ASSESSMENT OF EFFICIENCY AND CONDITION BASED OPTIMUM LOADING OF TRANSMISSION LINES A dissertation submitted to the Department of Electrical Engineering, University of Moratuwa in partial fulfillment of the requirements for the Degree of Master of Science by WITHANAGE DON ANANDA JAYASIRI CHANDRAKUMARA Supervised by: Prof. HYR Perera and Eng. LAS Fernando Department of Electrical Engineering University of Moratuwa, Sri Lanka September 2005 University of Moratuwa 11111111111111111111111111111111111 84772 ! i..o Designing of transmission lines in Sri Lanka has been done considering average weather conditions through out the year. Whereas in the real situation, weather conditions are seasonally varying. Therefore, based on the seasonal variation of weather condition in Sri Lanka, existing transmission network can be optimally loaded delaying future construction of transmission lines. Abstract Transmission lines in any transmission network is the critical part or the one of the major limiting factors for power transfer capability of the transmission network. The thermal power transfer capability of Overhead Transmission lines is primarily a function of the height of the conductor above the ground. This height affects the safety of the public and is therefore clearly specified in legislation. Different methods for determination of Power Transfer capability of transmission lines are available. These include deterministic and various probabilistic approaches. The latter include a model simulating condition that affect the safety of the transmission line relating specially to the conductor position from which a measure of safety is developed. This measure can be used by designers to optimally design the transmission line from current loading point of view. The deterministic approach has been used by most utilities around the world, as it is quick and simple. That method assumes bad cooling conditions that will result in the line design temperature being achieved. Probabilistic methods use actual weather data and conditions prevailing on the line to determine the likelihood or probability of a certain condition. In this project, condition was taken as the conductor temperature rising up to the design temperature, which is 75 degree Celsius. IV I DECLARATION The work submitted in this dissertation is the result of my own investigation, except where otherwise stated. It has not already been accepted for any degree, and is also nol being concurrently submitted in whole or part to any University or Institution for any other degree. J Chandrakumara 29.11.2005 We I I endorse the declaration by the candidate ~ Prof. HYR Perera ~~ Eng. LAS 'Fernando A;. "' ; ' ..._ . "'·- vi ~ " CONTENTS Declaration Abstract Acknowledgement List of figures List ofTables List of Symbols Chapters 1. Introduction 1.1 General Background 1.2 Goals 1.3 Methodology used to accomplish goals 2. Operation of Transmission Network 2.1 Transmission Network in Sri Lanka 2.2 Arrangement of the transmission Network in Sri Lanka 2.3 Loading pattern of selected lines j 3. Methods used to design Transmission Line Current rating 3.1 Methods for determination of current rating 3.1.1 Probabilistic methods available 3.1.2 Deterministic Method 4. Assessment of efficiency of Transmission Lines 4.1 Design values of current carrying cap_ii~ty of Selected transmission lines " 4.2 Utilization ofTransmission Lines 4.3 Calculation of annual efficiency 5. Case study 5.1 Collection of Data 5.2 Analyzing Data 5.2.1 Sample Calculation 5.3 Summary of Calculation 5.4 Assessment of possible current ratings 5.5 Criteria for selecting optimum current rating 5.6 Voltage variation at receiving end 5.7 Assessment of Power Loss 5.8 Effect on Sag due to optimum loading IV v VI Vll X 1 1 2 2 4 4 4 4 10 10 II II 14 14 14 15 16 ; ' 16 16 . 17~ ' ... ·- v 19 19 " 19 20 21 22 ~ ii 6. Conclusion and Recommendation 6.1 Conclusion 6.2 Discussion 6.3 Recommendation for future researches References Appendix A Calculation of annual average charge Flow along selected two lines Appendix B Data collected and calculation of Current ratings at Sites Appendix C Calculated optimum current ratings of Two selected transmission line Appendix D Daily average operating Amperes of Selected two Transmission lines Appendix E Financial analysis on power loss along The transmission line Appendix F Different conductor types used in the Transmission network in Sri Lanka j Aj, ' 35 35 39 39 51 42 44 48 60 62 65 ... _ -.1 ;1 ._ ~ / Ill Acknowledgement I wish to express my appreciation and sincere thanks firstly to the University of Moratuwa for providing me the opportunity of following the Master's Degree Program in Electrical Engineering and Professor HYR Perera, Head of the Department of Electrical Engineering, University of Moratuwa and Mr. LAS Fernando, Deputy General Manager (Transmission Operation and Maintenance), Ceylon Electricity Board, who guided and encouraged me as Project Supervisors to achieve the goals of the project despite their load of work and responsibilities. Their advice and insight were immeasurable. j I would extend my sincere gratitude for Transmission Design branch, System Control Center and my colleagues and brother Engineers of Transmission Operation & Maintenance Branch of the Ceylon Electricity Board. Special thanks go to my subordinates in the Hot Line Maintenance unit of the Ceylon Electricity Board. While I regret for my inability to specifically mention individuals, I am grateful to all the staff of the University of Moratuwa and my colleagues who were helpful in numerous ways to make my endeavor a success. Last, but not least, I thank my beloved wife Thamara and children Kalana and Imalsha for their affection, appreciation, support and understanding towards me in achieving the aspiration. A~ "' .,.; ...._, .... v ~ / v List of Figures Figure Page 2.1 Present average Loading patterns of 220 kV Kotmale - Biyagama Transmission line 6 2.2 