Medium voltage drives

Due to the power semiconductor devices that are avail-able with maximum blocking voltages of 6.5 kV, the drive market with power ratings of up to 25 MW is domi-nated by voltage-source in

An overview of the common

converter topologies and power

semiconductor devices

B Y M A R C H I L L E R , R A I N E R S O M M E R ,

& M A X B E U E R M A N N

semiconductor devices used in medium-voltage drive systems This article provides

a general overview of the common converter topologies

available on the market and their corresponding major

characteristics The different topologies are compared and

evaluated with respect to the number of semiconductors

and the complexity

Due to the power semiconductor devices that are

avail-able with maximum blocking voltages of 6.5 kV, the drive

market with power ratings of up to 25 MW is

domi-nated by voltage-source inverters (VSIs) in insulated gate-bipolar transistor (IGBT) and insulated gate-commutated transistor (IGCT) tech-nologies For

high-er powhigh-er

demands and special applications, thyristor converters are still frequently used

Medium-Voltage Drives Over the past few years, a market for medium-voltage drives has developed, which is growing at a rapid pace This market is mainly driven by rising energy costs and by the energy-saving potential offered by variable-speed drives The boom in the raw materials market has resulted

in an increased demand for medium-voltage drives over the entire power and voltage range (drives for rolling mills, gas compressors, extraction pumps, etc.)

The use of medium-voltage converters is not only limited to applications in the high power range Especially

in the American and Asian markets, medium-voltage drives are used in a power range down to several 100 kVA

In the small and medium power range of approximately

300 kVA to 2 MVA, the output voltage is based on the

Digital Object Identifier 10.1109/MIAS.2009.935494

©CREATAS

22

standard voltages available throughout the world: 2.3 and

4.16 kV in the North American and 6–6.6 kV in the Asian

and Russian markets Even 10 kV drives are emerging for

applications that have to operate on weak line supplies In

Europe, low-voltage drives up to 690 V have become

prev-alent in this power range

In the case of high rating drives from 2 to 60 MVA, the

output voltage can usually be freely selected and is

deter-mined by cost optimization of the whole drive system,

consisting of line transformer, converter, cables,

switch-gear, and motor

Because of the various applications, the wide voltage and

power range for medium-voltage drives and the rapid

devel-opment of the available power semiconductor devices,

numerous different converter topologies have been

devel-oped for medium-voltage applications in recent decades

Unlike the low-voltage range, where the two-level (2L)

volt-age-source converter has become the dominant solution, a

large number of different converter topologies are available

in the medium-voltage market The spectrum varies from

IGBT converters with low- and high-voltage IGBTs through

IGCT converters to conventional thyristor converters

Converter Topologies

Figure 1 provides a general overview of the converter

top-ologies available on the market for medium-voltage

appli-cations Table 1 shows the essential performance data and

the power semiconductor devices used in the various

top-ologies The data only refer to the basic converter

configu-rations of the converter topologies being presented

Possible parallel or tandem connections of power sections

or step-up transformers at the converter output are not

taken into account because this would give a misleading

impression of the actual performance of each topology in conjunction with the power semiconductors being used Moreover, Table 1 only focuses on load-side inverters Pos-sible configurations for line-side converters have not been taken into consideration

The circuit topologies generally used in medium-voltage industrial converters can be roughly divided into three categories:

These categories not only differ in their actual circuit topology but also by the power semiconductors that are used—an essential difference In VSIs, asymmetrically blocking turn-off devices (IGBTs and IGCTs) with anti-parallel free-wheeling diodes of 1.7–6.5 kV are used, whereas the 1.7-kV IGBTs are only used in cellular con-verters [SC-HB(2L)] For CSIs, symmetrically block-ing, turn-off gate-commutated transistors (GCTs) are employed, whereas for load-commutated inverters (LCIs) and CCs, electrically fired, symmetrically blocking thyris-tors are used

All self-commutated VSIs, CSIs, and all line-commu-tated CCs are basically suitable for feeding induction and synchronous motors In contrast, LCIs can only be operated in conjunction with overexcited synchronous motors since this type of motor is able to provide the reactive power necessary for the commutation of the motor-side thyristors

Voltage-Source Inverters

The VSI can be basically divided into two categories:

converters in a delta connection and converters in a star

Medium-Voltage Drives for Industrial Applications

Voltage-Source

Inverters (VSIs)

Current-Source Inverters (CSIs)

Cycloconverters (CCs)

Delta Circuit

• Load tated Inverter (LCI)

• With Common Star-Point (CC-S)

• Open Circuit (CC-D)

Synchronous Motors (SMs)

• Three-Level

Neutral Point

Clamped

(3L-NPC)

• Four-Level

Flying Capacitor

(4L-FC)

• Series Connected H-Bridge Cells — Two-Level Cells SC-HB (2L) — Three-Level Cells SC-HB (3L)

• Self-Commutated Current Source Inverter (CSI)

