Presentation on theme: "Power Electronics Chapter 3 AC to DC Converters (Rectifiers)"— Presentation transcript:
1 Power ElectronicsChapter 3AC to DC Converters(Rectifiers)
2 Outline 3.1 Single-phase controlled rectifier 3.2 Three-phase controlled rectifier3.3 Effect of transformer leakage inductance on rectifier circuits3.4 Capacitor-filtered uncontrolled rectifier3.5 Harmonics and power factor of rectifier circuits3.6 High power controlled rectifier3.7 Inverter mode operation of rectifier circuit3.8 Realization of phase-control in rectifier circuits
6 Basic thought process of time-domain analysis for power electronic circuits The time-domain behavior of a power electronic circuit is actually the combination of consecutive transients of the different linear circuits when the power semiconductor devices are in different states.(3-2)ωt = a ，id= 0(3-3)
7 Single-phase half-wave controlled rectifier with freewheeling diode Inductive load (L is large enough)VTia)Tu12LRdVD(3-5)(3-6)(3-7)(3-8)Maximum forward voltage, maximum reverse voltageDisadvantages:Only single pulse in one line cycleDC component in the transformer current
8 3.1.2 Single-phase bridge fully-controlled rectifier Resistive loaddRTu12iabVT34a)For thyristor: maximum forward voltage, maximum reverse voltageAdvantages:2 pulses in one line cycleNo DC component in the transformer current
10 3.1.2 Single-phase bridge fully-controlled rectifier Inductive load (L is large enough)(3-15)CommutationThyristor voltages and currentsTransformer current
11 Electro-motive-force (EMF) load With resistorDiscontinuous current id
12 Electro-motive-force (EMF) load With resistor and inductorWhen L is large enough, the output voltage and current waveforms are the same as ordinary inductive load.When L is at a critical value(3-17)
13 3.1.3 Single-phase full-wave controlled rectifier Transformer with center tapComparison with single-phase bridge fully-controlled rectifier
15 Another single-phase bridge half-controlled rectifier Comparison with previous circuit:No need for additional freewheeling diodeIsolation is necessary between the drive circuits of the two thyristors
16 Summary of some important points in analysis When analyzing a thyristor circuit, start from a diode circuit with the same topology. The behavior of the diode circuit is exactly the same as the thyristor circuit when firing angle is 0.A power electronic circuit can be considered as different linear circuits when the power semiconductor devices are in different states. The time-domain behavior of the power electronic circuit is actually the combination of consecutive transients of the different linear circuits.Take different principle when dealing with different loadFor resistive load: current waveform of a resistor is the same as the voltage waveformFor inductive load with a large inductor: the inductor current can be considered constant
17 3.2 Three-phase controlled (controllable) rectifier 3.2.1 Three-phase half-wave controlled rectifier(the basic circuit among three-phase rectifiers)3.2.2 Three-phase bridge fully-controlled rectifier(the most widely used circuit among three-phase rectifiers)
18 3.2.1 Three-phase half-wave controlled rectifier Resistive load, a = 0ºCommon-cathode connectionNatural commutation point
21 Resistive load, quantitative analysis When a 30º, load current id is continuous.When a > 30º, load current id is discontinuous.(3-18)(3-19)Average load currentThyristor voltages(3-20)1- resistor load inductor load3- resistor-inductor load
22 Inductive load, L is large enough Load current id is always continuous.Thyristor voltage and currents, transformer current(3-18)(3-23)(3-24)(3-25)
23 3.2.2 Three-phase bridge fully-controlled rectifier Circuit diagramdbaciuVT135462TnloadCommon-cathode group and common-anode group of thyristorsNumbering of the 6 thyristors indicates the trigger sequence.
31 Quantitative analysis Average output voltageFor resistive load, When a > 60º, load current id is discontinuous.Average output current (load current)Transformer currentThyristor voltage and currentSame as three-phase half-wave rectifierEMF load, L is large enoughAll the same as inductive load except the calculation of average output current(3-26)Electronics(3-27)(3-20)Power(3-28)(3-29)31
32 3.3 Effect of transformer leakage inductance on rectifier circuits In practical, the transformer leakage inductance has to be taken into account.Commutation between thyristors thus can not happen instantly, but with a commutation process.
