Transmission & Grid Services — HVDC, FACTS, Interconnects, RTO/ISO Planning

The transmission grid moves bulk electric power from where it is generated to where it is consumed. In the United States, that means roughly 700,000 miles of high-voltage transmission (≥69 kV) operating at 60 Hz across three weakly synchronized interconnections (Eastern, Western, ERCOT-Texas), divided among 130+ Balancing Authority Areas (BAAs), coordinated within seven Regional Transmission Organizations / Independent System Operators (RTOs/ISOs) plus several large non-RTO regions, and overseen by the Federal Energy Regulatory Commission (FERC) and the North American Electric Reliability Corporation (NERC, with eight regional entities including RF, SERC, MRO, WECC, NPCC, TRE, RFC, FRCC). Europe runs synchronously across continental ENTSO-E with separate Nordic and UK/Ireland subsystems. China and India operate their own continental-scale grids. This note maps the equipment, planning processes, and frontier technologies that make bulk power transfer possible.

US grid topology

• Three interconnections — Eastern Interconnection (the largest, spanning from the Rockies eastward and into eastern Canada), Western Interconnection (WECC; from the Rockies west, into BC and Alberta and into Baja California), Electric Reliability Council of Texas (ERCOT; most of Texas; deliberately not synchronously interconnected to keep out of FERC jurisdiction). Eastern + Western are tied via small HVDC back-to-back ties; ERCOT to Eastern via a 600 MW HVDC tie at Welsh substation and to Mexican CFE network.
• RTOs/ISOs — CAISO (California), ERCOT (Texas), MISO (Midcontinent — 15 states from Manitoba to Louisiana), SPP (Southwest Power Pool — Plains states), PJM (Pennsylvania-New Jersey-Maryland Interconnection — 13 states + DC, the largest by load), NYISO (New York), ISO-NE (New England 6 states).
• Non-RTO regions — Southeast (vertically integrated utilities — Southern Company, Duke, TVA, NextEra/FPL, Dominion); Pacific Northwest (Bonneville Power Administration BPA, PacifiCorp, PGE, Avista, Idaho Power, Puget Sound Energy); mountain states (Xcel, PacifiCorp, NV Energy, Tucson Electric, Salt River Project, Western Area Power Administration WAPA). The Southeast operates SEEM (Southeast Energy Exchange Market, launched November 2022) for sub-hourly bilateral matching. WECC operates WEIM (Western Energy Imbalance Market launched 2014; CAISO-operated; ~24 BAA participants) and WEDAM (Day-Ahead Market, launching 2025-26; SPP competing with Markets+ for Western day-ahead participation).
• Balancing Authorities (BAAs) — 130+ entities responsible for moment-to-moment supply-demand balance within their footprint. Examples: BPA, MISO, PJM, CAISO, NYISO, ERCOT, SPP, plus utility BAs like Duke Energy Carolinas, Tampa Electric, Tucson Electric.
• NERC + ERO Enterprise — North American Electric Reliability Corporation; FERC-certified ERO Electric Reliability Organization with eight Regional Entities: ReliabilityFirst (RF), SERC, MRO, WECC, NPCC, Texas RE (TRE), Florida (FRCC absorbed into SERC 2019), Western Electricity Coordinating Council Reliability Coordinator. NERC publishes mandatory Reliability Standards (TOP, IRO, PRC, CIP cybersecurity, etc.) enforced with civil penalties up to USD 1M/day/violation.

Voltage levels

• Transmission — 230 kV, 345 kV, 500 kV (most US west-of-Mississippi backbone), 765 kV (American Electric Power AEP pioneered 1969 Mountaineer Marietta; PJM service area). EHV / UHV (Ultra High Voltage) 800-1100 kV pioneered in China (State Grid Corporation of China’s Zhundong-Wannan ±1100 kV HVDC and 1000 kV AC Changji-Guquan) and Brazil (Itaipu Furnas 765 kV; Belo Monte ±800 kV HVDC). The economic case for higher voltage strengthens with longer distance because losses scale as I²R and current falls inversely with voltage at constant power.
• Subtransmission — 69-138 kV; regional distribution to substations and large industrial customers.
• Primary distribution — 4-35 kV (12.47 kV most common 4-wire wye system in US; 13.8 kV common in industry; 25 kV and 34.5 kV in lower-density and underground areas).
• Secondary distribution — 120/240 V single-phase (US residential split-phase from a center-tapped distribution transformer), 480 V three-phase (US commercial/industrial), 208 Y/120 V three-phase wye, 600 V (Canada commercial).

