Lecture 28 360 chapter 9_ power electronics inverters
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Lecture 28 360 chapter 9_ power electronics inverters
- 1. EEE 360 Energy Conversion and Transport George G. Karady & Keith Holbert Chapter 9 Introduction to Motor Control and Power Electronics 10/22/14 360 Chapter 9 Power Electro1nics
- 2. Lecture 28 10/22/14 360 Chapter 9 Power Electro2nics
- 3. 9.5.1 Voltage Source Inverter with Pulse Width Modulation 10/22/14 360 Chapter 9 Power Electro3nics
- 4. 9.5.1 Voltage Source Inverter • The amplitude of the harmonics can be reduced by using the pulse width modulation (PWM) technique. • The basic concept of the PWM method is the division of the on-time into several on and off periods with varying duration. • The rms value of the ac voltage is controlled by the on-time of the switches,. 10/22/14 360 Chapter 9 Power Electro4nics
- 5. 9.5.1 Voltage Source Inverter • The most frequently used PWM technique is sinusoidal pulse width modulation. • This approach requires a bridge converter with IGBT or MOSFET switches shunted by an anti-parallel connected diode. • The diode allows current flow in the opposite direction when the switch is open. 10/22/14 360 Chapter 9 Power Electro5nics
- 6. 9.5.1 Voltage Source Inverter • These freewheeling diodes prevent inductive current interruption • This provides protection against transient overvoltage, which may cause reverse breakdown of the IGBT and MOSFET switches. • The typical circuit diagram is shown in Figure 9.44. 10/22/14 360 Chapter 9 Power Electro6nics
- 7. 9.5.1 Voltage Source Inverter Rload Idc Vdc Lload Vac S1 S4 S3 S2 Figure 9.44 Single-phase voltage source converter. 10/22/14 360 Chapter 9 Power Electro7nics
- 8. 9.5.1 Voltage Source Inverter • During the positive cycle, S1 and S2 are switched by the high frequency pulse train shown in Figure 9.45. • During the negative cycle, the pulse train switches S3 and S4. • The load inductance integrates the generated pulse train and produces a sinusoidal voltage (Vac) and current wave, as shown in Figure 9.45. 10/22/14 360 Chapter 9 Power Electro8nics
- 9. 9.5.1 Voltage Source Inverter • The width of each pulse is varied in proportion to the amplitude of a sine wave. A typical PWM waveform is shown in Figure 9.45. • The switches in this converter are controlled by gate pulses. • The gate signal contains several pulses distributed along the half-cycle. 10/22/14 360 Chapter 9 Power Electro9nics
- 10. 9.5.1 Voltage Source Inverter PWM Output Voltage Load Current Load Voltage 0s 10ms 20ms 30ms 40ms 50ms I(V5)*10 V(L1:1,VOUT-) V(V5:+,V5:-) Time 400 0 -400 Figure 9.45 Gate pulse input signal, and ac voltage and current outputs of a pulse width modulation (PWM) converter. 10/22/14 360 Chapter 9 Power Electro1n0ics
- 11. 9.5.1 Voltage Source Inverter • The control circuit produces the gate pulse train by generation of a triangular carrier wave and a sinusoidal reference signal. • The two signals are compared, and when the carrier wave is larger than the reference signal, the gate signal is positive. 10/22/14 360 Chapter 9 Power Electro1n1ics
- 12. 9.5.1 Voltage Source Inverter • When the carrier wave is smaller than the reference signal, the gate signal is zero. • This results in a gate pulse with variable width. • Figure 9.46 – (a) shows the carrier wave and reference sine wave; – (b) depicts the resulting gate signal with variable width pulses. It has to be noted that several other methods are used for generation of PWM signals 10/22/14 360 Chapter 9 Power Electro1n2ics
- 13. 9.5.1 Voltage Source Inverter 1.0V 0V -1.0V Carrier wave Reference signal 0s 5ms 10ms 15ms 20ms 25ms V(PWM_TRI1.E1:IN+) V(PWM_TRI1.Vtri:+) Time (a) Triangular carrier wave and sinusoidal reference signal 10/22/14 360 Chapter 9 Power Electro1n3ics
- 14. 9.5.1 Voltage Source Inverter Gate pulse with variable width 0s 5ms 10ms 15ms 20ms 25ms Time V(PWM_TRI1:s) 1.0V 0V -1.0V (b) Variable-width gate pulse signal Figure 9.46 Pulse width modulation (PWM) signals. 10/22/14 360 Chapter 9 Power Electro1n4ics
- 15. 9.5.1 Voltage Source Inverter • The frequency of the reference sine wave determines the frequency of the generated ac voltage. • The amplitude of the ac voltage can be regulated by the variation of the reference signal amplitude. 10/22/14 360 Chapter 9 Power Electro1n5ics
- 16. 9.5.1 Voltage Source Inverter • The amplitude of the fundamental component of the ac voltage is: V = V = ac V mV dc dc control carrier V • The modulation index is the ratio of the peak-to- peak ac voltage (2Vac) to the dc voltage. 10/22/14 360 Chapter 9 Power Electro1n6ics
