Articles

Large Metro Rail Projects – Earthing and Bonding Case Study

Safearth were recently engaged to identify and manage the earthing and bonding risks on several large metro rail projects. They developed detailed earth system designs for bulk power supplies and traction systems and created high-level strategies and philosophies for the project. This required a comprehensive understanding of the issues and complexities involved with underground tunnels and working collaboratively with all key stakeholders.

Managing earthing related risk in the rail environment requires approaches significantly different to typical HV power systems. For example, hazardous voltages may occur under fault conditions but also due to the flow of traction currents under normal operations. The current draw from moving trains combined with currents injected by regenerative breaking, can result in power system operation that not only changes with time but also changes physical locations. In many ways, it is like having substations and earth faults following a travel itinerary or timetable.

The animation below represents the rail voltage rise along a 15km section of twin tracks showing the current draw and movement of trains. Each of the blue lines represents stations where trains are required to accelerate and decelerate. The yellow lines represent the locations of trains with train movements in both the city bound and country bound directions.

It may be seen from the animation that as trains accelerate and motor between stations there is a corresponding voltage impressed on the rail. It may also be seen that the rail voltages change polarity as trains decelerate into a station using regenerative braking. The system rail voltages are continually changing depending on the operation of the traction system. This animation represents one scenario and would change based on train frequency, travel times, station dwell times as well as equipment degraded modes.

In worse case scenarios, these voltages and currents can result in hazardous touch voltages, electrolysis and interference, low frequency induction (LFI) and electromagnetic interference (EMI). Understanding these system currents and associated rail voltage rises, for normal operation, abnormal operation (such as degraded modes) and under fault conditions, is necessary to develop sound earthing and bonding designs to protect people, plant, and equipment.

Typical earthing approaches applicable to above ground meshed power systems are often difficult to implement in constrained tunnel environments where parallel exposure lengths are significant and available real estate is minimal. This combined with the likely presence of people adds considerable complexity to earthing designs.

The Challenge

Large metro rail projects are situated in high-density CBD environments and typical earthing strategies cannot be easily implemented due to the highly constrained environment. In addition, earthing, bonding, and electrolysis often have competing requirements. For example, earthing mitigation measures often demand low resistance between equipment and earth to facilitate the passage of AC current whereas electrolysis mitigation measures often demand high resistance.

To manage stray and leakage currents, it is necessary to provide separation between systems and this requires electrical insulation as well as physical separation. This is difficult in underground tunnels where systems are highly interconnected, in close proximity, and offering limited space. Trying to find the right balance in these projects was complicated, requiring careful planning and reassessment to limit leakage current. Adequate separation between systems was of key importance, as stray current or voltages could be transferred from one system to another resulting in electrolysis or harm to people or equipment.

Separation and insulation are difficult when it is required in structural components such as structural steel work and reinforced concrete structures. It is relatively easy to install an electrical isolation section in a fence, however, to achieve isolation in large structural members requires special consideration. Insulation requirements must also be balanced against earthing requirements associated with electrical equipment. It is often necessary to achieve low resistances to maintain safe voltages and ensure correct operation of protection equipment and these requirements are often in direct conflict with one another.

Many of these systems are cross discipline, designed and constructed by different parties and possibly under different contractual arrangements and as such managing the interfaces with close coordination was critical.

The Solution

Dynamic modelling of the traction systems was important to understand the earthing and bonding performance under a range of power systems and traction system operational modes. This understanding allowed for the development of earthing and bonding philosophies that guided the detailed designs and interface requirements.

Working closely with designers throughout the design process was essential to make certain that cross interface issues were coordinated and managed and to ensure designs were consistent. It was also important to understand that interfacing was a two-way street and often requirements and constraints from other packages required consideration and inclusion in the earthing and bonding designs.

Communicating with the construction team was also vital to ensure designs were correctly understood and implemented. Staged commissioning testing, not just at completion but progressively throughout the project construction phases, was undertaken to confirm design compliance and to confirm earthing and bonding strategies were effective, particularly across interdisciplinary or contractual boundaries.

Final commissioning at the end of the project will be critical as it will be the final confirmation that the design objectives specified in the initial design phases have been correctly implanted and achieved. This confirmation is often a key requirement and prerequisite to energisation and critical to the successful delivery of the project.

Conclusion

The projects are ongoing and ensuring that the earthing systems are correctly configured on these large-scale metro projects is paramount to ensure public safety and the reliability of rail systems and services. This requires careful consideration of earthing and bonding risks throughout the entire project life cycle.

Safearth’s continuing commitment to these projects is focused on the successful coordination of multi discipline packages across multiple project teams to ensure consistent implementation of strategies, not just during the design phase but also through the construction phase and final commissioning.

Safearth consultant: Lachlan McKerrow – Principal Engineer IL, SME Rail