HVDC: High-Voltage Direct Current

HVDC competes with AC transmission when distance is great, when an asynchronous interconnection is needed, or for submarine/long underground cables (where AC capacitive charging current limits run length to roughly 80 km submarine).

LCC (line-commutated converter) HVDC

• Thyristor-based; mature technology since the 1950s (Gotland 1954 first commercial HVDC; ABB).
• Requires a strong AC system at both ends for commutation; can’t black-start; consumes 40-60% of rated power in reactive support during operation.
• Highest power ratings: ±1100 kV Zhundong-Wannan (Xinjiang to Anhui, China; 12 GW; 3284 km; commissioned 2019; longest and highest-voltage HVDC ever).
• ±800 kV UHVDC widely deployed in China for west-east transfer: Jinping-Sunan, Yunnan-Guangdong, Xiluodu-Zhejiang, Hami-Zhengzhou.
• North American examples: Pacific DC Intertie (PDCI; Celilo OR to Sylmar CA; ±500 kV; 3,100 MW; LADWP + BPA + PacifiCorp; commissioned 1970, since upgraded to 3,220 MW), Nelson River Bipoles I/II/III (Manitoba Hydro; ±450/500 kV), Square Butte (North Dakota lignite → Minnesota Power), Champlain-Hudson Power Express (CHPE) (Quebec to NYC; ±400 kV; 1,250 MW; 339 miles all underground/underwater; in construction 2024-26 target; Hydro-Québec power to ConEd serving NYC).

VSC (voltage-source converter) HVDC

• IGBT-based; introduced commercially by ABB as HVDC Light (1999 Gotland 50 MW pilot); Siemens calls it HVDC Plus.
• Can black-start; full active and reactive power control independently; doesn’t need a strong AC system; smaller converter footprint suitable for offshore platforms.
• MMC (Modular Multilevel Converter) topology (Siemens Marquardt 2003 patent) is now standard for high-power VSC HVDC, replacing earlier two- and three-level designs. Allows scalable voltages and dramatically reduced harmonics.
• Major projects:
  • North Sea Link (NSL) — UK to Norway; National Grid + Statnett; ±525 kV VSC; 1,400 MW; 720 km (~558 km submarine + onshore); commissioned October 2021.
  • Viking Link — UK to Denmark; National Grid + Energinet; ±525 kV VSC; 1,400 MW; 765 km (longest interconnector subsea cable in the world); commissioned December 2023.
  • IFA-2 — UK to France; National Grid + RTE; 1,000 MW; commissioned January 2021.
  • Western Link — Scotland to Wales/England; National Grid + SP Energy Networks; ±600 kV; 2,250 MW; 422 km; commissioned 2017 (with extended commissioning issues into 2019).
  • German offshore wind connectors — TenneT operates many platform-to-shore connections: DolWin1/2/3/5/6, BorWin1-5, HelWin1/2, SylWin1, NordLink (DE-NO 1400 MW 2021). LanWin (~2030+) further additions.
  • South-West Link — Sweden Sydvästlänken; ±300 kV VSC; 1200 MW; long commissioning issues 2015-19.
  • Caithness-Moray — Scottish Highlands; ±320 kV VSC; 1,200 MW; SSEN; 2018.
  • Réseau de transport d’électricité (RTE) — France offshore wind connectors.

HVDC manufacturers

The HVDC supplier market is concentrated. Hitachi Energy (formerly ABB Power Grids; Hitachi acquired 80.1% from ABB 2020 USD 11B; remaining 19.9% acquired 2022) is the largest. Siemens Energy (spun from Siemens AG 2020) is second. GE Vernova (spun from GE 2024; includes Alstom Grid acquired 2015) third. Mitsubishi Electric, Toshiba, NR Electric (China), C-EPRI / NARI (China), Sifang (China), Hyosung (Korea), Hyundai Electric (Korea) also active.

FACTS (Flexible AC Transmission Systems)

Power-electronic devices that enhance AC transmission capability without building new lines.

