A high altitude electromagnetic pulse (HEMP) has the potential to cripple power systems. A HEMP is a nuclear detonation, occurring at least 30 km above the surface of the earth. HEMPs emit harmful radiation in the form of electromagnetic waves. A HEMP can be broken down into three energetic waves, realized as geoelectric fields, E1, E2 and E3. This paper focuses on the impacts of HEMP E3 to power systems. Provided by the International Electrotechnical Commission for unclassified use is a single HEMP E3 reference field. The E3 field could be prone to slight variations due to various atmospheric conditions and weapon characteristics. As a result, studying the impacts of one specific field may be incomplete. In this paper, slight variations to the referenced field are made in order to better understand the potential power system impacts and sensitivities to various geoelectric field characteristics. The results show that the system's response is the most sensitive to changes to the geoelectric field's magnitude.