A SIGNIFICANT amount of the energy used by rolling stock is retained in the vehicle in the form of kinetic energy. During deceleration, this kinetic energy is withdrawn and on electrified lines it can be fed back to the grid. However, this is not the case with diesel and older electric trains which have traction equipment that is not designed for regeneration. Unfortunately, this also partly applies to modern electric vehicles, as the commonly-used dc traction power supplies for trams, metros and commuter railways normally cannot fully absorb all of the energy. The braking resistor thus fuels a significant part of the withdrawn kinetic energy.
Considering rolling stock energy, at best 60-70% of the energy used for accelerating the vehicle may be recovered, although the actual value is in many cases below this, particularly for trams, and metro and commuter trains. However, these vehicles operate many diagrams with short distances between stops that have to be covered at relatively high speeds, which result in constant changes between acceleration and deceleration.
The lower the mass of a vehicle, the lower its kinetic energy at the same speed. This also implies that lower kinetic energy requires less energy for accelerating and decelerating the vehicle. Reducing the vehicle’s mass by 10% will result in around 1% less energy being required - significantly lowering operating costs. Reducing the mass of the vehicle is therefore one of the most important objectives when designing or refurbishing rolling stock. This not only applies to the vehicle in general, but also to its different systems, notably the auxiliary power converter (APC).
APCs, sometimes also called static inverters (SIV), are a vital part of any train. They take their power from the traction power supply, usually from overhead catenary or third rail, via a common train line powered by the locomotive or generator set, and supply the consumer of electrical power onboard the train with suitable voltage.
APC requirements are particularly exacting as the mass of the APC is expected to be reduced even though power ratings typically increase. This is especially important for trams, and metro and commuter trains where APCs are commonly installed redundantly due to the need for high availability.
One of the key features of an APC is the galvanic separation between the power source and the load for safety and reliability reasons. There are different technical approaches for the galvanic separation.
A conventional APC consists of a three-phase inverter followed by a transformer to ensure galvanic separation. For such a system, the frequency required by the ac loads - usually 50 or 60Hz - defines the operating frequency of the separation transformer, which is the heaviest part of the APC, often accounting for more than 50% of the unit’s total weight.
The key design idea of a medium-frequency APC is to run the separation transformer at much higher frequencies. With today’s power electronics technology, operating frequencies of a few 10kHz (medium frequency) are achieved even for power ratings of 150kVA or more. This reduces the mass of the separation transformer from a few hundred kilos to tens of kilos and therefore significantly lowers the APC’s total weight. This technology is called medium-frequency galvanic separation.
To run the separation transformer at a higher frequency requires a different APC configuration. A medium-frequency auxiliary power converter APC now typically consists of an input inverter powering the separation transformer with a medium-frequency ac voltage. At the separation transformer output this medium-frequency ac voltage is rectified and fed into the dc link of the output three-phase inverter which directly supplies the connected loads with the necessary voltage and frequency.
Medium-frequency galvanic-separation APCs have also become much smaller. Today the total volume occupied by one APC beneath a train is much less than 1m3, which is only around half the size of a conventional APC. This reduction in size allows the medium-frequency APCs to be standardised, which not only reduces manufacturing costs, but is also one of the best approaches to reduce risk and improve reliability thanks to constant improvements to the units.
Initially, the greater number of components necessary and the increased complexity made the medium-frequency APCs less reliable than conventional APCs. While this was certainly true for the first generation of medium-frequency APCs, developments during the last 25 years have led to significant improvements in reliability. Today’s third generation of medium-frequency APCs can achieve the same level of reliability as conventional systems.
At the same time, efficiency has increased significantly. While conventional APCs hardly reach more than 90% efficiency, due to the compromises necessary to save weight, today’s third generation with medium-frequency galvanic separation can achieve a peak efficiency of 95% or more.
This improvement is a testament to the maturity of today’s medium-frequency APC and has encouraged railway operators around the world to accept this technology, making medium-frequency galvanic separation the technology of choice.
The increased complexity of medium-frequency APCs is an advantage because, apart from allowing standardisation, their small size enables other features to be integrated. For example, our Smart Converter 3 product line offers two separate dc outputs: one to supply the dc loads with a constant dc voltage, and the other to provide the battery with an independently-controlled dc voltage. Additional features, such as state-of-charge monitoring for the battery or even a mode to supply the ac output with battery power, can be integrated easily.
While medium-frequency APCs were first used on trams and metro trains they are now being adopted for commuter trains. A recent example is CAF’s New Generation Sprinter (SNG) commuter EMU for Netherlands Railways (NS). This train uses two or three Smart Converter 3 medium-frequency APCs powering a common train ac bus supplying all electrical consumers with the energy needed. This redundancy helps to maintain train availability.
Key driver
Advances in power electronics are the key driver in the development of APCs with medium-frequency galvanic separation. The technology is mature and service-proven. However, further advances in power electronics will also benefit APCs and particularly with the introduction of new power semiconductors such as silicon carbide (SiC) devices, which will allow new design approaches.
The design of an APC is always a compromise between the lowest possible weight and efficiency. The introduction of medium-frequency APCs made it possible to decrease weight significantly while increasing efficiency. The new power semiconductors can now be used to either reduce dimensions or weight further or to increase efficiency.
Today, the dimensions and weight of medium-frequency APCs are defined to a high degree by the necessary current conducting capabilities and vehicle interfaces. While a further reduction is certainly possible, the potential seems limited and the necessary effort out of proportion. On the other hand, the potential for an increase of efficiency seems more promising.
The requirements for the next generation of medium-frequency APCs are not yet completely clear. Should the dimensions and weights be reduced still further, or is an increase in efficiency a better objective? Perhaps there is no general answer. In any case, further analysis is needed together with the train manufacturers and operators to provide an answer and further improve railway energy efficiency.
