Multi level inverter

Multi level Inverter- classification-

1. 1 Multi Level Inverter Dr. A. Ravi Professor/EEE FRANCIS XAVIER ENGINEERING COLLEGE TIRUNELVELI- India

2. Multilevel Inverters - Introduction 2

3. Multilevel Inverters - Introduction 3 Drawbacks of two-level VSIs for MV Drives H i g h d v /dt i n th e i n v erter outp u t v oltage – as hi g h as 10,000V/µs Motor harmonic losses This can be solved by adding properly tuned LC filter. It has some disadvantages Increased manufacturing cost Fundamental voltage drop Circulating current between the filter and DC circuit

4. Multilevel inverter output voltage: (a) two-level and (b) nine-level. 4 Multilevel Inverters - Introduction

5. Multilevel Voltage Source Inverter One phase leg of general n-level inverter 5

6. Multilevel Voltage Source Inverter Three phase multi level inverter 6

7. Multilevel Voltage Source Inverter 7 Schematic of single pole of multilevel inverter by a switch Each capacitor has the same voltage Em , which is given by m level inverter need (m-1) capacitors

8. Multilevel Voltage Source Inverter 8 The actual realization of the switch requires bidirectional switching devices for each node. The topological structure of multilevel inverter must (1) have less switching devices as far as possible , (2) be capable of withstanding very high input voltage for high-power applications, (3) have lower switching frequency for each switching device.

9. Multilevel Voltage Source Inverter 9 Typical output voltage of a five-level multilevel inverter.

10. Multilevel Voltage Source Inverter 10 The general structure of the multilevel converter is to synthesize a near sinusoidal voltage from several levels of dc voltages, typically obtained from capacitor voltage sources . As the number of levels increases, the synthesized output waveform has more steps, which produce a staircase wave that approaches a desired waveform . Also, as more steps are added to the waveform, the harmonic distortion of the output wave decreases, approaching zero as the number of levels increases . As the number of levels increases, the voltage that can be spanned by summing multiple voltage levels also increases.

11. Multilevel Voltage Source Inverter 11 The output voltage during the positive half-cycle can found from where SFn is the switching or control function of nth node and it takes a value of 0 or 1. Generally, the capacitor terminal voltages E1, E2, call have the same value Em .

12. Diode-clamped multilevel inverter; Flying-capacitors multilevel inverter; Cascade multilevel inverter. 12 Types Multilevel Inverter

13. Multilevel Voltage Source Inverter 13 Multi-level inverters are the preferred choice in industry for the application in High voltage and High power application Advantages of Multi-level inverters Higher voltage can be generated using the devices of lower rating. Increased number of voltage levels produce better voltage waveforms and reduced THD. Switching frequency can be reduced for the PWM operation.

14. Multilevel Converter Topologies 14

15. Diode Clamped (NPC) 3-level Inverter Three-phase three-level diode-clamped converter also called NPC converter

16. 16 Diode Clamped Multilevel Inverter

17. 17 Diode Clamped Multilevel Inverter -1 phase 1. For an output voltage level v ao = Vdc , turn on all upper-half switches Sa1 through Sa4. 2 . For an output voltage level v ao = 3Vdc/4, turn on three upper switches Sa2 through Sa4 and one lower switch S a1 ‘ 3. For an output voltage level v ao = Vdc /2, turn on two upper switches Sa3 through Sa4 and two lower switches S a1 ‘ and S a2 ’ 4. For an output voltage level v ao = Vdc /4, turn on one upper switch Sa4 and three lower switches Sa1 through Sa3 5 . For an output voltage level v ao = 0, turn on all lower half switches S ‘ a1 , through S a4 ’

18. 18 Diode Clamped Multilevel Inverter -1 phase Diode-Clamped Voltage Levels and Their Switch States

19. High-voltage rating for blocking diodes: In an m-level leg, there can be two diodes, each seeing a blocking voltage of 19 Features of Diode Clamped Multilevel Inverter where m is the number of levels; k goes from 1 to 1m - 22; Vdc is the total dc-link voltage . the number of diodes required for each phase is N D as (m-1)(m-2)

20. High-voltage rating for blocking diodes : Capacitor voltage unbalance: 20 Features of Diode Clamped Multilevel Inverter

21. Major advantages of the diode-clamped inverter When the number of levels is high enough, the harmonic content is low enough to avoid the need for filters . • Inverter efficiency is high because all devices are switched at the fundamental frequency. • The control method is simple.

