Black Starts: How the Grid Gets Restarted

Black Starts: How the Grid Gets Restarted
Black Starts: How the Grid Gets Restarted

Gripped as we are at the time of this writing by a historic heatwave, it’s hard for those of us in the western United States to picture a time when cold and ice reigned across the land. But really, it was only about four months back that another bit of freakish weather was visited across most of the country, including places ill-equipped to deal with the consequences. The now-fabled “February Freeze” left millions, mostly in Texas, scrabbling about in the dark and cold as a series of cascading engineering failures took apart their electrical grid, piece by piece, county by county.

The event has been much discussed and dissected, as an event with such far-reaching impact should be. Like much discussion these days, precious little of it is either informed or civil, and that’s not good news for those seeking to understand what happened and how to prevent it from happening again, or at least to mitigate the effects somewhat. Part of that is understandable, given the life-disrupting and often life-threatening situations the disaster forced people to suddenly face. It’s also difficult for people to discuss an event so widespread in its scope and impact — there’s just too much for anyone to wrap their head around.

To make the present discussion a little easier, we’ll be focusing on one aspect of the February grid crash that’s often bandied about but rarely explained: that the Texas grid was mere minutes away from collapsing completely, and that it would have taken weeks or months to restore had it been able to slip away. Is that really possible? Can the power grid just “go away” completely and suddenly? The answer, sadly, is yes, but thankfully a lot of thought has been put into not only preventing it from happening but also how to restart everything if it does happen, by performing what’s known as a “Black Start.”

All That Overhead

By some measures, the planet’s electrical grid is the largest and most complex machine we’ve ever built. It seems a fair claim; in the USA and Canada alone, the transmission grid has over 120,000 miles (190,000 km) of lines that stretch across the entire continent. And that’s just the system to move bulk electricity from where it’s produced to substations located near population centers; add in the millions of miles of cable that form the distribution system that connects individual customers, and you begin to see the sheer size of the system. Then consider that this system has over 10,000 generation plants, each of which has to be synchronized with all the other plants regardless of demand, and the complexity involved starts to reveal itself.

That’s one big machine. Every power plant and transmission line in the conterminous United States. Everything in this map has to work in unison, regardless of load or local conditions. Source: US Energy Information Agency

With all the equipment in place to make the power grid work over such a wide area, it’s hard to recall that this was not always the case. The North American grid of today grew bit by bit, starting mainly in the population centers of the east and midwest, electrifying first the cities and later spreading out into the rural areas. Building on existing systems allowed power companies to not only leverage the hard-won knowledge of what works and what doesn’t work when stringing up wires and connecting customers, but also to provide the power needed for their new generation facilities. The simple fact is that it takes power to make power, and that’s the heart of the black start problem.

Power generation is simple in theory, and we all learned the basics at one time or another — turn potential energy into kinetic energy to spin a magnet inside a big coil of wire. But the details are where the complexity lies. For example, in a coal power plant, milling the raw coal to the proper size to be used as fuel in the boilers takes power, as do the conveyors that feed the boilers, the actuators that control the valves, the sensors and control systems that regulate the speed of the turbines, and the switchgear that connects the generators to the grid. It takes power to make power, and a power plant can use a significant fraction of its own power. When a plant can turn out 700 megawatts or more, the overhead load needed to run — or restart — a plant can be huge.

Aside from the equipment needed to fuel and control the power plant, there’s another piece of the black start puzzle that may come into play: excitation current. Most power plants use self-excited generators, meaning a small amount of the current they produce is used to power the field coils of the generator, creating the powerful magnetic field needed to generate electricity. Once a self-excited generator spins to a stop, there’s no current available to excite the field coils. For short outages, that’s generally not a problem, as the residual magnetism of the iron in the generator’s rotor will be enough to start a weak flow of current, which will then excite the field coils and allow the generator to come up to full capacity.

Extended outages, however, may cause a rotor’s magnetic field to weaken enough that it will need a little help getting going. Black start procedures need to account for this eventuality by providing a means to “flash” the field windings with external power. The process for smaller generators is very similar, and it’s worth keeping in mind for anyone who stores a generator without actually taking it out and using it occasionally. Just keep in mind that for a power plant, it’s going to take much more than a hand drill to flash the windings.

Generators? What Generators?

Assuming an idled power plant is still connected to the transmission system, and assuming other plants within the region are still operating, the black start process is pretty simple — just take the power from the grid using switchgear and transformers the plant has for just this purpose. But in the case of a regional catastrophe — like the Texas winter storm, where condensate at natural gas wellheads and in the lines supplying power plants froze solid, pinching off fuel to dozens of operators — plants over a wide area may all go down together, making it impossible to import the power needed to restart. This condition is called “islanding”, and this is where the power plant’s black start procedure comes into play.

Utilities are understandably reluctant to share details publicly, but black starts of islanded plants are generally a cascade of operations where successively larger emergency generators are brought online, until enough power is available to black start the main generator. The process usually starts with diesel generators, which produce enough power to keep the power plant’s lights and control systems on. This will allow operators to start a larger generator, perhaps a gas turbine cogenerator, which then provides enough power to run the pumps, valves, feeds, and switchgear of one of the main generators in the plant. Once one of those is turning, the rest of the generators in the plant can start, and service can be restored.

All the procedures for black starting an islanded power plant are carefully documented, and the plans are supposed to be practiced at regular intervals to make sure everything works. Unfortunately, when crunch time came in Texas last February, and multiple power plants were islanded, the black start process was anything but smooth. One report said that nine out of thirteen generators designated as primary black start units were found to be not working, while six of the fifteen generators designated to back up the primary black start generators were also down. The problems with generators ranged from freeze damage to fuel issues, including the inability of trucks to deliver fuel oil on icy roads.

At the risk of oversimplifying a complex and far-flung series of events, for want of a set of tire chains, Texas came astonishingly close to losing their power not just for a couple of days, but for weeks or possibly months. It didn’t happen in this case, but only just barely and by several strokes of luck. We’ve no doubt that a lot of engineering skill and ingenuity went into getting the reluctant black start generators back online, too, so hats off to everyone who worked hard to avert the catastrophe. Hopefully, this will serve as a wake-up call, and that some thought will be put into how to better engineer the whole black start system, not only in Texas but at every power plant in the world.