Transcript DC to AC Conversion ( INVERTER ) - ENCON

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
NAA-2002
1
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
2
MLI (3)
V1
S1
D1
Dc1
S2
D2
Dc4
S3
D3
S4
D4
VDC/4
V2
VDC/4
V
D
C
Dc2
V3
Vo
VDC/4
V4
Dc3
Dc6
Dc5
S5
D5
S6
D6
S7
D7
S8
D8
VDC/4
V5
NAA-2002
3
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
4
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 5level DCMI
Power device
Output Phase Voltage (Vo)
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
index
NAA-2002
5
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
6
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
7
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-toback with a DC capacitor link (suitable
for specific applications only – UPFC,
frequency changer, phase shifter)
NAA-2002
8
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
9
MLI (10)
 nl-level ICMI has nl levels output phase
voltage and (2nl-1) levels output line voltage
VDC
3VDC/4
VDC/2
S1
D1
S2
D2
S3
D3
S4
D4
Vo
VDC/4
NAA-2002
S5
D5
S6
D6
S7
D7
S8
D8
10
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
11
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
Power device
Output Phase Voltage (Vo)
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
index
NAA-2002
12
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
13
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
14
MLI (15)
 Consists of (nl–1)/2 or h number of singlephase 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
15
MLI (16)
S11
S21
VDC
Vm1
S31
Vphase (Vo)
S41
Module 1
S12
S22
VDC
Vm2
S32
S42
Module 2
S1h
S2h
Vmh
VDC
S3h
0
S4h
Module h
NAA-2002
16
MLI (17)
 Power devices switching states of a 5-level
MSMI
Power 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
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
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
NAA-2002
0
0
-2VDC
17
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
18
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
19
MLI (20)
– Feed each MSMI module from a
capacitively smooth fully controlled threephase 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
20
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
21
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)
NAA-2002
22