Understanding The Electric Grid #1
The Bulk Power System's Need For Resource Diversity
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The Bulk Power System’s Need for Resource Diversity
Planning a reliable power system is a complex task, and the electrical, geographical, and environmental considerations that must be accounted for are numerous. The combination of these factors creates a system whose security is constantly threatened, and one of the most impactful design considerations, through which we can bolster the resilience of our bulk power system, is adequate resource diversity. Procuring generation from power plants that have a range of unique operational characteristics is critical for everything from day-to-day operations to mitigating the effects of black swan events. The defining nature of black swan events is that they are a surprise, and that they have devastating effects on the systems they impact. Earthquakes, extreme winter storms, and heatwaves all pose severe threats to the reliability of our electric grids. It is ultimately during these unusual times of distress that system design matters most. Every consideration when planning the power grid should be made with the worst case scenario in mind. Restoring a bulk power system from a widespread black out is time consuming, expensive, and most importantly - deadly. In this article, I will walk you through just a few basic examples of how having a diverse generation fleet results in increased reliability by relying on each unit type’s unique operational capabilities.
First, let us look at the Eastern Interconnection, which spans from Saskatchewan down to Louisiana, and from Florida up to Nova Scotia. The vast geographical coverage of the bulk power system exposes it to threats related to each of the unique geographies it runs through. This geographical diversity though, is also what helps strengthen the grid by diluting the risks associated with any one control area. What this also means is that there is no one size-fits-all solution for engineering a reliable electric grid, and that an isolated event in one control area could be devastating if that area does not have unique neighbors to rely on. Managing the bulk power system is a truly collaborative effort, and this strength in numbers approach is what allows your phone to charge at any moment, despite the many contingencies that occur on the system every day. This feature of the grid means that homogeneity poses a serious risk; the same goes for our generation fleets. This means that having a diverse fleet of generation units across the country is critical for guaranteeing the reliability of the bulk power system.
Different Power Plants Serve Different Roles
Every piece of equipment on the bulk power system plays a critical role, but without generation units there would be no power. Power plants are the instruments with which system operators balance system demand and generation. This balance is a delicate one, and when the two pieces are perfectly matched the system’s frequency sits at 60.00Hz. Many pieces of equipment, including generators, on North America’s interconnections are designed to operate within a specific range around 60Hz, and large frequency deviations can severely damage this equipment. When there is insufficient generation to meet demand the system’s frequency decreases, and in order to prevent units’ protective relays from tripping them offline, the current online generation must respond to rebalance the system. Our generation fleet must also be capable of meeting demand in the face of extreme weather, scheduled maintenance, and system contingencies. The nature of the grid requires different power plants to have a range of characteristics including, but not limited to: fuel type, response rate, maximum and minimum outputs, and voltage support
Serving Base load
Base load represents the system demand that is constant day and night; it is effectively the floor of demand that must be met 24/7, 365. In the chart above, you can see that PJM’s average weekday demand never dips below the ~60,000 megawatt-hour range. The demand below this level represents base load, and is ever present on the system.
Power plants that reliably serve base load run off fuel supplies that are procured with mid to long term contracts, are capable of remaining online for months at a time, and have high output capabilities. The most common forms of base generation units include nuclear and coal power plants. These generation units can output upward of 1,000 megawatts for months on end without maintenance. Thanks to the high energy density of uranium fuel, a nuclear reactor can potentially fuel up once and slowly deplete its uranium stockpile over the course of decades. Base load units benefit from their easy to store fuels, and are capable of securing a supply that can ensure power production on a time horizon beyond that of any other resource. Having power plants that can reliably output a large amount of power is critical for meeting the system’s base load, and without them blackouts would be all too common.
Regulation
Regulating units are those that respond to minor imbalances in generation and load, and to do so these units must constantly adjust their power output. A key feature of regulating units is their responsiveness, which allows them to ramp up or down quickly to new schedules (aka base points). This ability to ramp up or down quickly is crucial for meeting system demand that changes every time someone flicks a light switch.