- 17. 9.5.1 Voltage Source Inverter Freewheeling diode • The inverter interrupts the current several times each cycle. • The interruption of an inductive current would generate unacceptably high overvoltage. • This overvoltage generation is eliminated by providing freewheeling diodes connected in parallel with the switches. 10/22/14 360 Chapter 9 Power Electro1n7ics
- 18. 9.5.1 Voltage Source Inverter Frdeieowdheeeling • When the switches open, the current, if inductive, is diverted to the diodes, as shown in Figure 9.47 Current when switches S1 and S2 closed and S3 and S4 open Current when switches S1 and S2 open and S3 and S4 open 10/22/14 360 Chapter 9 Power Electro1n8ics Rload Vdc Vac Idc Lload S1 S4 S3 S2 Figure 9.47 Freewheeling diode operation.
- 19. 9.5.1 Voltage Source Inverter Rload Current when switches S1 and S2 closed and S3 and S4 open Current when switches S1 and S2 open and S3 and S4 open Idc Vdc Lload Vac S1 S4 S3 S2 Figure 9.47 Freewheeling diode operation. • The diagram shows the current path when switches S1 and S2 are closed, and switches S3 and S4 are open. • When switches S1 and S2 open (now all switches are open), the current diverts through the diodes of switches S3 and S4. • This current diversion prevents the interruption of inductive current. 10/22/14 360 Chapter 9 Power Electro1n9ics
- 20. 9.5.2 Line Commutated Inverter 10/22/14 360 Chapter 9 Power Electro2n0ics
- 21. 9.5.2 Line Commutated Inverter • The current source inverter operation requires both ac and dc sources • In addition to an inductance that maintains the dc current constant or at least assures continuous dc current. • The inverter operation requires a delay angle between 90° and 180°. 10/22/14 360 Chapter 9 Power Electro2n1ics
- 22. 9.5.2 Line Commutated Inverter • Figure 9.48 shows bridge circuit generated waveforms for 90° and 170° delay angles. – (a) shows that the delay angle between the ac voltage – The square-shaped ac current is 90° – The average dc voltage is zero – Because the magnitude and duration of the positive and negative segments of the dc voltage are identical. 200 160 120 80 40 0 40 80 120 160 200 0 30 60 90 120 150 180 210 240 270 300 330 360 w×t deg (a) Delay angle of 90° Vac(t) V Vdc( t,a) V Iac( t,a) A 10/22/14 360 Chapter 9 Power Electro2n2ics
- 23. 9.5.2 Line Commutated Inverter • Figure 9.48 shows bridge circuit generated waveforms for 90° and 170° delay angles. – (b) shows that the delay 200 160 120 80 40 0 40 Vac(t) V Vdc( t ,a) V Iac( t,a) A angle between the ac voltage 80 and the square-shaped ac 120 160 current is 170°, – The average dc voltage is negative – Because the duration of the positive part of the dc voltage is almost non-existent (i.e., 10° out of 180°). . This implies that the power flows 200 0 30 60 90 120 150 180 210 240 270 300 330 360 w×t deg •(b) Delay angle of 170° from the dc to the ac side 10/22/14 360 Chapter 9 Power Electro2n3ics
- 24. 9.5.2 Line Commutated Inverter • The practical use of the line-commutated inverter requires the regulation of the dc voltage to maintain constant dc current. • Figure 9.49(a) shows the bridge converter circuit that produces the voltages shown in Figure 9.48. • Using the Thévenin equivalent, the converter can be replaced by a dc source and impedance, as shown in Figure 9.49(b). • The average dc current in this equivalent circuit is: - V V I dc_inv dc_source dc R = 10/22/14 360 Chapter 9 Power Electro2n4ics
- 25. 9.5.2 Line Commutated Inverter • The practical use of the line-commutated inverter requires the regulation of the dc voltage to maintain constant dc current. • Figure 9.49(a) shows the bridge converter circuit that produces the voltages shown in Figure 9.48. Vdc_inv Idc Th1 Th3 Vac Vdc_inv Vdc_source A) Circuit diagram 10/22/14 360 Chapter 9 Power Electro2n5ics Vdc_source Vac AC Th4 Th2 Idc Voltage R difference Voltage difference A B Figure 9.49 Single-phase line-commutated inverter B) Equivalent circuit
- 26. 9.5.2 Line Commutated Inverter Vdc_inv Idc Figure 9.50 Voltage difference during inversion. 10/22/14 360 Chapter 9 Power Electro2n6ics Vdc_source Voltage difference R • The converter delay angle and the dc source voltage must be controlled simultaneously to maintain a constant voltage difference and dc current. • In a practical circuit, the required voltage difference is generally small • Consequently, both the inverter-produced voltage and the source voltage must be negative, as shown in Figure 9.50.
