Mario Garsi, Business Development Engineer, TT Electronics
High performance and higher efficiency are common trends in most technologies, but particularly in transportation applications where the potential for significant environmental impact on a global scale is high. Greener, cleaner ideals are taking the lead in defining new goals for how technology must balance performance with environmental considerations. Electric vehicles and next-generation electric trains are working to meet the common goal to minimise their carbon footprint on Earth.
It’s an effort that demands attention to every facet of train design and development – with new systems facing constraints of size, weight, and integration with legacy systems. Following a common trend demonstrated successfully by high-speed national trains, railway scenarios such as subway, city, and intercity light rail systems are focusing on optimising all performance aspects of their traction and auxiliary units. This means tapping into advances in optoisolators that enable smarter designs and improved performance – scalable across fleets of trains, diverse routes, and broad geographic regions.
Getting familiar with optoisolators
Energy efficiency is all about the effective transfer of electrical signals.Optoisolators play a role here, sitting between circuits as a means to transfer information. Importantly, they provide high isolation voltage between the two edges of the transfer – for example, a low voltage microprocessor communicating with a high voltage board used on an electrical engine. Optoisolators consist of at least one LED emitter and a receiver (sensor) and achieve communication by means of light in air – enabling inherent high voltage insulation and functioning in contrast to a mechanical part connected by copper wiring.
Consider that typically, a design has wide voltage differences between emitters and receivers. For example, the emitter may operate as the compute centre of the vehicle, making the calculations required for engine control, acceleration, or deceleration. This part of the system works at low voltage, such as 3.3, 5 or 12 volts, as it simply does not demand significant amounts of power. On the other end of the design, the working engine today requires up to 1,500 volts, which may rise to 3,000 volts in the future.
Optoisolators not only handle this voltage disparity but have been developed to withstand the rigorous, less controlled environmental conditions common to all types of railway and rolling stock scenarios. Solar panel applications offer a highly effective proof of concept for this level of performance in that they are devices that must function with unwavering reliability in conditions of wind and rain, hot and cold shifts in weather, heavy shock and vibration, and other external factors such as dirt and debris.
Thelevel ofIP resistance, or certified resistance to water entering the device from external sources, developed in solar panel applications validates these components as suitable for long-term, outdoor performance.
Taking a look at light rail traction and auxiliary units
The basic block of subway or light rail traction and auxiliary units features a subway locomotive traction drive or auxiliary system primarily composed of three parts: traction/auxiliary inverter; traction motor/auxiliary unit; and traction control system/auxiliary control unit. An IGBT (Insulated-gate Bipolar Transistor) switch operates “On” and “Off” settings and is mounted into the high-speed inverter system. Optoisolators, including an optic fibre transmitter and receiver, separate low voltage data from high voltage systems within the train via circuit coupling.
As underlined in the above block diagram, the incoming next-generation platform will be based on the 3000 Vdc or higher bus voltages to improve efficiency and save energy. High Voltage High Speed (HVHS) isolators will be key component for these high-performance platforms working outdoor in harsh environment and will become even more so in the future.
The main components of an optoisolator are:
· An emitter, either LED or VECSEL (vertical external cavity surface-emitting laser)
· A receiver such as photodiode, phototransistor, or photo IC
· Dielectric material
· Connecting leads
· High voltage plastic encapsulating housing
The main features of optoisolators working in this environment are:
· Data transfer rate up to 2 Mbit/sec (in some cases even more)
· Creepage distance path 24mm
· Operating temperature -40 °C to +100°C
· Voltage spike noise immunity up to 30KV/us dv/dt
· Voltage suppression from 10 kV up to 30 kV
· UL and VDE certificate (optional)
The electrical parameter used to measure the feature mentioned above is called CTI (Comparative Tracking Index).This specific Index measures the electrical breakdown (tracking) properties of insulating material. The tracking measurement is an electrical breakdown on the surface of an insulating material wherein an initial exposure to electrical arcing heat carbonises the material. Carbonised areas are more conductive than the pristine insulator, increasing current flow, resulting in increased heat generation, and eventually, the insulation becomes completely conductive.
For this reason, to withstand the new boom in light rail applications, manufacturers of optoisolators started to develop new products able to achieve CTI ≥ 600V – the most optimal rate achievable today.
Click image to enlarge
Figure 2: Optoisolator Image
Keeping on track for energy efficiency
In addition to all of these specific technical factors, it has become increasingly important to offer long-term outdoor performance. Lab validation is no longer enough, and optoisolators must perform in extreme physical settings with high humidity and many other environmental factors.
Solar panel applications have validated the environmental capabilities of current optoisolators, acting as a groundwork application for greater use in light rail settings. As short distance trains such as subways and underground city transportation follow the lead of national rail systems, they are quickly working toward more efficient, higher voltage engines that offer greater efficiency on a large scale. With a 3,000-volt engine – instead of 1,500 – rail operators can move the same number of people and goods the same distance while using less power consumption based on optimised engine designs.
Taking the lead from solar panels and now other next-gen energy applications such as windmills, designers can tap into optoisolators proven for reliable performance in rugged environments with little maintenance. Windmills on an ocean platform or a light rail system with non-stop demands for moving people and cargo…each of these scenarios benefits long-term from proven components designed to perform non-stop in variable, outdoor settings. High efficiency equals reduced consumption, whether it’s the consumption of power or maintenance resources – a positive, visible, and sustainable trend in high-speed trains as well as commuter trains common in Europe and Asia, and part of North America.