Most of us are connected to the power grid, and we take for granted that power will just always be available. But to discuss how the grid needs to evolve to meet future needs, including the changes as a result of new technologies and renewable energy, we will need to dive into some moderately technical topics. Because of the complex interrelationships between the components of the grid, and their operation, it is hard to organize into a logical path. We need a good overview of the pieces and how they work together, in order to understand discussions of the impacts, benefits, and costs of different directions our utilities may go, and how they will affect us today and in the future.
To start, we need to start with some vocabulary and the basics. This is the quick overview; further basic info is available on Wikipedia. Our focus here will be more functional—how it works, how it operates, how the different options trade off against each other.
To do that, I'm going to focus on the role of stored energy and how it relates to power flows and robustness. To keep it in a logical order, I'll roughly follow the flow, first of power, then of history, and end with a couple of recent examples of problems and solutions.
- Generation
- Transmission
- Distribution
- Load
- Operational Constraints
- Reserves, Failures, and Recoveries
- Capacity Factor
- Storage
- Aliso Canyon Natural Gas Blowout
- Elon Musk's Bet
- References
Let's look at a simplified diagram.
Simplified Grid
Generation
The basic task of the grid is to reliably deliver electrical power from generation to load. Power is generated by taking an energy source and in one or more steps, converting it into electrical energy. Common examples:
- Thermal ➡︎ steam ➡︎ rotary ➡︎ electrical. This is one of the most familiar and common. The source of thermal energy can be fossil fuels, concentrated solar, nuclear, geothermal, or waste heat from industrial processes. The heat is used to create steam under pressure. the pressure is converted to rotation by a turbine, which turns a rotary generator;
- Solar ➡︎ electrical. Solar panels convert electricity the sun's photon energy to electricity at a quantum level. Also called photovoltaic or PV.
- Combustion ➡︎ rotary ➡︎ electrical. The most common is the natural gas combustion turbine.
Transmission
To get the power from the point of generation to the load, we need to be able to transmit the electricity over long distances. For reasons that we will discuss later, this is done by converting it to AC 3-phase power at 50 Hz or 60 Hz (depending on system). (Many generators produce AC 3-phase at the required frequency directly).
Because it is AC, we can use a transformer to boost the voltage while cutting the current by the same factor. Doubling the voltage allows the same power to be transmitted at 1/2 the current. This, in turn, reduces the transmission losses to 1/2 * 1/2 = 1/4. Transmission line voltages range from 69 kV to 765 kV, or even higher, with the higher voltages being suitable for transmitting higher amounts of power or longer distances. The high-voltage transmission lines carry the power to substations, where the power can be directed with switches and transformers to other substations in the transmission network, and finally to distribution substations.
Distribution
At the distribution substations, transformers convert the power down to a lower-but-still-high voltage, between 2.3 kV and 39 kV. These lower voltages are safer in proximity to people and the equipment is less expensive. The higher current needed at the lower voltage is acceptable because of the shorter distance the power is delivered. The final stage is for the voltage to be lowered to the voltage required at the customer service drop, commonly 110 VAC or 220 VAC.
Load
Loads are anything that uses electrical energy from the grid, from a simple night light in a child's bedroom to giant arc furnaces melting huge vats of iron.
Loads are not constant. They follow a somewhat predictable daily cycle, but the cycle varies according to weather and other factors that a utility needs to take into account in making forecasts for what power will be needed. There are sudden events, such as switching on equipment, or sudden shutdown of a factory due to an emergency or equipment failure. Grid operators employ meteorologists to predict the impact of weather on both load and generation, and most power is purchased 1-7 days in advance.
The grid operators must be able to compensate for these changes by a combination of adding or removing generated power, or drawing power from storage or putting it into storage. These are accomplished by means of power purchases on the real-time spot market, and by issuing commands to direct the generation and flow of power. These decisions are normally made on a 5-minute basis.
Operational Constraints
There are several key operating principles to keep in mind with our grid. In our simplified grid diagram, we have several generators feeding power into the grid, and several loads taking power from the grid. For our loads to receive reliable power, there are several constraints that must be met:
- The power put into the grid must at all times exactly match the load plus system losses. This can come from generation or from storage. In particular, note that the power added cannot exceed the demand.
- The voltage must be kept constant, even as loads or generators are added or removed. Both too high and too low are unacceptable. In most cases, the voltage must be kept within ±5% even when generators go offline, and must change <8% within that range.
