As the energy system decarbonizes, an increasing amount of our electricity will be generated by intermittent renewable sources such as wind and solar. The importance of energy storage in a renewables-intensive energy system is often talked about. What is discussed less often is the need for frequency stability in the alternating current (AC) supplied. Maintaining a consistent frequency is critical for the safe and reliable operation of the infrastructure that supplies electricity as well as for the equipment attached to it. Current grids rely heavily on the inertia of the large rotating turbines and generators in conventional power stations to provide this frequency stability. As these sources are replaced by renewables that lack this rotating inertia, alternative methods of maintaining frequency stability will be required. This article explores the importance, challenges and solutions associated with this issue.
Why Do We Use AC Power?
Most electrical transmission and distribution grids carry electricity as AC. AC has become established as the standard way of delivering mains power because AC voltage can be easily stepped up or down to different levels using transformers. Changing the voltage within the grid is particularly important for efficient transmission and distribution of electrical energy.
Electrical power is equal to current multiplied by voltage. For a constant power, when the voltage is increased, the current therefore decreases. The amount of power that is dissipated as heat in a wire, known as the line loss, is equal to the resistance of the wire multiplied by the square of the current. Therefore, the best way to decrease line loss is to decrease the current, which entails increasing the voltage. If the voltage is increased by a factor of x, the line loss will decrease by x2. Another way of looking at this is that the electricity can be transmitted by x2 times the distance with the same line loss.
Transformers work by placing two coils in close proximity to couple their electromagnetic fields. The AC input current passes through one coil, causing a changing magnetic field. This changing magnetic field then induces an AC current in the other coil. Adjusting the number of windings in the two coils causes the output voltage to be higher or lower than the input voltage. However, this only works if the current is constantly changing, and therefore AC is required for the use of a transformer.
Although generator designs typically produce AC, it would be just as easy for generators to be designed to produce direct current (DC). AC is useful for power transmission, but in many other areas it is simply an inconvenience, especially with modern electronics and semiconductor devices. Virtually all electronic devices require DC, as does LED lighting and the small electric motors used in household appliances. This means that all of these devices require not only a transformer to step down from mains voltage; they also need an AC-to-DC converter. This conversion is carried out by a rectifier. Since AC is constantly changing direction, diodes are often used to produce DC, which flows in a single direction. Diodes are like one-way valves that allow currents to pass in only one direction. Further filtering may be carried out using energy storing and smoothing components such as capacitors and inductors.
Although AC provides a simple way of increasing voltage for long-distance transmission, having DC transmission along the lines provides advantages as well. For instance, since it doesn’t transfer reactive power (power that can do no useful work), DC transmission can offer lower line losses compared to AC. In fact, when large quantities of power are transmitted over very long distances, high voltage DC (HVDC) or ultra high voltage DC (UHVDC) is used. For example, the Changji-Guquan electrical superhighway uses 1,100 kV UHVDC to transmit 12 GW over a distance of 3,293 km. The development of HVDC has been enabled by solid-state power converters that generally use thyristors.
With improvements in semiconductor switching, DC-to-DC converters are becoming more efficient and are commonly used in electronic devices, solar panels and many other applications. These converters often use switches to generate high-frequency AC and rectify it back to DC.
Despite the advent of DC-to-DC converters, AC power transmission is still used for legacy reasons. Converting all of the power generation, transmission and distribution infrastructure to work with DC would be a huge undertaking. All the end-use devices would also have to be converted at the same time. The enormity of such an undertaking means we’re likely to be using AC for the foreseeable future.
The Importance of Frequency Stability and Power Quality
Traditionally, electricity has been produced by large turbines driven by steam or hydro power, which then drive an AC generator. Most electricity is still generated this way. The AC generator consists of an electromagnetic coil, known as the rotor, which rotates within a series of fixed heavy copper bars collectively called the stator. When the magnetic field generated by the rotor moves past one pair of diametrically opposite stator bars, an electrical current is induced between them. The rotor has a positive and a negative pole, so every 180 degrees the rotor induces a current in a different direction, causing the current in the stator bars to alternate back and forth. Every rotation amounts to one AC cycle. If the generator is rotating at 3,000 rpm it will produce an alternating current with a frequency of 50 Hz. Standard generators have three sets of copper bars in the stator, which each produce their own sinusoidal voltage, leading to the three phases in the electrical distribution system.
