In a large bowl, mix together the dry ingredients - sooji flour salt, baking soda and baking powder very well.
In another bowl, whisk well the yogurt without forming any lumps.
Add the yogurt to the dry flour mixture and mix very well.
Add the water to the mixture and mix well.
Add the oil to the flour mixture and stir everything well to form the batter for plain rava idli. The consistency for rava idli batter is quite thick, however if the batter is too thick to handle, you may add a few spoons of water and mix the mixture very well to make the batter of the right consistency. Make sure not to add too much extra water, as the idli may not come right if the batter is thin.
Add about 2 tablespoons of batter in each idli mould, fill all moulds until the batter is finished. Since we use baking powder in this recipe, the idlis will rise so make sure not to add too much batter in the moulds. If the idli moulds are small, you may need to reduce the amount of batter used in each mould.
Heat very little water in the idli cooker and once the water starts boiling, place the idli stand inside the idli cooker.
Close the cooker and cook in low heat for about 12 to 14 minutes.
Open and see if the idlis are cooked and soft. Once the idlis are cooked, keep the cooker away from the hot stove and allow few minutes for the idlis to cool down.
Once the idlis are cooled for a few minutes, use a spoon (you may dip the spoon in cold water which will make your work easy) to take out each idli from the moulds.
Transfer the soft and delicious plain rava idlis to the serving dish, serve with any chutney, sambar or any vegetarian curry as you wish.