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1

Classical Multilevel ConvertersGabriel H. P. Ooi, Ziyou Lim, and Hossein Dehghani Tafti

1.1 Introduction

Power electronic converters are classified mainly based on the current (CS) or voltagesource (VS). In the early days around the 1980s, current source inverters (CSI) werepopular when thyristor-based semiconductors were first developed [1]. The CSIsare also known as load-commutated inverters (LCI) in the industry, which mainlycomprise the gate turn-off thyristor (GTO) or the integrated gate-commutated thyristor(IGCT) in the circuit. Usually, CSIs are operated under short-circuit conditions usingcurrent-controlled switching devices; hence, a low switching frequency is required.Besides, the gate driver circuitry design can become too complicated.

In 1964, Ray E. Morgan had proved that the performance of an inverter will be moreefficient for a fast switching operation where switching losses are greatly reducedthrough the experimented bridge chopper inverter. Despite the IGCT allowing a higherswitching frequency (up to a few kilohertz) than the GTO, the switching losses areconsiderably high. Hence, the switching frequency for the IGCT is typically limited toa few hundred hertz (around 500 Hz).

In the mid-1990s, insulated-gate bipolar transistors (IGBTs) designed to beopen-circuit voltage-controlled semiconductor devices for fast switching purposes hadbecome available in the market. As a result, IGBTs would have been more favorablein the voltage source inverter (VSI) where a lower switching loss, higher efficiency,and a higher reliability are achieved. Moreover, the VSI has a low current harmonicdistortion due to the high switching frequency operation. The VSI is used in industrialapplications due to the numerous advantages, and therefore, multilevel VSI is the primefocus of research and will be further discussed in this chapter.

1.2 Classical Two-Level Converters

The configuration of the conventional three-phase two-level VSI as shown in Fig. 1.1 hasbeen commonly used in the early 1960s for motor drives. The 2L-VSIs were constructedusing thyristor-based switching devices during those times until the 1990s when IGBTswere used to replace the thyristors for faster switching purposes.

Advanced Multilevel Converters and Applications in Grid Integration, First Edition.Edited by Ali I. Maswood and Hossein Dehghani Tafti.© 2019 John Wiley & Sons Ltd. Published 2019 by John Wiley & Sons Ltd.

COPYRIG

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4 1 Classical Multilevel Converters

Phase APhase B

Phase C

–ve

+ve

abc

Sx2

Sx1

Ix(t)

Vdc CdcThree-phase

acload

Cxf

LxfRxf

Figure 1.1 Traditional three-phase two-level voltage source inverter (2L-VSI).

The topology of the three-phase 2L-VSI is simple in nature, consisting only of twoswitching devices in each phase leg, thus allowing the cost of implementation to berelatively low. Although a fast switching operation of up to 20 kHz can be performedwith the IGBTs to reduce the switching losses in the 2L-VSI, there is an implication ofhigh stress in motor winding especially when a long cable is connected between theinverter and the motor [2]. The increase in switching frequency operation causes highdV/dt (large voltage spikes) due to the characteristic impedances of the cable; hence, ithas a detrimental impact on the motor.

However, electromagnetic interference (EMI) becomes a major issue when the powerelectronic appliances are operating at a switching frequency range between 10 kHzand 30 MH. Since power electronic converters usually operate below the frequency of10 MHz (typically around 10 kHz to 150 kHz), EMI is spread by the conduction of thewires. Hence, the conducted EMI is the key concern in the design of power convertersinstead of the radiated EMI. Thus, the EMI filter is required to suppress the conductedEMI for the 2L-VSI with a 20 kHz switching frequency so that the filter size can also beminimized.

Apart from that, there is a limitation for the 2L-VSI to drive high power (≥100 MW)applications due to the available voltage ratings of the semiconductor devices in the mar-ket. Based on the configuration of the 2L-VSI, each of the semiconductor devices has tosupport the total amount of DC voltage supply. Therefore, the design of 2L-VSI becomescomplex when a number of semiconductor components are series-connected togetherin order to withstand high DC voltage [3, 4]. As a result, the reliability of the 2L-VSI willalso be greatly affected due to the increase in the number of switching devices requiredin the circuit.

