There is a strong global trend in research and product development to replace SF6 as insulation gas in high-voltage equipment. We have done this for Hivoduct. The key design features are protected by PCT filing.
Replacing SF6 gas requires a new product design - as it is fundamental to the performance of high-voltage products.
We did the new design from the bottom-up and developed a new generation of pressurized air-insulated Hivoduct products. This was accomplished by incorporating the learnings 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
Bring the high-voltage transmission underground - just like HV cables. Use a "slim gantry" design.
-> Visible only at the ends when it connects to existing lines.
Reduce GWP of the insulation gas
Use air for insulation (80% NO2, 20% O2, dry).
-> Air has no GWP (Global Warming Potential). (SF6 has a GWP of 23000)
Reduce electromagnetic field emissions
Use a coaxial aluminum tube arrangement (GIL) with conductive and grounded enclosures.
-> No outside electric fields. Minimum outside magnetic fields.
Use 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.
Sufficient 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 design. Below are some key points for the development.
Using the reduced generic dielectric performance of air compared to SF6 (this is the main reason why SF6 has been used), we obviously had to compensate with higher pressure, larger gap and smoother, more efficient design.
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. One can see that the insulation gap has to be increased by 1.5 cm to use air.
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.
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 components portfolio:
The set of Hivoduct components can be freely combined with each other to provide the required product characteristics. Proper selection and combination is part of the engineering process.
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 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, 2 Hivoduct G10 systems with 900 MW each fit into an 800 mm tunnel and can be installed and removed independently.
By using the compact and flexible components, Hivoduct can be inserted into non-straight pipes and tunnels.
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 aluminum enclosure.
The magnetic field emissions depend on the actual current flow through the conductor. Hivoduct 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 Hivoduct 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 Hivoduct fades away within a few meters. Therefore, if a Hivoduct is 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 is the best technical option if low outside electromagnetic fields are required for any given current or power transmission capacity.
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 is different - by using air for insulation, having a different design, different manufacturing and installation methods and a different business model. However, the basic 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/