A method for improving the electric field distribution in a high voltage direct current cable includes providing a cable that includes a conductor, a protective jacket, a shield layer disposed inside the jacket, and an insulation layer disposed inside the shield layer, surrounding the conductor. The insulation layer includes ethylene-propylene rubber, talc, and montmorillonite.
GOVERNMENT RIGHTS STATEMENT
 This invention was made with government support under contract number DE-AR0000231 awarded by the U.S. Department of Energy. The government has certain rights in the invention.