PROTECTION of staff from voltage differences, and of electrification equipment from the effects of lightning strikes and stray current, is being obtained by SNCF using ABB’s hybrid voltage limiter.
It is common practice to avoid direct earthing of the rails in DC traction systems, to reduce the risk of stray currents causing corrosion of nearby drainpipes, bridges and fences. However, this can lead to unacceptable potential rises, resulting in high touch voltages in equipment which is not used to carry current, such as support masts. Unless low voltage limiters are fitted between conductive structures and the traction current return path, railway staff can be at risk in the event of failures in the overhead line or pantographs. In addition, reliable over-voltage protection is needed to protect electrical equipment from voltage surges from lightning strikes and switching.
The HVL has a metal oxide varistor as a surge arrester, connected in parallel with two anti-parallel thyristors which reduce to harmless levels the touch potentials to which staff could be exposed. Over-voltages generated by lightning strikes with a duration of 10ms are limited by the MO-varistor, while those longer than 0·4ms are limited by the thyristors to less than 3V.
Sealed in a relatively small 3·8 kg package suitable for outdoor installation, the HVL is effectively maintenance-free, and will continue to provide protection without needing to be reset after an activation. The first HVL installation was at Montaur in France, where it has replaced equipment which was requiring monthly attention from SNCF staff.