I can't refuel my power plants, when my food storage is cross 650k it automatically decreasing and..

Food and fuel are consumed by your troops. (fuel by vehicles, food by others) the more troops you have the higher the consumption. When the consumption is more that your rate of production the amount of food or fuel in your warehouses will decrease.

But there is a cut off point, the game will stop consumption when your level drops to a fixed amount (650k in your case). This is to stop you running out completely. The cut off point will raise as you level up.

You can use the fuel that you have to fill the power stations. this will drop your fuel level down and you will produce more up to the cut off point. you can also top up your fuel in the ware house with stored box items from rewards. but you need to use any added fuel quickly otherwise it will be consumed by the troops.

You can reduce your number of troops to reduce consumption and continue upgrading buildings to raise the production rate. But in general if you are playing a lot and are able to keep earning reward boxes that contain food and fuel then you will be able to maintain a stock that you can use when needed.

You can also buy boxes with stored power from the commercial hub from time to time to have a back up stock of that too.