UNIVERSITY OF NAIROBI

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1 UNIVERSITY OF NAIROBI COLLEGE OF ARCHITECTURE AND ENGINEERING DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING PROJECT TITLE: UPRATING OF TRANSMISSION LINES-A CASE STUDY OF 132KV DANDORA JUJA RD LINES I & II PROJECT INDEX: 105 NAME: WANGARI KEITH MACHARIA REG. NO.: F17/40182/2011 SUPERVISOR: DR. C. WEKESA EXAMINER: PROF. M. MANG OLI Project report submitted in partial fulfillment of the requirement for the award of Bachelor of Science Degree in Electrical and Electronic Engineering of the University of Nairobi. DATE OF SUBMISSION: 16 TH MAY

2 DECLARATION OF ORIGINALITY 1) I understand what plagiarism is and I am aware of the university policy in this regard. 2) I declare that this final year project report is my original work and has not been submitted elsewhere for examination, award of a degree or publication. Where other people s work or my own work has been used, this has properly been acknowledged and referenced in accordance with the University of Nairobi s requirements. 3) I have not sought or used the services of any professional agencies to produce this work 4) I have not allowed, and shall not allow anyone to copy my work with the intention of passing it off as his/her own work. 5) I understand that any false claim in respect of this work shall result in disciplinary action, in accordance with University anti-plagiarism policy. Signature: Date: - 2 -

3 DEDICATION I dedicate this to my family, friends and all those who helped me through. Thank you for your unwavering love and support

4 ACKNOWLEDGEMENT. I would like to acknowledge the department of Electrical and Information Engineering for entrusting me with this project. I thank my supervisor, DR. C. Wekesa for guiding me throughout this endeavour. His insightful guidance cannot go unmentioned. I would also like to thank my family for their hard work and dedication in ensuring that I have the chance to pursue this degree. I would also like to thank my friends and fellow classmates who believed in me and encouraged me to always push on. Last but not least, I would like to thank God for the gift of life, health and all the blessings that have enabled me to come this far and to finish this project

5 TABLE OF CONTENTS Contents UNIVERSITY OF NAIROBI DECLARATION OF ORIGINALITY DEDICATION ACKNOWLEDGEMENT TABLE OF CONTENTS ABSTRACT CHAPTER Introduction Main objective Specific objective Problem statement CHAPTER 2: LITERATURE REVIEW Growth in demand for power in the past present and the near future Right of way hinderances (ROW) Methods of increasing power flow through a transmission line Definition of uprating/upgrading Use of series capacitors and FACTS devices Construction of high surge impedance line Enhanced system and equipment monitoring Conversion of 3-phase systems to 6-phase systems Expanding Existing Transmission Capacity Technology Fundamentals of Power Transfer Limits Surge Impedance Loading Thermal Limits System Limits Increasing Thermal Limits Improved Transmission Structures Uprating Conductor hardware and accessories

6 2.7 Conductor selection CHAPTER 3.METHODOLOGY Introduction Formulation Overhead line rating calculations Sag-Tension calculations CHAPTER 4: RESULTS AND DISCUSSION Results Discussion CHAPTER 5: CONCLUSION AND RECOMMENDATION Conclusion Recommendation References Appendix Abbreviations

7 ABSTRACT Due to the increase in demand for power, uprating of overhead transmission lines has now become the most common way of solving this progressive increase in demand. It is not always necessary to construct new transmission lines so that we can add the transmission capacity of the existing transmission line.uprating of overhead line by increasing its current carrying capacity allows an increase in its power transfer capability. This report examines the technical facts considered before the current carrying capacity of a conductor can be increased, it also describes the methods of uprating the transmission line and the factors to consider. The report also considers a case of an existing two 132kv lines from Dandora to Juja road substations. The transformer in the Dandora substation is rated 200MW but each line is currently carrying 143 MW and can carry a maximum of 165Mw each when overloaded, thus this report will also give a solution on how to maximize power from the transformers in the substation to the other substation or to the load Centre.The two parallel lines are always operated at or should carry 80% of the line rating for reliability and contingency too. In our case the lines are operating normally at 71.5% the rated power and when the power is increased to 80% the lines are overloaded which shouldn t be the case. The two lines on study are part of the many lines under the utilities company The Kenya Power and Lighting Company Ltd (KPLC).KPLC is a key player in the electricity sector with the mandate to purchase bulk power from Kenya generating company (kengen), Independent power producers (IPP) and transmits it to its load centers via the transmission lines, distributes and retails the electricity to customers throughout Kenya. The source of generation are hydro-electric, Geothermal, Thermal, wind and Biomass. The Kenya power transmission system is an interconnection of high voltage transmission lines. This interconnection is known as the Transmission network or the Grid. The Kenyan grid currently consists of 132KV and 220KV lines and substations.400kv lines are currently under constructions and will connected to the system in the future. The total length in kilometers of the 220kv is 1,331KM while the total length in kilometers of 132KV lines is 2,211KM. The two 132kv lines under study which runs from Dandora to Juja road substations measures 2KM each

8 1 CHAPTER Introduction The increase in power demand has caused flow of power to increase in the existing electrical transmission lines too. There are several factors which have caused this increase: Firstly, the technological growth has greatly led to the increase in demand for electrical power. Most of the industrial machines, houses, household appliances, electronic gadgets etc. uses power. The affordability and availability of the appliances and gadgets have caused a significant demand for power nowadays as compared to some years ago, and the growth is expected to go on as time goes by. Also increase in population and the government subsidising the power connection fee has also led to many homes being connected the national grid. Secondly, the removal of state restrictions and regulation in regards to achieving vision 2030 has led to increase in demand. I.e. nowadays the connection fee can be paid in instalments over years through the monthly power bills and this gives each homestead in the country a chance to be connected to the grid. Also the consequent changes on generation points connected to the transmission system has caused major changes in the power flows across transmission lines.ie when the demand gets high during peak hours the generation points increase the power generated to meet the load demand and this in turn affects the conductor design properties. The losses in the lines also causes injection of extra power in the transmission system to cater for the losses and meet the demand and this too results in overloading the conductor. As a result of the increased power demand, some lines are operated near their ampacity limit. Ampacity or thermal rating is the maximum current a conductor can carry to meet it design features to avoid its destruction and to meet its safety standards. An excess conductor temperature may result in exceeding the required conductor sag with consequent dangerous reduction in the clearances to the ground.all these effects caused by an excessive current could put public safety at risk. In order to solve these problems new lines could be constructed. However, the high population growth which has led to high population density has resulted to the high intensive use of land. This has caused only a small piece of land put aside for electrical lines. As a consequence, the legislation authorization and ruling on the right of way (ROW) for the transmission lines, the public presentation of the project, the commissioning of the project and the tendering process can take a lot of years while the demand still - 8 -

9 continues to increase therefore there is great pressure to increase the power flow in existing right of ways using existing infrastructure as far as possible. Traditionally, the upgrading of the line has been used in order to increase the line rating. The upgrading involves increasing the line voltage or the number of conductors. The main problem of these methods is the need to strengthen the towers. For this reason, methods without the need to strengthen the towers that allow to increase line power flow securely and safely, close to its ampacity limit, have been developed.one of the option is to increase the ampacity of the line by conductor replacement. The new conductor needs to have better properties as compared to the previous one such as lower sagtemperature relations. I.e. the conductors sag isn t affected much by the line temperature. Another solution is real time monitoring of the conductor where factors such as weather conditions i.e. wind and ice loading conditions in areas which experiences winter are considered, conductor temperature, sag and tension are also considered. 1.2 Main objective One of the major reasons for uprating an existing transmission line is to maximize utilization of the existing corridor for transfer of power. In situations where construction of new lines is obstructed by the presence of ecologically sensitive areas, forests or urban habitations, uprating of the existing transmission line is one of the most appropriate solutions to meet the power flow requirements. In this project the main objective is to uprate the existing 132kv Dandora Juja road lines I & II which currently have 400mm² ACSR (Aluminium core steel reinforced) conductor and replace it with similar or small size of conductor which has almost twice the current capacity of ACSR. This will be a high temperature-low sag conductor (HTLS) or high capacity conductor. Below are examples of HTLS conductors: 1. Aluminium Conductor Composite Core (ACCC) conductor. 2. Aluminium conductor core reinforced (ACCR) conductor. 3. Aluminium conductor steel supported conductor (ACSS) conductor. 4. GAP(Aluminium-zirconium strands which are steel reinforced) conductor They all have better thermal characteristics as compared to ACSR conductor 1.3 Specific objective In situations where demand for power is increasing or is expected to increase in the near future, it is highly recommendable to increase the power flow capacity of the existing transmission line in the corridor through uprating rather than to construct a new line. This can enable achievement of the required power demand at a very less cost and in a very short duration as compared to construction of new lines. In this case - 9 -

