Interfacing a Flywheel to the Analog Model Power System

August 29, 2005 Department of Electrical and Computer Engineering

University of Idaho

Sponsors & Mentors

Dr. Herb Hess Dr. Brian Johnson

Instructor

Dr. Brian Johnson

Flywheel Team

Gavin Abo

abo6715@uidaho.edu

Nate Stout stou6862@uidaho.edu

Nathan Thomas thom5938@uidaho.edu

TABLE OF CONTENTS List of Figures and Tables......................................................................................................ii Abstract ..................................................................................................................................iii Project Description.................................................................................................................1

Background Information............................................................................................1 Problem Statement .....................................................................................................2 Objectives ..................................................................................................................2 Constraints .................................................................................................................2 Functional Specifications...........................................................................................3 Solution Method.........................................................................................................3

Status......................................................................................................................................4 Method of Solution ................................................................................................................4

Technical Description ................................................................................................4 Theoretical Basis........................................................................................................5

Test Plan.................................................................................................................................8 Appendix A: Figures..............................................................................................................A-1 Appendix B: Pictures .............................................................................................................B-1 Appendix C: Specifications ...................................................................................................C-1 Appendix D: Bill of Materials ...............................................................................................D-1 Appendix E: Parts Ordered ....................................................................................................E-1 Appendix F: Individual Reports.............................................................................................F-1

i

List of Figures and Tables

Figure 1. Signal Flow for a Detected Sag ..............................................................................3 Figure 2. Space Vector PWM Vector Diagram .....................................................................7 Figure 3. PWM Variable Width Example..............................................................................7 Figure 4. Overall Design Schematic ......................................................................................A-1 Figure 5. AMPS Side Switch Diagram..................................................................................A-2 Figure 6. Induction Motor Side Switch Diagram...................................................................A-2 Table 1: Specifications...........................................................................................................C-1 Table 2: Bill of Materials.......................................................................................................D-1

ii

Table 3: Known Converter Data ...........................................................................................E-1

Abstract Title: Interfacing a Flywheel to the Analog Model Power System Authors: Gavin Abo, Nate Stout, and Nathan Thomas Date: May 12, 2005 Department of Electrical and Computer Engineering University of Idaho

iii

Progress on the sag correction interface to AMPS over the Spring 2005 semester

have come up with an initial design and have begun refining the design. Several proof of

concept designs have been shown. Most of the subsystems have been researched and the

purchasing process has begun. The purchased items should arrive over the summer for

work in the fall. The programs are a work in progress and will be programmed over the

Summer and Fall of 2005.

Project Description

Background

In the mid-1990s, the University of Idaho acquired the Analog Model Power

System (AMPS) from Idaho Power for educational and research use [1]. Idaho Power is

an electric utility provider to about 883,000 people in southern Idaho and eastern Oregon

[2]. The AMPS system was originally constructed by Idaho Power to test relays and

breakers for equipment and system protection. In addition, the AMPS system was used

to model part of Idaho Power’s own transmission and distribution system. Over the years,

the University of Idaho has made several modifications to the donated system to

incorporate the following:

1. A fault matrix, in which three faults can be placed on the system either simultaneously or in an evolving manner. 2. The ability to load impedance faults. 3. SEL (Schweitzer Engineering Laboratories) relays for system protection.

The AMPS is currently located in the basement (room G10) of the Buchanan

Engineering Laboratory (BEL) on the University campus in Moscow, Idaho. It has been,

and still is a valuable tool for students and researchers by providing insight into the

workings of a power transmission system. The capacity of this system is continually

being increased by the addition of subsystems such as the current topic of interest, a

flywheel voltage sag correction system.

A flywheel is well known for efficient mechanical energy storage in its rotating

momentum, which can then be applied to a generation source that converts mechanical

energy into an electrical energy output to a system. Thus, a flywheel is expected to be a

practical alternative power source for inline (series) voltage sag correction for the AMPS.

The energy storage system should be useful in keeping the voltage on the system within

1

given tolerances and protecting equipment that is required to operate within a very

narrow voltage range. Furthermore, it could provide a model for observation and

analysis, which would be valuable as an educational and research tool.

