Bi-Directional Inverter and Energy Storage System

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Bi-Directional Inverter and Energy Storage System

Submitted to fulfill the requirements of:

Texas Instruments Analog Design Contest

by

Derik Trowler and Bret Whitaker

May 2008

University of Arkansas

College of Engineering

Department of Electrical Engineering



University of Arkansas Department of Electrical Engineering ii

ABSTRACT

This report presents a scaled down energy storage system for peak load shaving applications.

The design includes a bidirectional inverter along with a dc-dc converter capable of interfacing a battery

bank with the ac power grid. The main goals of the project included the implementation of two modes of

operation: a battery discharge mode where current is being fed into the grid and a battery charging mode

in which current is pulled from the grid and put into the batteries. A secondary goal of the design was to

ensure that the current being injected into grid was at or near unity power factor.

The results of the project were successful as current was injected into the grid with near unity

power factor by utilizing a hysteresis current control method. The current waveform was seen to be

discontinuous, which was most likely caused by the inductance value used to filter the output current.

Difficulty in designing the output filter was to be expected since hysteresis control has an inherent

variable switching frequency. Regardless of this fact, the system maintained the desired RMS output

current and thus proved the functionality of the system in discharge mode. The bidirectional capability of

the system was also proven by recharging the battery bank with no hardware changes. Testing results

showed that all the requirements were met as the system proved to function as a scaled down energy

storage system.



University of Arkansas Department of Electrical Engineering iii

TABLE OF CONTENTS

1. INTRODUCTION ....................................................................................................................................... 1

1.1 Motivation for an Energy Storage System ........................................................................................... 1

1.2 Scope of the Design ............................................................................................................................ 3

2. THEORETICAL BACKGROUND .............................................................................................................. 4

2.1 Introduction .......................................................................................................................................... 4

2.2 Discharge Mode .................................................................................................................................. 5

2.3 Charge Mode ....................................................................................................................................... 5

2.4 Concluding Remarks ........................................................................................................................... 5

3. HARDWARE DESIGN OVERVIEW .......................................................................................................... 6

3.1 Introduction .......................................................................................................................................... 6

3.2 DSP ..................................................................................................................................................... 6

3.3 Analog Signal Conditioning ................................................................................................................. 7

3.3.1 Current Sensors ............................................................................................................................ 7

3.3.2 Voltage Sensors............................................................................................................................ 8

3.4 Digital Signal Interface......................................................................................................................... 9

3.5 Power Electronics .............................................................................................................................. 10

3.5.1 Design of the dc-dc Converter .................................................................................................... 10

4. SOFTWARE DESIGN OVERVIEW ........................................................................................................ 13

4.1 Introduction ........................................................................................................................................ 13

4.2 Discharge Mode Control .................................................................................................................... 13

4.2.1 Signal Conditioning ..................................................................................................................... 14

4.2.2 PI Control .................................................................................................................................... 15

4.2.3 Hysteresis Control....................................................................................................................... 16

4.3 Charge Mode Control ........................................................................................................................ 18

4.3.1 Trickle Charge Control ................................................................................................................ 18

5. IMPLEMENTATION ................................................................................................................................ 19

5.1 Introduction ........................................................................................................................................ 19

5.2 Main Power PCB ............................................................................................................................... 19



University of Arkansas Department of Electrical Engineering iv

5.3 Gate Driver PCBs .............................................................................................................................. 20

5.4 Signal Conditioning PCB ................................................................................................................... 20

5.5 Complete Build-Up ............................................................................................................................ 22

6. RESULTS ................................................................................................................................................ 23

6.1 Introduction ........................................................................................................................................ 23

6.2 Charge Mode Results ........................................................................................................................ 23

6.3 Discharge Mode Results ................................................................................................................... 24

6.4 Conclusion from Results ................................................................................................................... 26

6.5 Future Work ....................................................................................................................................... 26

REFERENCES ............................................................................................................................................ 28

ACKNOWLEDGEMENTS ........................................................................................................................... 29



University of Arkansas Department of Electrical Engineering v

LIST OF FIGURES

Figure 1: Estimated Grid Load Profile. .......................................................................................................... 2

Figure 2: Estimated Grid Load Profile with BES Installed. ............................................................................ 2

Figure 3: System Block Diagram. ................................................................................................................. 4

Figure 4: Main Circuit Components. ............................................................................................................. 4

Figure 5: System Interface Block Diagram. .................................................................................................. 6

Figure 6: DSP Evaluation Board. .................................................................................................................. 7

Figure 7: Current Sensing Network. .............................................................................................................. 8

Figure 8: Dc Voltage Sensor (left) and Ac Voltage Sensor (right). ............................................................... 9

Figure 9: Digital Interface. ............................................................................................................................. 9

Figure 10: Simulation Schematic. ............................................................................................................... 11

