Wide area synchronous grid explained

A wide area synchronous grid (also called an "interconnection" in North America) 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).[1]

Neighbouring interconnections with the same frequency and standards can be synchronized and directly connected to form a larger interconnection, or they may share power without synchronization via high-voltage direct current power transmission lines (DC ties), solid-state transformers or variable-frequency transformers (VFTs), which permit a controlled flow of energy while also functionally isolating the independent AC frequencies of each side. Each of the interconnects in North America is synchronized at a nominal 60 Hz, while those of Europe run at 50 Hz.

The benefits of synchronous zones include pooling of generation, resulting in lower generation costs; pooling of load, resulting in significant equalizing effects; common provisioning of reserves, resulting in cheaper primary and secondary reserve power costs; opening of the market, resulting in possibility of long term contracts and short term power exchanges; and mutual assistance in the event of disturbances.[2]

One disadvantage of a wide-area synchronous grid is that problems in one part can have repercussions across the whole grid.

Properties

Wide area synchronous networks improve reliability and permit the pooling of resources. Also, they can level out the load, which reduces the required generating capacity, allow more environmentally-friendly power to be employed; allow more diverse power generation schemes and permit economies of scale.[3]

Wide area synchronous networks cannot be formed if the two networks to be linked are running at different frequencies or have significantly different standards. For example, in Japan, for historical reasons, the northern part of the country operates on 50 Hz, but the southern part uses 60 Hz. That makes it impossible to form a single synchronous network, which was problematic when the Fukushima Daiichi plant melted down.

Also, even when the networks have compatible standards, failure modes can be problematic. Phase and current limitations can be reached, which can cause widespread outages. The issues are sometimes solved by adding HVDC links within the network to permit greater control during off-nominal events.

As was discovered in the California electricity crisis, there can be strong incentives among some market traders to create deliberate congestion and poor management of generation capacity on an interconnection network to inflate prices. Increasing transmission capacity and expanding the market by uniting with neighbouring synchronous networks make such manipulations more difficult.

Frequency

In a synchronous grid, all the generators naturally lock together electrically and run at the same frequency, and stay very nearly in phase with each other. For rotating generators, a local governor regulates the driving torque and helps maintain a more or less constant speed as loading changes. Droop speed control ensures that multiple parallel generators share load changes in proportion to their rating. Generation and consumption must be balanced across the entire grid because energy is consumed as it is produced. Energy is stored in the immediate short term by the rotational kinetic energy of the generators.

Small deviations from the nominal system frequency are very important in regulating individual generators and assessing the equilibrium of the grid as a whole. When the grid is heavily loaded, the frequency slows, and governors adjust their generators so that more power is output (droop speed control). When the grid is lightly loaded the grid frequency runs above the nominal frequency, and this is taken as an indication by Automatic Generation Control systems across the network that generators should reduce their output.

In addition, there's often central control, which can change the parameters of the AGC systems over timescales of a minute or longer to further adjust the regional network flows and the operating frequency of the grid.

Where neighbouring grids, operating at different frequencies, need to be interconnected, a frequency converter is required. HVDC Interconnectors, solid-state transformers or variable-frequency transformers links can connect two grids that operate at different frequencies or that are not maintaining synchronism.

Inertia

See main article: Inertial response. Inertia in a synchronous grid is stored energy that a grid has available which can provide extra power for up to a few seconds to maintain the grid frequency. Historically, this was provided only by the angular momentum of the generators, and gave the control circuits time to adjust their output to variations in loads, and sudden generator or distribution failures.

Inverters connected to HVDC usually have no inertia, but wind power can provide inertia, and solar and battery systems can provide synthetic inertia.[4] [5]

Short circuit current

In short circuit situations, it's important for a grid to be able to provide sufficient current to keep the voltage and frequency reasonably stable until circuit breakers can resolve the fault. Many traditional generator systems had wires which could be overloaded for very short periods without damage, but inverters are not as able to deliver multiple times their rated load. The short circuit ratio can be calculated for each point on the grid, and if it is found to be too low, for steps to be taken to increase it to be above 1, which is considered stable.

Timekeeping

For timekeeping purposes, over the course of a day the operating frequency will be varied so as to balance out deviations and to prevent line-operated clocks from gaining or losing significant time by ensuring there are 4.32 million on 50 Hz, and 5.184 million cycles on 60 Hz systems each day.

