Warm　standby　has　been　widely　applied　to　power　generation　systems　to　ensure　safe　and　reliable　operation　with　lower　energy　consumption　and　shorter　leading　time　compared　with　the　case　of　hot　standby　and　cold　standby,　respectively.　Since　there　is　a　series　of　intermediate　states　between　perfect　functioning　and　complete　failure　for　generating　units,　multi-state　characteristics　emerge　in　power　generation　systems.　Considering　different　parameters　of　state　transition　time　distributions　before　and　after　activation　of　standby　units,　as　well　as　generating　units　with　arbitrary　state　transition　time　distributions,　this　entails　a　need　to　propose　a　reliability　evaluation　technique　to　handle　multi-state　power　generation　systems　considering　warm　standby　with　arbitrary　state　transition　time　distributions.　An　analytic　method　based　on　multi-state　decision　diagram　(MSDD)　is　proposed　in　this　paper　for　the　reliability　analysis　of　the　proposed　systems　considering　successful　activation　probabilities　of　warm　standby　units.　The　MSDD-based　technique　is　applicable　to　multi-state　power　generation　systems　with　arbitrary　state　transition　time　distributions　for　generating　units.　The　proposed　method　enables　the　automated　calculation　of　the　system　reliability　and　expected　surplus　performance.　Furthermore,　the　optimum　sequence　of　activating　warm　standby　units　that　maximizes　system　reliability　is　formulated　based　on　the　proposed　automatic　reliability　calculation　method.　Illustrative　examples　are　provided　to　validate　the　effectiveness　of　the　proposed　technique.