Is one allowed to refill a electric samovar which is almost empty, with much cold water, on yom tov? It will almost certainly trigger the heater to go on,and if the samovar has a light signal, the light will most probably go on. Thank you

If as you say, as a direct result of pouring in cold water the samovar will immediately turn on the heating element and the light, it is forbidden. Needless to say on the forthcoming Chag it is forbidden in any case since it falls on Shabbat. Chag Same'ach.