22. Major Disadvantages of the diode-clamped inverter • Excessive clamping diodes are required when the number of levels is high . • It is difficult to control the real power flow of the individual converter in multi converter systems .

23. Diode Clamped (NPC) 4-level and 5- level Inverters 23

24. Diode Clamped (NPC) 4-level and 5- level Inverters SWITCH STATUS V AN FOUR-LEVEL INVERTER S 1 S 2 S 3 S 1 ’ S 2 ’ S 3 ’ 1 1 1 3E 1 1 1 2E 1 1 1 E 1 1 1 FIVE-LEVEL INVERTER V AN S 1 S 2 S 3 S 4 S 1 ’ S 2 ’ S 3 ’ S 4 ’ 1 1 1 1 4E 1 1 1 1 3E 1 1 1 1 2E 1 1 1 1 E 1 1 1 1 24

25. Diode Clamped (NPC) multilevel Inverters Component Count of Diode-Clamped Multilevel Inverters Voltage Level m Active Switches 6(m-1) Clamping Diodes a 3(m-1)(m-2) DC Capacitors (m-1) 3 12 6 2 4 18 18 3 5 24 36 4 6 30 60 5 7 36 90 6 a All diodes and active switches have the same voltage rating. 25

26. Diode Clamped (NPC) multilevel Inverters 26 Disadvantages Uneven loss distribution in the devices In a fundamental cycle, the conduction period of the inner devices is more than the outer devices. This causes unequal losses in devices in a leg. The fluc t ua t ion o f th e dc b u s mi d p o int voltage Additional clamping diodes. Complicated PWM switching pattern design

27. Flying Capacitor 3-level Inverter 27

28. Flying Capacitor 3-level Inverter S a1 S a2 S a3 S a4 Pole voltage, V aO 1 1 V dc /2 1 1 1 1 1 1 -V dc /2 28

29. 29 Flying Capacitor 5 -level Inverter

30. Flying Capacitor 5-level Inverter 30 Switching State Pole voltage, V AN S 1 S 2 S 3 S 4 1 1 1 1 4E 1 1 1 3E 1 1 1 1 1 1 1 1 1 1 1 2E 1 1 1 1 1 1 1 1 1 1 1 E 1 1 1

31. Flying Capacitor Multilevel Inverters Component Count of Flying Capacitor Multilevel Inverters Voltage Level m Active Switches 6(m-1) Clamping Diodes DC Capacitors m  2 (m  1)  3 * (  k) k  1 3 12 5 4 18 12 5 24 22 6 30 35 7 36 51

32. Features of Flying-Capacitors Inverter 32 Large number of capacitors : Balancing capacitor voltages:

33. The major advantages of the flying-capacitors inverter can be summarized as follows Large amounts of storage capacitors can provide capabilities during power outages . These inverters provide switch combination redundancy for balancing different voltage levels. Like the diode-clamp inverter with more levels, the harmonic content is low enough to avoid the need for filters. Both real and reactive power flow can be controlled. 33

34. The major disadvantages of the flying-capacitors inverter can be summarized as follows: An excessive number of storage capacitors is required when the number of levels is high. High-level inverters are more difficult to package with the bulky power capacitors and are more expensive too . The inverter control can be very complicated, and the switching frequency and switching losses are high for real power transmission. 34

35. Multilevel (3-level) Cascaded H- Bridge Inverters - with equal voltages 35

36. Multilevel (3-level) Cascaded H- Bridge Inverters - with equal voltages 36 Switching State Pole voltage, V AN S 1A S 2A S 3A S 4A 1 1 E 1 1 1 1 1 1 -E

