2009 International Conference on Education Technology and Computer
A Novel Gui Simulation Program Designed for Teaching of Underground Power Cables
Yunus BİÇEN
Faruk ARAS
Department of Industrial Electronics, Duzce University TR-81010 Uzunmustafa Düzce, Turkiye
[email protected]
Department of Electrical Education, Kocaeli University TR-411380 Umuttepe Izmit, Turkiye
[email protected] numerous possibilities of giving the users a practice and tuition in the wider aspects of power system [4, 6]. The specific computer programs provide important gains to the students on topics of electrical applications. However there are not enough simulation programs examined with the special topics of electrical engineering education. Available commercial programs have not any educational aspects for students, and their costs are very high. For these reasons, a program is created using GUI (Graphical User Interface), without commercial concern, for graduate students who choose the power cable course which has been given at department of electrical education in Kocaeli University since 2002 [7]. The first computer simulation program including only steady state analysis of power cable was studied in 2006 [7]. The program has been implemented with light of the view of the students and added new modules such as transient analysis for variable load or time entering alternatives and wide power cable library. Thus it can be also used by different academic level groups and engineers in power system industry.
Abstract—This paper presents a novel computer simulation program designed to aid teaching process in course of underground power cables for engineering students. The calculation of power cable ratings uses several different formulas and long analytical processes based on IEC 60287 and IEC 60853 standards. Therefore it is difficult to examine different variations such as power cable properties, installations and operation conditions, during the restrictive lesson time. For these reasons, a program has been developed to use for teaching process of the power cable course. The program allows the users to effectively learn the rating of underground power cables under different operation conditions with illustrative figures. The program details are presented and explained in this paper. Keywords-underground power cables, transmission line, engineering education, computer simulations
I.
INTRODUCTION
Today, many educational computer programs have been developed as a classroom teaching aid or as an individual self-study utility for outside the classroom. These programs invigorate the teaching and learning [2]. In addition, an educational program should have some features unlike industrial commercial programs, such as easy usage, comprehensive useful menu, alternative choices, self-study specialties, guidance and advanced help et cetera [3]. The properties are important for fast comprehending and spending their time to gain an intuitive feel for the system, as much as possible [4]. On the other hand, computer simulation programs have played an important role in learning process of engineering students with a better understanding of topics [5]. Technical education is completely affected by changing technology and, it should be updated in conformity with improvements. However, providing the experiment set up systems very difficult and costly, especially for high voltage and larger power systems in electrical engineering education. Moreover, the power systems have a lot of mathematical equations which should be explained with theory and problems [6]. Therefore, well-designed programs has to be used in engineering education for the solution of much complex systems which can be analyzed and projected with
978-0-7695-3609-5/09 $25.00 © 2009 IEEE DOI 10.1109/ICETC.2009.65
II.
BASICS OF POWER CABLE RATING
Power cables are the most important devices of power transmission and distribution systems and particularly used in densely populated urban areas due to safety, reliability and aesthetical consideration. At present, high voltage underground XLPE cable systems up to 500 kV which circuit lengths up to 40 km are in operation worldwide. Insulated power cables consist of a solid cable core, a metallic shield and a non-metallic outer covering with semiconductor layers as shown in Figure1, and the heat generated in the cable is dissipated to the environment trough the layers which are important for calculating ampacity (ampere carrying capacity). Tins =T1 : Main insulation thermal resistance (K.m/W) Tprt =T3: Thermal resistance of Insulation shield including the other insulation parts (K.m/W) Text=Ta+Td =T4 : External thermal resistance (K.m/W) Wc : Power cable conductor loss (W/m)
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Soil Sand
θ c − θ a − Wd .TB
I=
Rac .T A
III.
The first model was proposed for calculating ampacity of underground cable by Neher-McGrath in 1957 [1]. The Neher-McGrath Model has been widely accepted for over 50 years. Today, the greater majority of utilities and cable manufactures have been using the IEC standards based on the Neher-McGrath Model. An assessment on ampacity calculation has been presented comparing analytic and numerical methods [8]. IEC standards, steady-state (60287) and transient (60853), uses thermal model of power cable system included thermal resistances (Tth) and thermal capacitances (Qth) similar to electrical circuits [9,10]. Thermal model of a cable system is shown in Figure 2.
