Tech Insights

Weighing the Trade-Offs of Overhead and Underground Transmission

December 06, 2023 by Shannon Cuthrell

How do overhead and underground transmission technologies stack up regarding efficiency, reliability, and route constraints? Let’s unpack the key distinctions. 

As countries worldwide add transmission lines or upgrade existing infrastructure to support large-scale wind and solar projects, they must decide: Should the lines be located overhead or underground? It’s not an easy decision, as land use, conditions, and grid restraints are all factors.

 

Australia’s 109-mile-long Murraylink transmission line is connected to two converter stations in Victoria and South Australia. It’s one of only a handful of inter-regional HVDC transmission projects in the country.

Australia’s 109-mile-long Murraylink transmission line is connected to two converter stations in Victoria and South Australia. It’s one of only a handful of inter-regional HVDC transmission projects in the country. Image used courtesy of APA Group

 

A report prepared by Curtin University and the University of Queensland, Australia, may provide some guidance. The report compares the technical aspects of overhead and underground transmission infrastructure up to 500 kV and evaluates the pros and cons of high-voltage alternating current (HVAC) and direct current technologies (HVDC) in efficiency and applications. 

 

HVAC Underground Transmission: Technical Advantages and Limitations

Underground lines are necessary in certain sections of the transmission grid in urban and environmentally significant areas. For example, international studies show that undergrounding short sections (up to 12 miles) is required to overcome space limitations in cities. At these short distances, AC is cheaper because it supports high voltage levels without requiring converter stations, unlike HVDC. 

HVAC underground cable transmission is limited to a shorter route length (31 miles for 500 kV) than overhead lines because of the high electrical capacitance of the cables. Generally, AC electricity flows through power wires from the plant to the point reaching the consumer. AC is flexible to changing voltages, with some limitations. When carrying large amounts of electricity long-distance, AC can change the direction in which electricity flows, causing losses in the transmission process. 

However, one of the challenges of underground transmission is the significant charging currents associated with highly capacitive cables. AC systems need reactive power compensation plants and terminal equipment to mitigate these energy losses, making them particularly demanding economically. 

Reactive power compensation plants, such as shunt reactors or static var compensators (SVCs) at termination points, offset energy losses by stabilizing the voltage during load variations. These plants can be expensive, as compensation is required for circuits at 50% to 100% of the critical length—about 31 to 43 miles for extra high voltage cables above 345 kV. 

 

Examples of 500 kV HVAC and 525 kV HVDC underground cable installations.

Examples of 500 kV HVAC and 525 kV HVDC underground cable installations. Image used courtesy of Curtin University - Figure 3 (Page 16)

 

The report outlines a few methods to install transmission cables. They’re typically installed in buried conduits or ducts in trenches 3.9 to 4.9 feet deep. Duct installation is more flexible in construction because it reduces the time in which trenches are left open, offering additional safety benefits. Cables are placed into the ducts with lengths around 1,640 to 3,280 feet. 

Developers could also pursue the cheaper direct-buried cable option, in which cables are laid in an open trench and back-filled after completion. 

 

Considerations for HVAC Overhead Transmission Lines

Overhead transmission lines are significantly longer than underground lines, traversing routes across hundreds of miles. HVAC is ideal for lines up to 372 miles and requires less power compensation than underground cables. HVAC enables connections with multiple generators and delivers bulk electricity supply to customer load centers. 

HVAC systems generally have a service life of 60 to 80 years, thanks to well-established maintenance practices and reliability with structures, conductors, and insulators. 

In environmentally sensitive or urban areas, overhead HVAC cables have been used at shorter distances due to the same charging current and capacitive characteristics as underground systems. Still, reactive power compensation plants may be required in longer lines, but to a lesser extent than equivalent-rated underground lines. Shunt reactors and SVCs are designed to limit temporary overvoltage when a line is energized or switched out of service. 

Flexibility is another key benefit, as HVAC overhead lines are the most economical option for adding future connections to the line. 

However, one essential challenge in overhead transmission lines is securing easements that allow construction or maintenance access and space to clear vegetation. Typical easement widths vary from 98 feet for 132 kV lines to 229 feet for a double-circuit 500 kV line. 

 

HVDC: Long-Distance Power Transfer for Overhead or Underground Lines

Unlike length-constrained HVAC cable systems, HVDC transmission is best suited for long point-to-point connections, applicable for both overhead and underground transmission. Since DC electricity flows in one direction, it can carry power over long distances. HVDC is more cost-effective than equivalently rated 500 kV AC at route lengths spanning 434 miles of land. 

HVDC is particularly advantageous for offshore wind installations where submarine cables link two regions or systems. HVDC enables interconnection for asynchronous AC grids. This is useful in offshore or onshore cable connections where the route length exceeds an HVAC transmission cable with equal power transfer capacity. Offshore HVDC networks are economical within 37 to 60 miles from the mainland. 

However, a significant disadvantage of HVDC is that large, costly AC/DC converter stations are required at terminal connection points to the main grid. HVDC systems need specialized substations to carry electricity over long distances without significant power losses. Converter stations transform AC to DC when transmitting power from the offshore wind farm. Then, the power gets converted back to AC for onshore distribution.

Transistor-based voltage source converters (VSCs) are an increasingly popular converter technology for HVDC transmission in offshore wind and grid interconnections. Thyristor-based line-commutated converters are mainly used in overhead DC lines and high-power transmission systems with DC voltages topping 800 kV. 

 

A converter station for the NordLink HVDC project.

A converter station for the NordLink HVDC project. Image used courtesy of Hitachi Energy

 

VSC HVDC systems are commonly used in interregional transmission for European offshore and onshore zone interconnections. For example, VSCs were recently deployed in the 387-mile-long NordLink interconnector project, transmitting wind and hydropower between northern Germany and southern Norway. The project required several converter stations and over 372 miles of submerged and underground cables. 

Long-distance projects are particularly challenging with competing environments on either coast. NordLink’s Norwegian converter station was built in the mountains, where workers had to break and soften the rock-hard ground. On the German side, sponge-like ground was the major problem, requiring months to lay the foundation for a 10,000-ton converter station. The builders considered different construction methods for each soil type. 

In another example, Germany’s upcoming underground SuedLink project will install converter stations across 7 hectares. At around 435 miles, it’s expected to be the world’s longest underground transmission cable, transporting wind power from northern to southern Germany. Hitachi Energy will supply a converter station capable of transferring up to 2 GW, enough to power 5 million households. The link will transmit electricity for 341 miles underground at 525 kV. 

Like NordLink, SuedLink’s route is a massive undertaking, with 1,739 to 3,479 miles of cable length crossing 20,000 plots of land, 50 motorway and rail crossings, and additional waterways and protected areas.