Your question is poorly framed. You may have asked, "How can electricity BE stored in power generation station?" or perhaps, "IS electricity stored in power generation station?" A more specific question would result in much better answers. Having said this I will attempt to give you an answer to my interpretation of your question.
Most fossil fuel generation plants do not specifically store the power they produce. As an example, any fuel used in a typical power plant produces heat, which in turn produces heat to turn water to steam. This in turn spins a generator to produce electrical power. If you consider the fuel only, you may say that the power is "stored" in the fuel, in that it has not yet been released or transformed into electrical energy but remains stored in the fuel. In that sense, this would be how the "electricity to be" is stored.
This analogy may help. If you own a car, how is the energy stored for you so that it is ready when you want to go for a drive. The answer would be the gasoline in your fuel tank. You can look at a power plant the same way. To go further, if your car is idling, it is burning some fuel but at a low rate. This is also true of a power plant used to generate electricity. We burn some fuel to keep the generator spinning so that if you turn on your lights, we are ready to supply you with electrical power. Just as in your car, if you are sitting at a red light, your car is idling waiting for you to step on the accelerator to move forward as soon as a green light comes on. You are now using up some of your "stored" fuel to accelerate your vehicle.
If you are speaking of the "Electrical Energy" that is produced by any form of power plant, fossil fuel, nuclear, photo-voltaic, wind, pumped storage, geothermal, etc. The only storage capability we have at this time are batteries, capacitors, a few super coils and magnetic bottles. So they are few and not very effective for large power production.
At this time it is easier to store the fuel for later conversion. In some cases we may store the heat as with a solar plant that heats salt which is later used to create steam and turn a turbine, etc.
I hope this helps,
Newton1Law
One concept is to use "pumped storage" for large scale application. A hydroelectric facility is paired with a generation facility. During periods of low demand, the generation facility would use excess power to run the turbines as pumps to pump water from below to a higher elevation in the dam. During periods of high demand, the water would be released to run the turbines (which were previously acting as pumps) to generate the added electricity needed. You may want to google the term pumped storage generation facility to find locations where this concept is being used.
It is not stored.
