Islanding is the intentional or unintentional division of an interconnected power grid into individual disconnected regions with their own power generation.
Intentional islanding is often performed as a defence in depth to mitigate a cascading blackout. If one island collapses, it will not take neighboring islands with it. For example, nuclear power plants have safety-critical cooling systems that are typically powered from the general grid. The coolant loops typically lie on a separate circuit that can also operate off of reactor power or emergency diesel generators if the grid collapses. [1] [2] Grid designs that lend themselves to islanding near the customer level are commonly referred to as microgrids. In a power outage, the microgrid controller disconnects the local circuit from the grid on a dedicated switch and forces any online distributed generators to power the local load. [3] [4]
Unintentional islanding is a dangerous condition that may induce severe stress on the generator, as it must match any changes in electrical load alone. If not properly communicated to power line workers, unintentional island can also present a risk of electrical shock. Unlike unpowered wires, islands require special techniques to reconnect to the larger grid, because the alternating current they carry is not in phase. For these reasons, solar inverters that are designed to supply power to the grid are generally required to have some sort of automatic anti-islanding circuitry, which shorts out the panels rather than continue to power the unintentional island.
Methods that detect islands without a large number of false positives constitute the subject of considerable research. Each method has some threshold that needs to be crossed before a condition is considered to be a signal of grid interruption, which leads to a "non-detection zone" (NDZ), the range of conditions where a real grid failure will be filtered out. [5] For this reason, before field deployment, grid-interactive inverters are typically tested by reproducing at their output terminals specific grid conditions and evaluating the effectiveness of the anti-islanding methods in detecting island conditions. [4] [6]
Intentional islanding divides an electrical network into fragments with adequate power generation in each fragment to supply that fragment's loads. [7] [8] In practice, balancing generation and load in each fragment is difficult, and often the formation of islands requires temporarily shedding load. [9] [10] Synchronous generators may not deliver sufficient reactive power to prevent severe transients during fault-induced island formation, [11] and any inverters must switch from constant-current to constant-voltage control. [12]
Assuming P≠NP, no good cut set criterion exists to implement islanding. Polynomial-time approximations exist, but finding the exactly optimal divisions can be computationally infeasible. [8] [9]
However, islanding localizes any failures to the containing island, preventing failures from spreading. [13] In general, blackout statistics follow a power law, such that fragmenting a network increases the probability of blackouts, but reduces the total amount of unsatisfied electricity demand. [14]
Islanding reduces the economic efficiency of the wholesale power market, [10] and is typically a last resort applied when the grid is known to be unstable but has not yet collapsed. [8] In particular, islanding improves resilience to threats with known time but not location, such as terrorist attacks, military strikes on electrical infrastructure, or extreme weather events. [15]
Following the 2019 California power shutoffs, there was a rise in interest in the possibility of operating a house's electrical grid as an island. While typical distributed generation systems are too small to power all appliances in a home simultaneously, it is possible for them to manage critical household power needs through traditional load-frequency control. Modules installed in series between the generator and large loads like air conditioners and electric ovens measure the island power frequency and perform automatic load shedding as the inverter nears overload.[ citation needed ]
This section relies largely or entirely upon a single source .(April 2024) |
Automatically detecting an island is the subject of considerable research. These can be performed passively, looking for transient events on the grid; or actively, by creating small instances of those transient events that will be negligible on a large grid but detectable on a small one. Active methods may be performed by local generators or "upstream" at the utility level. [16]
Many passive methods rely on the inherent stress of operating an island. Each device in the island comprises a much larger proportion of the total load, such that the voltage and frequency changes as devices are added or removed are likely to be much larger than in normal grid conditions. However, the difference is not so large as to prevent identification errors, and voltage and frequency shifts are generally used along with other signals. [17]
The active analogue of voltage and frequency shifts attempts to measure the overall impedance fed by the inverter. When the circuit is grid-connected, there is almost no voltage response to slight variations in inverter current; but an island will observe a change in voltage. In principle, this technique has a vanishingly small NDZ, but in practice the grid is not always an infinitely-stiff voltage source, especially if multiple inverters attempt to measure impedance simultaneously. [18] [19]
Unlike the shifts, a random circuit is highly unlikely to have a characteristic frequency matching standard grid power. However, many devices, like televisions, deliberately synchronize to the grid frequency. Motors, in particular, may be able to stabilize circuit frequency close to the grid standard as they "wind down". [20]
At the utility level, protective relays designed to isolate a portion of the grid can also switch in high impedance components, such that an islanded distributed generator will necessarily overload and shut down. This practice, however, relies on the expensive widespread provision of high-impedance devices. [21] [22]
Alternatively, anti-islanding circuitry can rely on out-of-band signals. For example, utilities can send a shut-down signal through power line carrier communications or a telephony hookup. [23] [24]
Certain passive methods are uniquely viable with direct current generators, such as solar panels.
