Islanding

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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[edit]

Intentional islanding divides an electrical network into fragments with adequate power generation in each fragment to supply that fragment's loads.[7] In practice, balancing generation and load in each fragment is difficult, and often the formation of islands requires temporarily shedding load.[8][9] Synchronous generators may not deliver sufficient reactive power to prevent severe transients during fault-induced island formation,[10] and any inverters must switch from constant-current to constant-voltage control.[11] However, islanding localizes any failures to the containing island, preventing failures from spreading. 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.[12]

Islanding is rare, in part because it reduces the economic efficiency of the wholesale power market.[9] Implementing islanding also lacks for good cut set criteria; finding the optimal divisions is computationally infeasible. However, polynomial-time approximations are known.[8]

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Home islanding[edit]

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]

Detection methods[edit]

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]

Inverter-specific techniques[edit]

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]

Distributed generation controversy[edit]

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]

Safety concerns
If an island forms, repair crews may be faced with unexpected live wires.
End-user damage
Distributed generators may not be able to maintain grid frequencies or voltages close to standard, and nonstandard currents can damage customer equipment. Depending on the circuit configuration, the utility may be liable for the damage.
Controlled grid reconnection
Reclosing distribution circuits onto an active island may damage equipment or be inhibited by out-of-phase protection relays. Procedures to prevent these outcomes may delay restoration of electric service to dropped customers.

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]

References[edit]

  1. ^ Autorité de sûreté nucléaire. "Îlotage provoqué des deux réacteurs à la centrale nucléaire de Saint-Alban". ASN (in French). Retrieved 2019-02-25.
  2. ^ "Centrale nucléaire de Fessenheim : Mise à l'arrêt de l'unité de production n°2". EDF France (in French). 2018-07-14. Retrieved 2019-02-25.
  3. ^ Saleh, M.; Esa, Y.; Mhandi, Y.; Brandauer, W.; Mohamed, A. (October 2016). "Design and implementation of CCNY DC microgrid testbed". 2016 IEEE Industry Applications Society Annual Meeting. pp. 1–7. doi:10.1109/IAS.2016.7731870. ISBN 978-1-4799-8397-1. S2CID 16464909.
  4. ^ a b "IEEE 1547.4 - 2011". IEEE Standards Association Working Group Site & Liaison Index. IEEE. Retrieved 3 March 2017.
  5. ^ Bower & Ropp, pg. 10
  6. ^ Caldognetto, T.; Dalla Santa, L.; Magnone, P.; Mattavelli, P. (2017). "Power Electronics Based Active Load for Unintentional Islanding Testbenches". IEEE Transactions on Industry Applications. 53 (4): 3831–3839. doi:10.1109/TIA.2017.2694384. S2CID 40097383.
  7. ^ Mureddu, Mario; Caldarelli, Guido; Damiano, Alfonso; Scala, Antonio; Meyer-Ortmanns, Hildegard (2016-10-07). "Islanding the power grid on the transmission level: less connections for more security". Scientific Reports. 6 (1): 34797. doi:10.1038/srep34797. ISSN 2045-2322.
  8. ^ a b Pahwa, S.; Youssef, M.; Schumm, P.; Scoglio, C.; Schulz, N. (2013). "Optimal intentional islanding to enhance the robustness of power grid networks". Physica A. 392 (17). Elsevier: 3741–3754. doi:10.1016/j.physa.2013.03.029 – via Kansas State repository.
  9. ^ a b Yang, Bo; Vittal, Vijay; Heydt, Gerald T. (30 Oct 2006). "Slow coherency based controlled islanding". IEEE Transactions on Power Systems. 21 (4): 1840–1847. doi:10.1109/TPWRS.2006.881126.
  10. ^ Katiraei, F.; Iravani, M. R.; Lehn, P. W. (Jan 2005) [31 May 2005]. "Micro-grid autonomous operation during and subsequent to islanding process" (PDF). IEEE Transactions on Power Delivery. 20 (1): 248–257. doi:10.1109/TPWRD.2004.835051. TPWRD-00103-2003.
  11. ^ Balaguer, Irvin J.; Qin Lei; Yang, Shuitao; Supatti, Uthane; Peng, Fang Zheng (Jan 2011) [10 Dec 2010]. "Control for grid-connected and intentional islanding operations of distributed power generation". IEEE Transactions on Industrial Electronics. 58 (1): 147–157. doi:10.1109/TIE.2010.2049709 – via Academia.edu.
  12. ^ Scala, Antonio; Lucentini, Pier Giorgio De Santis; Caldarelli, Guido; D'Agostino, Gregorio (1 June 2016) [13 Oct 2018]. "Cascades in interdependent flow networks". Physica D. 323. Elsevier: 35–39. arXiv:1512.03088. doi:10.1016/j.physd.2015.10.010.
  13. ^ [1]
  14. ^ [2]
  15. ^ 1996 System Disturbances (PDF) (Report). Princeton, NJ: North American Electric Reliability Council. Aug 2002. pp. 58–61.
  16. ^ Bower & Ropp.
  17. ^ Bower & Ropp, pp. 17–19.
  18. ^ Bower & Ropp, pg. 24
  19. ^ "Negative-Sequence Current Injection for Fast Islanding Detection of a Distributed Resource Unit", Houshang Karimi, Amirnaser Yazdani, and Reza Iravani, IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008.
  20. ^ Bower & Ropp, pg. 20
  21. ^ CANMET, pg. 12-13
  22. ^ Bower & Ropp, pp. 37–38.
  23. ^ Bower & Ropp, pg. 40
  24. ^ CANMET, pg. 13-14
  25. ^ Bower & Ropp, pp. 20–21.
  26. ^ Bower & Ropp, pp. 28–29, 34.
  27. ^ Bower & Ropp, pg. 22
  28. ^ Bower & Ropp, pg. 26
  29. ^ "Technical Interconnection Requirements for Distributed Generation" Archived 2014-02-07 at the Wayback Machine, Hydro One, 2010
  30. ^ "California Electric Rule 21 Supplemental Review Guideline" Archived 2010-10-19 at the Wayback Machine
  31. ^ Jonathan Sher, "Ontario Hydro pulls plug on solar plans", The London Free Press (via QMI), 14 February 2011
  32. ^ Verhoeven, pg. 46
  33. ^ CANMET, pg. 45
  34. ^ Bower & Ropp, pg. 13
  35. ^ CANMET, pg. 3
  36. ^ CANMET, pg. 9-10
  37. ^ Risk analysis of islanding of photovoltaic power systems within low voltage distribution networks. 2002. CiteSeerX 10.1.1.114.2752.
  38. ^ CANMET, p. 48.

Bibliography[edit]

Standards[edit]

  • IEEE 1547 Standards, IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems
  • UL 1741 Table of Contents, UL 1741: Standard for Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources

Further reading[edit]

External links[edit]

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