Challenges

Trains are supplied at 1,500 V DC from overhead wires, then the DC current returns to its supply via the train wheels and steel rail tracks. The rails are intended to be insulated from the general body of the earth, however, the insulation is not infallible. Some return current passes back to the traction substation via routes other than rails. This is referred to as 'stray' traction current.

Stray traction current can cause corrosion of steel foundations and pipes at any location where it exits steel structures into the earth.

The risk of stray traction current arises due to the way the Sydney Trains network, HV aerial lines at 33 kV or 66 kV tend to run in parallel with the railway lines. As such, the overhead earth wire of a HV aerial line is a potential route for stray traction current to return to the traction substation.

Aurecon’s challenges were:

To prevent stray traction current from leaving the overhead earth wire via the steel reinforced concrete foundation of the steel pole
Allow the overhead earth wire to perform its two functions, firstly lightning protection of substation equipment and underground cables, and secondly prevention of dangerous voltages during fault conditions in the AC network

A device meeting this objective is on the one hand supposed to be 'blocking' current (the DC traction current) and on the other hand 'allowing' current (the lightning surges and AC fault currents). Any such device must be extremely hardy, as it will be directly in the path of lightning strikes to the overhead earth wire.

Finally, the arrangement needed to offer cost and maintenance advantages to the operator Sydney Trains, at least sufficiently to make up for the disadvantages of timber.

Solution

The solution consisted of using an existing safety device in an innovative way. The product, ABB HVL 120-0.3, (a Hybrid Voltage Limiting device) was developed for personal protection from touch voltages, and protection of equipment from over-voltages. It is typically mounted at the base of a mast for the overhead contact lines in the railway.

In our solution for HV aerial poles, this technology has been adapted to block stray traction current rather than primarily act as a safety device.

The manufacturer was initially doubtful their device was suitable in this circumstance. However, they realised the enhanced capability once the project team had fully described the intention of the innovation, provided concept drawings, and discussed the specifications in detail.

Outcome

Completed in 2017, the Novo Rail Strathfield to Hornsby 2 kV-11 kV Feeder Upgrade, Stage 1 HV aerial upgrade has now been constructed and commissioned into service.

The major benefits from this approach were:

Costly late-night traffic management and transport planning for poles longer than a standard trailer was avoided during construction, with the steel poles arriving in segments
The risk of delays due to discovered geotechnical conditions was taken out of the construction programme, as the steel poles are not directly embedded like a timber pole must be. Rather, the foundation can be constructed ahead of time and the depth modified as required without any danger of miscalculation or human error in ordering the wrong pole length.
The operator-maintainer Sydney Trains has an improved asset with an increased asset life and reduced labour maintenance requirements (no rot or termites)
Aesthetic improvements with a modern, consistent galvanized steel appearance across all structures

For the rail network operator Sydney Trains, this innovation opens up the potential for steel poles to be used for any HV aerial feeder upgrades in their network, as well as for individual pole replacements as required. Sydney Trains has recognised the reduced ongoing maintenance compared to timber poles contributing to a safer, more efficient and more reliable rail network.

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