• SVC (Static VAR Compensator) — thyristor-controlled reactor (TCR) and/or thyristor-switched capacitor (TSC) combinations; provides controllable reactive power for voltage support. Late 1970s onward; ABB/Hitachi/Siemens/GE all build. Hundreds installed worldwide.
• STATCOM (Static Synchronous Compensator) — VSC-based reactive power compensator; ±100-500 MVAr typical; faster response than SVC; smaller footprint; doesn’t require capacitor/reactor banks for the same dynamic range. Increasingly the preferred choice for new reactive support installations; supports weak-grid integration of inverter-based resources.
• Series compensation — capacitor banks in series with a long transmission line; reduces effective line reactance, increasing power transfer capability. Risk: subsynchronous resonance (SSR) between the series capacitor and torsional modes of nearby thermal generator shaft systems; the classic event is Mohave Generating Station 1970 (Nevada/Arizona; series caps + Western intertie + Mohave units → catastrophic shaft damage). Modern protection includes capacitor bypass at SSR detection.
• TCSC (Thyristor-Controlled Series Capacitor) — series compensation with variable thyristor control; addresses SSR risk; commonly cited example Slatt 500 kV (BPA 1993).
• UPFC (Unified Power Flow Controller) — combines series and shunt VSCs sharing a DC link; full independent control of voltage magnitude, angle, and active/reactive power flow on a line. AEP Inez 320 MVA (1998) the first US installation.
• Phase-shifting transformers (PSTs) — older mechanical-tap-changer technology that controls active power flow on parallel paths. Many PSTs in operation (e.g., Meeden 1700 MVA Netherlands; PSE-T Polish-German border).

Conductors

• ACSR (Aluminum Conductor Steel Reinforced) — aluminum strands around steel core; the workhorse of HV transmission since the 1930s.
• ACSS (Aluminum Conductor Steel Supported) — annealed aluminum on steel; can operate at higher temperature (180-200°C continuous) than ACSR (~75-100°C continuous) without losing strength; useful for reconductoring uprates.
• ACCC (Aluminum Conductor Composite Core) — CTC Global’s product (now CTC Global Corporation); polymer composite core (carbon + glass fiber + epoxy); much lower thermal sag than steel core; can operate at 180-200°C with sag similar to ACSR at 75°C, enabling significant ampacity increase on existing right-of-way and tower geometry.
• ACCR (Aluminum Conductor Composite Reinforced) — 3M’s product; aluminum-matrix ceramic-fiber composite core; high-temp, low-sag.
• HTLS (High-Temperature Low-Sag) reconductoring — replacing existing conductor with ACCC, ACCR, or ACSS on the same towers to add 50-100% capacity without rebuilding the line. Often dramatically cheaper than greenfield transmission; recent FERC Order 1920 (May 2024) and Notice of Proposed Rulemaking on Grid Enhancing Technologies (GETs) including HTLS, dynamic line ratings, advanced power flow control are increasing policy attention to this category.
• Bundled conductors — multiple sub-conductors per phase (twin, triple, quad) for EHV and UHV to reduce corona losses and increase ampacity.

Insulators

• Porcelain — historical default; brittle; can shatter on impact.
• Glass — used widely in Europe and elsewhere; failure mode is visible (cap shatters; remainder still mechanically intact, so easier inspection).
• Composite (polymer) — silicone rubber or EPDM weather sheds over fiberglass core rod. Lighter, more hydrophobic, more pollution-tolerant. Now standard for new transmission insulators globally.
• Pollution and contamination — Equivalent Salt Deposit Density (ESDD) and Non-Soluble Deposit Density (NSDD) characterize site contamination severity (IEC 60815); influences insulator leakage distance requirements.
• Corona discharge — partial discharge from sharp points or contamination at high voltage; produces audible noise (the characteristic crackle), radio interference, ozone, ultraviolet, and small power losses. Mitigated by larger conductor diameter, bundled conductors, corona rings on insulator end fittings.

