Insulation design

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. 

Mechanical design

The key items for mechanical design are: 

• Cope with air pressure up to 9 bar to cover a wide range of rated voltage levels.
  Note that multiples of this pressure are specified for bursting pressure type tests (e.g by SVTI, EN 12952, EN 12953, EN 13445, ASME, etc.)
• Use less sealings and a double sealing system for air tightness over the lifetime of the product.
• No welded flanges: As gas-tight welding of thin-walled aluminum tubes is a tricky process (in the factory and specifically on-site) and non-destructive disassembly is not possible, we avoided welded flange connections.
• No bolted flanges: Typically, a bolted GIL flange requires 16 bolts + 32 washers + 32 nuts.  This sums to a tremendous amount of small parts and manpower requirement for assembling an extended GIL.
  -> Example for 1000 m of a GIL with bolted flanges:
  3 phases * 1 Flange/5m * 1000m * (16+32+32) = 48'000 items.
  In addition, checking the proper tightening of each bolt is a process prone to failure.
  Therefore, we avoided bolting for Hivoduct flanges.
• Use flexible elements so the Hivoduct can be laid e.g. along a road or tunnel and follow the bending and up's and down's of the road.
  This includes flexible angle pieces and compensators to handle thermal expansions during operation.
• Seal the flanges against water or dust ingress.
• Enable quick assembly and disassembly for re-arranging, extensions, maintenance and repair. A feature specifically useful for test- or lab installations where frequent non-destructive disassembly and rearranging is required.

Hivoduct components portfolio: 

• Straight busduct:   Any length <= 5m
• Fixed angle piece: 25°, 45°, 90°
• Variable angle piece: Flexible angle +- 10° in all directions
• Length compensator:  dL = 0-70 mm
• Lateral dismantling piece: To remove a section of a Hivoduct line for maintenance while the rest can stay in place.
• Bushing interface: To connect to High-Voltage lines
• Fixation elements:
  To install, align and fix the 3 phases on-site
• Air filling and pressure monitoring system
• Assembly and disassembly tools

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.

Installation design

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: 

• Low losses: To avoid spacing between phases for thermal reasons and to avoid the need for parallel lines or cables.
  Low losses are engineered by providing sufficient conductor cross section. 
  
• Compact dimensions: To use less space and require smaller trenches or conduits. Compact outer dimensions are achieved by avoiding flange bolts.
• Long sections of Hivoduct can be inserted into confined spaces such as pipes, troughs, conduits or below tunnel pavements without the need for access for workers or machinery. 
• Minimum bending radius. Hivoduct can have any bending radius (even < 1m) and therefore go around corners or follow roads or other parallel infrastructure.
  Note: Engineering effort increases for "curvy" installations . 
• Passive internal arc protection. The rare event of an internal arc fault should not be noticeable on the outside. This enables Hivoduct to share infrastructure space with roads, railways, communication infrastructure, conduits, tunnels, bridges and others.
• Fire protection. Hivoduct are made of 99.8% aluminium. Only the spacers are flammable material. If the pressurized air leaks, it is still air. This enables co-location with critical infrastructure.
• The (optional) supply duct may be used for low-voltage cables, fiber-optic cables, air supply or others in parallel to Hivoduct.

See more details in the "products" tab.

Application examples: 

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.