Transcription
Power ElectronicsIntroductionY. BaghzouzEE 442-6421-1
Power Electronics: an OverviewPower electronics is an interdisciplinary subject within electrical engineering.1-2
Power Electronic SystemA power electronic system consists of power electronic switchingdevices, linear circuit elements, digital circuits, microprocessors,electromagnetic devices, DSPs, filters, controllers, sensors, etc .Ideal switch: controls energy flow with no loss.a) when on, it has zero voltage drop and will carry any current,b) wen off, it blocks the flow of current regardless of the voltage.Desired Features:a) 100% efficiencyb) 100% reliability1-3
Power Converter “Converter” is a general term - an AC/DC converter isshown above. Rectifier Mode of operation when power from AC to DC Inverter Mode of operation when power from DC to AC Power converters can also convert DC-to-DC AC-to-AC Practical switching devices are selected based on theirpower handling rating – the product of their voltage andcurrent ratings – rather than their power dissipation ratings.1-4
Types of Semiconductor Switches1-5
Power Electronic Applications1-6
Example: Fluorescent LightingThe line-frequency AC is converted to DC, then to high-frequency AC.1-7
Example: Switch-Mode Power Supply Transistor is operated in switch mode (either fully ONor fully OFF) at high switching frequency. Electrical isolation achieved by high-frequencytransformer (smaller, lighter and more efficient) Result: compact and efficient power supply1-8
Example: Adjustable Speed Drives Conventional drive wastes energy across the throttlingvalve to adjust flow rate.Using power electronics, motor-pump speed is adjustedefficiently to deliver the required flow rate.1-9
AC Motor Drive Converter 1 rectifies line-frequency AC into DCCapacitor acts as a filter; stores energy and decouplesthe two converters. Converter 2 inverts dc to variable frequency AC – asneeded by the motor.1-10
Pure Electric and Plug-In Hybrid VehiclesElectric Drive System: DC-AC ConverterBattery Charger: AC-DC converter1-11
Example: Renewable Power Generation (PV)PV Inverter: DC-AC Converter1-12
Example: Renewable Power Generation (Wind)The rectifier-inverter converts variable-frequency AC to fixed line-frequency AC.1-13
Example: HVDC Transmission Because of the large fixed costnecessary to convert ac to dc and thenback to ac, dc transmission is onlypractical in specialized applications– long distance overhead powertransfer ( 400 miles)– long underwater cable powertransfer ( 25 miles)– providing an asynchronous means ofjoining different power systems.1-14
Power electronic circuits are non-linear. Periodic waveforms but often not sinusoidal analyticalexpressions in terms of Fourier components1-15
Fourier Analysis1-16
Example of simple non-sinusoidal periodicsignals1-17
Current Decomposition Current decomposition of into fundamental (is1) anddistortion current (idis):is (t ) is1 (t ) ish (t ) is1 (t ) idis(t )h 1whereish (t ) a h cos(h 1t ) bh sin(h 1t ) 2 I sh cos(h 1t h )Herein,I sh ah2 bh22, h arctan(bh / ah )1-18
RMS Value and Total Harmonic Distortion The rms value of a distorted waveform is equal to thesquare-root of the sum of the square of the rms value ofeach harmonic component (including the fundamental).2I s I s21 I sh2 I s21 I dish 1 Total Harmonic DistortionI s2 I s21I disTHD (%) 100 100I s1I s11-19
Power and Power Factor Average (real) power:P V Ih 0,1,.h shcos( h ) Apparent Power: S Vs I s Power factor:PPF S Case of sinusoidal voltage and non-sinusoidal current:P Vs1I s1 cos( 1 )Vs1I s1 cos( 1 ) I s1I s11PF cos( 1 ) DPF DPF2Vs1I sIsIs1 (THD) Displacement Power Factor:DPF cos( 1 )1-20
Current-voltage in an inductor and capacitor In an inductor, the voltage is proportional to the rate of change ofcurrent. An inductor cannot sustain a constant voltage for along time. In a capacitor, the current is proportional to the rate of change ofvoltage. A capacitor cannot sustain a constant current for a longtime.div Ldt1 ti v (t ) d t i (t 0 )L t0dvi Cdt1v Ct i d t v(t )t001-21
Inductor response in steady-state to Tt0vd t 01-22
Capacitor response in steady-state t o Tt0id t 01-23
Switching Function (switch control)Switch ONSwitch OFF T: periodto : time delay (used in rectifiers)f 1/T : switching frequencyD: duty ratio - fraction of time during which the switch is ON1-24
Example 1: Simple Power ConverterAssume the diode is ideal, and let Vac 170 sin (377t) , and R 120 Ω1. Average value of output voltage Vout 54.11 V2. Average (DC) current Io 0.45 A3. Average (active) power supplied by source P 60 W4. RMS value of current 0.707 A5. RMS value of fundamental current 0.5 A6. Phase angle of fundamental current 0 deg7. RMS Value of distortion current 0.5 A8. Apparent power S 84.85 VA9. Non-active power 60 (?)1-25
Assignment 1: Diode wit RL LoadRepeat the previous example after placing and inductor L 200 mH (use anymanual our computer tool). Verify the values below:1. Average value of output voltage Vout 50 V2. Average (DC) current Io 0.417 A3. Average (active) power supplied by source P 47.18 W4. RMS value of current 0. 627 A5. RMS value of fundamental current 0. 441 A6. Phase angle of fundamental current -27.1 deg7. RMS Value of distortion current 0.446 A8. Apparent power S 75.24 VA9. Non-active power 58.61 (?)1-26
Assignment 2: Diode with RC LoadRepeat the previous example after placing and capacitor C 150 uF in parallelwith the 120 Ohm resistive load (use any manual our computer tool).1. Average value of output voltage Vout . V2. Peak value of capacitor current . A3. Average (active) power supplied by source P .W4. RMS value of source current .A5. RMS value of fundamental current (source) . A6. Phase angle of fundamental current (source) . deg7. RMS Value of distortion current (source) .A8. Apparent power S .VA1-279. Non-active power (?)
Obey KCL and KVLclosed KVL and KCL place necessary constraints on the operationof switches.– In the case of voltage sources, switches must not act tocreate short-circuit paths among unlike sources.– In the case of KCL, switches must not attempt tointerconnect unequal current sources.1-28
High-frequency vs. Low-frequencyTransformers Nickel-steel is used (instead of silicon-iron) to reduce corelosses at high frequency. Fewer copper winding turns are needed to produce thesame flux density.– An increase in frequency permits a large increase in powercapacity.– For the same power capacity, a high frequency transformer ismuch smaller, cheaper, more efficient and lighter than a 60 Hztransformer.Example: 60 Hz, 120/24 V, 36 VA, B 1.5 T, core loss 1 W, N1/N2 600/120, weight 500 g. 6 kHz, 120/24, 480 VA, B 0.2 T, core loss 1 W, N1/N2 45/9, weight 100 g.1-29
Power Electronics: an Overview Power electronics is an interdisciplinary subject within electrical engineering. 1-3 Power Electronic System A power electronic system consists of power electronic switching devices, li
