Virtual Laboratory for Power Quality Study

Procedia - Social and Behavioral Sciences 191 ( 2015 ) 2798 – 2802

1877-0428 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Selection and peer-review under responsibility of the Organizing Committee of WCES 2014 doi: 10.1016/j.sbspro.2015.04.438

ScienceDirect

WCES 2014

Virtual Laboratory for Power Quality Study

Monica Iovan

*, Flavius Dan Surianu

, Florin Molnar-Matei

Abstract

The paper presents a virtual laboratory for studying power quality events like three-phase voltage dips, which is used by students to learn and test their knowledge about the subject. The virtual laboratory is needed because in real life, in power quality domain, it is very difficult to generate power quality events in order to see the effects or to analyze their characteristics. The virtual laboratory is designed to give the students the possibility to learn first the theoretical aspects of the subject, then to apply the knowledge they accumulate to several signals, in order to get the characteristics of the event and in the end to test their obtained results with signals characteristics. There are two major advantages of using the virtual laboratory: first is that the students can see different events, with different characteristics, and in this way, they understand better the phenomenon; and the second is that a classical laboratory for studying three phase voltage dips is very expensive, the main elements (programmable voltage sources) having a very high cost.

© 2014 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of the Organizing Committee of WCES 2014. Keywords: electrical simulation, electrical education, power quality, voltage dip;

1. Introduction

It is known that in some domains it is not always possible to exercise on real situations due to the cost or more important due to safety and security. This is also the situation in the power quality domain where is difficult to generate power quality events in order to see the effects and analyse them, so power quality laboratories are using different virtual laboratories or simulation tools. As seen in the literature from the electrical domain, such virtual laboratories are used more often and become even more important in the modern teaching methods (Atanasijevic-Kunc et al., 2011), being able to find virtual laboratory for teaching electronics (Mosterman et al, 1996; Oakley, 1996), power systems (Patton & Jayanetti, 1996; Baloi & Pana, 2011; Coroiu et al., 2011).

* Monica Iovan. Tel.: +40 256 403428 E-mail address: [email protected]

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

This paper aims to present a virtual laboratory developed for students to understand and learn the voltage dip characteristics and to analyse a generated three phase voltage dip waveform. The paper starts from a previous work (Molnar-Matei et al., 2013) where the authors present a mathematical model for single phase voltage dip generator. The laboratory combines different learning methods so the students improve their knowledge and understand better the voltage dips concepts and characteristics.

As it follows, Section II presents some theoretical aspects of the voltage dips; Section III presents the implementation of the developed virtual laboratory, and finally Section IV presents the final conclusions.

2. Voltage Dips

In this period, when power energy is always present in our lives we start seeming electricity more like a right we have and this require a high level of quality. One of the actual biggest quality problems that exist is voltage dip. The modern world use now so many automated processes which are very sensitive to the voltage level that the appearance of a voltage dip produce significant financial losses. Voltage dips are usually caused by faults on the power system and are defined as a decrease in root mean square (RMS) voltage under 0.9 p.u. having duration from half a cycle to tens of seconds (IEC61000-4-30, 2008). The parameters for a voltage dip are:

x The duration of a voltage dip is the time in which the voltage is below a threshold (IEC61000-4-30, 2008). In some cases it can be used two thresholds: one for dip starting (typically 90% of nominal voltage) and one for dip ending (typically 92% of nominal voltage).

x The residual voltage, obtained by calculating RMS voltage over a multiple of half of cycle.

x Phase shift, name used in (IEC61000-4-30, 2008) and (Djokic & Milanovic, 2006). Other authors (Bollen & Gu, 2006) use the term phase angle jump. Through the phase angle jump it is understood the difference between the phase angle before and during the dip.

x The point on wave, which represents the moment on the waveform when appears or ends the dip. In (Djokic et al., 2006) the authors considered that this parameter should be used to compute the voltage dip duration. The residual voltage and the dip duration are represented graphically in Fig. 1.

Fig. 1.Typically voltage dip amplitude and duration.

In the virtual laboratory a voltage dip signal is generated for each student using a simulation module that was created in such a way that could simulate a real voltage dip. The simulation module was developed using the following mathematical model for generate a single phase voltage dip (1). After that, the generated signal is affected by the introduced noise and harmonics as described in (2).

° ° ° ° ¯ ° ° ° ° ® ­   ˜ ˜  d      ˜ ˜ d    ˜ ˜  d      ˜ ˜ d ˜ ˜ t t ft t N A t t t t j t j t Y t Y t j t N t t t ft t N A t t t t j t j t Y t Y t j t N t t ft t N A t y T S T T T T M S T T T S 2 , ) 2 sin( ) ( 1 2 2 ) 2 ( 2 ) 2 ( 2 ) 2 ( 2 ) 2 ( 1 ) ( 2 ) ( 2 1 , ) 2 sin( ) ( 2 1 1 ) 1 ( 1 ) 1 ( 1 ) 1 ( 1 ) 1 ( 2 ) ( 1 ) ( 1 , ) 2 sin( ) ( 1 ) ( (1) 0 10 20 30 40 50 60 70 0 0.1 T ime [s] 0.2 0.3 D uration R esidual voltage Volta ge [kV]

–

¦

For three phases voltage dips the variation of amplitude and angle for each phase are determined automatically by selecting one of the seven types corresponding to ABC classification (Bollen & Gu, 2006).

Regarding ABC classification, seven voltage dip types are presented and are grouped in three groups, based on the number of phases with most severe voltage drop. (Bollen et al., 2004)

x Type A: the three voltage drops have the same amount. These events will be referred to below as three-phase drops.

x Type C, E and G: two retained voltages are much smaller than the third one, these voltage sags are referred to as two-phase drops.

x Type B, D and F: one voltage drops much more than the two other voltages (single-phase drops).