For example, inverters typically generate a phase shift when islanding. Inverters generally match the grid signal with a phase locked loop (PLL) that tracks zero-crossings. Between those events, the inverter produces a sinusoidal output, varying the current to produce the proper voltage waveform given the previous cycle's load. When the main grid disconnects, the power factor on the island suddenly decreases, and inverter's current no longer produces the proper waveform. By the time the waveform is completed and returns to zero, the signal will be out of phase. However, many common events, like motors starting, also cause phase jumps as new impedances are added to the circuit. [25]
A more effective technique inverts the islanding phase shift: the inverter is designed to produce output slightly mis-aligned with the grid, with the expectation that the grid will overwhelm the signal. The phase-locked loop then becomes unstable when the grid signal is missing; the system drifts away from the design frequency; and the inverter shuts down. [26]
A very secure islanding detection method searches for distinctive 2nd and 3rd harmonics generated by nonlinear interactions inside the inverter transformers. There are generally no other total harmonic distortion (THD) sources that match an inverter. Even noisy sources, like motors, do not effect measurable distortion on a grid-connected circuit, as the latter has essentially infinite filtration capacity. Switched-mode inverters generally have large distortions — as much as 5%. When the grid disconnects, the local circuit then exhibits inverter-induced distortion. [27] Modern inverters attempt to minimize harmonic distortion, in some cases to unmeasurable limits, but in principle it is straightforward to design one which introduces a controlled amount of distortion to actively search for island formation. [28]
Utilities have refused to allow installation of home solar or other distributed generation systems, on the grounds that they may create uncontrolled grid islands. [29] [30] In Ontario, a 2009 modification to the feed-in tariff induced many rural customers to establish small (10 kW) systems under the "capacity exempt" microFIT. However, Hydro One then refused to connect the systems to the grid after construction. [31]
The issue can be hotly political, in part because distributed generation proponents believe the islanding concern is largely pretextual. A 1999 test in the Netherlands was unable to find distributed-generation islands 60 seconds after grid collapse. Moreover, moments when distributed generation only matched distributed loads occurred at a rate comparable to 10−6 yr−1, and that the chance that the grid would disconnect at that point in time was even less, so that the "probability of encountering an islanding[ sic ] is virtually zero". [32]
Unintentional islanding risk is primarily the case of synchronous generators, as in microhydro. A 2004 Canadian report concluded that "Anti-islanding technology for inverter based DG systems is much better developed, and published risk assessments suggest that the current technology and standards provide adequate protection." [33]
Utilities generally argue that the distributed generators might effect the following problems: [34] [35]
The first two claims are disputed within the power industry. For example, normal linework constantly risks exposure to live wires, and standard procedures require explicit checks to ensure that a wire is dead before worker contact. Supervisory Control and Data Acquisition (SCADA) systems can be set to alarm if there is unexpected voltage on a purportedly-isolated line. A UK-based study concluded that "The risk of electric shock associated with islanding of PV systems under worst-case PV penetration scenarios to both network operators and customers is typically <10−9 per year." [36] [37] Likewise, damage to end-user devices is largely inhibited by modern island-detection systems.
It is, generally, the last problem that most concerns utilities. Reclosers are commonly used to divide up the grid into smaller sections that will automatically, and quickly, re-energize the branch as soon as the fault condition (a tree branch on lines for instance) clears. There is some concern that the reclosers may not re-energize in the case of an island or that an intervening loss of synchrony might damage distributed generators on the island. However, it is neither clear that reclosers are still useful in modern utility practice nor that breaker-reclosers must act on all phases. [38]
Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines that facilitate this movement form a transmission network. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. The combined transmission and distribution network is part of electricity delivery, known as the electrical grid.
In electrical engineering, the power factor of an AC power system is defined as the ratio of the real power absorbed by the load to the apparent power flowing in the circuit. Real power is the average of the instantaneous product of voltage and current and represents the capacity of the electricity for performing work. Apparent power is the product of root mean square (RMS) current and voltage. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power may be greater than the real power, so more current flows in the circuit than would be required to transfer real power alone. A power factor magnitude of less than one indicates the voltage and current are not in phase, reducing the average product of the two. A negative power factor occurs when the device generates real power, which then flows back towards the source.
A power inverter, inverter, or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opposite of rectifiers which were originally large electromechanical devices converting AC to DC.
Distributed generation, also distributed energy, on-site generation (OSG), or district/decentralized energy, is electrical generation and storage performed by a variety of small, grid-connected or distribution system-connected devices referred to as distributed energy resources (DER).
A Flexible Alternating Current Transmission System (FACTS) is a family of Power-Electronic based devices designed for use on an Alternating Current (AC) Transmission System to improve and control Power Flow and support Voltage. FACTs devices are alternatives to traditional electric grid solutions and improvements, where building additional Transmission Lines or Substation is not economically or logistically viable.
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages. They are a common component of the infrastructure. There are 55,000 substations in the United States.