10 study the maximum power that each line should carry is 200 MW, currently the line carries 143MW due to thermal constraints. Since the transformers at Dandora substation are rated 200 MW each, there is no power increase planned in near future hence the need to uprate the two lines to Juja Road substation. 1.4 Problem statement Challenge caused by increase in demand for affordable electricity has caused many system planners and transmission engineers to try hard to find economical ways to reduce grid congestion and improve grid reliability. In many cases, grid congestion costs the utilities and the customers they serve millions of shillings annually.restrictions on permitting new lines and advances in conductor and related technologies have changed the options available to planners and engineers trying to solve these problems until now. However, there have been limited design tools available that can help to show the differences between multiple conductor solutions. Power grid congestion is a situation where the existing transmission line or distribution line is unable to accommodate the required load during high demand periods. Grid congestion affects the reliability and also causes a decrease in efficiency because line losses increases greatly under high load conditions. If the transmission lines are operated near their thermal limits, there would be a substantial loss during high load conditions. An example of this was in 2004 in the western US where congested transmission lines in California were unable to carry low-cost hydroelectric power from the northwest to the southwest. In addition to grid congestion, which is typically a function of a transmission conductor's propensity to sag as it heats up because of its electrical resistance and thermal properties, congested transmission lines also impact grid reliability. Should an adjacent line fail or be taken out of service, the lines that remain in service can quickly become overloaded. This can lead to a cascading outage, as was observed in the eastern US in Current North American Electric Reliability Corp. (NERC) initiatives may be further tightening acceptable conductor sag limits as it has become apparent, in many cases, that aged conductors and ever changing under-build may compromise safe clearances. One of the most economical ways of addressing congestion is the use of a highcapacity or high-temperature conductor. A widely adopted high-capacity conductor, Aluminum Conductor Composite Core (ACCC), uses a high-strength, light-weight carbon and glass fiber core that have a low coefficient of thermal expansion, virtually eliminating thermal sag. The core's decreased weight compared to a steel core also allows the incorporation of some 28 percent more aluminum in any given conductor size. The added aluminum content decreases conductor resistance, so even under heavily loaded conditions line losses are minimized

11 An ACCC conductor can carry twice the current of a conventional aluminum conductor steel reinforced (ACSR) conductor. While an aluminum conductor steel supported (ACSS)-high temperature version of ACSR-conductor is also capable of carrying twice the current of a conventional ACSR conductor, because of its electrical resistance, it does so at much higher operating temperatures. In many cases the ACSS conductor's thermal sag can prevent its higher capacity from being fully realized. In either case, under any load condition, the ACCC conductor's added aluminum content and decreased resistance reduces line losses by 30 to 40 percent or more. The ACCC conductor's high strength, low coefficient of thermal sag, superior selfdamping and resistance to load fatigue, corrosion resistance and other attributes can help improve project economics on new and reconductoring projects in any environment. Using ACCC to increase the capacity of an existing line can reduce or eliminate the need to reinforce existing structures, which can save millions of shillings and permitting challenges. For new lines, using ACCC can allow greater spans between fewer and shorter structures, saving time and money, along with the added and long-term energy efficiency benefits. While increased capacity and reduced thermal sag have obvious advantages as reduced line losses also translate into large savings for the utility company, the consumer and the environment. When projects are considered as a whole, the early choices made by planners and engineers can reduce the number of new structures or modifications to existing towers, resulting in project savings that often exceed the cost of the conductor entirely

12 2 CHAPTER 2: LITERATURE REVIEW. 2.1 Growth in demand for power in the past present and the near future The increase in population growth, improved standards of living and advanced technology in the country has caused a great rise in demand for power in the recent years. The demand is also anticipated to increase as shown in the graph below thus the need to increase the installed capacity. To increase the installed capacity we would require constructing new transmission lines and also using some methods of increasing the power transfer capabilities. Figure 1. Growth in demand for power in Kenya. 2.2 Right of way hinderances (ROW) To add power transfer capacity construction of new transmission lines has been the traditional method used. However the public opposition for the construction of new lines is also increasing. This is due to allocation of land for other amenities like roads, railway lines and other public facilities. The increase in population has also led to increase in population density thus fewer paths for transmission line construction. The clearances has also to be catered for to keep the public safe but the increase in constructions especially in urban areas is also becoming a major obstacle for utilities

13 companies. Environmental factors and acquiring legal rights for overhead transmission lines have also become a challenge too. 2.3 Methods of increasing power flow through a transmission line. The following are methods of increasing power flow through a transmission line: 1) Uprating of transmission lines. 2) Upgrading of transmission lines. 3) Use of series capacitors and FACTS (flexible alternating current transmission systems) devices. 4) Construction of high surge impedance line such as expanded bundle, compact lines. 5) Enhanced system and equipment monitoring. 6) Conversion of 3-phase systems to 6-phase systems Definition of uprating/upgrading. Power flow in a 3-phase system is given by P= 3*V*I. Where: V is the line voltage. I is the line current. Thus the power flow in a transmission line can either be increased by increasing the line voltage V or the line current I. The modification of line that results to a higher current carrying capacity is referred to as thermal uprating and the modification of the line that allows the line to operate at a higher voltage is referred to as voltage upgrading Reasons for uprating/upgrading. The main reason for uprating/upgrading a transmission line is to fully utilize the existing path for transfer of power. In circumstances where construction of new lines is hindered by ROW issues, ecologically sensitive areas, forests and even urban areas, uprating /upgrading the existing transmission line is the most economical method of increasing the power transfer capability of the transmission line. Also in areas where the power demand is expected to grow in the future, it is highly advisable to increase the power flow of any existing transmission line through uprating or upgrading rather than construction of a new line. This can help in achieving the power demand cheaply and in less time Comparison between uprating and upgrading. Uprating is the best solution for increasing the power carrying capacity of a transmission line where power flow is limited by thermal limitations such as in short

14 lines where power flow is as much as twice the Surge Impedance Loading (SIL). In transmission lines where stability reasons is the main concern especially in the long lines, the increase in the power carrying capacity of the line is solved through upgrading.increasing the line voltage also reduces the line per unit reactance. Upgrading also leads to a reduction in the voltage drop along the line thus improves the voltage control. Upgrading also leads to an increase in MVA rating of the transmission line in relation to uprating. However upgrading requires more capital investment, more power outage time for construction thus resulting in poor reliability and also replacement of some substation equipment. This is because the electrical clearance of the line will have to be increased to cater for the increased sag if the same conductor is to be used. The existing structure will also need some modification to cater for clearances between the conductors. Thus it is advisable to prioritize uprating to upgrading if the reliability, time and cost factors are to be considered. The substation is already rated at 132KV and there is no expected increase in the line voltage thus this also positions uprating of the line as the best method to increase the power transfer capability Use of series capacitors and FACTS devices Series capacitors. The use of series capacitors for compensation part of the inductive reactance of long transmission line will increase the transmission line capacity. It also increases the transient stability margins, optimizes load sharing between parallel transmission lines and reduces the overall system losses. Transmission line compensation means modification in electric properties of the transmission line to increase the power transfer capability. In series compensation, the main objective is to reduce the transfer reactance of the line at power frequency by means of series capacitors. This increases the system stability which in turn increases the power transfer capability of the line. Series capacitors can be connected at one or both ends of the line. The line ends are the locations of the capacitors. Mid-point series compensation is more effective in the case of very long transmission lines. Series capacitors located at the line ends create more complex protection problems than those installed at the center of the line. The power transfer along a transmission line is shown below:

15 Figure 2. Transmission line without series compensation The active power over the uncompensated transmission line is given by P= ( ES ER )* sin ð Xt Where: ES- sending end voltage ER- receiving end voltage Xt- transfer reactance of the transmission line ð Load angle. Higher voltage gives higher power flow limit. Higher voltage for the same power gives lesser current thus reducing I²R losses. Series compensation has been applied to mostly long transmission lines and other locations where the transmission distances are great and where large power transfers over this distances is required. Modern high voltage and extra high voltage transmission lines are series compensated to improve the power system performance, to enhance power transfer capacity, to enhance power flow control and voltage control and to decrease the capital investment. Figure 3. Transmission line with series compensation. The active power transferred by the compensated transmission line is given by: P= ( ES ER )* sin ð Xt Xc

16 The effects of series compensation are: The lower line impedance improves the system stability. The lower line impedance improves the voltage regulation. Adding the series capacitance provides a method of controlling the division of load among several lines. Increasing the loading capacity of a line improves the utilization of the transmission system and therefore returns on the capital investment. Increase in power capacity as compared to uncompensated line FACTS based devices A flexible alternating current transmission system (FACTS) is a system composed of static equipment used for the A.C transmission of electrical energy. It enhances controllability and increases the power transfer capability of the network. In series compensation, the facts based device is connected in series with the power system. It works as a controllable voltage source. All transmission lines experiences series inductance. When a large current flow this causes a large voltage drop and to compensate this, series capacitors are connected to decrease the effect of inductance. It also improves the power factor. In shunt compensation the power system is connected in shunt with the FACTS. It works as a controllable current source Construction of high surge impedance line. The magnitude of power that a given transmission line can carry safely depends on various factors. These factors can be categorized into thermal and surge impedance loading limits. For long lines the capacity is limited by its SIL level. A decrease in line inductance and surge impedance would in turn increase the surge impedance loading (SIL) which would result in increase in power transfer capability. The surge impedance which is also known as the characteristic impedance is given by the equation below. Zo= L C Where: L is the per unit impedance. C is the per unit capacitance. The SIL is equation is shown below SIL (MVA) = VLL2 Zₒ