Substation equipment

• Power transformers — step up (GSU Generator Step-Up at power plants), step down (transmission to subtransmission), interface transformers between voltage levels. Three-phase or single-phase (banks of three at the largest ratings; ease of transport and spare strategy). Oil-immersed (mineral oil — increasingly natural ester / FR3 vegetable oil for fire safety and biodegradability), dry-type (smaller ratings, indoor).
• Switchgear:
  • AIS (Air-Insulated Switchgear) — open-air bus and equipment; large footprint; common in transmission substations.
  • GIS (Gas-Insulated Switchgear) — SF6-insulated; ~10× smaller footprint; favored in urban, offshore, and constrained sites. SF6 has a global warming potential of ~23,500× CO2 over 100 years; leakage during operation and end-of-life is a substantial industrial GHG source. The industry is transitioning to SF6-free alternatives: Hitachi Energy EconiQ (dry air for medium voltage; C5-PFK / 3M Novec fluoroketone-based for high voltage), Siemens Energy Blue (clean air / Vacuum + clean air), GE g3 (fluoronitrile + CO2). IEC + IEEE standards updating for fluorine-free alternatives; EU F-gas Regulation revision (2024) and California SB 1383 / CARB regulations driving phase-out.
  • Hybrid switchgear — IS Insulated Switchgear and packaged solutions for medium/high transmission.
• Circuit breakers — interrupt fault currents up to ~80 kA at transmission voltages. SF6 breakers dominate ≥72.5 kV; vacuum interrupters dominate medium-voltage (≤38 kV) and are extending into 72.5 kV and 145 kV (Hitachi, Siemens, GE introductions 2020s). Operating mechanisms: spring, hydraulic, pneumatic. Operating times typically 2-5 cycles total (33-83 ms at 60 Hz).
• Disconnectors / isolators — provide visible mechanical break; not load-break; only for isolation when bus is de-energized.
• Surge arresters — metal-oxide varistor (MOV; ZnO blocks); divert lightning and switching transient currents to ground. EPRI-developed BIL (Basic Insulation Level) coordination.
• Instrument transformers — CTs (Current Transformers) and PTs (Potential Transformers, also called VTs Voltage Transformers; CVTs Capacitor Voltage Transformers / CCVTs Coupling Capacitor Voltage Transformers for HV applications). Optical / fiber CTs (Rogowski coils, electronic CTs) emerging for high-voltage and asymmetric current measurement.

Protection and control

• Protective relaying — devices that detect abnormal conditions and command breakers to open. Functions standardized by ANSI/IEEE device numbers: 21 distance, 51 overcurrent, 67 directional, 87 differential, 27 undervoltage, 81 frequency, 25 sync-check, 50BF breaker failure.
• Distance protection (21) — measures impedance from relay to fault; used on transmission lines; zones reach successively further. Mho, quadrilateral, polygonal characteristics.
• Differential protection (87) — compares currents at two ends of a protected zone; trips on Kirchhoff imbalance. Line differential, transformer differential (with second-harmonic restraint for inrush), bus differential, generator differential.
• Pilot protection — communications-assisted scheme (POTT permissive overreaching transfer trip, DCB directional comparison blocking, DTT direct transfer trip) for high-speed line clearing; teleprotection over fiber or microwave.
• Major relay vendors — SEL Schweitzer Engineering Laboratories (Pullman WA; founded 1982 by Edmund O. Schweitzer III; ~6,000 employees; dominant US market share for transmission protection), Hitachi Energy (Relion line), Siemens Energy (Reyrolle / SIPROTEC), GE Vernova (Multilin/Reason — Power Management / Bus Differential), NR Electric (China; major share in Chinese and global markets), Sifang (China), ABB now Hitachi.
• Synchrophasors / PMUs (Phasor Measurement Units) — GPS-time-synchronized voltage and current phasor measurements at 30-120 samples/sec, defined by IEEE C37.118 / IEC 60255-118. Enable wide-area monitoring (WAMS), state estimation, oscillation detection, post-event forensics. NASPI (North American SynchroPhasor Initiative) coordinates US deployment; FNet/GridEye (University of Tennessee Knoxville; Yilu Liu) deployed thousands of low-cost frequency disturbance recorders at distribution outlets giving wide-area frequency visibility.
• SCADA / EMS — Supervisory Control and Data Acquisition + Energy Management System. Master vendors: OSI (Open Systems International; acquired by Emerson 2020 then by Lutech / GE Vernova 2022; rebranded GE Vernova EMS), Siemens Spectrum Power, Hitachi Energy Network Manager / e-terra, GE Vernova ADMS, ETAP, Survalent, Schneider Electric ADMS. ICCP / IEC 60870-6 TASE.2 for inter-control-center data exchange.
• DERMS (Distributed Energy Resource Management System) — orchestrates rooftop solar, batteries, EVs, controllable loads on distribution feeders. AutoGrid (acquired by Schneider Electric 2022), Itron, Smarter Grid Solutions (acquired by Mitsubishi Electric 2020), Siemens, Schneider Electric, Spirae, Generac Concerto (formerly EnerNoc).

Grid services (cross-reference)

The transmission system delivers — and increasingly is paid for — several distinct services beyond bulk energy transfer.

