Harmonic mitigation in traction drives
Hans Bängtsson
Faculty opponent: Professor Roland John Hill, University of Bath, Great Britain
In railway traffic the low friction between wheel and
rail causes long braking distances, normally much longer than the sight distance of the
driver, i.e. if the driver starts the braking when a problem on the track is discovered,
it might be too late to brake the train. Therefore, safe railway traffic can not only rely
on the driver. It is necessary to have a signalling or even an automatic surveyor system,
especially in densely populated areas, which supervises the positions of different trains
along the track and organises the traffic.
Such a system uses train detection systems to control whether a certain section of the
track is occupied or not. Train detection systems can unfortunately be disturbed by
harmonics, generated by the drive system of a rail vehicle. To reduce the risk of
disturbance, the choice of modulation strategy and the use of line filters are critical
tasks. In the design stage of the filter, it is necessary to be able to predict the
generation of harmonics.
This work presents fast algorithms for calculation of harmonics, generated by a traction
drive system. The traction drive system, here described, is based on three phase induction
motors, fed from voltage source machine and line converters. Non-ideal commutations are
taken into account, which is urgent as these will generate undesirable low frequency
harmonics.
It is possible to compensate the effects of the non-ideal commutations, and in this work
following
compensation methods are investigated:
- Dead time compensation, where the differential between the integral of the inverter
output voltage, or rather the output flux, and the voltage time area reference, is fed
back to the control system.
- Position asymmetry compensation, in which the DC-component in the machine converter
phase currents is fed back to the control system.
- Compensation of a remaining DC-bias in Hall-effect current transducers. In the method,
the fundamental component in the DC-link current is fed back to the control system.
The compensation methods are found to effectively reduce the low frequencies. Thus the
weight of the filters onboard the trains can be minimised, which considerably saves energy
and costs of components.
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