Figure 11: Boost Converter Voltage with 35% Duty Cycle Simulation. ...................................................... 11

Figure 12: Battery Current during Charge Mode Simulation. ...................................................................... 12

Figure 13: Discharge Mode Control Block Diagram. .................................................................................. 13

Figure 14: Signal Conditioning Block Diagram. .......................................................................................... 14

Figure 15: PI Control Block Diagram. ......................................................................................................... 15

Figure 16: Simulated Output of Boost Converter ........................................................................................ 16

Figure 17: Hysteresis Control Block Diagram. ............................................................................................ 17

Figure 18: Simulated Hysteresis Output Current with Reference and Band .............................................. 17

Figure 19: Trickle Charge Control. .............................................................................................................. 18

Figure 20: Populated Main Power Board. ................................................................................................... 20

Figure 21: Signal Conditioning Board. ........................................................................................................ 21 Figure 22: Completed Project. .................................................................................................................... 22

Figure 23: Completed Project. .................................................................................................................... 22

Figure 24: Trickle Charge. ........................................................................................................................... 23

Figure 25: Voltage and Current Waveform with Gate Signals. ................................................................... 24

Figure 26: Voltage and Current Waveforms with Boost Converter Output. ................................................ 25

Figure 27: Grid Voltage and Current Waveform. ........................................................................................ 26



University of Arkansas Department of Electrical Engineering 1

1. INTRODUCTION

1.1 Motivation for an Energy Storage System

The demand for energy will continue to increase as long as world population increases and

people continue to demand a higher standard of living. The challenge lies in providing this energy from

dependable and sustainable sources while maintaining respect for the environment. Coal-fired power

and other fossil fuel based energy sources are a proven source for the needed energy; however, they

also cause undesirable effects on the environment. While it is clear that renewable energy is not the

immediate answer to the problem, it can certainly play a role in the solution to global energy needs when

used in conjunction with traditional sources of energy.

Renewable energy currently faces several drawbacks on its track to become the sole source of

electric power generation. One major drawback is its dependency on geographic location. For example,

the best locations to harvest solar energy lie in the desert regions of earth’s surface. However, most of the energy consumers do not reside in these arid regions. Wind power also faces the same geographic

problem. The best available wind energy in the United States lies in the Midwestern and Great Plains

states [1]. Again, these states are not where most of the nation’s energy consumers are located.

Another drawback that renewable energy suffers from is its intermittent nature. Wind energy has been

known to cause major brown-outs because of unexpected drops in wind speed. When this happens,

coal-fired power plants are expected to pick up the tab for the extra needed energy. However, coal-fired

power plants cannot ramp up their generation fast enough to counteract the effects of a lack of sufficient

wind. Therefore, an energy storage system is needed to work with renewable energy sources in order to

counteract intermittent generation.

Another issue that the electric power grid faces is peak demand loading periods. These periods

of time are when energy demand is at its highest and generally happen during the hours of 5 PM to 7 PM

as shown in Figure 1. During these hours, power plants must ramp up generation in order to keep up

with demand. Energy is expensive for the power utility to produce during these hours because the

increased generation may come from high cost processes. These increased prices are usually passed

down to commercial and industrial customers. Most residential customers currently pay a flat rate;

however, improved metering technologies will allow utility companies to start charging different rates at

different time periods. In contrast, energy demand drops well below the baseline power generation duringthe late night and early morning hours. Energy during these hours is cheap to generate for the power

utility and also cheap for consumers to purchase. It can be seen that a way to eliminate the peaks and

troughs of the power consumption trend is needed in order to help make energy more economical.




Figure 1: Estimated Grid Load Profile.

It is clear that an energy storage system is needed in order to solve the problems associated with

both peak demand loading and the intermittent nature of renewable energy. An approximate view of the

effects of a battery energy storage system (BES) can be seen in Figure 2. An effective BES system can

provide the extra energy needed during the peak energy consumption periods as well as when renewable

energy sources go offline. When used in conjunction with renewable and coal-fired power generation,

distributed energy storage systems can help make the power grid more efficient and cost effective.

Figure 2: Estimated Grid Load Profile with BES Installed.

0

0.2

0.4

0.6

0.8

1

12:00 AM 4:00 AM 8:00 AM 12:00 PM 4:00 PM 8:00 PM 12:00 AM

Grid

Demand

Time, h

L o a d , p u p

e a k

0

0.2

0.4

0.6

0.8

1

12:00 AM 4:00 AM 8:00 AM 12:00 PM 4:00 PM 8:00 PM 12:00 AM

Grid Demand

Grid Demand

After Peak

Load Shaving L o a d , p u p

e a k

Time, h

BES System

Charge

BES System

Discharge




1.2 Scope of the Design

This senior design project was focused on building a scaled down battery energy storage system.

