epri tr1002159

214
Development of a New Multilevel Converter-Based Intelligent Universal Transformer: Design Analysis Technical Report L I C E N S E D M A T E R I A L WARNING: Please read the License Agreement on the back cover before removing the Wrapping Material.

Upload: elsaorduna

Post on 09-Nov-2015

249 views

Category:

Documents


3 download

DESCRIPTION

Reporte

TRANSCRIPT

  • Development of a New MultilevelConverter-Based Intelligent UniversalTransformer: Design Analysis

    Technical Report

    LI

    CE

    NS E D

    M A TE

    RI

    AL

    WARNING:Please read the License Agreementon the back cover before removingthe Wrapping Material.

  • EPRI Project Manager F. Goodman

    EPRI 3412 Hillview Avenue, Palo Alto, California 94304 PO Box 10412, Palo Alto, California 94303 USA 800.313.3774 650.855.2121 [email protected] www.epri.com

    Development of a New Multilevel Converter-Based Intelligent Universal Transformer: Design Analysis

    1002159

    Final Report, March 2004

  • DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

    (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR

    (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT.

    ORGANIZATION THAT PREPARED THIS DOCUMENT

    EPRI PEAC Corporation

    Enertronics, Inc

    ORDERING INFORMATION Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 Willow Way, Suite 278, Concord, CA 94520, (800) 313-3774, press 2 or internally x5379, (925) 609-9169, (925) 609-1310 (fax).

    Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc.

    Copyright 2004 Electric Power Research Institute, Inc. All rights reserved.

  • iii

    CITATIONS

    This report was prepared by

    EPRI PEAC Corporation 942 Corridor Park Blvd. Knoxville, TN 37932

    Principal Investigators A. Maitra A. Mansoor B. Vairamohan

    Enertronics, Inc 2204 Hardwick Street Blacksburg, VA 24060

    Principal Investigator J. Lai

    This report describes research sponsored by EPRI.

    The report is a corporate document that should be cited in the literature in the following manner:

    Development of a New Multilevel Converter-Based Intelligent Universal Transformer: Design Analysis, EPRI, Palo Alto, CA: 2004. 1002159.

  • v

    REPORT SUMMARY

    This report provides a comprehensive design analysis of a high-voltage multilevel converter-based intelligent universal transformer (IUT). The proposed IUT design includes back-to-back, interconnected, multi-level converters coupled to a switched inverter circuit via a high-frequency transformer. The input of the universal transformer can be coupled to a high-voltage distribution system such as 4.16-kV with the use of existing high-voltage silicon insulated-gate-bipolar transistor (IGBT) devices, and the output of the universal transformer can be coupled to low-voltage applications such as 240/120 V.

    Background A significant opportunity exists to replace conventional transformers with a more sophisticated multi-functional device, the intelligent universal transformer. This approach would radically alter the way electric utilities serve their respective customers and expand the capabilities of a distribution transformer from primarily a voltage-transformation device to an integrated electrical customer interface. The IUT could broaden traditional service offerings, provide key operational benefits, satisfy a myriad of customer requirements for power quality, and at the same time provide functionalities for advanced distribution automation.

    Objectives To provide a comprehensive design analysis to verify the functionality of a high-voltage

    multilevel converter-based IUT design that uses high voltage power electronics To estimate the efficiency, cost, and reliability of the proposed design To refine the long-term roadmap for the development of the universal transformer with better

    cost estimates and schedule for development work in 20042007.

    Approach The design analysis work presented in this report is a part of a multi-year effort to develop a high-voltage, multilevel converter-based IUT based on an all-solid-state approach. The project team built on the work done in 2002 to assess the feasibility of replacing current distribution transformers with the IUT. The team selected circuit topology and active and passive components for the power stage and used simulations to evaluate the ability of the power stage design to provide the desired functionalities. The team estimated the cost of the proposed system based on existing component costs and estimated component cost for different degrees of market penetration and estimated system reliability using industry standard reliability methods. They developed cost estimates and a schedule for development work in 20042007 and identified the key functionalities and criteria that an IUT must meet to gain market acceptance.

  • vi

    Results While there are inherent risks in the development of the IUT, its potential is enormous. As compared to the conventional copper-and-iron based transformer, the high-voltage multilevel solid-state transformer will reduce transformer size and eliminate oil, aid in implementing standardized transformer designs, enhance power quality, and increase functionality. Enhanced functionality and flexibility easily justify its greater cost. The IUTs power electronics can be designed with modularity and expandability; and, since the cost of power semiconductor and associated controls has exponentially decreased over the past few decades, the IUTs economics can be expected to improve with time and maturation. The thorough design and performance analysis of the IUT design provided in this report shows which of the functional specifications can be achieved and what will be the tradeoffs in cost, efficiency, and reliability to achieve additional functionalities. The report details the power electronics circuits and associated open-loop and closed-loop controller designs that are required in an IUT to enhance transformer functionality. The report presents the IUT power stage design in detail, an AutoCAD drawing, specifications based on a designated KVA and input/output voltage range, an IUT reliability assessment, modeling and simulation results for IUT functionality verification, and performance verification of both power stage and controller designs. The report provides a detailed functional specification, written in a technology-neutral manner so as to provide designers with maximum flexibility in achieving desired functionalities. At a minimum, the functionality of the proposed design should meet the basic functionality of a conventional distribution transformer in terms of its capability for voltage transformation and provide operational benefits in terms of standardization of distribution transformers with respect to input/output voltage and kVA rating. This report describes in detail the possible phases for design and development of the proposed IUT technology along with schedules, important milestones, and budgets estimates for development work in 20042007. The economic viability of the IUT is also evaluated in this report.

    EPRI Perspective Pressures are increasing on distribution system operators to provide a higher-quality and more reliable product on demand when customers need it and at an acceptable price point. To respond to these needs, EPRI has a body of work under way to advance distribution automation. The IUT is a cornerstone device of advanced distribution automation. By integrating a smart device into the transformer, the main delivery point for most customers, a distribution utility can broaden its traditional service offerings, realize key operational benefits, and satisfy a myriad of customer requirements for improved power quality.

    Keywords Distribution transformer Intelligent universal transformer Power quality Power electronics High voltage multi-level converter Solid-state designs Semiconductor devices

  • EPRI Licensed Material

    EXECUTIVE SUMMARY

    Project Overview This report describes the findings of the research performed to assess the requirements for a high-voltage multilevel converter-based intelligent universal transformer (IUT). A high-voltage multilevel converter-based IUT was proposed for the next-generation distribution transformer (see Figure 1).

    S a1

    S a2

    S a3

    S a4

    Sc1

    Sc2

    Sc3

    Sc4

    Low-voltage IGBT based inverter 8 kV rms 11.3 kV pk

    L f

    S b1

    S b2

    S b3

    S b4

    0

    6.6kV

    C2

    C1

    6.6kV

    v in

    i a

    Lo

    Co

    Lac

    Cac

    Sr13

    Sr14

    Sr11

    Sr12

    8 kV16.6 kVA

    Lf

    6.6 kVSr11

    Sr12

    Sr13

    Sr14

    Sr23

    Sr24

    Sr21

    Sr22

    Sr21

    Sr22

    Sr23

    Sr24

    Si3

    Si4

    Si1

    Si2

    Si1

    Si2

    Si3

    Si4

    -6.6 kV

    0 120/240 V60 Hz

    120V400 Hz

    1r 1i2

    3 45

    6

    7

    IsolatedDC/DC

    converter48 Vdc+

    _

    A 3-Level Scheme

    A 5-Level Scheme

    Figure 1 Proposed Multi-Converter-Based Intelligent Universal System

    vii

  • EPRI Licensed Material

    The design analysis work presented in this report is a part of a multi-year effort to develop the IUT based on the all-solid-state approaches. It builds on the work done in 2002 to assess the feasibility of replacing current distribution transformers with a more sophisticated device, the IUT. The scope of last years activity was to assess the requirements for a next-generation universal transformer, identify the application areas, and evaluate the economic and technical considerations for different technologies and design options for a universal transformer. Also, it was identified that use of high-voltage (HV) power electronics in designing the primary side of an all-solid-state transformer offers significant advantage in reliability and cost over low-voltage (LV) designs.

