Frequency Converter Technology for Aircraft Power Systems

This paper presents an overview of frequency converter technology in qualitative terms, with the objective of conveying insight into the rationale for selecting particular power circuit arrangements. The tradeoffs involved in the design process are discussed, especially with regard to factors affecting size and performance. At present, the practical choice for an aircraft converter is between a cycloconverter and a rectifier-dc link-inverter. HE introduction of high-power semiconductor devices has enabled the development of efficient static power conversion equipment that can perform the frequency trans- formation required to match a variable-speed generator to constant-frequency load in an aircraft power system. Over the past 20 years, considerable effort has been expended in the in- vestigation of different circuit approaches, reduction of size, and refinement of controls to achieve the desired perform- ance. Present variable-speed, constant-frequency (VSCF) sys- tems can deliver power of a quality equal to or better than that available from a conventional constant-speed drive system. The most important controlled semiconductor device for high-power applications is the thyristor. When the anode is positive with respect to the cathode, a low-power control signal applied to the gate electrode will trigger the device from a blocking to a conducting state. However, gate control cannot turn the current off (except in specially designed devices); the circuit in which the thyristor operates must in some way extinguish the current and then apply reverse voltage for a short time before forward voltage can be reapplied. This external process is known as commutation, and is conveniently classified as either natural commutation or forced commutation. When the power source is ac, natural commutation for thyristor devices can be obtained if the circuit is constrained to operate within the limits of the cyclic voltage reversals. With dc sources, at least some of the devices must be force commutated. Since forced commutation requires additional circuit components, it is obviously preferable to employ a naturally commutated converter, provided that it can satisfy the per- formance objectives. When forced commutation is necessary, additional commutation components can be avoided if power transistors can be used, since they can be forced off by removing or reversing their base current. However, tran- sistors are limited in their power rating so that converters employing transistors are similarly restricted. Phase-Controlled Converters