The design was required to utilize power electronics to interface a battery bank with the grid. The system

was required to operate in two modes, with a majority of the focus on the “discharge mode” in which

power is drawn from the batteries and injected into the grid. The design was also required to recharge

the battery bank from the grid without making any hardware changes during a “charge mode” of

operation. The intent of the design was to provide a proof of concept for the system to allow later

development in capacity and complexity.




2. THEORETICAL BACKGROUND

2.1 Introduction

The design was specified to use the same hardware in two modes of operation and thus have

bidirectional power flow functionality. The discharge mode was specified as the process of extracting

energy from the battery bank and using it to supplement the grid. This was accomplished by boosting the

battery bank voltage to the necessary level and then converting it to ac with the proper frequency and

phase needed in order to inject current into the grid. This mode required a way to synchronize the

inverter output current with the grid voltage in order to ensure a near unity power factor and thus minimize

reactive power. Alternatively, the charge mode of operation utilizes the grid to recharge the battery bank

and store energy. This is accomplished by rectifying the grid voltage and regulating the amount of current

flowing into the batteries. The discharge mode is explained in greater detail in Section 2.2 while the

charge mode is given in Section 2.3. Figure 3 shows a system block diagram while Figure 4 shows the

general circuit schematic to be realized.

Figure 3: System Block Diagram.

Figure 4: Main Circuit Components.

Vgrid

1 5

4 8

1 2

VBAT

1 2

G1 G3

G2GBoost

G40

H-Bridge InverterBidirectional Buck/Boost Converter

FilterGBuck




2.2 Discharge Mode

At the core of the design shown in Figure 4 is an H-Bridge inverter in series with a bidirectional

dc-dc converter. In the discharge mode, the bidirectional buck/boost converter is used to boost the

battery voltage to a level higher than the output of the transformer so that current will be allowed to flow

from the batteries into the grid. The inverter is used to chop up the DC voltage from the batteries into an

unfiltered ac voltage. The chopped ac voltage is then passed through an output filter in order to smooth

out the current waveform passing into the grid. The current is finally passed through a step-up

transformer which provides isolation while stepping the voltage up to 120 V RMS for direct interface with the

grid.

Since the voltage waveform is determined by the grid, the inverter will be of the current controlled

type. A hysteresis control method was selected for this system because of its ease of implementation.

This method works by setting a band around a reference signal and turning on and off switches according

to when the current crosses the band boundary. Additionally, the boost converter was controlled by using

a proportional-integral (PI) control strategy.

2.3 Charge Mode

The benefit of the charge mode lies in the fact that it only adds one additional switch to those

required for the discharge mode. The charge mode utilizes the freewheeling diodes on the inverter as a

bridge rectifier while the dc-dc converter regulates the amount of current that is allowed to flow into the

batteries. This aspect of the design was the easiest to implement since it only requires the modulation of

a single switch and does not require any special phase locking considerations. This mode was

considered a secondary goal to some extent for this reason. The battery charging was accomplished

through a simple trickle charge method.

2.4 Concluding Remarks

The design encompasses several aspects of electrical engineering including power electronics,

signal processing, control systems, and digital systems. Various voltages and currents throughout the

system had to be measured and conditioned into a form that could be read by an analog-to-digital

converter (ADC). Furthermore, the design required several digital outputs in order to provide driving

signals to the switches utilized in the design. The control system required several algorithms to work in

conjunction to achieve the final result (i.e. the boost controller and inverter). The overall system

presented an opportunity to explore the many aspects within the electrical engineering field and led to an

increased array of knowledge and experience.




3. HARDWARE DESIGN OVERVIEW

3.1 Introduction

Figure 5: System Interface Block Diagram.

The major challenges presented in the hardware design of the project were brought out in

interfacing the DSP with the power electronics. For example, when measuring the ac voltage, it was not

sufficient to utilize a simple resistive voltage divider since most ADCs are not tolerant of negative

voltages. Hence, a voltage level shifting circuit was required in order to remedy this problem. A similar

problem occurs when measuring current levels as simple current sensing resistors are not an optimal

choice for this application. Another issue that occurred with both current and voltage measurements was

the need for electrical isolation between the power electronics and the digital system. The signal flow

diagram for the overall system can be seen in Figure 5. The following sections are broken down as

follows: Section 3.2 covers the DSP hardware interface, Section 3.3 overviews all analog measurements

while Section 3.4 looks at the digital interface aspect of the design. Note that this section views the

hardware from an on-paper approach while Section 5 views the actual printed circuit board (PCB)

implementation of the system.

3.2 DSP

The DSP chosen for the design was the TMS320F2808 manufactured by Texas Instruments, Inc.

