Load Control Devices on Overhead Transmission Lines
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174 LOAD CONTROL DEVICES ON OVERHEAD TRANSMISSION LINES Working Group 22.06 December 2000 LOAD CONTROL DEVICES ON OVERHEAD TRANSMISSION LINES PREPARED BY WORKING GROUP 22.06 Members of the Working Group : Elias GHANNOUM - Convenor (Canada) Joel ANGELINI (France), Jim FERGUSON (United Kingdom), Svein FIKKE (Norway), Edwin GOODWIN (United States), Ramon GRANADINO (Spain), Trevor JACOBS (New Zealand), Friedrich KIESSLING (Germany), Joao Felix NOLASCO (Brazil), Jan ROGIER - New Convenor (Belgium), Pavel FRONEK (Czech Republic). Corresponding Members : D. CHOUDHRY (India), Farid KHADRI (Algeria), Jeong Boo KIM (Korea), Tony PLOG (Netherlands), W. Neil PIERCE (Australia), Helmut STRUB (Switzerland). LOAD CONTROL DEVICES ON OVERHEAD TRANSMISSION LINES Working Group 22.06 TABLE OF CONTENTS 1. SUMMARY 3 2. INTRODUCTION 4 3. CLASSIFICATION OF LCD 5 3.1. Load Release Devices 5 3.2. Load Reduction Devices 5 4. TECHNICAL DATA RELATED TO AVAILABLE LCD 5 4.1. Special Elongated Fittings : Japanese Experience 6 4.1.1. Type I 6 4.1.1.1. Principle 6 4.1.1.2. Design Characteristics 8 4.1.1.3. Mechanical Performance 8 4.1.1.4. Electrical Performance 8 4.1.1.5. Service Experience 8 4.1.2. Type II 9 4.2. Controlled Sliding Clamps 10 4.2.1. French Experience 10 4.2.1.1. Principle of the Controlled Sliding Clamps 10 4.2.1.2. Calibration 11 4.2.1.3. Design of a Controlled Sliding Clamp 12 4.2.1.4. Test 13 4.2.1.5. Device Efficiency 16 4.2.2. Belgian Experience 18 4.2.3. Romanian Experience 18 5. SERVICE EXPERIENCE OF LCD IN DIFFERENT COUNTRIES 19 E:\174\Lcd1099c\LCD1099C.DOC 1 E:\174\Lcd1099c\LCD1099C.DOC 2 LOAD CONTROL DEVICES ON OVERHEAD TRANSMISSION LINES 1. SUMMARY Electricity utilities need to design overhead lines to withstand potential cascade failures of towers, which could occur as a result of exceptional icing or rupture of conductors. Some utilities simply design the lines with section towers at frequent intervals and other utilities use special devices called « load control devices » (LCD), the function of which is to limit the stresses applied to the towers in cases of exceptional longitudinal loads. These devices are usually based on one of the following principles: - Releasing the conductor from the tower, - Sliding the conductor through suspension clamps to balance the load, - Elongating the conductor fitting to increase the apparent length of the conductor and decrease its tension, - Elastic deformation of the crossarms on the towers. This report describes the characteristics and performance of elongatable fittings occasionally used in Japan and the controlled sliding clamps used in France. Some Japanese utilities have replaced conventional suspension sets with Suspension-Tension sets with elongatable fittings on existing lines under which buildings appeared after construction. These devices were occasionally used in the past in Japan and they make it possible to raise the height of conductors (in normal service) without replacing or modifying the existing suspension towers. The first type of device is based on the unfolding of the fitting, triggered by the rupture of calibrated shear pins. The loads are damped by an impact-absorbing element. The second type of device is based on sliding the shackle in a groove designed into the yoke plate. In France, EDF systematically installs controlled sliding clamps on suspension towers. Each tower of a line is designed for a conventional asymmetric ice load. The sliding load of the clamp is chosen in a range of standard values such that if the actual load from the asymmetric ice load reaches the calculated load then the conductor will slide through the clamp. The suspension tower is thus protected from failure from exceptional longitudinal loads and at the same time the tower cost is not increased. E:\174\Lcd1099c\LCD1099C.DOC 3 The results of a survey show very varied utilisation of the LCDs in different countries: either not used at all, occasionally used or systematically used. The choice of device is based on many reasons and is linked to different local practices and parameters: - Countries which systematically use LCDs, such as France, Belgium and Romania, give one or more of the following reasons : * The global cost of a line is reduced, * The efficiency of these devices has been checked in test stations and on several occasions in the field. - Countries which use these devices in a non systematic way, such as Japan, do it in specific cases, like on existing lines to avoid the replacement of the towers when buildings are built under them, - Countries, which never use LCD, give one or several of the following arguments: * The towers are systematically designed for specified longitudinal loads, * Icing is unknown or very rare in the country, * The efficiency of the devices is uncertain, * The increase of the conductor sag, generated by an LCD, is unacceptable in the view of public safety, * The use of controlled sliding