged at constant current

to a voltage slightly below the rated voltage using a 30V,2A power supply and discharged

through a 5 load. The data was logged using YOKOGAWA MX100 Datalogger. To

monitor the leakage resistance, the cells were charged to 2.5V and disconnected from external

circuits. The voltages across the cells were monitored over a weeks period at 24 hour interval.

Table 5.1 gives the values of the 12 ultracapacitors being used. The values logged were used

in excel and line of best fit was constructed and the values found. The values for the above

12 capacitors were plotted using excel and the plot is shown in figure 5.1

5.2 Ultracapacitor Voltage Monitoring Circuit

The voltage monitoring circuit was tested with a comination of resistor, LEDs and diodes

of different ratings. The results which were consequential in choosing the components used

in the ultracapacitor board are summarised in figure 5.2 The characteristic required should

be such that at over 2.5V, the ultracap should exhibit a significant leakage current to bring

the voltage down to 2.1 or 2V, at which the leakage current should be almost zero. As per

the figure 5.2, the characteristic is satisfied by a series combination of 1 LED, 1.5 resistor

and 1 diode, which is hence chosen.

33

34 Chapter 5. Experimental Results

Table 5.1: Ultracapacitors Parameters

S.No Capacitance(F) Leakage resistance(k)

1 123.43 9.528

2 123.77 10.26

3 131.33 9.838

4 146.11 9.094

5 148.14 9.646

6 150.69 10.24

7 152.72 10.63

8 153.37 11.42

9 158.05 10.11

10 158.78 11.4

11 159.62 12.6

12 160.43 16.97

Figure 5.1: Ultracapacitors Charging Plots

5.3. Closed Loop Control Results 35

Figure 5.2: Ultracapacitors Voltage Management Circuit

5.3 Closed Loop Control Results

5.3.1 Buck Converter - Ultracapacitor Charging Test Results

The following describe the waveforms captured on scope

1. Blue waveform - DC bus Voltage

2. Brown waveform - Ultracapacitor current (Sensing circuit output)

3. Red waveform - Ultracapacitor Voltage

5.3.1.1 Charging at 1A

The input voltage was kept fixed at 24V. The charging was done with a charging current

reference of 1A. The scope waveform shows the capacitor getting charged from 7V to 23V in

almost 175secs. This is validated below. For an ultracapacitor current of 1A, output from

sensing circuit should be 2.67V which can be seen on scope. Figure 5.3 shows the charging

at 1A.

I = Cdv

dt(5.1)

Iadc = 2.5 + (0.167 1) = 2.67V (5.2)t = dv C

I= (23 7) 12.5

1= 200s (5.3)


Figure 5.3: Charging of Ultracaps - 1A

5.3.1.2 Charging at 2A

The input voltage was kept fixed at 24V. The charging was done with a charging current

reference of 2A. The scope waveform shows the capacitor getting charged from 4V to 23V

in almost 90secs. This is validated below. For an ultracapacitor current of 2A, output from

sensing circuit should be 2.83V which can be seen on scope. Figure 5.4 shows the charging

at 2A.

Figure 5.4: Charging of Ultracaps - 2A

I = 2A (5.4)

Iadc = 2.5 + (0.167 2) = 2.83V (5.5)


t = dv CI

= (23 4) 12.52

= 125s (5.6)


5.3.2 Boost Converter Test Results

First the boost converter was tested with the controller programmed to take a 4V input and

generate a 6V output with the latter as the reference voltage. The following describe the

waveforms captured on scope

1. Green Waveform : Input Voltage,

2. Blue waveform : Output Voltage.

The above waveforms are each 2V/div for figures 5.5,5.6,5.7 and at 10V/div for 5.8,5.9 and

5.10.

1. Figure 5.5 shows output voltage of the boost converter whiich was at slightly less than

4V reaching 6V in nearly 1sec. The voltage spikes on the waveforms are switching

spikes and power supply noise.

Figure 5.5: Boost Converter Constant Voltage Control - 4.7V input, 6V output, 20ohm load

2. The figure 5.6 shows a step change in input voltage from 4V to 5.7V. The output

voltage increases momentarily and settles down at 6V in about 1sec.

3. Figure 5.7 shows a step change in voltage from 5.6V to 4.5V, the output decreases

alongwith the input and finally settles down at the set reference of 6V.