This can, rarely, lead to problems. In 2018 Kosovo used more power than it generated due to a row with Serbia, leading to the phase in the whole synchronous grid of Continental Europe lagging behind what it should have been. The frequency dropped to 49.996 Hz. Over time, this caused synchronous electric clocks to become six minutes slow until the disagreement was resolved.[6]

Deployed networks

Name Countries Covers/NotesOrganization/Company data-sort-type="number" Generation capacity !data-sort-type=numberYearly generation !Year/Refs
Northern Chinese State Grid Northern China1700 GW5830 TWh2020[7]
Continental Europe (UCTE) (minus Ireland, Sweden, Finland, Lithuania, Latvia, Estonia, Cyprus and Eastern Denmark) 24 European countries, serving 450 millionENTSO-E. 859 GW 2569 TWh 2017[8]
Eastern US (except most of Texas) and eastern Canada (except Quebec and Newfoundland and Labrador)610 GW 1380 TWh2017
Serving over 1.4 billion people425 GW 1844 TWh 2023
11 countries of former Soviet Union serving 240 million337 GW 1285 TWh 2005[9] [10]
Chinese southern grid320 GW 1051 TWh2019[11]
Western US, western Canada, and northern Baja California in Mexico265 GW 883 TWh 2015[12]
National Interconnected System (SIN) Electricity sector in Brazil150 GW 410 TWh(2007)2016
Nordic countries (Finland, Sweden-except Gotland, Norway and Eastern Denmark) serving 25 million people93 GW 390 TWh
National Grid (Great Britain)Great Britain's synchronous zone, serving 65 million83 GW(2018)336 TWh 2017[13]
Iran National GridIran and Armenia, serving 84 million people82 GW 2019[14]
Southern African Power Pool SAPP serves 9 out of 12 SADC countries and small regions of Angola, Malawi, and Tanzania80.9 GW289 TWh2020[15]
Most of Texas; serves 24 million customersElectric Reliability Council of Texas (ERCOT) 78 GW 352 TWh (2016)[16] 2018[17]
National Electricity MarketAustralia's States and Territories except for Western Australia and the Northern Territory (Tasmania is part of it but not synchronised)50 GW 196 TWh 2018[18]
Quebec42 GW 184 TWh
Java-Madura-Bali System (JAMALI)JAMALI System serves 7 provinces (West, East, and Central Java, Banten, Jakarta, Yogyakarta, and Bali), serving 49.4 million customers. (Part of ASEAN Power Grid project)40.1 GW (2020)[19] 163 TWh (2017)[20] 2021
Argentina except Tierra del Fuego39.7 GW129 TWh 2019[21]
National Electrical SystemMain Chilean grid31.7 GW 75.8 TWh2022[22]
Sumatera SystemSumatera System serves 8 provinces (North, West, South Sumatera, Aceh, Bengkulu, Lampung, Jambi, and Riau) and Bangka Island, serving 17 million customers. (Part of ASEAN Power Grid project)14.7 GW(2020)[23] 32.1 TWh(2016)2022[24]
Irish Grid Ireland and Northern Ireland.7.3 GW(2022)[25] 29.6 TWh 2020[26]
SIEPACThe Central American Electrical Interconnection System serves Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua and Panama6.7 GW2020[27]
Khatulistiwa SystemSarawak state and the northwestern part of West Kalimantan (Part of ASEAN Power Grid project)Heads of ASEAN Power Utilities/Authorities (HAPUA)5.5 GW2017
South West Interconnected SystemWestern Australia4.3 GW17.3 TWh 2016[28]
A partial table of some of the larger interconnections.

Historically, on the North American power transmission grid the Eastern and Western Interconnections were directly connected, and was at the time largest synchronous grid in the world, but this was found to be unstable, and they are now only DC interconnected.[29]

Planned

DC interconnectors

Interconnectors such as High-voltage direct current lines, solid-state transformers or variable-frequency transformers can be used to connect two alternating current interconnection networks which are not necessarily synchronized with each other. This provides the benefit of interconnection without the need to synchronize an even wider area. For example, compare the wide area synchronous grid map of Europe (in the introduction) with the map of HVDC lines (here to the right). Solid state transformers have larger losses than conventional transformers, but DC lines lack reactive impedance and overall HVDC lines have lower losses sending power over long distances within a synchronous grid, or between them.

Planned non-synchronous connections

The Tres Amigas SuperStation aims to enable energy transfers and trading between the Eastern Interconnection and Western Interconnection using 30GW HVDC Interconnectors.