The most important reason for regulating the frequency of the AC supply is that if there are different frequencies within the grid, it will damage equipment. Frequency regulation is, therefore, not so much about achieving an accurate frequency as it is about synchronizing all the equipment so that it operates smoothly together. This is of great importance to the generators themselves, but industrial motors and many other pieces of equipment are also affected. It just so happens that agreeing on a fixed frequency is the easiest way to achieve this synchronization. The frequency across a national grid must be controlled to within less than 1 percent.
There are many other potential issues with power quality beyond frequency instability, including voltage surges, power outages and noise. In some cases, these are related to—and can be caused by—frequency instability. They may also have other causes within the supply system or locality. Local disturbances may be caused by things like the noise from electric motors propagating through the electrical system. Power outages, or blackouts, involve a complete loss of electrical power. This can be caused by a fault at any stage in the electrical supply, including generation, transmission, distribution and substations.
Voltage stability is typically controlled to within 5 percent throughout the distribution system. This is achieved by producing and absorbing reactive power at the grid level. Instability is often referred to as a voltage surge, a poorly defined term that may refer to any number of different overvoltage conditions. Within IEEE 1100-2005, these conditions are specified more precisely as transients, swells and dips:
- Transient: An overvoltage or undervoltage condition lasting from microseconds to a few milliseconds. A transient spike is the overvoltage condition, which is often caused by inductive or capacitive loads being turned on or off.
- Impulsive transient: A sharp rise in overvoltage, typically caused by a lightning strike or a motor being turned off.
- Oscillatory transient: An alternating pattern of swelling and shrinking of voltage that takes place very rapidly.
- Swells and Dips: Voltage fluctuations typically deviating by 5 to 10 percent from the nominal voltage, lasting longer than a transient, and usually for a few cycles. A swell is an overvoltage condition, and a dip or sag is an undervoltage condition.
Longer undervoltage conditions, lasting for minutes or hours, are known as brownouts. These conditions are sometimes implemented intentionally to effectively ration power consumption in an emergency. Different types of devices behave very differently in a brownout condition. Resistance-based devices, such as heaters and incandescent lights, simply consume less power, reducing their output in proportion to the reduction in voltage. This is not harmful to these devices. Some types of electric motors will behave in the same way, but others can draw additional current, which can cause overheating with the risk of burnout. Power supplies and digital devices are less predictable; some can adapt without problems, while others will malfunction in such conditions.
At the other end of the scale, very high-frequency disturbances—at significantly higher frequencies than the grid frequency—are known as noise. In a time-domain plot of current or voltage, a perfect AC supply will be a smooth sinusoidal wave. Noise will appear as rough spikes within this wave. Noise is caused by many factors. Thermal noise, caused by resistance within wires, is an ever-present but usually very minor effect. Local loads such as welders and motors can lead to much more significant noise. Although often difficult to detect, noise can increase heating in equipment, accelerating the rate of wear and potentially leading to failure.
Related to noise, harmonics are high-frequency disturbances at integer multiples of the grid frequency. Various nonlinear loads can cause harmonics—for example, fluorescent lighting, rectifiers, power supplies and variable-speed motors. Harmonics can be a particular issue for motors, causing vibration and torque pulses, placing additional load on bearings, producing overheating, and reducing efficiency.
Filters and surge suppression may be required to protect equipment from poor power quality. Surge suppression removes transients and swells, while filters smooth out noise and harmonics.
Surge protectors can block current using inductors that damp out sudden changes in current, or capacitors that damp out sudden changes in voltage. However, most surge protectors short-circuit when overvoltage occurs, diverting current back into the power distribution lines to be dissipated by resistance in the wires. To provide effective protection, the short must respond very rapidly when an overvoltage occurs. This is achieved using a spark gap, a discharge tube or a semiconductor device. The maximum let-through voltage is known as the clamping voltage and can be as high as twice the nominal voltage. Surge protectors are also specified with a response time and an energy rating—meaning the maximum power they can absorb before failure occurs. Some surge protectors will cut the power when they fail, while others will continue to supply power without protection, typically indicating failure is occurring with a warning light.
