NAA-2002 1

4.7 MULTILEVEL INVERTERS (MLI)

Main featureAbility 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

NAA-2002 2

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 VDC/(nl-1) ( nl nodes on DC bus, nl levels of output

phase voltage , (2nl-1) levels of output line voltage)

NAA-2002 3

MLI (3)

VDC/4

VDC/4

VDC/4

VDC/4

V1

V5

V4

V3

V2

V DC

Dc1

Dc2

Dc3

Dc4

Dc5

Dc6

Vo

S1

S2

D1

S8

S7

S6

S5

S4

S3

D5

D4

D3

D2

D8

D7

D6

NAA-2002 4

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

NAA-2002 5

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

Output Phase Voltage (Vo) Power device

index V1

V2

V3

V4

V5

S1 1 0 0 0 0

S2 1 1 0 0 0

S3 1 1 1 0 0

S4 1 1 1 1 0

S5 0 1 1 1 1

S6 0 0 1 1 1

S7 0 0 0 1 1

S8 0 0 0 0 1

NAA-2002 6

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

NAA-2002 7

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

NAA-2002 8

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)

NAA-2002 9

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

NAA-2002 10

MLI (10) nl-level ICMI has nl levels output phase

voltage and (2nl-1) levels output line voltage

3VDC/4 Vo

S1

S8

S7

S6

S5

S4

S3

S2

VDC/2 VDC/4VDC

D1

D7

D6

D5

D4

D3

D2

D8

NAA-2002 11

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

NAA-2002 12

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 Output Phase Voltage (Vo)

Power device

index V1

V2

V3

V4

V5

S1 1 0 0 0 0

S2 1 1 0 0 0

S3 1 1 1 0 0

S4 1 1 1 1 0

S5 0 1 1 1 1

S6 0 0 1 1 1

S7 0 0 0 1 1

S8 0 0 0 0 1

NAA-2002 13

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

NAA-2002 14

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

NAA-2002 15

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

NAA-2002 16

MLI (16)

Vphase (Vo)

S11 S21

S31 S41

S1h S2h

S3h S4h

S12 S22

S32 S42

VDC

Module 1

Module 2

Module h

Vm1

0

VDC

VDC

Vm2

Vmh

NAA-2002 17

MLI (17) Power devices switching states of a 5-level

MSMIPower devices index Output voltages

S11 S21 S31 S41 S12 S22 S32 S42 Vm1 Vm2 Vo

1 0 0 1 1 0 0 1 +VDC +VDC +2VDC

1 0 0 1 1 1 0 0 +VDC 0 +VDC

1 0 0 1 0 0 1 0 +VDC 0 +VDC

1 0 0 1 0 1 1 0 +VDC VDC 0

1 1 0 0 1 0 0 1 0 +VDC +VDC

1 1 0 0 1 1 0 0 0 0 0

1 1 0 0 0 0 1 0 0 0 0

1 1 0 0 0 1 1 0 0 VDC VDC

0 0 1 1 1 0 0 1 0 +VDC +VDC

0 0 1 1 1 1 0 0 0 0 0

0 0 1 1 0 0 1 0 0 0 0

0 0 1 1 0 1 1 0 0 VDC VDC

0 1 1 0 1 0 0 1 VDC +VDC 0

0 1 1 0 1 1 0 0 VDC 0 VDC

0 1 1 0 0 0 1 0 VDC 0 VDC

0 1 1 0 0 1 1 0 VDC VDC -2VDC

NAA-2002 18

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)

NAA-2002 19

MLI (19)

Type of multilevel inverter DCMI ICMI MSMI

Main power devices (nl – 1) x 2 (nl – 1) x 2 (nl – 1) x 2

Main diodes (nl – 1) x 2 (nl – 1) x 2 (nl – 1) x 2

Clamping diodes (nl - 1) x (nl - 2) 0 0

DC bus capacitors (nl – 1) (nl – 1) (nl – 1)/2

Balancing capacitors 0 (nl – 1) x (nl – 2)/2 0

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

NAA-2002 20

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

NAA-2002 21

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.

NAA-2002 22

MLI (22)

Modulation strategies for multilevel invertersStep 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)