    As compared to the conventional copper-and-iron based transformer, the HV multilevel solid-state transformer will not only reduce transformer size and eliminate oil but also will enhance the power quality performance and increase the functionality. Conceptually, the following functions that do not exist in the conventional transformer can be obtained with the proposed IUT circuit:

    Identifying Opportunities to Add Value Using All-Solid-State Transformer Designs

    While the risk involved in an all-solid-state approach could be much higher, the potentials are also enormous. Motivations for possible market transformation using all-solid-state transformer designs include:

    Ability to Provide a Wide Range of Services

    The new approaches based on innovative, multi-functional, and modular transformer designs can be used to expand the capabilities of a conventional distribution transformer from primarily a voltage-transformation device to an integrated electrical customer interface that has the capability to meet the customers requirement and is therefore a viable market opportunity. This pioneering approach has tremendous market advantages for the industry in that utilities can provide not only the traditional ac service and cater to power quality and reliability needs for specific customers, but also can create new service opportunities by providing whatever service the customer desires (dc power, high-frequency ac power, sag protection, harmonic filtering, ability to convert single-phase service to three-phase for powering certain types of equipment, and so on).

    1. Opportunity to provide new service opportunities The development of a universal transformer will provide an opportunity to offer dc service, high-frequency ac service, ability to convert single-phase service to three-phase for powering certain types of equipment, and premium power services with enhanced power quality and reliability tailored to individual customer requirement. Opportunities for premium-quality service will be mainly targeted towards industrial and high-end commercial customers. Options for dc requirement currently exist primarily in the Internet and telecommunication sector, with potential widespread application of dc because of the proliferation of digital loads.

    2. Opportunity to meet customers power quality and reliability requirements A universal transformer is proposed to use modern power electronics technology to serve as the energy buffer in between the source and load to avoid direct impact from either load-to-source or

    viii

  • EPRI Licensed Material

    source-to-load. In addition, with the flexibility of power electronics switching, it also has the potential to meet enhanced power quality and reliability tailored to individual customer requirement such as voltage sag compensation, instantaneous voltage regulation, harmonic compensation, outage compensation, capacitor switching protection, single-phase protection, and options for energy storage.

    Ability to Provide Improved Operational Benefits

    Utilities need to make long-term capital investments in all-solid-state-based transformer technologies that will completely change distribution transformer design to achieve additional operational benefits such as:

    1. Opportunity to implement standardized transformer design There are fixed and variable costs in maintaining an inventory of distribution transformers. One of the primary functional specifications for the universal transformer is standardization of product classes compared to the existing practice based on multiple kVA size, primary voltage, and secondary voltage. Realization of this primary functional specification through the development of a solid-state distribution transformer should result in significant reduction in inventory cost. It is possible to significantly reduce inventory costs by introducing standardized transformer designs.

    2. Opportunity to considerably reduce the size and weight of magnetic components As compared to the conventional copper-and-iron-based transformer, the disclosed, multi-level, solid-state transformer contains only high-frequency AC components. The resulting core size is small as compared to that which is designed for 60-Hz operations.

    3. Opportunity to dramatically reduce environmental concerns by introducing designs that do not use mineral oil or other liquid dielectrics Handling and disposal of transformer oil is definitely costly to utilities. Moreover, leak and spill cleanup (in terms of cost) is another additional large-ticket item. The universal transformer design will eliminate the need for oil, which is one of primary environmental issues related to oil-immersed transformer design. The use of oil-free universal transformers will portray a positive image in the community and also significantly reduce environmental risk and regulatory exposure.

    4. Opportunity to provide reactive support The universal transformer designs will have the ability to dynamically compensate for reactive power requirements of the load in order to minimize the effect of flicker from rapidly fluctuating loads such as arc furnaces, rolling mills, and so forth. In addition, the design should have the capability to correct power factor when applied in the power-factor-correction mode.

    5. Opportunity to provide other advanced distribution automation functions and thereby reduce operations and maintenance costs The operations-and-maintenance (O&M) cost reduction are potentially achievable with a universal transformer through improved communication capabilities by adopting the UCATM communication architecture for the universal transformer. Advanced distribution automation provides opportunities to develop new applications for condition monitoring and asset management purposes. One potential area is monitoring of distribution transformers in order to minimize unnecessary maintenance and replacements and thus enhance network operation efficiency. The intelligent universal transformer could have the ability to act as a sensor of voltage, current, and power factor; an

    ix

  • EPRI Licensed Material

    aid in real-time voltage regulation; and a provider of advanced distribution automation functions. Approximately 13% of the total distribution resource usage is for feeder and line maintenance (see [12]). Assuming a 2% reduction in the total O&M cost, universal transformer technology can potentially save $1.3 million for a utility with total annual distribution cost of $500 million.

    However, all new products face barriers to entry, particularly when competing with entrenched, mature technologies. The disadvantages of the IUT technology include:

    Need suitable high-voltage, low-power IGBT or alternate device to be competitive with conventional transformers cost

    LV-IGBTs are mature products. Using LV-IGBTs, a significant reduction in price as a result of mass product would not be possible.

    Will need five to 10 years for product development and field application. Lower efficiency than the conventional transformer. Need more device and components for design. Risky in terms of reliability. Requires power electronics on the high-voltage side. Higher EMI than conventional transformer. Lack of customer acceptance and willingness to try new untested technologies Project Objectives It is quite clear that, even with the higher risk involved in all-solid-state designs, the preferred research path should aim for the quantum leap in distribution transformer technology that is possible with the all-solid-state approach rather than an incremental improvement achieved through combination of existing technologies. The overall objective of this years activity was to leverage previous years research and provide a comprehensive design analysis to verify the functionality that can be achieved with the conceptual high-voltage multilevel converter-based IUT design and estimation of efficiency, cost, and reliability of such a proposed design.

    Conceptually, the solid-state power-conversion-based distribution transformer will be much more expensive than the conventional one. However, the electronics can be designed with modularity and expandability. When considering the enhanced functionality and the flexibility, the added cost can be easily justified. In fact, the cost of power semiconductor and associated controls has exponentially decreased over the past few decades. Whether or not the IUT can be economically viable is also evaluated in this report.

    This preliminary research is the first step before embarking on the development of the hardware and associated controls for a prototype universal transformer. This is the most critical stage because the commitment to a certain design topology will ultimately lead to hardware and controller development and lab prototype. Critical issues that are addressed in this report, include:

    x

  • EPRI Licensed Material

    Selection of the circuit topology for the power stage Selection of active devices and passive components and calculation for semiconductor device

    voltage and current ratings and inductor and capacitor values based on certain assumptions or requirements on ripple, ride-through, or control response

    Simulation of the performance of the power stage using industry-accepted design packages for power electronics design analysis

    Verification of basic functions, such as voltage balancing for a multiple-level stack, with hardware experiments

    Estimation of the cost of the proposed system based on existing component costs and estimated component cost for different degrees of market penetration

    Estimation of the system reliability using industry standard reliability calculation methods such as MIL-HDBK-217F Reliability Prediction of Electronic Equipment

    Determination of design packaging and thermal management systems Evaluation of the design with respect to its ability to meet the different desired functionalities Development of a more refined EPRI long-term roadmap (see Figure 2 through Figure

    5) for the development of the intelligent universal transformer with better cost estimates and schedule for the development work in 20042007