1.3 The Need for Multilevel Converters

Multilevel power converters, a new breed of power electronic converters, are beingdeveloped and evolved since the 1980s. The ability to overcome the limitations and the

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1.4 Classical Multilevel Converters 5

challenges faced in the traditional 2L-VSI topology has been favorable in many industrialhigh-power applications.

True to their name, multilevel converters synthesize the output voltage into a multiplestepped level (staircase-like) waveform. By doing so, the voltage stress across each semi-conductor device is greatly reduced. Additionally, by higher number of output voltagestepped levels, the voltage harmonic is decreased. Thus, the size of the output filter canbe further reduced or even eliminated. In addition, good output power quality can beachieved even with a much lower switching frequency (less than 10 kHz). Hence, theproblems of EMI and high switching losses can be avoided.

Multilevel converters have drawn great research interest and attention over the lastdecade because of their numerous advantages. Despite having been considered asproven mature technologies in many industries, multilevel converters still encountermany challenges, such as unbalanced capacitor voltages when a higher number ofoutput voltage stepped levels is intended to be achieved. Therefore, an investigation ofthe existing multilevel topologies used in industries will be discussed in the followingsections of this chapter. The advantages along with the drawbacks of each multilevelconverter topology will be elaborated as well. These will serve in providing a clearervision to remain focused on the research area and alternative solutions can be proposedto improve the performance of multilevel converters.

1.4 Classical Multilevel Converters

1.4.1 Multilevel Diode Clamped Converters

The three-level neutral point clamped inverter (3L-NPC), shown in Fig. 1.2, was createdby Nabae and colleagues [5] as a multilevel power converter in 1981. The expansion ofthe 3L-NPCI is discussed in [6] and the first 10 kVA four-level inverter prototypes areinvestigated in [7].

Based on the fundamental 3L-NPC (Fig. 1.2), the three-level output voltage steppedwaveform is synthesized through both the diodes clamped to the middle of both thedc-link capacitors. The middle connection of the dc-link capacitors is referred to as theneutral point and also served as the output phase voltage reference point. Since theoutput voltage waveform is synthesized from the dc-link capacitor voltage, the numberof diode elements required to be clamped will increase when a higher number of out-put voltage level is desired. Therefore, the topology of 3L-NPC evolves into a multileveldiode clamped inverter (MDCI).

The number of switches i and clamping diodes D required for the m-level (mL) outputphase voltage of the MDCIs can be determined using equations (1.1) and (1.2), respec-tively:

iswitches(MDCI) = 2[(mL) − 1] (1.1)

Ddiodes(MDCI) = 2[(mL) − 2] (1.2)

The number of dc-link capacitors Q needed for mL-MDCIs can be known based onthe following equation (1.3):

Qcapacitors(MDCI) = (mL) − 1 (1.3)

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6 1 Classical Multilevel Converters

Phase A

mth- level

Phase BPhase C

–ve

+ve

Three-phaseac

load

ab

c

Sx1

Sx(n–1)

Sxn

Sx1′

Sxn′

Ix(t)

Sx(n–1)′

Vdc

Cdc1

Cdc(n–1)

Cdcn

Figure 1.2 Three-phase multilevel diode clamped inverter (MDCI).

Since there are no passive elements required for the MDCIs (excluding the dc-linkcapacitors), only conduction losses and switching losses are accounted for the clampeddiodes and switches in the converter.

When a higher number of output voltage stepped levels is desired, a higher number ofcapacitors will be needed in the dc-link. Theoretically, the dc VS should be distributedequally among the number of dc-link capacitors. However, in practical cases, the param-eters of both active and passive elements can never be identically the same, due to which,an unbalanced condition occurs in the dc-link capacitor voltages. Besides, the amountof charging and discharging current flowing through the diodes clamped between eachpair of dc-link capacitors is unequal because of the switching strategy used [6].

The unbalanced condition of the dc-link capacitor voltages makes the MDCI topologybecome disadvantageous and challenging to achieve a higher number of output phasevoltage levels. Several methods have been proposed to overcome the dc-link unbalancedcondition based on an active control [8], an active balancing circuit [9–11], or a passiveRLC technique [12, 13].

