1. NAA-20021 4.7 MULTILEVEL INVERTERS (MLI)  Main feature  Ability to reduce the voltage stress on each power device due to the utilization of multiple levels on the DC bus  Important when a high DC side voltage is imposed by an application (e.g. traction systems)  Even at low switching frequencies, smaller distortion in the multilevel inverter AC side waveform can be achieved (with stepped modulation technique )  3 main MLI circuit topologies

2. NAA-20022 MLI (2)  Diode-clamped multilevel inverter (DCMI)  Extension of NPC  Based on concept of using diodes to limit power devices voltage stress  Structure and basic operating principle  Consists of series connected capacitors that divide DC bus voltage into a set of capacitor voltages  A DCMI with nl number of levels typically comprises (nl-1) capacitors on the DC bus  Voltage across each capacitor is V DC /(nl-1) ( nl nodes on DC bus, nl levels of output phase voltage, (2nl-1) levels of output line voltage)

3. NAA-20023 MLI (3)

4. NAA-20024 MLI (4)  Output phase voltage can assume any voltage level by selecting any of the nodes  DCMI is considered as a type of multiplexer that attaches the output to one of the available nodes  Consists of main power devices in series with their respective main diodes connected in parallel and clamping diodes  Main diodes conduct only when most upper or lower node is selected  Although main diodes have same voltage rating as main power devices, much lower current rating is allowable  In each phase leg, the forward voltage across each main power device is clamped by the connection of diodes between the main power devices and the nodes

5. NAA-20025 MLI (5)  Number of power devices in ON state for any selection of node is always equal to (nl-1)  Output phase voltage with corresponding switching states of power devices for a 5- level DCMI

6. NAA-20026 MLI (6)  General features  For three-phase DCMI, the capacitors need to filter only the high-order harmonics of the clamping diodes currents, low-order components intrinsically cancel each other  For DCMI employing step modulation strategy, if nl is sufficiently high, filters may not be required at all due to the significantly low harmonic content  If each clamping diode has same voltage rating as power devices, for nl-level DCMI, number of clamping diodes/phase = (nl-1) x (nl-2)  Each power device block only a capacitor voltage

7. NAA-20027 MLI (7)  Clamping diodes block reverse voltage (Dc1, Dc2, Dc3 block VDC/4, 2VDC/4 and 3VDC/4 respectively)  Unequal conduction duty of the power devices  DCMI with step modulation strategy have problems stabilizing/balancing capacitor voltages  Average current flowing into corresponding inner nodes not equal to zero over one cycle  Not significant in SVC applications involving pure reactive power transfer

8. NAA-20028 MLI (8)  Overcoming capacitor voltage balancing problem  Line-to-line voltage redundancies (phase voltage redundancies not available due to structure)  Carefully designed modulation strategies  Replace capacitors with controlled constant DC voltage source such as PWM voltage regulators or batteries  Interconnection of two DCMIs back-to- back with a DC capacitor link (suitable for specific applications only – UPFC, frequency changer, phase shifter)

9. NAA-20029 MLI (9)  Imbricated cell multilevel inverter  Capable of solving capacitor voltage unbalance problem and excessive diode count requirement in DCMI  Also known as flying capacitor multilevel inverter (capacitors are arranged to float with respect to earth)  Structure and basic operating principle  Employs separate capacitors precharged to [(nl-1)/(nl-1)xVDC], [(nl-2)/(nl-1)xVDC] …{[nl-(nl-1)]/[nl-1]xVDC}  Size of voltage increment between two capacitors defines size of voltage steps in ICMI output voltage waveform

10. NAA-200210 MLI (10)  nl-level ICMI has nl levels output phase voltage and (2nl-1) levels output line voltage

11. NAA-200211 MLI (11)  Output voltage produced by switching the right combinations of power devices to allow adding or subtracting of the capacitor voltages  Constraints : capacitors are never shorted to each other and current continuity to the DC bus capacitor is maintained  5-level ICMI – 16 power devices switching combinations (SWC). To produce VDC and 0 (1 SWC – all upper devices ON, all lower devices ON), VDC/2 (6 SWC), VDC/4 and 3VDC/4 (4 SWC)  Example - capacitor voltage combinations that produce an output phase voltage level of VDC/2

