Computer‐assisted Interactive Learning for Teaching Power Flow Problem

www.jitae.org Journal of Information Technology and Application in Education Volume 4, 2015 doi: 10.14355/jitae.2015.04.004

Computer‐assisted Interactive Learning for Teaching Power Flow Problem Wenhao Zhang*1, Haiying Jiang2 Electrical Engineering, Tongji University, 4800 Cao’an Rd, Shanghai, China *1

xiwen@tongji.edu.cn; 2jianghy@tongji.edu.cn

Abstract This paper presented a computer‐assisted method to teach and understand power flow calculation techniques. The education method is facilitated by specially designed user‐friendly software PowerWorld simulator. The Simulation is to help students fully understand power flow calculation through the fulfilment of the following educational objectives: 1) use of Powerworld to solve the power flow problem of a small power system as well as a real‐world power system; 2) complete understanding of the algorithms involved; 3) analysis of the results, identification and justification of the differences among different algorithms; and 4) extension of calculation methodologies. These methods have been proven effective in the education of students studying power system dispatching at Tongji University(TJU), China.

Keywords Power Flow; PowerWorld Simulator; Power Engineering Education

Introduction The need for power network analysis techniques under steady state conditions appears in many applications of power engineering. The real time control functions in modern energy management system, such as monitoring, generation control and security assessment and control, utilize network analysis methods under steady state conditions. It is desirable to “know” the operating state of the system from a combination of measurements and analysis procedures. Conceptually, we classify the analysis problems into three categories: (a) power flow, (b) state estimation, and (c) optimal power flow.

The power flow problem is to assume that at a given instant of time, the control settings are known. In addition, the generating unit real power outputs, the electric loads and the tie line flows are also known. Then the power flow problem is defined as the procedure by which, given the above information, the circuit flows and voltage levels everywhere in the system are computed.

Power flow is the basis to study state estimation and optimal power flow problems. It’s explained in detail in the course of “Power System Analysis” and furtherly introduced in “Power System Dispatching Automation”.

Power System Courses at TJU “Power system engineering” is a professional compulsory course and “Power system dispatching automation” is an elective course of Department of electrical engineering. Power system is the main service object of electrical engineering, electric power dispatching automation system is the main technical mean of modern power system operation and management, to ensure the safe and economic operation of the power system, so the study of this course is to enable students to the requirements of the theory to understand and grasp the dispatching automation, performance and operation characteristics, and at the same time to complete the experiment, deepening the understanding of the provisions of the device, and the cultivation of practical ability. As the professional knowledge necessary to work in the automation of electric power system management, scheduling, operation, design, development.

Applications software is all computer programs which are utilized in the control and operation of a power system.

Depending on the needs of a particular system, applications software may vary.

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Journal of Information Technology and Application in Education Volume 4, 2015 www.jitae.org

Power Flow Calculation Educational Tool PowerWorld Simulator is a power system simulation package designed from the ground up to be user‐friendly and highly interactive. Simulator has the power for serious engineering analysis, but it is also so interactive and graphical that it can be used to explain power system operations to non‐technical audiences.

Main Feature Simulator consists of a number of integrated products. Its core is a comprehensive, robust Power Flow Solution engine capable of efficiently solving systems of up to 100,000 buses. This makes Simulator quite useful as a stand‐ alone power flow analysis package. Transmission lines can be switched in (or out) of service, new transmission or generation can be added, and new transactions can be established. Simulator’s extensive use of graphics and animation greatly increases the users’ understanding of system characteristics, problems, and constraints, as well as of how to remedy them.

The base package also contains all the tools necessary to perform integrated economic dispatch, area transaction economic analysis, power transfer distribution factor (PTDF) computation, short circuit analysis, and contingency analysis. All of the above features and tools are easily accessible through a consistent and colorful visual interface.

These features are so well integrated.

Graphical User Interface GUIs are increasingly used to provide users of computer simulations a friendly and visual approach, and help students master this tool by visualizing each step of the solution process. PowerWorld Simulator has two distinct modes, Edit Mode and Run Mode. The Edit Mode is used to construct new simulation cases or to modify existing cases, while the Run Mode is used to perform the actual power system simulation.

FIG.1 POWERWORLD SIMULATOR USER INTERFACE

Brief Presentation In PowerWorld Simulator, the student can create a new case or start with an existing case (sample cases) to be familiar with the different features. To build a online diagram, the student should master the basic knowledge on power system modelling. In the edit mode, all the related elements have been provided in the simulator, including buses, substations, generators, loads, transmission lines, transformers, series capacitors, switched shunts, etc.

For the calculation of transmission power flows, students have alternative solution methods. The options are: 1) RECTNEWT: For Rectangular Newton‐Raphson method; 2) POLARNEWTON: For Polar Newton‐Raphson method; 3) GAUSSSEIDEL: For Gauss‐Seidel method; 4) FASTDEC: For Fast Decoupled method; 5) DC: For DC method; 6) ROBUST: For ROBUST method. The default method is RECTNEWT. Teaching Method The objective of the power flow calculation of the Power System Dispatching Automation at TJU is to present several power flow calculation methods to the students and allow them to practice with various system configurations in order to understand the advantages and disadvantages of each solution method.

At the beginning of the semester, a user guide of PowerWorld Simulator is distributed to the students together with educational examples with detailed parameters involved. The teaching is implemented as follows. The instructor presents in the class the power flow calculation theories together with some simple arithmetic examples.

