VIENNA RECTIFIER

INTRODUCTION

Power Factor :Measure of how efficiently electrical power is consumed

Lower the power factor,higher the current drawn

Energy wastage and increase in cost of equipment

Harmonic currents created due to switching action

Harmonic distortion of current waveform leading to EMI problems and cross-interferences

POWER CONVERTERS(SINGLE PHASE)

Voltage follower approach,working in discontinuous conduction mode

Use of passive filtersConverters with improved energy processingMultilevel convertersResonant converters

THREE PHASE TOPOLOGIES

Converters with passive filtersBoost derived converters:Six switch

converter,Four switch converter,Three switch converters,Single switch converters

Buck derived converters:Six switch converter,Single switch converter

Modular topologies

VIENNA RECTIFIER

Invented in 1993 by Prof. Johann W. KolarDeveloped at Technical university,ViennaType of three switch boost derived converterThree phase three-level three-switch PWM

rectifier with controlled output voltage

FEATURES

Three-wire input, no connection to neutralUnidirectional power flow with continuous

sinusoidal currentHigh power densityLow conducted common-mode EMI emissionsOperational even in presence of unbalanced

mains or only two phasesMalfunction in control circuit does not

manifest itself in short circuit of output or PFC front end

TOPOLOGY

WORKING

Semiconductor switch in each phase legSwitch controls the current by controlling the

magnetization of the inductorAdjusting pulse width that turns ON the MOSFET,

corresponding line current is forced to be sinusoidal and in phase with the voltage

Switched on, inductor is charged,driving the current through the bidirectional switch.

Deactivating the switch increases causes the current to bypass the switch and flow through the freewheeling diodes resulting in a negative voltage across the inductor and draining it

CONDUCTION STATES (iR>0,iB<0,iY<0)

CONTROL METHODS

1.FUZZY CONTROLSwitching function segmented in 12 zones

corresponding to 0 to 360 phase angle of power frequency

Fuzzy control matrix of statesEach state represents a PWM look-up table for

switch signals. Fuzzy rules to identify the state from measured

value of input and output voltages and select the appropriate pulse width table to generate PWM signals

2.CONSTANT FREQUENCY INTEGRATION CONTROL

Control by sensing either the inductor currents or the switching currents

Control key equation:Vm(1-dx)=Rs|ix| for x=a,b,c where

Rs=equivalent current sensing resistance

dx=duty ratio of switch

Vm=output of feedback error compensator

ix=phase currentRealized by one integrator with reset as well as

some logic and linear components

3.DIGITAL CURRENT CONTROL

Suitable for high frequenciesData processing using DSPs or FPGAs Calculation of duty cycle of one phase while

holding other two constant during a switching period

Current signal sampled in the middle of PWMAccordingly,duty cycle updated at the start of

next PWM cycle(Uniformly sampled modulation)

4.CONTROL USING SYNCHRONISED CARRIER

Shape of input current depends on switching states of other phases

Triangular signal waveform used as carrier signal

Provides better performance than sawtooth waveform

Coupling of phases leads to closer approximation of ideal sinusoidal shape and lower harmonics

ADVANTAGES

Low distortionNo need for a neutral wireReduction in voltage stress and switching

losses of power semiconductorsWide mains voltage rangeHigh efficiency of the order of 97.5%

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