Overhead High Voltage Direct Current (HVDC) transmission lines are known for their superior power-carrying capabilities compared to underground cables. The differences between the two mainly stem from their design, thermal management, and cost factors. By understanding these distinctions, one can appreciate why overhead lines are often preferred for long-distance power transfer.
Key Takeaway
- Overhead HVDC lines operate at higher voltages, allowing for greater power transfer and reduced line losses. (1)
- Thermal management is more efficient in overhead lines, enabling higher current-carrying capacities compared to underground cables.
- Overall costs, installation ease, and maintenance accessibility make overhead HVDC lines the more economical choice for long-distance transmission.
Voltage and Power Capacity
Higher Voltage Tolerance
Overhead HVDC lines can operate at voltages up to ±800 kV, enabling them to transmit significantly more power compared to underground systems, which are limited by insulation and cooling requirements. For example, a ±525 kV HVDC line is capable of transmitting up to 2,000 MW, while a 500 kV HVAC overhead line usually handles around 3,000 MW.
Reduced Losses
HVDC systems avoid the capacitive and inductive losses that are common in AC transmission, making them more efficient for long-distance power transfer. (2) This efficiency is particularly notable in overhead lines, which face fewer technical constraints than underground cables.
Thermal and Physical Constraints
Cooling Efficiency
One significant advantage of overhead lines is their ability to dissipate heat naturally through airflow. This allows them to support higher current loads. In contrast, underground cables are insulated and buried, leading to heat retention, which in turn limits their ampacity.
Route Length Limitations
Overhead HVDC lines can extend up to 2,500 km with minimal reactive compensation. Underground cables, however, are generally limited to 40–80 km due to technical and cost barriers, making them less viable for long-distance applications.
Cost and Infrastructure
Capital Costs
The initial investment for overhead HVDC lines is generally 1.5 to 3 times less than that for underground systems. This discrepancy arises from the simpler installation process and reduced material requirements for overhead lines. Conversely, underground cables incur higher costs due to excavation, jointing, and insulation needs.
Converter Stations
Both overhead and underground HVDC systems require converter stations at each end. However, the efficiency gains from overhead systems often offset these costs, especially for distances exceeding 600 km. Underground systems face additional expenses for reactive compensation and fault repairs.
Reliability and Maintenance
Fault Recovery
Overhead lines have a clear advantage when it comes to maintenance. They can be repaired in hours to days, while underground systems may take weeks due to the challenges of fault location and accessibility. The ease of access for overhead lines translates to reduced downtime.
Exposure Risks
While overhead lines are susceptible to weather-related risks, underground cables can suffer from excavation damage and longer-term degradation of insulation, which can complicate repairs and increase costs.
Key Comparisons
| Factor | Overhead HVDC | Underground HVDC |
| Voltage Capacity | Up to ±800 kV | Limited by insulation and cooling |
| Power Transfer | 2,000 MW (per circuit) | ~1,000 MW (requires multiple cables) |
| Cost per km | $1–2.5M (transmission only) | $2–3M (2–3 times overhead) |
| Max Route Length | 2,500 km | 80 km (with compensation) |
| Thermal Management | Air-cooled, higher ampacity | Limited by soil conductivity |
FAQs
Why do overhead HVDC transmission lines generally have higher power transmission capacity than underground HVDC cables?
Overhead HVDC transmission lines can carry more power than underground HVDC cables mainly because of better cooling. Air-cooled conductors release heat into the open air, while underground cables struggle with soil heat dissipation. This thermal limitation directly affects the current carrying capacity. With better cooling, overhead lines can handle higher current density and ampacity, resulting in greater megawatt capacity and deliverable power over the same transmission distance.
How do thermal limitations affect the current carrying capacity of underground cables compared to overhead lines?