Satish Samineni, a past graduate student at the University of Idaho, started the

flywheel sag correction project. He began the project with a model simulation that

showed that the project was feasible using PSCAD computer simulation. Our project is

to actually build and implement in hardware the model simulation that Satish completed

for his Master of Science [3]. However, some modifications to his design must be made

since the simulation design was for a shipboard power system rather than the AMPS

system [4].

Problem Statement

The AMPS currently does not have the ability to correct for voltage sags on the

model system.

Objectives

To further improve the capabilities of the AMPS, a flywheel voltage sag

correction system will be interfaced to AMPS to automatically correct for voltage sags.

Constraints

The system will only have to account for balanced sags (equal voltage drops on

all three phases), however it must be able to be upgraded for unbalanced sag correction in

future modifications. It will also have to be able to run continuously, correcting for sags

when they occur and keeping energy in the flywheel the rest of the time. The system

must be capable of bi-directional power flow in for switching from running the motor

from AMPS to putting voltage on AMPS to correct for a voltage sag.

2

The system will have a maximum voltage it can provide to the AMPS. Because

our flywheel will be losing energy when correcting sags, it will not be able to correct for

a sag indefinitely. After correcting for a sag, the flywheel will have to be brought up to

rated speed again. This will limit how fast we can correct for cascaded sags. There are

no specific size or weight requirements that have to be met.

Functional Specifications

The system will interface a flywheel to the AMPS. It will automatically correct

for voltage sags that can be initiated in the AMPS. This will further facilitate the learning

experience for students and enable them to experiment with different technologies in the

power industry.

Solution

The system will require two AC/DC converters. One will act as a rectifier and the

other as an inverter depending on which way the power is flowing. Each converter

requires six Integrated Bipolar Gate Transistors (IGBTs) and a Digital Signal Processor

(DSP). Tier Electronics will probably provide the converters.

Figure 1 - Signal Flow for a Detected Sag

3

The AMPS will start and keep the flywheel spinning. Space vector Pulse Width

Modulation (PWM) will be used to control the motor while for spinning the flywheel.

When a sag is detected, the flow of power will be reversed, and the flywheel will be

turning the motor as a generator for the duration of the sag. During this time, the

converters will be using sine wave PWM to control how much voltage we put on the

AMPS. When the sag is over, the system will return the default setting of the motor

spinning the flywheel.

Status

Currently, the simulation by Satish of the entire sag correction system is designed

and working. Team Hydrofly also has working simulations of space vector PWM and the

sag detector. Capacitor shorting bars are also designed, tested, and working in order to

make sure the capacitors are not holding a charge while being worked on, as seen in

Picture 3 in Appendix B.

Method of Solution

Technical Description

The system will interface the flywheel to the AMPS using two AC/DC converters

using IGBTs with anti-parallel diodes as seen in Figure 4 in Appendix A. Each converter

will have a DSP that will control the PWM and monitor system voltages and currents

closest to the side it is in control of. When correcting for a sag, the correction voltage

will be kept in phase with the AMPS voltage by using a phase lock loop (PLL).

The system will use two forms of PWM, depending on which way the power

needs to be flowing. Each form of PWM is used to control the voltage of the converters.

Space vector will be used for providing voltage to the motor because it uses the DC bus

4

more efficiently, gives fewer harmonics, and it is specifically used in variable speed drive

applications. The speed of the motor needs to be varied in order to control the energy

flow into the flywheel. Sine wave PWM will be used to control the correction voltage

because it has a higher switching frequency and it can be operated independently on each

phase. This is important for future modification of this design.

When the system is spinning the flywheel, board 2 will be a rectifier and board 1

will be an inverter (Figures 5&6 in Appendix A). Board 1 will be using space vector

PWM to control the motor. This scheme will be using different combinations of eight

basic vectors, spaced 60 degrees apart, to form our desired vector (Figure 2 below).

The DSPs will have to be reprogrammed to work for the design. Board 1’s DSP

will be programmed with space vector PWM. Board 2’s DSP will be programmed with

sine wave PWM. The sine wave will be generated with a look-up table and the triangle

wave will be generated with a counter. A phase lock loop will be used to keep the

injected voltage needs in phase with the AMPS voltage.

