2015 International Future Energy Challenge Proposal

Topic B: Battery Energy Storage with an Inverter

That Mimics Synchronous Generators

Team members: Sabahudin Lalic, David Hooper, Nerian Kulla,Kevin Bisson, and Shawn Maxwell

University of Connecticut

Department of Electrical and Computer Engineering

September 30, 2014

I. Objective

IFEC 2015 will provide members of the team with invaluable experience in the design,

testing, and critical thinking necessary to meet and exceed the goals set by the challenge. These

skills are crucial to the development of professional engineers and will serve to help each

member grow. In addition, team skills that will be acquired through the project are a necessary

part of engineering and most effectively gained through practical real world experience.

II. Team Background

The team will consist of four undergraduate students, one graduate mentor with advising Dr.

Sung-Yeul Park. The competition will be considered a senior design project for three of the

undergraduate students.

Dr. Sung-Yeul Park received the M. S and Ph.D. degrees in electrical and computer

engineering from Virginia Tech, Blacksburg, in 2004 and 2009, respectively. From 2004 to 2009,

he was a graduate research assistant at the future energy electronics center (FEEC), Virginia

Tech. Dr. Park received several international paper awards including a third paper award in IAS

2004, a best paper award in IPCC 2007, an outstanding writing award in the International Future

Energy Challenge (IFEC) in 2007 and a Torgersen Research Excellence Award at College of

School in Virginia Tech in 2009. Dr. Park joined as an assistant professor in electrical and

computer engineering department and as an associate member of Center for Clean Energy

Engineering, University of Connecticut in 2009. His research interests include digital power

conversion, energy storage, distributed generation integration, and microgrid applications.

Shawn Maxwell received the B.S degree in electrical engineering from the University of

Hartford in the spring of 2014. His skills include programming, soldering, PCB design, and

reverse engineering. He was published and presented at the ASEE conference at the University

of Bridgeport in 2014. In the fall he started graduate education at the University of Connecticut

in addition to a graduate assistantship at the Center for Clean Energy Engineering, University of

Connecticut. His research interests include embedded microcontroller systems, real time

operating systems, and digital communications.

Kevin Bisson graduated from H.C. Wilcox Technical High School from the Electronics

Technology trade in 2011. During his time there he worked with semiconductors, integrated

circuits, operational amplifiers, power supplies, combinational and sequential logic circuits, and

basic computer theory. They also built simple PCBs and frequently used DMMs, function

generators, oscilloscopes, and soldering irons. He is currently a junior in electrical engineering at

the University of Connecticut.

Nerian Kulla received the Associate Degree as Automotive Technician form Naugatuck

Valley Community College in spring 2008. He begun working as an Automotive Technician

where he used technical skills to diagnose and repair electrical and mechanical automotive

systems. In spring 2010 he started the undergrad education at the University of Connecticut

where he is currently a senior pursuing a B.S degree in Electrical Engineering. He is interested in

Power Electronics and Nanotechnology.

David Hooper is a senior at the University of Connecticut seeking a double major in

electrical and computer engineering. After graduating, he hopes to continue his education by

attending graduate school. His top academic interests include power electronics systems and

digital systems design. In the past, David has worked at two separate government-contracting

jobs, and he plans to pursue a career in government-based electrical engineering.

Sabahudin Lalic graduated from Bullard-Havens Technical High School with a Diploma in

the Electrical Trade. During his four years in High-School, he did Residential and Commercial

wiring for several Habitat for Humanity projects. In addition, experiments were done with

Transformers and 3-phase Relays. After graduating from High School, he continued his

Electrical Trade Apprenticeship by joining the Local 488 Electrical Union Apprenticeship

Program, while at the same time beginning his degree in Electrical Engineering at Husatonic

Community College. He is currently a junior in Electrical Engineering at the University of

Connecticut.

Past IFEC Participation and Experience of University of Connecticut

As a student at Virginia Tech, Dr. Sung-Yeul Park took part in the 2007 IFEC competition. In

2011, the UConn team participated and made finalist with their design for a 5kW EV charger. In

2013, UConn participated once again with their microinverter design but wasn’t invited to the

final competition.