- The frequency must be maintained within strict standards. This is necessary to allow the generators to remain in synchrony, so their power is in phase and adding, rather than out of phase, partially or complete canceling. For most systems in the US, the North American Electric Reliability Corporation mandates the frequency be maintained within ±0.017 Hz, except systems such as large hydro can deviate by up to 0.034 Hz.
- The total accumulated cycle deviation due to frequency errors are also tightly controlled. That is if the frequency runs high for a period, it must run slow to compensate.
- The rate of change of frequency errors is also limited.
- For rotating generators, voltage and frequency are intimately related. If a generator slows because of additional load (higher current), both the frequency and the voltage drop.
- The power factor must be controlled throughout the grid within a range of 0.95 leading to 0.95 lagging, so that real power is transferred, and not just shuttled around the system. This amounts to ±842 µs.
- There must be sufficient operating reserve to allow adding or removing power in response to unpredicted changes in load.
- There must be sufficient contingency reserve to allow recovery from the loss of any generator or transmission unit. The presence of large generators in the system requires large contingency reserves.
- Every component of the system has operating limits—a maximum current and voltage that cannot be exceeded. Components such as transmission lines may allow for overcapacity by a specified amount for a specified short interval.
- Generators have a minimum start time—how long it takes to put them into operation. For some, this can depend on how much time has passed since they last ran.
- Generators have a maximum ramp rate—how quickly the can vary their output up or down.
- Generators have minimum power levels, below which they cannot operate.
Reserves, Failures, and Recoveries
A grid can never be operated reliably at 100% capacity. In addition to the possibility of increased load, generators shut down for many reasons, from planned maintenance to catastrophic breakdown.
When a generator or transmission line unexpectedly goes down, there must be enough slack in the system to pick up the load. If a 3 GW nuclear or fossil fuel plant goes offline, or lightning damages a 3 GW transmission line (such as the Pacific Intertie from Oregon to Los Angeles), that power has to be available elsewhere, or load will have to be shed.
The inertia of the rotating generators and synthetic inertia provided by electronic inverters will be the first to pick up the load. The extra load will slow the generator, causing the frequency to drop as well. If a generator has additional capacity available, automatic controls may kick in to compensate.
Generators that are offline but ready to go may be switched in, but generators that depend on heat (including all based on burning fuel) require time in minutes to hours. The fastest are the direct-cycle gas turbines used in peaker plants to handle peak demand and transitions, but they require on the order of 10 minutes to come online.
More efficient plants, like combined-cycle gas turbines and other systems with a steam component, take even longer.
To fill the gap, system operators may purchase additional power on the regional spot market if it's available. The spot markets operate on intervals ranging from five minutes to a day-ahead market.
Capacity Factor
As can be seen above, generators do not run at full capacity. Between being held in reserve, reduced generation for lack of load, or reduced for operational reasons, such as maintenance, refueling, lack of sun or wind no generator operates at 100% capacity indefinitely. (Some nuclear reactors have operated above 100% nameplate capacity for periods exceeding one year, but not over the life of the reactor).
The ratio of what a generator produces over time, vs what its nominal capacity is, is called the capacity factor. This can range from near zero for a kept in reserve and used infrequently, to a little over 100% for some nuclear reactors operating above their nameplate rating for extended periods between maintenance and refueling.
A common complaint about wind and solar is that they are intermittent and cannot run at full capacity at all times. But as we have seen, the same is true of other plants. With scheduled and unscheduled maintenance, conventional plants' available capacity is partly predictable, partly unpredictable.
Storage
Inertia
Energy storage over a variety of time scales has always been important in the grid. The first form of storage was simply the inertia of the large, massive generators. Edison's first design, the Jumbo, used in his Pearl Street Station in New York City, weighed 25 tonnes and included heavy flywheels in its design.
Today's large turbine/generator combinations are even more massive, and this inertia plays a critical role in maintaining voltage and frequency stability. Additional inertia can be added to the system in the form of synchronous condensers, which are essentially generators allowed to rotate freely with power borrowed from the grid, returning it when needed.
Reactive Power
In an AC system, if the current and voltage are 90° out of phase, no useful power is transferred. Instead, energy flows first in one direction, than in the other. It is only when current and voltage are working together that useful power is transferred.
Loads and equipment that work with magnetic fields, such as motors and transformers cause the current to lag behind the voltage, as the changing magnetic fields induce an opposing voltages. Transmission lines also produce magnetic fields, but also act as capacitors, storing energy in the electric fields between the lines and the earth.