This particular DSP features a 100 MHz clock speed, built-in PLL, 16 enhanced PWM outputs, and two 8-

channel 12-bit ADCs [2]. The combination of the PLL and enhanced PWM module sets this DSP apart

from most DSPs. In addition, the DSP is sold as an evaluation board (Figure 6) which includes all the




external circuitry required for optimal functionality. The numerous functions of this DSP make it an ideal

choice for grid-connected power systems.

Figure 6: DSP Evaluation Board.

3.3 Analog Signal Conditioning

Analog signal conditioning was a very important aspect of the design as it was the only means of

communicating circuit behavior to the DSP. The signals received by the ADC module of the DSP must be

conditioned so that they are between 0-3 V as this is the safe area of operation (SOA) for the ADC.

Additionally, the measurement circuitry was required to electrically isolate the corresponding signal from

the DSP. The design called for a voltage measurement on either side of the dc-dc converter as well as

on either side of the transformer. In addition, current measurements were needed at the low voltage side

of the dc-dc converter as well as at the grid interface. In summary, there are 4 voltage measurements

and 2 current measurements throughout the system. The current sensors are described in detail in

Section 3.3.1 while the voltage sensors are given in Section 3.3.2.

3.3.1 Current Sensors

The current sensing circuitry utilized was an Allegro Microsystems, Inc. part number ACS712.

This Hall-Effect current sensor and its external circuitry are capable of sensing a ±30 A current and




converting it into a 0-5 V isolated analog signal [3]. A simple voltage divider circuit was then used to

convert the resulting signal into a 0-3 V signal to be used by the ADC. The current sensing network is

given in Figure 7.

Figure 7: Current Sensing Network.

3.3.2 Voltage Sensors

The design included a total of 4 voltage measurements; two were dc measurements while the

remaining sensors measured ac voltages. An isolating op-amp (Texas Instruments P/N: ISO122JP) was

used to electrically isolate all voltage measurements from the DSP. This op-amp is a unity gain devicethat can measure up to a 50 kHz bipolar signal, assuming the supply rails are not exceeded in which case

the device will saturate [4]. All op-amps were powered by means of an isolated complementary ±15V dc-

dc converter (Texas Instruments P/N: DCH010515DN7). For dc measurements, the isolating op-amps

were accompanied by the appropriate voltage dividers in order to scale the anticipated levels down to

voltages usable by the ADC. The ac voltages required additional level shifting circuitry to insure that the

ADC was never biased into the negative voltage levels. This was accomplished by injecting dc voltage

into the inverting op-amp (Texas Instruments P/N: OPA2131UA) configuration as shown in Figure 8 by

means of a voltage reference IC (Texas Instruments, Inc. P/N: REF3312AIDBZT). Figure 8 gives the

details for both types of voltage transducers used in the design.

I_IN

I_OUT

A_GND

R210k

IP+11

IP+22

IP-13

IP-24

GND5FIL6

VI7VCC8

ACS712

R16.8k

C11nF

C20.1uF

SIG_I

VCC_+5V




Figure 8: Dc Voltage Sensor (left) and Ac Voltage Sensor (right).

3.4 Digital Signal Interface

The digital aspect of the design included interfacing the DSP with 6 gate driving circuits. This

portion of the design was fairly straight-forward except that the TMS320F2808 is 3.3 V CMOS compatible

while the gate driver ICs are +5 V logic level compatible. As a result, a 74LVX3245 logic level translator

was used as a buffer between the two incompatible logic levels. Additional logic circuitry was also added

for safety concerns arising from the possibility that two switches in the same H-bridge leg could be

simultaneously turned on, causing a short circuit. The circuitry designed to eliminate this possibility is

shown in Figure 9. In summary, the digital interface of the design gave a hardware safety feature which

eliminated the possibility of causing a short circuit while still interfacing the DSP with gate driver circuitry.

Figure 9: Digital Interface.

SIG_V1

R112k

R23k

V_+15VV_-15V

A_-15V A_+15V A_GND

V_GNDV1

1

1

TP1

Test Point

+Vs11

-Vs12

Vout7

Gnd28

+Vs29-Vs210

Vin15Gnd116

U1

ISO122

A_GND

SIG_V3

R8

2.49k

R7

10k

R54.99k

R66.04k

In1

Out2

Gnd3

U7

REF3312

+3

-2

V+8

V-4

OUT1

U5-1

OPA2131UA

A_+15V

V3_+15V

A_-15V

A_-15V

V3_-15V

A_GND A_+15V

V3_GNDV3

+Vs11

-Vs12

Vout7

Gnd28

+Vs29-Vs210

Vin15Gnd116

U3

ISO122

A_GND A_+5V

11

TP3

Test Point

1 2

U15A

7404

3 4

U16B

7404

1 2

U14A

740412

1312

U17A

7411

345

6

U17B

7411

~ENABLE

SIGNAL_1

SIGNAL_2

R33.3k

VCC

OUT_1

OUT_2




3.5 Power Electronics

The power electronics design of the project included the dc-dc converter, H-bridge inverter, and

all other accompanying magnetic components. The dc voltage was chosen to be 36 V which required the

use of three 12 V lead-acid batteries in series. The step-down transformer used gives a 36 VRMS value

which equates to a 36√2 VPK or about 51 V. Therefore, the purpose of the dc-dc converter was to boost

the battery voltage to a level higher than 51 V for proper discharge mode operation. In retrospect, the dc-

dc converter must then step down the 51 V dc rail in order to limit the battery charging current during the

charge mode. Section 3.5.1 gives the design steps taken to design the dc-dc converter.