In Table 1 there are presented the complex values of the voltages on the phases and the phase diagram for each voltage dip. It was noted with U the effective voltage on the R phase from the ante perturbation regime, and with h the voltage dip amplitude represented in percent from voltage U.

Table 1. ABC Classification for voltage dips

Voltage Dip Type Phase diagram Phase voltages Voltage Dip Type Phase diagram Phase voltages

A R S T U hU 1 3 U hU j hU 2 2 1 3 U hU j hU 2 2     E R S T U U 1 3 U hU j hU 2 2 1 3 U hU j hU 2 2     B R S T U hU 1 3 U U j U 2 2 1 3 U U j U 2 2     F     R S T U hU 1 1 U hU j 2 h U 2 12 1 1 U hU j 2 h U 2 12       C R S T U U 1 3 U U j hU 2 2 1 3 U U j hU 2 2     G       R S T 1 U 2 h U 3 1 3 U 2 h U j hU 6 2 1 3 U 2 h U j hU 6 2        D R S T U hU 1 3 U hU j U 2 2 1 3 U hU j U 2 2    

3. Implementation of Virtual Laboratory

The simulator is based on a computational Web platform that was created using an ASP .net, which is remotely communicated to a database server that stores all the information sent by students, or created by the simulation algorithm. Each student has a username and password to connect to the environment.

In order to assimilate the voltage dips characteristics, the student use this application following an education process called closed-loop process controls system (Dal, 2013) that guides them through the learning period. The teaching module is subdivided into three tutorials. The first module presents the problem and the available technical information.

3.1. Course content

In this way the laboratory was implemented starting with the objectives and pre requisites of the course. Students taking Power Quality have to use their knowledge acquired over their previous studies, like FFT that is usually discussed in the course Signal Processing. The prerequisite summarized knowledge and skills (abilities), which specifically include the following: Knowledge on MATLAB, Knowledge on FFT (fast Fourier transform), LabView, Programming languages, Excel or as skills they need: teamwork to analyse the results with a predefined schedule and deadlines and self-learning skills to solve problems they receive as project homework.

The main course learning objectives are that students: are required to learn and then to apply this knowledge to characterize a voltage dip signal.

After that the instructor coordinates and organizes the activities by giving introductory lectures and providing resources. The teacher uses the virtual laboratory in order to present a bibliography for the study. Also the students find more information in the self-study activity they have, using the Theoretical Aspects part of the virtual laboratory.

The first part of the result is obtained from the answers to the quiz, the students have to take. Teacher evaluates the results and informs the student about his score.

3.2. Project work

A good way to help students to achieve these competencies is to set project-oriented task, consisting of case study they have to solve, putting into practice the knowledge they got from the teacher. Also, feedback is required from students at the end of training period. In one of the topics from this part the students are able to visualize some completed analyses, designed to exemplify the main concepts of voltage dips. After that each student will download, from the Assignments window, a generated voltage dip signal. This signal is generated using the mathematical model described above, and exported in Excel format. In the same time the information regarding the signal and the voltage dip characteristics are saved in a local database. The signal generator use as input parameters random set of data. Each input parameter is generated as a value in a range of possible entries, as described in Table 2.

Table 2. Voltage dip parameters range Dip Amplitude [pu] 0.6 – 0.8 Harmonics

φ[º] -30 – 30 Amplitude [%] 0 – 5

Dip duration [s] 0.01 – 1 φ[º] 0 – 360

Voltage variation [%] -10 – 10 f [Hz] 150, 250,…, 1250 Frequency variation [%] -2 – 4 Noisy Frequency

Dip Type a, b, c, d, e, f, g Amplitude 0 – 5

Number of Harmonics 0 – 10 φ[º] 0 – 360

Number of Noisy Frequencies 0 – 10 f [Hz] 0 – 10000

They will have to analyse the signal in order to get the following information: x The real voltage and the nominal voltage

x Voltage dip amplitude x Voltage dip duration x Point on wave x Phase angle jump

x Voltage dip type according to the ABC classification

In order to do this the student has use the methods described in Theoretical Aspects, and to implement them in using the knowledge about FFT, programming languages and Excel.

3.3. Learning Evaluation

After finishing the homework project the student has to fill-in his results in the Check Results window. During submit, it will be displayed the correct results taken from the database and the student’s results are sent to the teacher. At the end of the module, students have to write a final report describing their choices and the results obtained. This has to be as a work group where the students have to collect and analyse the results they got. For this they have to create a report containing information about number of dips for each type, amplitude and duration range. Afterwards, students can provide some explications regarding the possible causes that may produce such sets of voltage dips.

4. Conclusions and Future Work

The tool gives students the possibility to analyse different voltage dips, which would take too much time in a real laboratory. It is clear that this learning module improved students’ understanding of the topic.

Student assessment supports the conclusion that the proposed analyses framework offers a useful mechanism in student practical work for understanding and explaining the observed phenomena. One of the advantages of the tool presented here is that it allows the instructor to set up simulationswith no effort, which obviously helps in showing many different cases.

Finally, in surveys, students provided positive evaluations of the value of this type of tool in their professional training. Future development of the tool will focus on extending the modules to other power quality phenomena. Acknowledgements

This work was partially supported by the strategic grant POSDRU 107/1.5/S/77265 (2010) of the Ministry of Labor, Family and Social Protection, Romania, co-financed by the European Social Fund – Investing in people. References

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