Presentation on theme: "Power Electronics Chapter 6 AC to AC Converters ( AC Controllers and Frequency Converters )"— Presentation transcript:
1 Power ElectronicsChapter AC to AC Converters ( AC Controllers and Frequency Converters )
2 Classification of AC to AC converters ElectronicsSame frequencyvariable magnitudeAC powerAC powerVariablefrequencyAC powerPowerAC controllersFrequency converters(Cycloconverters)AC to AC converters2
3 Classification of AC controllers Phase control: AC voltage controller(Delay angle control)Integral cycle control: AC power controllerAC controllerPWM control: AC chopper(Chopping control)On/off switch: electronic AC switchPWM: Pulse Width Modulation
4 Classification of frequency converters ElectronicsPhase control: thyristor cycloconverter(Delay angle control)Frequency converter(Cycloconverter)PWM control: matrix converter(Chopping control)PowerCycloconverter is sometimes referred toin a broader sense—any ordinary AC to AC converterin a narrower sense—thyristor cycloconverter4
5 Outline 6.1 AC voltage controllers 6.2 Other AC controllers 6.3 Thyristor cycloconverters6.4 Matrix converters
6 6.1 AC voltage controllers 6.1.1 Single-phase AC voltage controller6.1.2 Three-phase AC voltage controllerApplicationsLighting controlSoft-start of asynchronous motorsAdjustable speed drive of asynchronous motorsReactive power control
7 6.1.1 Single-phase AC voltage controller Resistive loadOu1oiVTwtElectronicsRu1oiVT2PowerThe phase shift range (operation range of phase delay angle):0 a p7
8 Resistive load, quantitative analysis RMS value of output voltageRMS value of output currentRMS value of thyristor currentPower factor of the circuit(6-1)(6-2)(6-3)(6-4)
9 Inductive (Inductor-resistor) load, operation principle 1oiVT2The phase shift range: a p
10 Inductive load, quantitative analysis Differential equationSolutionConsidering io=0 when wt=a+qWe have(6-5)(6-6)(6-7)The RMS value of output voltage, output current, and thyristor current can then be calculated.
11 Inductive load, when a < The circuit can still work.The load current will be continuous just like the thyristors are short-circuit, and the thyristors can no longer control the magnitude of output voltage.The start-up transient will be the same as the transient when a RL load is connected to an AC source at wt =a (a < ).Start-up transient
12 Electronics Power Harmonic analysis There is no DC component and even order harmonics in the current.The current waveform is half-wave symmetric.The higher the number of harmonic ordinate, the lower the harmonic content.a = 90 is when harmonics is the most severe.The situation for the inductive load is similar to that for the resistive load except that the corresponding harmonic content is lower and is even lower as is increasing.60120180Fundamental357a/( °)In*%204080100ElectronicsPowerCurrent harmonicsfor the resistive load12
13 6.1.2 Three-phase AC voltage controller Classification of three-phase circuitsY connectionLine-controlled ∆ connectionBranch-controlled ∆ connectionNeutral-point-controlled ∆ connection
14 3-phase 3-wire Y connection AC voltage controller 'abcuiUa0'VT536421For a time instant, there are 2 possible conduction states:Each phase has a thyristor conducting. Load voltages are the same as the source voltages.There are only 2 thyristors conducting, each from a phase. The load voltages of the two conducting phases are half of the corresponding line to line voltage, while the load voltage of the other phase is 0.
15 3-phase 3-wire Y connection AC voltage controller Resistive load, 0 a < 60a4p325uao'abact1VT6
16 3-phase 3-wire Y connection AC voltage controller Resistive load, 60 a < 90ap4325uao'abact1VT6
17 3-phase 3-wire Y connection AC voltage controller Resistive load, 90 a < 150ap4325uao'abacVT16
18 6.2 Other AC controllers6.2.1 Integral cycle control—AC power controller6.2.2 Electronic AC switch6.2.3 Chopping control—AC chopper
19 6.2.1 Integral cycle control —AC power controller MLine periodControl period=*Line period24OConductionangleN3uo1,iwtURu1oiVT2Circuit topologies are the same as AC voltage controllers. Only the control method is different.Load voltage and current are both sinusoidal when thyristors are conducting.