Present average Loading patterns of 220 kV Kotmale- Anuradapura Transmission line 7 2.3 Specimen Daly Load Curve of Transmission System in Sri Lanka 7 2.4 Map of selected two transmission lines 8 2.5 Sketch of the Transmission Network in Sri Lanka 9 l 5.1 Calculated Optimum Loading Patterns of 220 kV Kotmale - Biyagama Transmission line form January to March 27 5.2 Calculated Optimum Loading Patterns of 220 kV Kotmale- Biyagama Transmission line form April to September 28 5.3 Calculated Optimum Loading Patterns of 220 kV Kotmale - Biyagama Transmission line form October to December 29 5.4 Calculated Optimum Loading Patterns of 220 kV Kotmale - A'pura Transmission line form January to April 30 5.5 Calculated Optimum Loading Patterns of 220 kV Kotmale- A 'pura Transmission line form May to September 31 5.6 Calculated Optimum Loading Patterns of 220 kV Kotmale - A 'pura Transmission line form October to December 32 --~ "' ,.,. ~ - :.-... v / VI List of Tables Table 2.1 a Existing 132k V transmission network of Sri Lanka 2.1 b Existing 220 k V transmission net work of Sri Lanka 4.1 Design values of current carrying capacity of selected transmission lines 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13 Designed parameters of selected two transmission lines Sag corresponding to different temperatures Calculated Optimum Loading pattern of220 kV Kotmale- . Biyagama Transmission line from January to March .t Calculated Optimum Loading pattern of220 kV Kotmale- Biyagama Transmission line from April to September Calculated Optimum Loading pattern of 220 k V Kotmale - Biyagama Transmission line from October to December Calculated Optimum Loading pattern of220 kV Kotmale- A' pura Transmission line from January to April Calculated Optimum Loading pattern of220 kV Kotmale- A' pura Transmission line from May to September Calculated Optimum Loading pattern of220 kV Kotmale- A' pura Transmission line from October to December Calculated optimum current ratings and corresponding Voltage drop and power loss of 220 kV Kotmale - Biyagama Transmission line from January to March Calculated optimum current ratings and corresponding Voltage drop and power loss of220 kV Kotmale - Biyagarna Transmission line from April to September Calculated optimum current ratings and corresponding Voltage drop f and power loss of220 kV Kotmale Biyagama Tr~mission line from October to December Calculated optimum current ratings and corresponding Voltage drop and power loss of220kV Kotmale Anuradapura Transmission line from January to April Calculated optimum current ratings and corresponding Voltage drop and power loss of220kV Kotmale - Anuradapura Transmission line from Page 5 6 14 16 25 27 28 29 30 31 32 33 33 33 ... - 34 May to September ..,. ?4 Calculated optimum current ratings and corresponding Voltage drop. 5.14 6.1 and power loss of220kV Kotmale Anuradapura Transmission line.Jrom October to December Calculated Optimum Current rating of220 kV Kotmale -Biyagarna Transmission line ~ 34 36 VII Table Page 6.2 Calculated Optimum Current rating of220 kV Kotmale - Anuradapura Transmission line 36 6.3a Recommended optimum current ratings of220kY Kotmale - Biyagama Transmission line from January to March 37 6.3b Recommended optimum current ratings of220kV Kotmale- Biyagama Transmission line from April to September 38 6.3c Recommended optimum current ratings of 220k V Kotmale- Biyagama Transmission line from October to December 38 6.3d Recommended optimum current ratings of220kY Kotmale- Anuradapura Transmission line from January to December 38 AI Actual average flow of charge along 220 kY Kotmale -Biya{ama A2 Bl B2 B3 B4 CI C2 C3 C4 C5 C6 C7 C8 C9 CIO Cll Transmission line Actual average flow of charge along 220 kY Kotmale - Anuradapura Transmission line Data collected and calculated current ratings at Kotmale Data collected and calculated current ratings at Biyagama Data collected and calculated current ratings at Anuradhapura Data collected and calculated current ratings Mahailukpallama Probability Distribution of current ratings for 220 kV Kotmale- Biyagama Transmission line from January to March Probability Distribution of current ratings for 220 kV Kotmale- Biyagama Transmission line from January to March Probability Distribution of current ratings for 220 kV Kotmale- Biyagama Transmission line from April to September Probability Distribution of current ratings for 220 kV Kotmale- Biyagama Transmission line from Aprit,..to September Probability Distribution of current ratings for 220 k'V Kotmale - Biyagama Transmission line from October to December Probability Distribution of current ratings for 220 kV Kotmale - Biyagama Transmission line from October to December Probability Distribution of current ratings for 220 kV Kotmale- Anuradapura Transmission line from January to April Probability Distribution of current ratings for 220 kV Kotmale- Anuradapura Transmission line from January to April ,.,_ Probability Distribution of current ratings for 220 kV Kotmale - , Anuradapura Transmission line from May to September Probability Distribution of current ratings for 220 kV Kotmale- Anuradapura Transmission line from May to September Probability Distribution of current ratings for 220 kV Kotmale- Anuradapura Transmission line from October to December 42 43 44 45 46 47 48 49 50 51 52 53 ; ' 54 .. 