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What are Multipulse AC‐DC Converters?

• Multipulse converters are converters providing morethan six pulses of DC voltage per cycle from AC inputOr the converter having more steps in AC input currentthan that of six pulse bridge rectifier supply current.

• Bridge rectifier is the basic block required for ACDCconversion, however, full‐wave and half‐wave rectifiersare also used up to 120kW ratings.

• Phase shifting transformers are used to derive multiplephase supply from three‐phase AC mains usingdifferent combinations of transformer windings such asstar, delta, zigzag, fork, polygon, etc.

Problems in AC‐DC Converters

• The major problems in AC‐DC converters are injection ofharmonics at input and output .

• Harmonics cause dielectric, thermal or voltage stress,which cause premature ageing of electrical insulation.

• Voltage distortion at point of common coupling takes placedue to the voltage drop of harmonic currents flowingthrough system impedances.

• Capacitor bank overloading due to system resonance.• Interference on telephone and communication lines due to

noise induced from the power conductors.• Poor power factor.• Equipment damage from voltage spikes created by high

frequency resonance resulting from notching.

Power Quality Standards in AC‐DCConverters

• IEEE Standard IEEE‐519. established in 1981 as the“Recommended Practices and requirements forHarmonics Control in Electrical Power System” givinglimits on current and voltage distortion and revised in1992

• IEC 61000 The electric equipments have beencategorized in four groups: Class‐ A, B, C and D, for thepurpose of harmonic current limitation.

• IEEE‐1531 (issued in 2003) gives necessary guidelinesfor specifications for the use of passive shunt harmonicfilters in low and medium voltage electric powersystems

Current Distortion Limits for General DistributionSystems(120 V Through 69 000 V), IEEE‐519 Std.

Current Distortion Limits for General SubtransmissionSystems (69001 V through 161 000 V), IEEE‐519 Std.

Limits for Class A equipment, IEC‐61000 Std.

Advantages of Multi‐pulse AC‐DCConverters

• The performance parameters such as total harmonic‐distortion (THD) of AC mains current and ripple factorof output DC voltage improve, simultaneously.

• The improvement is independent of supply frequencyvariation, unlike passive filters.

• Minimal or no control required as Diodes and/orthyristors are mainly used.

• Economic, maintenance free and efficient.• Phase shifting transformers are used to derive multiplephase supply from three‐phase AC mains usingdifferent combinations of transformer windings such asstar, delta, zigzag, fork, polygon, etc.

Application Potential• Non‐Isolated Uncontrolled Rectifiers :

– Front end of Switched mode powers like‐ SMPS, UPS, AC‐DC motor drives, dcservo drives.

– aircraft VSCF (Voltage source controlled frequency) systems and aircraftmaintenance systems using 60Hz/400Hz converter systems.

– DC motor drive system with regenerative capacity.– Some power supplies of large rating also use non‐isolated dual converters

• Isolated Uncontrolled Rectifiers :– The railways working on DC.– The welding equipment working on high Frequency– HVDC systems.– Battery energy storage systems.– Adjustable speed synchronous motor drive of large power ratings for

applications such as mining.– Arc furnaces

Typical power supply system of DC arc furnaces

• TFS = Thyristorfiring scheme;

• CAD = Conduction Angle Determinator;

• FAD = Firing Angle Determinator;

• PLL = phase Locked Loop

A model of twelve pulse railway rectifier system

Load commutated current source inverter fed synchronous motor

Advantages of Multipulse Converter

• PASSIVE WAVESHAPING

• LOW COST

• RUGGED STRUCTURE

• LESS LOSSES

• HIGH EFFICIENCY

• LESSER RATING TRANSFORMER REQD IN AUTOTRANSFORMER BASED TOPOLOGIES

CLASSIFICATION OF MULTIPULSE AC‐DC CONVERTERS based on phase shiftPhase Shift = 600 / Number of Six‐Pulse Converters• Pulse Based Configurations• Twelve‐Pulse Converters 00 and 300 Phase Shift• Twelve‐Pulse Converters Based on +150 and ‐150Phase Shift

• Twenty‐Four‐Pulse Converters Based on 150Phase Shift

• Eighteen ‐Pulse Converters Based on +200 and ‐200 Phase Shift

• Thirty‐Pulse Converters Based on 120 Phase Shift

Multipulse Diode Rectifiers

Six‐pulse diode rectifier with a resistive load.

Six‐pulse diode rectifier with a capacitive load.

Discontinuous Current Operation

Continuous Current Operation

Definition of THD and PF

Per‐Unit System

THD and PF of Six‐Pulse Diode Rectifier

SERIES‐TYPE MULTIPULSE DIODE RECTIFIERS

• 12‐Pulse Series‐Type Diode Rectifier

12‐Pulse Series‐Type Diode Rectifier

SEPARATE‐TYPE MULTIPULSE DIODE RECTIFIERS

• 12‐Pulse Separate‐Type Diode Rectifier

12‐Pulse Separate‐Type Diode Rectifier

Application of the 12‐pulse separate‐type diode rectifier in a cascaded Hbridge

multilevel inverter‐fed drive.

Phase‐Shifting Transformers

Y – Δ Transformer

Y – Z TRANSFORMER

Y/Z‐1 Transformers

Y/Z‐2 Transformers

Turns Ratio for Y/Z Transformers

Δ/Z TRANSFORMERS

Δ/Z‐1 Transformer

Δ/Z‐2 Transformer

Turns Ratio for Δ/Z Transformers

Examples of phase‐shifting transformers for multipulse rectifiers.

• For 12 Pulse Rectifiers

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