33 Commutation process analysis Circulating current ik during commutationCommutation angleOutput voltage during commutationub-ua = 2·LB·dia/dtik: Idia = Id-ik : Idib = ik : Id(3-30)
34 Quantitative calculation Reduction of average output voltage due to the commutation processCalculation of commutation angleId ,gXB ,gFor a 90o , a , g(3-31)(3-36)
35 Summary of the effect on rectifier circuits Single-phase full waveSingle-phase bridgeThree-phase half-waveThree-phase bridgem-pulse recfifier①②CircuitsConclusionsCommutation process actually provides additional working states of the circuit.di/dt of the thyristor current is reduced.The average output voltage is reduced.Positive du/dtNotching in the AC side voltage
36 3.4 Capacitor-filtered uncontrolled (uncontrollable) rectifier Emphasis of previous sectionsControlled rectifier, inductive loadUncontrolled rectifier: diodes instead of thyristorsWide applications of capacitor-filtered uncontrolled rectifierAC-DC-AC frequency converterUninterruptible power supplySwitching power supply3.4.1 Capacitor-filtered single-phase uncontrolled rectifier3.4.2 Capacitor-filtered three-phase uncontrolled rectifier
42 3.5 Harmonics and power factor of rectifier circuits Originating of harmonics and power factor issues in rectifier circuitsHarmonics: working in switching states—nonlinearPower factor: firing delay angle causes phase delayHarmful effects of harmonics and low power factorStandards to limit harmonics and power factor3.5.1 Basic concepts of harmonics and reactive power3.5.2 AC side harmonics and power factor of controlled rectifiers with inductive load3.5.3 AC side harmonics and power factor of capacitor-filtered uncontrolled rectifiers3.5.4 Harmonic analysis of output voltage and current
43 3.5.1 Basic concepts of harmonics and reactive power For pure sinusoidal waveformFor periodic non-sinusoidal waveformorwhereFundamental componentHarmonic components (harmonics)(3-54)(3-55)(3-56)
44 Harmonics-related specifications Take current harmonics as examplesContent of nth harmonicsIn is the effective (RMS) value of nth harmonics.I1 is the effective (RMS) value of fundamental component.Total harmonic distortionIh is the total effective (RMS) value of all the harmonic components.(3-57)(3-58)
45 Definition of power and power factor For sinusoidal circuitsActive powerReactive powerQ=U I sinjApparent powerS=UIPower factor l =cos j(3-59)(3-61)(3-60)(3-63)(3-62)(3-64)
46 Definition of power and power factor For non-sinusoidal circuitsActive powerP=U I1 cosj1Power factorDistortion factor (fundamental-component factor)n =I1 / IDisplacement factor (power factor of fundamental component)l1 = cos1Definition of reactive power is still in dispute.(3-65)(3-66)
47 Review of the reactive power concept The reactive power Q does not lead to net transmission of energy between the source and load. When Q ¹ 0, the rms current and apparent power are greater than the minimum amount necessary to transmit the average power P.Inductor: current lags voltage by 90°, hence displacement factor is zero. The alternate storing and releasing of energy in an inductor leads to current flow and nonzero apparent power, but P = 0. Just as resistors consume real (average) power P, inductors can be viewed as consumers of reactive power Q.Capacitor: current leads voltage by 90°, hence displacement factor is zero. Capacitors supply reactive power Q. They are often placed in the utility power distribution system near inductive loads. If Q supplied by capacitor is equal to Q consumed by inductor, then the net current (flowing from the source into the capacitor-inductive-load combination) is in phase with the voltage, leading to unity power factor and minimum rms current magnitude.ElectronicsPower47
48 3.5.2 AC side harmonics and power factor of controlled rectifiers with inductive load Single-phase bridge fully-controlled rectifier
49 AC side current harmonics of single-phase bridge fully-controlled rectifier with inductive load (3-72)wheren=1,3,5,…(3-73)ConclusionsOnly odd order harmonics existIn 1/nIn / I1 = 1/n
50 Power factor of single-phase bridge fully-controlled rectifier with inductive load Distortion factorDisplacement factorPower factor(3-75)(3-76)(3-77)
52 AC side current harmonics of three-phase bridge fully-controlled rectifier with inductive load (3-79)where(3-80)ConclusionsOnly 6k1 order harmonics exist (k is positive integer)In 1/nIn / I1 = 1/n
53 Power factor of three-phase bridge fully-controlled rectifier with inductive load Distortion factorDisplacement factorPower factor(3-81)(3-82)(3-83)
54 3.5.3 AC side harmonics and power factor of capacitor-filtered uncontrolled rectifiers Situation is a little complicated than rectifiers with inductive load.Some conclusions that are easy to remember:Only odd order harmonics exist in single-phase circuit, and only 6k1 (k is positive integer) order harmonics exist in three-phase circuit.Magnitude of harmonics decreases as harmonic order increases.Harmonics increases and power factor decreases as capacitor increases.Harmonics decreases and power factor increases as inductor increases.