    Identification of the key functionalities and criteria that need to be met in order for this technology to gain market acceptance

    xi

  • EPRI Licensed Material

    Figure 2 Projections of Possible Phases and Individual Tasks for Design and Development (20042007) of the Proposed HV Multi-Level-Converter-Based IUT Design

    xii

  • EPRI Licensed Material

    Jan 2004 Sep 2008Jan 2005 Jan 2006 Jan 2007 Jan 2008

    1/04 - 4/ 04Initiate forum to select

    potential vendors

    4/04 - 6/054.16kV IUT Bench Model

    - Completed

    1/06 - 12/ 0613.8kV IUT Alpha Model

    Field Prototype- Completed

    10/06 - 05/0713.8kV, 5 IUT Beta Models

    Field Prototype- Completed

    BENCH MODELDevelopment Schedule

    FIELD PROTOTYPEDevelopment Schedule

    05/07 - 11/07IUT Field Installation &

    CommissioningField Data Collection

    - Start

    1/07 - 05/07Site Selection & Investigation

    Field Demonstration- Start

    11/07 - 08/08IUT Field Data Collection

    & Analysis

    FIELD Deployment Schedule

    1/0613.8kV IUT

    Field Prototype- Start

    Figure 3 EPRIs Long-Term Roadmap for the Proposed HV Intelligent Universal Transformer Development Schedules and Important Milestones

    Total Cost of Design

    $0

    $500

    $1,0

    00

    $1,5

    00

    $2,0

    00

    $2,5

    00

    $3,0

    00

    $3,5

    00

    $4,0

    00

    $4,5

    00

    $5,0

    00

    $5,5

    00

    $6,0

    00

    $6,5

    00

    $7,0

    00

    $7,5

    00

    3-level power circuit

    HV-gate drive

    Gate drive interface

    Sensor conditioning

    60-Hz inverter

    400-Hz inverter + 48 V dc/dc converter

    HF transformer

    DSPs

    Auxiliary power supply

    Heat sink and packing

    Total

    (Cost Estimates)

    Cost estimate for the 3-level 4.16-kV IUT Cost estimate for the 5-level 15-kV IUT

    $4,614 $7,084

    Figure 4 Overall Cost Estimate in the 3-Level 4.16kV and 5-Level 15kV IUT

    xiii

  • EPRI Licensed Material

    Figure 5 EPRIs Long-Term Roadmap for the Proposed HV Intelligent Universal Transformer Detailed Budget Estimate for Development Work in 20042007

    xiv

  • EPRI Licensed Material

    ACKNOWLEDGMENTS

    The authors wish to acknowledge the support of Dr. Siriroj Sirisukprasert of Virginia Tech for his contributions and assistance throughout this project. In particular, we would like to thank Dr. Allen R. Hefner and Dr. Ranbir Singh of National Institute of Standards and Technology (NIST) for their insightful thoughts and comments on the development of high-voltage semiconductor devices.

    xv

  • EPRI Licensed Material

    xvii

    CONTENTS

    1 INTRODUCTION ....................................................................................................................1-1 Project Scope........................................................................................................................1-1 The Good Old Distribution Transformer..............................................................................1-3 Why Look at Anything Else? .................................................................................................1-5 Can the Opportunity to Offer a Wide Range of Services Be Realized Through Incremental Improvements in Conventional Transformers?..................................................1-8

    Short-Range Market Driver Improved Power Quality and Reliability .............................1-9 Short-Range Market Driver Powering Three-Phase Loads From a Single-Phase Service............................................................................................................................1-12

    Woodworking Workshops ..........................................................................................1-12 Metalworking Workshops...........................................................................................1-12 Vehicle Servicing .......................................................................................................1-12 Farming......................................................................................................................1-12

    Mid-Range Market Driver DC Distribution ...................................................................1-12 Long-Range Market Driver High-Frequency AC Power ..............................................1-14 Expanding Traditional Service Offering Through a Sophisticated Device ......................1-14

    Can the Opportunity to Offer Improved Operational Benefits Be Realized Through Incremental Improvements in Conventional Transformers?................................................1-16

    Operational Benefits Reduced Inventory Through Standardization ............................1-17 Operational Benefits Advanced Distribution Automation.............................................1-19 Operational Benefits Environmental ............................................................................1-20 Operational Benefits Reactive Compensation.............................................................1-20

    Emerging Power Electronic-Based Transformer Designs ...................................................1-20 Window of Opportunity for All-Solid-State Transformer Designs Using HV, Low-Current Power Electronic Switches ................................................................................1-24

    Organization of the Report ..................................................................................................1-25

  • EPRI Licensed Material

    xviii

    2 DESIGN OF THE POWER STAGE ........................................................................................2-1 2.1 Basic Principle of Diode-Clamp Multilevel Converter ......................................................2-1 2.2 Design Calculation for the Front-End AC/DC Multilevel Converter .................................2-5

    A. Selection of DC Bus Capacitor Voltage Level..............................................................2-6 B. Size of DC Bus Capacitor ............................................................................................2-7 C. Size of Boost Inductor..................................................................................................2-7

    2.3 Design Calculation for the DC/DC Multilevel Half-Bridge Converter ...............................2-8 A. Transformer Design .....................................................................................................2-8 B. Output Filter Capacitor Design.....................................................................................2-9 C. Output Filter Inductor Design .....................................................................................2-10

    2.3 Design Calculation for the DC/AC Inverter Output Filters .............................................2-12 A. Determine Inverter Output Filter Inductor...................................................................2-16 B. Determine Inverter Output Filter Capacitor ................................................................2-17

    2.4 Design Calculation for the 400-Hz Inverter Output Filters.............................................2-17 A. Determine Inverter Output Filter Inductor...................................................................2-17 B. Determine Inverter Output Filter Capacitor ................................................................2-18

    2.5 Power Stage Design for Three-Level 4.16-kV System Active Front-End Converter .....2-19 A. Selection of the DC Bus Capacitor Voltage Level......................................................2-19 B. Size of DC Bus Capacitor ..........................................................................................2-19 C. Size of Boost Inductor................................................................................................2-20

    2.6 Physical Layout of the IUT ............................................................................................2-20 A. Power Stage Layout...................................................................................................2-20 B HV-IGBT Gate Drive and Interface Layout..................................................................2-21 C. Physical Layout of the Complete IUT System............................................................2-22

    3 MODELING AND SIMULATION FOR PERFORMANCE VERIFICATION ............................3-1 Active Front-End Control Design...........................................................................................3-1

    A. Open-Loop Transfer Functions of AFE ........................................................................3-2 B. Feedback Control Design for AFE ...............................................................................3-4 C. Current Loop Compensator Design .............................................................................3-4 D. Voltage Loop Compensator Designs ...........................................................................3-7 E. PWM Generator and DC Link Voltage Balancing Techniques...................................3-10 F. Simulation Results for the Three-Level Active Front End Converter ..........................3-14

    3.2 Modeling and Simulation of the 60-Hz DC/AC Inverter .................................................3-15

  • EPRI Licensed Material

    xix

    A. Behavior Circuit Modeling ..........................................................................................3-16 B. Inverter Controller Design With a Simple Voltage Loop.............................................3-17 C. Inverter Control Design With Feed-Forward Voltage Loop ........................................3-18

    3.3 Performance Verification for the Entire IUT System......................................................3-20 A. Voltage-Sag Compensation .......................................................................................3-21 B. Outage Compensation ...............................................................................................3-25 C. Instantaneous Voltage Regulation Under Load Transients .......................................3-27 D. Effects of Unbalanced Load, Overload, and Lagging Power Factor ..........................3-28 E. Effects of Nonlinear Loads .........................................................................................3-30 F. Performance Verification for the 400-Hz Inverter .......................................................3-32

    3.4 Modularization for Three-Phase IUT Design .................................................................3-33

    4 RELIABILITY ASSESSMENT OF THE PROPOSED IUT SYSTEM......................................4-1 Background of Reliability Assessment ..................................................................................4-1 Quantifying Reliability............................................................................................................4-2 Various Reliability Standards ................................................................................................4-5

    MIL 217.............................................................................................................................4-5 Bellcore.............................................................................................................................4-5 NSWC (Naval Surface Warfare Center) ...........................................................................4-5 RDF 2000 .........................................................................................................................4-5 PRISM ..............................................................................................................................4-5

    Comparison of Different Standards .......................................................................................4-6 Reasons for Choosing MIL 217.............................................................................................4-7 Demerits of MIL 217..............................................................................................................4-8 Application of MIL 217 in Various Stages of IUT...................................................................4-8

