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Wide area synchronous grid
Regional electrical grid

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 CESA system in 2008, over 350,000 megawatt hours were sold per day on the European Energy Exchange (EEX).

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.

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

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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 2000–2001 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

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.45

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

NameCountriesCovers/NotesOrganization/CompanyGeneration capacityYearly generationYear/Refs
Northern Chinese State Grid ChinaNorthern ChinaState Grid Corporation of China1700 GW5830 TWh20207
Continental Europe Synchronous Area (CESA, formerly UCTE grid) European Union (excluding  Ireland,  Sweden,  Finland,  Cyprus, Region Zealand and Capital Region of Denmark) Bosnia and Herzegovina  Montenegro  North Macedonia  Kosovo  Serbia  Switzerland  Morocco  Algeria  Tunisia  Turkey  Ukraine  Moldova35 countries in Europe and North Africa, serving 450 millionENTSO-E859 GW2569 TWh20178
National Grid (India) IndiaServing over 1.4 billion peoplePower Grid Corporation of India470 GW1844 TWh2023
Eastern Interconnection United States  CanadaEastern US (except most of Texas) and eastern Canada (except Quebec and Newfoundland and Labrador)610 GW1380 TWh2017
IPS/UPS Russia (Excluding Kaliningrad and Sakhalin)  Belarus  Kazakhstan  Kyrgyzstan  Uzbekistan  Tajikistan  Georgia  Azerbaijan  Mongolia8 countries of former Soviet Union and Mongolia, serving 210 million337 GW1285 TWh2005910
China Southern Power Grid ChinaSouthern China320 GW1051 TWh201911
Western Interconnection United States  Canada  MexicoWestern US, western Canada, and northern Baja California in Mexico265 GW883 TWh201512
National Interconnected System (SIN) BrazilElectricity sector in Brazil150 GW410 TWh

(2007)

2016
Synchronous grid of Northern Europe Norway  Sweden  Finland  DenmarkNordic countries (Finland, Sweden-except Gotland, Norway and Eastern Denmark) serving 25 million people93 GW390 TWh
National Grid (Great Britain) United KingdomGreat Britain's synchronous zone, serving 65 millionNational Grid plc83 GW

(2018)13

336 TWh201714
Iran National Grid Iran  Armenia  TurkmenistanIran and Armenia, serving 84 million people82 GW201915
Southern African Power Pool Angola  Botswana  Democratic Republic of the Congo  Eswatini  Lesotho  Mozambique  Malawi  Namibia  South Africa  Tanzania  Zambia  ZimbabweSAPP serves 9 out of 12 SADC countries and small regions of Angola, Malawi, and Tanzania80.9 GW289 TWh202016
Texas Interconnection United StatesMost of Texas; serves 24 million customersElectric Reliability Council of Texas (ERCOT)78 GW352 TWh (2016)17201818
National Electricity Market AustraliaAustralia's States and Territories except for Western Australia and the Northern Territory (Tasmania is part of it but not synchronised)National Electricity Market50 GW196 TWh201819
Quebec Interconnection CanadaQuebecHydro-Québec TransÉnergie42 GW184 TWh
Java-Madura-Bali System (JAMALI) IndonesiaJAMALI 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)PLN40.1 GW (2020)20163 TWh (2017)212021
Argentine Interconnection System ArgentinaArgentina except Tierra del Fuego39.7 GW129 TWh201922
National Electrical System ChileMain Chilean grid31.7 GW75.8 TWh202223
Sumatera System IndonesiaSumatera 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)PLN14.7 GW

(2020)24

32.1 TWh

(2016)25

202226
Irish Grid Ireland  United KingdomIreland and Northern Ireland.EirGrid7.3 GW

(2022)27

29.6 TWh202028
SIEPAC Panama  Costa Rica  Honduras  Nicaragua  El Salvador  GuatemalaThe Central American Electrical Interconnection System serves Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua and Panama6.7 GW202029
Khatulistiwa System Malaysia  IndonesiaSarawak 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 System AustraliaWestern Australia4.3 GW17.3 TWh201630

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.31

Planned

  • Unified Smart Grid unification of the US interconnections into a single grid with smart grid features.
  • SuperSmart Grid a similar mega grid proposal linking UCTE, IPS/UPS, and Mediterranean grid.
  • ASEAN Power Grid plan to connect all ASEAN Grids. The first step is connecting all mainland ASEAN countries with Sumatra, Java, and Singapore Grid, then Borneo Island and Philippines.

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

  • Energy portal

References

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