As cities grow, energy needs increase. Simultaneous development associated with population growth means that over time, real estate increases in cost while diminishing in availability.
This creates the emerging trend in the energy industry, which is moving away from vast, sprawling outdoor substations and overhead transmission towers, to the redevelopment of existing sites with more compact and enclosed substations, and moving to underground transmission and distribution circuits.
To help strengthen and provide power supply security for Auckland’s CBD and the transmission network north of Auckland, national grid operator Transpower and distribution utility Vector initiated the construction of two new substations at Hobson Street, Auckland.
The site’s spatial and associated constraints meant that major innovation was required across many facets of the substation’s complex design. Many conflicting issues in substation design required new and original approaches to solve and mitigate risks around seismic performance, fire, site security, plant layouts, HV cabling and 3D drafting.
Aurecon, as lead project design consultant, was responsible for the key aspects of design and technical construction support for the two new substations.
Hobson Street Substation, once a power generation facility for powering Auckland’s early public transport system – trams, has seen significant development over the past 100 years. The new Transpower substation at Hobson Street, forms a vital link in the upgrade of the 220 kV grid to cater for growing power demand north of Auckland.
The site at Hobson Street was an ideal location for a new GXP substation, being close to the load centre of Auckland CBD, with the existing Hobson-Penrose cable tunnel portal at the site, along with an existing zone substation on site.
The use of Vector’s Penrose to Hobson Street Tunnel as an essential part of the new supply route, allowed both Transpower and Vector to reinforce Vector’s supply into the CBD.
The site overall was approximately 3450 m2 of which around 50 per cent was covered in existing buildings, Hobson Street car park and garden area. The remaining 50 per cent of the total site area was allocated to the new development including:
As the existing site was an operational Vector sub-transmission and zone substation, it was required to be kept live throughout the redevelopment.
The project presented many complex challenges, including:
A description of particular design solutions and features incorporated to achieve improvements in the key areas of maintainability, operability and safety follows.
The close proximity of an adjacent multi-story hotel, existing critical live substation and CBD/public spaces surrounding the site and potential 15 storey development directly over part of the site, required innovative explosion and fire control measures.
While transformer failure is a relatively rare occurrence, the consequence can be extreme. When a transformer fails an explosion and/or fire can result.
To assess the appropriate transformer enclosure design to cope with the pressure rise and temperatures associated with a transformer explosion and fire, Aurecon undertook a detailed assessment of the behaviour of transformer explosions.
The results of this assessment fed directly into the enclosure design, where the key outcome was the process of how to classify transformer fires and explosions. Once that classification was detailed and explained, a method for mitigating the effects of transformer explosions and fire could be defined. Explosions from transformer failures were classified as deflagration explosions and further classified as aerosol or mist explosions.
Although an aerosol explosion appears violent, it is considered to be a low pressure explosion compared to a detonation. Even though the explosion is relatively low pressure and prolonged compared to other explosions, if the pressure is not relieved and exceeds the strength of the bounding walls, damage and creation of projectiles can result, causing even further damage.
The usual way of treating such an explosion is to vent the explosion such that a build-up of pressure does not occur. Industry standards1 define the relationship between available vent area to release pressure build-up and the strength of the walls for the enclosure, which was applied for the transformer enclosures at Hobson Street.
The challenge with the Hobson Street site and transformer enclosures was to find the right balance between feasibility of wall strength and open area available for pressure relief venting. With the transformer enclosures being small areas encircled by other buildings, it was challenging to finding enough spare wall area to enable adequate venting while keeping a reasonable limit on wall strength and not venting into public areas or on adjacent assets.
Further to this, the vented areas selected were in architecturally prominent locations on the buildings, so required visual treatment, whilst also being necessary to keep both rain and nesting birds from entering; in effect needed to be covered, whilst providing natural ventilation to the enclosures and also needing to be demountable to facilitate transformer removal in future.
The innovation applied to solve this problem of conflicting constraints was to develop a design of demountable steel structures with hinged louvre panels rated to open at a threshold pressure associated with a rapid pressure rise from a transformer explosion.