Even though many research studies have contributed to resolve the unbalanced con-dition for three- to five-level MDCI, [8] makes the control a little more complex or morelosses are incurred from the active balancing circuit [9–11] and the passive RLC tech-nique [12, 13]. Additional costs are also incurred to implement these proposed methodsfor a higher number of output phase voltage levels.

These factors could be the reason that MDCI with more than a three-level outputphase voltage is still not available in any of the industrial markets yet. Currently, 3L-NPCis the only MDCI topology popularly available in the market. This is due to its simplicityin design, low costs, and high reliability as compared with other multilevel topologies.

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1.4 Classical Multilevel Converters 7

The 3L-NPC is widely used in industrial medium power motor drives as well as powerinverters for wind and solar systems.

1.4.2 Multilevel Flying Capacitor Converter

A multilevel flying capacitor inverter (MFCI) as shown in Fig. 1.3 was named “the ver-satile multilevel commutation cell” originally in 1992 [14]. MFCI offers an alternativeapproach for multilevel conversion. Based on its original name, the topology of theMFCI is constructed with the multiple cells concept. By increasing the number of cellsin the MFCI, a higher number of output phase voltage levels can be obtained as well. Thedesign does not only allow the topology to be versatile but also achieves the objective ofa multilevel approach with the property of voltage sharing among the switches.

The MFCI has a similar structure as the MDCI, where capacitors are clamped betweenthe switches instead of the diode elements. Hence, each cell in the MFCI consists of twosemiconductor devices with a capacitor clamped. The total number of switches i andcapacitors p required for mL-MFCI can be determined based on the number of cells(ncells) implemented as shown in equations (1.4) and (1.5), respectively:

iswitches(MFCI) = ncells × 2 (1.4)pcapacitors(MFCI) = ncells − 1 (1.5)

Based on the topology of the MFCI as shown in Fig. 1.3, the synthesized output phasevoltage levels are not dependent on the dc-link capacitors. Hence, the MFCI, unlike theMDCI, will not face the same challenge of the unbalanced dc-link capacitor voltageswhen a higher number of output phase voltage levels is desired.

CxpthCx2 Cx1

Sx1

Sx(n–1)

Sxn

Sxn′

Sx(n–1)′

Sx1′

Cell 1

Cell n

Ix(t)

Phase A

mth- level

Phase B

Phase C

–ve

+ve

Three-phaseac

load

a

b

c

Vdc Cdc

Figure 1.3 Three-phase n-cell multilevel flying capacitor inverter (MFCI).

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8 1 Classical Multilevel Converters

However, more capacitors are needed for the mL-MFCI with a higher voltage level.Capacitors usually lead to a large inrush of current during the start-up period. Therefore,pre-charge circuits are needed to avoid high current during the transient period whichmay damage the power rating of the semiconductor devices [15]. In order to minimizethe current from overstressing the switches during the initial operation, the commonpractice in any converter is to place a resistor in between the DC source and the dc-linkcapacitors. High losses may be experienced during this process, thus a switch is also con-nected in parallel to the pre-charge resistor and will be turned on after pre-charging. Soexcess losses can now be avoided during normal operations by shorting the pre-chargeresistor through the switch.

Besides that, capacitors are passive elements that also serve as energy storage devices.The amount of energy stored in a capacitor will affect the voltage quality across it. Theanalytical and experimental results obtained in [16] have proven that a higher carrierfrequency can reduce or minimize the capacity of the flying capacitors, and low voltageripples can be achieved. However, a higher carrier frequency will limit the MFCI’s highpower conversion and also lead to issues such as EMI.

Besides that, a large amount of energy stored may affect the losses of the MFCI con-verter in addition to the switching and conduction losses of the switches. The reliabilityof capacitors, especially electrolytic capacitors required for a higher power rating isanother major concern in the MFCI converter. Additionally, the space taken up by anumber of flying capacitors will also increase as compared with the size of the diodesclamped in the MDCI.

As a result, these mentioned factors are a great disadvantage in designing highermL-MFCI for industrial use. Therefore, the MFCI is currently attractive for highspeed drives and those applications that require low current ripples. The MFCI is onlyavailable in ALSTOM.