55 56 57 58 VIII Table Page Cl2 Probability Distribution of current ratings for 220 kV Kotmale- Anuradapura Transmission line from October to December 59 OJ Average Operating data of220 kV Kotmale - Biyagama Transmission line 60 02 Average Operating data of220 kV Kotmale - Anuradhapura Transmission line 61. E Financial analysis on power loss along the transmission line 62 F Different ACSR Conductor types used in Transmission network 65 j .. ;. " ; ' ~ · :,.. .... yl ~ " IX List of Symbols Symbols Qc Qr Qs ta tc tr R d do Pr v ~f kr K: Ka e a Qs A' e He Zc z1 He We Ww s 0 f E T a L Description Conductor current, A Convected heat loss, W/ft Radiated heat loss, W/ft Heat gain from the sun, W/ft Ambient temperature, °C Average temperature of conductor, °C Air film temperature, °C AC resistance, n/ft Conductor diameter, in Conductor diameter, ft. Density of air, lb/ft3 Velocity of air stream, ftlh Absolute viscosity of air, lb/h j Thermal conductivity of air at temperature trW/ft. Temperature of conductor, K Ambient temperature, K Coefficient of emissivity, 0.23 to 0.91 Coefficient of solar absorption, 0.23 to 0.91 Total solar and sky radiated heat, W/ff Projected area of conductor- d/12 Effective angle of incidence of the sun's rays, degrees Altitude of sun, degrees Azimuth of sun, degrees Azimuth of line, degrees Elevation of conductor above~ level, ft Conductor weight ' Wind force on conductor Catenary length along conductor Sag Stress or T I A Young's modules Tension of the conductor ; ' Coefficient of linear expansion of conductor .... ~ · Span "' ,.~ X Chapter 1 Introduction 1.1 General background Transmission lines are the most essential and more expensive part in any national grid. Since a transmission line need to travel across the country in different terrain having different weather conditions, designing and construction of such transmission line is extremely tedious and expensive. Currently, transmission network of Sri Lanka is operating on two t]Ilsmission Voltages namely, 132 kV and 220 kV. Presently most of the hydro generation is confined to the central hill region of the country, and requires to be transmitted to load centres where population density is high and industries are based. The country is experiencing about 1 0% of Electricity Demand growth while 73 % of total houses have presently been electrified. Therefore in order to cater for future demand, generation and transmission capacities are required to be increased accordingly. In order to increase the transmission capacities, it is required to construct new transmission lines. As we are aware that Sri Lanka is a small island and already considerable area has been utilized for existing Transmission Network. In future, construction of such transmission lines is much more difficult due to objections coming from environmentalists and the general public. Therefore it is badly needed to find alternatives to increase the transmission capacities of the existing Transmission Network f This project deals with investigating the possibilities t{ asses the present operating efficiency or utilization factor of Transmission lines and to schedule a optimum loading pattern of the selected two transmission lines from the set of existing transmission lines. ' The loading of a transmission line is governed by the Current carrying capacity of the conductor strung on the transmission line. In the transmission network of Sri Lanka, Aluminum Conductor Steel Reinforced (ACSR) has been used having different ratings. Those conductors are normally identified by the name of animals such as Zebra~oat ..... Lynx etc. (see table C 1 in Appendix C) ..,. ~ ,/ There are two basic types of conductors available namely homogeneous & non- homogeneous conductors. Homogeneous conductors can be categorized further in to two types namely All Aluminum Conductors (AAC) and All Aluminum Alloy Conductor (AAAC). Non-homogeneous conductors can be categorized into two types namely Aluminum Conductor Steel Reinforced (ACSR) and Aluminum Conductor Alloy Reinforced (ACAR). At present, National Grid of Sri Lanka is having two Transmission networks. One with 132 kV High Voltage lines and the other with 220 kV Extra High Voltage network. 132 kV Network is having nearly 42 transmission lines with a total of 1500 km length. The other network is having 8 transmission lines with total length of350 km. Most of the transmission lines in the 132 kV Transmission network is fairly old therefore those lines are now planned to be replaced by 220 kV Transmission lines. Therefore the investigating the possibilities for optimum loading of such transmission lines is not worth. Therefore the study was focused mainly on the 220 kV Transmission network. The case study in project has been confined to selected two transmission lines, one running from south to north and the other running from east to west to represent total220 kV network. (See figure. 2.1) j 1.2 Goals The main objective of this study is to assess the present efficiency of the two selected transmission lines and prepare a schedule for optimum loading pattern for the selected transmission lines from the existing transmission network without exceeding designed criteria given below, (a) Minimum Ground Clearance (b) Maximum allowable conductor temperature (c) Hardware properties The design criteria (a) and (b) can not be violated due to the fact that the safety of the public is governed by the criteria given in Technical Specification for transmission lines [11]. 1.3 Methodology used to accomplish goals .. ~ " The following methodology was used to accomplish goals a. Study of the transmission network , The operating criteria and variation of the demand through out day of the transmission network were studied. The most important transmission lines for the study were selected. • · b. Study on literature on transmission lines :. .. Study the past and present practice of designing of transmission lines "in the Ceylon Electricity Board typical transmission line design, [7]. Similarly, other standard and available formal and informal methods of designing of transmission lines in other countries were also studied. 