Introduction:
In the realm of modern transportation, electrified railway systems have become a backbone of efficient and sustainable commuting. To ensure the smooth operation of electric trains, robust and reliable traction systems are imperative. However, these systems are susceptible to transient overvoltages that can arise from lightning strikes, switching operations, and faults in the power supply. Surge arresters stand as the guardians of these systems, deflecting and dissipating transient energy to prevent catastrophic failures. This article delves into the multifaceted process of selecting surge arresters for traction systems, encompassing energy and rated voltage calculations, line length dependency, considerations for ungrounded systems, dealing with Temporary Overvoltage (TOV) conditions continuously, as well as handling, and installation guidelines.
With the introduction of ever-accelerating speeds, increasing power of locomotives, phasing out of diesel locos, increase in lightning activities, and newer traction power systems like 2x25kV with new-age projects like NCRTC, NHSRCL, DFCCIL, and many others – protecting the system has become even more crucial and correct selection of arresters is a crucial step towards ensuring reliability.
Energy Calculations:
Surge arresters, often referred to as surge protectors or lightning arresters, function by absorbing and redirecting transient energy away from sensitive equipment. To ensure their effectiveness, the energy-handling capacity of the surge arrester must exceed the potential transient energy that could be introduced during surge events. As per IEC 60099-5, the energy (W) absorbed by the surge arrester during switching surges can be calculated using the formula:
Generally, the total energy W’ is 2xW and is represented in kJ (kilo Joules) as 2 strokes of switching surges are considered. However, in case of traction, the factor may even be 4 in case the locomotive movement within the section is very frequent. Moreover, a safety factor of 25% is generally added.
It must be noted that the Energy capacity required is directly proportional to the line length (L) and inversely proportional to the surge impedances (Zs), and any change in these parameters would result in a change in the energy capacity required.
In recent developments, the Indian Railways have already switched to 42kV Class-4 arresters for locomotives due to the same reasons.
Considerations for TOV and Ungrounded Systems:
In certain cases, traction systems may utilize ungrounded systems to minimize service disruptions caused by ground faults. While ungrounded systems (ungrounded systems with respect to the power transformer) can reduce immediate interruptions, they pose challenges during transient events. Temporary Overvoltage (TOV) conditions can emerge during system faults, causing voltages to rise above anticipated normal levels. Surge arresters for ungrounded systems should be selected carefully, considering the increased likelihood of single-phase-to-ground faults that can escalate into severe system-wide issues. Surge arrester selection must account for the unique characteristics of ungrounded systems to provide effective protection.
For example: In traditional railway systems of 25/27.5kV, the maximum permissible voltage is 29kV. By applying the factor of 1.4 for solidly grounded systems, we arrive at a rated voltage of 29 x 1.4 = 40.6, rounded off to the nearest multiple of 3 as 42kV.
However, in the case of ungrounded systems of the same voltage levels, the factor becomes 1.73, and the appropriate rated voltage becomes 29 x 1.73 = 50.6, rounded off to 51kV.
As per IEC 60099-5, “Because the overvoltage levels on distribution systems are not well monitored and in many cases not well known, they are assumed as the worst case during a system ground fault. Since ground faults are the most common cause of temporary overvoltages on a distribution system and the magnitude of the voltage rise on the unfaulted phase is seldom known, it is assumed for arrester sizing purposes to be the worst case. The worst case voltage rise depends on the system neutral configuration.”
To counter these situations, the NHSRCL has installed an EVT (Earthing Voltage Transformer) to monitor the voltage spikes on the health line in case of a phase-ground fault.
Zero-crossing operation and co-relation to voltage spikes:
New-age circuit breakers have the ability to operate in zero-crossing conditions – which limits the rise of the voltage / avoids the voltage-doubling effect. However, in case of no zero-crossing ability, care must be taken when conducting the insulation-coordination study. The spike in voltages has been found to be multi-fold higher in the case of non-zero crossing breakers vs. zero-crossing breakers.
Handling and Installation Considerations:
Proper handling and installation of surge arresters are pivotal for their effective operation. Surge arresters should be stored and transported according to manufacturer guidelines to prevent physical damage or contamination. It is very common to see arresters being kept on the ground in a puddle of water, which then goes through all kinds of weather conditions while staying there for days in a row. Such situations must be avoided to ensure the longer life of the arresters.
Lastly, Earthing resistances are a make-or-break characteristic for the proper functioning of the arrester. As per RDSO’s TI/MI/0030 related to the installation and commissioning of the arresters – the maximum permissible earthing resistances are 10 Ohms at AT, 0.5 Ohms at TSS, and 2 Ohms at SP/SSP. Any increase in these resistances causes a large voltage drop when lightning discharge current passes through thereby stressing the insulation voltage withstand capabilities of the surge arrester.
Conclusion:
The above just skims the surface of the vast field of arrester selection which takes into account more than 25 factors (as per IEC 60099-5) for electrical and mechanical selection – like Ferranti effect, lightning intensity, number of locomotives present in the section at a given time, frequency of trains, load rejection conditions, among many others parameters.
In conclusion – Selecting surge arresters for traction systems is a multifaceted process that demands careful consideration of energy and rated voltage calculations, line length dependencies, system grounding, TOV conditions, and installation practices. By adhering to these considerations, railway operators can establish a robust defence against transient overvoltages, ensuring the reliability, safety, and efficiency of electrified railway systems. Surge arresters, as silent guardians of modern transportation, play an indispensable role in maintaining the seamless operation of electric trains in an ever-evolving world.