    Design Stage ....................................................................................................................4-8 Lab Prototype Stage.........................................................................................................4-9 Final Production Stage .....................................................................................................4-9

    MIL 217 Analysis Methods ....................................................................................................4-9 Parts Count Analysis ........................................................................................................4-9 Part Stress Analysis .......................................................................................................4-10

    Reliability Analysis of the Proposed IUT Design Using Item Toolkit ...................................4-11 An Introduction to Item Toolkit........................................................................................4-13 Parameter Values Used in the Reliability Analysis.........................................................4-18

    Results of Reliability Analysis..............................................................................................4-21

  • EPRI Licensed Material

    xx

    ABB Power Electronics-Based Distributed Transformer .....................................................4-23 Comparison of Results of Both Designs..............................................................................4-26

    5 CIRCUIT DESIGN AND OVERALL COST ESTIMATION......................................................5-1 Active-Front-End Power Circuit .............................................................................................5-2 HV Gate Drive Circuit ............................................................................................................5-3 Gate Drive Interface ..............................................................................................................5-5 Sensor Conditioning Circuit...................................................................................................5-6 Inverter Circuit .......................................................................................................................5-7 Cost Estimate for the 400-Hz Inverter and DC/DC Converter.............................................5-10 Cost Estimate for the Entire IUT..........................................................................................5-10

    6 UNIVERSAL TRANSFORMER ROADMAP PLANNING.......................................................6-1 Opening New Frontiers for Next-Generation Transformers ..................................................6-1 Overall Roadmap Technology Characteristics How to Realize the Vision in the Future....................................................................................................................................6-6

    Following the General Trends to Support the Vision Direction.........................................6-6 Long-Term Research Timeline for Any All-Solid-State Design ......................................6-8

    Long-Term Research Timeline for any HV Multi-Converter-Based Intelligent Universal Transformer Design...............................................................................................................6-9

    Solid-State Designs Using HV Power Electronic Switches and Diodes A Key for Success ..........................................................................................................................6-10 The Unique Requirement of High-Voltage Low-Current Power Electronics for Distribution Transformer Application ..............................................................................6-13

    Silicon-Based HV-Insulated Gate Bipolar Transistors (IGBTs)..................................6-15 Basic Structure......................................................................................................6-15 High-Voltage Insulated Gate Bipolar Transistors ..................................................6-16 Cost Projection for Commercially Available Silicon-Based Insulated Gate Bipolar Transistor ..................................................................................................6-17

    Moving Beyond HV IGBT Why Wide-Band-Gap Semiconductor Materials and Why Not .....................................................................................................................6-20

    What are Wide Band Gap Materials?....................................................................6-20 Band Gap and Operating Temperature.................................................................6-22 Power Device Figure of Merit ................................................................................6-22 SiC Device Figure of Merit ....................................................................................6-23

    Cost Projection for SiC Devices.................................................................................6-25 IUT Development with Wide-Band Gap Material ..................................................6-26

  • EPRI Licensed Material

    xxi

    Critical Technology Areas...............................................................................................6-27 EPRIs Long-Term Roadmap for the Proposed HV Multilevel Converter-Based Intelligent Universal Transformer ........................................................................................6-28

    Phase I: Intelligent Universal Transformer Prototype Development, Testing, and Functionality Verification.................................................................................................6-31

    Major Deliverables for the Phase I.............................................................................6-37 Phase II: Intelligent Universal Transformer Field Prototype Development, Testing, and Functionality Verification..........................................................................................6-38 Phase III: Intelligent Universal Transformer Field Deployment, Field Unit Data Collection, and Functionality Verification........................................................................6-40 Phase IV: Intelligent Universal Transformer Technology Commercialization (2008-2010) ..............................................................................................................................6-42

    Development of Comprehensive Functional Specifications for the Proposed HV Multilevel Converter-Based Intelligent Universal Transformer ............................................6-42

    Design Guideline for Proposed IUT Based on Conventional Dry-Type Distribution Transformer Standards...................................................................................................6-42

    Usual Service Condition.............................................................................................6-43 Overload Requirement for IUT...................................................................................6-43

    Generic Specification......................................................................................................6-44 Standardization..........................................................................................................6-44 Power Quality and Reliability .....................................................................................6-44 Reactive Compensation.............................................................................................6-45 Communications ........................................................................................................6-45 Efficiency ...................................................................................................................6-45 Reliability ...................................................................................................................6-45 Environment...............................................................................................................6-46 Cost ...........................................................................................................................6-46

    Technical Specifications .................................................................................................6-46 Voltage and Current Waveforms Under Transient Conditions...................................6-49

    A. Load Step .........................................................................................................6-49 B. Voltage Sag ......................................................................................................6-50 C. Outage..............................................................................................................6-51 D. Nonlinear Load .................................................................................................6-52

    7 REFERENCES .......................................................................................................................7-1

  • EPRI Licensed Material

    xxiii

    LIST OF FIGURES

    Figure 1-1 Distribution Transformer ...........................................................................................1-4 Figure 1-2 Efficiency Trends Over Several Years for 75-kVA, Three-Phase Distribution

    Transformers......................................................................................................................1-6 Figure 1-3 Options for Power Quality Mitigation at Different Levels ........................................1-10 Figure 1-4 Cost of Power Quality Solution Versus Knowledge of Equipment Sensitivity.........1-11 Figure 1-5 Typical Powering Configuration for Telecommunication Equipment ......................1-13 Figure 1-6 Standard Delivery for Various Requirements Today ..............................................1-15 Figure 1-7 New Expanded Service Opportunity.......................................................................1-16 Figure 1-8 Importance of Standardization................................................................................1-17 Figure 1-9 Variations in Distribution Resource Usage .............................................................1-18 Figure 1-10 Realizing the High-Frequency AC Link Stage in a Design ...................................1-20 Figure 1-11 Simplified Schematic for Power Electronics-Based Transformer Design .............1-22 Figure 1-12 Closing the Device Design Loop A Key for Building Optimal Application-

    Specific Power Devices....................................................................................................1-25 Figure 2-1 Multilevel Diode-Clamped Converters: (a) Three-Level and (b) Five-Level..............2-2 Figure 2-2 Complete Circuit Diagram of a Three-Level IUT.......................................................2-3 Figure 2-3 Half-Bridge-Based Three-Level IUT .........................................................................2-4 Figure 2-4 Combination With Full-bridge 3/5-Level Converter and Half-Bridge 3/5-Level

    Inverter That Allows the Use of Semiconductor Devices With a Lower Voltage Blocking Level ....................................................................................................................2-5

    Figure 2-5 Simulated DC Bus Capacitor Voltage and Boost Inductor Current in Three Line Cycles.........................................................................................................................2-8

    Figure 2-6 Zoomed-in Simulated Line Current to Show the Inductor Ripple Content ................2-8 Figure 2-7 Physical Size of the Transformer Cores ...................................................................2-9 Figure 2-8 Rectifier Output Inductor Current Ripple ................................................................2-10 Figure 2-9 Simulation Diagram of the Three-Level DC/DC Converter .....................................2-10 Figure 2-10 Switch Control Signals for the Three-Level DC/DC Converter .............................2-11 Figure 2-11 Transformer Primary Voltage and Secondary Output Voltage Before and

    After the Filter Inductor.....................................................................................................2-11 Figure 2-12 Output Filter Capacitor Voltage and Inductor Current Waveforms .......................2-12 Figure 2-13 The DC/AC Inverter With Split Outputs ................................................................2-12 Figure 2-14 Sinusoidal PWM Method ......................................................................................2-13 Figure 2-15 Inverter Output Voltage Before and After Filtering ...............................................2-13

  • EPRI Licensed Material

    xxiv

    Figure 2-16 Harmonic Contents of the Inverter Output Voltage Before Filtering .....................2-14 Figure 2-17 Dual Modulation Method With Two Sets of Duty Cycles da and db .......................2-14 Figure 2-18 Inverter Output Voltage With Dual Modulation Method ........................................2-15 Figure 2-19 Harmonic Contents of the Dual Modulated Inverter Output Voltage Before