20 Spectrum of the current in AC power controller There is NO harmonics in the ordinary sense.There is harmonics as to the control frequency. As to the line frequency, these components become fractional harmonics.Harmonic order as to control frequencyHarmonic order as to line frequency51234121461080.60.50.40.30.20.1In/0m
21 6.2.2 Electronic AC switchCircuit topologies are the same as AC voltage controllers. But the back-to-back thyristors are just used like a switch to turn the equipment on or off.
22 6.2.3 Chopping control—AC chopper Principle of chopping controlThe mean output voltage over one switching cycle is proportional to the duty cycle in that period. This is also called Pulse Width Modulation (PWM).AdvantagesMuch better output waveforms, much lower harmonicsFor resistive load, the displacement factor is always 1.Waveforms when the load is pure resistor
24 6.3 Thyristor cycloconverters (Thyristor AC to AC frequency converter) Another name—direct frequency converter (as compared to AC-DC-AC frequency converter which is discussed in Chapter 8)Can be classified into single-phase and three-phase according to the number of phases at output6.3.1 Single-phase thyristor-cycloconverter6.3.2 Three-phase thyristor-cycloconverter
25 6.3.1 Single-phase thyristor-cycloconverter Circuit configuration and operation principlePNuZoOuoaP=0=p2wtOutput voltageAverage output voltage
26 Single-phase thyristor-cycloconverter Modes of operationtOuo,i12345PNRectificationInversionBlocking
28 Modulation methods for firing delay angle Calculation methodFor the rectifier circuitFor the cycloconverter outputEquating (6-15) and (6-16)ThereforeCosine wave-crossing method(6-15)(6-16)(6-17)(6-18)Principle of cosine wave-crossing method
29 Calculated results for firing delay angle =0.1a/()Output voltage phase anglewt1201501803060900.20.30.80.91.0p23Output voltage ratio (Modulation factor)
30 Input and output characteristics Maximum output frequency: 1/3 or 1/2 of the input frequency if using 6-pulse rectifiersInput power factorHarmonics in the output voltage and input current are very complicated, and both related to input frequency and output frequency.0.80.60.40.2g=1.0Input displacement factorLoad power factor (lagging)Load power factor (leading)
31 6.3.2 Three-phase thyristor-cycloconverter The configuration with common input line
32 Three-phase thyristor-cycloconverter The configuration with star-connected output
33 Three-phase thyristor-cycloconverter Typical waveformsOutput voltage200t/msInput current withSingle-phase output200t/msInput current with3-phase output200t/ms
34 Input and output characteristics The maximum output frequency and the harmonics in the output voltage are the same as in single-phase circuit.Input power factor is a little higher than single-phase circuit.Harmonics in the input current is a little lower than the single-phase circuit due to the cancellation of some harmonics among the 3 phases.To improve the input power factor:Use DC bias or 3k order component bias on each of the 3 output phase voltages
35 Features and applications Direct frequency conversion—high efficiencyBidirectional energy flow, easy to realize 4-quadrant operationVery complicated—too many power semiconductor devicesLow output frequencyLow input power factor and bad input current waveformApplicationsHigh power low speed AC motor drive
36 6.4 Matrix converter Circuit configuration a b c u v w S S a) b) Input OutputabcuvwS123Sija)b)
37 Matrix converter Usable input voltage a) Single-phase input voltage b) Use 3 phase voltages to construct output voltagec) Use 3 line-line voltages to construct output voltage
38 Features Direct frequency conversion—high efficiency Can realize good input and output waveforms, low harmonics, and nearly unity displacement factorBidirectional energy flow, easy to realize 4-quadrant operationOutput frequency is not limited by input frequencyNo need for bulk capacitor (as compared to indirect frequency converter)Very complicated—too many power semiconductor devicesOutput voltage magnitude is a little lower as compared to indirect frequency converter.