Global challenges as urbanization and climate change are steering focus on electrical railway systems. Modern electric trains need to be energy efficient, environmentally friendly and comfortable in order to reinforce the shift towards rail.
As identified during the requirements phase the essence of a successful product in the rail sector shall fulfill all of the above.
PTC´s answer to the market demands is the PBX/L product characterized by low weight, low volume, high efficiency and avoidance of environmental disturbance by high frequency noise.
Increased energy efficiency directly leads to lower energy consumption. Reduction in weight contributed linearly to the reduction of kinematic energy which must be spent to move the train. Small volume allows installation in the limited space in modern trains which are becoming more and more complex. Low noise emission promotes vehicle acceptance in urban areas.
PTC relies on a combination of proven technologies and its strength in innovation:
Proven Medium-frequency potential separation:
Typically, the AC as well as DC power supply network in trains must be galvanically insulated to the supply network.
For this PTC employs medium-frequency potential separation. The isolation transformer is connected to a series resonant converter which can operate in the medium frequency range (several kHz) due to its low loss switching abilities. Medium frequency transformers are much lighter than conventional isolation transformers typically operated at 50 Hz.
As the frequency of the electric current increases, the iron cross-section can be made smaller while the power transmission remains the same. The isolation transformer can be operated with an optimized switching frequency. As result the weight and volume of the APS can dramatically be reduced.
Proven Active input filter:
APS directly fed from the catenary network must be equipped with an input filter. On the one hand this filter removes harmonic currents (generated by the converter switching) to avoid conducted emissions that could interfere with the railway signalling. On the other hand, it protects the converter from HV transient overvoltage’s. In usually applied L-C Filter topologies the filter choke must be designed with high inductance to limit the input current caused by supply voltage transients.
PTC expands the L-C topology with an IGBT, which connects a pre-charge resistor in series with the filter choke when the upper limit of the input current or of the filter capacitor voltage is exceeded. With this active transient overvoltage protection, a filter choke with lower inductance can be selected. This again results in reduced weight and volume of the APS. This principle is also used for pre-charging the input stage capacitors during start-up.
Innovation SIC:
With faster switching speed and much lower conduction and switching losses, Silicon-Carbide (SiC) semiconductor devices enables more efficient and compact design of high power rated converters. PTC has successfully developed the PBX/L product to take advantage of SIC technology and at the same time master the challenges of high di/dt and dv/dt associated with fast switching. Volume and weight of magnetic components as the SUC choke, MF-transformer and LVPS transformer were reduced due to higher switching frequencies in the respective converter stages.
The overall efficiency of the APS increases even with increased switching frequency.
Since the high switching frequencies are outside the listening frequency range, high frequency noise (beeping) is avoided.
Due to low losses of the SIC devices the converter topology could be simplified. In contrast in conventional topologies using IGBT´s reduction of switching voltage by serialization of semiconductor devices in order to reduce switching losses is not necessary. So it is possible that a single SIC MOSFET works at the input facing the complete input voltage range. Simple topologies with less semiconductor devices lead to higher reliability of the system.
Daniel Paprosch, System Engineer
PTC Rail OEM GmbH.