To bring the power back into balance, it is necessary to store energy from one part of the cycle and move it to where it is needed. This can be done with capacitors (which store energy in their electric fields) and inductors (which store it in magnetic fields). To offset constant phase angles, fixed reactors or capacitors may be used. To accommodate varying phase shifts, switched banks of capacitors or inductors may be used.
Pumped Hydroelectric Storage
In the 1890s, the first pumped hydroelectric stations were introduced in Italy and Switzerland, reaching MW scale in 1909 in Schaffhausen, Switzerland. By 1929, 40 pumped hydroelectric storage facilities were in operation in Europe, and the first was opened in the United States.
The first US plant was the Rocky River system in New Milford, Connecticut. It uses water from the Housatonic River, raised 61 meters in elevation to Candlewood Lake when the Housatonic water is high.
The use of storage allowed utilities to make better use of their existing generators, raising their capacity factor, and thus reducing the additional generation capacity which would have been needed.
The demand for storage rose dramatically with the introduction of nuclear power. Because nuclear plants take a long time to adjust their power levels, and can do so only within a limited range, storage was needed to allow their capacity to be used. Capital and operational costs would continue even when shut down, so the economic viability of nuclear plants depended on ensuring a consistent market for their power, while avoiding over-dependence on it. Pumped hydroelectric fit the role perfectly.
However, the early 21st century saw a major decline in gas prices, which cut into the economic advantages of pumped hydro and storing power. Combined with the difficulties in siting and the long approval and construction cycles for pumped hydro, pumped hydro development slowed.
The only real competition pumped hydro had up to this point was compressed air storage in underground caverns, but only two such installations have ever been produced.
Then, in 2016, everything changed.
Batteries
Batteries had long been under development, with small and pilot installations. But two dramatic events brought them onto the world stage.
Aliso Canyon Natural Gas Blowout
On 2015-10-23, well SS-25 in SoCalGas's Aliso Canyon natural gas storage facility sprung a massive leak deep underground. Located above the north end of the San Fernando Valley in Los Angeles, the Aliso Canyon field is in close proximity to the Porter Ranch residential community. It is a long-depleted oil field originally developed by J. Paul Getty, consisting of 114 wells. Well SS-25 was drilled in 1953.
About 114,400 tonnes of natural gas was vented to the atmosphere, and the flow was not stopped until 2016-02-11. The field was not cleared for use until after extensive repairs and testing, plugging several of the wells which failed the testing. One was even found to be leaking.
Much of Los Angeles sits atop old abandoned oilfields, including Playa del Rey, next to Los Angeles International Airport, which is another SoCalGas natural gas storage field. The loss of Aliso Canyon had a ripple effect throughout the gas distribution system, causing the other storage fields to be depleted to dangerously low levels. These fields supply peak demands for natural gas to both homes and gas-powered peaker power plants, such as the Scattergood plant on the other side of LAX, threatening both gas and electrical shortages.
The loss of California's largest gas storage field led to major limitations on the use of gas generators to meet peak electrical needs.
On 2016-01-06, Governor Brown declared a state of emergency, and utilities and regulators scrambled for a solution. The first step was conservation measures, including a ban on new gas connections. To replace the power capacity lost due to the lost natural gas storage, the decision was made to add electrical storage. Approval was granted in May for tenders to put out, and three vendors were selected.
Southern California Edison selected Tesla for their Mira Loma site, with a 20 MW/80 MWhr battery that was installed and in operation 88 days later.
SCE also selected Greesmith Energy for their AltaGas Pomona Energy Facility, with another 20 MW/80 MWhr battery.
San Diego Gas & Electric selected AES for their Escondido substation, who supplied a 30 MW/120 MWhr battery which was briefly the largest battery in the world.
While these were not the first grid batteries, the rapid and cost-effective deployment of production plants demonstrated that the grid battery market was here.
Elon Musk's Bet
Meanwhile, as these were being installed, on 2016-09-28, a storm struck South Australia, and in a 40-second interval beginning at 16:17:33, two separate tornadoes knocked out a single 275 kV transmission line and a double 275 kV line 170 km apart. The resulting repeated voltage swings in the grid caused protection circuitry on several wind farms to disconnect from the grid after 9 events. This caused loss of 456 MW of power, or 52% of total wind generation, which in turn put increased power imports on a connector transmission line from Victoria, causing it to recognize a loss-of-synchronization, and trip, removing a further 900 MW in 600 ms.
At this point, South Australia was a power island, with inadequate generation to meet load, and too quickly to shed load to match generation and maintain synchronization. As load was shed and reserves called upon, synchronization was lost as voltages dropped at different rates, and it all collapsed about 1 second later.