3.5.1 Design of the dc-dc Converter

The dc-dc converter was required to boost 36 V up to ≥51 V during forward mode and perform

the inverse during reverse mode. The topology used is basically a half-bridge converter with additional

filtering circuitry. The design steps include solving the equations in [5] for the buck converter portion of

the design.

For a buck converter operating in the continuous region:

The duty cycle is given by:

The current at the continuous-discontinuous boundary is given by:

Assuming that the switching frequency was given as 50 kHz, the value of the inductance needed was

approximately 36 μH. From this, the capacitance values can be calculated from:

In this equation, f c signifies the cut-off frequency which was chosen to be about 2 orders of magnitude

lower than the switching frequency. This yields the need for a 2700 μF capacitance for the low voltageside. The high voltage side of the dc-dc converter only depends on the amount of ripple allowed in the

voltage output. For design simplicity, this value was oversized at 2700 μF so that identical parts could be

used. Figures 10 through 12 show the dc-dc converter schematic used in simulations and their results.




Figure 10: Simulation Schematic.

Figure 11: Boost Converter Voltage with 35% Duty Cycle Simulation.

1 2L

36uH

VBAT

36VDC

RG1

10

RG2

10

C2

2700u

C1

2700uF

RBAT

0.03

V4

51VDC

V3

0

Z1

Z2

V2




Figure 12: Battery Current during Charge Mode Simulation.




4. SOFTWARE DESIGN OVERVIEW

4.1 Introduction

The software controls for this system were developed using MATLAB/Simulink. The 2808 DSP

selected for this project is compatible with the Target for TI C2000 library which allows ANSI C code to be

generated directly from the Simulink model. Code Composer Studio interfaces directly with Simulink to

provide a method for loading the code onto the DSP boards. The two modes of operation were designed

as two separate sets of code which were independently loaded and run from the DSP ’s RAM. It should

be noted that a dynamic control between operating modes is a feature that would be included in a full-

scale system but is outside the scope of this project.

4.2 Discharge Mode Control

The discharge mode can be characterized as having two independent algorithms operatingsimultaneously to control the system. The boost converter utilized a PI controller to regulate the

increased voltage to a desired level. The H-bridge inverter used a hysteresis control to chop up the

boosted voltage and regulate the current flowing into the grid. The PI control was initialized whenever the

battery bank is connected and a voltage greater than 32 V was sensed by the DSP. The hysteresis

control begins whenever the boost voltage becomes greater than 54 V. The system was designed to

continue running until stopped manually by disconnecting the battery bank and grid.

Figure 13: Discharge Mode Control Block Diagram.

buck gate

enable

gate 3

gate 6

gate 4

gate 5

ePWM_2A

WA

C280x

ePWM

Signal Conditioning

In1 Out1

SaturationRate Transition 1

Internal Signal Conditioning

V1_in

V2_in

V4_in

I2_in

batt_V_out

boost_V_out

ref_out

current_out

Hysteresis

trigger

ref_sig

current

pos_gates

neg_gates

F2808 eZdsp

Enabled

PWM Control

V2_in pwm

Enable Hysteresis

boost_V enable

Enable Boost

batt_V enable

Digital Output 4

C280x

GPIO DO

GPIOx

Digital Output 2

C280x

GPIO DO

GPIOx

Digital Output 1

C280x

GPIO DO

GPIOxConstant

0

ADC

C280x

ADC

A0

A1

A3

A5

V4

I2

V2

V1




4.2.1 Signal Conditioning

The first major subsystem in the discharge mode controls is the signal conditioning block which

can be seen in Figure 14. This subsystem takes each ADC output and recreates the actual voltage or

current corresponding to the specific measurement before the hardware signal conditioning alters it. The

signal conditioning first converts the uint16 data to int32 so that it can contain negative values. A shift is

implemented for V4 and I2 because these values represent AC measurements and this allows for the

midpoint to again be zero. This data is then converted to a fixed point representation before being

amplified back to the real world amplitude. An infinite impulse response filter (IIR) is used for the voltage

signals to remove as much noise as possible but the same is not done for the current signal I2. This is

because the frequency of this current is variable so the filter’s 3 -dB frequency would have to be quite

large and thus negate any noise cancelling effects. The ADC outputs are now properly conditioned with

real world amplitudes to be used by the subsequent blocks for controlling the system.