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Here, the students are required to calculate these examples by hand.

Then, the use of PowerWorld simulator is presented, using attractive slide presentations and animation graphics.

Each student uses one of the lab computers to build the given systems and solve the power flow problem. The instructor, aided by 1 advanced postgraduate student, facilitates the students to solve this problem. In brief, the computer‐assisted interactive teaching strategy is as follows: 1) study a problem; 2) study a solution; 3) solve by hand; 4) verify by computer; and 5) repeat for different algorithms. At the end, students are required to simulate a more complicated system and submit the reports. The students can see the solution of the problem, on a step‐by‐ step basis and this interactive learning environment has been proved very effective in the classroom, since most engineering students are active learners.

Results and Discussion Educational Objectives The core educational objective of making students able to operate at the higher levels is achieved through the fulfillment of the following individual educational objectives[1]: 1) use of simulator to solve the power flow problem of a small power system (5‐bus) as well as a IEEE 14‐bus power system; 2) analysis of the results and understanding of process to build admittance matrix, especially for branches with transformers; 3) complete understanding of the solution method of different algorithms, especially the assumptions and formation of Jacobian matrix; 4) analysis of the results, identification and justification of the differences among different computer aided solution methods, and valid the results by hand calculation; 5) extension of power flow calculation methodologies. This objective is dedicated to the students who are interested to make their dissertation in this field.

5‐Bus Test System The students are given the line data and the bus data of 5‐bus test system[2], shown in the left part of Fig. 2. Bus 5 is the slack bus. All parameters are given as per unit (p.u.) values on a 100‐MVA power base.

(a) 4

2

3

5 slack

200 MW

370 MW

100 Mvar

130 Mvar

258 MW 230 Mvar

1 160 MW 80 Mvar

(b) FIG.2 (a)5‐BUS TEST SYSTEM AND PARAMETERS; (b)SIMULATION MODEL IN RUN MODE

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Journal of Information Technology and Application in Education Volume 4, 2015 www.jitae.org

The simulation model is built by using the blocks in simulator. After input the data, the system in Run Mode is shown in the right part of Fig. 2. The arrow size also illustrates the power flow in an iconic view.

1: k

z

kz

kz k 1

k2z 1 k (a) (b)

FIG.3 (a)EQUIVALENT CIRCUIT OF TRANSFORMER BRANCH; (b)TRANSFORMER PARAMETER INTERFACE

During the model building, the transformer options settings are worth to be noticed. Fig. 3(a) shows a transformer branch and its equivalent circuit[3]. In Fig. 3(b), we can see the ʺFrom Busʺ , ʺTo Busʺ, and the ＂Turns Ratioʺ, these blanks must be carefully input according to the system parameters in Fig. 2. If the bus terminals are disordered, the admittance matrix would be wrong and consequently the power flow calculation results would be quite different.

Simulation Results More detail can be found by opening the model explorer dialog. Table 1 shows the admittance matrix. And Table 2 lists the results of power flow simulation. The bold numbers show the voltage of Bus 1 is under the threshold.

TABLE 1 ADMITTANCE MATRIX TABLE 2 BUS VOLTAGES

Bus

1

2

3

4

5

Bus

Nom( kV)

PU Volt

Volt (kV)

1

1.38 ‐ j6.29

‐0.62 + j3.90

‐0.75 + j2.64

0

0

1

138

0.86215

118.977

‐4.78

2

‐0.62 + j3.90

1.45 ‐ j66.98

‐0.83 + j3.11

j63.49

0

2

138

1.07792

148.752

17.85

3

‐0.75 + j2.64

‐0.83 + j3.11

1.58 ‐ j35.74

0

j31.75

3

138

1.03641

143.025

‐4.28

4

0

j63.49

0

j66.67

0

4

138

1.05

144.9

21.84

5

0

0

j31.75

0

‐j33.33

5

138

1.05

144.9

0

Angle (Deg)

Conclusions A computer‐assisted interactive learning environment has been introduced and successfully used in the teaching of power system dispatching automation course of the seventh semester of the four‐year undergraduate curriculum of electrical engineering at Tongji University, China. Students learn very easily how to use the PowerWorld simulator to build models and test different solution methods. Students found the course more interesting and study the education material very positively and the overall course is evaluated higher after this tool was introduced.

REFERENCES

[1]

Pavlos S. Georgilakis, George A.Orfanos, Nikos D. Hatziargyriou. ʺComputer‐Assisted Interactive Learning for Teaching Transmission Pricing Methodologiesʺ, IEEE Transactions on Power System, Vol.29, No.4, July2014, pp.1972‐1980

[2]

Zhenxiang Han, Guoyan Wu. ʺPower System Analysis.ʺ Zhejiang University Press, 1993, pp.128‐131

[3]

Yangzan He. ʺPower System Analysis.” Huazhong University of Science and Technology Press, 2002, pp.170‐171

Wenhao Zhang, was born in Shanxi Province, China in 1982. He received the Bachelor degree in Harbin Institute of Technology in 2003, Master degree in Xi’an Jiaotong Unviersity in 2006, and Ph.D degree in Myongji University, South Korea in 2010, respectively. His research interests include power system protection, reliability assessment and fault diagnosis. He currently work as an Assistant Professor at Tongji University.

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