Research Overview

Background: The detonation of a nuclear weapon at high altitude or in space (~30 km or more above the earth’s surface) can generate an intense electromagnetic pulse (EMP) referred to as a high-altitude EMP or HEMP. HEMP can propagate to the earth and impact various ground-based technological systems such as the electric power grid. Depending on the height of the explosion above the earth’s surface and the yield of the weapon, the resulting HEMP can be characterized by three hazard fields, denoted as E1 EMP, E2 EMP, and E3 EMP. 

The International Electrotechnical Commission (IEC) defines the three HEMP hazard fields based on their distinct characteristics and time scales:  The early time component (E1 EMP) consists of an intense, short-duration electromagnetic pulse characterized by a rise time of 2.5 nanoseconds and amplitude on the order of tens of kV/m (up to 50 kV/m at the most severe location on the ground).

The intermediate time component (E2 EMP) is considered an extension of E1 EMP and has an electric field pulse amplitude on the order of 0.1 kV/m and duration of one microsecond to approximately ten milliseconds.

The late time component (E3 EMP) is a very low frequency (below 1 Hz) pulse with amplitude on the order of tens of V/km with duration of one second to hundreds of seconds. E3 EMP is often compared with severe geomagnetic disturbance (GMD) events; however, the intensity of E3 EMP can be orders of magnitude more severe, and E3 EMP is much shorter in duration than GMD events, which can last for several days.

Potential impacts of HEMP vary depending on the component (E1 EMP, E2 EMP, or E3 EMP) that is responsible for the resulting disruption or damage.

The geographic area exposed to varying levels of E1 EMP fields can be quite large, as the area of coverage is characterized by the line of sight from where the weapon is exploded to the horizon. For example, a detonation at 200 km can affect a circular area of on the order of 3 million square miles. However, not all areas included within the circular region experience the maximum electric field, and strength of the field falls off with distance from the ground zero location. The incident E1 EMP can couple to overhead lines and cables, exposing connected equipment to voltage and current surges (referred to as the conducted threat). The resulting E1 EMP can also radiate equipment directly (referred to as the radiated threat). Potential impacts from E1 EMP on the electric transmission system include disruption or damage of electronics such as digital protective relays (DPRs), communication systems, and supervisory control and data acquisition (SCADA) systems.

The characteristics of E2 EMP are often compared with nearby lightning strikes. However, it is important to understand that E2 EMP does not couple to overhead lines or cables in the more traditional sense of how lightning strikes a transmission tower or a conductor. Rather, E2 EMP couples to conductors through the air, like E1 EMP. This coupling mechanism is similar to how the field created by a nearby lightning strike couples to an overhead transmission line. Because the amplitude of the incident E2 EMP field is quite low (0.1 kV/m), impacts to the transmission system are not expected to occur.

E3 EMP induces low-frequency (quasi-dc) currents in transmission lines and transformers. The flow of these geomagnetically induced currents (GICs) in transformer windings can cause magnetic saturation of transformer cores, which causes transformers to generate harmonic currents, absorb significant quantities of reactive power, and experience additional hotspot heating in windings and structural parts. Potential impacts of E3 EMP on the bulk power system can include voltage collapse (regional blackout) and transformer damage due to additional hotspot heating.

When the EPRI EMP research project was launched, publicly available data on the HEMP threat, potential impacts of HEMP on the electric transmission system, and field-tested E1 EMP mitigation options specific to substations were limited. Additionally, there were differences between the findings of EMP research conducted during the 1980s through the early 1990s by the Department of Energy (DOE) and others and more recent findings communicated by the former Commission to Assess the Threat to the United States from Electromagnetic Pulse Attack (former EMP Commission). Because of these differences and the potential impacts that a HEMP attack could have on society, EPRI launched a three-year research project in April 2016 to provide electric utilities and other stakeholders with a technical basis for making more-informed decisions regarding the potential impacts of HEMP on the electric transmission system and potential options for mitigating possible impacts. By the conclusion of the project, the research was financially supported by more than 60 U.S. utilities.

The full report is found on link: EMP report 05082019.pdf


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