Figure 14: Signal Conditioning Block Diagram.

current_out

4

ref_out

3

boost_V_out

2

batt_V_out

1

uint 16 to int 32

data conversion 3

int32(SI)

uint 16 to int 32

data conversion 2

int32

(SI)

uint 16 to int 32

data conversion 1

int32(SI)

uint 16 to int 32

data conversion

int32

(SI)

To Fixed Point 3

In1 Out1

To Fixed Point 2

In1 Out1

To Fixed Point 1

In1 Out1

To Fixed Point

In1 Out1

Set Reference

Signal Amplitude

In1 Out1

Recreate Output

Current

In1 Out1

Recreate Boost

Voltage

In1 Out1

Recreate Battery

Voltage

In1 Out1

Normalize Signal

To One

In1 Out1

IIR Filter 2

In1 Out1

IIR Filter 1

In1 Out1

IIR Filter

In1 Out1

Eliminate Signal

Conditioning Shift

2048

Eliminate Signal

Conditioning Shift 1

2048

Add 3

AddI2_in

4

V4_in

3

V2_in

2

V1_in

1RE-SHIFTED

RE-SHIFTED

RE-SHIFTED

RE-SHIFTED FIXED POINT

- PER UNIT




4.2.2 PI Control

The PI Control of the boost converter utilizes the standard configuration for this type of control

which can be seen in Figure 15. The voltage at the output of the boost converter is compared to a

reference of 58V. The actual voltage is subtracted from the reference to create an error signal which is

then propagated through the control system. The block labeled Gain 3 is the proportional term (P) and

was selected to have a gain value of 0.01. The blocks labeled Discrete-Time Integrator 1 and Gain 4

represent the integrated term (I) which has a gain value of 2. The P and I terms are then added together

and conditioned before being sent to the ePWM block. The conditioning transforms the data back into the

uint16 data type and the saturation block limits the range of pulse widths. The ePWM block accepts a

value from 0 to 100 as an input which represents the duty cycle percentage and outputs the

corresponding PWM signal. This control allows for a constant boost voltage to be maintained as the

loading of the converter changes with the changing ac current output. The simulated output of the boost

converter during full system operation can be seen in Figure 16.

Figure 15: PI Control Block Diagram.

pwm

1

Vref

58

Rate Transition 1

Gain 4

In1 Out1

Gain3

In1 Out1

Float to IQN

A Y

IQN

IQmath

Discrete-Time

Integrator 1

K Ts

z-1

Enable V2_in

1

Error

DC




Figure 16: Simulated Output of Boost Converter

4.2.3 Hysteresis Control

The hysteresis control of the H-Bridge compares the current output of the system to a reference

band. The block diagram for this system can be seen in Figure 17. The grid voltage was sampled and

used as the reference signal so that the output current would be in phase with the grid voltage. This

prevents the need for phase-locked loops and simplifies the controls and circuitry for the system. The

reference band was set so that the upper limit was the reference signal plus one and the lower limit was

the reference minus one. The system works so that the positive switches (S3 and S6) turn on whenever

the output current becomes less than ref - 1 and the negative switches (S4 and S5) turn on when the

output current becomes greater than ref + 1. The AND gates in this subsystem force the outputs to be

disabled until certain conditions are met. The first condition is the trigger for the hysteresis system. This

occurs whenever the output of the boost converter exceeds 54 V. The second condition is the dead band

for the system. This prevents any switches from turning on when the reference signal is within +/- 0.3 of

zero amplitude. A simulated view of the hysteresis control and output can be seen in Figure 18 where the

purple signal is the reference and the yellow is the output current.




Figure 17: Hysteresis Control Block Diagram.

Figure 18: Simulated Hysteresis Output Current with Reference and Band

neg_gates

2

pos_gates

1

hysteresis width

1

Relational

Operator 3

<

Relational

Operator 2

>

Relational

Operator 1

<

Relational

Operator

<

Logical

Operator 4NOT

Logical

Operator 3

OR

Logical

Operator 2

AND

Logical

Operator 1

AND

Float to IQN 2

A Y

IQN

IQmath

Float to IQN 1

A Y

IQN

IQmath

Float to IQN

A Y

IQN

IQmath

Enabled

Subsystem

In1 Out1

Constant2

.3

Constant1

-.3

current

3

ref_sig

2

trigger 1

upper bound

lower bound




4.3 Charge Mode Control

The method of charging the battery was chosen to be trickle charging. This is neither an

advanced nor an overly desired method of charging however it is easily implemented and acts as a proof

of concept for the design. The main focus of the project has been on the discharging mode because of

the fact that battery chargers are readily available commercially whereas a system that implements an

equivalent discharging mode is less commonplace.