17 VLL is the line-line voltage of the transmission line Enhanced system and equipment monitoring Installation of tension and sag monitors can help in monitoring the transmission line. On a cool windless day when the air temperature and wind speed is low, and there is no sun, the MVA rating may be higher than that of a hot windy day. The line rating will vary in such a way that is partly predictable or partly random. Also the load variation monitoring can help in determining when the lines will carry their maximum limits. I.e. During peak times the demand is high thus the lines can be put to their maximum capacity while during off-peak times when the demand is low the lines can be operated below their limits. Thus the power transfer capability is maximized only when the demand is high Conversion of 3-phase systems to 6-phase systems. Six phase system is one of the multiphase power systems. Due to harmonics effects and some other reasons six phase systems and six phase machines are not common but six phase transmission lines are popular due to the following reasons: 1. Increased power transfer capability. 2. Better voltage regulation. 3. Better efficiency. 4. Greater stability and reliability. 2.4 Expanding Existing Transmission Capacity Technology Complex technology is necessary to increase the power flow capacity on existing power equipment (overhead lines and power transformers), power circuits (multiple power equipment elements in series), and power system interfaces (multiple parallel power circuits connecting power system regions). The following three issues are basic to all approaches: 1. For overhead lines, increase in power flow capacity is dependent on line length, original design, environmental regulations, the condition of structures and the type of conductors used. Increase in a line s thermal rating could range from between 5% to 100%. 2. Overhead lines are only part of the transmission line path (circuit). The lines are terminated at substations by air disconnects, circuit breakers, and line traps. The power flow through all of the circuit elements must be limited to avoid damaging the line or the terminating equipment. The maximum allowable power flow over this circuit may be limited by any one of the circuit elements. According to the currently

18 used rating method, a facility rating must be the minimum of all ratings between substations. 3. Increase in maximum allowable power flow through a component circuit or circuit element does not necessarily yield a higher rating. This is because increased power flow on an improved element may interfere with another element s limits. Transmission circuit ratings are often developed on a system basis, rather than on an individual line basis. This is because the maximum power flow on the transmission system is a function of the overall system topology (transmission lines, transformers, generation, series and shunt compensation, and load). Many non-thermal system considerations (such as sag, tension and voltage) can also limit the maximum power flow on a specific transmission circuit. The overall limit may be set between operating areas, irrespective of ownership or individual lines, and may change during a day based on system conditions. Increasing the capacity on a single line by 100% would not necessarily increase the system capacity by this amount. A separate parallel facility may have a constraint after the flow increases by only 25%. 2.5 Fundamentals of Power Transfer Limits The following describe technical aspects of electric power transfer that will help those evaluating alternative strategies for increasing the transfer capability of the grid Surge Impedance Loading The surge impedance loading (SIL) of a power transmission line is the nominal power flow capacity based on the design characteristics for the line and its operating voltage. SIL is governed more by the overall geometry of the line and its operating voltage and less by the conductor size. SIL is independent of the line length. SIL is not the maximum that a particular line can carry, but rather a benchmark that can be used to compare lines of different designs and voltage rating. SIL is a useful concept to compare different transmission lines. SIL (MVA) = Vll2 Zₒ VLL is the line-line voltage of the transmission line. Zₒ is the lines characteristic impedance which is a function of the line s inductance and capacitance. For an overhead transmission line, typical surge impedance is around 300 ohms, compared to a cable, which may be 50 ohms or less. At 345 kv, the SIL of an overhead line is on the order of 400 MW. Short lines may be able to carry 800 MW or more. Long lines of the same construction may be limited to less than 400 MW by system considerations. Underground transmission cables always operate very far

19 below SIL because of limitations on heat dissipation. As a result, underground transmission cables are a net source of reactive power (vars) to the system. Reactive loading and losses can become a limiting problem if a significant number of the lines are loaded above their SIL. As loading increases appreciably above SIL for many lines in the system, the reactive losses will increase in relation to the square of the current and the line reactance. Adding high-capacity lines instead of improving the power transfer capability of the system could further increase the reactive losses and consequently further hinder power transfers Thermal Limits Thermal limits are the maximum flows that can be permitted through a transmission circuit, either on a continuous basis or for a short duration, based on the circuit design. The design parameters include the conductor type, conductor bundles, ambient temperature, wind speed, ice loading, and span length. The thermal limitation is critical in cases of lower voltage lines of 80 km or less. At extra-high voltage (345 kv and above), environmental considerations, such as corona discharge and field effects, dictate line designs and usually result in high thermal capabilities, which can exceed the realistic power transfer. For extra-high voltage transmission, line terminating equipment, such as wave traps and substations, impose a thermal limit rather than the line itself. Consequently, thermal limits are significant only for short lines at 138 kv and below. The process of selecting a thermal rating for an overhead line can be fairly complex or simple. Ratings are published by conductor manufacturers for a range of conservative weather assumptions and conductor temperature limits. Ratings can also be determined from field measurements of sag, wind direction and strength, solar insolation, and other variables. As power flow increases in a bare overhead power line, the conductors, connectors, and associated hardware are heated because of the ohmic losses. Typically, lines that are thermally limited are the shorter lines in the system and the economic cost of electrical losses may be tolerable. However, potential damage to conductor systems or safety concerns occasioned by violation of minimum clearances remains a concern and must be catered for. Thermal ratings for overhead lines are defined in amperes or megavolt amperes (MVA) with an associated duration and, possibly, by frequency of occurrence. Consequently, one line may have a continuous thermal rating of 100 MVA; a 4-hour, long-time emergency rating of 115 MVA; and a 15-minute, short-time emergency rating of 130 MVA. The system operator would understand these ratings to mean that the power flow on this line could reach but not exceed 100 MVA indefinitely. Also, if the flow exceeds 100 MVA, but is less than 115 MVA, the operator must reduce the

20 flow to below 100 MVA within the next 4 hours. If the flow exceeds 115 MVA, the operator must reduce it to below 100 MVA within 15 minutes. The temperature limits on these lines typically serve to limit the loss of conductor tensile strength to less than 10% over the life of the line. It may be possible to exceed the thermal limits of lines and accept some loss of life provided safe clearances are maintained especially for lines that are scheduled for replacement or upgrade in the near future System Limits System limits are functions of transmission line reactance in relation to the overall power system. Series reactance, shunt admittance, and their combination can alter system transfer limits. System planners have long recognized this relationship, particularly where there are prospects of changing the line surge impedance, either by adding equipment (e.g., series capacitors) or by modifying the line itself (e.g., reconductoring, voltage upgrading). Transmission line series inductive reactance is determined by conductor size, phase spacing, number of conductors, relative phasing (double-circuit lines), and line configuration. In long high-voltage overhead transmission lines, the series reactance is larger than the series resistance and is dominant. For this reason, simple reconductoring of many long transmission lines, with no change to structure geometry, results in only minor changes in system power flows Increasing Thermal Limits The thermal limits of the conductor are the limiting factor in the capabilities of transmission lines. As electricity flows through a transmission line, heat is produced due to the flow of current through the resistance of the conductor. As the current flowing through the conductor increases, additional heat is produced, which causes the conductors temperature to increase. The temperature is a function of the electrical current and the environmental conditions (temperature, humidity, and wind speed). If the conductor becomes too hot, one of two problems may result. 1. Excessive heat may permanently damage the conductor. Each transmission line has a maximum amount of power that can flow over it without damage. 2. Increasing temperature may cause the line to physically sag below design levels, resulting in increased risk of injury to the public and conductor damage as well as line outages. The line must not touch anything including the ground. Physical sag of the line can be reduced by using certain types of conductors. Other important constraints are the level of electric and magnetic fields produced (e.g., electric fields increase as the conductor gets closer to the ground), the maximum structure loads during occasional high wind and ice loads, and the maximum temperature at which the energized conductors are allowed to operate. Given standard