1.4.3 Multilevel Cascaded H-Bridge Converter

An alternative multilevel approach was proposed by Zheng et al. [17] in 1996 whichused a similar concept of modularity as the MFCI; hence, flexibility for this multilevelconverter can be allowed. The multilevel converter was named the multilevel cascadedH-bridge inverter (MCHBI) based on the topology configuration shown in Fig. 1.4.

Each cell comprises four switches to form a conventional full-bridge (H-bridge) VSIand an isolated dc capacitor. A higher output phase voltage level can be achieved by cas-cading more cells together. The staircase-like voltage waveforms can only be synthesizedusing square pulses instead of chopped waveforms (pulse-width modulation – PWM)that are commonly used in other multilevel converters. Hence, this results in the expe-rience of higher conduction losses in each switching device.

The MCHBI is advantageous when a higher number of output phase voltage levelis attained (more than 11-level), and the output voltage waveform appears closer to apure sinusoidal-like waveform. Thus, lower harmonic distortions can be achieved andthe output filter size can also be reduced. However, the total number of switches i andcapacitors p required will also be increased depending on the number of cells requiredbased on equations (1.6) and (1.7), respectively:

iswitches(MCHB) = ncells × 4 (1.6)

pcapacitors(MCHB) = ncells (1.7)

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1.4 Classical Multilevel Converters 9

Phase A

mth - level

Phase B

Phase C

Three-phaseac

load

a

b

c

Sx–C1–2Sx–C1–1

Sx–C1–4Sx–C1–3

Sx–Cn–2Sx–Cn–1

Sx–Cn–4Sx–Cn–3

Ix(t)

Cell 1

Cell n

Cell (n–1)

Vdcn

Vdc1

Cxn

Cx1

m

Figure 1.4 Three-phase n-cell multilevel cascaded H-bridge inverter (MCHBI).

The MCHBI does not have the problem of unbalanced capacitor voltages like theMDCI and also does not require excessive flying capacitors like the MFCIs to achievemultilevel conversion. Nevertheless, the MCHBI experiences a large circulating currentin each cell due to its configuration. In addition, the MCHBI also requires isolated dcsources to supply across the dc capacitors in each cell. This makes MCHB very disad-vantageous; hence, an alternative solution like a phase-shifted transformer is requiredto provide isolated dc voltages for each cell.

Phase-shifted transformers are usually hard to manufacture, which necessitates thata costlier solution be implemented. Besides that, high losses and heat dissipated areincurred from this bulky transformer when the load is not operated at its rated power.Even though a phase-shifted transformer takes up a large volume and space of the con-verter, many industries use the MCHBI to achieve a higher output phase voltage levelfor high power motor drives.

1.4.4 Modular Multilevel Converter

The modular multilevel inverter (M2I) as shown in Fig. 1.5 was introduced [18, 19] andthe prototype of the 2MW power rating with 17L output voltage was experimented andverified [20]. The configuration of M2I is similar to that of the MCHBI; the only dif-ference between them is the placement of the DC source. Based on Fig. 1.5, M2I onlyrequires a single DC source unlike the MCHB with many isolated DC sources. Hence,M2I is highly suitable for HVDC systems.

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10 1 Classical Multilevel Converters

Each cell consists of two semiconductor devices and a capacitor which forms a singlephase half-bridge VSI (chopper cell). The configuration of each cell can also be done withfull-bridge VSI (bridge cell) same as those used in the MCHB modular cell. Hence, thenumber of switches i for both chopper cell and bridge cell configuration can be deter-mined based on the number of cells with equations (1.8) and (1.9), respectively:

iswitches(M2I−Chopper) = ncells × 2 (1.8)

iswitches(M2I−Bridge) = ncells × 4 (1.9)

While the capacitors p required for both chopper cell and bridge cell configurationscan be known based on equation (1.10):

pcapacitors(M2I) = ncells (1.10)

A bulky phase-shifted transformer is not required, but M2I experiences a greater cir-culating current than the MCHBI. Therefore, a single coupled or center-tapped inductoris required for each phase leg, connected between the positive and negative dc rail con-nected modular cells [21–23] as shown in Fig. 1.5.