Regulating units are also called on to help relieve transmission constraints by re-dispatching around congestion in order to reduce power flows on overloaded equipment. These overloads must be reduced quickly, often in less than 15 minutes. Two common types of regulating units include hydro facilities and combined cycle combustion turbines. Hydro facilities easily adjust their wicket gates in order to increase or decrease the amount of water spinning the turbine, and thus change their output. Combustion turbines can also change their rate of fuel injection quickly which allows them to vary their output on a moment’s notice.
The Niagara Power Project is one of New York’s most important power plants thanks to its size, speed, and reliability.
Regulating units must also have enough headroom to move their output up or down (usually in the range of tens of megawatts). With strict upper and lower operating limits on most generation units, this means that medium to large generators are best suited for regulation, and hydro and combined cycle combustion turbines often fit this description. A 500 megawatt combined cycle plant can operate at say 400 megawatts, and safely ramp up 100 megawatts or down 100 megawatts in a few minutes. This ability to vary output quickly is critical for balancing generation and ever-changing demand, and ultimately keeping your lights on.
Quick Start
Quick start units serve a critical reserve role in reliable grid operations. These units are capable of coming online, and within just minutes they can ramp up to their full power output. Such units tend to be smaller (tens of megawatts of capacity) combustion turbines that are powered by oil or gas. Quick start units are often needed for relieving system constraints and responding to contingencies. When transmission lines either become overloaded, or have future studied overloads, operators must reduce those power flows within short time frames. When contingencies actually occur, such as a transmission line or power plant tripping out of service, online generation must respond immediately, but if the online generation has no more reserves, then quick start units are called on to replenish them while slower starting units are brought online. Balancing authorities are required to maintain strict levels of reserves that are capable of maximizing their output in 10 minutes or less. When the reserves of online generation are depleted, quick start units are the last line of defense against rolling blackouts.
Black Start
Black start units’ capabilities are seldom used, but when they are, their unique characteristics are invaluable. Black start units are those which are able to come online and produce power without the use of auxiliary power (aka a reliance on the grid). In the event of a system wide black out, black start facilities are the first units called on to re-energize the system and restore power to customers. In New York State, three of the most important generation units all have black start capabilities: Niagara Power Project, St. Lawrence Power Project, and the Blenheim-Gilboa Project. All three of these hydro units have black start capabilities. Under normal operating conditions these hydro units are powerful, fast responding, and are incredibly versatile.
The pumped-storage facility at Gilboa is unlike any other generation plant in New York. Its upper reservoir stores potential energy in its water that is ready to provide power at a moments notice. During normal operations, Gilboa produces power when needed (typically during the day) and pumps water back up at night in order to arbitrage differences in energy prices during high load and low load periods.
As helpful as these hydro units are during normal operating conditions, it is their black start capabilities that set them apart. During times of restoration, that are typically accompanied with emergencies and disasters across a geographic region, the most important generation facilities are those that can come online and reach maximum output without a reliance on the grid (which in times of restoration does not exist). Niagara, Gilboa, and St. Lawrence are the first units called on to re-energize the backbone of New York State’s transmission system, and these units catalyze a domino effect that ultimately allows the electric grid to be put back together again.
So, the bulk power system’s generation fleet is composed of power plants that consume a range of fuel types, have unique operational characteristics, and have various limitations, but in concert, their diversity ultimately strengthens the electric grid. Ideal power sources in one location on the grid may be useless in another, and each node on the system must be studied in its own context. When designing a power system, ensuring reliability under normal conditions is a relatively pedestrian task, but ensuring reliability during a hurricane, a polar vortex, or any other time of emergency is a whole different story. Withstanding such extreme events requires a generation fleet diverse enough such that reduced or zero output from one type of resource does not jeopardize the whole system. As we continue to improve our bulk power systems, we must constantly remember the need for a wide range of operational features from our generation units.
Many of the topics mentioned in this write-up will be covered in greater detail in the future, so if you are interested in learning more about the electric grid and keeping up with the state of the grid, then make sure to subscribe below! If there are any particular topics that you are interested in or questions you want answered then feel free to reach out to me any time. And whether you enjoyed this letter, learned a few things, or hated it, please subscribe and share!
Pete
Apparent Power