21 worst-case weather conditions, the thermal rating of an existing line is determined by the maximum allowable conductor temperature. Thus, uprating (adding more current to increase power transfer capability) such lines without reconductoring normally requires getting ways to maintain electrical clearances above the ground when operating at a higher conductor temperature. To protect against problems resulting from thermal overloads on transmission lines utilities companies install relays. A relay senses the amount of power flowing through a transmission line and operates a circuit breaker to interrupt the power flow on the line. If it exceeds the thermal limit of the line then the power will then flow through parallel paths. The increased loading along the parallel paths creates the potential for an overload condition on other transmission lines. If the system is not properly designed, operated, and maintained, thermal overloads can lead to cascading outages of transmission lines and system breakup. Transmission line capacity can be increased through improvements in transmission tower design (to compensate for physical sag) and increases in conductor current capabilities (to withstand more heat). The ability to accurately determine the conductor thermal condition at any point in time (monitoring) is also helpful in maintaining the line Improved Transmission Structures Adaptations can be made to accommodate physical sag resulting from increased transmission capacity on existing lines. Ground clearance on specific spans can be increased by installing additional structures mid-span, if the ground contours and permitting restrictions allow. Clearance can also be increased by modifying the existing structures to raise the conductor attachment points. Digging the ground in between the support structures can also be used as a method of increasing the clearance. Alternatively existing structures can be replaced with taller structures. These methods do not increase conductor tension, minimizing the need to replace angle and dead-end structures. Increasing ground clearance typically results in only modest increases in allowable ampacity (electricity through the line) before cost becomes high Uprating Basics of uprating Thermal uprating is based on the fact that the tower structure / geometry, air gap clearances and conductor bundle configuration are generally limited by the voltage of the transmission line. If we keep the line voltage constant and we vary the line current to a greater value we may be able to increase the power transfer capability of the transmission line without possible need of changing the tower structure/ geometry. Thermal uprating methods are cheapest and less time consuming as compared to other methods

22 Methods of uprating. Increase in ampacity of a transmission line may be achieved through the following methods Increase conductor rating by changing the thermal rating criteria The maximum operating temperature of an ACSR conductor can be reached without getting to the annealing point of the aluminium strands. This results to an increase in the MVA of the line. The ACSR conductor experiences a loss in its composite strength if operated above 95 C for an extended period of time. The strength of the steel core is not affected for temperatures below 300 C. The increase in maximum sag of the conductor due to an increase in the maximum operating temperature of around 5-10 C is only marginal. I.e. the approximate increase in sag is 0.2 m for 5 C increase and 0.4 m for 10 C increase while the approximate increase in MVA for a 400KV DC line is 150MVA and 300MVA respectively. If the increase in sag of the transmission line lies within the safety margin, there is no need for construction of a new tower structure or refurbishment of the existing tower. The above method has an advantage which is: There will be no purchase of any new conductor. No line outages. Thus it is therefore the most cost effective and effortless method of increasing the MVA rating of a transmission line. If the electrical clearance corresponding to the new higher current carrying conductor is not enough then the following should be done: i. The supports structure must be raised. ii. The conductor tension should be increased. iii. The suspension clamp positions should be changed. This method may not be economical since it results to a small increase in the MVA rating of the transmission line. It is the best solution where only a marginal increase in power flow capacity of the transmission line is required. It also helps in getting rid of cost and time of a new transmission line construction. Conductor temperature ( C) MVA rating Maximum sag (m) Span (m) Tension (N)

23 Table 1. An indication in the variation in sag and MVA rating as a function of conductor temperature. The calculations were done considering an ambient temperature of 30 C. The conductor parameters were: Stranding (No/diameter. mm) 6/4.1 mm Break load (N) Cross-sectional (area sq.mm) - 75 Modulus of elasticity (N/mm²) Coefficient of expansion- (I C) 1.91 * 10^-5 Weight (Kg/Km) 320 Conductor diameter (mm) 12.3 Calculating conditions: Safety factor 2 Radial ice thickness (mm) -0 Wind pressure (N/mm²) 400 Initial conductor temperature ( C)

24 Figure 4. A graph showing how an increase in temperature results to an increase in MVA rating Dynamic environment rating. Installation of tension and sag monitors can help in dynamic line rating. On a cool windless day when the air temperature and wind speed is low, and there is no sun, the MVA rating may be higher than that of a hot windy day. The line rating will vary in such a way that is partly predictable or partly random. The cost of monitoring the system may be a small percentage as compared to the construction of a new line. It also has the advantage of no service outage. This method does not change the maximum capacity of the existing conductor but it allows the maximum utilization of the conductor

25 Increasing the conductor area The increase in the aluminium or aluminium content of the existing conductor results to a decrease in the conductor resistance.it is shown by the formula below. R= ρl A Where: R is the resistance. ρ is the conductor resistivity. L is the conductor length. A is the conductor area. This result to an increase in ampacity of the conductor. It can be achieved by the following two methods: i. Adding new conductors to the existing conductor. ii. Replacing the existing conductor with new conductor of different size and shape. ACSR Dog 100 sq.mm ACSR Zebra 400 sq.mm ACSR curlew 600 sq.mm Temperature( C) Current (A) Temperature( C) Current (A) Temperature( C) Current (A) Table 2. How the increase in conductor size affects the current carrying capability of a conductor at different working temperatures

26 Figure 5. A graph showing how conductor cross-sectional area affects its current carrying capability There are certain factors to consider before increasing the conductor area a) The higher the conductor area the greater the weight. An increase in conductor weight results to an increase in vertical load and an increase in tension too if the conductors sag is to be limited. The increase in tension will lead to tower reinforcement. The increased area may also increase the wind loading on the conductor thus increasing the vertical and transverse load on the tower structure. To get rid of increased outer conductor s diameter, conductor with trapezoidal strands can be used instead of circular strands. In this conductor the aluminium area of the conductor is increased without overall increase in diameter. This helps in increasing the ampacity with less effect on mechanical loading of the transmission line rather than using the latter

27 b) The increase in weight leads to an increase in the conductors sag. If the conductor tension values are limited to those of the existing conductor, the following can be done to cater for the increased sag: Modifying the towers to increase clearance. Installation of new towers in areas with critical spans. Installation of negative sag devices along the conductor. The device is altered by the changes in temperature. When there is an increase in temperature, the conductor expands and there is an increase in its length. The device changes geometrically to cater for the increase in sag. As the conductor temperature decreases the sag decrease too and the device retains its normal shape. Excavation of key areas to increase ground clearance Reconductoring using a conductor of higher ampacity. Reconductoring is the process of replacing the existing conductor with a higher ampacity one so that the thermal rating of the existing transmission line can be increased. Reconductoring using a conductor of larger diameter than the current one can lead to an increase in wind loading. Increase in wind loading will require improvement of the existing structure to cater for the excess weight. Reconductoring using conductor with the same diameter as original conductor but with higher thermal rating can lead to higher ampacity with no need to reinforce the existing tower and structures. Therefore the line rating is limited by the following factors: The properties of the conductor material The environmental conditions surrounding the conductor. The ground clearance of the line. Today, most overhead transmission lines are aluminum conductor steel reinforced (ACSR) conductors. Steel can withstand temperatures up to 300 C with no changes in its properties. Aluminum, however, experiences deterioration in mechanical properties (annealing) when the temperature is higher than 90 C. The temperature is a function of the electrical current and the environmental conditions. The replacement conductors can be classified into two categories: 1. Conductors to be operated at temperatures at moderated temperatures (Temperatures < 100 C.) 2. Conductors to be operated at high temperatures (Temperatures >100 C.)

28 Conductors for operation at moderate temperatures. There are three types of conductors that operate at moderate temperatures: a) AAAC (All aluminium alloy conductors). b) Aluminium conductor aluminium alloy reinforced. (ACAR). c) High conductivity AAAC. The AAAC conductors have a higher strength to weight ratio as compared to ACSR. If they are operated at the same percentage of the rated breaking strength with ACSR, they can operate at a higher temperature than ACSR with the maximum sag for the design catered for. Operating at the same percentage of rated breaking strength however leads to much higher ratio of horizontal tension to the conductor unit weight and this can cause problems to lines sensitive to vibration. ACAR combines strands made from aluminium alloy and EC grade aluminium. The use of EC grade aluminium increases the conductivity of the conductor. If the number of the alloy strands is also increased, the mechanical strength of the conductor is also increased. The use of ACAR to substitute ACSR depends mainly on the allowable operating tension Conductors for operation at higher temperatures By use of AAAC and ACAR conductors to replace ACSR conductor may not be an economical solution since there is no much increase in current carrying capacity of the transmission line as compared to the cost of replacing the line. Use of higher operating temperatures conductors can be a better solution and can result in a significant increase in thermal rating of the existing conductor; it is even twice in some situations. The conductors can be classified into two categories: 1. High temperature conductors 2. High temperature low-sag conductors High temperature conductors This type of conductors has the ability to operate at temperatures of at least 150 C. Their sag increases linearly with increase in temperature. Among the most commonly used high temperature conductors are: TACSR (thermal resistant aluminium conductor steel reinforced) This conductor is made up of an inner steel core consisting of galvanized steel wires and outer aluminium layers composed of aluminium-zirconium alloy strands

