Distribution System Modeling and Analysis with MATLAB® and WindMil® 5th Edition

Book Preface

We are now in the beginning stages of the smart grid, and distribution systems are getting smarter as automation becomes more prevalent. This is possible due to smart meters, otherwise known as advanced metering infrastructure. The US Energy Information Administration reported that 102.9 million smart meters have been deployed by 2020 and that 88% of these smart meters are for residential customers in distribution circuits [1]. At the very start, we want to emphasize that this text is intended to only develop and demonstrate the computer models of all the physical components of a distribution system. As the text develops the component models it will become clear that this thing we called “load” is the weak link in the overall anal-ysis of a distribution system. Smart meters give access to much more data than was traditionally captured; however, they still have not been utilized to their full capacity. At the present time, the only true information available for every customer remains energy in kilowatt-hours consumed during a specified period. This topic is addressed in Chapter 2. The problem with load is that it is constantly changing. Computer pro-grams can and have been developed that will very accurately model the components, but without real load data, the results of the studies are only as good as the load data used. As the smart grid is developed, more accurate load data will become available, which will provide for a much more accurate analysis of the operating conditions of the distribution system. What needs to be emphasized is the smart grid must have computer programs that will very accurately model all the physical components of the system. The purpose of this text is to develop very accurate models of the physi-cal components of a distribution system.
In the model developments, it is very important to accurately model the unbal-anced nature of the components. Programs used in the modeling of a transmission system assume that the system is a balanced three-phase system. This makes it pos-sible to model only one phase. That is not the case in the modeling of a distribution system. The unbalanced nature of the distribution system must be modeled. This requirement is made possible by modeling all three phases of every component of the distribution system.
The distribution system computer program for power-flow studies can be run to simulate present loading conditions and for long-range planning of new facilities. For example, the tools provide an opportunity for the distribution engineer to opti-mize capacitor placement to minimize power losses. Different switching scenarios for normal and emergency conditions can be simulated. Short-circuit studies provide the necessary data for the development of a reliable coordinated protection plan for fuses, recloser, and relay/circuit breakers.
So, what is the problem? Garbage in, garbage out is the answer. Armed with a commercially available computer program it is possible for the user to prepare incor-rect data and as a result, the program outputs are not correct. Without an understand-ing of the models and a general “feel” for the operating characteristics of a distribution system, serious design errors and operational procedures may result. The user must fully understand the models and analysis techniques of the program. Without this knowledge the garbage in, garbage out problem becomes very real.
The purpose of this text is to present the reader a general overall feeling for the operating characteristics of a distribution system and the modeling of each compo-nent. Before using the computer program, it is extremely important for the student/engineer to have a “feel” for what the answers should be. Engineers once used a slide rule for engineering calculations prior to the advent of hand calculators. The beauty in using a slide rule was you were forced to know what the “ballpark” answer should be. We have lost that ability thanks to hand calculators and computers but under-standing the ballpark answer is still a necessity.
It has been very interesting to receive many questions and comments about previ-ous editions of the text from undergraduate and graduate students in addition to prac-ticing engineers from around the world. That gets back to the need for the “feel” of the correct answer. New students need to study the early chapters of the book to develop this “feel”. Practicing engineers will already have the “feel” and perhaps will not need the early chapters (1, 2, and 3). In developing the fifth edition of the book, we have retained most of the contents of Chapters 1, 5, 7, 8, and 11. We have added the concept of electric vehicles as loads and their impact on the grid. This is visited in Chapter 2 at a high level and in a more detailed analysis-based treatment again in Chapter 9. We modified Chapters 3, 6, and 10 to give the book a quicker and more direct treatment of the concept of iterative power flow. Lastly, in Chapter 4, we devel-oped a new model for tape-shielded underground cables, which we believe to be more accurate than the previous model.
This textbook assumes that the reader has a basic understanding of transformers, electric machines, and transmission lines. At many universities, all these topics are crammed into a one-semester course. For that reason, a quick review of the needed theory is presented as needed.
There are many example problems throughout the text. These examples are intended to not only demonstrate the application of the models but also to teach a “feel” for what the answers should be. The example problems should be studied very carefully since they demonstrate the application of the theory just presented. Each chapter has a series of homework problems that will assist the student in applying the models and developing a better understanding of the operating characteristics of the component being modeled. Most of the example and homework problems are very number intensive. In previous versions, all the example problems had used a software package called “Mathcad” [2]. Since distribution engineers will soon need the skills in both computer programming and data science in addition to traditional electrical engineering, we have elected to convert all example problems to MATLAB [2]. All example code is provided for instructors, students, and practicing engineers. We have decided to keep the Mathcad iterative routines in the textbook, as Mathcad provides a fantastic visual flow of how a program works.
As more and more components are developed, and the feeder becomes more com-plicated, it becomes necessary to use a sophisticated distribution analysis program. Milsoft Utility Solutions has made a student version of “WindMil” [4] available along with a user’s manual. The user’s manual includes instructions and illustrations on how to get started using the program. Starting in Chapter 4, there is a WindMil  assignment at the end of the homework problems. A very simple system utilizing all the major components of the system will evolve as each chapter assignment is com-pleted. In Chapter 10, the data for a small system is given that will allow the student/engineer to match operating criteria. The student version of WindMil and the user’s manual can be downloaded from the Milsoft Utility Solutions website homepage. The address is:

