In this paper polyphase transformer arrangements with reduced AVA capacities are presented for harmonic current reduction in high power rectifier utility interface systems. Based on the concept of an autotransformer, a proposed twelve-pulse redfier system is realized with a resultant kVA rating of 0.18P0 @U). In this arrangement the rectifier diodes each conduct for 120 degrees per cycle and thefiflh and seventh harmonics are absent from the utility input line current. In the second scheme an eighteen-pulse rectifier is realized with the AVA rating of 0.16P0 @U), rectifier diode conduction of 120 degrees per cycle andfiph, seventh, eleventh and thirteenth harmonic cancellation in the utility line currents. Additional applications for the proposed polyphase transformer arrangements include twelve and eighteen-pulse systems feeding multiple six-pulse nonlinear loah, such as ac motor drives, with an appropriate phase shifl, and this arrangement substantially reduces the utility line current harmonics. For example, it is shown that the fiflh and seventh harmonics generated by two separate six-pulse nonlinear loads are subtracted, and this contributes to reduced distortion. Simulation results verifiing the proposed concept are presented.
A transformer performs many functions such as voltage transformation, isolation, and noise decoupling, and it is an indispensable component in electric power distribution systems. However, at low frequencies (60/50 Hz), it is a bulky and expensive component. In this paper, the concept of electronic transformers is further extended and explored for its suitability in power distribution systems. It should be noted that from the input/output behavior, the electronic transformer and the conventional transformer are identical. Possible topologies employing static converters connected on the primary and secondary sides are explored to realize high-frequency operation of the magnetic core. To assist the commutation process, a four-step switching has been developed which does not require the use of snubbers. Reduced size, losses, higher efficiency, and better voltage regulation are some of the advantages of this approach. A 10-kVA design example along with experiment results are discussed. It is shown that a transformer designed with a conventional grainoriented silicon-steel core can process three times the power at 1-kHz operating frequency as compared to 60 Hz. The proposed method is scalable in voltage/current with the currently available insulated gate bipolar transistor (IGBT) devices connected in series without special snubbers.Index Terms-Electronic transformers, four-step switching, high-frequency switching.
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