Analytical Approach of the AC-DC Interactions in Aerospace Electric Power Networks
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Abstract:
The trend to more electrical embedded functions aboard future aircrafts makes it of crucial importance to reconsider long used solutions for the electric power distribution. We consider here a mixed ac/dc architecture and we propose a methodology to evaluate the interactions between the ac and dc sides (sometimes called "HVDC" (high-voltage direct current) networks) of such a network, through rectifiers, possibly associated with autotransformers. An analytical approach has been selected and the obtained relationships appear to be very useful for the designers of these networks: they are applicable to choose the characteristics of the architecture, as well as to adapt already existing standards for the specific needs of such solutions.Keywords:
Autotransformer
In order to enlarge the application scope of autotransformer and increase its turn ratio,a new phase-shift angle extent for autotransformer was given through analyzing the requirement of multi-pulse rectifier.An autotransformer was designed with the phase-shift angle π/2 and applied to the 12-pulse rectifier.The input current,load voltage and magnetic components capacity of the new autotransformer were also calculated.Theoretical analysis,simulation and experimental results show that the magnetic components capacity of the new autotransformer is only 63% of the output power,and the autotransformer has a better symmetry and least windings.
Autotransformer
Rectifier (neural networks)
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Autotransformer
Simulation software
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This paper presents four new step-up and step-down 18-pulse direct asymmetric autotransformer rectifiers. The converters are designed to operate with variable frequency power supplies (400-800Hz), while meeting tight harmonic, power factor and output voltage requirements. The analysis takes into consideration the total number of windings used to achieve the step-up and step-down voltage gain-which determines the autotransformer structure and performance-the relative size of the windings as well as the total KVA ratings, and the existence of secondary-side series windings-which impact the commutation process of the diode bridges. A complete modeling procedure of the autotransformers is additionally presented with closed form expressions for all winding currents. Experimental results obtained with laboratory prototypes are used to validate the correct operation and high-performance of the proposed autotransformer rectifier units, where the polygon and delta-polygon autotransformer rectifiers showed the best results.
Autotransformer
Rectifier (neural networks)
Commutation
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Besides YNa-connected high-voltage and medium-voltage windings, autotransformers in high-voltage networks usually also have a tertiary winding. It is a delta-connected low-voltage winding intended for loading and sized mostly for one third of autotransformer rated power. The paper describes winding loads imposed on an autotransformer with tertiary by loads at autotransformer's medium-voltage and tertiary terminals, and also by loads at its high-voltage and tertiary terminals. It is shown that, if in the autotransformer design the tertiary load is ignored, the common winding can become overloaded. The solution is a control of the common winding current by an over-current relay. The relay current is obtained by current transformers at high-voltage and medium-voltage terminals connected at their secondaries through an intermediate current transformer, and it corresponds to the common winding current.
Autotransformer
Delta-wye transformer
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The article exposes a method of calculation of current distribution within autotransformer system of power supply of single and double track railways with voltage between wire network and rails that differs from the voltage between feeding wire and rails. The presented equations result in simplified formulae that allow calculating the load on transformer’s units. The calculations based on formulae show that compensating current doesn’t influence the load of transitional autotransformers. The load on the autotransformer which neighbors substation on one track depends on the load on the autotransformer which neighbors the substation on the second track.
Autotransformer
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With the development of extra-high voltage power grid technology,more and more high-voltage and large-capacity transformers have been used in power grid. In order to reduce the cost,more autotransformers are used. Compared with traditional transformer,the primary and secondary sides of the autotransformers have not only the coupling of magnetic field but also the connection of the electric circuit. Manufacturers still use the traditional empirical formula to solve the electromagnetic problem of high voltage,large-capacity autotransformers. Therefore,in order to research the electromagnetic problem of autotransformer,the field-circuit coupling mathematical model for the autotransformer is derived and the FE program is made. In this paper,a 500 k V EHV autotransformer is taken as the example and the magnetic field distribution and winding current under the second side short-circuited condition are analyzed by the FE program in Fortran and the commercial software to verify the accuracy of the proposed mathematical model.
Autotransformer
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The universal method of voltage transformation from EHV levels to lower voltage systems is through the use of the autotransformer. The characteristic of the autotransformer is profoundly influenced by the tertiary winding or the lack of it. This paper discusses the influence of the tertiary winding and its placement, fault magnitude and short circuit duty, autotransformer loading, overexcitation, protection of unloaded tertiaries, loading of paralleled autotransformers, and the use of single-phase units while presenting a circuit for studying unequal impedances in paralleled banks.
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Autotransformer
Representation
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A very simple AC-line voltage autotransformer based on switched capacitors (SC) is presented. Compared to classical autotransformers or various AC voltage stabilisers it is lightweight and small. Various configurations for SC-autotransformers and possible different switching algorithms are pointed out. The theoretical background for the simplest SC-autotransformer is developed from basic circuit theory. Its characteristics are investigated mathematically and numerically. Designer's guidelines are deduced and a prototype SC-autotransformer is designed. The autotransformer and minimum control circuitry for its operation are constructed, and the basic characteristics are measured. The theoretical and measured waveforms agree very well. By controlling the charging/discharging intervals of the capacitors-the output voltage is controlled. A special characteristic of the simplest SC-autotransformer is its capability to operate either in step-up or step-down mode using the same configuration.
Autotransformer
Line (geometry)
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