The concept allowed for natural ventilation to the enclosure, whilst keeping birds and weather out, all fabricated in demountable steelwork frames fixed back to the concrete structure. The hinged louvre panels provide the free open area required to alleviate pressure build up, where the individual louvres and whole panels must resist the design blast pressure without failing. The panels are fixed to the primary steel frame with hinges along one edge to allow the panels to open outwards fully, and with breakable shear-fixings on the other edge. The shear-fixings are to be designed with a breakable bolt such that when the pressure on the panel reaches a pre-defined level (below the blast pressure), the fixing will break and the panel will swing freely outwards. This is because the full free-area of the wall is required for pressure venting to achieve the designed blast pressure.
A number of methods were employed to mitigate the effects of fire in the enclosure design. Of these methods, one was to install a high expansion foam fire suppression system in the transformer enclosures at Hobson Street substation. The system includes the first set of units of this size to be put into an electrical substation in New Zealand and operates above New Zealand building code standards for fire protection.
The basis of operation for the foam suppression system is upon activation, the unit uses fire water mains pressure, combined with foam concentrate to drive a diffuser that creates the foam which is rapidly dumped on the fire. The foam effectively smothers the fire, by reducing free air movement necessary for combustion, water content of the foam turns to steam which dilutes the oxygen concentrations and cools to levels that no longer support combustion, also prevents fire spread through insulation qualities of the foam.
The added benefit of choosing a foam system over other fire suppression systems (such as water mist, water deluge, gas suppression), is that there is less potential for oil contaminated, hazardous run-off due to the reduced volume of water present in the foam (due to high expansion ratios), also due to the fact that the foam is partially self-supporting it does not require a totally sealed area to contain the foam.
Similarly, the enclosure does not need to have air tight boundaries as opposed to inert gas systems. This allows the other conflicting constraints to be met, including the ventilation of the enclosures. Further to this the foam itself, which is relatively self-supporting, its efficacy of suppression is not affected by wind and openings. The foam system allows suppression of transformer fires, whilst not compromising on the other criteria required for the design of the transformer enclosures at Hobson Street.
The detection system requires two out of three positive sensors to operate before activating the foam system and tripping the transformer and radiator fans. The electrical conductivity of the foam is low, but because of the voltages present in the transformer enclosure, the design of the system directly trips the transformers prior to foam release.
To help coordinate disciplines and visualise the design in such a way as to avoid complicated site construction issues, 3D drafting and modelling software REVIT and NAVISWORKS were innovatively used together throughout the substation design. The combination of the various 3D design outputs when incorporated into NAVISWORKS permitted a whole site and walk through views, internally and externally, inclusive of equipment layouts, to assess the impact of the tight construction tolerances, installation conditions and risks associated with maintainability, operability and safety prior to construction.
The route complexity and the number of HV cable circuits proposed to be run in the project required a new approach to cable layout. After a number of iterations to ensure the correct methodology was used, significant progress was made in laying out cable routes, individually in 3D with the prescribed minimum cable radii.
This was of significant benefit to highlight and visualise the numerous three dimensional cable transitions and bends, effectively testing and checking the installation fit prior to installation. This gave the client confidence in the design and also an opportunity to engage with cable installers at an early stage (whilst in design and prior to construction) to work through installation methodologies for the cables and obtain their feedback.
The redevelopment at Hobson Street has seen a major transformation of the site into a grid exit point and a sub-transmission substation as part of the Transpower and Northland (NAaN) project. The NAaN project was recently completed early 2014, where Hobson Street substation was expected to be on the project critical path and one of the final pieces of infrastructure to be completed to commission the NAaN project. As such, Hobson Street was designed, constructed and commissioned under a tight schedule, where design and construction overlapped. The redevelopment project was completed ahead of schedule and ahead of other sections of the NAaN cable link. Aurecon is pleased to be able to share in our client's achievement with the successful delivery of this project.
1. Specifically, National Fire Protection Association (USA) standard on explosion protection by deflagration venting - NFPA 68.