A. Program Algorithm The algorithm starts with inputs of data and choice the options included cable, installation and operation properties. The data are defined with variables in the program process for users. While the program is making complex procedures for calculating ampacity, it has a simple interface for users. In addition, the stages of the program follow a meaningful and sequential structure for easing the perception. First, users must choice a power cable form and its installation conditions. Afterwards the appeared box according to choosing stage must be filled in form of numerical values. These operations continue until the result table stage. However, the result window is the stage which is selected to steady state or transient analysis of power cable. Both result stages and analysis stages have graphical support in addition numerical values. In every stage, users can benefit from HELP menu or guidance text on window. The program has ability to make successive repetitions. Hence, users can make successive comparisons by changing one or more data.
Thermal circuit of a power cable
When the conductor is energized, heat loss (I2.Rac) is generated within the cable [10,11]. And others are the dielectric losses in the insulation and losses in the metallic parts of the power cable. The thermal circuit is then modeled by an analogous electrical circuit in which voltages are equivalent to temperatures and currents to heat flows [9,10]. In Figure 2, the thermal capacitances cause time delay of the temperature rise and considered in the transient operation with heat sources and thermal resistance. The thermal resistances are only used in calculation of ampacity in the steady state operation excepted thermal capacitances. In this case, equation (1) can be written by considering an insulated power cable [9]. ⎛ T1 ⎞ + T3 + T4 ⎟ ⎝2
⎠
θ c − θ a = Wc [T1 + (1 + λ )(T3 + T4 )] + Wd ⎜
TA
STRUCTURE OF THE PROGRAM
The simulation program is created by using GUI (Guide User Interface) added with m-files and Simulink files. M-file execute the program algorithm according to user action. Mfiles are also mainly iterative calculator determined power cable ampacity and initial values for transient analysis.
Figure 1. Sample power cable (a), parts (b)
Figure 2.
(2)
B. Capabilities of the Program The program gives 180 different combinations and options to users such as cable shape, cables materials and installation conditions are preferred prevalent applied in industry. They are present under the following titles. • • • • •
(1)
TB
In this equation, θc is maximum operating temperature of conductor (oC), θa is ambient temperature (oC), Wc is conductor loss (I2.Rac), (W/m), Wd is dielectric loss (W/m), λ: shield loss factor, T1, T3, T4 are thermal resistances of insulation, covering and external environment of the cable, respectively and depend on a few parameters such as structure of the cable, insulation materials, depth of laying, laying type, and thermal characteristics of the soil and other materials. The cable ampacity can be found from equation (1) as follow;
• • • •
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Structure design of underground power cables Install configuration (directly buried, duct b., pipe) Various environment conditions (soil, sand, concrete, thermal backfill) Analysis of underground power cables under various operation conditions Iterative techniques based on IEC-60287 standards for steady state. Steady state analysis for various structure and thermal properties of cable and surroundings with thermal circuit models Thermal model based on IEC-60853 standards for transient and also temperature is determined by transient analysis. Ampacity calculation Transient analysis for determining cable temperature on variable load current Graphical notation on window for both steady state and transient analysis
C. Program Modules The program includes three main modules as windows. The first one is selection module which is placed several options about power cable properties, cables form and environment conditions. Second module is the stage inputs of data related with power cables and installation conditions. The last module displays the results of analyzing. 1) First Module: The installation conditions and cable structure are very important in ampacity calculation [12]. As an example, a window of this module is shown in Figure 3. The insulation type of the cable is chosen by user and also, HELP button can guide to user on every stage as shown in Figure 3.
Steady state analysis • Effect of changing values of the cable and environment parameter • Effect of different cable parameters Transient analysis • Short-duration • Short-duration with step load • Long-duration • Long-duration with step load a) Steady State Analysis: Power cable ratings are computed according to the selected configuration and inputs by the program user. As shown in Figure 5, calculated ampacity and important parameters of the sample power cable such as a.c resistance of conductor, dielectric loss, loss factors of metallic layers and, thermal resistances of insulation parts of the cable can be viewed on the result window as shown at Result Table. All the results computed in steady state analysis are also used at transient analysis. Analysis blocks are circled in two different kinds of analyses (1, 2), as shown in Figure 5.