• Frequency response — primary (governor / FFR Fast Frequency Response from inverter-based resources; arrests frequency excursion within seconds), secondary (Regulation Reserve / AGC Automatic Generation Control; 4-second EMS dispatch to restore frequency to 60 Hz), tertiary (Reserves: Spinning, Non-Spinning, Supplemental; 10-30 minute capability).
• Inertia — synchronous generators provide kinetic energy via their rotating mass. Inverter-based resources (solar, wind, batteries) without grid-forming control provide essentially none; this is creating system-stability concerns in heavily renewable systems (ERCOT, Ireland, South Australia, California). Synthetic / virtual inertia from grid-forming inverters is the emerging solution.
• Voltage support and reactive power — generators, synchronous condensers (older fossil units retired and converted), STATCOMs, SVCs, capacitor banks. Increasingly purchased through market-based mechanisms.
• Black start — restart capability from total system collapse without grid power. Designated black-start units (typically hydro, dual-fuel CTs, dedicated gas units) compensated through reliability-must-run or capacity payments.
• Capacity — guaranteed availability of generation/demand response/storage to meet system peak load; PJM Reliability Pricing Model, ISO-NE FCM, NYISO ICAP, MISO PRA all market-clear capacity.
• (See capacity-and-ancillary-services-markets for market mechanics.)

Transmission planning

• RTEP/MTEP/ITP processes — RTOs run regional transmission expansion plans (PJM RTEP, MISO MTEP, SPP ITP, CAISO TPP, NYISO CRP, ISO-NE RSP). 10-15 year planning horizons; identify reliability needs (NERC Reliability Standards TPL-001 etc.), economic needs (production cost benefit), and public-policy needs (state RPS, federal IRA-driven renewable additions).
• FERC Order 1000 (2011) — required incumbents to open transmission planning to non-incumbents; eliminated federal Right of First Refusal; cost allocation reform requiring beneficiary-pays methodology with planning region default and inter-regional process. Mixed effectiveness; many regions established cost-allocation methods but inter-regional project flow has been minimal.
• FERC Order 2023 (July 2023) — interconnection queue reform; transition from serial to cluster studies (queued projects studied in groups annually rather than sequentially), enforceable study deadlines with penalties, increased site control and commercial readiness requirements at posting. Aimed at unclogging the 2.6+ TW US interconnection queue (~95% solar, wind, storage; LBNL annual queue reports).
• FERC Order 1920 (May 2024) — long-term regional transmission planning; 20-year horizon required; ex-ante cost allocation with state engagement; first major reform to long-term planning since Order 1000. Effective 2025 with regional compliance filings due July 2025.
• NIETC (National Interest Electric Transmission Corridor) designation + DOE Section 1222 — DOE authority under EPAct 2005 + IIJA 2021 + IRA 2022 to designate national interest corridors and partner on federal transmission lines (limited use). FERC backstop siting authority in NIETCs under Federal Power Act 216 (revived by IIJA 2021 amendment after Piedmont 2014 decision invalidated earlier process).
• State siting — transmission siting remains primarily a state matter (Public Utility Commissions / Public Service Commissions, certificate of public convenience and necessity processes). Interregional projects facing multi-state opposition are notoriously hard (Plains and Eastern Clean Line abandoned 2017; SunZia AZ/NM cleared after 17 years and finishing 2025-26).
• Vertical-integration regions vs RTO markets — Southeast utilities own generation and transmission and self-plan within state PUC oversight, with limited interregional market activity beyond the SEEM exchange.