    Filtering ............................................................................................................................2-15 Figure 2-20 Simulated Inductor Current and Resistive Load Current ......................................2-16 Figure 2-21 Inverter Output Voltage Waveforms After Filtering...............................................2-17 Figure 2-22 Simulated Inductor Current and Resistive Load Current ......................................2-18 Figure 2-23 Inverter Output Voltage Waveforms After Filtering...............................................2-18 Figure 2-24 Verification of Capacitor Ripple Voltage ...............................................................2-19 Figure 2-25 Verification of Inductor Current Ripple..................................................................2-20 Figure 2-26 Three-Dimensional View of the Five-Level Half-Bridge Converter Power

    Stage................................................................................................................................2-21 Figure 2-27 Gate Drive Interface Board...................................................................................2-22 Figure 2-28 Physical Layout of the Complete Five-Level Half-Bridge Converter.....................2-23 Figure 3-1 Schematic of the Active Front End of IUT ................................................................3-1 Figure 3-2 Closed-Loop Control Block Diagram for AFE ...........................................................3-4 Figure 3-3 Bode Plot of the Open-Loop Transfer Function........................................................3-5 Figure 3-4 Phase Plot of the Open-Loop Control-to-Current Transfer Function

    Associated With Digital Delay ............................................................................................3-6 Figure 3-5 Bode Plots of Current Loop Gain..............................................................................3-7 Figure 3-6 Bode Plots of the Voltages Loop Transfer Function With Current Loop Closed.......3-8 Figure 3-7 Bode Plot of Voltages Loop Gain .............................................................................3-9 Figure 3-8 Three-Level Diode-Clamped Converter as an Active Front End Using Single-

    Pole, Triple-Throw Switches to Represent the Phase Legs.............................................3-10 Figure 3-9 State Machine of the Proposed PWM Technique...................................................3-11 Figure 3-10 PWM Generator Block Diagram ...........................................................................3-12 Figure 3-11 Flowchart of PWM Generator ...............................................................................3-13 Figure 3-12 Pulse Width Modulation as Function of Duty Cycle..............................................3-14 Figure 3-13 Unity Power Factor Is Maintained Regardless of Load Conditions.......................3-14 Figure 3-14 Regulated DC Link Voltage ..................................................................................3-15 Figure 3-15(a) Full-Bridge DC/AC Inverter Circuit ...................................................................3-16 Figure 3-16(b) Behavior Circuit Model of a Full-Bridge Inverter Circuit ...................................3-16 Figure 3-17 Control System of a Full-Bridge DC/AC Inverter With a Simple Voltage Loop.....3-17 Figure 3-18 Transient Response With Simple Voltage Control Loop.......................................3-18 Figure 3-19 The Full-Bridge Inverter Control System With Feed-Forward Control ..................3-19 Figure 3-20 Transient Response With Feed-Forward Voltage Loop Control ...........................3-20 Figure 3-21 Complete IUT for System Simulation ...................................................................3-21 Figure 3-22 AFE Converter Simulation Diagram Showing Step Functions for Voltage-

    Sag Simulation .................................................................................................................3-22

  • EPRI Licensed Material

    xxv

    Figure 3-23 Inverter Simulation Diagram Showing Fixed Load Under Voltage-Sag Operation .........................................................................................................................3-22

    Figure 3-24 Voltage-Sag Simulation Results ...........................................................................3-24 Figure 3-25 Responses of DC Bus Voltages and Input Current After Voltage Sag .................3-25 Figure 3-26 Outage Compensation Simulation Results...........................................................3-26 Figure 3-27 Responses of DC Bus Voltages and Input Current After Outage.........................3-27 Figure 3-28 Simulation for Instantaneous Voltage Regulation Under Load Transients ...........3-28 Figure 3-29 Simulation Under Unbalanced Load Condition.....................................................3-29 Figure 3-30 Simulation Results Under 100% Overload ...........................................................3-29 Figure 3-31 Harmonic Compensation ......................................................................................3-30 Figure 3-32 Time-Domain Voltage and Current Waveforms of a Nonlinear Load ...................3-31 Figure 3-33 Harmonic Contents of the Nonlinear Load ...........................................................3-31 Figure 3-34 Inverter With Open-Loop Control..........................................................................3-32 Figure 3-35 Inverter With Closed-Loop Control .......................................................................3-32 Figure 3-36 Transient Response for the 400-Hz Output ..........................................................3-33 Figure 3-37 Modular IUT Design for Three-Phase Connection................................................3-33 Figure 3-38 Simulated Three-Phase IUT Input Voltage and Current Under Load-Step

    Condition ..........................................................................................................................3-34 Figure 4-1 Typical Reliability Bathtub Curve.............................................................................4-2 Figure 4-2 Cumulative Distribution Failure Function F(t) for the Two Transformers ..................4-4 Figure 4-3 Schematic of Intelligent Universal Transformer Circuit Model ................................4-12 Figure 4-4 Block Diagram Representation of Intelligent Universal Transformer (IUT)

    Design ..............................................................................................................................4-12 Figure 4-5 Snapshot Showing Different Regions in the Item Toolkit .......................................4-14 Figure 4-6 Section of IUT Schematics Showing Block 1r ......................................................4-16 Figure 4-7 Dialog Window of Item Toolkit Showing the General View (Left) and Physical

    View (Right) Section.........................................................................................................4-16 Figure 4-8 Dialog Window of Item Toolkit Showing the Application View Section...................4-17 Figure 4-9 Grid View of Item Toolkit Showing the Blocks and Their Failure Rate ...................4-17 Figure 4-10 Failure Rate of Different Blocks of IUT .................................................................4-22 Figure 4-11 Contributions of Each Block of IUT Towards Total Failure Rate of the

    System .............................................................................................................................4-23 Figure 4-12 Schematics of ABBs Power Electronics-Based Distributed Transformer ............4-24 Figure 4-13 Schematics of Intelligent Universal Transformer Divided Into Three Distinct

    Sections ...........................................................................................................................4-26 Figure 4-14 Cumulative Distribution Function of Failure Function F(t) for the IUT, ABB,

    and Traditional Transformer.............................................................................................4-28 Figure 4-15 Comparison Showing the Failure Rate (Failures per Million Hours) of IUT

    Design and ABB Design...................................................................................................4-29 Figure 5-1 Partition of Circuit Boards and Major Circuit Components .......................................5-1

  • EPRI Licensed Material

    xxvi

    Figure 5-2 Associated Cost of Individual Factors in the Two Design Options .........................5-12 Figure 6-1 Proposed Multi-Converter-Based Intelligent Universal System................................6-4 Figure 6-2 A Stage-Gate Approach for New Product Development ..........................................6-7 Figure 6-3 A Generic Technology Roadmap for All-Solid-State Based Distribution

    Transformers......................................................................................................................6-9 Figure 6-4 Available HV Semiconductor Devices and Their Applications................................6-11 Figure 6-5 DARPAs R&D Development Plan for HV Device...................................................6-12 Figure 6-6 HV Device Roadmap and Development Projection Based on DARPAs R&D

    Program ...........................................................................................................................6-12 Figure 6-7 Requirements for the Next Generation HV Semiconductor Switches and

    Diodes ..............................................................................................................................6-13 Figure 6-8 Are Present HV Semiconductor Devices Suitable for IUT Application? .................6-14 Figure 6-9 Cost Comparison for Off-the-Shelf Insulated Gate Bipolar Transistors (IGBTs)

    A Direct Vendor Quote ..................................................................................................6-15 Figure 6-10 Insulated Gate Bipolar Transistor (IGBT) Structure and Symbol: (a) Basic

    IGBT Structure, (b) IGBT Equivalent Circuit and Symbol.................................................6-16 Figure 6-11 Ilustrations of Insulated Gate Bipolar Transistors (IGBTs): (a) Discrete TO-

    247 Package Rated Up to 1.2 kV and 50 A and (b) HV-IGBT Module Rated Up to 6.5 kV and 600 A..............................................................................................................6-16