2 c. Study on loading parameters of transmission lines The parameters of a transmission line, which govern the loading of a line, were studied [6]. The ways and means to control such parameters to improve the loading of a transmission lines were investigated. d. Selection of sample lines. Two transmission lines for a case study were selected in such a way that both transmission lines can reasonably represent total transmission network of Sri Lanka. The operating pattern of both transmission lines was studied. e. Collection of data The data which control the parameters of transmission line loading for the selected two transmission lines were collected on reasonable interval for reasonat11e period. Present operating data of selected two transmission lines were also collected. f. Identification and developing of a model A model was developed based on accepted design practices to calculate the current rating using data collected. A probabilistic approach was developed based on reasonable criteria to select optimum current ratings for a three-hour period of time. g. Study on effects due to loading on calculated optimum ratings The voltage regulation, power loss and variation of sag of the transmission lines were calculated for the estimated optimum current ratings to recommend exact optimum current rating. --~ " ;' ~ · :.~ .. yl ~ ,/ 3 Chapter 2 Operation of transmission network 2.1 Transmission network in Sri Lanka The transmission network in Sri Lanka is operating in two Voltage levels 132 kV and 220 kV. Presently 132 kV Transmission network is being upgraded to 220 kV in order to improve the power transfer capability and to reduce the transmission losses. (Transmission network in Sri Lanka is shown in Figure 2.5) 2. 2 Arrangement of the transmission network in sA Lanka There are about 50 Transmission lines in commercial operation island wide. The length of the 220 kV Transmission Network is about 350 km and that of 132 kV systems is about 1500km. (List of the transmission lines is given in Table 2.1 a and 2.1 b). 2.3 Loading Pattern of selected transmission lines At Present, almost all the transmission lines are utilized to their full capacity. Some of the transmission lines are operating at its maximum permitted Ampere rating during peak hours causing bottle necks to the system leading to system failures when one of the transmission lines fails. Therefore, any possibility to permit extra power flow through such transmission lines can eliminate bottlenecks and high stresses when operating the system at peak demand. Presently Sri Lanka transmission network experiences peak demands at daytime and nighttime. During daytime, system experiences two peak demands, first peak demand occurs between 05.30 hrs and 07.30 hrs. That is mainly due to morning activities in houses. The other daytime peak demand occurs between 9.30 hrs and 12.30 hrs due to co.ncement of office and other related activities. The highest peak demand in the system occurs in the night between 18.30 and 21.30 hrs. The night peak demand occurs mainly due to lighting load in the country (Daily Load Curve on 15.01.2004 is given in Figure 2.4 for reference). During peak demand in the system, whole transmission network is fully utilized. Therefore network is now being operated in full capacity. "" The average loading pattern of selected two transmission lines, 220 kV Kotmale - Biyagama Transmission line and 220 kV Kotmale- Anuradapura Tr~wiss~ line are shown in Figure 2.2 and Figure. 2.3 respectively. The loading patterns of both transmission lines follow the same pattern of Daily load curve. In additi:J· ~- r>j~ v ~ ~v ~ ~ ~ ~ ~ ')., ., Time of day Figure 2.3 Daily Load Curve on 15th Jan. 2004 -:..•- v' .;' .._ /., 7 G ~ \ ~~~ 2•1-:\C::.' 1o)OOWW\'--. ) .. c.-\~~.: \ Anuradapura ' A~ ' Biyagama Figure 2.4 -Map of selected1Wo transmission lines Leg~nd llti'Y ~,..,_. hilt~ u•r 2000 ~ - • · 210'-V lii)C'"• Pw.,_..,.. t1'200$ tla'Y lnr .. £t,,,.,..u ot 2000· tl1~Y l~t~e . UNtt'O'~ CaDit 1llkV L..nc. P~N'IH ty 1~ ~ t20.V GS . (U1ollf'l9 u tJ 2000 C ~YC.S ~CIDf1'00S ~ 1u .. v GS . E14t"'t •• et 2000 (5; 1ll•~CS ,..,._.,100~ Q Hroro Poet 1 S....t.tCI" E u•c~n9 e HyOro ro.tr S~:O'I ~d • . .;_. ltlf'~~,.._~ , Stat*"Eu.t""'jj E -· TNnuf~( ''"'*",......,.. ........ ~· /"""''' ) -~ loteO~i''U vi ~ " ... \ \ \ A.aN ihiU ,.:· :{.!' r;.;;;.; ----":~. ' Legend 2zo-.v """' Et<~hl'lc; u or tOOO U Ot.V ltnt P~.a f'W'IofO 0, 2'00) ll2tV lr~t h •11"'9 u 0' lOOO 1l2'\V l ll'\f IJI'IOct;r~C"tlf' 133Y llfllf ,lollt.NO ... 20t» ~ - UO&Y GS Ec-11"'9 u 01 2003 c ZZOtV CS ,._ .... X!O~ ..: 1l1t.VCS: lt.•..,.-9Ut~t:\ll00 r:; 1ll•VGS ...._.,.,101:~ ~ H¥oro 'oe-t ' .St ollO'\ [m1r~ e Wyflto fto-t ' S' a'oon ~td ... ~m.al ,. .... · $ 1l t ll0f'l Eu•••; e ~"'1!\&1 f...-t• SWI\YI P\.1Mt4 0 50km ~ . ,. '>'•··/ -' ""'' ~=.· _) ·-· ~"'~-~~ ..... ~· ~ ... .. "' F~gure 2.5 Sketch oftransmrss10n network of Sri Lanka in year 2005 Chapter 3 Methods used to determine transmission line Current ratings The power transfer on transmission lines affects the sag of the conductors and hence the height of the conductor above the ground. This in turn affects the safety of the public. The determination of the allowable power transfer is thus not only a function of properties of the conductor but also of the public safety. It is thus essential that the designers are aware of the factors that affect the safety as well as the types of accidents or factors that are pertinent to the utility. l 3.1 Methods for determination of conductor current rating Basically following methods have been adopted by most of the utilities all over the world for determination of current carrying capacity of a transmission line conductor [2],[3). a. Probabilistic approach b. Deterministic approach The deterministic approach assumes certain bad cooling conditions (low wind speed, high ambient temperature etc.) and calculates the current that would result in the line design temperature. The line temperature or design temperature is the temperature at which the height of the conductor above the ground is minimum permissible. The deterministic approach has been used by utilities for number of years. The draw back of the method is that it does not address the safety or the relationship between the safety and the power transfer capability. Power utilities at present are designing and operating .. tpeir lines and power systems " based on inter alia, the allowable current (or ampacity) that can flow down the line. This current is usually calculated using deterministic approach assuming bad cooling conditions. It is assumed that by limiting the current the safety criteria will be met and the line will not contravene any regulations. ..;' It is also known, however, that this condition may result at some stage in the conductor exceeding the line design temperature causing line to be under clearance. What is needed therefore is the quantification of the safety ofthe design. :.