IET Publication: Guide to Earthing and Bonding for AC Electrified Railways Authors Dr R D White and Allen McDonald - week 2

Application of the book ‘Guide to Earthing and Bonding for AC Electrified Railways’?

‘Guide to Earthing and Bonding for AC Electrified Railways’?is due to be published by the IET 2022 in Spring 2022

Week 2 – Case Studies 1, 2 & 3

?Hi everybody this is week 2 of a series of articles based on the application of earthing and bonding requirements and these requirements have been defined and explained in the book.

This series of LinkedIn articles are designed to show the necessity for good Engineering Management and in particular the integration design processes to enable the safe and reliable operation of AC electrified railways.

?Seven specific case studies are provided these are actual examples where railway projects have not identified the requirements for integration of the earthing and bonding of railway systems. Each case study identifies the causal factor (Hazard cause), the potential Hazard (e.g. danger to humans), the Hazard consequences and the design recommendations.

?Case Study 1: AC earth currents and induction into lineside cables

?The characteristics of induction of an earth return system are addressed in Sections 2,3 & 5 ‘Guide to Earthing and Bonding for AC Electrified Railways’.

?Most railway networks operate their own data communication and telecommunication system with this being laid in trunking beside the running rails. With the development of new railway digital networks, the design will include optical fibres and copper circuits. The railway system produces high levels of traction current which flow in the electrification system, this flow of current is responsible for producing electromagnetic interference into its own and third-party telecommunication networks which are parallel to the railway.

During the commissioning of a 25kV electrified railway, the telecommunications systems failed catastrophically due to induced voltage on the lineside copper circuits, causing the surge arrestors on long lineside cables to exceed their rating and operate due to this excessive induced voltage. This induction also created a touch voltage Hazard for railway maintainers working on the systems.

The causal factor established that the designers failed to apply the correct calculations of the mutual coupling which includes an earth return system. The results of the design modelling were initially overly optimistic and recommended copper cable lengths that were 3 to 4 times greater than they should have been.?This design requirement is a complex modelling task undertaken by specialists, and this is necessary to ensure that the individual railway electrical systems are effectively and safely integrated.

?The engineers investigated the failures and subsequently undertook correct induction calculations using earth return and proposed modifications to shorten the length of the copper circuits. This required the addition of data isolation transformers and enhancements to the immunisation of lineside conductors against the effects of the mutual coupling from the 25kV electric traction system.?The modifications significantly improved system reliability by reducing the susceptibility of the axle counters to induced voltage disturbance.

?The consequence of this system failure was that the commissioning and opening of the railway were delayed by 3 months. Therefore, the commercial loss included the cost of a modification to the lineside cables and the loss of income revenue over 3 months. The project suffered from reputational damage, loss of revenue, increased capital and operational costs.

?Case Study 2: High impedance 25kV earth fault to a non-conducting bridge deck

?The characteristics of non-conducting bridges are addressed in Section 5&7 ‘Guide to Earthing and Bonding for AC Electrified Railways’.

?Where there is inadequate spatial clearance between AC overhead lines and non-conducting bridges, there is a risk of 25kV earth fault and therefore the requirement to provide additional bonding. Some railway administrations fit additional external longitudinal copper or steel strips that are bonded to the traction return system. These fittings are usually installed on non-conducting tunnel walls, concrete decks of overline bridges. This provides mitigation against structural damage from a dewired pantograph, a falling live overhead conductor, should there be a breakage in the OCS, or due to birds roosting on the overhead contact system.

The most likely hazard to the overline bridge is when a traction return bond between the bridge copper strips to the return system (rails) ?is inadvertently removed. This can occur during track maintenance or re-railing and where there is an adjacent live road. The causal factor is usually a dewired pantograph or more likely a bird strike that created an earth fault to the non-conducting tunnel lining or bridge deck. With the intended conductive return path has been disconnected, the voltage of the bridge can reach typically 2kV- 5kV, and there is subsequent damage to the structure including fire, damage to utilities and conductive services and danger to humans.

Where such a failure occurred an ageing DC conductor rail metro system was operating on the overline bridge. Longitudinal services (ageing, conducive sheathed cables) of the metro system were installed on cable trays/hangers on the inside of the parapet of the overbridge.

The most probable causal factor was a bird roosting on the overhead line, where there was insufficient clearance to the bridge deck. The bird enabled a 25kV fault current into the overline bridge, the current took the best conductive path to earth, and this was via the conductive armour of the cables to a signalling equipment room and the ‘earthy’ running rails of the third-party infrastructure. The damage to the infrastructure of the DC railway was significant, with several track circuits damaged and requiring replacement. This failure also created a touch voltage (indirect) hazard for railway maintainers working on the electrical systems connected to the bridge.

With all accidents, there are often multiple causal effects and in this situation that was the case. Firstly, bonding a bridge requires redundancy such that if a bond becomes disconnected the protection of the infrastructure is not compromised. Secondly, it is NOT recommended that conductive assets of a third party are mounted on an overline bridge, conductive assets should therefore be isolated from the bridge conductive structure. Thirdly it was very inappropriate for a bird to roost under the bridge.

The commercial loss included the replacement of track circuits on the DC metro.

Technical Recommendations

·??????Where bridges are required to be bonded due to the presence of 25kV overhead lines, the bonding of the bridge should have redundancy;

·??????Where bridges are required to be bonded due to the presence of 25kV overhead lines, the electrical services and metallic utilities on the bridge should not be bonded to the conductive bridge parapets.

. Operation and maintenance procedures are required that will ensure that bonds are not damaged or disconnected inadvertently.

?Case Study 3 Earth Potential Rise (EPR)

The characteristics of the earth potential rise of an HV Tower is addressed in Section 3, 5 and 9 ‘Guide to Earthing and Bonding for AC Electrified Railways’.

?HV systems (400kV Power lines) that are close to a railway can have a disturbing effect, particularly on low voltage signalling and control systems, following a short circuit or a flashover.

In a storm an AC electrified railway network suffered 1000s lightning strikes which resulted in a significant number of the electrical system and signalling system failures. Part of the railway is in a long tunnel (5 km), and there was an electrical feed (‘clean’ earth) from the tunnel portal to the signalling equipment room 2.5km inside the tunnel.

The causal factor was a 400kV tower 50Hz earth fault. The earth fault was caused by lightning striking the HV tower causing the insulators on the HV towers to flashover, creating a 50Hz EPR (Earth Potential Rise) throughout the mountain and in the tunnel itself. This EPR created a potential difference between the tunnel structure earth and the signalling ‘clean’ earth which was referenced to the substation at the tunnel portal.

This voltage difference was sufficient to cause local flashovers in the signalling equipment room and thereby disturb the control of the axle counters.?This voltage difference also created a touch voltage hazard for railway maintainers working on the systems.

Engineers investigated the failures subsequently installed overvoltage protection devices between the tunnel structure (rail) and the signalling ‘clean’ earth.?This did mitigate the touch voltage hazard to maintainers.?However, it did not necessarily wholly mitigate the risk of disturbance to the signalling axle counters.

?Technical Recommendations

·???For human protection the instalment of an overvoltage protection device between the signalling ‘clean’ earth and the traction or tunnel earth.

?Recommendations for future earthing options

·???The implementation of an integrated earthing system (traction return, structure earth, LV earth and signalling earth) would provide significant improvement of immunisation against disturbance from lightning and switching transients;