12. NAA-200212 MLI (12) VDC - VDC/2 VDC – 3VDC/4 + VDC/4 VDC - 3VDC/4 +VDC/2 – VDC/4 3VDC/4 – VDC/2 + VDC/4 3VDC/4 – VDC/4 VDC/2  Power devices switching states of a 5-level ICMI

13. NAA-200213 MLI (13)  General features  With step modulation strategy, with sufficiently high nl, harmonic content can be low enough to avoid the need for filters  Advantage of inner voltage levels redundancies - allows preferential charging or discharging of individual capacitors, facilitates manipulation of capacitor voltages so that their proper values are maintained  Active and reactive power flow can be controlled (complex selection of power devices combination,  switching frequency/losses for the former)  Additional circuit required for initial charging of capacitors

14. NAA-200214 MLI (14)  Assuming each capacitor used has the same voltage rating as the power devices, nl-level ICMI requires: (nl – 1) x (nl – 2)/2 auxiliary capacitors per phase (nl – 1) main DC bus capacitors  Unequal conduction duty of power devices  Modular structured multilevel inverter (MSMI)  Referred to as cascaded-inverters with Separate DC Sources (SDCs) or series connected H-bridge inverters  Structure and basic operating principle

15. NAA-200215 MLI (15)  Consists of (nl–1)/2 or h number of single- phase H-bridge inverters (MSMI modules)  MSMI output phase voltage Vo = Vm1 + Vm2 + …….. Vmh Vm1 : output voltage of module 1 Vm2 : output voltage of module 2 Vmh : output voltage of module h Structure of a single-phase nl-level MSMI

16. NAA-200216 MLI (16)

17. NAA-200217 MLI (17)  Power devices switching states of a 5-level MSMI

18. NAA-200218 MLI (18)  General features  Known to eliminate the excessively large number of bulky transformers required by the multipulse inverters, clamping diodes required by the DCMIs and capacitors required by the ICMIs  Simple and modular configuration  Requires least number of components  Comparison of power devices requirements per phase leg among three MLI (assuming all power devices have same voltage rating, not necessary same current rating, each MSMI module represented by a full-bridge, DCMI and ICMI use half-bridge topology)

19. NAA-200219 MLI (19)  Flexibility in extending to higher number of levels without undue increase in circuit complexity simplifies fault finding and repair, facilitates packaging  Requires DC sources isolated from one another for each module for applications involving real power transfer  Adaptation measures have to be taken in complying to the separate DC sources requirement for ASDs applications

20. NAA-200220 MLI (20) –Feed each MSMI module from a capacitively smooth fully controlled three- phase rectifier, isolation achieved using specially designed transformer having separate secondary windings/module –Employ a DC-DC converter with medium to high frequency transformers (between rectifier output and each MSMI module input), allows bidirectional power flow  Isolated DC sources not required for applications involving pure reactive power transfer (SVG)  pure reactive power drawn, phase voltage and current 90º apart  balanced capacitor charge and discharge

21. NAA-200221 MLI (21)  Originally isolated DC voltages, alternate sources of energy (PV arrays, fuel cells)  Advantage of availability of output phase voltage redundancies  Allows optimised cyclic use of power devices to ensure symmetrical utilization, symmetrical thermal problems and wear  Design of power devices utilization pattern possible  Overall improvement in MSMI performance – high quality output voltage etc.

22. NAA-200222 MLI (22)  Modulation strategies for multilevel inverters  Step modulation  Space vector modulation  Optimal/programmed PWM technique  Sigma delta modulation (SDM)  High-dynamic control strategies  Multilevel hysterisis modulation strategy  Sliding mode control based on theory of Variable Structure Control System (VSCS)