    Figure 6-12 Voltage and Current Waveforms of a High-Voltage Insulated Gate Bipolar Transistor (HV-IGBT) Switching at 2-kV and 700-A Conditions .......................................6-17

    Figure 6-13 Cost Trend for Silicon-Based Insulated Gate Bipolar Transistor ..........................6-19 Figure 6-14 Cost Trend and Projection for Silicon-Based Insulated Gate Bipolar

    Transistor + Diode............................................................................................................6-19 Figure 6-15 Decrease in System Volume Through Utilization of SiC.......................................6-21 Figure 6-16 Projections of Possible Phases and Individual Tasks for Design and

    Development (20042007) of the Proposed HV Multilevel Converter-Based IUT Design ..............................................................................................................................6-30

    Figure 6-17 EPRIs Long-Term Roadmap for the Proposed HV Intelligent Universal Transformer Development Schedules and Important Milestones .................................6-30

    Figure 6-18 EPRIs Long-Term Roadmap for the Proposed HV Intelligent Universal Transformer Detailed Budget Estimate for Development Work in 20042007..............6-31

    Figure 6-19 Schematic of the Proposed IUT Design ...............................................................6-47 Figure 6-20 Output Voltage Shows Glitches Under Load Transient, but the Magnitude

    Remains Constant............................................................................................................6-50 Figure 6-21 Input Current Increases by Three Times During 50% Voltage Sag, but the

    Output Voltage Is Not Affected.........................................................................................6-51 Figure 6-22 Input Current Increases by Five Times, and the Output Voltage Sees 2% Dip

    After a 2-Cycle Outage.....................................................................................................6-52 Figure 6-23 Inverter Output Voltage Under Nonlinear Load Conditions ..................................6-53

  • EPRI Licensed Material

    xxvii

    LIST OF TABLES

    Table 1-1 Characteristics of Distribution Transformers Typically Used in United States ...........1-5 Table 1-2 Typical Costs Data for Three-Phase, Pad-Mounted Transformer..............................1-6 Table 3-1 Power Stage Parameters for the Active Front End....................................................3-2 Table 3-2 Possible Switching States of Three-Level Diode-Clamped AFE Converter.............3-11 Table 4-1 Reliability of Components Versus Number of Years of Operation of the

    Components.......................................................................................................................4-4 Table 4-2 Comparison of Various Reliability Models .................................................................4-6 Table 4-3 Components Used in the IUT Design ......................................................................4-13 Table 4-4 Parameter Values Required by the Component Diode............................................4-18 Table 4-5 Parameter Values Required by the Component IGBT.............................................4-19 Table 4-6 Parameter Values Required by the Component Capacitor......................................4-20 Table 4-7 Parameter Values Required by the Component Transformer .................................4-20 Table 4-8 Parameter Values Required by the Component Inductor ........................................4-21 Table 4-9 Results of the Reliability Prediction of IUT Design Using Item Toolkit.....................4-22 Table 4-10 Component List of ABB Design .............................................................................4-24 Table 4-11 Results of the Reliability Prediction of ABB Design Using Item Toolkit .................4-26 Table 4-12 Comparison of Total Components Used in ABB and IUT Design..........................4-27 Table 4-13 Comparison of the Failure Rates of ABB and IUT Design, Block-Wise.................4-27 Table 5-1 Bill of Materials for Three-Level 4.16-kV Power Circuit Board...................................5-3 Table 5-2 Bill of Materials for the Five-Level 8-kV Power Board ...............................................5-3 Table 5-3 Bill of Materials for the Gate Driver............................................................................5-4 Table 5-4 Bill of Materials for the Gate Drive Interface ..............................................................5-6 Table 5-5 Bill of Materials for the Sensor Conditioning Circuit...................................................5-7 Table 5-6 Bill of Materials for the Inverter Board .......................................................................5-8 Table 5-7 Cost Estimate for the Three-Level 4.16-kV IUT.......................................................5-11 Table 5-8 Cost Estimate for the Five-Level 15-kV IUT ............................................................5-11 Table 6-1 Operational Benefits An Overall Comparison .........................................................6-5 Table 6-2 Power Quality and Reliability Benefits An Overall Comparison..............................6-5 Table 6-3 Options for Expanding Services Opportunities An Overall Comparison.................6-6 Table 6-4 Comparison of Price for Silicon Insulated Gate Bipolar Transistor Over the

    Last Five Years From Different Vendors..........................................................................6-18 Table 6-5 Key Properties of Wide-Band Gap Semiconductor Materials ..................................6-21

  • EPRI Licensed Material

    xxviii

    Table 6-6 Basic Material Properties of Si and SiC...................................................................6-24 Table 6-7 Figure of Merit Comparing Si and SiC Devices .......................................................6-24 Table 6-8 SiC Device Cost and Performance Comparison......................................................6-26 Table 6-9 IUT 4.16-kV Projected Bench Model Development Schedule (Assumes

    Funding Availability) .........................................................................................................6-32 Table 6-10 IUT 15-kV Projected Bench Model Development Schedule (Assumes

    Funding Availability) .........................................................................................................6-33 Table 6-11 Budget Estimate Summary for 4.16-kV Bench Model Development .....................6-35 Table 6-12 Budget Estimate Summary for 15-kV Bench Model Development ........................6-35 Table 6-13 Budget Estimate by Task and Labor Category for 4.16-kV Bench Model

    Development ....................................................................................................................6-36 Table 6-14 Budget Estimate by Task and Labor Category for 15-kV Bench Model

    Development ....................................................................................................................6-37 Table 6-15 Projected IUT 15-kV Field Prototype Model Development Schedule ....................6-39 Table 6-16 Budget Estimate Summary for 15-kV Field Prototype Development .....................6-40 Table 6-17 Projected IUT Development Schedule During Field Deployment, Field Unit

    Data Collection, and Functionality Verification.................................................................6-41 Table 6-18 Budget Estimate Summary During Field Deployment, Field Unit Data

    Collection, and Functionality Verification .........................................................................6-41

  • EPRI Licensed Material

    1 INTRODUCTION

    Project Scope The design analysis work presented in this report is part of a multi-year effort to develop the IUT based on the all-solid-state approaches. It builds on the work done in 2002 to assess the feasibility of replacing current distribution transformers with a more sophisticated device, the IUT. The 2002 report [11] determined the technical and economic feasibility, clarified optional technology/design paths, estimated the development cost, and identified early market entry opportunities. Also, it was identified that use of HV power electronics in designing the primary side of an all-solid-state transformer offers significant advantage in reliability and cost over low-voltage (LV) designs.

    It is quite clear that, even with the higher risk involved in all-solid-state designs, the preferred research path should aim for the quantum leap in distribution transformer technology that is possible with the all-solid-state approach rather than an incremental improvement achieved through combination of existing technologies. As compared to the conventional copper-and-iron based transformer, the HV multilevel solid-state transformer will not only reduce transformer size and eliminate oil but will also enhance the power quality performance and increase the functionality. Conceptually, the following functions that do not exist in the conventional transformer can be obtained with the proposed IUT circuit:

    1. Voltage sag compensation. When the input source voltage drops for a short period, the universal transformer can compensate for the deficit and maintain constant output voltage. The total period of compensation, as a function of the amount of energy storage, can be adapted to the specific need of the customer.

    3. Instantaneous voltage regulation. If the input source voltage fluctuates due to power system transient or other load effects, the universal transformer will maintain constant output voltage because it has the energy buffer.

    4. Outage compensation. Similar to voltage sag compensation, the universal transformer can provide full voltage compensation for the period needed by the built-in energy storage.

    5. Capacitor switching protection. In general, a power-factor-correction capacitor switching produces voltage transient on the nearby utility line. With the universal transformer, the voltage transient will not propagate to the secondary (load) side.

    6. Harmonic compensation. Nonlinear loads produce harmonic-distorted current that tends to propagate back to the primary side of the transformer. The universal transformer will maintain a clean input current with unity power factor.