Distribution and grid modernization

• Smart meters / AMI (Advanced Metering Infrastructure) — two-way digital meters with periodic interval data (15-min, 5-min, even 1-min). ~70%+ of US residential customers have AMI 2024. Vendors Itron, Landis+Gyr, Aclara (Hubbell), Honeywell, Sensus (Xylem).
• SCADA + ADMS (Advanced Distribution Management System) — combines distribution SCADA, OMS Outage Management System, DMS Distribution Management System (volt-VAR optimization VVO, conservation voltage reduction CVR, FLISR Fault Location Isolation Service Restoration). Major vendors Schneider Electric, Hitachi Energy, GE Vernova, Siemens, Survalent.
• Substation automation — IEC 61850 (open standard for substation communications); GOOSE messaging for fast protection signals; sampled values for digital instrument transformers.
• DER (Distributed Energy Resources) interconnection — IEEE 1547-2018 (revised standard) requires advanced inverter capabilities (volt-VAR, frequency-watt, ride-through). California Rule 21 implements at state level; Hawaii Rule 14H led the country. Smart inverter requirements increasingly required everywhere.
• Microgrids / islanding — local grids that can disconnect (island) from the bulk grid during outages. Mil/DoD bases (ESTCP-funded projects), hospitals, universities, critical facilities. CERTS Consortium for Electric Reliability Technology Solutions.
• Reliability metrics — SAIDI (System Average Interruption Duration Index, customer-minutes outage / total customers), SAIFI (System Average Interruption Frequency Index), CAIDI = SAIDI/SAIFI, MAIFI (Momentary; <5 minutes). Major event days (typically storm-driven) reported separately. US 2023 SAIDI excluding MEDs ~150 min/yr; including MEDs ~440 min/yr (EIA Form 861).
• Storm restoration — mutual aid (EEI / APPA mutual assistance), pre-staging, vegetation management (largest single distribution opex line item — 30-50%+ of distribution O&M). Tree-trimming cycles 3-5 years typically; faster on critical circuits.
• Undergrounding — placing distribution lines underground for storm resilience. Florida post-storm hardening (Duke Energy Florida, FPL, TECO programs targeting 100% undergrounding of laterals over decades) most aggressive US. Costs ~5-10× overhead for new construction; conversion of existing overhead even more.
• FLISR (Fault Location Isolation Service Restoration) — automated reclosers, sectionalizers, and ADMS algorithms reduce outage extent and duration. Self-healing claim.

Grid-scale storage on transmission system

(See _index energy-storage-systems Tier 3 note for full detail; brief here.)

• Pumped hydro storage — ~22 GW US (~25 GW total including DOE Form 860). Largest US site: Bath County Pumped Storage Station (Bath County VA; Dominion + Allegheny; 3,003 MW; ~24 GWh storage; commissioned 1985; uprated through 2018). Ludington Pumped Storage (Michigan; Consumers Energy + DTE; 1,872 MW; on Lake Michigan; commissioned 1973). Globally ~165 GW PHS dominates grid storage capacity by far.
• Battery energy storage (BESS) — explosive growth. EIA-860/923 data: ~26 GW operating end-2024 (vs ~9 GW end-2022 — ~50% annual growth). Geographic concentration: California ~9 GW, Texas ~6 GW, Arizona ~2 GW, Nevada ~1 GW. CAISO routinely sees batteries provide 25%+ of evening peak supply through summer 2024-25.
• Moss Landing Energy Storage Facility (Monterey County CA) — Vistra-owned (acquired Dynegy 2018); peak 750 MW / 3,000 MWh (one of world’s largest). Major thermal runaway event January 16 2025 in Phase I Vistra building destroyed ~300 MW of capacity (~30,000 LG Chem battery modules); air quality monitoring + Pacific Gas & Electric coordination + ongoing forensics. Earlier fires September 2021 and February 2022 in Phase I (PG&E side). Major industry inflection on safety standards.
• Long-duration energy storage (LDES, 10+ hour) — emerging category:
  • Form Energy — iron-air battery (Fe + O2 ↔ Fe-oxide); ~100-hour duration; first commercial deployment Eccles 10 MW / 1 GWh in Lincoln Township WV (FirstEnergy / Mon Power; commissioning 2024-25); Great River Energy 1.5 MW / 150 MWh Cambridge MN earlier pilot.
  • Highview Power — Liquid Air Energy Storage (LAES); CRYOBattery; UK Carrington Village 50 MW / 250 MWh under construction 2024.
  • Hydrostor — Advanced Compressed Air Energy Storage (A-CAES); Goderich Ontario pilot operational; Willow Rock 500 MW Kern County CA, Silver City 200 MW NSW Australia in development.
  • Energy Vault — gravity (crane lifting concrete blocks; conceptually pumped hydro analog without water); EV1 demonstration in Switzerland; subsequent business pivot to battery integration.
  • EnergyDome — CO2 battery (compress CO2 to liquid, discharge through turbine); 20 MW Sardinia commercial demonstration 2024.
  • Antora Energy — thermal storage in carbon blocks heated to 2400°C; discharge via thermophotovoltaic cells; multi-day duration target. Series B 2023, scaling.
  • Malta — Brayton heat-pump thermal storage (molten salt + cold storage); pilots.
  • ESS Inc. — iron flow battery; commercial deployments at SMUD, Burbank, others.
• Compressed air (CAES) — McIntosh AL 110 MW (Alabama Electric Cooperative 1991), Huntorf Germany 290 MW (1978) historical; A-CAES emerging.
• Flow batteries — vanadium redox (VRB Energy, Largo Clean Energy), zinc-bromine (Redflow), iron flow (ESS Inc.). Long-life, deep cycle.