- 27. 9.5.2 Line Commutated Inverter • The dc current can be maintained constant by keeping the voltage difference constant. • Consequently, if the delay angle increases, Vdc_inv is reduced; • The maintenance of constant dc current and voltage difference requires the appropriate reduction of the Vdc_source. Vdc_inv Idc 10/22/14 360 Chapter 9 Power Electro2n7ics Vdc_source Voltage difference R Figure 9.50 Voltage difference during inversion.
- 28. 9.5.2 Line Commutated Inverter Vdc_inv Idc Figure 9.50 Voltage difference during inversion. 10/22/14 360 Chapter 9 Power Electro2n8ics Vdc_source Voltage difference R • If the firing angle increases beyond 90°, – Vdc_inv becomes negative; – The maintenance of constant dc current and voltage difference requires the change of Vdc_source polarity. • In this case, the power flows from the dc to the ac circuit,
- 29. 9.5.2 Line Commutated Inverter Vdc_inv Idc Figure 9.50 Voltage difference during inversion. 10/22/14 360 Chapter 9 Power Electro2n9ics Vdc_source Voltage difference R , • The current and voltage have – the same direction in the dc source (generator) – opposite directions in the inverter (load) as demonstrated
- 30. 9.5.3 High Voltage DC transmission 10/22/14 360 Chapter 9 Power Electro3n0ics
- 31. 9.5.3 High Voltage DC transmission • High-voltage dc lines are used to transport large amounts of energy over a long distance. • A representative application is the Pacific DC Intertie, which interconnects the Los Angeles area with Oregon. • The voltage of the DC Intertie is ±500 kV and the maximum energy transport is 3100 MW. • More than one hundred dc transmission systems operate around the world, one of the oldest and most famous is the cable interconnection between England and France. 10/22/14 360 Chapter 9 Power Electro3n1ics
- 32. 9.5.3 High Voltage DC transmission Transformer Inductance Inductance AC filter 10/22/14 360 Chapter 9 Power Electro3n2ics AC filter DC filter DC filter DC Line (+) Converter DC Line (-) Converter Transformer Figure 9.51 Concept of high-voltage dc transmission
- 33. 9.5.3 High Voltage DC transmission • Figure 9.51 show a simplified connection diagram for a high-voltage dc (HVDC) system. • The major elements are two converter stations interconnected by a dc transmission line. • The converter station can operate in both inverter and rectifier modes, which permits energy transfer in both directions. • Each converter station contains two converters. 10/22/14 360 Chapter 9 Power Electro3n3ics
- 34. 9.5.3 High Voltage DC transmission • The two converters are connected in series at the dc side. The series connection node (middle point) is grounded. • One of the converters generates the positive, and the other produces the negative dc voltage. • The harmonics are filtered at both the ac and dc sides. 10/22/14 360 Chapter 9 Power Electro3n4ics
- 35. 9.5.3 High Voltage DC transmission • Each converter is typically supplied by a transformer. The HVDC system uses two different transformer types at each converter station. – One of the converters is supplied by a wye-wye transformer – The other is connected to a wye-delta transformer. • This produces a 30° phase shift between the dc voltage outputs of the two converters. • The phase shift produces a smoother dc output voltage. 10/22/14 360 Chapter 9 Power Electro3n5ics
- 36. 9.5.3 High Voltage DC transmission • Figure 9.52 shows the converters in a HVDC station. • Each converter contains six high voltage valves, with several hundred thyristors connected in series. • The valves are shielded by rounded aluminum electrodes as shown in the Fig 9.52 10/22/14 360 Chapter 9 Power Electro3n6ics
- 37. 9.5.3 High Voltage DC transmission Figure 9.52 Valve hall of a DC converter station. 10/22/14 360 Chapter 9 Power Electro3n7ics