4.3.1 Trickle Charge Control

The trickle charge control scheme is very simple compared to those previously mentioned. A

block diagram for this control can be seen in Figure 19. This control scheme works by outputting a

constant duty cycle signal to control the amount of current that flows into the battery. This system is

enabled whenever the grid becomes connected and the DSP senses at least 40 V at the output of the

grid voltage rectifier. This triggers the constant value to be sent to the ePWM module which will then

output a constant 50 kHz waveform with a 10% duty cycle. This method allows current to slowly enter the

battery and gradually build a charge. This control scheme is designed like most trickle battery chargers in

that the battery and grid must be disconnected to stop the charging process.

Figure 19: Trickle Charge Control.

gate 3 = GPIO 4

= header pin 13

gate 6 = GPIO7

= header pin 21

gate 4 = GPIO27

= header pin 15

gate 5 = GPIO13

= header pin 17

enable = GPIO 1

= header pin 10 boost gate = GPIO 2

ePWM_2A

WA

C280x

ePWMSignal Condi tioning 3

V2_in rect_V_out

Signal Condi t ioning 1

In1 Out1

Saturation

Rate Transition 1

F2808 eZdsp

Enabled

PWM Control

pwm

Enable Buck 1

rect_V enable

Digital Output 4

C280x

GPIO DO

GPIOx

Digital Output 2

C280x

GPIO DO

GPIOx

Digital Output 1

C280x

GPIO DO

GPIOx

Constant3

0

Constant2

0

Constant1

0

ADC

C280x

ADC

AV2




5. IMPLEMENTATION

5.1 Introduction

Given the overall complexity of the system, the design included a total of 8 PCBs excluding the

DSP board in order to distribute the various components. This included a main power board, a signal

conditioning board, and 6 identical IGBT gate driver boards. Although the resulting design was somewhat

bulky, separating the various components out on different PCBs significantly decreased troubleshooting

time. This point was later reassured when it was determined that a second version of the signal

conditioning board was required for EMI and safety concerns. Had all the parts been placed on a single

PCB, then the construction would have basically restarted from scratch at that point. The components

external to the PCBs were mounted as close as possible to PCB to decrease wire length. All in all, the

system was built with testing and troubleshooting considerations in mind.

The following sections briefly overview the actual hardware constructed for the design. The

sections are broken down by the PCB that contains the part of the system in question. Section 5.2 covers

the main power PCB which contains the power electronics devices and provides the foundation on which

the rest of the design is built. Section 5.3 overviews the 6 identical gate driver boards used to interface

with the IGBTs. The scope of Section 5.4 pertains to the signal conditioning board which serves the role

of being a buffer stage for the DSP board. The DSP board was a purchased product therefore its design

is not covered here.

5.2 Main Power PCB

The main power PCB was used to house all the power devices included in the design. This

included two +5 V power supplies used by the digital and analog circuitry. These power supplies were

isolated via transformers in order to eliminate ground loops. Both power supplies were of the linear

topology and utilized +5 V regulators (Texas Instruments P/N: UA7805CKCS). In addition, the signal

conditioning PCB and all the gate driver boards were placed on the main board. This included all the

IGBTs, freewheeling diodes, filter capacitors, and the dc-dc converter inductor. The current sensing

circuitry was placed on the main board since the sensors must lie in the main current path. Also, the

voltage dividers used by the voltage sensors were placed on this board in order to keep the high-voltage

and high-current signals away from the signal conditioning board. The main board was basically

designed to be a foundation for the power devices and as a mounting for the other PCBs. The populated

main board can be seen in Figure 20.




Figure 20: Populated Main Power Board.

5.3 Gate Driver PCBs

The gate driver PCBs were small 1.5” X 1.5” PCBs used to drive the IGBTs. Each gate driver

was placed on its own board in order to ease replacement in case one was to fail. In addition, removing

the gate driver boards from the main board allowed for easy testing of gate signals without actually

switching a device. The six gate driver boards can easily be seen in Figure 20. In summary, the 6 gate

driver boards were a small removable module that enabled easy testing and troubleshooting.

5.4 Signal Conditioning PCB

The signal conditioning board housed the digital interface and the analog signal conditioning

circuitry. It was built as a buffer for the DSP board by providing isolation with all analog circuitry and a

+3.3 V to +5 V logic level translator. The signal PCB was designed to mount to the main board using

stand-offs and header pins. The DSP board was connected to the signal board by a similar method. The

signal board can be seen in Figure 21.




Figure 21: Signal Conditioning Board.




5.5 Complete Build-Up

The complete circuit and component build-up can be seen in Figures 22 and 23.

Figure 22: Completed Project.

Figure 23: Completed Project.