Theoretical Basis

A voltage sag is a short term drop in voltage. A drop of only 10% can cause

sensitive loads to misoperate or shut down completely [3]. Process and fabrication plants

take a lot of time to restart after shutting down completely. They lose production time

and therefore lose money.

Loads that draw large starting currents being connected into the system or

electrical faults are the most common causes of voltage sags [3]. A flywheel can correct

voltage sags in a system so that these critical loads never see the sag.

5

A flywheel is well known for efficient mechanical energy storage in its rotating

momentum, which can then be applied to a generation source that converts mechanical

energy to electrical energy output to a system. Thus, a flywheel is expected to be a

practical alternative power source for inline (series) voltage sag correction for the AMPS.

The energy storage system should be useful in keeping the voltage on the system within

given tolerances and protecting equipment that is required to operate within a very

narrow voltage range. Furthermore, it could provide a model for observation and

analysis, which would be valuable as an educational and research tool.

A flywheel stores an amount of energy proportional to the moment of inertia and

the rotational speed squared.

(1.1)

The moment of inertia for our flywheel is:

(1.2)

where m is the mass of a flywheel and r is the radius.

Once the flywheel is spinning, very little energy is required to keep it spinning.

Space vector PWM is used to control the speed of the induction motor, which in turn

controls the energy going to the flywheel.

Space vector PWM involves making an abc to dq0 transformation using the

following matrix where Sas, Sbs, and Scs add to be the three-phase vector, and Sqs, Sds,

S0s is the two phase equivalent.

(1.3)

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Different combinations of the eight vectors created by switching are used to make

the desired vector as seen in Figure 2. This desired vector represents the three-phase

voltage we need on the motor.

Figure 2: Space Vector PWM Vector Diagram

Sine wave PWM involves overlapping a sine wave with a triangle wave and

running them through a comparator. Whenever the sine wave has a higher value than the

triangle wave, the comparator outputs a logic one. If the triangle wave has a higher value

than the sine wave, the comparator outputs a logic zero. This makes a variable width

square wave, as seen in Figure 3 below, with an underlying sine component.

Figure 3: PWM Variable Width Example

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Test Plan

Measure the DC bus voltage with a multimeter and compare to board measurement.

Verify switching sequence with an oscilloscope. Measure the flywheel speed with a tachometer and compare to position encoder

measurement. Measure the frequency of SVPWM and SPWM with a frequency counter or

oscilloscope. Calculate the energy of the flywheel using measured data. Verify the maximum sag correction duration of 1.5 s. Verify that 37 % sag is corrected for its duration to 0.95 per unit with an

oscilloscope. Verify a sag response within 2 cycles with an oscilloscope. Verify that a 4 sample per cycle rate can initiate a sag correction within set

response time. Verify functionality of the PLL. Display results from converter (likely using HyperTerminal through RS232

communication). Verify functionality of the sensors (LEMS).

8

Bibliography

[1] AMPS User Guide. University of Idaho. Moscow, ID. [Online]. Available:

http://www.ece.uidaho.edu/hydrofly/documents/AMPS_User_Guide.pdf

[2] About Us. Idaho Power Company. [Online]. Available:

http://www.idahopower.com/aboutus/

[3] S. Samineni. Modeling and Analysis of a Flywheel Energy Storage System for

Voltage Sag Correction. University of Idaho. Moscow, ID. [Online]. Available:

http://www.ece.uidaho.edu/hydrofly/documents/Flywheel/Satish%20Thesis.pdf

[4] S. Samineni, B. Johnson, H. Hess, and J. Law, “Modeling and Analysis of a Flywheel

Energy Storage System with a Power Converter Interface,” presented at the

International Conference on Power Systems Transients (IPST), New Orleans,

USA, 2003.