29 Figure 6. A figure showing the cross sectional area of a TACSR conductor. The aluminium alloy has a higher resistivity when compared to hard drawn aluminium but it can be operated to temperatures of up to 210 C thus increasing the power carrying capacity. There are different types of aluminium alloy conductors used in the formation of this conductor. The alloys have the following properties: Aluminium alloy. Conductivity (%) Tensile strength (MPa) Continuous operating temperature ( C) EC TAL Current carrying capacity (%) This type of conductor finds applications in the following areas: In transmission lines where the current levels have to be 1.5 to 1.6 times higher than the capacity of a normal ACSR. In overhead lines in areas where corrosion caused by contact of two different metals may occur. In overhead transmission lines where low sag is not a limiting factor. In transmission lines which can be operated safely at a continuous temperature of 150 C. In reconductoring without necessarily modifying the tower

30 Table 3.The table below shows the technical comparison of ACSR panther and TACSR conductor. properties ACSR panther TACSR panther Cross- sectional area (mm²) Conductor Diameter (mm) Weight (kg/km) DC Resistance at 20 C temperature (ohms/km) Maximum Operating Temperature ( C) Voltage Level (kv) Line length (km) 1 1 Span (m) Maintaining same ampacity in TACSR conductor Calculation temperatures ( C) Current to be maintained (A) AC resistance (ohms/km) Line losses (kw/circuit) Power transferred (MW/circuit) Ampacity at maximum operating temperature in TACSR conductor. Calculation temperatures ( C) Current (A) AC resistance (ohms/km) Line losses (kw/circuit) Power factor Power transferred (MW/circuit) Sag at the mentioned temperature above and 0% wind (m) Tension to be maintained at 32 c and 100% wind. The assumptions made: Coefficient of emissivity= 0.6 Wind velocity = 0.6 m/s. Solar absorption coefficient = 0.5 Average ambient temperature = 48 C. Constant of mass temperature coefficient of resistance of conductor per C =0.004 for both conductors. Wind pressure = kg/m²

31 Solar radiation = 1200 W/m². From the above table the following remarks can be made: 1) TACSR can operate at a maximum temperature of 150 C while ACSR operates at 85 C thus this boosts the current carrying capacity by 112%. The ACSR carries 420 Amps by while TACSR carries 893 Amps at their maximum operating temperatures. The difference in currents: =473A ( 473 ) 100 = 112.6% 420 2) The power transferred in MW by TACSR conductor at its maximum operating temperature is 112% higher than that of ACSR. Maximum ACSR panther power at its maximum operating temperature 83.8 MW. Maximum ACSR panther power at its maximum operating temperature 178 MW. Difference =94.2 MW ( ) 100=112.4% High conductivity AAAC conductor (Al59) The high conductivity all aluminium alloy conductor made of aluminium magnesium-silicon alloy strands has been widely used by utilities companies in the world. This conductor can carry 25-30% more current as compared to ACSR conductor of the same size while the sag remains the same and the working tension is lesser than that of ACSR.it also has lower resistance than ACSR thus the losses are reduced. It also has a higher corrosion resistance as compared to alloy series of AAAC conductors. When comparing AL59 conductor with conventional AAAC and ACSR conductors, the following points can be made: Its ultimate tensile strength is lesser when compared to the above conductors but it can be strung in the same tension. If the span and working tension is maintained as the same, it will have lower sag as compared to the above conductors. Thus this conductor can be used in uprating the existing lines or in construction of new lines. The table below shows options for reconductoring existing ACSR with AL59 conductor

32 Table 4. The table below shows technical comparison of ACSR Dog and AL59 (19/2.84) properties ACSR Dog AL59 (19/2.84) Conductor Diameter (mm) Weight (kg/km) DC Resistance at 20 C temperature (ohms/km) Voltage Level (kv) Line length (km) 1 1 Span (m) Maintaining same ampacity in AL59 conductor Calculation temperatures ( C) Current to be maintained (A) AC resistance (ohms/km) Line losses (kw/circuit) Ampacity at maximum operating temperature in both conductors. Calculation temperatures ( C) Current (A) AC resistance (ohms/km) Line losses (kw/circuit) Power factor Power transferred (MW/circuit) Sag at the mentioned temperature above and 0% wind (m) Tension to be maintained at 32 c and 100% wind. The assumptions made: Coefficient of emissivity= 0.6 Wind velocity = 0.6 m/s. Solar absorption coefficient = 0.5 Average ambient temperature = 45 C. Constant of mass temperature coefficient of resistance of conductor per C =0.004 for ACSR and for AL59. Wind pressure = kg/m². Solar radiation = 1200 W/m². From the above table the following remarks can be made:

33 1. The weight of AL59 conductor in kg/km is 16.24% less as compared to the ACSR conductor. Al 59 weight per km 330 ACSR dog weight per km- 394 Difference: = *100=16.24% The decrease in weight shows that there is no need for structure modification when reconductoring. 2. The DC resistance at temperature of 20 C of AL59 conductor is 12% lesser as compared to that of ACSR thus increasing AL59 s ampacity while reducing its line losses. ACSR dog D.C resistance at 20 C AL 59 D.C resistance at 20 C Difference: = =11.35%. 3. While maintaining the same ampacity, the line losses of AL59 conductor are 13.28% lesser when compared to that of ACSR. ACSR dog line losses AL 59 line losses Difference: =11.26 kw =13.28% 4. When both conductors are operated at their maximum temperatures, AL59 conductor carries 42% more current when compared to ACSR conductor. ACSR dog current- 288 AL 59 current -409 Difference: = = 42.01%. 5. The power transferred by the AL59 conductor is 42% more than that of the ACSR conductor at their maximum operating temperature. The power transfer at maximum operating currents. ACSR dog MW/cct AL Difference: =12.07 MW/cct *100=41.99%

34 6. While maintaining the tension of ACSR at 32 and 100% wind, the increase in sag is for an increase in 121 amperes. The difference in sag between ACSR dog and AL 59 at maximum temperature and 0% wind. Difference: =0.585 The difference in current at maximum operating temperature =121 A High temperature-low sag conductors This type of conductors has the ability to operate continuously at temperatures of at least 150 C. Their increase in sag is not linear at all temperatures due to knee-point temperature. Knee-point temperature is the temperature which the core carries all the tension in the conductor. The conductor experiences sag due to the expansion of the steel core alone (The coefficient of linear expansion of steel conductor is lower than the complete conductor). The higher the thermal expansion of the aluminium causes all its stress to be carried by the steel core. Beyond the knee point temperature, the new conductor coefficient will be the same as that of the core resulting in low sag when operated at high temperatures. Replacement of ACSR conductors in existing transmission line with such types of conductors can therefore lead to an increase in the line carrying capacity without the need for tower modification. Commonly used HTLS conductors INVAR The INVAR conductor consists of a core of iron and nickel alloy which has a low coefficient of thermal expansion. The outer layer of INVAR conductor is composed of aluminium-zirconium alloy. This type of conductor can be operated at temperatures of around 200 C at low sag. This type of conductor has the following advantages: Its current carrying capacity is 100% or more when compared to a conventional ACSR conductor with the same diameter. It has lower sag than ACSR conductor under same ampacity due to its INVAR core. Modification or reinforcement of the existing line is less or not required if the INVAR conductor has the same diameter as the existing ACSR conductor to be replaced

35 Figure 7. A figure showing the cross-sectional area of a typical INVAR conductor. The numerical data shows the option of using INVAR conductor for the purpose of reconductoring in place of existing ACSR conductor. Table 5. The table below shows the technical comparison of ACSR moose and INVAR conductor. properties ACSR moose INVAR Conductor Diameter (mm) Weight (kg/km) DC Resistance at 20 C temperature (ohms/km) Maximum Operating Temperature ( C) Voltage Level (kv) Line length (km) 1 1 Span (m) Maintaining same ampacity in INVAR conductor Calculation temperatures ( C) Current to be maintained (A) AC resistance (ohms/km) Line losses (kw/circuit) Power transferred (MW/circuit) Ampacity at maximum operating temperature in INVAR conductor. Calculation temperatures ( C) Current (A) AC resistance (ohms/km) Line losses (kw/circuit)

36 Power factor Power transferred (MW/circuit) Sag at the mentioned temperature above and 0% wind (m) Tension to be maintained at 32 c and 100% wind. The assumptions made: Coefficient of emissivity= 0.6 Wind velocity = 0.6 m/s. Solar absorption coefficient = 0.5 Average ambient temperature = 48 C. Constant of mass temperature coefficient of resistance of conductor per C =0.004 for both conductors. Wind pressure = kg/m². Solar radiation = 1200 W/m². From the above table the following remarks can be made: 1. Under the same diameter, the unit weight i.e. weight per km of INVAR conductor is much less than that of ACSR conductor thus when reconductoring there is no need for tower refurbishment or modification. INVAR weight/km-1950 kg ACSR moose weight/km-2004 kg Difference: =54 kg 2. The DC resistance at 20 C of INVAR conductor is 3.49% lesser when compared to that of ACSR conductor thus increasing its ampacity and reducing its line losses. INVAR DC resistance at 20 C ACSR moose DC resistance at 20 C Difference: = *100=3.49% 3. INVAR conductor can operate at a maximum of 210 C while ACSR conductor operates at maximum temperature of 75 C thus the increase in its current carrying capacity is almost 170%. At maximum operating temperature ACSR current -726A INVAR current 1957A Difference: =1231A *100=170%

