Building or upgrading high-voltage lines becomes increasingly difficult due to environmental concerns and impact on neighbourhood. Using underground high-voltage cables can mitigate some related problems - but adds new ones like thermal limitations, joints, magnetic field emissions and specific earthing and reactive power compensation requirements.
Hivoduct pressurized air cables are a new technology with better technical parameters than OHL and XLPE cables. They use pressurized air for insulation and a coaxial tube-in-tube conductor design - similar to GIS busducts.
GIS busducts and GIL (=Gas insulated lines) today mostly use SF6 as insulation gas. There is a strong global trend in research and product development to replace SF6 in these applications. Hivoduct pressurized air cables replace GIS exit busducts and GIL with the same performance but without using SF6.
Replacing SF6 gas requires a new product design - as it is fundamental to the performance of high-voltage products.
Hivoduct has done a new design from the bottom-up and developed a new technology "pressurized air cables" which cover the MV and HV range. This was accomplished by incorporating the learnings from over 50 years of gas-insulated high-voltage products and adding a number of new and unique features.
Our main guideline for development was our mission:
Reduce the visual impact
Pressurized air cables are mostly layed underground - like XLPE cables.
-> Visible only at the ends when it connects to existing lines or substations.
No GWP of the insulation material
Pressurzied air cables use air for insulation (80% NO2, 20% O2, dry).
Reduce electromagnetic field emissions
Pressurized air cables use a coaxial aluminum tube arrangement (like GIL) with rigid, conductive and grounded enclosures.
-> No outside electric fields. Minimum outside magnetic fields.
Pressurized ai cables have an aluminum conductor & enclosure with sufficient cross-section and optimum diameter ratio. Use distant spacers to keep them coaxial. Optimize all together with air pressure. Optimize 3-phase arrangement. Optimize on-site laying options and minimize digging. Enable co-location with existing infrastructure.
-> No copper. No XLPE. No concrete pouring. No need for double systems. Less digging and filling.
Large conductor cross-section leads to less resistance and therefore less losses. Smaller epsilon_r and bigger air gap leads to less AC losses (capacitive currents). Smooth and round dielectric design avoids corona losses. Double sealing system avoids pressure losses.
The result from this development process is the Hivoduct pressurized air cable design. Below are some key points for the development.
Using the well-known and researched dielectric performance of air , we designed optimal conductors, enclosures, spacers and shields to work with air pressures up to 10 bar.
The calculation shows an example for a rated voltage of 145 kV - which according to IEC 62271 standard requires an impulse withstand voltage (BIL) of 650 kV. To include some margin, a typical design may be done for ~700 kV.
Air is used with increased pressure and larger insulation gap.
The electric field stress is highest on the surface of the conductor. We've chosen a closest to optimal diameter ratio of OD/ID = 2.7 = e.
Single-phase enclosures ensure the most ideal coaxial arrangement and best use of the air insulation space.
The key items for mechanical design are:
Hivoduct cable components portfolio:
The set of cable components can be freely combined with each other to provide the required product characteristics.
Proper selection and combination is part of the engineering process.
An on-line configurator helps to choose the right options and technical parameters and instantly provides a budgetary price.
For underground or enclosed applications, it is key to require the least space and least effort for conduits, laying, digging and trenching. The main design features to enable this are:
See more details in the "products" tab.
Hivoduct pressuriuzed air cables in pre-casted concrete trench or in a tunnel below pavement or in a pipe underground.
Trenching can be avoided using Micro-tunneling technology with concrete pipes. The roller system allows quick installation and removal.
Lowest outside electromagnetic fields
Single-phase coaxial conductor arrangements with conductive and grounded enclosures have the general advantage of featuring the least outside electromagnetic fields:
The electric field is completely shielded by the closed, conductive, grounded aluminum enclosure.
The magnetic field emissions depend on the actual current flow through the conductor. Enclosures are conductive and solidly grounded. Actual current in the conductor induces similar enclosure currents which will cancel out most of the outside magnetic field. This is why the outside magnetic field of pressurized air cables always remains significantly lower than that of HV lines or cables.
See an example simulation here: G10 triangular arrangement with 3*3150 A at 50 Hz, layed 1 m underground. Green area is 1 to 3 microTesla. Red area is 10 microTesla.
The remaining outside magnetic fields from pressurized air cables fade away within a few meters. Therefore, if laid e.g. 1 m below ground or deeper, a very low limit value of 1 uT may be achieved during average current flow on ground level directly above.
Hivoduct pressurized air cables are the best technical option if low outside electromagnetic fields are required for any given current or power transmission capacity.
 Innovation to reduce the environmental impact of HV transmission lines: High-efficiency pressurized air cable technology. W. Holaus, IEEE Workshop 12/2021
 Pressurized air insulated cables: A novel, compact GIL design for 12 kV- 420 kV: Design, Simulation, and Test results. W. Holaus et. al. CIGRE 08/2022
 Test- und Betriebserfahrung mit einer 145-kV-Druckluftkabel-Pilotanlage. W. Holaus, GIS Anwenderforum Darmstadt 09/2022
 Pressurized Air Cables - a new technology for sustainable energy transmission 12 kV - 420 kV. W. Holaus, VDE Berlin 11/2022
Gas Insulated Lines (GILs) are coaxial arrangements of a tube-shaped conductor centered inside a tube-shaped gas-tight enclosure. An insulation gas at elevated pressure is used to isolate the conductor from the enclosure. They have a long history in high-voltage transmission and its characteristics are well known.
Hivoduct aircables are different - by using air for insulation, introducing flexibility like other cables, having a different design, different manufacturing and installation methods. However, the physical working principle is the same - therefore some background information below for readers new to this topic.
For detailled information: Hermann Koch: Gas Insulated Transmission Lines ISBN 978-0470665336
A good summary on GIL technology and characteristics is provide here by Mike Rycroft:
Several manufacturers offer GIL products
Existing GIL products use SF6 or SF6 gas mixtures or other specific synthetic insulation gas mixtures to provide the main insulation between the high-voltage conductor and the enclosure. Research for a variety of gas alternatives is ongoing.
Traditional GIL designs use on-site welding of enclosure tubes or bolted flanges to connect single tubes into extended GIL sections.
Since the advent of high-voltage DC transmission, evaulation of GIL technology for this application case is ongoing:
Studies on electromagnetic fields
There are numerous studies on electromagnetic fields of different transmission technologies. It is a common understanding that GIL technology offers the least outside field emissions, as they use a conductive and solidly grounded enclosure. Example: http://www.emfs.info/sources/underground/gil/