55 3.5.4 Harmonic analysis of output voltage and current (3-85)where(3-86)Output voltage of m-pulserectifier when a = 0º(3-87)
56 Ripple factor in the output voltage Output voltage ripple factorwhere UR is the total RMS value of all the harmoniccomponents in the output voltageand U is the total RMS value of the output voltageRipple factors for rectifiers with different number of pulses(3-88)(3-89)m23612∞gu（%）48.218.274.180.994
57 Harmonics in the output current (3-92)where(3-93)(3-94)(3-95)
58 Conclusions for a = 0ºOnly mk (k is positive integer) order harmonics exist in the output voltage and current of m-pulse rectifiersMagnitude of harmonics decreases as harmonic order increases when m is constant.The order number of the lowest harmonics increases as m increases. The corresponding magnitude of the lowest harmonics decreases accordingly.
59 For a ¹ 0ºQuantitative harmonic analysis of output voltage and current is very complicated for a ¹ 0º.As an example, for 3-phase bridge fully-controlled rectifier(3-96)
60 3.6 High power controlled rectifier 3.6.1 Double-star controlled rectifier3.6.2 Connection of multiple rectifiers
61 3.6.1 Double-star controlled rectifier CircuitWaveforms When a = 0ºDifference from 6-phase half-wave rectifier
62 Effect of interphase reactor (inductor, transformer) (3-97)(3-98)
63 Quantitative analysis when a = 0º (3-99)(3-100)(3-101)(3-102)
65 Comparison with 3-phase half-wave rectifier and 3-phase bridge rectifier Voltage output capabilitySame as 3-phase half-wave rectifierHalf of 3-phase bridge rectifierCurrent output capabilityTwice of 3-phase half-wave rectifierTwice of 3-phase bridge rectifierApplicationsLow voltage and high current situations
66 3.6.2 Connection of multiple rectifiers ElectronicsConnection of multiple rectifiersTo increase the output capacityTo improve the AC side current waveform and DC side voltage waveformLarger output voltage: series connectionLarger output current: parallel connectionPower
68 Phase-shift connection of multiple rectifiers Series connection12-pulse rectifier realized byseries 3-phase bridge rectifiers
69 Quantitative analysis of 12-pulse rectifier VoltageAverage output voltageParallel connection:Series connection:Output voltage harmonicsOnly 12m harmonics existInput (AC side) current harmonicsOnly 12k±1 harmonics existConnection of more 3-phase bridge rectifiersThree: 18-pulse rectifier (20º phase difference)Four: pulse rectifier (15º phase difference)
70 Sequential control of multiple series-connected rectifiers Circuit and waveforms of series-connectedthree single-phase bridge rectifiers
71 3.7 Inverter mode operation of rectifiers Review of DC generator-motor systemshould be avoided
72 Inverter mode operation of rectifiers Rectifier and inverter mode operation of single-phase full-wave converter
73 Necessary conditions for the inverter mode operation of controlled rectifiers There must be DC EMF in the load and the direction of the DC EMF must be enabling current flow in thyristors. (In other word EM must be negative if taking the ordinary output voltage direction as positive.)a > 90º so that the output voltage Ud is also negative.
75 Inversion failure and minimum inversion angle Possible reasons of inversion failuresMalfunction of triggering circuitFailure in thyristorsSudden dropout of AC source voltageInsufficient margin for commutation of thyristorsMinimum inversion angle (extinction angle)bmin=d +g+q′ （3-109）
76 3.8 Realization of phase-control in rectifier circuits ObjectHow to timely generate triggering pulses with adjustable phase delay angleConstitutionSynchronous circuitSaw-tooth ramp generating and phase shiftingPulse generatingIntegrated gate triggering control circuits are very widely used in practice.
78 Waveforms of the typical gate triggering control circuit ElectronicsPower78
79 How to get synchronous voltage for the gate triggering control circuit of each thyristor D,y115-11TRTSuABCabc-sasbscUABFor the typical circuit on page 20, the synchronous voltage of the gate triggering control circuit for each thyristor should be lagging 180º to the corresponding phase voltage of that thyristor.