These devices can provide protection against transients, swells, noise and harmonics. Frequency stability is not so easily controlled by the end user. It is, therefore, a critical requirement for the grid.
The Role of Inertia in Maintaining Frequency Stability
A rotating generator supplying the grid is electromechanically coupled to the grid. Assuming constant demand, if more mechanical power is supplied to the generator, it will speed up, causing the AC frequency to increase. In the same way—assuming there is constant power input—if electrical demand increases, the greater electromagnetic load on the generator will cause it to slow down. The AC frequency would then decrease.
The rate at which these changes take place depends on the inertia of the generator, turbine and any other rotating components attached to it. If the rotating components have no mass, they could change speed instantly in response to an imbalance between the electrical load and the mechanical power input. The inertia of large heavy turbines and generators therefore acts as a damper to slow the rate of frequency changes. This provides enough time for the input power to be controlled and therefore maintain a constant AC frequency.
Conventional thermal power generation provides excellent frequency regulation, combining the inertia of very heavy rotating components with the ability to rapidly control power by adjusting the supply of steam to the turbine. This is equally true of both fossil fuel and biomass-fired power plants. In large power stations, the damping provided by the rotating inertia means that a significant change in frequency will take more than one second to occur, which is sufficient time for adjustments in the steam supply to compensate for the change.
To maintain frequency stability, the rotational speed of every generator connected to the national grid must by synchronized. This can be extremely challenging. As electrical demand varies within the grid, it affects the electromagnetic load on the generator and subsequently the input mechanical power required to maintain a constant speed. If the power is not controlled to account for this, the speed of rotation will change, and the grid frequency will fluctuate.
The Challenges of Maintaining Frequency Stability
As wind, solar and other distributed and renewable sources are beginning to replace large, centralized power stations, it is becoming more difficult to achieve frequency stability. There are two major reasons for this. First, there is now a far larger number of small generators, many of which are not directly operated by the grid. This makes controlling them far more complex. Second, most of these small generators provide no inertia, meaning that a much more rapid control response is required to effectively maintain frequency stability.
Although wind turbines have physical inertia, they are not coupled to the grid frequency in the same way as other power generators and therefore do not provide inertia for frequency stability. They do provide some frequency control capability since wind output can be regulated down or held back.
Solar panels can be rapidly switched on and off to provide good frequency regulation. However, because solar panels are dispersed very widely and are not directly operated by the grid operator, achieving this kind of control is especially challenging.
Most currently operational nuclear reactors were designed to provide baseload power. This means they cannot be controlled for frequency, although they do provide inertia. More modern designs of nuclear reactors will provide greater flexibility of operation with frequency regulation capabilities.
Energy Storage and Power Quality Solutions
Renewables-intensive energy systems will require different types of energy storage that are able to buffer supply and demand over differing time periods. These can broadly be categorized as frequency regulation, daily or weekly fluctuations, and seasonal variation. There is, however, significant synthesis between these provisions. For example, battery storage is likely to play a significant role in buffering daily or weekly fluctuations, but it can also provide frequency stability. Similarly, flywheel storage can buffer daily or weekly fluctuations while also adding significant inertia to the system. Pumped-storage hydroelectricity has very good frequency stability.
Within the seasonal energy domain, there is also support for frequency stability. Hydrogen-fueled gas turbine power plants have both inertia and frequency stability. Biomass-fueled power plants are also likely to play a significant role in providing power during periods of peak demand, and they have all of the frequency-regulating benefits of conventional fossil fuel power plants.
Use of UHVDC enables economic transmission of large quantities of power across borders and even in intercontinental transmission. This provides more efficient load leveling between differently generating and consuming regions. It also allows large quantities of power to be stored through the greater use of pumped-storage hydroelectricity.
Conclusion
Electrical generation is moving away from large, centralized steam turbines, and toward a distributed system with many small renewable generators. This makes achieving frequency stability considerably more challenging. Ancillary services to provide frequency stability will be required, and the price of such services is not always considered when evaluating the cost of renewable power. Perhaps the most challenging aspect of achieving frequency stability will be controlling the many independently operated generators distributed across the grid.