Islanding

Source: Wikipedia, the free encyclopedia.

Islanding is the intentional or unintentional division of an

power generation
.

Intentional islanding is often performed as 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

distributed generators to power the local load.[3][4]

Unintentional islanding is a dangerous condition that may induce severe stress on the generator, as the generator must match any changes in

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

Intentional islanding divides an electrical network into fragments with adequate

constant-voltage control.[12]

Assuming

computationally infeasible.[8][9]

However, islanding localizes any failures to the containing island, preventing failures from spreading.

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

terrorist attacks, military strikes on electrical infrastructure, or extreme weather events.[15]

Home islanding

Following the

overload.[citation needed
]

Detection methods

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 shift detection 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

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

power line carrier communications or a telephony hookup.[23][24]

Inverter-specific techniques

Certain passive methods are uniquely viable with direct current generators (inverter-based resources), such as solar panels.

For example, inverters typically generate 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

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

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

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

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
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

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.[citation needed
]

It is, generally, the last problem that most concerns utilities.

phases.[38]

References

  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. Archived from the original on 2019-02-26. Retrieved 2019-02-25.
  3. 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. S2CID 40097383
    .
  7. PMID 27713509
    .
  8. ^
    ISSN 1751-8695
    .
  9. ^
    Kansas State
    repository.
  10. ^
    doi:10.1109/TPWRS.2006.881126
    .
  11. doi:10.1109/TPWRD.2004.835051
    . TPWRD-00103-2003.
  12. doi:10.1109/TIE.2010.2049709 – via Academia.edu
    .
  13. doi:10.1109/TSG.2010.2082577
    .
  14. doi:10.1016/j.physd.2015.10.010
    .
  15. doi:10.1109/ISGT45199.2020.9087788
    .
  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. CiteSeerX 10.1.1.114.2752
    .
  38. ^ CANMET, p. 48.

Bibliography

Standards

  • 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

External links
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