III. Project Introduction

The development and advancement of alternative energy sources, which are novel but can

exhibit intermittency not tolerable in conventional electricity grids, is of great interest in order to

solve the looming energy supply problem. In order to deal with these issues and seamlessly

integrate these sources into the grid, it is necessary to design a special power inverter and battery

energy storage solution with flexibility, reliability, efficiency, and cost in mind.

The goal for the International Future Energy Challenge is thus to design a storage and

inverter solution to bridge 4 lead acid batteries in series with the grid. The system needs to be bi-

directional to either charge the battery off the grid, or supply energy from the battery to the grid.

It must have protection for the battery in case of fault conditions. Increasing efficiency, power

density, and reducing manufacturing cost are key goals. The output will mimic synchronous

generators to facilitate connection to the grid by means of frequency, voltage, active power and

reactive power control and offer seamless transition from grid to stand-alone mode. The required

specifications of the system as found on the IFEC request for proposals can be found in Table 1.

Table 1: Specifications

Dimensions <1 LiterWeight <1 kgManufacturing Cost <US$0.5/WOverall Energy Efficiency >95%Battery Input Voltage 48V DCOutput Power Rating 500 VA continuousOutput Voltage 230V AC (rms, single phase)Output Frequency 50 Hz

IV. Design

A. Hardware

The design of the hardware will consist of two main components: topology and devices.

A generalized block diagram of the hardware system can be seen in Fig. 1.

Fig. 1 Overall power stage and control block diagram

1. Topology

A two-stage bidirectional topology was chosen to simplify testing as well as the modularity

of the design. While a single stage would offer better efficiency due to minimized switching

devices and thus conduction losses, ultimately the increased complexity of the design would

offset those advantages. The first stage or front-end converter consists of a DC-DC boost

converter to generate the higher voltage necessary from the 48V battery input in grid connected

and standalone modes. This stage will generate over 350V ~ 400V DC or so in order to account

for losses in the following stage. In battery charging mode, the DC-DC converter will be

configured as a buck converter in order to reduce the voltage to levels necessary to charge the

battery. The next part or back-end inverter is a DC-AC inverter composed of a full-bridge to

generate the required synchronous 230V AC rms in grid connected and standalone modes. In

battery charging mode, this stage will rectify the AC grid voltage into DC to be fed into the

previous DC-DC converter. LC filtering will follow to meet output requirements, EMI

compliance, and remove higher order harmonics that are byproducts of the digital switching

modulation. Since isolation would require the use of a transformer that would lower overall

system efficiency, a non-isolated design was favored. Table 2 shows the summary of topology

options.

Table 2: Topology Options

Topology Option Pros ConsSingle Stage Higher Efficiency, Less

ComponentsControl complexity is high

Two Stage Control complexity is low Lower Efficiency,More Components

Isolated Protection Lower EfficiencyNon-Isolated Higher Efficiency No ProtectionSoft Switching Higher Efficiency More Components,

More Complicated DesignHard Switching Less Components, Simpler Design Lower Efficiency

2. Devices

When taking in the consideration the key specifications for our system we need to think of

some of the available devices that will meet our requirements such as: Operation voltage 36V to

60V, efficiency > 95%, output power up to 500W. One of the elements that will have an impact

in our design is the switching device. Therefore we have to take in consideration some of the

switching devices available to meet our specifications. Two of the switching devices that could

benefit our design requirement are: IGBTs, MOSFETs. When dealing with high voltage-high

current, IGBT type semiconductors are a good alternative. They can handle high voltage up to

1500V and it can also handle high current. They are small in size and inexpensive. However,

switching speed is not as fast as that of a power MOSFET but it is faster than a BJT. In particular

the switching off speed is what slows them slower. The next switching device to consider is the

power MOSFET. The power MOSFET shares most of the features of the IGBT but it has a

higher switching speed. It is easy to control in low voltage applications. If a dual active bridge is

to be used in our design, the MOSFET is known to achieve ZVS only if turning on current is

negative. Turn on current of all the MOSFETS will always be negative with proper dead time

and magnetizing inductance, and ZVS will always be achieved. MOSFETs experience some

power in conduction but in most cases it could be negligible and there won’t be a huge dent in

efficiency. However, there is an advantage according to the MOSFET structure, it has an internal

anti-paralleled diode and there is no need for an external diode. The important point is that if the

gate voltage of MOSFET (switching state) becomes high, MOSFET will conduct the current

regardless of the current direction. Another difference in MOSFET is that it does not have the

constant term of voltage drop in its conduction period.

