Post on 31-Dec-2015
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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
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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)