Figure 3. Parameter choice popup menu
2) Second Module: According to the selected parameters in the first module, definition of parameters and edit box for inputs of data with auxiliary figures are placed in this module as shown in Figure 4. In addition, abbreviations on auxiliary figures help users. Figure 5. Result window and analysis blocks
The first analysis block numbered 1 shows graphically the effects of changing values of selected parameters on ampacity. Using this module, the following effects can be examined by selection of any parameters; • • • • • • • • •
Figure 4. The window of data inputs and auxiliary figures
3) Third Module: In this module, the results are analysed under two main stages according to the inputs and the selections. The stages are given as follows;
Ambient temperature (oC) Diameter of conductor (mm) Insulation thickness (mm) Frequency (Hz) Spacing between conductors (mm) Cables buried depth (mm) Soil thermal resistance (K.m/W) Cable ac resistance (Ω/m) Conductor temperature (oC)
All of above parameters affect the cable ampacity directly. For example, ambient temperature is an important parameter for ampacity calculation since it depends on season or climate of each country. In winter season, soil
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ambient temperature is 18oC in Australia while it is -5oC in Canada [13]. Other important parameter is soil thermal resistance. Thermal characteristics of the soil affect the thermal resistance of the soil as shown in Table 1, and increase of its value reduces the ampacity of power cable. Therefore, thermal properties of the surrounding soil, environment temperature and the burying depth of the cables alter the ampacity excessively. [13, 15]. The other above listed parameters must be considered to determine the installation and operation conditions.
(a)
(b)
Figure 7. Analysis results of parameter types changing (a,b) TABLE I. Thermal resistivity (K.m/W)
0.7 1,0 2,0 3,0
THERMAL RESISTIVITY OF SOIL [13] Soil conditions
Very moist Moist Dry Very dry
b) Transient Analysis: In the stage of transient analysis, the thermal capacitances of the cable parts and soil must be considered [10,13]. Thermal model is produced by the program according to inputs of cable types and installation conditions. Computed parameters in steady state stage can be also used in transient analysis and viewed in a window included power cable losses (Wc, Ws, Wt), thermal resistances and thermal capacitances of cable layers and environment (T1,...,T4’’ ), (Qc,…,Qd), Van-wormer coefficients (p,p’,p” ) as shown in Figure 8. Before starting analysis, one of the short or long duration for transient analysis must be selected.
Weather conditions
Continuously moist Regular rainfall Seldom rains Little or no rain
At Steady State Analysis, after the selecting a parameter in the window in Figure 5 to examine its effect on ampacity, a new window is opened as shown in Figure 6. The variation of the parameter versus ampacity can be seen when its ranges are input in the window. As an example, the range input window and plot of ambient temperature versus ampacity are shown in Figure 6.
Figure 8. Data window block for transient analysis Figure 6.
However, as an alternative, input of step load is also available in the same window by clicking the “continue for step load function” button, the window Figure 9 is opened. Transient analysis is carried out according to the selections. For transients lasting longer than one hour, the capacitance of the insulation is divided between the conductor and the shield positions. it has been found necessary to divide the insulation into two portions having equal thermal resistances for the shorter durations. The thermal capacitance of each portion of the insulation is then assumed to be located at its boundaries, using the Van Wormer coefficient according to the Van Wormer method [9,10].
Steady State analysis window
In the second analysis block (2) in Figure 5, effect of different cable parameters such as type of conductor and insulation, shield material and installation shape on ampacity is examined. Although the types of the parameters change, the values of the parameters range are constant during the computation process. Figure 7(a) and Figure 7(b) show the effect of the conductor type and insulation type on ampacity, respectively.
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IV.
CONCLUSIONS
The new designed program has very useful modules which support the development of knowledge and skills of the engineering students in power cable ratings included 180 different combinations and options. It can offer the user a wide of learning opportunities not always available in commercial programs. It concludes that the graphical language of the program allows the user to construct a power cable and to analyse it under any operation and installation conditions freely. REFERENCES [1] Figure 9. Variable load ranges input window
[2]
Normally, in steady state condition, current is inputted as a constant value that it is taken maximum value (p.u. =1, % 100 load factor) at the initial choices specified by user. But in transient stage, the load current of power cable can be defined as a step function of time like a square wave. Consequently, the losses related metallic layers except dielectric loss vary with cable current changes in time. On the other hand, the temperature of the power cable depends on the current changes because of the loss of conductor and the other losses. In addition, external heat sources or other cables in cable route cause the temperature rise of the referenced cable which will be analysed. Therefore, by the program, the temperature for each cable is obtained at each time step adding to its own temperature [9].
[3]
[4]
[5]
[6] [7]
[8]
[9]
[10]
[11]
[12] Figure 10. Variable load and temperature changing response [13]
For step load function, user must enter the range of time and variable loss of the power cable in the course of time. When the analysis is started, Simulink model is operated and the results which are obtained using the principle of superposition are graphically viewed on the screen. Power cable temperature rises or decreases are depend on the combination of thermal capacitances and resistances formed by the model with step load function [11,14].
[14]
[15]
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