Table. 3 shows the summary of three devices. It appears that the best choice for the switching

device would be GaN (Gallium Nitrate). However, due to the lack of availability we would have

to look at the other two choices. The other two switching devices are the IGBT and the power

MOSFET. There are different reasons why main these devices are suitable for our application.

First, they are easily attainable, they are cost effective and on top of all they meet our

specifications. They both can work at a peek of 500W output power and they can both handle

12A and 230V. Second, they are both controllable via PWM. So to find out what better works for

us we have to look at what sets them apart. Unlike the MOSFET the IGBT is a switching device

that can handle high voltage and high current but it lacks the ability to switch at a high frequency

comparable to the MOSFET. This can result in larger inductor and capacitor to compensate for

the off time of the switch. In addition, the cost of the components will increase and weight of our

design will increase as well while we are limited to 1kg. So to address this we can use the power

MOSFET on the DC-DC conversion since and benefit from the high frequency switching cycles.

On the other hand we can employ the IGBT on the inverter side since the grid current alternates

at a low frequency of 50Hz ~ 60 Hz depending. Finally, both switches are capable to perform at a

high efficiency of around 95% or higher when ZVS and ZCS controls are used.

Table 3: Device Options

IGBT MOSFET GaN

DC-DC Conversion Good Better Preferred

DC-AC Inversion Better Good Preferred

High operating Temp. Good Ok 20º-200ºC

Switching Power loss Ok Good Better

Switching Freq. <20kHz >200kHz >1GHz

Voltage Rating >1000V <250V <1000V

Current Rating >500A <200A >200A

Operating Power >>500W ≤500W 2.5kW

Output Impedance Low Medium Low

Gate Drive Input Voltage 3-10V 4-8V ~5V

B. Software

1. Control

The software consists of the control algorithm that manages each of the three modes of

operation. Closed loop feedback in the form of a digital signal processor (DSP) that tracks

voltage, current, phase, and power makes up the software aspect of the design. The DSP that was

chosen is the TI TMS320F28335. This specific processor was chosen for its high performance

(up to 150 MHz, 32 bit architecture), flexibility (hardware floating-point unit, 16x16 multiply),

and I/O (88 I/O pins, 18 PWM channels, serial peripherals, 12 bit 16 channel ADC). Grid mode

is accomplished by synchronization via reading voltage, current and phase at the grid side in

order to match output voltage amplitude, current, and phase of the inverter before connection to

the grid. A phase locked loop (PLL) provides a convenient method to obtain the phase of the

grid. Control of the DC-DC converter and DC-AC inverter is accomplished by pulse width

modulating (PWM) the switching devices to generate the correct output based on feedback. In

addition, the configuration of the switching devices allows processor controlled grid to battery

charging. This ensures seamless bi-directionality and mode transitions. Battery charging and

protection is accomplished by monitoring the current and voltage at the battery side and utilizing

constant current (CC) and constant voltage (CV) to ensure safe charging of the lead acid

batteries. While more complicated charging schemes are available, CC and CV are sufficient and

simple to implement, thus reducing cost. A ½C charging rate is desirable to limit charging time to

two hours and thus would require our hardware to sustain 1kW of power. Stand alone mode is

accomplished by current, voltage and phase droop control in order to control real and reactive

power delivered to the load. Fig. 2 shows the overall control block diagram in possible

operational modes: charge mode, discharge mode, grid-connected mode, and standalone mode.

Gc(s) is the compensator transfer function, Gp(s) is the plant transfer function, H is the

sensor gain, and all * is to indicate the reference value or target value in Fig. 2.

