P35.019: HVDC Overhead Transmission Lines
Objective
As new high‑voltage direct‑current (HVDC) transmission lines are planned and constructed, and existing lines refurbished, there is a growing need to better understand the performance of HVDC transmission systems and their associated line components. While the industry has operating experience with HVDC lines constructed more than 30 years ago, significant knowledge gaps remain related to component selection, application, aging mechanisms, and long‑term performance under modern operating conditions. Information derived from field experience and focused research is essential to support informed specification, evaluation, and application of HVDC line components, as well as to improve the performance and reliability of existing HVDC assets.
HVDC transmission lines exhibit electric field and environmental effects that differ fundamentally from those of HVAC transmission lines. For example, corona from HVDC lines may produce audible noise, electromagnetic interference, ozone generation, and spark discharges similar to HVAC lines; however, the presence of space charge is unique to DC operation and can influence electric field distributions, ground‑level effects, and line performance. These differences require specialized consideration in the design, operation, inspection, and maintenance of HVDC transmission systems.
The objective of this project is to provide utilities with the knowledge, technical information, and practical tools needed to support the design, inspection, operation, and maintenance of HVDC transmission lines, with the goal of improving system reliability, safety, and long‑term performance.
This project is intended to address the following industry knowledge gaps:
- Component performance characterization and benchmarking for new HVDC line designs and refurbishment applications.
- Guidance on component specification and selection for new construction and refurbishment of HVDC transmission lines.
- Improved understanding of safe live‑work methods, tools, and procedures applicable to HVDC environments.
- Improved characterization and assessment of electrical effects in the vicinity of HVDC transmission lines, including space‑charge‑related phenomena.
Research Value
This project is expected to deliver meaningful benefits to utilities, funders, and the public by improving the planning, design, operation, and permitting of HVDC transmission infrastructure. Anticipated benefits include:
- Improved reliability and resilience of electric service through enhanced selection and application of HVDC line components, leading to improved system performance and reduced risk of in‑service issues or failures.
- Reduced life‑cycle and implementation costs by providing guidance to optimize the design and operation of HVDC lines with respect to electrical effects, helping utilities avoid overly conservative designs while maintaining safety and compliance.
- Improved public acceptance and regulatory confidence by providing a sound scientific and technical basis for assessing HVDC electrical effects, thereby supporting permitting, siting, and stakeholder engagement for new and refurbished HVDC transmission lines.
Approach
This project is intended to provide engineering tools, guidelines, and methodologies that address both component performance and the electrical effects associated with overhead HVDC transmission lines. In 2027, the following tasks are anticipated as part of this project:
HVDC Transmission Line Reference Information: This task intends to update the overhead line specific HVDC reference documents that were previously published by EPRI in the early 1990’s. This document details the design characteristics of existing lines, and describes the testing conducted and results obtained in determining the corona and electric field characteristics of HVDC transmission lines. In 2027 further chapters are expected to be updated and added to this document.
Update HVDC Line Design Guide: This task, updated each year, collects relevant information on HVDC line designs from various completed HVDC research tasks. Topics will include design aspects pertinent to HVDC and will identify areas of design that are similar between dc and ac. The guide is updated each year with new information.
Update HVDC Electrical Effects Module in TLWorkstation: Based on the results and data obtained from the reduced-scale, full-scale, and operational line measurements, the existing algorithms used in the EPRI TLWorkstation software tool intend to be updated and the user interface improved.
Ground Electrode and Electrode Line Maintenance and Refurbishment: In 2027, this task intends to evaluate inspection, maintenance, and refurbishment strategies for HVDC ground electrodes and associated electrode lines that are approaching or have reached end of design life. The work includes documenting degradation mechanisms, proven maintenance and refurbishment practices, and technical considerations to support asset life extension, reliability, and risk reduction for HVDC electrode systems.
Perform Live-Line Tool Testing: This task intends to test live-line tools under HVDC conditions. The plan is to investigate the effects of space charge and corona to determine their impact on the flashover performance and voltage distribution along the length of the tool. Additional work may also be done to determine the accuracy of voltage detection devices under HVDC conditions.
Update HVDC Reference Book (The Olive Book): An electronic version of the updated HVDC reference book will be made available to project funders. This reference book, updated each year, contains information on overhead lines and converter stations.
Perform Hardware and Insulator Corona Tests: When specifying corona requirements for HVDC insulator hardware assemblies, many utilities use the equivalent peak ac voltage for testing. Currently, no HVDC hardware corona test specifications exist. This task intends to develop test specifications based on laboratory testing and develop guidelines to assist in specifying corona tests for HVDC hardware. In 2027 the intent is to review numerical simulations and to update the testing protocols.
Provide Tools and Resources on The Transmission Resource Center: The following calculators, tools, result summaries, and references are planned to be available on the HVDC Lines Transmission Resource Center:
| Resource Title | Resource Type |
|---|---|
| HVDC Voltage Detector Testing | Results Summary |
| HVDC Line Performance | Reference |
| HVDC Corrosion Research Questions and Objectives | Reference |
Anticipated Deliverables
| Deliverable | Type |
|---|---|
| HVDC Reference Book (The Olive Book) | Reference Book |
| TLW Gen 2 – HVDC Electrical Effects Module | Software |
| Ground Electrode and Electrode Line Maintenance and Refurbishment | Technical Update |
| HVDC Transmission Line Design Guide | Technical Update |
| HVDC Transmission Line Parameter and Testing Guide | Technical Update |
Past EPRI Work on Topic
| Product ID | Title | Description | Published Date |
|---|---|---|---|
| 3002032812 | HVDC Line Parameter and Testing Guide | This technical update modernizes an earlier EPRI HVDC reference, drawing on insights from the EPRI–HVDC The Dalles Project and reflecting EPRI’s broader efforts to advance HVDC and FACTS technologies in support of increased transmission capacity, renewable integration, and AC/DC planning decisions. This update includes an updated Introduction, the EHV‑DC Test Center chapter, and Appendix A covering global HVDC configurations. | December 2025 |
| 3002029614 | HVDC Anomalous Flashovers: A Study of the Anomalous Flashovers on Manitoba Hydro's HVDC System | This technical update investigates the phenomenon of anomalous flashovers on Manitoba Hydro’s High Voltage Direct Current (HVDC) system, focusing primarily on their characteristics, underlying causes, and the significant challenges they pose. The report details historical and more recent research efforts that have provided valuable data on flashover events. | December 2024 |
| 3002027144 | Impact of Corrosion due to Operation of HVDC Transmission Systems: HVDC Corrosion Key Considerations | This report examines why corrosion mechanisms in HVDC systems differ fundamentally from those in HVAC systems, with emphasis on identifying damage, quantifying severity, and evaluating mitigation options. | December 2023 |