Major transmission projects and grid operators

United States and Canada

• Texas CREZ (Competitive Renewable Energy Zones) — USD 6.8B, ~3,600 miles of 345 kV; built 2010-2014; enabled the explosion of west Texas wind (and later solar) by transporting it to load centers. Considered the most successful single transmission build for renewables to date.
• Plains and Eastern Clean Line — abandoned 2017; Oklahoma-Tennessee HVDC for wind; couldn’t secure full state-by-state siting consensus.
• SunZia — New Mexico to Arizona; 550 mi 525 kV; wind-focused; Pattern Energy lead developer (acquired Southern California Edison’s share 2022); permitted 2023 after 17 years; energization phases 2025-26.
• Grain Belt Express — Kansas wind to Indiana; Invenergy (acquired from Clean Line 2018); partially under construction 2024+ after Missouri PSC approval 2024.
• Power Pathway / Colorado — Xcel Energy Public Service Colorado; 560 mi 345 kV; under construction.
• TransWest Express — Wyoming wind to Nevada/California; 730 mi DC + AC; under construction 2024+.
• Champlain Hudson Power Express (CHPE) — Hydro-Québec → New York City; ±400 kV VSC HVDC; 339 mi all underground/underwater; 1,250 MW; in construction; commercial operation target 2026.
• Clean Path NY — upstate NY renewables → NYC; FERC Section 216 / state co-ordination; under development.
• MISO Long Range Transmission Plan (LRTP) — Tranche 1 ~USD 10.3B 18 projects approved July 2022 — the largest single transmission portfolio approval in US RTO history; Tranche 2 ~USD 21.8B 24 projects approved December 2024.
• PJM CIR (Capacity Interconnection Rights) and offshore wind tie-ins — NJ TransCo (PSEG + Linden); Maryland POE Public Offer Evaluation; New York Public Policy Transmission projects.
• HQ-NE Phase II (New England Clean Energy Connect / NECEC) — Quebec to Lewiston ME; 145 mi 320 kV HVDC; legal challenges + 2021 Maine referendum + 2023 court reinstatement; energization target 2026.

China and Asia

• UHV (Ultra-High Voltage) DC backbone — State Grid Corporation of China invested USD 100B+ since 2009 in UHV DC and AC transmission to move power from western/northern resource-rich regions (wind + solar + hydro + coal) to eastern/southern load centers.
• Zhundong-Wannan (Changji-Guquan) — ±1100 kV DC, 12 GW, 3,324 km; commissioned 2019; world’s highest voltage and largest capacity HVDC project.
• Jinping-Sunan — ±800 kV, 7.2 GW, 2,090 km; hydro from Sichuan to Jiangsu.
• CSPG (China Southern Power Grid) — ±800 kV multiterminal projects in Yunnan and Guangdong.
• India — POWERGRID Corporation of India; ±800 kV North-East-Agra; long EHV AC; planning HVDC for IRES (Inter-Regional Energy Sharing).

Europe

• European TSOs — TenneT (Netherlands + northwestern Germany; major offshore wind connector developer), Amprion, 50Hertz (eastern Germany), TransnetBW (Baden-Württemberg), RTE Réseau de Transport d’Électricité (France; the largest single TSO by network length in Europe), National Grid Electricity Transmission / NESO (UK; FSO Future System Operator carved out from National Grid plc October 2024 as the public-sector National Energy System Operator NESO), Terna (Italy), REE / REE Networks (Spain — Red Eléctrica), REN (Portugal), Statnett (Norway), Svenska Kraftnät (Sweden), Energinet (Denmark), Fingrid (Finland), Elia (Belgium), APG (Austria), Swissgrid (Switzerland).
• ENTSO-E — European Network of Transmission System Operators for Electricity; coordinates 39 TSOs from 35 countries.
• Project of Common Interest (PCI) list — EU-designated cross-border infrastructure prioritized for permitting and EU CEF funding.

Africa

• African Power Pools — WAPP (West African Power Pool), SAPP (Southern African Power Pool; Cahora Bassa-South Africa HVDC ±533 kV, 1,920 MW Mozambique-South Africa key asset), EAPP (Eastern; including Ethiopia GERD-related HVDC connections), CAPP (Central), NAPP (Northern; Algeria-Morocco-Tunisia + Mediterranean HVDC ties to Spain/Italy under development).
• DESERTEC concept — North African solar to Europe via HVDC undersea; never fully realized but motivated several individual project studies (TuNur Tunisia, Xlinks Morocco-UK in development 2024+ aiming 3.6 GW 4,000 km undersea).