Underground HVDC cables face serious thermal limitations because soil doesn’t remove heat as well as air does. This poor soil heat dissipation restricts the cable’s ampacity (how much electrical current it can safely carry). When cables get too hot, their insulation can fail. Overhead line conductors, being surrounded by moving air, stay cooler naturally. This cooling difference is why overhead HVDC transmission can handle higher current density and power density than underground installation.
What role does the converter station cost play when deciding between overhead HVDC transmission and underground cables for long-distance power transfer?
Converter station cost is a big factor in any HVDC system, whether overhead or underground. These stations convert between AC and DC at each end of the line. While these costs are similar for both options, the higher price of underground HVDC cables often pushes the total project cost much higher than overhead line cost. This affects the economic break-even distance where HVDC makes sense over HVAC, especially for grid interconnection projects connecting remote power generation to cities.
How do maintenance accessibility and outage duration compare between underground cables and overhead lines in HVDC systems?
Maintenance accessibility is much easier with overhead HVDC transmission. When problems happen, crews can see and fix issues quickly on overhead lines. Underground HVDC cables require digging to access, making repairs difficult and slow. This difference directly affects outage duration – overhead line repairs might take hours or days, while underground cable jointing and repairs can take weeks. This maintenance challenge affects overall transmission reliability in the power system operation.
What are the differences in land use and right-of-way (ROW) requirements between overhead lines and underground cables?
Overhead HVDC transmission needs a wider right-of-way (ROW) with a larger line footprint than underground HVDC cables. (3) The transmission corridor for overhead lines must be kept clear of tall trees and buildings. Underground cables need a narrower energy corridor but require more intensive construction to install. While overhead lines have greater visual impact on the landscape, underground cables cause more short-term land disruption during installation. These factors affect the environmental impact of both transmission infrastructure options.
How do HVDC vs HVAC systems compare in terms of transmission efficiency and losses over long distances?
HVDC systems have lower transmission line losses than HVAC for long-distance power transfer. AC systems suffer from skin effect and reactive compensation needs due to capacitive charging current. DC lines don’t have these problems, allowing better peak voltage utilization. In AC lines, only the RMS voltage (about 70% of peak) transfers power. HVDC systems can use the full line-to-line voltage continuously, improving transmission efficiency. This advantage grows with transmission distance, making HVDC better for connecting different grid frequency differences.
Why might utilities upgrade from ACSR conductors to HTLS conductors in overhead transmission lines?
Utilities often upgrade from traditional ACSR conductors (aluminum conductor steel reinforced) to HTLS (high-temperature low-sag) conductors for ampacity increase. This conductor upgrade can boost power transfer capability without building new towers. HTLS conductors handle higher temperatures with less conductor sag, allowing greater current carrying capacity. This double circuit conversion strategy helps overcome transmission system constraints. While more expensive than standard conductors, HTLS options often cost less than completely new transmission line design or DC line reconstruction.
How does a bipole configuration improve the reliability and capacity of HVDC transmission systems?
A bipole configuration in HVDC link design uses two conductors with opposite voltages (positive and negative), plus sometimes a ground return. This setup improves system stability and voltage stability compared to single-pole systems. If one pole fails, the other can still carry partial power, improving transmission reliability. Bipole systems also maximize voltage rating utilization and high voltage insulation performance, doubling power transfer efficiency compared to monopole designs. This arrangement is common in modern HVDC systems for grid interconnection across asynchronous connection points.
Conclusion
In the realm of power transmission, overhead HVDC lines clearly outperform underground cables. Their ability to operate at higher voltages, coupled with lower losses and costs, makes them the preferred option for long-distance applications. While underground systems may have their place in specific scenarios, such as in environmentally sensitive areas, the advantages of overhead lines in terms of efficiency, maintenance, and overall reliability cannot be overstated. Understanding these factors can significantly influence power system planning and infrastructure decisions in the future.
References
- https://cencepower.com/blog-posts/hvdc-transmission-systems
- https://eepower.com/technical-articles/an-in-depth-comparison-of-hvdc-and-hvac/
- https://www.hydro.mb.ca/guides/transmission-line-right-of-way/