6. RESULTS

6.1 Introduction

This section gives the results of the project and then states some conclusions as to what was

accomplished and what aspects could be improved upon. The results are broken down by the mode of

operation: Section 6.2 shows the results for the charging mode while Section 6.3 gives the discharge

mode results. Section 6.4 summarizes the conclusions gained from the project while Section 6.5

explores possibilities for future work.

6.2 Charge Mode Results

As expected, the charging mode was easily implemented by modulating the dc-dc converter for

buck mode. The switch was modulated using a constant duty cycle in order to slowly charge the battery

bank from the gr id. This method is similar to how a commercially available “trickle” battery charger functions. An illustration of trickle charging with a 10% duty cycle can be seen in Figure 24.

Figure 24: Trickle Charge.




6.3 Discharge Mode Results

The major challenge presented in the discharge mode was to modulate the H-bridge switches in

such a way that the current injected into the grid was in phase with the grid voltage. This aspect of the

design was successful as shown in Figure 26. The positive gate signals (O-Scope Ch2) are shown to

only modulate during the positive cycle while the negative signals (O-Scope Ch3) modulated during the

negative cycle only.

Figure 25: Voltage and Current Waveform with Gate Signals.

A non-ideal characteristic of the system can be seen in the output current waveform (O-Scope

Ch4). The problem is that the filter inductor was operating in the discontinuous mode, evident by the

choppy current waveform. The probable solution would be to increase the inductance value to help

smooth the current waveform; however, the switching frequency for hysteresis control is variable and

makes filter design difficult. Another possible cause for the large di/dt in the current waveform is the




difference in voltage between the transformer secondary and the output of the boost converter. The

rectified voltage of the transformer secondary is 36√2 V or about 51 V dc. A higher differential voltage

between this value and the output of the dc-dc converter yields a higher di/dt in the current waveform

when a switch pair is turned on. Figure 26 shows the output voltage of the boost converter (O-Scope

Ch2) and the inverter output current simultaneously while Figure 27 shows only the grid side voltage and

current. Regardless of the discontinuous current, the desired RMS value for the output current was

maintained throughout the operation of the discharge mode. In summary, the inverter successfully

injected current in to the grid at near unity power factor thus proving the functionality of the system during

this mode of operation.

Figure 26: Voltage and Current Waveforms with Boost Converter Output.




Figure 27: Grid Voltage and Current Waveform.

6.4 Conclusion from Results

The project was an overall success since all the goals were met in the end. Current was fed into

the grid that was in phase with the ac mains voltage waveform. The current exhibited some non-ideal

qualities; however, this distortion can most likely be fixed by simply replacing the output inductor. The

project also proved the capability of recharging the batteries with no hardware changes and thus proved

the capacity for bidirectional power flow through the system. The successful demonstration of operating

modes shows that the system meets all the goals that were set at the beginning of the project.

6.5 Future Work

Any project should always leave one with new knowledge and a desire to take that new

knowledge to the next level. The characteristics of this system presented an opportunity to explore many

aspects within electrical engineering and allowed the design team members to gain a diverse array of

knowledge and experience. All in all, the project was a very challenging and complex project that turned




out to be very rewarding and successful in the end. The project also led to many more questions and

ideas as to how to improve the system functionality.

The most obvious future work pertaining to this project would be to redesign the output filter

inductor. This will be a challenging task as a filter is more difficult to design for the variable switching

frequencies inherent in a hysteresis control scheme. While hysteresis control is a proven method for grid

connected inverters, the problems that arise from having a variable switching frequency make it less

desirable. As a result, exploring other control methods would prove useful.

In addition to new control methods, there are several other interesting topics to explore such as:

Implementing a control that can dynamically switch between charge and discharge mode

depending on the peak load demand.

Eliminate the need for a step-up transformer by increasing the battery bank voltage.

Perform system efficiency and reliability tests.




REFERENCES

[1] American Wind Energy Association, “Top 20 States with Wind Energy Resource Potential,”

http://www.awea.org/pubs/factsheets/Top_20_States.pdf.

[2] Texas Instruments, Inc., “TMS320F280x Data Manual,” SPRS230J , Sept. 2007.

[3] Allegro MicroSystems, Inc., “AC712 Datasheet,” ACS712-DS, Rev. 7, 2007.

[4] Texas Instruments, Inc., “ISO122 Datasheet,” PDS-857F, Nov. 1993.

[5] Ned Mohan, Tore M. Undeland, and William P. Robbins, Power Electronics: Converters, Applications,

and Design, 3rd ed. Hoboken: John Wiley & Sons, Inc., 2003.



ACKNOWLEDGEMENTS

We would like to thank Diogenes Molina of the University of Arkansas for assistance in programming. We

would also like to thank Mr. Ray Hayes and American Electric Power for their financial support of this

project.

Bi-Directional Inverter and Energy Storage System

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