Costs

• Overhead AC transmission — typical US 345 kV ~USD 1-3M/mile; 500 kV ~USD 2-4M/mile; 765 kV ~USD 3-5M/mile (excluding substations).
• HVDC overhead — converter stations USD 200-500M+ each end; line cost similar to comparable AC voltage.
• HVDC underground / submarine — USD 3-7M/mile cable + USD 200-500M converters each end; for long-distance, the higher cable cost is offset by lower losses and ability to bury through populated areas.
• EHV substations — USD 50-300M for transmission switchyards depending on configuration (breaker-and-a-half, ring bus, double bus double breaker) and voltage class.
• Cost allocation — politically charged; FERC Order 1000 beneficiary-pays principle has divergent implementations across regions. Inter-regional cost allocation has been particularly difficult (PJM-MISO Joint Operating Agreement; MISO-SPP Coordinated System Plan).

Grid-scale challenges (2024-2026)

• Interconnection queue backlog — ~2.6 TW (terawatts) of generation and storage in US interconnection queues (LBNL 2024 report); ~95% solar + wind + storage. Average time from queue entry to commercial operation has lengthened from 2 years (2000s) to 5+ years (2020s).
• Permitting — federal NEPA review, USFWS Endangered Species Act consultations, USACE Section 404 water crossings, BLM/NPS/USFS land-use approvals, FAA/FCC tall-structure marking, state PUC certificates, county zoning, easement assembly, eminent domain (where available; severely constrained in many states for interregional projects). Permitting reform (Manchin-Barrasso EPRA Energy Permitting Reform Act 2024 introduced, not enacted) ongoing legislative debate.
• Interregional coordination — RTOs/ISOs have historically optimized internally; inter-RTO seams have been a structural bottleneck. FERC Order 1000 made limited progress; FERC Order 1920 may move further.
• Climate adaptation — heat-wave-driven thermal de-ratings on overhead lines (Phoenix 50°C summer 2024 led to ratings reductions); wildfire ignition liability (PG&E Camp Fire November 2018 → bankruptcy January 2019 → CPUC AB 1054 California Wildfire Fund USD 21B fund 2019; Edison International USD 1.1B Thomas Fire settlement 2019, USD 2.4B Woolsey Fire settlement 2024; Hawaiian Electric Maui Lahaina wildfire August 2023 → near-bankruptcy + AUgust 2024 USD 4B settlement); flood-zone substation hardening; vegetation management intensification.
• Cybersecurity — NERC CIP (Critical Infrastructure Protection) standards mandatory for BES Cyber Systems. Specific incidents: Ukraine 2015 Sandworm attack; Colonial Pipeline May 2021 (oil pipeline not grid but cascading); various probing of US utilities ongoing (Volt Typhoon 2023 CISA disclosure).
• Inverter-based resource (IBR) stability — Odessa TX disturbance May 2021 (192 MW solar tripped during transmission fault recovery) + similar Odessa events 2022 + cascading inverter trip events at Texas-NM border; NERC IBR Strategy 2022 and IRPTF Inverter-Based Resource Performance Task Force standards work; IEEE 2800-2022 grid-forming and grid-following IBR standard.

Adjacent

• electricity-markets — LMP locational marginal pricing, day-ahead and real-time markets, congestion revenues, FTRs financial transmission rights.
• capacity-and-ancillary-services-markets — frequency response services, regulation, reserves, capacity auctions PJM RPM / ISO-NE FCM / NYISO ICAP.
• carbon-markets-and-compliance — RGGI and California cap-and-trade interactions with dispatch and transmission, EPA Clean Power Plan / 111(d) rules.
• _index — energy-storage-systems Tier 3 note for battery chemistry, BMS, cycle life, fire safety; power-electronics Tier 3 for IGBT, GaN/SiC, MMC topology; high-voltage-engineering Tier 3 for insulation coordination, lightning, surge protection.
• _index — IPCC AR6 mitigation pathways, net-zero scenarios requiring 2-3× transmission expansion (Princeton Net-Zero America, NREL Solar Futures Study).
• insurance-and-actuarial — wildfire liability insurance, utility credit, catastrophe bond exposure of transmission and distribution assets.
• _index — Federal Power Act sections 215 + 216 + 219, FERC jurisdiction, state vs federal siting, PURPA, EPAct 2005, IIJA 2021, IRA 2022 transmission and grid provisions.