The M2I would offer a more interesting and an attractive larger power applicationas compared with the MCHBI, but the implementation and design of either a singlecoupled or center-tapped inductor would make the M2I a bit complex and tedious. The9L-M2I was first commissioned by SIEMENS for the HVDC PLUS transmission system.The M2I converters are also commercialized for the HVDC systems by other companieslike ABB (HVDC Light) and ALSTOM (HVDC MaxSine).

Phase A

mth- level

Phase B

Phase C

–ve

+ve

Three-phaseac

loada

b

c

Vdc

Sx–C1–1

Sx–C1–2Cell 1

Cell 2

Ix(t)

Cell (n–1)

Sx–Cn–1

Sx–Cn-2Cell n

Cxn

Cx1

Figure 1.5 Three-phase n-cell modular multilevel inverter (M2I).

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1.4 Classical Multilevel Converters 11

1.4.5 Multilevel Active Neutral Point Clamped Inverter

An alternative multilevel inverter as shown in Fig. 1.6 was designed in 2005 by Bernetet al. [23, 24] and was named “the active neutral point clamped converter (ANPC)”initially. The original design was proposed to overcome the unequal loss distributionamong the switches in 3L-NPC.

Later, the origin was expanded with multiple cells and multilevel ANPC was developedthereafter [25]. A higher number of output phase voltage levels can be achieved with theaddition of commutation cells like those in the MFCI. Therefore, each cell comprises twoswitches with a capacitor clamped.

The total number of switches i and capacitors p in each phase leg can be known basedon the number of cells implemented using equations (1.11) and (1.12), respectively:

iswitches(ANPC) = 6 + (ncells × 2) (1.11)

pcapacitors(ANPC) = ncells (1.12)

The configuration of the ANPC as shown in Fig. 1.6 is basically a combination of bothMDCI and MFCI. As a result, the multilevel ANPC has kept the advantageous charac-teristics of both MDCI and MFCI. This shows that the ANPC will not be affected by thedc-link capacitors when the number of output phase voltage level is increased. The mod-ularity of commutation cells allows the flexibility of the ANPC in extending to a highernumber of voltage levels without encountering the unbalanced dc-link condition.

The ANPC in this case does not require any bulky phase-shifted transformers, mul-tiple isolated dc sources or a balancing circuit for dc-link capacitor voltages. Hence,the ANPC offers an attractive method for medium-to-high industrial motor drives ascompared with the other previously mentioned multilevel converters. A back-to-backconfiguration of 5L-ANPC is available in the current industrial market, ABB, for regen-erative motor drives.

Three-phaseac

loada

b

c

Phase A

Phase B

Phase C

+ve

Sx2

Sx4

Sx3

Sx1

Cx1Ix(t)

Cxn

Cell (n–1)Cell 1 Cell n

Sx6

Sx5 Sx–C1–1

Sx–C1–2

Sx–Cn–1

Sx–Cn–2

mth- level

Vdc

Cdc1

Cdcn

–ve

Figure 1.6 Three-phase n-cell multilevel active neutral point clamped inverter.

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12 1 Classical Multilevel Converters

1.5 Multilevel Applications and Future Trends

Multilevel converter topologies are well-known for its applicability in medium-to-highpower drives and are popularly used for the industrial motor drives application.Moreover, many academia research studies have also explored the application potentialof these multilevel converters in other areas such as transportation, renewable energy,utilities grid, and even low power Class D amplifiers as shown in Table 1.1 [26].

Multilevel converter applications play an important role in our daily lives and powerelectronic converters are a likely trend. Since the cost of semiconductor devices havebeen depreciating, more industries have also been investing a lot in developing newmultilevel converters for other possible areas that are not listed in Table 1.1 such asmore electric aircraft (MEA), heat ventilation and air conditioning (HVAC), and others.