clamps causes severe damage to ACSR conductors. In conclusion, the utilities that want to go further in the field of load control devices, should concentrate on the following aspects: reliability, safety, and estimation of the cost effectiveness. With respect to this last aspect, the experience in some countries show that, for the future, the modifications and upgrading of existing lines are a potential field of application for LCDs. 2. INTRODUCTION The most serious overhead line failure, which could have severe consequences affecting the grid operation, is the cascade failure of several transmission towers. This cascade failure is usually initiated by the failure of a single tower or by the breaking of conductors and is the result of insufficient design strength of towers to withstand high longitudinal loads. The choice of solution to avoid such damage will depend upon a compromise between the risk and consequences of a cascade failure and the extra cost of construction specified by the designer. The most frequent solutions to limit or avoid series tower failures are, either to use section supports at regular intervals (also called anti-cascading towers), or to provide enough longitudinal strength to the suspension towers. In the second solution, the use of LCD can make it possible to limit considerably the magnitude of the longitudinal loads and thus the cost of the towers. After a general presentation of the different categories of LCD, this document describes some devices which are currently available. It then summarises the experience of the countries that took part in the survey conducted as part of SC22 WG 06 of CIGRE. E:\174\Lcd1099c\LCD1099C.DOC 4 3. CLASSIFICATION OF LCD The task assigned to an LCD is to avoid major and costly line failures, which could occur in the case of certain exceptional loads, by releasing or limiting the stresses applied to a main line- component. Practically, LCD are generally designed to protect the towers from exceptional stresses applied by the conductor in the case of unusual icing, cascading, etc. Considering the existing devices, two categories of Load Control Device can be defined: 3.1 Load release devices In this case, the conductor load that could cause the failure of towers is totally released from the tower so that the conductor system is not controlled (or a very limited control). Different types of devices exist: -Devices in which the conductor grip within the tower attachment fitting is released, such as release type suspension clamps (no more control of the longitudinal tension), -Devices in which the conductor or the fitting itself become detached from the tower, by means of mechanical fuses such as shear pins or safety bolts, associated with a system (such as a loose jumper) to prevent the conductor from falling down to the ground. 3.2 Load reduction devices In this case, the conductor load that could cause tower failure is reduced to an acceptable value and the system maintains control of the conductors. Different types of devices exist: -Devices acting on the fastening of the conductor within the fitting, such as controlled sliding clamps, in which the conductor slides when the longitudinal load exceeds a specified value, -Devices which act on the length of the fittings (unfolding or sliding elements) so as to increase the apparent length of the conductor. -Devices which enable elastic deformation or swinging of the crossarms of the tower to absorb longitudinal load. Dynamic damping devices constitute another category of control equipment. These act on the origin of the stress and are designed to stop or reduce dynamic phenomena such as conductor vibration and galloping: damping jumpers, Stockbridge dampers, spacer-dampers etc. These devices cannot really be considered as LCD and this report will not address them further. 4. TECHNICAL DATA RELATED TO AVAILABLE LCD This chapter gives the principles and characteristics of some important load reduction devices of two types: - Special elongating fittings - Controlled sliding clamps E:\174\Lcd1099c\LCD1099C.DOC 5 4.1. SPECIAL ELONGATING FITTINGS: JAPANESE EXPERIENCE These devices were occasionally used in the past in Japan. This is mainly due to the increase in restrictions to the building of new lines and increasing urbanisation around the existing lines. In Japan, new buildings are often built under existing lines and Japanese utilities must find solutions to compensate for the lack of distance from the lines to the ground and the buildings without replacing or raising towers, if possible. One solution is to replace the conventional suspension systems with Suspension-Tension Insulator Assemblies. These devices can also be used to improve the insulation level of the lines, by increasing the number of insulators without lowering the height of conductors. In these cases, the existing suspension insulator set is replaced by two tension sets suspended under a special suspension fitting, which in normal service is shorter than the conventional suspension set.