The primary objective of this project is to review the current knowledge of “best practices” in designing EHV overhead lines. Primary focus here will be the mechanical aspects of line design. The information should be presented to TODEM members in a way that they can understand. It is also expected that the document will provide a quick reference guide so that a less experienced engineer can understand the issues and the technical content and follow the EHV line design process in a transparent manner. Another objective of this project is to educate the younger engineers who are joining our workforce at an increasing rate. The knowledge base for EHV AC line design is getting smaller due to the dramatic changes that have occurred throughout the industry over the past 25 years and many experienced engineers leaving the profession primarily due to retirement and/or restructuring of the industry.
The main objective of section 7 is to write a section that will review the best practices that the industry is currently following with respect to mitigations of various vibration issues of EHV lines. The section will cover three basic vibration issues that the line normally encounters:
- Aeolian vibrations,
- Galloping and
- Wake induced oscillations.
This section will deal with those aspects of line vibration issues and identify the “best practices” engineers must follow to avoid any premature failure of the line.
This section will primarily focus on the vibration issues and challenges of EHV line design and best practices to mitigate these challenges using practical methods. Overhead lines are in constant motions and these motions are normally categorized in three specific areas: Aeolian vibration, Galloping motions and Wake induced oscillations. Aeolian vibration is normally associated with low amplitude high frequency oscillations of the conductor when the wind causes alternate shedding of wind-induces vortices from the top and bottom sides of the conductor. The galloping motion is associated with very high amplitude motions of the conductor with a low frequency of oscillation. This type of motion is caused when the wind imparts energy on an asymmetric ice covered conductor. Wake induced (sub span) oscillations happens within phases in bundle configurations, when the flow around a sub conductor is altered due to the presence of the other sub conductor in the system. Aeolian vibration is known to cause fatigue damages (strand breakage) near the clamp area (at the suspension point) while uncontrolled galloping motions can cause cross arm and or structural member failure due to excessive dynamic force. Line may also fail due to “flashover’ due to phases clashing in the air. Mitigations are normally provided with adequate damping of the system to ensure that the imparted wind energy is dissipated effectively through various types of dampers for Aeolian vibration, many types of anti-galloping devices and interphase spacers for galloping motions and spacer dampers for wake induced motions respectively.