37 4. The power transferred in MW by INVAR conductor at its maximum operating temperature is 169.6% more as compared to that of ACSR conductor. Power at maximum operating temperature. ACSR conductor- 439 MW/cct INVAR conductor MW/cct Difference: = *100=169.6% ACSS (Aluminium conductor steel supported) The construction of this type of conductor is the same as that of ACSR except that the aluminium strands are fully annealed. The annealed or 0-tempered aluminium strands have a higher conductivity than hard drawn aluminium. The hard drawn aluminium conductivity is 61.2% while that of annealed aluminium is 63% as compared to copper which has 100% conductivity. The aluminium strands don t take any mechanical load thus can be operated at temperatures in the order of 200 C without loss in their strength. When the complete conductor is stressed, the aluminium elongates and transfers the entire load to the steel core. The conductor finds use in the following applications: In areas where the current to be carried by a conductor is doubled under the same tower loadings i.e. without the need to refurbish the tower. ACSS with the same diameter as ACSR can be used to fulfill this condition. When the conductor is used in new lines, there can be reduction in cost of the components i.e. the tower structures due to decreased sag in the same amount of power transfer

38 Figure 8. A figure showing the cross sectional area of a typical ACSS conductor with trapezoidal aluminium strands. The numerical data shows the option of using ACSS conductor for the purpose of reconductoring in place of existing ACSR conductor. Table 6. The table below shows the technical comparison of ACSR panther and ACSS lark conductor. Properties ACSR panther ACSS lark Conductor Diameter (mm) Weight (kg/km) DC Resistance at 20 C temperature (ohms/km) Maximum Operating Temperature ( C) Voltage Level (kv) Line length (km) 1 1 Span (m) Maintaining same ampacity in ACSS conductor Calculation temperatures ( C) Current to be maintained (A) AC resistance (ohms/km) Line losses (kw/circuit) Power transferred (MW/circuit) Ampacity at maximum operating temperature in ACSS conductor. Calculation temperatures ( C) Current (A)

39 AC resistance (ohms/km) Line losses (kw/circuit) Power factor Power transferred (MW/circuit) Sag at the mentioned temperature above and 0% wind (m) Tension to be maintained at 32 c and 100% wind. The assumptions made: Coefficient of emissivity= 0.6 Wind velocity = 0.6 m/s. Solar absorption coefficient = 0.5 Average ambient temperature = 48 C. Constant of mass temperature coefficient of resistance of conductor per C =0.004 for both conductors. Wind pressure = kg/m². Solar radiation = 1200 W/m². From the above table the following remarks can be made: 1. The DC resistance at 20 C of ACSS is 2.63% lesser than the ACSR resistance thus the increase in ampacity. DC resistance at 20 C ACSR panther ACSS lark Difference: = *100=2.63% 2. When operated at the same ampacity, the ACSS line loses are 2.89% lesser than that of ACSR. Under the same ampacity ACSR panther line losses 90MW/cct ACSS lark line losses MW/cct Difference: = *100=2.888% 3. The ACSS conductor can operate at a maximum temperature of 250 C while the ACSR operates at 75 C thus increasing its current carrying capacity by 181%. At maximum operating temperature ACSR current -420A ACSS current

40 Difference: =760A *100=180.95% 4. The power transferred at maximum conductor operating temperatures in ACSS is 181% higher than ACSR. At maximum operating temperature ACSR power ACSS power Difference: = *100=181% GAP conductor This type of conductor involves a small gap maintained in between inner steel core and outer aluminium-zirconium alloy layers. The steel core is the only one that is tensioned and it carries the entire mechanical load. The increase in conductors sag is determined by the coefficient of expansion of the steel core at all temperatures. The core has low sag when operated at high temperatures in the order of 200 C. Importance of these conductors They have ultra-high strength steel core with temperature resistance of up to 250 C The high temperature grease allows aluminium to move freely over the core and protect the core from long term corrosion; the grease is resistant to temperatures of up to 300 C. The major drawback of these conductors is that the installation process is complex. This conductor does not need modification of existing tower structure if it were to replace ACSR conductor of the same diameter

41 Figure 9. A figure showing the cross sectional area of a typical GAP conductor. The numerical data shows the option of using GAP conductor for the purpose of reconductoring in place of existing ACSR conductor. Table 7. The table below shows the technical comparison of ACSR zebra and GAP conductor. Properties ACSR zebra GAP Conductor Diameter (mm) DC Resistance at 20 C temperature (ohms/km) Maximum Operating Temperature ( C) Voltage Level (kv) Line length (km) 1 1 Span (m) Maintaining same ampacity in GAP conductor Calculation temperatures ( C) Current to be maintained (A) AC resistance (ohms/km) Line losses (kw/circuit) Power transferred (MW/circuit) Ampacity at maximum operating temperature in GAP conductor. Calculation temperatures ( C) Current (A) AC resistance (ohms/km)

42 Line losses (kw/circuit) Power factor Power transferred (MW/circuit) The assumptions made: Coefficient of emissivity= 0.6 Wind velocity = 0.6 m/s. Solar absorption coefficient = 0.5 Average ambient temperature = 48 C. Constant of mass temperature coefficient of resistance of conductor per C =0.004 for both conductors. Wind pressure = kg/m². Solar radiation = 1200 W/m². From the above table the following remarks can be made: 1. GAP conductor can operate at its maximum temperature of 210 C as compared to that of ACSR at 75 C thus boosting up the current by 165% nearly 1.65 times. At their maximum operating temperature. ACSR zebra current 641A GAP current 1696 Difference: =1057A *100=165% 2. The power transferred in MW of GAP conductor at its maximum operating temperature is 165% higher as compared to that of ACSR. At maximum operating temperature. ACSR zebra power MW/cct GAP MW/cct Difference: = *100=165 % ACCC (Aluminium conductor composite core) This type of conductor consists of an outer core composed of o-tempered /annealed aluminium trapezoidal strands and a carbon fibre composite inner core. The resistivity of annealed aluminium is less than that of hard drawn aluminium or thermal resistant aluminium alloys thus this leads to reduced line losses. The conductor can be operated to temperatures of up to 180 C at lower sag. The carbon fibre core is 30% lighter thus reduces its weight per unit length thus there is no need for structural modification if the conductors diameter is the same as the one to be replaced

43 The carbon fibre core also has a lower coefficient of thermal expansion thus the sag is low at high operating temperatures hence increased current carrying capacity and no need to increase the clearance. Figure 10. A figure showing a typical cross- sectional area of an ACCC conductor. The ACCC conductor has the following important features: It has reduced thermal sag when compared to ACSR conductor. It has reduced line losses when compared to ACSR conductor. The annealed aluminium strands are in trapezoidal form and this result to an additional aluminium content which improves the line conductivity while reducing its losses. It can reduce the cost of upgrading existing lines or in construction of new lines due to its greater strength, reduced sag and its increased current carrying capacity. It does not corrode, rust or cause electrolysis with aluminium conductor or the tower components thus it is suitable for polluted and zones near the ocean. The conductor finds applications in the following areas: 1. Where reduction of line losses is required. Under equal loading conditions they reduce the line losses by 30-40% compared to other conductors of the same diameter and weight. 2. Ideal for reconductoring on existing line and upgradation of existing line. They increase capacity by improving line clearance and reduce strain on structure thus increasing their life. The numerical data shows the option of using ACCC conductor for the purpose of reconductoring in place of existing ACSR conductor. Table 8. The table below shows the technical comparison of ACSR panther and ACCC conductor. Properties ACSR panther ACCC Casablanca Conductor Diameter (mm) Weight (kg/km)

44 DC Resistance at 20 C temperature (ohms/km) Maximum Operating Temperature ( C) Voltage Level (kv) Line length (km) 1 1 Span (m) Maintaining same ampacity in ACCC conductor Calculation temperatures ( C) Current to be maintained (A) AC resistance (ohms/km) Line losses (kw/circuit) Power transferred (MW/circuit) Ampacity at maximum operating temperature in ACCC conductor. Calculation temperatures ( C) Current (A) AC resistance (ohms/km) Line losses (kw/circuit) Power factor Power transferred (MW/circuit) Sag at the mentioned temperature above and 0% wind (m) Tension to be maintained at 32 c and 100% wind. The assumptions made: Coefficient of emissivity= 0.6 Wind velocity = 0.6 m/s. Solar absorption coefficient = 0.5 Average ambient temperature = 48 C. Constant of mass temperature coefficient of resistance of conductor per C =0.004 for both conductors. Wind pressure = kg/m². Solar radiation = 1200 W/m². From the above table the following remarks can be made: 1. The DC resistance at 20 C of ACCC conductor is 26.33% less than ACSR thus increasing ampacity and reducing line losses. The D.C resistance at 20 C ACCC Casablanca ACSR panther

