Low voltage limiters combine a metal oxide surge arrester without gaps and a thyristor-based low voltage limiter ensure effective protection of personnel and equipment in DC traction systems.
ABB's new generation HVL (hybrid voltage limiter) low voltage limiters, that combine a MO (metal oxide) surge arrester without gaps and a thyristor-based low voltage limiter, have been developed to ensure effective protection of personnel and equipment in DC traction systems. They fulfil all the requirements for voltage limitation while reducing touch potentials to harmless levels. * The need for low voltage limiters - in DC traction systems stray currents can flow in the ground from the rails to other metallic structures (such as drainpipes, bridges, fences and so on) where they can cause corrosion and considerable erosion of the metal.

To avoid this problem, it is common practice not to have direct earthing of the rails where possible.

However, this can lead to unacceptable potential rises and too high touch voltages, both in normal train operation but especially in the case of a failure such as a broken overhead line or derailed pantograph.

In such installations, protection for personnel has to be provided by the use of low voltage limiters (LVL).

* The need for over voltage protection - in addition to the touch voltages caused by failure in the supply system, impulse over voltages caused by lightning or switching can occur on the rails.

These impulse over voltages can travel along the rails to the substations and damage the electrical equipment.

So in addition to personnel protection, reliable over voltage protection has to be provided for the electrical systems.

* The ABB solution - two devices in one - ABB's solution has been to develop a new device - the HVL (hybrid voltage limiter).

The HVL combines the required personnel protection against touch voltages with protection of electrical equipment against lightning over voltages.

The HVL contains a metal oxide (MO) varistor in parallel to two anti-parallel connected thyristors.

Over voltages generated by lightning with a duration of some 10 microseconds are limited by the MO-varistor.

Over voltages with a time duration of 0.4 milliseconds and longer are limited by the thyristors to a voltage below 3V.

The HVL is completely encased in silicon.

* About ABB - ABB is a leader in power and automation technologies that enable utility and industry customers to improve performance while lowering environmental impact.

The ABB Group of companies operates in around 100 countries and employs about 104,000 people.