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Appendices

Appendix A: Figures

Appendix B: Pictures

Appendix C: Specifications

Appendix D: Bill of Materials

Appendix E: Parts Ordered

Appendix F: Individual Reports

Appendix A: Figures

Figure 4: Overall Design Schematic

A-1

Appendix A: Figures

Figure 5: AMPS Side Switch Diagram

Figure 6: Induction Motor Side Switch Diagram

A-2

Appendix B: Pictures

Picture 1: AMPS System Board

Picture 2: Flywheel

B-1

Appendix B: Pictures

Picture 3: Capacitor Shorting Bars

Picture 4: IM Nameplate Data

B-2

Appendix C: Specifications

Table I: Design Specifications

The AMPS 3 Phase, 208 V, 60 Hz, 5 kVA Series Transformers 240V/240V, 7.5 kVA LC Filters To be determined DC Bus Voltage 450 V max DC Bus Capacitance 2 x 250V 1000 µF (grounded between the 2) Flywheel Inertia 0.911 kg-m2 Induction Machine Ratings 208 V, 32.6 A, 60 Hz, 10 hp, 4 pole SVPWM Switching Frequency 1 kHz SPWM Switching Frequency 10.8 kHz Maximum Sag Correction Duration 1.5 s Maximum Magnitude of Sag Correction 37% (or 63% of rated) @ 0.95 per unit Sag Correction Response Time Within 2 cycle Magnitude Sag Correction Tolerance Within 0.95pu ± 0.05pu of rated 2 Tier Converters 6 IGBTs 75A, JTAG (software not

included), etc. DSP Program Language C with inline ASM from TI Sample Rate for Voltage Correction 4 samples per cycle Flywheel Speed Sensor Position Encoder

C-1

Appendix D: Bill of Materials

Table 2: Bill of Materials

Quantity Item Manufacture Unit Price

SubTotal

2 AC/DC Converters Tier Electronics $1,700 $3,400 3 Single Phase Transformer Hammond Power

Solutions $80 $240

1 Design Poster/Report Binding

UI Commons Copy Center

$30 $30

1 DSP Software Texas Instruments $250 $250 6 Voltage Transducer Digi-Key Corporation $37 $222 6 Current Sensor Digi-Key Corporation $21 $126 2 1000 µF Capacitors Digi-Key Corporation $8 $16 1 Miscellaneous Unknown $541 $541

Total $4,825

D-1

Appendix E: Parts Ordered

Low Voltage DC to AC Converter

Table 3: Known Converter Data Item Value Fan Voltage 48V DC Max DC Bus Voltage 700V IGBT rated current 75A rms Board Max Current 25A rms Power to board 5V DC Board Max Voltage ≥208V line-to-line

Have ordered two of these units to make an AC/AC system as of May 12, 2005.

E-1

Appendix F: Individual Reports

Individual Report: Gavin Abo My main contributions for the semester are as follows: -Developed and coded the web site for the Hydrofly Team. -Presented during the Design Review. -Communicated with Tier Electronics to receive quotes for the Flywheel design. -Implemented animation for the SVPWM for the Prototype Demonstration. -Scheduled rooms for sponsor meetings and meeting with Satish. -Took care of the purchasing of the DC/AC converters from Tier Electronics. -Helped write the final report for the Flywheel Team. Minor contributions for the semester are as follows: -Helped with writing the White Paper. -Helped with writing the Proposal. -Helped with preparing the Design Review Presentation. -Helped with creating simulations for the Prototype Demonstration.

F-1

Appendix F: Individual Reports

Individual Report: Nate Stout

This semester I contributed to the following aspects of the design project -Helped with writing of the white paper, proposal, progress reports, funding proposal, and final report. -Helped with the Simulation of the sag detector and the SVPWM for prototype demonstration. -Helped with writing and presentation of the Design presentation. -Obtained keys to BEL G10 and GJL Rm. 102. -Contacted Applied Power Systems about driver boards. -Obtained Business Card for Bruce Smetana of Isothermal Systems Research, INC. -Helped to troubleshoot Fuel Cell problems and bring it back up to operating condition.

F-2

Appendix F: Individual Reports

Individual Report: Nathan Thomas

This semester I worked on the following:

• Drawing the overall circuit diagram • Programming and animating a general space vector model in MathCAD • Programming the sag detection model in Simulink • Writing the proposal, final report, and weekly progress reports • Writing the design presentation • Contacting people about getting a AC/DC converter • Developing Specifications for AC/DC converters

F-3