45 Difference: = *100= By maintaining same ampacity with ACSR conductor, the line losses of ACCC conductor are % lesser. By maintaining the same ampacity ACCC losses MW/cct ACSR losses 90 MW/cct Difference: = *100=27.59% 3. ACCC conductor maximum operating temperature is 175 C while ACSR conductor operates at maximum temperature of 75 C. This boosts its current carrying capacity by almost %. At their maximum operating temperature. ACCC current-1119a ACSR current 420A Difference: =699A *100=166.43% 4. The power transfer in MW of ACCC conductor at its maximum operating temperature is % higher as compared to that of ACSR conductor. At their maximum operating temperature ACCC power MW/cct ACSR power 83.8 MW/cct Difference: = *100=166.45% ACCR (Aluminium conductor composite reinforced) This type of conductor consists of outer aluminium-zirconium alloy strands and an inner composite core. The composite core consists of aluminium and aluminium oxide matrix. The conductor can be operated continuously to temperatures of up to 200 C

46 Figure 11. A figure showing the cross-sectional area of a typical ACCR conductor. Table 9. The table below shows the technical comparison of ACSR panther and ACCC conductor. properties ACCR 336-T16 ACSR panther Total diameter (mm) Cross-sectional area (mm²) Weight (kg/km) Resistance 20 C (ohms/km) Resistance 75 C (ohms/km) Voltage (Kv) Current (A) power (MVA) The assumptions made: Coefficient of emissivity= 0.6 Wind velocity = 0.6 m/s. Solar absorption coefficient = 0.5 Average ambient temperature = 48 C. Constant of mass temperature coefficient of resistance of conductor per C =0.004 for both conductors. Wind pressure = kg/m². Solar radiation = 1200 W/m². 1. The weight per unit km of ACCR conductor is less by 64 kg when compared to ACSR conductor thus there is no need for tower reinforcement when reconductoring ACCR weight per km-198 ACSR weight per km

47 Difference: =64 kg 2. At the same conductor cross sectional area, the ACCR current carrying capability is 78% more than that of ACSR ACCR current-906a ACSR current- 509A Difference: = *100=77.99% 3. At the same conductor cross sectional area, the ACCR power carrying capability is 78.16% more than that of ACSR. ACCR power-207.2mw ACSR power MW Difference: =90.9 (90.9)/(116.3)*100= 78.16% 2.6 Conductor hardware and accessories Uprating of the transmission line to higher current carrying capacity sometimes involves replacement of associated hardware e.g. towers and conductor accessories e.g. insulators etc. to cope with the increased current. The hardware is designed in such a way that they have good heat dissipation characteristics due to the increase in operating temperatures thus making the operation safe. Magnetic heating of the clamp as a result of increased current passing through the conductor also increases which results to an increase in magnetic losses. To solve this problem ferrous clamp may be replaced with non-ferrous clamps to reduce the magnetic losses. Also due to the increase in conductor carrying capacity, the cross-arms distance might also be increased to cater for insulation. The tower length might also be increased to cater for clearances effect. 2.7 Conductor selection Conductor selection can have a significant effect on both the short and long term economic performance of transmission line projects. Since conductors are one of the major cost components of a line design, selecting an appropriate conductor type and size is essential for optimal operating efficiency. Due to this reason, a number of systematic approaches for conductor selection have been developed. The choice of appropriate conductor depends on:

48 Electrical load requirements. The projections in load growth. Support structure requirements. Environmental considerations Set regulations. Cost of the project. Some physical and economic factors that affect the choice of conductors are: Increasing the conductor diameter results to increase in its weight, thus the wind loading is increased on the support structures hence the need to reinforce them. Choosing a conductor with a higher resistance increases the power losses along the line. Thus the conductor selection should be based on the total system cost not the unit cost of the conductor and the economic values it will offer as compared to other conductors. From the conductors discussed above, we see that they all have better characteristics than ACSR conductor. For the purpose of reconductoring, we have to choose the conductor with outstanding properties among them all. Figure 12. A graph showing how the increase in temperature affects the cable sag of different types of conductors

49 Among all the conductors, the ACCC conductor has lowest sag with increase in temperature thus this shows that there is no need for structure modification to increase the clearance when reconductoring if the conductors of the same cross sectional area are to be used. Figure 13. A graph showing how increase in temperature affects the current carrying capability of the different typesof conductors

50 3 CHAPTER 3.METHODOLOGY 3.1 Introduction This chapter gives the details on how reconductoring using a conductor of higher ampacity as the best uprating method of an existing transmission line. The factors to consider were the right of way hinderances, line losses, need for tower reinforcement due to increase or decrease in the conductortension and weight, increase in power transfer capability and ground clearance. The overall project economics was also a determining factor. 3.2 Formulation Overhead line rating calculations Operating current I (A)= SQRT((convection loss-(solar absorption coefficient*intensity of solar radiation*overall conductor diameter)+(pi*conductor emissivity*stefans constant*overall conductor diameter)*((conductor temperature rise above ambient+ambient temperature+273)^4 (ambient temperature+273)^4)) /a.c resistance at operating temperature and current) A.C resistance at operating temperature and current Ra.c (ohms/cm)= direct current resistance Intensity of solar radiation Si (watts/cm²)=(intensity of solar radiation/10000) Overall conductor diameter d(cm)=(diameter(mm)/10) Convection loss Hc (watts/cm)= IF(wind velocity normal to conductor>= 20, *(wind velocity equivalent*conductor diameter)^0.448 *conductor temperature rise above ambient, *(overall conductor diameter)^0.669 *(conductor temperature rise above ambient)^1.233 Conductor operating temperature ( C)=(conductor temperature rise above ambient+ambient temperature) Wind velocity equivalent Veq (cm/sec)=wind velocity normal to conductor*(barometric pressure/760)*(293/(273+ambient temperature) Barometric pressure b (mm Hg)=(10.33-(altitude above sea level/900) )*1000/13.6 Resistance at operating temperature Rt (Ω/cm)=resistance at 20 C*(1+temperature coefficient of resistance*(conductor operating temperature-20)) Line rating S (MVA)=number of conductors in bundle*( 3*line voltage*operating current)/

51 Line losses = phase current² * phase resistance Calculation constants: Coefficient of emissivity =0.4 Wind velocity=0.6m/s Solar absorption coefficient=0.6 Wind pressure= kg/m² Solar radiation=1200w/m² Radiative heat gain= W/m Radiative heat loss= W/m Convective heat loss= W/m Joule heat gain= W/m Stefans constant=5.67*10^-12 watts/cm² Intensity of solar radiation=0.1 watts/cm² Ambient temperature=35 C Wind velocity normal to conductor=60 cm/sec Frequency=50Hz Temperature coefficient of resistance ρ (k^-1)=

52 3.2.2 Sag-Tension calculations Figure 14. A figure showing the conductor sag at two different tower points. A- Tower A. B- Tower B. Deflection is the same as the conductor sag h- difference in height between tower A and B. a- Distance from tower A to the conductors lowest point. b- Distance from tower A to the conductors lowest point. Ta- conductor tension at tower A. Tb- conductor tension at tower B. The ruling span formula is based on the fundamental assumption that the attachments of the conductor to suspension structures between dead-end structures are flexible enough to allow for longitudinal movement to equalize the tensions in adjacent spans to the ruling span tension. If the temperature of a line segment with unequal spans is raised uniformly, conductor in each span elongates in response to the temperature change. This elongation increases the sag, thereby decreasing the tension. If the suspension insulators remained stationary (without any rotation), there would be a

53 tension difference in adjacent spans of different lengths. However, the suspension clamps displace longitudinally to provide force resolution at each suspension clamp. For level spans, sag and slack in each suspension span at temperature T call be calculated from the following parabolic equations. Sag = Di, t = (w*si²)/ (8*HR, t) = DR,t * (Si/SR)² Slack = βi,t = Li,t-Si,t= (8* Di²,t)/(3*Si²) = (Si³*w²)/(24*H²R,t) Rate of slack = βi,t / Si= (8*D²i,t)/(3*Si²) = (Si²*w²)/(24*H²R,t) Ruling Span = SR = (S³1 + S³ S³n)/(S1 + S Sn) The rate of slack at temperature T can be calculated from the following equations: (Rate of slack)i,t =βi,t/si=(li,t Si,t)/Si Where Li,t = Li[1+α(T-Tₒ) +(HR,t-HR)/E*A] Si,t =Si + (ᶑi - ᶑi-1),t Substituting for Li,t and Si,t in the rate of slack equation: Βi,t/Si=(Li*LR,t/(Si*LR)-1-(ᶑi - ᶑi-1),t/Si) Finally the change in the rate of slack and span length due to a change in temperature from Tₒ to T can be calculated from the equations below: Change [βi,t/si]tₒ-t = change [βr,t/ SR] Tₒ-T - (ᶑi - ᶑi-1)/Si) Where D-sag S-span length L-conductor length Β-slack Tₒ- conductor stringing tension H- Horizontal conductor tension w- Conductor unit weight A- Conductor cross-sectional area E- Conductor modulus of elasticity α- Coefficient of thermal expansion ᶑ - longitudinal horizontal movement of an infinitely flexible suspension clamp. Subscripts ₒ- at temperature Tₒ t- at temperature T R- Ruling span

