Basic questions about power grid stability with variable loads and sources

Question

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:

1. 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.

2. 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?

3. 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?

4. 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?

5. 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.

Answer 1

In a traditional grid, the frequency stability is maintained by the rotating mass of the generator set. If the generator suddenly becomes easier to spin, or more difficult to spin, (because you have switched a switch on or off), the steam turbine and generator may gradually get faster or slower -- like a moving car at the top or bottom of a hill. Short term frequency stability just depends on the rotating weight: if you have a couple of slow rotations, you give it some more steam, and at the end of the day you expect (and are required) to have had the correct number of rotations over the last 24 hours. Sometimes the weight wasn't enough, and extra rotating weight is added: flywheel installations.

With modern electronics, it is not necessary to use rotating weight. Electronic devices can react to the slope of the alternating current, adding more energy to make the voltage and current rise and fall faster, or shunting energy away to make the voltage and current rise and fall slower, not having to take the average over a few cycles and then add or reduce steam to accelerate or decelerate the turbine.

1. Gensets slow down when you add load, just like any other engine.

2. Gensets are regulated to maintain constant speed. If you have hills, it may confuse the speed regulation, just as your speed control on your car can be confused. There are specifications and requirements on how fast you can change the load on a grid without the grid falling over: if you operate a load outside those specifications and expectations, you could be going downhill when the speed control is trying to make the car go faster, then be going uphill when the speed control is trying to make the car go slower. This is called 'control theory', and is a characteristic of the 'control system', of which the generators and solar/wind are only a small part.

3. In normal operation, load made up of lots of houses is just one large load. If something goes wrong, different parts of the load may go out of synchronization with each other or with the supply. You may consider it as a 'loosely coupled' system of multiple weights and springs: you can overdrive part of the system to make it over-oscillate, or stop, or go faster or slower.

4,5) For grid control, frequency stability is not a 'different thing' than voltage stability or current stability. They are all aspects of the same thing. Adding storage to a grid doesn't magically give it more voltage stability or phase stability any more than adding a large gas tank to your car does.

Wind and Solar have less rotating mass than 'conventional' does, which removes one form of stability. But solar can be switched off and started up in with sub ms delay. The control system has to be engineered to correspond to the different characteristics, which may include batteries and electronic frequency regulation.