    1-1

  • EPRI Licensed Material

    Introduction

    7. Single-phasing protection. If the input power source has a missing phase or running under single-phase condition, the universal transformer will have a detection circuit and shut the output to prevent the system from running under an abnormal source condition.

    8. DC output. The universal transformer has a dc/ac three phase-leg inverter circuit that can be configured to become an interleaved three-leg dc/dc converter, thus providing dc output if necessary.

    9. 400-Hz output. The output of the universal transformer is a dc/ac inverter that can be configured for 400-Hz output rather than conventional 60-Hz or 50-Hz output.

    10. Variable-frequency output. Similar to the 400-Hz output function, the frequency of the universal transformer output could be a variable that is set by the user.

    With the above listed functionalities, it is clear that the next generation distribution transformer requires substantial power electronics and control elements. In this report, a high-voltage multilevel converter-based IUT is proposed for the next generation distribution transformer. The proposed IUT design includes back-to-back, interconnected, multi-level converters coupled to a switched inverter circuit via a high-frequency transformer. The input of the universal transformer can be coupled to a high-voltage distribution system, with the use of existing high-voltage silicon IGBT devices, and the output of the universal transformer can be coupled to low-voltage applications.

    The scope of this years activity was to conduct a comprehensive design analysis of a multilevel converter-based IUT, using HV power electronics on the input side. Details of the power electronics circuits and the associated open-loop and closed-loop controller designs that are required in an IUT to enhance the transformer functionality are provided. The detailed IUT power stage design, its specification based on a certain KVA and input/output voltage range (1-phase 15kVA), AutoCAD drawing, reliability assessment, modeling and simulation results for IUT functionality verification, and performance verification of both power stage and controller designs are also presented in this report. This design analysis will then serve as a blueprint for the development of the IUT and form the basis for competitive bidding (in 2004) for product development in 20042007. A more refined EPRI long-term roadmap for the development of the universal transformer with better cost estimates and schedule for the development work in 20042007 is also outlined in this report.

    Conceptually, the solid-state power-conversion-based distribution transformer will be much more expensive than the conventional one. However, the electronics can be designed with modularity and expandability. When considering the enhanced functionality and the flexibility, the added cost can be easily justified. In fact, the cost of power semiconductor and associated controls has exponentially decreased over the past few decades. Whether or not the IUT can be economically viable is also evaluated in this report. Also, this report identifies the key functionalities and criteria that need to be met in order for this technology to gain market acceptance.

    Critical issues that are addressed in this report, therefore, include:

    Selection of the circuit topology for the power stage

    1-2

  • EPRI Licensed Material

    Introduction

    Selection of active devices and passive components and calculation for semiconductor device voltage and current ratings and inductor and capacitor values based on certain assumptions or requirements on ripple, ride-through, or control response

    Simulation of the performance of the power stage using industry-accepted design packages for power electronics design analysis

    Verification of basic functions, such as voltage balancing for a multiple-level stack, with hardware experiments

    Estimation of the cost of the proposed system based on existing component costs and estimated component cost for different degrees of market penetration

    Estimation of the system reliability using industry standard reliability calculation methods such as MIL-HDBK-217F Reliability Prediction of Electronic Equipment

    Determination of design packaging and thermal management systems Evaluation of the design with respect to its ability to meet the different desired functionalities Development of a more refined EPRI long-term roadmap for the development of the

    intelligent universal transformer with better cost estimates and schedule for the development work in 20042007

    Identification of the key functionalities and criteria that need to be met in order for this technology to gain market acceptance

    Subsequent sections provide a brief overview of present-day distribution transformers based on type and design characterization, ongoing technology improvements in transformer designs, and possible market drivers for the universal transformer, a device that is expected to provide a range of functionalities beyond those available with conventional transformers.

    The Good Old Distribution Transformer

    In the early 1900s, as an alternative to Thomas Edisons DC power system, Westinghouse introduced an AC power system based on a transformer technology that can easily step up or step down AC voltage levels as required for power transmission, distribution, and retail consumption. Today, these good old distribution transformers (see Figure 1-1) are still used at virtually every site where utility power is delivered for use. Distribution transformers, whether mounted overhead on a power pole or mounted on the ground, reduce the voltage to a more usable level, typically 480/277 V (for three-phase application) or 208/120 V or 240/120 V (for single-phase application).

    1-3

  • EPRI Licensed Material

    Introduction

    PAD MOUNTED TRANSFORMERS POLE-MOUNTED TRANSFORMERS

    DISTRIBUTION TRANSFORMER

    Figure 1-1 Distribution Transformer

    Table 1-1 lists the characteristics of distribution transformers typically used in the United States. Utilities and commercial and industrial users purchase more than 1 million new distribution transformers annually. The vast majority of distribution transformers on the utility-owned distribution system are liquid-immersed (purchased using total-owning cost criteria, accounting for cost-of-energy losses), while those used in commercial and industrial applications are predominantly dry-type. Liquid-immersed transformers are the predominant type of transformer and represent the oldest technology with the most established performance record. In contrast to the dry-type units, which use the natural convection of air for cooling, liquid-filled transformers rely on oil or other liquid dielectric medium circulating across their coils for cooling.

    1-4

  • EPRI Licensed Material

    Introduction

    Table 1-1 Characteristics of Distribution Transformers Typically Used in United States

    Transformer Type Phase

    Primary Voltage (kV)

    Secondary Voltage (V)

    Capacities (kVA)

    Liquid-immersed 1 34.5 and below 600 and below 1500

    Liquid-immersed 3 34.5 and below 600 and below 32500

    Dry-type 1 34.5 and below 600 and below 15833

    Dry-type 3 34.5 and below 600 and below 152500

    Dry-type 1and 3 34.5 and below 600 and below 0.2515

    For purposes of this report, distribution transformers are defined as all transformers that perform the final transformation from electric utility power distribution line voltages (4 to 34.5k V) to final lower secondary utilization voltages (120 to 480 V) suitable for customer equipment. These transformers typically range in size starting from 10 kVA, single-phase to 3000 kVA, three-phase. High-voltage (that is, 69- to765-kV) transformers are considered power transformers, not distribution transformers, and thus are not included here.

    Why Look at Anything Else?

    Given the growing environmental, economic, and governmental concerns for building new power-generation facilities, utilities continue to look for cost-effective ways to defer new power generation while meeting their customers growing demand for electricity. Even a true visionary like Thomas A. Edison could not have imagined the level of sophistication that would be required of equipment used for the transmission and distribution of electric power a little more than a hundred years after its invention. Nor could he have anticipated all of the complex issues facing the electrical industry as it moves into the 21st Century. As the distribution transformer enters its second century of service, it is not easy to predict whether this device is here to stay or how its evolution will proceed.

    The positive attributes of conventional distribution transformers, well documented for years, include low cost, high reliability, and high efficiency. They are among the most reliable of components in power-conversion systems, with an ability to handle 500 times the power and 15 times the voltage of their turn-of-the-20th Century ancestors. Their weight per unit of power has dropped by a factor of 10, efficiency typically exceeds 99%, and they have an average lifetime of 30 years or more. Figure 1-2 shows efficiency trends for distribution transformers over the past several years. Standard costs for three-phase, pad-mounted transformers range from $40 to $100/kVA for sizes ranging between 50 and 150 kVA (see Table 1-2).

    1-5

  • EPRI Licensed Material

    Introduction

    SOURCE: Barnes, P., Van Dyke, J., McConnell, B., and S. Das, Determination Analysis Of Energy Conservation Standards For Distribution Transformers, ORNL 6847, Oak Ridge, TN, July 1996Van Dyke, J., Barnes, P., McConnell, B., and S. Das, Supplement T o The Determination Analysis Of Energy Conservation Standards For Distribution Transformers, ORNL 6925, Oak Ridge, TN, Sept. 1997

    Efficiency @ Maximum Load (Liquid Type)

    Efficiency @ 15% Load (Liquid Type)

    Efficiency @ 15% Load (Dry Type)

    Efficiency @ Max. Load (Dry Type)

    Figure 1-2 Efficiency Trends Over Several Years for 75-kVA, Three-Phase Distribution Transformers

    Table 1-2 Typical Costs Data for Three-Phase, Pad-Mounted Transformer

    Cost ($) Size (kVA)

    12.5 kV 34.5 kV

    75 7,749 10,584

    150 9,450 11,605

    300 11,718 15,574

    500 13,608 20,034

    750 21,357 21,377

    1000 25,515 28,350

    NOTE: Above costs include necessary cable terminations, pads, miscellaneous material and transformer, but no primary or secondary cables.

