October 2014 Page 91

October 2014 Page PB

Signalling

Updating rail signalling systems should involve evaluating the DC systems that power them - to ensure they provide the proper foundation for safety, explains Mathias Lang

The growth of the rail system in Britain is leading to expanded routes and system-wide updates and improvements. For many operators this also

represents an opportunity to revisit their rail signalling systems, as well as the DC power systems that support them, to ensure that these foundational systems reliably and effectively improve rail safety. For while the railways are extremely safe, the increase in train

cars and passengers combined with aging equipment and new conductor distractions means there is no room for complacency. Many operators are upgrading their rail signalling systems. An example of this is Great Western’s plans to spend £350 million to upgrade 1960’s-era signalling technology on its main line. The project is part of a broader £5 billion plan to expand services on the main line to accommodate more travellers. A lot has changed since the 1960’s, and

one of the significant technology updates has been to the power systems that are critical to signalling system upgrades. DC power systems consist of a rectifier that converts AC power from the mains to the DC power that is needed by the equipment. There’s also a back-up power that can use batteries or diesel generators. The two are tied together via a controller that monitors system health and battery back-up health and can charge the batteries and switch between mains and back-up power system when needed.

The power behind the signal

October 2014 Page 93

Signalling

Time to changeJust like Great Western, many rail systems made their DC power system technology selections decades ago, and put in place rock-solid, if hard to manage, systems based on silicon-controlled rectifier (SCR) technology that has served railway systems well for many years. At that time SCR power conversion systems were getting competition from switched-mode rectifier (SMR) technology, which had just entered the market and had great advantages in terms of power conversion efficiency, lighter weight and small size. But SCR power systems had better performance and offered the best reliability at the time and became the de facto choice for railroads worldwide. Fast forward to today and SMR power systems are used widely in the telecom and other industries, and technology improvements have eliminated all of the initial performance challenges. This technology is now a viable choice for rail signalling systems thanks to its modularity, redundancy and high efficiency.

Modularity provides flexibility and redundancyThe modularity of SMR systems is due to their small size and light weight and has resulted in system flexibility and cost-effective redundancy, resulting in dramatically improved reliability. This modularity means that if a 100-amp system is needed at a site, an SMR power shelf might provide that power through two 50A rectifier modules that operate in parallel to provide the total required power. A third 50A module could be added in back-up mode in the event that one of the two main rectifiers failed.

This approach contrasts with monoblock power systems, in which a single rectifier provides the entire power load, with redundancy provided by a second, complete 100A rectifier. Generally, a modular system is less expensive. It also adds flexibility that is demonstrated by the ability to expand the capacity of a DC power system to accommodate new equipment, by adding new rectifier modules to a power shelf instead of replacing an entire monoblock system with a new, higher power system.

RedundancyRedundancy is a key issue in maximizing power system uptime. Modular systems provide N+1 redundancy; meaning only one module is required to back up all of the (N) modules in a power shelf. When the system detects a failure in a rectifier module it will immediately switch over to the back-up module without an interruption in service. SMR power modules weigh between four and 10 pounds and each one is hot swappable, so that a repairperson can replace that module without

any downtime. System controllers automatically recognise the new module and it starts to pick up its share of the load immediately. This dramatically lowers mean time to repair (MTTR) to less than 10 minutes.

High efficiency reduces cost and impact on environment All industries today are looking to be good corporate citizens and save on energy costs. Choosing a high-efficiency SMR DC power system technology to use for signalling systems will reduce power consumption and lower the impact on

the environment for rail operators. Best-in-class SMR technology has an AC-to-DC power conversion efficiency level of up to 95 per cent. What this means is that as the AC power comes into the system from the mains, only five per cent of that energy is lost (typically as heat) in the AC-to-DC conversion process. Here’s an example of how high-efficiency can reduce energy loss. Assume the case of a 125V, 100A DC power system that is 85 per cent efficient, but that is operating at 50 per cent of its capacity. That means its output is 6250W (125V x 50A). To generate 6250W of output, an 85 per cent efficient charger requires 7353W of input. That means 1103W are wasted in the conversion process. And those wasted watts are lost in the form of heat; that is nearly 4000 BTU/hour, which will mean additional costs for air condition or air movement system to keep the overall system cool. Compare that energy utilisation to a 95 per cent efficiency power system with the same capacity and you see that you only need 6578W of AC input to get the necessary 6250W of DC output. The losses have been cut to 328W - a reduction in waste of about 70 per cent versus the low-efficiency system.Not only does this save money for rail operators, but the reduced energy consumption also contributes significantly to reducing carbon emissions.

Safety most importantRail safety is the most important job of today’s rail operators, and many are revisiting their decades-old systems and looking to update them. When updating rail signalling systems it’s important to consider the system holistically and to evaluate the DC power systems that power them, to ensure they offer modularity, redundancy and high efficiency - and thus provide the proper foundation for safety.Mathias Lang is segment manager for Rail & Metroat Eltek Visit www.eltek.com

The modularity of SMR systems is due to their small size and light weight and has resulted in system flexibility and cost-effective redundancy, resulting in dramatically improved reliability