I am curious about integration of renewable energy in electricity grids. I am a mechanical engineer and understand energy and power but I have little knowledge about electricity. I will be thankful if you help me understand what can go wrong in a power grid when we add intermittent renewable energy sources, namely solar and wind. I found helpful information about inertia in answers to this question. I have also learned that stability in a power grid means stability in its voltage, frequency and phase. Here are my questions:
Suppose a simple AC circuit containing a source, a resistor and a switch. If the switch is suddenly turned on, electricity flows through the resistor. Is my understanding correct that in this experiment the frequency of the source tends to fall? Does this experiment have any effect on the source's voltage and phase? My reasoning for the decline of frequency is that the resistor dissipates some energy (as heat), which means some energy is taken from the generator, which means the generator is left with less energy to maintain its angular velocity with.
Suppose the switch in the circuit of question 1 is turned on and off repeatedly at irregular intervals. Does this experiment lead to different results than that of question 1?
Suppose a large number of circuits of question 1 connected in parallel. If the switches of the circuits are turned on and off randomly and repeatedly and independently from one-another, can we assume the ensemble of the circuits has the effect of a uniform load on the source?
In a grid based on conventional energy sources, I think of renewables as the switch in the above example. That is, when renewable electricity is available, I consider the switch is off, the renewable source is feeding the circuit, and the main (conventional) AC source is disconnected from the circuit. On the contrary, when renewable power is not available, the switch is turned on, and the main source is connected to the circuit and feeds the resistor. I understand electric loads are not just resistance, and I am considering them as things that consume a certain kWh of energy. Are my assumptions reasonable or am I missing something here? Is it reasonable to say that in such a grid the main challenge is to maintain frequency stability?
In a hypothetical grid based on solar and wind (that is, with little or no conventional sources), I am guessing the main challenge is to maintain voltage stability with minimum storage costs. Is my guess reasonable or am I missing something here? Is it true that voltage stability can be achieved by adding storage to the grid? What can be done for achieving frequency and phase stability?
Thank you for your time.