<. ~ The probabilistic approach uses the actual weather conditions and data prevailiqg on the line to determine the probability of a certain condition occurring. Such a cori'dition could be for example, conductor temperature rising above the design temperature. These methods have been developed to include a measure of safety of the transmission line. This can be used as a means of comparison of practices between utilities in all countries. 10 3.1.1 Probabilistic Methods Available There are three main probabilistic methods available at present. Method 1 The first is a method whereby the probability of an accident occurring can be quantified. The benefits of this method are that an absolute measure of safety is achieved. The draw back is that the nature of the parameters is extremely difficult to determine. In addition correlation between the parameters need to be determined. This could vary form country to country, [2]-[5). Method 2 I The second method uses the existing weather data to determine the temperature of the line conductors for a given current flow. The amount of time that the temperature exceeds the line design temperature can be determined for each current level. The utility can then decide on the current level based on the percentage of excursion or "exceedence". The advantages of this method are that it is relatively easy to determine the percentages and decide on a level by which to operate. The disadvantage is that there is no way of determining the difference in safety. An adaptation of above method is to simulate the weather data and the current flow to determine the cumulative distribution of the conductor temperature as a function of current. This curve could be used to determine the current and excursion leve~ [2)-[5). Method 3 The third method is to simulate the safety of a transmission line by incorporating all the factors that affect the safety of the line. From this method a measure of safety can be developed whereby the practices in different countries can be compared objectively . • The advantages of this method are that all factors ar~"tonsidered. The variation of occurrence of objects under the line can be related to the safety of the line_, [2]-[5]. 3.1.2 Deterministic Method ; ' Thermal Rating Parameters The temperature of an overhead conductor is primarily a function o( .cu~t flow, ambient conditions (wind speed and direction, air temperature, and solarj~diation), and the physical properties of the conductor (electrical resistivity, emissivity, absorptivity, and specific heat). Some of the important points that shall be considered when calculating thermal rating of conductors are as follows; • Ohmic (I2r) line loss, the main source of heat input to the conductor, is a function of electrical current and conductor resistance. Heat input from solar radiation is a II much smaller source, even during daylight hours, and is impacted by the conductor's absorptivity. • Heat is lost from the conductor primarily through convection and radiation. Air temperature and wind speed, and direction impact of convection loss, conductor emissivity controls radiation loss. • The temperature of the conductor is an indirect measurement of the thermal energy (heat) stored within the conductor. As thermal energy is added, much of it is stored within the conductor, raising the conductor temperature. This thermal storage capacity depends on the specific heat of the conductor material. The steady state current rating and short circuit capacity is calculated based on IEEE - 738 - 1986, [6]. The equations stated in this standard are as follows; Basic Heat balance equation Heat generated due to resistance of the conductor = fr Heat gained from sun = qs Heat loss due to convection = qc Heat loss due to radiation = qr Heat gained = Heat loss fr + qs = qc + qr Therefore, I = ..J(qc + qr- qs)/r j Forced convected Heat Loss is given by following empirical formula qcl = {1.01 + 0.37l(dprVIJ,J.r)052}kt(tc- ta) W/ft qc2 = 0.1695(dprY/J.J.r)06}kt(tc- ta) W/ft .. ,. " The maximum value of qc obtained from Eq (3.3) or (3.4) is used. Natural Convection Heat Loss at Sea Level is given by following empirical formula .... - (3.1) (3.2) (3.3) (3.4) qc = 0.072.d0·7\tc- ta)l.25 W/ft • (3.5) ~ Radiated Heat Loss is given by following empirical formula ~-.. yf qr = O.l38.d.e.{(kc/ 1 00)4 - (ka/1 00)4 } W/ft (3.6) 12 Solar Heat Gain is given by qs = a.Qs.(Sin9)A' 9 - Cos·1 {(Cos Hc).Cos (Zc- Z1)} Identification of Letter Symbols I Conductor current, A qt Convected heat loss, W/ft qr = Radiated heat loss, W/ft qs = Heat gain from the sun, W/ft ta = Ambient temperature, °C tc = Average temperature of conductor, °C tr = Air film temperature, °C r = AC resistance, Q/ft d - Conductor diameter, in do= Conductor diameter, ft. pr= Density of air, lb/ft3 V = Velocity of air stream, ftlh !lr = Absolute viscosity of air, lb/h kr = Thermal conductivity of air at temperature tr W /ft. Kc = Temperature of conductor, K Ka = Ambient temperature, K e = Coefficient of emissivity, 0.23 to 0.91 a = Coefficient of solar absorption, 0.23 to 0. 91 Qs = Total solar and sky radiated heat, W/ft? A'= Projected area of conductor - d/12 • 9 = Effective angle of incidence of the sun's rays, degrees ".t' He= Altitude of sun, degrees Zc = Azimuth of sun, degrees Z1 = Azimuth of line, degrees He = Elevation of conductor above sea level, ft. (3.7) (3.8) / ~ ~ - :..