ABB's new HVL (hybrid voltage limiter) low-voltage limiters, which combine an 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.
In DC traction systems stray currents can flow in the ground from the rails to other metallic structures (such as drainpipes, bridges or fences) 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 and 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).
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.
ABB's solution has been to develop a new device - the HVL (hybrid voltage limiter) that 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 with 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.
For more information, visit www.abb.com"