Power electronics is the application of electronics to the control and conversion of electric power.
Electric power quality is the degree to which the voltage, frequency, and waveform of a power supply system conform to established specifications. Good power quality can be defined as a steady supply voltage that stays within the prescribed range, steady AC frequency close to the rated value, and smooth voltage curve waveform. In general, it is useful to consider power quality as the compatibility between what comes out of an electric outlet and the load that is plugged into it. The term is used to describe electric power that drives an electrical load and the load's ability to function properly. Without the proper power, an electrical device may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality, and many more causes of such poor quality power.
A variable-frequency drive is a type of AC motor drive that controls speed and torque by varying the frequency of the input electricity. Depending on its topology, it controls the associated voltage or current variation.
This is an alphabetical list of articles pertaining specifically to electrical and electronics engineering. For a thematic list, please see List of electrical engineering topics. For a broad overview of engineering, see List of engineering topics. For biographies, see List of engineers.
A microgrid is a local electrical grid with defined electrical boundaries, acting as a single and controllable entity. It is able to operate in grid-connected and in island mode. A 'stand-alone microgrid' or 'isolated microgrid' only operates off-the-grid and cannot be connected to a wider electric power system. Very small microgrids are called nanogrids.
A phasor measurement unit (PMU) is a device used to estimate the magnitude and phase angle of an electrical phasor quantity in the electricity grid using a common time source for synchronization. Time synchronization is usually provided by GPS or IEEE 1588 Precision Time Protocol, which allows synchronized real-time measurements of multiple remote points on the grid. PMUs are capable of capturing samples from a waveform in quick succession and reconstructing the phasor quantity, made up of an angle measurement and a magnitude measurement. The resulting measurement is known as a synchrophasor. These time synchronized measurements are important because if the grid’s supply and demand are not perfectly matched, frequency imbalances can cause stress on the grid, which is a potential cause for power outages.
An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of a power system is the electrical grid that provides power to homes and industries within an extended area. The electrical grid can be broadly divided into the generators that supply the power, the transmission system that carries the power from the generating centers to the load centers, and the distribution system that feeds the power to nearby homes and industries.
In utility and industrial electric power transmission and distribution systems, a numerical relay is a computer-based system with software-based protection algorithms for the detection of electrical faults. Such relays are also termed as microprocessor type protective relays. They are functional replacements for electro-mechanical protective relays and may include many protection functions in one unit, as well as providing metering, communication, and self-test functions.
A photovoltaic system, also called a PV system or solar power system, is an electric power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and convert sunlight into electricity, a solar inverter to convert the output from direct to alternating current, as well as mounting, cabling, and other electrical accessories to set up a working system. Many utility-scale PV systems use tracking systems that follow the sun's daily path across the sky to generate more electricity than fixed-mounted systems.
An electrical grid is an interconnected network for electricity delivery from producers to consumers. Electrical grids consist of power stations, electrical substations to step voltage up or down, electric power transmission to carry power over long distances, and finally electric power distribution to customers. In that last step, voltage is stepped down again to the required service voltage. Power stations are typically built close to energy sources and far from densely populated areas. Electrical grids vary in size and can cover whole countries or continents. From small to large there are microgrids, wide area synchronous grids, and super grids.
A wide area synchronous grid is a three-phase electric power grid that has regional scale or greater that operates at a synchronized utility frequency and is electrically tied together during normal system conditions. Also known as synchronous zones, the most powerful is the Northern Chinese State Grid with 1,700 gigawatts (GW) of generation capacity, while the widest region served is that of the IPS/UPS system serving most countries of the former Soviet Union. Synchronous grids with ample capacity facilitate electricity trading across wide areas. In the ENTSO-E in 2008, over 350,000 megawatt hours were sold per day on the European Energy Exchange (EEX).
Ancillary services are the services necessary to support the transmission of electric power from generators to consumers given the obligations of control areas and transmission utilities within those control areas to maintain reliable operations of the interconnected transmission system.
Synchronverters or virtual synchronous generators are inverters which mimic synchronous generators (SG) to provide "synthetic inertia" for ancillary services in electric power systems. Inertia is a property of standard synchronous generators associated with the rotating physical mass of the system spinning at a frequency proportional to the electricity being generated. Inertia has implications towards grid stability as work is required to alter the kinetic energy of the spinning physical mass and therefore opposes changes in grid frequency. Inverter-based generation inherently lacks this property as the waveform is being created artificially via power electronics.
In an electrical grid, the short circuit ratio is the ratio of the short circuit apparent power (SCMVA) in the case of a line-line-line-ground (3LG) fault at the location in the grid where some generator is connected to the power rating of the generator itself (GMW). Since the power that can be delivered by the grid varies by location, frequently a location is indicated, for example, at the point of interconnection (POI):
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: CS1 maint: postscript (link) accidental islanding of a generator during transformer maintenance causes severe overfrequency on the island and requires manual control of the turbines to reintegrate with the larger grid