Abstract
M.Ing. (Electrical and Electronic Engineering)
The South African Railways covers a total distance of some 24 000 km, making use
of a signalling network of 1000 interlocking systems. This comprises of the order of
1000 signalling houses each with its own unlnterruptible power supply system of which
a DC·AC converter Is one of the components. Due to the requirements by Spoomet,
high frequency (ultrasonic) chopper technology had to be used. This treatise
describes the design and development of a 3 kVA composite DC·AC converter with
an ultrasonic AC link for this application. In the primary converter (centre tapped topology) ZVS Is achieved by good circuit layout and controlled transformer capacitance. As a result the performance of the converter was improved with regard to efficiency and switching transients compared to the conventional hard switched converter. The four pulse topology with an output LC·filter was used for the secondary converter. Due to the modulation technique being used, the load perceived the switching frequency as 20kHz while the actual switching frequency was only 10kHz. In this way switching losses were kept within reasonable limns as well as satisfying the low audible noise requirement. The stability of the LC·filter was achieved with a current feedback loop. The literature describes the capacitor current being used in the feedback loop. In this case the Inductor current was used with the advantage of output short circuit protection without an additional current measurement. Apart from the advantages of small weight and volume from a maintenance point of view, the composite DC·AC converter also achieved high efficiency (> 85%) over a wide load range (10%-100% of full load).