    Transformer purchasing decision impact overall efficiencies for transformers in use today. There are five different types of purchasing practices used by various market players in the distribution

    1-6

  • EPRI Licensed Material

    Introduction

    transformer market. These practices are first cost, total life-cycle owning cost (TOC), band of equivalence (BOE), over-sizing, and choice of winding materials. The last two practices are the only ones not based on economics, but they play an important role in transformer-purchase decision making for some end users.

    Transformer purchases are treated as capital expenditures for equipment with an expected life of 30-40 years; however, lack of capital causes most small and mid-sized end-users to favor the short-term purchasing criteria (that is, the first cost) with short payback periods (that is, 1-3 years). Purchases of transformers are often based on the first cost (without any consideration of long-term economics) when transformer evaluation and purchase decisions are not made, by the end-user. This is particularly true where purchase decisions are made on the basis of temperature rise and low first cost for commercial and industrial end-users buying dry-type, pad-mounted transformers. In addition, these users are not always aware of, and in some cases are uncertain about, the costs and benefits of using energy-efficient transformers.

    In recent years the increases in capital and operating costs for power plants, difficulties in citing new facilities, and concerns for the environment have forced utilities to evaluate energy efficiency. These concerns for energy efficiency have been translated by utilities into loss evaluations for their transformer and equipment purchases, expressed as dollars per kilowatt-hour saved. Higher the loss evaluation translates to more premiums to minimize energy losses. The loss evaluation rates (rates that a utility is willing to pay per watt reduction in rated core and conductor losses) needed to calculate TOC, are currently supplied by most electric utility purchasers by evaluating the specific application situation (for example, duty cycle, cost of capital, expected life). Most pole- and pad-mounted transformers are currently loss-evaluated, while almost no dry-type transformers are evaluated. In some cases, utilities also offer rebates to customers for undertaking loss evaluations and then monetary assistance to buy down a more expensive, more energy-efficient transformer where it meets utility savings criteria. For example, Bonneville Power Administration offers a one-time incentive of up to $0.15/kWh saved in the first year of operation to its utility and industrial customers. Unfortunately, these programs rarely extend to the smaller distribution transformers that are common in C&I facilities. Thus, most commercial, institutional, and light-duty industrial end users, for whom a transformer purchase decision is more peripheral to their business than it is for a utility, do not use loss evaluations such as TOC, which require extensive analysis and the input of many variables. Because of tightening in the availability of capital budgets these days, there is a growing trend even among utilities to use either some form of TOC or first-cost criteria for making liquid-filled transformer purchase decisions. The move away from the TOC purchasing criterion results in the selection of a less efficient transformer and hence reduces the energy conservation potential. Details of other criteria are provided in the 2002 report Feasibility Assessment for Intelligent Universal Transformer (EPRI, Palo Alto, CA: 2002. 1001698) and also documented in Determination Analysis Of Energy Conservation Standards For Distribution Transformers, Barnes, P., Van Dyke, J., McConnell, B., and S. Das, ORNL 6847, Oak Ridge, TN, July 1996.

    Were it not for these stationary, almost totally silent, highly reliable devices, the distance separating generators from customers would have been significantly greater, many households and industries would require their own substations, and electricity would have been a much less practical form of energy. Unfortunately, like other devices in modern electrical distribution systems, the conventional transformer also has some drawbacks. These drawbacks include voltage drop under load, inability to mitigate flicker, sensitivity to harmonics, environmental

    1-7

  • EPRI Licensed Material

    Introduction

    impacts when leaks of mineral oil occur, limited performance under DC-offset load unbalances, inability to convert single-phase service to three-phase for powering certain types of equipment, and no energy-storage capacity. One consequence of not having energy-storage capacity is that the output can be easily interrupted because of a disturbance at the input. Also, when the output load current generates harmonics and reactive power, the conventional transformer reflects them back to the input side.

    Owing to their bulky iron cores and heavy copper windings, conventional transformers are by far one of the heaviest parts in an electrical distribution system. The size and weight of the transformer is primarily a function of the saturation flux density of the core material and maximum allowable core and winding temperature rise [1]. The saturation flux density is inversely proportional to frequency, and hence increasing the frequency allows higher utilization of the steel magnetic core and reduction in transformer size. However, because the operating frequency of commercial power is ordinarily fixed (60 Hz in the United States), the volume and the weight of the distribution transformer cannot be reduced below its definite values. Whats more, they do experience continuous no-load or core losses that arise from being constantly energized and ready to serve a load.

    As pressure increases on electric service providers to provide a higher-quality and more reliable product on demand and at a price point that is acceptable to customers, there is desire to increase utilization of conventional transformers. It has been shown in prior studies [2, 3, 4] on distribution transformer performance, efficiency, and loss evaluations that even small changes in efficiencies can add up to a large net gain in energy savings. On the basis of these energy savings study results, many researchers are exploring the potential for high-energy-efficiency transformers over conventional copper and iron core transformers. While some major improvements have occurred in transformer technology from time to time (such as the introduction of grain-oriented core steel), other developments in the areas of core, winding, insulation, and dielectric liquids have provided only incremental improvements in transformer technology. The question, then, is this: Will present and emerging incremental improvements to conventional transformers provide a solution for the future? Or, is there a need for a technology replacement?

    Can the Opportunity to Offer a Wide Range of Services Be Realized Through Incremental Improvements in Conventional Transformers?

    While more than 1 million new distribution transformers are purchased annually, utility distribution transformers account for an estimated 61 billion kilowatt-hours (kWh) of the annual energy lost in the generation and delivery of electricity. Additional transformer losses in non-utility applications are estimated to be 79 billion kWh [2-3]. As mentioned in the preceding sections, the U.S. Department of energy (DOE) and many other institutions are focusing a great deal of time and money in research on the economic benefits of increased efficiency in distribution transformers.

    If increasing reliability and efficiency and lowering initial costs are the primary goals, then incremental improvements in conventional transformer technology may well be the desired approach. However, this economic gain will more than likely not be realized directly by many end users, because utility metering is typically located at the secondary of the transformer. End

    1-8

  • EPRI Licensed Material

    Introduction

    users could see some indirect savings because reducing losses in utility transformers should reduce the overall cost of electricity. However, there are other factors in todays market environment to consider, including increased sensitivities of customer equipment and process, increased need for DC power created by the next-generation DC loads, higher energy costs, increasingly stringent environmental regulations, and diminishing energy reserves.

    A conventional transformer lacks energy-storage capability, and thus the output can be easily interrupted due to the disturbance at the input. When the output load current generates harmonics and reactive power, the conventional transformer also reflects them back to the input side. To overcome these problems, modern power electronics technology can be used to serve as the energy buffer between the source and load to avoid direct impact from either load-to-source or source-to-load. With the flexibility of power electronics switching, it is also possible to provide universal voltage outputsuch as DC voltage or variable-frequency AC voltageto the load. The next sections identify possible market drivers for a sophisticated transformer, a device that is expected to provide a range of functionalities beyond those available with conventional transformers.

    Short-Range Market Driver Improved Power Quality and Reliability

    In todays environment, power producers need to mix and match their electrical service offerings to meet the customers changing requirements. The need for a sophisticated transformer that can employ modern power electronics to improve transformer functionality has increased because of the emerging digital economy, which requires a high degree of power quality and reliability. Matching the voltage requirement within a certain range was once and remains now the primary functional requirement for a transformer. However, the proliferation of sensitive loads in residential, commercial, and industrial customer segments