~ ... .., ~ / 13 Chapter 4 Assessment of efficiency of transmission lines Transmission l\etwork in Sri Lanka comprises around 50 different lines. Studying all these lines is beyond the scope of this project. As a case study, the following two transmission lines have been selected for the purpose of assessing the efficiency. I. 220 kV Kotmale- Biyagama double circuit duplex conductor transmission line / 2. 220 kV Kotmale- Anuradhapura Single circuit simplex conductor transmission Line Since the designing of transmission lines in Sri Lanka has been carried out using deterministic model, current carrying capacity of Overhead Transmission lines have two different ratings based on day and evening time through out the year. 4.1 Designed values of current carrying capacity of selected Transmission lines The present Daytime and Evening current ratings are given in table 4.1 I Name of the Transmission line Current carrying capacity (A) Day time Evening 220 kV Kotmale- Biyagama Transmission line 4*726 4*987 220 kV Kotmale- A' pura Transmission line 726 987 --:r Table 4.1 -Design values of current carrying capacity of selected transmission lines 4.2 Utilization of the transmission lines. ; ' The main purpose of a transmission line is to transfer bulk power from the point of generation to the point of delivery. It is required to-transfer reactive and active power along the transmission lines. Therefore, in assessing the efficiency or utilization fa~, it is more versatile to consider flow of current along the transmission line tHough it contributes to increase the transmission loss. Therefore, in assessing the efficiency,~ annual efficiency was calculated based on flow of charge {Ampere * time). (Please see table A 1 and A2 in appendix A) respectively for calculation of hourly flow of actual charges. 14 4.3 Calculation of annual efficiency of transmission lines. Annual efficiency = Annual actual flow of charge *I 00 annual expected flow of charges Annual actual flow of charge = te *C*K* Ae -t- td*C*K* Ad Where Ae Ad te td c K = Evening rating of the conductor of the line = Day time rating of the conductor of the line = time duration for evening rating of the conductor f = time duration for day time rating of the conductor ·" = Number of circuits ofthe line =Number of conductors per phase 4.3.1 220 kV Kotmale - Biyagama Transmission line Annual expected flow of charge = (18*2*726+6*2*987)*365 Ah Annual average actual flow of charge (See table A1 in appendix A) Annual efficiency (utilization factor) = 13862700 Ah = 1628861.4 Ah = 11.75 % 4.3.2 220 kV Kotmale - Anuradhapura Transmission line Annual expected flow of charge Annual average actual flow of charge ( see table A2 in appendix A ) Annual efficiency (utilization factor) = (18*726+6*987)*365 Ah = 6931350 M' = 1118148.8 Ah = 16.1 % -;..~ ... yl ; ' ~ · 15 Chapter 5 Case Study Case study was done on selected two transmission lines described in Chapter 2. Since the designing of transmission lines in Sri Lanka have mainJy been carried out by the Ceylon Electricity Board as per IEEE 738 -1993 model [9], the designed current ratings oftwo lines are as given below in table 5.1. Transmission line Area Length Conductor Max. operat. ' Current carrying I mm 2 krn type Temperature capacity (A) oc Day evening 220 kV Kotmale- Twin 4*726 4*987 Biyagama Tr. line 2*400 72 Zebra 75 220 kV Kotmale- A' 726 987 pura Tr. line 400 163 Zebra 75 Table 5.1 - Designed Parameters of two selected Transmission lines 5.1 Collection of data The Wind velocity and the ambient Temperature in three-hour intervals for the past 5 years were collected at following locations (i.e. nearly 25000 values per location). 220 kV Kotmale - Biyagama Double circuit Duplex condu~t9r transmission line runs from Kotmale Grid Substation and terminates at Biyagama "({(rid Substation. Therefore ambient temperatures and Wind velocities were collected at sites located at Kandy (Kotmale) and Colombo (Biyagama). (See Appendix B). 220 kV Kotmale- Anuradhapura single circuit Simplex conductor transmission ..-tine runs from Kotmale Grid Substation via Maradankadawala area and terminates at Anuradhapura Grid Substation. Therefore ambient temperatures and wind velocities were collected at sites located in Kandy (Kotmale), Mahailukpallama~nd Anuradhapura. (See tables B 1-B4 in Appendix B). .. •. 5.2 Analyzing Data As per Chapter 3, the thermal rating of conductor is calculated as follows; From the equation(3.2) I= ~(Qc + Qr- Qs) /r v ., " 16 J 5.2.1.Sample Calculation Sample calculations are presented for following data taken at Biyagama for 220 kV Kotmale - Biyagama Transmission Line. Data Time Wind Speed Ambient temperature Coefficient of emissivity' Coefficient of solar absorption' Conductor out side diameter Absolute viscosity of Air (f..lr)2 Density of Air (pr) 2 Conductor AC resistance at 75 ° C Thermal conductivity of Air Calculation = 15.00 hrs a.m. on 2000 (April- September) = 33150 ftJh = 31.86 °C = 0.5 = 0.5 - 28.62 mm = 1.126 inch 0.0478 lb/h = 0.0672 lb/ft3 - 2.5237x I o-5 0/ft = 0.00864 W/ft .J Ambient air temperature in Kelvin (Ka) = 31.8 + 273 =304.8 K Air film temperature (a) Convection Heat loss = [(75 + 26)/2] °C - 50.5 °C Since wind velocity is greater than zero the forced convection heat loss is given by the equation (3.3) and (3.4). = { 1.01 + 0.371(dprVIJ..tr)052 }k~tc- ta) = {1.01 + 0.371(1.1267x0.0672x33150/0.478)?-52 }0.00864(43.2) = 39.8 W/ft ft" q cl q c2 = 0.1695(dprVIJ..tr)0"6 }k~tc- ta) W/ft = 0. 1695(1.1267x0.0672x33150/0.0478)0·6}0.00864( 43.2) W/ft .... = 42.98 W/ft Select the largest value, Natural convection Heat loss = 0.072*(0.77)0 75(75 - 31.8)125 = 6.55 W/ft Therefore qc = 49.53 W/ft 1 values accepted for Sri Lanka 2 from IEEE 738 table ~ .... v _,."' 17 ] (b) Radiation Heat loss from equation (3.6) qr = 0.138.d.e.{(kc/100)4 - (k/100)4} W/ft qr= 0.138.xl.l2677x0.5{(348/100)4 - (304.8/lOOt} W/ft qr = 4.692 W/ft (c) Solar Heat gain from equation (3.7) = a.Q5.