What are Multipulse AC‐DC Converters? • Multipulse converters are converters providing more than six pulses of DC voltage per cycle from AC input Or the converter having more steps in AC input current than that of six pulse bridge rectifier supply current. • Bridge rectifier is the basic block required for ACDC conversion, however, full‐wave and half‐wave rectifiers are also used up to 120kW ratings. • Phase shifting transformers are used to derive multiple phase supply from three‐phase AC mains using different combinations of transformer windings such as star, delta, zigzag, fork, polygon, etc.
Problems in AC‐DC Converters • The major problems in AC‐DC converters are injection of harmonics 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 place due to the voltage drop of harmonic currents flowing through 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‐DC Converters • IEEE Standard IEEE‐519. established in 1981 as the “Recommended Practices and requirements for Harmonics Control in Electrical Power System” giving limits on current and voltage distortion and revised in 1992 • IEC 61000 The electric equipments have been categorized in four groups: Class‐ A, B, C and D, for the purpose of harmonic current limitation. • IEEE‐1531 (issued in 2003) gives necessary guidelines for specifications for the use of passive shunt harmonic filters in low and medium voltage electric power systems
Current Distortion Limits for General Distribution Systems(120 V Through 69 000 V), IEEE‐519 Std.
Current Distortion Limits for General Subtransmission Systems (69001 V through 161 000 V), IEEE‐519 Std.
Limits for Class A equipment, IEC‐ 61000 Std.
Advantages of Multi‐pulse AC‐DC Converters • The performance parameters such as total harmonic‐ distortion (THD) of AC mains current and ripple factor of output DC voltage improve, simultaneously. • The improvement is independent of supply frequency variation, unlike passive filters. • Minimal or no control required as Diodes and/or thyristors are mainly used. • Economic, maintenance free and efficient. • Phase shifting transformers are used to derive multiple phase supply from three‐phase AC mains using different combinations of transformer windings such as star, delta, zigzag, fork, polygon, etc.
Application Potential •
Non‐Isolated Uncontrolled Rectifiers : – Front end of Switched mode powers like‐ SMPS, UPS, AC‐DC motor drives, dc servo drives. – aircraft VSCF (Voltage source controlled frequency) systems and aircraft maintenance 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
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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 • •
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TFS = Thyristor firing 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 shift Phase 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 ‐150 Phase Shift • Twenty‐Four‐Pulse Converters Based on 150 Phase 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
18‐Pulse Series‐Type Diode Rectifier
24‐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
• For 18 Pulse Rectifiers
• For 24 Pulse Rectifiers
