EPRI’s free field illumination test facility in Charlotte, North Carolina
EPRI’s free field illumination test facility in Charlotte, North Carolina. The system produces a 50-kV/m, MIL-STD-461G/RS105 compliant E1 field that is used to test equipment’s susceptibility to radiated threats.

Impact of High-Altitude Electromagnetic Pulse on Transmission Systems

Differing outcomes left electric industry stakeholders wondering about the actual impacts from a HEMP attack on the electric grid and how to approach cost-effective mitigation.

Industry Challenge

When discussing high-altitude electromagnetic pulse (HEMP) research, it is important to understand the nature of HEMP. HEMP is created by detonating a nuclear weapon at high altitude or in space. The resulting HEMP consists of an initial short-duration pulse (E1) and an intermediate pulse with characteristics similar to those caused by nearby lightning strikes (E2), followed by a late pulse (E3) similar to a severe geomagnetic disturbance (GMD) event.

Since the 1950s, the U.S. military and others have recognized the potential for E1 to disrupt or damage electronic equipment; later, they began to understand the potential effects of E3. To address concerns during the Cold War, the U.S. Departments of Energy and Defense assessed the potential impacts of a HEMP attack on the electric grid. Although most of the early HEMP research was classified, the main unclassified findings were that E1 can damage unhardened assets over large geographic areas, but long-term blackouts would be unlikely due to limited damage to large power transformers from E3.

Following the Cold War, research by the former EMP Commission provided a different perspective. The Commission identified potentially dire HEMP impacts on the electric grid, with a widespread blackout possibly lasting for months or even years due to the combination of E1 impacts and widespread damage to large power transformers from E3.

These differing outcomes left electric industry stakeholders wondering about the actual impacts from a HEMP attack on the electric grid and how to approach cost-effective mitigation.

EPRI’s Response

To address these questions, the Electric Power Research Institute (EPRI) launched a three-year research project in April 2016 to provide the electric utility industry and other stakeholders with a technical basis for understanding the potential impacts of HEMP on the transmission system and to identify appropriate mitigation options.

Progress, Results & Next Steps

EPRI’s initial, three-year research concluded in April 2019. Its findings related to E1, E2, and E3 impacts and identified mitigation options are outlined in the final report.

A significant portion of the research included assessing the potential impacts of E1 on digital protective relays (DPRs) in substations. First, a modeling and simulation effort provided insight into the electrical stress that DPRs might be exposed to during an event. Next, researchers performed testing of DPRs to determine the thresholds (conducted and radiated) at which damage or disruption can occur. The impacts then were assessed by comparing the potential electrical stress with the withstand levels determined through testing.

The results of the assessment showed that modern DPRs can be damaged or disrupted by E1 and that impacts could be experienced over large geographic regions. This finding prompted laboratory testing and/or evaluation of several potential mitigation options, which included the following:

  • Shielded cables with proper grounding
  • Low-voltage surge protection devices and/or filters
  • Fiber optics-based protection and control and communications systems
  • Enhanced electromagnetic shielding of substation control houses
  • Grounding and bonding enhancements

It was found that these mitigation options adequately protected DPRs up to the 50-kV/m threat level.

The effect of the E2 component of HEMP was found to not have a significant impact on the transmission grid. Modeling of the E3 impacts showed that, depending on the target location, up to 21 large power transformers could be at potential risk of thermal damage and that voltage collapse (blackout) of a multi-state area is possible.

How to Use the Research

This research identified options for hardening DPRs against the potential impacts of E1, but additional research is needed to improve mitigation designs and understand potential unintended consequences. Collaborative research involving field evaluations of E1 mitigation in live substation environments with multiple utilities is underway. Mitigation options for E3 impacts were found to be similar to those used to address severe GMD events.

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