(Sin8)A' qs e = Cos-1 {(Cos Hc).Cos (Zc- Z1)} A'= d/12 = 1.126112 = 0.094 ft2 Qs = 75.75 w; te Local Sun Time Altitude J lc 10. a.m. 62 Noon 81 Average 71.5 Zl = 90 or 270 e = Cos-1 {(Cos 71.5).Cos (139- 90)} = 78.62 ° q5 = 0.5*75.75(Sin78.62)*0.094 I Azimuth Zc 98 180 139 .. ~ " q5 = 3.49 W/ft (this value is taken as constant for all calculations) Therefore, Steady State Current Rating I = ..J( qc + qr - qs)/r = -vl(49.53 + 4.692 - 3.49)/ 2.5237x l0"5 = 1418 A :..~ .. .., ; ' ~ ~ ., 18 5.3 Summary of Calculation Summary of calculations for 220 kV Kotmale - 8iyagama Double circuit Duplex conductor transmission line at Kotmale and Biyagama are given in Annex. B. The summary was prepared for three periods of the year, first period from January to March (first quarter), second period from April to September (second and Third quarter) and the last from October to December (fourth quarter) by considering the seasonal variations of the weather. Summary of calculations for 220 kV Kotmale - Anuradhapura Single circuit Simplex Conductor transmission line at Kotmale, Anuradhapura and Mahai lukpallama are given in annex B. The summary was prepared for three periods of the y_ear, first period from January to April., second period from May to September aJtti the last from October to December by considering the seasonal variations of the weather. 5.4 Assessment of possible current rating of Transmission lines 5.4.1 220 kV Kotmale - Biyagama Double circuit Duplex conductor transmission line The calculated average current ratings at Kotmale and 8iyagama at every three-hour intervals are presented in Tables B I and 82. For each and every three-hour interval, calculated average current ratings and the probability of their occurrence are shown in Appendix C. 5.4.2 220 kV Kotmale- Anuradhapura Single Circuit Simplex conductor Transmission line Similarly, the calculated average current ratings at Kotmale, Anuradhapura and Mahailukpallama are presented in Tables 8 I, 83 and 84. fPr each and every three- hour intervals, calculated average current ratings and the probhbility of their occurrence are shown in Appendix C. 5.5 Criteria for selection of optimum Current rating ; ' The optimum current ratings were decided based on following criteria, which was proposed by the author based on the distribution of the data collected. ~ -;...-.. (a) Expected current rating should have been possible at least for a mi.nimum o( 90 percent of total time of the period or (b) Next highest current rating is selected which has less than 90 percent of Total time and greater than 80 percent provided next lower value of the Current rating is around 95 percent of total time. (c) In addition to above, distribution pattern of calculated Ampere ratings throughout the period were also considered. ~ 't / /2 19 J The above criteria was mainly developed based on the current rating corresponding to 100 percent probability and the proposed optimum current rating may create a temperature difference of less than one degree Celsius when operating in proposed optimum current rating which occur with a maximum of 10 % probability. One degree Celsius shall make no change to the sag of the conductor. (Calculated optimum current ratings are highlighted in Appendix C.) Calculated Optimum loading Pattern of 220 kV Kotmale- Biyagama double circuit Duplex conductor transmission line for three periods of the year are given in figure 5.1. 5.2 and 5.3. Calculated Optimum loading Pattern of 220 kV Kotmale - Anuradapura single circuit simplex conductor transmission line for three periods of the year are given in figure 5.4, 5.5 and 5.6. j 5.6 Voltage Variation at the receiving end of the Transmission line Due to operation of the transmission line on calculated optimum current rating which will be a higher Current rating than the designed rating, there will be a possibility of violating the standard voltage regulation in the receiving end grid substation. Therefore it is necessary to calculate the Voltage drop along the transmission line when the line is operated on calculated optimum current rating. Voltage drop along the transmission line is given by the following equation [1 0]. Vd = l(R Cos

" ' - \ ' . S l . . . . . . - ' i l i ~· - 15.00- 18.00 Freq. I Prob. events I 31 21.00- 24.00 Range Freq. Pro b. A 1200 0 0.00 1101 - 1150 0 0.00 1051 - 1100 1 0.03 1001 - 1050 1 0.03 951 - 1000 1 0.03 6 0.19 6 0.19 0.29 0.00 31 24.00 -3.00 Cum. Pro b. 0.00 0.00 0.03 - 0.06 - 0.10 0.29 0.48 1.00 Table Cl2 -Probability distribution of possible current ratings of220 kV Kotmale Anuradhapura Transmission Line from October to December on three hourly basis 59 llllQ..'WOO I •I' ! ........ M:-'1 _ .. APPENDIX - D : Daily operating currents of selected two transmission lines The operating currents of both 220 kV Kotmale - Biyagama and 220 kV Kotmale - Anuradhapura transmission lines were collected hourly for the year 2004. Average currents of each transmission line are tabulated below. ttime 1" Quarter tznd Quarter ~rd Quarter 14m Quarter Current (A) !current (A) !Current (A) !current (A) 1.00 148.59 138.17 133.40 130.03 2.00 148.59 151 59 119.48 129.17 3.00 151 .79 144.23 92.57 . 128.59 4.00 150.46 137.58 92.45 ; 129.47 5.00 135.39 139.33 137.48 129.47 6.00 138.18 158.92 144.04 135.46 7.00 128.41 152.23 163.95 130.36 8.00 132.43 154.86 155.96 145.78 9.00 182.25 205.55 252.69 180.88 10.00 188.01 203.84 252.69 175.13 10.30 160.49 - - 204.27 252.69 175.13 11.00 162.56 204.27 297.01 156.79 11 .30 183.55 192.22 292.66 156.79 12.00 201.47 186.47 299.65 199.39 12.30 197.77 186 47 297.44 191.42 13.00 187.92 189 67 273.47 179.29 13.30 187.92 189 67 275.69 189.01 14.00 186.06 193 67 303.23 189.01 14.30 182.82 194 18 303.23 189.01 15.00 185.57 194.18 283.11 188.43 15.30 183.17 194.18 283.11 188.43 16.00 175.95 194.18 24J~54 190.05 16.30 178.35 179.34 270~01 190.05 17.00 182.52 177.47 273.85 259.70 17.30 176.26 177.03 253.60 255.03 18.00 159.55 178.50 252.41 212.23 18.30 159.55 177.14 249.81 238.23 19.00 197.36 178.88 323.12 318.27 20.00 292.18 185.01 - 358.51 346.13 21 .00 263.26 149.11 311 .07 275.76 22.00 191.04 152.26 312.72 191.05 23.00 192.27 144.67 216.53 172~77 24.00 156.32 _.