Research On Unbalanced Output Voltage In Multi-phase Rectifier Transformer And Its Effect

Output voltage unbalance of rectifier transformer is a common power qualiyproblem in multi-phase rectifier, which may result in noncharateristic harmonics ininput current of rectifier inequality of conduction current of rectifier devices, andripple coefficient increasing of load vltage. Therefore, the problem of outputvoltage unbalance attachs more and more attention. With the increasing inrequirement of power quality and wide application of multi-phase rectifier, how tosuppress the effect output voltage unbalancex of rectifier transformer on mutli-phase rectifier has become a research hot point. Setting the six-phase rectifier as anexample, this paper analyzes quantificationally the effect of voltage balance onmulti-phase rectifier, and introduces the corresponding suppressing method. Themain content is summaried as follow.This paper sets up the decoupling asymmetrical modle of six-phase rectifiertransformer, and qualiatative analyzes the effect of asymmetry of transformer onsix-phase rectifier. According to the characteristic of coupling circuit of rectifiertransformer, the relation among branch voltage, node voltage, branch current andnode current is established, and furtherfore, the relation between node voltage andnode current is calculated, and the corresponding node admittance matrix isobtained. With the matrix, the decoupling asymmetrical modes of delta-connectedautotransformer, double-star rectifier transformer and△/Y/△transformer are setup. These modes are easy to realize, and contain the connection characteristic andelectrical parameter, which can be used to simulate the effect of asymmetry oftransformer configuration on voltages across windings and output voltages.Simulation and experimental result show the proposed model can simulate theeffect of asymmetry configuration on voltages across windings and output voltages.In order to simplify the model of multi-phae rectifier, the relations betweenoutput voltage of rectifier transformer and transformer configuration, and systeminput voltages, are analyzed. Asymmetry coiguration of transformer and unbalanceof input voltage are the main reasonsr resulting in unbalance of output voltage ofrectifier transformer. Via analizing the effect of winding turn variation on its outputvoltage, the relation between output voltage unbalance factor of transformer and its configuration, and the variation rule of voltage unbalance factor with turn ratio oftransformer is obtained. The effect of over-phase and under-phase on multi-phaserectifier is analyzed. Although voltage unbalance factor is zero under over-phaseand under-phase, the THD of input voltage and ripple coefficient of load voltageboth become larger because the output voltages do not meet the requirement ofmulti-phase rectifier. The effect of input voltage on output voltage of transformerunder symmetry configuration of transformer is also analyzed. The theoreticalanalysis shows the transformer with symmetrical configuration does not change theoutput voltage unbalance factor of transformer. Using the aforementioned analysis,the multi-phase rectifier can be simplified to be the three-phase half-wave rectifierin parallel and in series connection.In order to analyze the effect of configuration asymmetry of autotransformerand unbalance of input voltage, the analytical model of multi-phase rectifier isproposed. From the analysis, the six-phase half-wave rectifier can be simplified astwo three-phase half-wave rectifiers in parallel connection, and six-phase full-bridge rectifier can be simplified as four three-phase half-wave rectifiers in paralleland in series connection. Therefore, three-phase half-wave rectifier is the basiccircuit of six-phase rectifier. According to the characteristic of three-phase half-wave rectifier, the operation process is devided into two segments, which arenormal conduction and commutation. The diffrential equations of the two segmentsare listed. The solution of time domain is obtained through Laplace transformer.Simulation shows the results from analytical model is accord with results fromMatlab. Becauofse the operation time of the proposed model is shorter than that ofMatlab, the proposed model has higher application value. Meanwhile, simulationresults show the asymmetry configuration of transformer and unbalance of inputvoltage both result in noncharacteristic harmonic in input current.This paper proposed a topology of step-up autotransformer. The propsedtransformer is combined with the harmonic blocking resulting from the asymmetryconfiguration of tranformer and voltage unbalance. Via analyzing the requirementof multi-phase reactor on rectifier transformer, a group of phase-shift angles areintroduced. To set the phase-shift equal to π/2as an example, a novel step-up delta-conneted autotransformer is designed. When the proposed transformer is combinedwith the harmonic blocking reactor, a new step-up multi-phase rectifier is proposed. From the viewpoint of assuring system symmetry and suppressing noncharacteristicharmonic, the optimal configuration of harmonic blocking reactor is obtained.Compared with other multi-phase rectifiers, the proposed rectifier has less kVArating of magnetic devices and higher system symmetry. Simulation andexperimental show the proposed multi-phase rectifier can effective suppress thenoncharacteristic resulting from the asymmetry configuration of tranformer andvoltage unbalance.