Chapter 4: DC to AC Conversion (Inverters)

1. Chapter 4: DC to AC Conversion (Inverters)

3. Overview • General concept • • Single-phase inverter (square wave and PWM) • • Harmonics • • Modulation • • Three-phase inverter

4. DC to AC Converter (Inverter) • DEFINITION: Converts DC to AC power by switching the DC input voltage (or current) in a pre-determined sequence so as to generate AC voltage (or current) output. • General block diagram (VSI & CSI) TYPICAL APPLICATIONS: – Un-interruptible power supply (UPS), Industrial (induction motor) drives, Traction, HVDC

6. Example of Inverter Application Uninterruptible Power Supply (UPS)

7. Example of Inverter Application Uninterruptible Power Supply (UPS)

8. Simple Square-wave Inverter • To illustrate the concept of AC waveform generation

9. AC Waveform Generation

10. AC Waveform Generation 5th

11. Harmonics Filtering • Output of the inverter is “chopped AC voltage with zero DC component”. It contain harmonics. • An LC section low-pass filter is normally fitted at the inverter output to reduce the high frequency harmonics. • In some applications such as UPS, “high purity” sine wave output is required. Good filtering is a must. • In some applications such as AC motor drive, filtering is not required.

12. Variable Voltage Variable Frequency Capability • Output voltage frequency can be varied by “period” of the square-wave pulse. • Output voltage amplitude can be varied by varying the “magnitude” of the DC input voltage. • Very useful: e.g. variable speed induction motor drive

13. Output Voltage Harmonics / Distortion • • Harmonics cause distortion on the output voltage. • • Lower order harmonics (3rd, 5thetc.) are very • difficult to filter, due to the filter size and high filter • order. They can cause serious voltage distortion. • • Why need to consider harmonics? • – Sinusoidal waveform quality must match TNB supply. • – “Power Quality” issue. • – Harmonics may cause degradation of equipment. Equipment need to be “de-rated”. • • Total Harmonic Distortion (THD) is a measure to determine the “quality” of a given waveform.

14. Total Harmonics Distortion (THD) V∞,

15. Total Harmonics Distortion (THD)

16. Fourier Series • Study of harmonics requires understanding of wave shapes. Fourier Series is a tool to analyze wave shapes.

17. Harmonics of Square-wave (1)

18. Harmonics of Square-wave (2)

19. Spectra of Square Wave • Spectra (harmonics) characteristics: – Harmonic decreases with a factor of (1/n). – Even harmonics are absent – Nearest harmonics is the 3rd. If fundamental is 50 Hz, then nearest harmonic is 150 Hz. – Due to the small separation between the fundamental an harmonics, output low-pass filter design can be very difficult.

20. Quasi-Square Wave (QSW)

21. Harmonics control Function??Purpose???

22. Quasi-Square Wave (QSW)

23. Example : • A full - bridge single phase inverter is fed by square wave signals. The DC link voltage is100 V. The load is R =10 Ωand L = 10mH in series. Given : Inverter output freq = 50 Hz • Calculate : • the THDv using the "exact“ formula. • the THDv by using the first three non - zero harmonics • the THDi by using the first three non - zero harmonics • Repeat (b) and (c) for quasi - square wave case with α = 30○ a) 48.3 % b) 41.4 %

24. Example : A single phase square-wave inverter has an R-L load with R = 30 and L = 10 mH. The inverter output frequency is 500 Hz. a) Determine the value of DC source to establish a load current which has a fundamental frequency of 5 Arms. b) Determine the total harmonics distortion (THD) of load current for the first two non-zero harmonics. a) 241. 2 b) 15.2 %

25. Example : 27.3 %

26. Half-bridge inverter • Also known as the “inverter leg”. • Basic building block for full bridge, three phase and higher order inverters. • G is the “centre point”. • Both capacitors have the same value. Thus the DC link is equally “spilt” into two. • The top and bottom switch has to be “complementary”, i.e. If the top switch is closed (on), the bottom must be off, and vice-versa. • Suitable for low power inverter. Big capacitor size and not economic, for high power rating.

27. Shoot through fault and “Dead-time” • In practical, a dead time as shown below is required to avoid “shoot-through” faults, i.e. short circuit across the DC rail. • Dead time creates “low frequency envelope”. Low frequency harmonics emerged. • This is the main source of distortion for high-quality sine wave inverter.

29. Single-phase, Full-bridge • Full bridge (single phase) is built from two half-bridge leg. • The switching in the second leg is “delayed by 180 degrees” from the first leg.

31. Three-Phase Inverter • Each leg (Red, Yellow, Blue) is delayed by 120 degrees. • A three-phase inverter with star connected load is shown on the right 

32. Three Phase Inverter Waveforms

33. Three phase inverter waveforms

34. Three Phase Inverter Waveforms (6 steps output VL-N)

35. 3-phase Inverter - Analysis Rtotal = R + R/2 = 3R/2 iTotal = Vs/Rtotal = 2Vs/3R Van = Vcn = iTotal.R/2 = Vs/3 Vbn = - iTotal.R = - 2Vs/3

36. 3-phase Inverter - Harmonics Example: VDC = 100 V, Output fundamental frequency = 60 Hz, Load R-L series Y- connected with R = 10 ohm and L = 20 mH, determine the THDi. Answer : THDi = 7 % • THDi is load dependent. • The output voltage magnitude can only be controlled by VDC, and switching frequency controls the output frequency. • Harmonics of order three and multiples of three are absent

42. Pulse Width Modulation (PWM) • • Objective PWM • - Control of inverter output voltage • - Reduction of harmonics • • Disadvantages of PWM • - Increase of switching losses due to high PWM frequency • - Reduction of available voltage • - EMI problems due to high-order harmonics • • Control of Inverter Output Voltage • - PWM frequency is the same as the frequency of Vtriangular • - Amplitude is controlled by the peak value of Vcontrol (Ma) • - Fundamental frequency is controlled by the frequency of Vcontrol

43. Pulse Width Modulation (PWM)

44. Pulse Width Modulation (PWM) Triangulation Method (Natural Sampling) – Amplitudes of the triangular wave (carrier) and sine wave (modulating) are compared to obtain PWM waveform. Simple analogue comparator can be used.(Introduced by Scnohung & Stemmler) – Basically an analogue method. Its digital version, known as REGULAR sampling is widely used in industry.(Introduced by Bowes)

45. PWM Types • Natural (sinusoidal) sampling (as shown on previous slide) – Problems with analogue circuitry, e.g. Drift, sensitivity, temperature, interference, etc. • Regular sampling – simplified version of natural sampling that results in simple digital implementation • Optimized PWM – PWM waveform are constructed based on certain performance criteria, e.g. THD. • Harmonic elimination/minimisation PWM – PWM waveforms are constructed to eliminate some undesirable harmonics from the output waveform spectra. – Highly mathematical in nature • Space-Vector Modulation (SVM) – A simple technique based on volt-second that is normally used with three-phase inverter motor drive

46. Modulation Index, Ratio

47. Modulation Index, Ratio • Modulation Index determines the output voltage fundamental component • Modulation ratio determines the incident (location) of harmonics in the spectra.

48. Regular Sampling (Asymmetric)

49. Asymmetric and symmetric regular sampling