__ 145.34 '---- 166.04 70.98 Table 01 -Average operating data of 220 kV Kotmale- Biyagama transmission Line ·~ /~ 60 ' •• .._IC ~· 1st Quarter 2nd quarter 3rd Quarter 4th Quarter time Current (A) Current (A) Current (A) Current (A) 1.00 120.62 108.20 114.4 67.85 2.00 120.62 108.20 114.4 56.51 3.00 120.62 108.20 114 4 56.51 4 00 120.36 108.20 114 3 56.51 5.00 156.61 107.36 132.0 68.92 6.00 180.68 130.08 155.4 80.74 7.00 181 .16 116 93 149 0 86.17 • 8.00 142.87 106 25 124.6 79.22 9.00 145.08 107.69 126.4 "' 76.64 10.00 139.10 106.02 122 6 83.08 11 .00 147.94 104.94 126.4 86.78 12.00 148.06 106.56 127.3 86.46 13.00 146.27 106.56 126.4 86.68 14.00 173.89 97.78 135 8 74.67 15.00 173.89 101 83 137 9 81 .31 16.00 177.99 100.95 139.5 81 .31 17.00 161 .33 95.53 128.4 81 .74 18.00 159.58 95 28 127 4 89.06 19.00 241 .96 152.03 197.0 133.74 20.00 336.52 210.31 273.4 139.39 21 .00 311 .86 164.31 238.1 108.43 I I I 22.00 205.65 132 12 168 9 67.52 23.00 154.86 101 .77 128.3 62.43 24.00 132.19 91 .77 112.0 . 62.41 ---- --- --~ Table 02- Average operating data of220 kV ~otmale -Anuradapura transmission line ... .... .., 61 APPENDIX - E : Financial Analysis E.l Financial Analysis on Power Loss of a transmission Line Energy losses in transmission lines may result either from thermal effect (er) or from corona losses. Usually corona loss is considered to be negligible compared to the thermal losses thus the corona loss has not been taken in to consideration in the loss calculation The cost of losses produced by heating current can be influenced by different factors, listed below. a. factors depending on the manner of transporting energy dJing the period i. line loading ii. utilization pattern iii. power factor b. factors of circuit lay out i. resistance of the circuit ii. parallel connection of circuits c. Energy cost i. Cost of energy Both types of above losses have two components: (a) Demand (capacity) Loss (b) Energy loss Annual cost of demand losses in year n (ADCn) and the annual cost of energy loss in year n (AECn) can be determined from the following equations . • ADCn = DL*DC*IDC*AFC*(I+RES)*(l+ESC)~ AECn = 8760*DL *LSF*EC*( I +ESC)" Present value of cost in yearn is given by, PV(ADCn) = (ADCn)/(l+R)" PV(AECn) = (AECn)/( I +R)" DL = Demand Loss = eR/1 000 kW DC= Demand Charge Rs. per kW IDC=Incremental Demand Charge AFC= Annual Fixed Charge ; ' ~ · :..~ .. ..., ~ / 62 RES= Generating Capacity Reserved LSF=Loss factor EC= Energy Charge Rs. per kWh ESC=Energy Inflation Rate R= time value of money per time period Annual demand cost is not considered for the calculation. For the sample calculation, select 220 kV Kotmale Biyagama transmission line. Approximate cost of construction of similar transmission line = Rs.2160 (million) . Normal life span of a transmission line is taken as 20 years Current (A) PowerMW Power loss Power Annual Energy Ainual MW loss% Cost Rs. Million energy cost saving due to new line 300.00 228.62 3.35 1.5 251 .9C 125.95 400.00 304.83 5.96 2.0 447.82 223.91 500.00 381.04 9.32 2.4 699.71 349.86 600.00 457.25 13.41 2.9 1007.5S 503.79 700.00 533.46 18.26 3.4 1371 44 685.72 800.00 609.66 23.85 3.9 1791 27 895.64 900.00 685.87 30 18 4.4 2267.08 1133.54 1000.00 762.08 37.26 4.9 2798.86 1399.43 - 1100.00 838.29 45.08 5.4 3386.62 1693.31 Table E - Financial calcu lation The above analysis shows that present value of cost saved is equal to cost of construction when operating the line with approximately 2.5 % power loss 2 .5 % Power Loss is acceptable. • .. .,. ' E.2 The other factors which are inestimable for a new construction I. Loss of land, which is a limited resource. 2. Effect on the aesthetic beauty of the area 3. Electromagnetic effects 4. Cost of environmental clearances ., ,._ yl PV of cost sav1ng 721 .69 1283.00 2004.6Jl 2886.74 3929 18 5131 .99 6495.17 8018.73 9702.67 ... ... / 63 E.3 Other options available to avoid above factors Underground transmission lines can avoid some of above effects. However, the cost of construction of such transmission lines are nearly four to five times higher than that of the overhead transmission lines. In addition, it requires at least 4-year to construct the line. Therefore, when we consider the cost of construction of underground transmission lines, the above financial analysis shows that 5.0% power loss is acceptable, for cases where the only alternative is an underground line. j .. ~ " .;' ~ :..~ .. yl 64 APPENDIX - F : Different conductor types used in The Transmission Network of Sri Lanka Code Name Tiger Coyote Oriole Lynx Goat Zebra Steel Stranding 7/2.36 7/1.91 7/2.69 7/2.79 7/3.71 7/3.18 Steel Area (mm2) 30.59 20.09 39.78 42.77 75.67 55.59 Steel core Diameter (mm) 7.08 5.73 8.07 8.37 11.13 9.54 Aluminum Stranding 30/2.36 26/2.54 30/2.69 30/2.79 30/3.71 54/3.18 Aluminum Area (mm2) 131.1 132.1 170.5 183.4 324.3 428.9 Total Area (mm~) 161.7 152.2 210.3 226.2 j 400.0 484.5 1 Overall Diameter (mm) 16.52 15.89 18.83 19.53 25.97 28.62 t I Greased Weight (kg/m) 0.602 0.522 0.782 0.842 1.489 1.621 I 1 Ultimate Tensile Strength (kg) 5914 4732 7730 8137 13838 13450 Modulus of Elasticity (kg/mm2) 8200 7700 8200 8200 8200 7000 Temperature Coefficient (per deg C) 17.8xl0- 18.8x10- 17.8xl0- 17.8x10- 17.8xl0- 19.8xl0- 6 6 6 6 6 6 DC Resistance (ohmslkm) 0.2204 0.2187 0.1694 0.1576 0.0891 0.0674' Current Rating at 54 °C Day (A) 178 180 199 204 244 253 Evening (A) 365 361 432 453 658 750 I Current Rating at 75 °C Day (A) 379 377 444 464 656 726 487 483 578 607 882 987 Evening (A) Current Rating at 90° Emergency (A) 554 550 658 690 1005 1112 Fault Current I Sec. (kA)' 12.7 11.9 16.5 17.8 31.5 34.3 ":t' Table F- ACSR Conductor types ..... 65 i I i