4.7 MULTILEVEL INVERTERS (MLI)

1
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
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)

3
MLI (3)
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

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

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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
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
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
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-1xVDC
• Size of voltage increment between two capacitors
  defines size of voltage steps in ICMI output
  voltage waveform

10
MLI (10)

• nl-level ICMI has nl levels output phase voltage
  and (2nl-1) levels output line voltage

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

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
  

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

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

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MLI (15)

• Consists of (nl1)/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
MLI (16)
17
MLI (17)

• Power devices switching states of a 5-level MSMI

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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)

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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
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
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
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)