It　is　a　feasible　way　to　transform　an　existing　XLPE　cable　line　from　AC　operation　into　DC　operation　for　uprating　the　transmission　capacity,　but　there　is　no　unified　standard　or　specification　for　the　transformation　design　till　now.　We　prepared　sheet　samples　by　spirally　cutting　the　XLPE　insulation　of　a　YJV22-8.7/10　kV-3×185　mm2cable　which　had　been　running　for　7　years,　and　the　breakdown　tests　were　performed　under　continuous-rising　and　step-up　voltages,　both　AC　and　DC,　with　the　same　conditions.　The　results　show　that,　compared　with　the　average　AC　breakdown　field　strength,　the　rate　of　continuous-rising　voltages,　from　2　kV/s,　0.5　kV/s　to　0.1　kV/s,　has　a　much　greater　impact　on　the　DC　breakdown　strength.　Besides,　with　the　increase　of　step　duration　from　2　min,　6　min　to　15　min,　the　AC　breakdown　strength　decreases　slightly,　while　the　DC　strength　increases　remarkably.　In　general,　with　the　increase　of　time,　AC　breakdown　strength　decreases　and　the　DC　breakdown　strength　increases,　which　lead　to　the　continuous　increase　of　the　DC/AC　ratio,　from　1.33　to　3.73.　Based　on　the　lower　confidence　limit　of　breakdown　strength　under　the　confidence　level　of　95%,　the　one-hour　breakdown　strengths　of　XLPE　samples　are　89.50　kV/mm　and　247.55　kV/mm,　and　the　voltage　endurance　coefficients　fitted　by　the　inverse　power　law　are　17　and　-11.51,　respectively,　for　AC　and　DC.　For　the　DC　breakdown　experiments　carried　out　by　applying　high　voltages　in　a　short　time,　the　relationship　between　the　breakdown　strength　and　time　does　not　conform　to　the　traditional　electrical-aging　life　law.　This　phenomenon　may　be　related　to　the　time-dependence　of　space　charge　effect,　which　needs　further　confirmation　by　the　following　research　work.　It　is　also　suggested　that　the　short-time　breakdown　test　data　based　on　sheet　samples　under　high　voltages　can　not　effectively　reflect　the　long-term　electrical-aging　performance　of　DC　insulation,　and　thus　can　not　be　directly　applied　for　the　insulation　design　of　XLPE　DC　cables.　©　2017,　High　Voltage　Engineering　Editorial　Department　of　CEPRI.　All　right　reserved.