54 4 CHAPTER 4: RESULTS AND DISCUSSION 4.1 Results As per the objective of the project, the current conductor ACSR canary 132 properties were taken from KPLC and its properties compared with other conductors of almost similar properties to see if reconductoring with a higher a capacity conductor would be economical. All the calculations were done considering each conductor maximum operating temperatures. The data was also used to come up with graphs. Table 10. The table below shows different conductors properties at their maximum working temperature. Conductor type Conductor diameter(m m) Conductor weight (kg/km) D.C C Conductor cross sectional area (mm²) Maximum operating temperature ( C) Maximum operating current (A) A.C resistance at 75 C (Ω/km) ACCR Grosbea k 636 ACCC Amsterda m 380 AAAC Upas BSEN GAP 310 Goose INVAR (ZTACIR ) ACSS stilt ACSR Canary Phase

55 current (A) Line- line voltage(kv) A.C phase resistance (Ω) Power S (MVA) Line losses I²R (KW) Line losses as a percentage of line power (%) Figure 15. A graph showing the power vs. the cross sectional area relations of different conductor types

56 Figure 16. A graph showing the conductor current vs. the conductor weight per km relations of different conductor types. Conductor ACCR Grosbe ak 636 ACCC Amsterd am 380 AAAC Upas BSEN GAP 310 Goose INVAR (ZTACI R) ACSS stilt ACSR Canary 132 Conductor diameter (mm) Conductor area (mm²) Conductor weight per km Safety factor Modulus of elasticity(n/m m²) Tensile strength (N)

57 Stringing tension (N) Ambient temperature ( C) Operating temperature ( C) Radial ice thickness (mm) Wind pressure (N/m²) Table 11. A table of various characteristics of different conductors used for span and stringing tension calculations. Figure 17a. A table showing the results of span vs. sag characteristics of different conductors from sag and tension calculator

58 Figure 17b. A table showing the results of span vs. sag characteristics of different conductors from sag and tension calculator. Figure 18. A graph showing the span vs. sag characteristics of different types of conductors

59 4.2 Discussion From the four uprating methods discussed earlier, that is: i. Increasing the conductor rating by changing the thermal rating criteria. ii. Dynamic environment rating. iii. Increasing the conductor area. iv. Reconductoring using conductor of higher ampacity. We saw that the last option is the one that offers the best method of increasing ampacity if the long term economic effects were to be put in place. The choice of conductor selection when reconductoring is also another great factor since tower and support structure strength has to be considered to ensure no other stress other than the required is applied to the structure after reconductoring. From the overhead transmission rating results, it is evident that all the conductors used for comparison has lower weight per km value than the current ACSR canary conductor thus this means that there will be no need for structure modification when reconductoring. It s caused by the lesser cross sectional area of the other conductors due to different manufacturing method and the type of material used. The conductors also have different maximum operating temperatures for ACCR, GAP, INVAR and ACSS is 200 C while that of ACCC and ACSR is 180 C and 75 C respectively. ACCR conductor has the highest current capacity at its maximum operating temperature when compared to the other conductors, 1342 A. it is followed closely by ACSS and ACCC conductors respectively. These conductors have lesser diameter than the current ACSR canary conductor. Due to the increase in operating temperature, when reconductoring using any of the listed conductors above, it would be necessary to use conductor clamps with a higher heat resistance. The increase in current leads to an increase in power transfer capability of the transmission line. ACCR conductor has the highest power carrying capability when operated at its maximum temperature according to our results, which is 306.9MVA. It is followed closely by ACSS and ACCC conductor which stands at MVA and MVA. ACSR conductor has the lowest power transfer capability among them all. The conductor line losses also vary according to their carrying capacity and the line phase resistance at their maximum operating temperatures. The line losses are given by I²R where I is the current and R the resistance. We can see ACCR has the greatest line losses but this is a small ratio of its power transfer capability when compared to other conductors. ACSR has the least line losses but when compared to its power capability, the ratio is greater than that of the other conductors. From the power to cross-sectional area graph characteristics, we already saw that increasing the conductor area is a method used to increase the power transfer

60 capability of a transmission line. However this statement doesn t translate to the results we got from the graph. This is because the ampacity of a transmission line depends on many other factors apart from increasing the cross-sectional area. It s evident that ACSR conductor has the largest cross-sectional area among the conductors while it has the least power transfer capability. The other conductors have lesser surface area but more power carrying capability than the current ACSR canary 132. Reconductoring using the other conductors will be beneficial due to the increased capacity with minimal or no tower modification. From the weight per km and conductor carrying capacity graph, ACSR canary has the greatest weight per km and carries the least current when compared to the other conductors. This factor is brought about by ACSR maximum operating temperature. The reduced weight per km means there will be no or minimal structure reinforcement or modification when reconductoring thus the other conductors offer a greater advantage when compared to ACSR. The decreased weight also means that there will be no extra pressure exerted on the tower structure. From the span versus sag characteristics graph, ACCC conductor has the lowest sag with span increase. It is followed closely by AAAC and ACCR conductor respectively. ACSR has greatest sag span characteristics, 0.6m at a span of 100m which was the basic span used for calculations.this leads to use of more materials in the tower construction to cater for the sag and clearance. This offers another advantage when reconductoring ACSR using the other conductors since they have lower sag than the latter. This means there will be no need for structure modification to cater for sag and clearance thus making it cheaper and faster to reconductor using the HTLS conductors to increase the power transfer capability

61 5 CHAPTER 5: CONCLUSION AND RECOMMENDATION 5.1 Conclusion The aim of the experiment was to study the methods of uprating a transmission line and from the tabular and graphical results we can say that the objective was met. We see that uprating the existing Dandora to Juja road lines I and II using a high temperature-low sag conductor (HTLS) such as ACCR would not only increase the power transfer capability of the lines but would also require less structural modification. This would lead to great savings when compared to the construction of a new line. The lesser line losses to power transfer ratio can be of great benefit to the utility company. Reconductoring process won t be affected by right of way hinderances since the tower structure is already in its way. This makes the process not only efficient method but also a faster way of increasing power transfer capability as compared to new line construction. 5.2 Recommendation For many years in our country the utility companies has adopted the use of traditional ACSR conductors, but the technological advancement has led to the introduction of HTLS conductors. Adoption of this technology in the construction of new transmission line or reconductoring the existing lines would lead to: Improved power transfer capability. Lesser line losses ratio compared to power transfer capability. Low sag in the HTLS conductors causes reduction in tower construction material since the conductor clearance won t be a hindering factor. The increased power carrying capability of HTLS conductor provides room for future connection due to the rate of increase in population. Replacement of the current ACSR conductor with ACCR since has the best power transfer properties among all the other conductors and its sag-span characteristics lie within the required range. This can lead to all the advantages stated above

62 6 References 1) The approach to thermal uprating of Transmission line by. S.P. Hoffmann, A.M. Clark. 2) Methods of increasing the rating of overhead lines by. I. Albizu, A. J. Mazon, I. Zamora 3) Effective grid utilization by. S. Balser, S. Sankar, R. Miller, T. Curry, A. Rawlins 4) A review on series compensation and its impact on performance of a transmission line by Himanshu M. Joshi, Nshant H. Kothari 5) High temperature conductors A solution in uprating overhead lines by R. Criado, I. Zamora, A. J. Mazon 6) Transmission line loadability improvement using FACT device by. R. H. Besdadiya, C. R. Patel, R. M. Patel 7) Sag and tension calculations for overhead transmission lines at high temperatures by. Mehran Keshavarzian, Charles H. Priebe 8) Maximizing the ratings of national grids existing transmission line using high temperature-low sag conductors by M. J. Tunstall, S. P. Hoffmann 9) Statistical approach to thermal rating of overhead lines for power transmission and distribution by. C. F. Price, R. R. Gibbon 10) Increasing the capacity of overhead lines in the 400KV Spanish transmission network real time thermal ratings by. F. Soto, D. Alvira, L. Martin 11) Determination of thermal rating and uprating methods for existing lines by. R. Stephen, D. Muftic 12) Uprating of transmission capacity in great Riyadh 132 KV transmission line grid system by adopting low sag and thermal rate up conductor by. A. Kikuchi, R. Morimoto, K. Mito, Y. Kimura, A. Mikumo 13) Powering up a nation by Essel infraprojects limited. Ramnatt house community centre, New Delhi

63 7 Appendix The overhead transmission line calculator is attached below. Figure 19a. A figure showing overhead line rating calculator. Figure 19b. A figure showing overhead line rating calculator