The key points here are:
- The sequence of events exceeded the contingency planning estimates of credible events in several ways.
- The wind farms that tripped were configured to lock out after 9 attempts. This safety feature, activated en-mass in response to the large event, amplified the impact to the point where the sequence became inevitable.
- The system inertia of 3,000 MW-s was only enough for 600 ms. Wind generation can be configured to provide inertia, but this was not common practice at the time.
Bringing the grid back up faced a number of challenges. Among them: You don't just turn on a power plant; fossil fuel power plants require electrical power to operate. (Nuclear plants require power to remain safe, as shown at Fukushima Daiichi, and nearly replicated at Daini). "Black start" power is one of 8 different "ancillary services" involving reserves of power that can be called on.
Power was restored to 80-90% of customers by midnight, but some involving substantial repairs to transmission and distribution were without power until 2016-10-11.
As a result of these events and subsequent analysis, many changes were made. Among them:
- Additional credible failures were considered in contingency planning, including loss of multiple wind farms.
- The wind farms were configured to be more tolerant of multiple grid failures
- Elon Musk's bet.
Conservative PM Turnbull made the claim that the cause was South Australia's reliance on renewable power (rather than the manner it was configured and operated). Lyndon Rive, Tesla's VP for the battery division, responded Tesla could deliver 100-300 MW of batteries in 100 days of receiving the order.
That led to this exchange over Twitter:
Which actually happened, becoming the Hornsdale Power Reserve, with 100MW/129 MWh capacity. This has proven its worth repeatedly over the two years it's been in service, providing ancillary services such as frequency and voltage regulation and adding substantially to the available inertia of the system. Experience has shown it to be faster and more precise than other alternatives and is now relied on as a front line response.
From subsequent legal filings, it is estimated the cost was approximately $90M AUD, with annual cost savings at around $40M AUD. The cost saving include being able to operate conventional plants more economically at higher capacity factor.
South Australia has since added another battery facility at Yorke, and others have been installed elsewhere as Australia works to update its grid to higher standards of reliability and robustness.
Battery storage is now mainstream technology, installed not from imposed mandates or subsidies, but from economics and concern for reliability—exactly the same reasons the first pumped hydro stations were built. But unlike pumped hydro, they can be deployed in months on existing sites. They arrive on site as shipping containers ready for connection, transportable by ship, train, and truck.
The world's largest battery (which required its own "gigafactory") is [edit: was slated to be in operation by 2018, but delayed to to a temporary spike in vanadium prices, 2019-11-10] in Dalian, China, where it helps to make use of the full power available from wind farms; they had been curtailing about 15% of generation when supply exceeds ability to deliver to load. (They also are remaking their power distribution, including UHV and UVDC transmission lines). This 200 MW/800 MWh battery uses vanadium flow technology, which has many advantages for grid usage over lithium ion. Shown here are is the Rongke Power Vpower2-A module + storage tanks at 500 kW/2000 kWh per module used in this battery.
Part of Australia's response has been to encourage rooftop solar, and to include residential storage in the mix; most are lithium ion, but one of the available Australian options is also a vanadium flow battery.
Concerns from a few years ago that we lack proven technology for economic and scalable storage are now clearly alleviated. It certainly will require a lot of expansion, but it can and is being done as part of the cycle of reinvestment; instead of replacing old fueled plants with new, investing in renewables with storage is proving to be more economical and to be deployable in far less time.
However, doing it at the most affordable cost while maintaining the highest standards of reliability, becomes more complicated as we go. In future articles, I will discuss the role of diversity—of supply type, geography, time, load, in minimizing the amount of storage required while maximizing the value obtained from our investments in generation.
Conclusion
I hope this article has given you a better sense of the landscape of power generation and distribution. I hope you'll return and read my next essays, which will cover some of these topics in more detail.
References
- Wikipedia: Electric Power Distribution
- Load factor calculator
- ABB Power Grids Capacitors and Filters
- NERC: Reliability Standards for the Bulk Electric Systems of North America
- CAISO: California ISO Planning Standards
- NERC: Reliability Guideline Reactive Power Planning
- FERC: Defining and Formulating Operating Reserve Requirements and Deployments
- EaTH Wiki: Pearl Street Station
- Popular Science, April 1930, Pg 50
- Popular Science, June 1930, Pg 60, "A Ten-Mile Storage Battery"
- ASCE: Rocky River Pumped Storage Hydraulic Plant
- AEMO: Black System, South Australia, September 28, 2016
- Rongke Power Vpower2-A Product Specification (500 kW/2000 kWh module)
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