With the involvement of the markets, the cost of implementation is always the primeconcern, which explains why some of the existing multilevel converters like MDCI andMFCI are not developed for higher multilevel output voltage due to the total numberof components required as shown in Fig. 1.7. Therefore, only multilevel topologies likeMCHB, MMI, and ANPC are developed with more than 6-level output phase voltages.Even though many academia research works have contributed alternative solutions toovercome the limitations of these existing multilevel converters to achieve higher mul-tilevel output voltages, but these proposed solutions are still not very welcome in theindustry. Thus, recently many research and development works are motivated to focuson designing new multilevel converters with a reduced number of component counts toachieve better power quality more efficiently.

Table 1.1 High-performance multilevel converter applications based on topology [26].

Multilevel topology

Application MDCI/ANPC MCHBI MFCI

Transportation Marine propulsion [78, 79, 82, 83] [78, 83] [83]Train traction [84–86] [80, 81, 84, 87–89] [209, 210]Automotive [90] [91–214] [215–218]

Renewableenergy

Solar photovoltaic [121, 181–185] [181, 186–193] [181]

Wind power [165, 194–198] [199] —Hydro power [224] — —

Utilities grid FACTS [225] [117, 225–227] [225]STATCOM [228–230] [55, 62, 192, 228,

231–234]—

UPFC/UPQC/DVR [245–247] [245, 248, 249] [250]HVDC [175] [55, 56, 180] —Active filters [235–239] [240–242] [243, 244]

Industrial/commercial use

Pumps/motors [219, 220] — —

Small signals Class D amplifier — [221–223] —

Note: The information in this Table 1.1 is extracted from [26]; hence, the reference numbering in the tablerefers to the references in [26].

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1.5 Multilevel Applications and Future Trends 13

03 5

MDCI

MFCI

MCHB

MMI - Chopper-Cell

MMI - Bridge-Cell

ANPC

7 9 11 13 15 17

No. of output phase voltage levels

19 21

10

20

30

Tota

l num

ber

of

com

ponen

ts p

er p

has

e le

g

40

50

60

70

80

90

100

110

Figure 1.7 Comparison of total component counts based on the respective multilevel topologies.(Note: The total number of components per phase leg presented in this figure includes dc-linkcapacitors for MDCI, MFCI, and ANPC and coupled inductors for MMI).

The advantages of reducing the amount of components needed do not only benefitthe industrial interests, but also help increase the global reliability of the converters.Low maintainability is required since lesser component counts are used in multilevelconverters. Furthermore, lower losses are incurred during the power conversion pro-cess; hence, a higher energy efficiency can be achieved. In addition, reducing componentcounts enable the converters to be of lighter weight and beneficial for many applicationswith space constraints such as MEAs, wind turbines, solar PV grid-connected systems,and HVAC systems.

As a result, the developing trends in new multilevel converters in the near future willbe involving cost, size, weight, losses, and power quality [27]. It is factually known thatthe advancement of multilevel converter designs is closely co-related with the develop-ment of power electronic devices as shown in Fig. 1.8.

1910 1920 1950 1960 1970 1980 1990 2000 2010

FUTURE

Gallium Nitride (GaN)

Active Neutral Point Clamped Inverter (ANPC)

(2005)

Multilevel Cascaded H-Bridge Inverter (MCHB)

(1996)

Modular Multilevel Inverter (M2I)

(2003)

Multilevel Flying Capacitor Inverter (MFCI)

(1992)

Neutral Point Clamped (3L-NPC)

(1981)

Gate Turn-Off Thyristor (GTO)

(Mid 1980s)Bulky Arc Mercury Valves

(First Power Rectification)

(Early 1910s)

Thyristor

(First Power Semiconductor)

(1950)

Integrated Bipolar-Gate Transistor (IGBT)

(Late 1990s)

Insulated Gate-Commutated Thyristor (IGCT)

(Mid 1990s)

Bridge Chopper (2L-VSI)

(1964)

Silicon Carbide (SiC)

Figure 1.8 The history of power electronics.

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14 1 Classical Multilevel Converters

As such, new electronic switching devices based on silicon carbide (SiC) and galliumnitride (GaN) shall be the upcoming technologies to support a higher switching fre-quency operation as well as to sustain a higher current handling capability. With that,a higher power density converter can be achieved. These will be served as guidelines inoptimizing the design of power electronic converters for both research and developmentstudies.

References

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References 15

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