THE POWER GRID HAS A MIND OF ITS OWN.

People who understand power systems are taking stock of the latest large-area blackout.  The fundamental problem a grid manager faces is that there's little by way of inventory.  “The grid is a synchronous system with no graceful failure mode.”  

One grid manager, the Nebraska Municipal Power Pool, explains.

Modern society requires electricity to be available on-demand around the clock. Fulfilling this need requires constant monitoring of electric demand on the electric grid and then matching that demand with electric load from various electricity generation sources.

Accomplishing this continual critical feat is a balancing act of epic proportions. It requires sources of energy that are “dispatchable” — an industry term essentially meaning a resource can be counted on when needed. This concept is critical in understanding why it takes a diverse mix of resources to operate the electric grid.

There are many sources of fuel that can produce electricity — the most typical being nuclear, coal, natural gas, hydro, wind and solar. Each of these resources has strengths and drawbacks when it comes to generating electricity. Some resources are more environmentally favorable while others are more reliable because they are dispatchable, or available when needed.

Dispatchable fuel resources include nuclear, coal, and natural gas. These fuel sources are highly reliable because each fuel is a constant supply. These are known as baseload resources.

Examples of non-dispatchable fuel resources include wind, solar and hydro-generated electricity (although some hydro generation can also be considered a baseload resource). These resources are environmentally beneficial because they produce no emissions, however, they are not always available — the wind doesn’t always blow, the sun doesn’t always shine, and, depending on location and weather conditions, water may not always be available to generate electricity.

The strategy for most utilities is to have a diverse mix of fuel resources, using both dispatchable and non-dispatchable forms of power supply to strike a balance between being environmentally conscious while not compromising reliability, ensuring a steady flow of electricity is always available.

Emmett Penney's summary for the Free Press of what might have gone wrong in Spain reinforces several of the points we examined earlier in the week.

Simply put, the power grid is like a giant game of tug-of-war. On one side, there’s demand, and on the other, there’s supply. Keeping the lights on involves making sure the rope between supply and demand remains taut. This tautness is called inertia.

Healthy inertia means healthy grid frequency, which in Europe means 50 hertz (in the United States it’s 60 hertz). In the version of events initially attributed to REN, something along one of its power lines disrupted the even flow of power between generators on its grid, which messed with the frequency, which likely tripped off more wind and solar, which slackened the rope, which probably caused the blackouts.

At least, that’s what we can infer based on what else was happening on the Spanish grid. As reported by Bloomberg’s Javier Blas, at the time of the blackout, the country had a slim percentage of dispatchable power generation in the mix. For those new to power sector terminology, dispatchable means sources that can switch on to meet demand when you need them.

That "healthy inertia" article offers a more intuitive explanation of the heavily mathematical notion of achieving stability by balancing electrical torque against mechanical torque.

Many generators producing electricity for the grid have spinning parts – they rotate at the right frequency to help balance supply and demand and can spin faster or slower if needed.

The kinetic energy ‘stored’ in these spinning parts is our system inertia. If there’s a sudden change in system frequency, these parts will carry on spinning – even if the generator itself has lost power – and slow down that change (what we call the rate of change of frequency) while our control room restores balance.

You don't want sudden changes in system frequency exceeding some very tight tolerances, and when everything is running properly, all the spinning parts are spinning at the same frequency; and the electrical torque exactly matches the mechanical torque.

The control room, however, might have to take a generator off line in the case of something disrupting the balance, to protect it from being overloaded.  What's interesting is that one way to introduce more inertia in a grid dependent on wind farms and solar fields is to create a latter-day rotary converter.

Renewables like wind and solar don’t synchronise with the grid in a way that provides inertia, so as the older coal and gas plants come off the system we need to find new ways to provide stability.

At [Britain's National Energy System Operator] we’re looking at a new approach – using either new assets or existing infrastructure that has been modified, to draw energy from the grid to power their turbines and create inertia, rather than inertia being a by-product of producing electricity.

So the same generators can continue providing inertia for the system, but with dramatically reduced need to burn fossil fuels.

That site also offers an intuitive explanation why restoring power after an outage isn't as simple as flipping a switch.

That wasn't the Iberian way, and when something went wrong, and the circuitry that protected the solar fields and wind farms from being overloaded took them off line, there weren't enough spinning parts to allow the control room to quickly begin restoring power after isolating and attempting to route around the problem.  (Interestingly, none of these analyses explain whether hydroelectric power is dispatchable or intermittent; it might not matter to the story, and putting the Iberian peninsula on the same grid as Switzerland and Norway would introduce a different set of troubles thanks to transmission line losses.)

Nuclear, cogeneration, and natural gas, all dispatchable resources, made up about 20 percent of the power mix. These resources also have the added benefit of supplying the grid with the steady inertia it needs to maintain proper frequency.

The opposite of dispatchable power is intermittent power, like wind and solar, which Spain has in spades—they made up about 80 percent of the mix just before the blackouts. These sources, especially photovoltaic solar, can’t supply the requisite inertia the grid needs. So when these “atmospheric oscillations” took place, the Spanish grid was already in a precarious spot because its dearth of dispatchable power made it more vulnerable to frequency disturbances.

Dispatchable resources have turbines that slowly switch off when they trip, which means the loss of inertia on the grid is less severe. When renewables trip, it’s like they blink out of existence.

There's yet another complication, which is that the current on the grid literally follows the path of least resistance, meaning that in the event of a failure somewhere on the grid, the consequences of the frequency disturbance might show up at some distance from the initial failure.  The system operator can run into the same problem starting back up, which is why once there's a widespread outage it's a while after the faults are identified and corrected before everybody's power is back.