Figure 5.6: Boost Converter Constant Voltage Control - Step Increase in Input

Figure 5.7: Boost Converter Constant Voltage Control - Step Decrease in Input


The boost converter was then tested to simulate the voltage decrement across the ultraca-

pacitor from 24V to 14V. This was done by varying a 60V,5A power supply from 24 to 14V,

although the load in the intended application is 6. Here the test was done with a 20,50W

resistor. The various test results are as follows

All the tests were done with a reference voltage of 25V.

1. Figure 5.8 shows the converter reaching steady state voltage of 25V from initial voltage

of slightly over 18V when the controller is turned on. The steady state voltage is

reached in almost 1sec.

Figure 5.8: Boost Converter Constant Voltage Control - 15V input, 25V output, 20ohm load

2. Figure 5.9 shows a step increase in the input from 15V to 22V. The output voltage

increases and finally settles down to 25V in about a sec.

Figure 5.9: Boost Converter Constant Voltage Control - Step Increase in Voltage


3. Figure 5.10 shows a step decrease in the input from 22V to 15V. The output voltage

decreases and finally settles down to 25V in about a sec. This is the condition that

happens when the ultracap is conducting, where the voltage decreases from 24 to 14V.

Figure 5.10: Boost Converter Constant Voltage Control - Step Decrease in Voltage


5.3.3 Boost Converter test results with ultracapacitor input

The following are the waveforms captured on scope:

1. Blue waveform - DC bus Voltage

2. Brown waveform - Ultracapacitor current (Sensing circuit output)

3. Red waveform - Ultracapacitor Voltage

5.3.3.1 Boost Converter Operation with 25W(23V, 20) load

Figure 5.11: Boost Converter Operation with 25W load

The figure 5.11 shows the discharge profile of ultracapacitors under a 25W (23V, 20)

load. It can be seen that the discharge takes place for almost 50secs, after which the PWM

is turned off because the ultracapacitor has dischared to its minimum voltage level which

has been set as 15V.

5.3.3.2 Boost Converter Operation with 50W(23V, 10) load

The figure 5.12 shows the discharge profile of ultracapacitors under a 50W (23V, 10) load.

It can be seen that the discharge takes place for almost 25secs, after which the PWM is

turned off because the ultracapacitor has dischared to its minimum voltage level which has

been set as 15V.


Figure 5.12: Boost Converter Operation with 50W load

5.3.4 Integrated Operation of Buck and Boost converter

The integrated opeartion is about creating the effect of voltage sag at the terminals of the

ultracapacitor and the converter and letting the ultracapacitor discharge with a 75W load

and the power supply would be turned on to charge the capacitors once again. Figures 5.13

Figure 5.13: Integrated operation - 75W(23V,7)

and 5.14 show the integrated operation of the Ride through system with a 75W load. It can

be seen that the voltage outage is detected in less than a second and the boost converter

operates for almost 9secs. Few more results were taken similar to figures 5.13 and 5.14

which are given below explaining the response to voltage outages. These figures show that

blackouts are detected in less than a second.


Figure 5.14: Integrated operation - 75W(zoomed view)

Figure 5.15: Integrated operation 1

5.4. Efficiency of the Power Circuit 45

Figure 5.16: Integrated operation 2

5.4 Efficiency of the Power Circuit

The Power circuit was tested in the buck mode with a 10 load for efficiency and with duty

ratios varying from 0.5 to 1. The results are tabulated as in table 5.2

Table 5.2: Efficiency of Power Circuit in Buck mode

Vin Iin Vo Io Dutycycle Efficiency

19 0.4 7.75 0.78 0.4 79

19 0.266 6.23 0.63 0.33 76.8

19 0.536 9.26 0.93 0.487 84.2

19.1 0.673 10.73 1.07 0.57 90

19.1 0.85 12.25 1.22 0.64 92.2

19.1 1.055 13.76 1.38 0.72 94

19.1 1.288 15.21 1.52 0.79 94

19.1 1.548 16.7 1.67 0.87 94.3


5.5 Thermal Test

A temperature rise test was done on the entire system by running the converter in buck

mode. The results were monitored using FLUKE TI20 THERMAL IMAGER. The

pictures taken using the thermal imager are shown below. It can be seen that the processor

dsPIC30F2023 is the hottest in the entire system. The current levels in the power circuit

were not too high to cause any significant copper loss in the inductor or conduction loss in

the conducting devices. The figure 5.17 shows the temperature rise of the half bridge and