Milsoft Utility Solutions
P.O. Box 7526
Abilene, TX 79608
Email: [email protected] Home page: www.milsoft.com

Unfortunately, there is a tendency on the part of the student/engineer to believe the results of a computer program. While computer programs are wonderful tools, it is still the responsibility of the users to study the results and confirm whether the results make sense. That is a major concern and one that is addressed throughout the text.
Chapter 1 presents a quick overview of the major components of a distribution system. This is the only section in the text that will present the components inside a substation, along with two connection schemes. The importance of the distribution feeder map and the data required is presented.
Chapter 2 addresses the important question, What is the “load” on the system? This chapter defines the common terms associated with the load. In the past, there was limited knowledge of the load and many assumptions had to be made. With the coming of the smart grid, there will be ample real-time data to assist in defining the load for a given study. Even with better load data, there will still be a concern on whether the computer results make sense. Electric vehicles are described as loads in Chapter 2 as well.
Chapter 3 introduces voltage drop and introduces the iterative power-flow pro-gram. This is done by introducing the “ladder” (forward/backward sweep) iterative method used by many commercial programs. This chapter covers the simplified approach to distribution systems analysis, assuming that distribution systems are bal-anced like that of their transmission level counterpart.
The major requirement of a distribution system is to supply safe reliable energy to every customer at a voltage within the American National Standards Institute (ANSI) standard, which is addressed in Chapters 4 and 5. The major goal of planning is to simulate the distribution system under different conditions now and into the future and assure that all customer voltages are within the acceptable ANSI range. Since voltage drop is a major concern, it is extremely important that the impedances of the system components be as accurate as possible. In particular, the impedances of the overhead and underground distribution lines must be computed as accurately as pos-sible. The importance of a detailed feeder map that includes the phase positions for both overhead and underground lines is emphasized.
Chapter 6 develops the models for overhead and underground lines using the impedances and admittance computed in earlier chapters. Included will be the “exact” model along with an approximate model. Introduced are the matrices required for the application of the ladder analysis method. Included in the chapter are methods of modeling parallel distribution lines.
Chapter 7 addresses the important concept of voltage regulation. How is it possi-ble to maintain every customer’s voltage within ANSI standards when the load is varying all the time? The step-voltage regulator is presented as one answer to the question. A model is developed for the application in the ladder technique.
Chapter 8 is one of the most important chapters in the text. Models for most three-phase (closed and open) transformer connections in use today are developed. Again, the models use matrices that are used in the ladder iterative technique. The impor-tance of phasing once again is emphasized.
Chapter 9 develops the models for all types of loads on the system. A new term is introduced that helps define the types of static load models. The term is “ZIP”. Most static models in a distribution system can be modeled as constant impedance (Z), constant current (I), constant complex power (P), or a combination of the three. These models are developed for wye and delta connections. A very important model developed is that of an induction machine. The induction motor is the workhorse of the power system and needs, once again, to be modeled as accurately as possible. Several new sections have been included in this chapter that develop models of the induction machine and associated transformer connection that are useful for power-flow and short-circuit studies. Induction generators are becoming a major source of distributed generation. Chapter 9 shows that an induction machine can be modeled either as a motor or a generator. Lastly, a thorough ZIP model is developed for Level 2 electric vehicle chargers. Electric vehicles are being deployed at a steady rate, and adding this energy requirement to the electrical sector from the previous fossil fuel transportation sector is sure to have an impact. Accurate electric vehicle modeling will be very important to future distribution engineers.
Chapter 10 puts everything in the text together for steady-state, power-flow, and short-circuit studies. The “ladder” iterative technique is introduced in Chapter 3 and then again in Chapter 6. This chapter goes into detail on the development of the lad-der technique starting with the analysis of a linear ladder network that is introduced in most early circuit analysis courses. This moves onto the non-linear nature of the three-phase unbalanced distribution feeder. The ladder technique is used for power-flow studies. Introduced in this chapter is a method used for the analysis of short-circuit conditions on a feeder.
Chapter 11 introduces the center-tapped transformer that is used for providing the three-wire service to customers. Models for the various connections are introduced that are used in the ladder iterative technique and short-circuit analysis. The WindMil assignments at the end of Chapters 10 and 11 allow the student/engineer to build, study, and fix the operating characteristics of a small distribution feeder.
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