Innovative trains
side, with the outputs connected in parallel on the dc side (Figure 3 see p72). This topology makes the solution scalable and provides scope for redundancy (the “M out of N” system). The incoming ac from the catenary passes through a filter inductor before entering the first converter module. Each module of the converter consists of an active front end (AFE) block and a dc/dc converter block (Figure 4 see p72). The AFE block is essentially an H-bridge that regulates the charging of link capacitors. This topology also allows for active power factor control. The cascade topology makes it possible to switch every module independently, allowing the switching patterns of the H-bridges to be
interleaved. If they are interleaved evenly (ie offset by 360 degrees/N, where N is the number of levels), the grid side of the converter sees an apparent (equivalent) switching frequency that is 2 N times higher than the actual switching frequencies of the individual H-bridges.
Lower harmonic distortion This high apparent switching
frequency, combined with the larger number of intermediate voltage levels, leads to a lower harmonic distortion than is possible with conventional traction converters, thereby reducing the need for input filtering. Medium-frequency transformers
provide galvanic isolation between the high voltage coming from the ac grid and the low voltage connecting the load, provide suitable voltage adaptation for the 1.5kV dc load voltage considering the 3.6kV intermediate dc- link voltage level, and help the IGBT (insulated-gate bipolar transistor) modules in the LLC resonant circuits to work in the soft switching mode. As overall size reduction increases the challenge from the dielectric point of view, this aspect has to be studied carefully.
In the Pett demonstrator, all nine transformers share the same oil-filled tank, as does the line inductor and the start-up charger.
Each of the nine medium-frequency transformers is a part of the associated dc/dc converter (Figure 3). By using the transformer’s leakage and magnetising inductances and the external circuit’s capacitors, a resonant LLC circuit is created (Lr, Lm and Cr as shown in Figure 4). The advantages of an LLC
circuit include: wide output-regulation range a reduction of switching losses on the
primary side through zero voltage switching (ZVS) over the entire load
range low turnoff current controlled by the design (not truly zero current switching
- ZCS) low-voltage stress and ZCS on the
secondary side diode rectifier, and load-independent operation at resonant frequency. As an LLC circuit is based on the principle of resonance, varying the switching frequency can control the output voltage. However, in the present Pett implementation, this has not been used and the LLC resonant dc/dc converter operates in the open loop with a fixed switching frequency of 1.75kHz, which is below the resonant frequency. The control targets can be
summarised as: maintaining sinusoidal input current near-unity power factor constant average dc C-link voltage,
and grid harmonic rejection.
The hardware is ABB’s ac 800PEC
controller, a platform that integrates fast and slow control functions. The existing Ee 933 traction transformer and GTO rectifier were removed to provide space for the new Pett cubicle, and some mechanical adaptations and electronic interface reworking was necessary to accommodate Pett.
74 IRJ September 2012
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