Figure 5.17: Thermal Test - Switches

LM2576 power supply IC. A black tape was put on the heat sink of the individual devices

to account for the emissivity of surfaces. The heat sink surface of the power supply IC was

black, not necessitating the use of tape. The figure 5.18 shows the temperature rise of the

processor board. The processors temperature rise is justified, as when program is flashed

into the processor, it draws significant power. The figure 5.19 shows the temperature rise of

the gate drive circuit, the black surfaces emissivity has been recorded in spite of not drawing

power.

5.5. Thermal Test 47

Figure 5.18: Thermal Test - Processor

Figure 5.19: Thermal Test - IR2110 and HCPL 3101

Chapter 6

Conclusions

The project on providing ride through for control power supply could be completed with the

following points noted during course of the project

1. The boost converter control was succcessfully tested and the results captured with

both power supply and the ultracapacitor input.

2. The charging of ultracapcitors in buck mode was tested at two current levels - 1A and

2A and the charging profile was found to be satisfactory. Voltage spikes of very high

magnitudes were experienced both at lower and higher voltage levels, which might have

a detrimental effect on the life of ultracapacitors.

3. The integration of buck and boost converter operations and hence testing of the whole

system could be completed successfully with the tests carried out by turning off the

power supply and allowing the ultracapacitor to feed the load and turning on the power

supply, whence it started charging back to the set reference.

The back up offered for various loads is shown in table 6.1.

The following issues might also be interesting.

1. Gate Drive Card : When charging was tried, with the capacitor already charged to

slightly more than half the reference voltage, the controller immediately saturated, but

the high side switch of the half bridge remained switched off. This finally turned out

to be an issue with the bootstrap capacitor on the Gate driver IR2110. If the low side

48

49

Table 6.1: Ride Through System Results

Load Backup Period

25W 50s

50W 25s

75W 9s

switch never turns on, there is no other way to charge the bootstrap capacitor. So an

isolated 12V power supply was used for charging this bootstrap capacitor.

2. Issues with dsPIC30F2023 target board : There was a plenitude of processor failures.

The reason is attributed to momentary overvoltages above its rated voltage of 5V

through the Analog input channels.So a 4.7V zener was used to avoid the processor

failures. The processor failures can also be reasoned due to the temperature rise. So

forced cooling (using fans) has to be adopted to avoid the processor failures. It might

also be because the target processor board layout is not satisfactory on the thermal side.

Sufficient space has to provided around the dsPIC30F2023 processor before connecting

it to the other components.

3. Another issue with the converter was that, the controller programs wouldnt run in

debug mode with the boost converter mode of operation and hence had to done in pro-

grammer mode. The reason is, in buck mode of operation, the main 12V power supply

powered both the converter and the processor with the MPLAB ICD2 experiencing

no undervoltages due to the switching. But in boost mode of operation, main power

supply is absent and the switching spikes got propogated through the current sense

resistor, which is connected to ground, whereby the debugger experience undervoltages

and shutdown in the middle of execution/running the target. This was solved by exe-

cuting the processor in a stand alone mode without the debugger and connecting the

debugger only when it was necessary to program.

4. The final issue being the switching spikes which need to be eliminated. This is because

of stray inductances due to the usage of long wires in various paths.

