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 land-based technological systems such as the electric power grid. Because of the extreme differences in views among experts regarding the potential impacts of HEMP on the electric power grid and the potential societal implications, the Electric Power Research Institute (EPRI) launched a three-year research project in April 2016 to investigate the potential impacts of a HEMP attack on the electric transmission system and to identify possible options for mitigating impacts.
You can access the entire report here: https://www.epri.com/research/products/3002014979
The video below is a summary of the EPRI study on the impacts that a high-altitude nuclear electromagnetic pulse (HEMP) would have on the power grid.
While it uses some technical talk and words at parts, even if you don't understand exactly what he says, you should be able to understand the end result.
This video is #10 on a playlist about the electrical grid, it's durability, and it's fail points. Watch here: https://www.youtube.com/playlist?list=PLTZM4MrZKfW-ftqKGSbO-DwDiOGqNmq53