PowerFlow

DirectionStage Mode Control Block Diagram

Discharge(Battery to

Grid)

Front-end

Converter

Boost

Back-end

Inverter

Stand-

alone

Grid Tie

Charge(Grid toBattery)

Front-end

Converter

C.C.

C.V.

Back-end

Inverter

Rectifier

Fig. 2 Control Block Diagram for each operational mode

2. User Interface

The user interface will consist of input in the form of a set of buttons to allow mode

switching between stand alone, grid connected, and battery charging modes. LEDs will notify the

user of which state the inverter is in, in addition to specifying normal operation or any fault

conditions, in addition to charge state. Digital I/O will also allow the system to be connected to

an external computer to monitor operation. Real time system metrics will also be displayed via

an onboard LCD screen.

V. Testing

A. Hardware- DC-DC, DC-AC: confirm operation of battery to grid (discharge) boost mode, and grid to

battery (charging) buck mode. Preliminary open loop testing to confirm that hardware is

properly configured.

B. Software (Control Algorithm)

- TI DSP evaluation board peripheral test firmware: confirm proper operation of ADC, GPIO,

PWM, Timer

- DC-DC converter open loop testing

- Charging operation (constant current, voltage, and power)

- DC-AC (Grid inverter operation) converter open loop and closed loop testing

VI. Schedule

The development schedule is as follows:

- September: Design all values of devices, including schematic, and parts list (1 week)

- October: Build footprint of parts (2 weeks), PCB layout (control board, and power stage

front and back end, 2 weeks)

- November: Board assembly (2 weeks), Initial open loop tests (implement software and

verify processor operation, 2 weeks)

- December: Closed loop tests

Table 4 describes the milestones of IFEC 2015 project.

Table 4. Project milestone

Research items2014 2015

8 9 10 11 12 1 2 3 4 5 6Proposal Preparation

Simulation of the proposed system

Design Prototype

Prototype performance test

Qualification reportOptimize Program

Revise and Optimize Hardware

Comprehensive Test and Packaging

VII. Power Electronics Lab Equipment and Facilities

The team has access to the high quality facilities shown in Fig. 3 that will allow the

design to be fully implemented and tested.

Fig. 3 Picture of energy storage test bed in the University of Connecticut

The resources of the lab make it ideal for this project. Some of its current assets including

software tools are listed:

Available equipment and facilities: ABC-150 450V/250A 150kVA bidirectional DC

power supply, AMX3120 12kW programmable AC power source, AVTRON 100kW resistive AC

load bank, 63802 Chroma 3 Phase 10kW AC/DC programmable load, Valence 12kWh Li-Ion

Battery, SAFT 12kWh Water cooled Ni-CD Battery, 208V and 480V 3 phase two power lines in

the laboratory, WT1800 Yokogawa power analyzer, Digital 4 channel high speed oscilloscope, 6

High voltage/high current oscilloscope probes

Simulation and Design tools: Altium PCB Design Tool, Psim simulation Tool, Matlab /

Simulink, COMSOL, Pspice, and ANSYS.

The programmable AC and DC power supplies in particular will be useful for acting as the

grid and battery during testing. In addition the AC/DC programmable load will allow us to test

the prototype under varying load conditions and characterize its output when used in tandem

with the power analyzer and oscilloscope. Our software suite will allow us to simulate and

design both the software and the hardware aspects.

VIII. Conclusion

The University of Connecticut would like to be a part of the 2015 International Future

Energy Challenge. UConn’s experiences with IFEC in 2011 and 2013, in addition to Dr. Sung-

Yeul Park’s participation in 2007 will give our team a distinct advantage. Our proposed schedule

and testing plan will allow us to complete the design within a timely manner, with focused

progress targets at each stage. The power electronics lab facility provided by the university is

stocked with all of the equipment we will need to design, simulate, build, and test our design. For

these reasons we believe that our team is qualified to participate in IFEC 2015.

IX. Signatures

Faculty Advisor: ______________________________________________

Student Team Leader: ______________________________________________