37. Multilevel (5-level) Cascaded H-Bridge Inverters - with equal voltages

38. Multilevel (5-level) Cascaded H-Bridge Inverters - with equal voltages PEGCRES 2015 38 Switching State V H1 V H2 Pole voltage, V AN S 11 S 31 S 12 S 32 1 1 E E 2E 1 1 1 E E 1 E 1 1 1 E 1 E 1 1 1 1 1 1 1 1 1 1 E -E 1 1 - E E 1 1 1 - E -E 1 - E 1 1 1 -E 1 -E 1 1 - E -E - 2E

39. Multilevel Cascaded H-Bridge Inverters – with equal voltages 39 The number of voltage levels in a CHB inverter can be found from m = (2 H + 1) where H is the number of H-bridge cells per phase leg. The voltage level m is always an odd number for the CHB inverter while in other multilevel topologies such as diode-clamped inverters, it can be either an even or odd number. The total number of active switches (IGBTs) used in the CHB inverters can be calculated by N sw = 6(m – 1)

40. Multilevel Cascaded H-Bridge Inverters (7 and 9-level) – per phase diagram 40

41. Multilevel Cascaded H-Bridge Inverters - with unequal voltages Per phase diagram 41

42. Multilevel Cascaded H-Bridge Inverters - with unequal voltages Voltage Level and Switching State of the Two-Cell Seven-Level CHB Inverter with Unequal dc Voltages 42

43. Cascaded H-Bridge Multilevel Inverters Component Count of Cascaded H-Bridge Multilevel Inverters Voltage Level m Active Switches 6(m-1) Clamping Diodes DC Sources 3 12 3 5 24 6 7 36 9 9 48 12 43

44. Features of Cascaded Inverter For real power conversions from ac to dc and then dc to ac, the cascaded inverters need separate dc sources. The structure of separate dc sources is well suited for various renewable energy sources such as fuel cell, photovoltaic, and biomass . Connecting dc sources between two converters in a back-to-back fashion is not possible because a short circuit can be introduced when two back-to-back converters are not switching synchronously. 44

45. 45 The major advantages of the cascaded inverter can be summarized as follows : • Compared with the diode-clamped and flying-capacitors inverters, it requires the least number of components to achieve the same number of voltage levels. • Optimized circuit layout and packaging are possible because each level has the same structure and there are no extra clamping diodes or voltage-balancing capacitors . • Soft-switching techniques can be used to reduce switching losses and device stresses.

46. 46 The major disadvantage of the cascaded inverter is as follows : • It needs separate dc sources for real power conversions, thereby limiting its applications .

47. key features of a multilevel structure The output voltage and power increase with number of levels. Adding a voltage level involves adding a main switching device to each phase . The harmonic content decreases as the number of levels increases and filtering requirements are reduced . With additional voltage levels, the voltage waveform has more free-switching angles , which can be preselected for harmonic elimination. In the absence of any PWM techniques, the switching losses can be avoided . Increasing output voltage and power does not require an increase in rating of individual device. 47

48. 48 Comparisons of Component Requirements per Leg of Three Multilevel Converters

49. Referenc e s 49 B. Wu, High-Power Converters and AC Drives, Wiley-IEEE Press, Piscataway, NJ, 2006. J. Rodriguez, J. S. Lai, and F. Z. Peng, Multilevel inverters: A survey of topologies, controls, and applications, IEEE Transactions on Industrial Electronics, 49(4), 724–738, August 2002. N . Mohan, Electronics: T . M . Und e la nd , an d W . P . Rob b in s , P o w er C on v er t er s , Applicatio n s, and Design, 3 ed n , Wiley, Hoboken, NJ, October 10, 2002. Rodriguez, S. Bernet, B. Wu, J. O. Pontt, and S. Kouro, Multilevel voltage-source-converter topologies for industrial medium-voltage drives, IEEE Transactions on Industrial Electronics, 54(6), 2930–2945, December 2007.

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