Appendix A

Schematics and Experimental Setup

A.1 Power Circuit Schematics

5

5

4

4

3

3

2

2

1

1

D D

C C

B B

A A

S

D

S

D

POWER CIRCUIT

GATE DRIVE CIRCUIT

NCC11

1u,63V

R9

10k21

TP8

1

U2

HPCL3101

1234 5

678

F1

FUSE

TP1

1

JP5

12

C20.1u

1

2

JP2

12

R11

0.1 ohms, 10W21

JP6

12

C310u

1

2

C9

1000u,63V

R64.7k

2

1

C12100u

JP3

12

R1

1.2k

21

M2IRF540N

TP6

1

JP1

12

M2

IRF540N

IR2110

U3

89

1011121314

7654321

NCVDDHINSDLINVssNC

HOVBVSNC

VCCCOM

LO

D3

15 V

12

U4

LM2576123456

123456

TP7

1

TP4

1

D1BYW29-150

31

TP2

1

JP3

12

Q22N2222

3

2

1

D2 15 V1 2

R2

4.7k

21

R34.7k

2

1

TP10

1

L2

680uH1 2

JP4

123

C40.1u

1

2

TP3

1

C510u

1

2

R7 1221

L1

300uH1 2

C60.1u

1

2

R8

1221

C131000u,35V

C710u

1

2

C101000u,63V

C10.1u

1

2

TP9

1

C80.1u

1

2

R4

1.2k

21

R1010k

2

1

JP7

12

U1

HPCL3101

1234 5

678

TP5

1

D11 1N5814

12

Q12N2222

3

2

1

R5

4.7k

21

H0M

GND

Vcap

N

Vbus

Isense

L0

GND3

VcapGND

GND3Vbus

GND2Vcc1

PWM1PWM2GND1

VpGND1

Vcc2GND3

VprocGND3

GND3Isense

L0

GND2

PWM2

Vcc

H0

GND1

Vcc1

PWM1

NGND2

GND2

Vp

M

Vp

GND1

GND3

GND3

Vcc2

Vproc

Vcc

Figure A.1: Power Circuit Schematic

50

A.2. Controller Schematics 51

A.2 Controller Schematics

Figure A.2: dsPIC Controller Board Schematic

[3]

52 Appendix A. Schematics and Experimental Setup

A.3 Ultracapacitor board - Top View

Figure A.3: Ultracapacitor Hardware - Top View

A.4. Ultracapacitor board - Side View 53

A.4 Ultracapacitor board - Side View

Figure A.4: Ultracapacitor Hardware - Side View


A.5 Power Circuit Hardware

Figure A.5: Power Circuit Hardware

A.6. Voltage and Current Sensing Circuit Hardware 55

A.6 Voltage and Current Sensing Circuit Hardware

Figure A.6: Sensing Circuit Hardware


A.7 Controller Circuit Hardware

Figure A.7: Controller Circuit Hardware

References

[1] R. Srinath , Digital control of solar photovoltaic converters, M.Sc. thesis, EE Dept,

IISc., Bangalore, December 2009.

[2] Kelly et al, Voltage regulator for contactor ride through ,IEEE transactions on Industry

Applications, Volume 36, NO.2, March/April 2000, Pages 697-704.

[3] C. Karrupuswamy , Bi-directional inverter with high frequency ac-link,, M.Sc. thesis,

EE Dept, IISc., Bangalore, March 2009.

[4] S. Giridharan, A novel transformer-less uninterruptible power supply, M.Sc. thesis, EE

Dept, IISc., Bangalore, September 1996.

[5] V. Ramanarayanan ,Course material on switched mode power conversion, IISc., Ban-

galore.

[6] G.S. Ramana Murthy, Design of inductors and transformers at power frequecies - A

modified Area-Product approach , M.Sc. thesis, EE Dept, IISc., Bangalore, March 1999.

[7] Robert W. Erickson and Dragan Maksimovic, Fundamentals of Power Electronics , II

edition, Springer India Academic Publishers, 2001.

[8] IRF540N datasheet,International Rectifier,http://www.irf.com.

[9] HCPL-3101 datasheet, Avago Technologies,http://www.avagotech.com.

[10] dsPIC30F Family Reference manual,http://www.microchip.com.

[11] IR2110 datasheet,International Rectifierhttp://www.irf.com.

57

58 References

[12] Ultracapacitor Application notes, datasheets and case studies, Maxwell Ultracapacitors,

http://www.maxwell.com.

[13] Ultracapacitor brochure, LS ultracapacitor.

[14] Camara et al, Design and New Control of DC/DC converters to Share Energy Between

Supercapacitors and Batteries in Hybrid Vehicles, IEEE transactions in Vehicular Tech-

nology, Vol.57, no. 5, September 2008, Pages 2721-2735.

[15] Jasvinder Singh Khoral,Power Factor Correction of Switching Power Supplies using

UC3845, M.E. project report, EE dept. IISc., Bangalore, June 1994.

[16] Sarmiento and Estrada,A Voltage Sag study in an Industry with Adjustable Speed

Drives, IEEE industry applications magazine, January/February 1996.

[17] Ragones plot,http://www.wikipedia.org.

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