See also

External links

Notes and References

  1. Web site: 30 October 2008 . EEX Market Monitor Q3/2008 . . . 2008-12-06 . 4 . 2011-07-10 . https://web.archive.org/web/20110710173200/http://www.eex.com/de/document/39600/20081027_EEX_Market_Monitor_Q3_2008_English.pdf . dead .
  2. Book: Haubrich , Hans-Jürgen . Operation of Interconnected Power Systems . Characteristics of interconnected operation . http://www.iaew.rwth-aachen.de/cms/upload/PDF/Vorlesungen/Denzel/chapter_1.1.pdf . 3 . . Dieter Denzel . 2008-10-23 . . 2008-12-06 . 2011-07-19 . https://web.archive.org/web/20110719081241/http://www.iaew.rwth-aachen.de/cms/upload/PDF/Vorlesungen/Denzel/chapter_0.1.pdf . dead . (See "Operation of Power Systems" link for title page and table of contents.)
  3. Web site: Technical Aspects of Grid Interconnection . 2023-12-14.
  4. Web site: Inertia and the Power Grid: A Guide Without the Spin .
  5. https://www.nrel.gov/docs/fy20osti/73856.pdf Inertia and the Power Grid: A Guide Without the Spin Paul Denholm, Trieu Mai, Rick Wallace Kenyon, Ben Kroposki, and Mark O’Malley
  6. News: Serbia, Kosovo power grid row delays European clocks . Reuters . Mar 7, 2018.
  7. Web site: Grid business, SGCC . www.sgcc.com.cn . 23 November 2021.
  8. Web site: ENTSO-E Statistical Factsheet 2017 . www.entsoe.eu . 2 January 2019.
  9. Feasibility Study: Synchronous Interconnection of the IPS/UPS with the UCTE . UCTE-IPSUPS Study Group . . 2008-12-07 . 2.
  10. Web site: IPS/UPS Overview . https://web.archive.org/web/20110728141249/http://www.ucte-ipsups.org/Pdf/Download/englisch/IPSUPS_Overview_Lebed.pdf . 2011-07-28 . . UCTE-IPSUPS Study presentation . 2005-04-20 . Brussels . 2008-12-07 . 4.
  11. Web site: Grid business, CSG . www.eng.csg.cn . 23 November 2021.
  12. https://www.wecc.biz/Reliability/2016%20SOTI%20Final.pdf 2016 State of the Interconnection
  13. Web site: Digest of UK Energy Statistics (DUKES): electricity. July 28, 2022. GOV.UK.
  14. Web site: Dalahoo Power Plant Adds 310 MW to Power Capacity. Eghtesad Online. 27 November 2019 . en. 2019-12-02.
  15. Web site: Annual Reports Southern African Power Pool . 2022-12-21 . www.sapp.co.zw.
  16. http://www.ercot.com/content/wcm/lists/89476/ERCOT2016D_E.xlsx Ercot 2016 Lists
  17. Web site: Quick facts . 818 . www.ercot.com .
  18. Web site: Electricity supply to regions of the National Electricity Market | Australian Energy Regulator . 2019-02-08 . 2019-02-09 . https://web.archive.org/web/20190209124008/https://www.aer.gov.au/wholesale-markets/wholesale-statistics/electricity-supply-to-regions-of-the-national-electricity-market . dead .
  19. Web site: Mulyana. Ridwan Nanda. 2021-02-23. Perwitasari. Anna Suci. PLN: Ada tambahan 3.000 MW pembangkit listrik di sistem Jawa-Madura-Bali tahun ini. 2021-04-24. kontan.co.id. id.
  20. Web site: synergy. 2017-04-28. Indonesia's Electricity Systems - Jawa-Madura-Bali System. 2021-04-24. Insights. en-GB.
  21. Web site: 12 June 2020. Informe anual 2019. 2019 Annual report. 2020-08-10. portalweb.cammesa.com. Compañía Administradora del Mercado Mayorista Eléctrico Sociedad Anónima. es-AR.
  22. Web site: 2018-07-30 . Sistema Eléctrico Nacional (SEN) I Coordinador Eléctrico Nacional (CEN) . 2022-12-21 . Coordinador Eléctrico Nacional Servimos a Chile con Energía . es-ES.
  23. Web site: synergy . 2017-05-29 . Indonesia's Electricity Systems – Sumatera Interconnected System . 2022-11-26 . Insights . en-GB.
  24. Web site: PLN Operasikan Sistem Interkoneksi 150 kV Sumatera-Bangka . 2022-11-26 . validnews.id . id.
  25. Web site: Generation capacity statement. ltd.uk. 21 September 2023.
  26. Web site: Wind Energy Powers Ireland to Renewable Energy Target. 28 Jan 2021 .
  27. Web site: Central American Electrical Interconnection System (SIEPAC). IRENA. 21 September 2023.
  28. https://westernpower.com.au/media/2308/facts-and-statistics-2015-16.pdf 2015/16 Facts and statistics
  29. When the Grid Was the Grid: The History of North America's Brief Coast-to Coast Interconnected Machine. Proceedings of the IEEE. January 2019 . 107 . 1 . 232–243 . 10.1109/JPROC.2018.2880938 . Cohn . Julie . 58005728 . free .
  30. Web site: Address at the 2006 International Conference of UHV Transmission Technology . https://web.archive.org/web/20080924201311/http://www.sgcc.com.cn/ywlm/gsyw-e/45953.shtml . 2008-09-24 . . UCTE-IPSUPS Study presentation . 2006-11-29 . Beijing . 2006-12-06.