How can overproduction of electric power be a problem to the grid?

Question

Recently, in Brazil, we had a major shutdown of the electrical grid of the entire country (wich is mostly interconnected across the majority of the states). After official investigation, the culprit was determined to be the excessive production of renewables, chiefly solar and wind power at a time of decreased net consumption. I just can't confidently understand how that can be. If the voltages and frequency of the several generators are paired (regardless of the power source), wouldn't the excess of power be just "available" for potential use? If I had a giant diesel generator turned on, can't it just drive a much smaller motor - in terms of power - and the remaining available power just be idle as potential at the generator's terminals? I understand it would be wasteful, but how is this situation a detriment to the continuity of the operation of the said generator?

Answer 1

It sounds like a real life demonstration of the saying 'failing to plan is planning to fail'.

A grid must be more or less balanced all the time, with the same power being generated as used. The moment to moment small deviations from exact equality are absorbed by all of the generators and motors on the system varying speed slightly, using the kinetic energy stored in the spinning machinery as a reserve.

As loads change, so the frequency of the grid changes slightly, and this is used as a signal to the various generators to add or subtract power, to maintain the balance. The control systems will normally try to control the frequency within tight limits, and take a large change in frequency to mean that there is a fault somewhere, and to disconnect things to protect generators from overspeed or over voltage.

In the early days of renewables, when they occupied a tiny fraction of the grid, their effect on control could be ignored. It would not be surprising then if they have been designed to be 'always on'. Surely if the sun is shining, you want the free (incrementally free) energy? The rest of the grid was big enough to absorb and control changes in renewable output.

Now with renewables making up many tens of percent of generating capacity, there is not enough conventional, controllable generation to switch off, if the load on the grid becomes too low. If the renewables cannot be switched off (or switched off fast enough, or in large enough quantities, or if the contract with the supplier doesn't include good enough financial incentive for them to stop supplying, whatever reason), then the grid frequency will rise, and the control systems will be tripped, and the grid will shut down.

In the bright future, when we have so much renewable capacity that it can still power the grid without storage on dull days, on sunny days we will have many times over-capacity. This is where the instantly controllable opportunistic electrolysers have been proposed to use this 'free' excess (when it's available) to make and store hydrogen, as a carbon-replacing feedstock for (some, like steel) industries, and to be burnt for long term energy storage to ride out long periods of dull, low wind, weather. Desalination is another good opportunistic load that can be switched on and off, and whose output can be stored.

Answer 2

No, because of Conservation of Energy, the amount of energy taken out of the grid is always exactly the same as the amount fed into it.

The energy taken out of a generator and fed into the grid is a braking force on the generator's turbine. So "idle potential" at the generator's terminals means that the shaft will speed up because the braking force is now weaker than the force turning the shaft.

To compensate for that, the operator needs to reduce whatever process generates the mechanical energy. Depending on what that is, that process can be slower or faster -- a flow of water can be reduced quick, while reducing a nuclear reaction takes time.

In any case, that energy will need to go somewhere. Small fluctuations aren't a problem, these just speed up and slow down all the generators (at the same time), that is a huge amount of metal, and the excess energy is stored that way, but for larger changes and over longer times you need to regulate how much energy is produced.

Answer 3

Why I installed solar panels on the roof to cover for blackouts due to renewables

There are two kinds of generators: grid-forming and grid-tied.

• Grid forming generator

Basically a low output impedance AC voltage source. It imposes its frequency and voltage. From your question, this is the kind of generator you had in mind. Connecting many of those in parallel with synchronized frequency and voltage forms what we call "the grid": a distributed, low impedance, fixed frequency AC voltage source.

The power it produces depends on what loads are connected to it. If no loads are connected, it won't produce any power.

Due to the huge rotational inertia, these alternators are very good at keeping frequency constant. They can also absorb very short power surges (milliseconds).

• Grid tied inverter

This is a completely different type of generator. It answers the problem: how do you install millions of independent solar arrays and inject that power into the grid. This kind of generator does not set the grid voltage and frequency. It is not grid-forming. Instead it adapts to the grid and injects power into it. This is done with control of the inverter switches, to inject current that is 180° out of phase with voltage. It looks a bit like emulating a negative resistor, although this isn't 100% accurate analogy.

Now as Neil says the issue is lack of planning. These inverters can control the power they inject into the grid, but in many jurisdictions renewable producers are paid by the injected kWh and have contracts specifying they can inject every watt they produce into the grid. In fact, if grid conditions prevent production, grid operators may have to pay penalties to large renewable producers.

This was done to improve profitability for renewables so we get more of them, however the cost and associated headaches from managing this forced power injection has not been eliminated, simply transferred to the grid itself.

Unlike grid forming generators, these inverters have no idea what loads are connected, so the power they produce only depends on what is available from solar/wind and to what value the limit is set in software. If the "injection limit" setting is not controlled according to grid conditions, they will keep injecting power even if no loads consume it.

This means there can be excess production, if there is more renewable power available than loads to consume it. Without a way to send an order to reduce production to enough inverters in the grid (maybe all), voltage and frequency increase, to the point the grid forming generators would have to absorb power instead of generating it. Then they shut down, and since there is no longer a grid, the grid-tied inverters also shut down, and you have a blackout.

Basically if you put enough renewable power that is not controllable on the grid, then the result is not controllable and it will crash and burn.

This was obvious from the beginning.

How to reconfigure millions of existing inverters to be controllable by grid operators in order to avoid overproduction and grid shutdown is going to be an... interesting question.

Answer 4

Produced energy must be equal to consumed energy.

Most of the electricity in a traditional electrical grid is produced by spinning generators. Generators are basically just magnets moving past coils. Each time the magnet passes the coil it induces an electric field. This is how you get the 50Hz/60Hz grid frequency: It’s directly related to how often the magnets are passing the coils i.e. how fast the generator is spinning.

When you draw more current from the grid the generators will have a harder time spinning, the voltage will drop and the frequency will drop.

Conversely when you draw less current the generators have an easier time spinning. But your coal and gas and nuclear power plants are still producing steam which still drives the generators. Same for hydro power plants: You still have water flowing over the turbines. Same for wind turbines: You still have wind driving the blades and the generators.

There are systems in place to regulate the mechanical input power into the generators (e.g. burn gas at a lower rate, turn wind turbine blades so they don't face the wind) but there will be limits to how fast and how effective you can regulate. If you don’t regulate your generators will just spin faster and faster and your voltage and frequency will just keep rising.

There can also be economic incentives to produce more electricity: If your wind turbines and solar panels are producing energy right now (basically for free), you’d be silly not to produce and sell as much as possible. Let somebody else shut down their power plant, you want to produce every MWh possible!