Induction Motors (IMs) Synchronous Motors (SMs)

Delta Circuit

∆

Induction Motors (IMs) Synchronous Motors (SMs)

Star CircuitY

1

Main classification of the basic topologies available in the market today for medium-voltage converters in industrial

connection The three-level neutral-point-clamped converter (3L-NPC, [4], Figure 2) and the four-level fly-ing-capacitor converter (4L-FC, [1], [5], Figure 3) are typical examples for delta-connected converters Common semiconductor devices used are IGBTs (in a module or press-pack package) and IGCTs for the 3L-NPC converter and IGBT modules for the 4L-FC The blocking voltages of the semiconduc-tors that are used vary from 3.3 to 6.5 kV for both types of converters To enable output voltages of up to 7.2 kV,

a direct series connection of IGBTs is also used for 3L-NPC converters

semiconductors of a valve arm is cal-culated approximately by

2

p

line-to-line motor voltage

Typical star-connected VSIs are the series-connected H-bridge (SC-HB) cellular converter with 2L H-bridges as shown in Figure 4 [SC-HB(2L)] or with three-level H-bridges [SC-HB(3L)] as shown in Figure 5 In this case, the actual drive converter is formed by connecting a number of single convert-ers in series A freely configurable number of n cells are connected to form

a phase, and the three phases are con-nected to a star point Converters of this type available on the market have

IGBT (1.7–3.3 kV-modules) and IGCT (4.5–6 kV) are used as semi-conductor valves The voltage load

arm is approximately calculated by

2

p

4 in Figure 4)

It is a characteristic feature of the delta connection that the blocking voltage rating is higher and the current rating is lower for a given output power compared with the star-con-nected topology Therefore, SC-HB topologies permit the use of low-blocking semiconductors (e.g., 1.7 kV)

in the medium-voltage range with a reduced number of cells nZ

Another major difference between star- and delta-connected circuits are

VSI 3L-NPC

Typically: 0

24

the different line infeed configurations In the case of

star-connected converters, a separate three-phase infeed

(indi-vidual transformer secondary winding) is required for each

cell Depending on the number of cells, this can result in

very complex transformers On the other hand, the delta

connection has a concentrated dc link, so that simple diode

rectifiers with standard transformers can be used The

common dc link also permits the implementation of

mul-timotor drives where several motors can be operated from

a single dc link (e.g., in rolling-mill applications)

An important advantage of cellular converters in a star

connection results from the small step sizes in the output

voltage waveform of the converter This allows standard

line motors to be used without any output filter For the

3L-NPC, on the other hand, a reinforced motor insulation

is required or a sine-wave filter in combination with

standard line motors

Further, the dimensioning of the dc-link capacitors is

also influenced by the basic converter topology In

delta-connected, VSIs, relatively small capacitances can be used

in the dc link Assuming full active power at the output,

the amount of energy stored in the dc link would

theoreti-cally permit constant output power—fed just from the

capacitive energy stored in the dc link—for only about

5–10 ms Because the star-connected cellular converters

fea-ture a single-phase cell output, the reactive power pulsating

at twice the output frequency requires a relatively large

dc-link capacitance to limit the dc-link voltage ripple Therefore, the energy storage effort in the dc link of star-connected circuits is four to ten times higher compared with delta-connected circuits This factor depends both on the leakage inductance of the transformer, the resulting coupling of the individual cells, as well as essentially on the capacitor technology being used (e.g., dry film and electro-lytic) In this topology, the dc-link capacitors therefore rep-resent a considerable percentage of the converter costs

Valve Arm

Ud

2

Ud

2

2

The 3L-NPC converter with a 12-pulse diode infeed.

Valve Arm

Ud

3

The 4L-FC converter with a 12-pulse diode infeed.

M

A1A

1

A2

A3

B1B1

B2

B3

C1C1

C2

C3

D1

D1

A2

B2

C2

D2

B3

C3

D3

D2

D3

Valve Arm

4

A3

SC-HB cellular converter with SC-HB(2L) per cell; four SC

M

A1

A1

A2

C1

C2

B1

B2

A2

B1

B2

C1

C2

Valve Arm

5

SC-HB cellular converter with SC-HB(3L) per cell; one cell

Current-Source Inverters

The self-commutated CSI (Figure 6) differs from the LCI

(described below) by the turn-off components that are

used on the motor-side converter This enables the

opera-tion of both synchronous and inducopera-tion motors The

pulsewidth modulation results in almost sinusoidal motor

currents This requires a real dc link with sufficient

magnetically stored energy Because the dc link does not

permanently conduct the peak value of the ac output

cur-rent, the dynamic response of this converter is

compara-tively low The ac filter capacitors must be used at the

output terminals, which are capable of conducting the

dc-link current at any time and therefore permit

commuta-tion of the turn-off power semiconductors [3] These

capacitors also operate as output filters and reduce the

voltage stress on the motor windings

As a turn-off semiconductor, symmetrically blocking

GCTs are generally used The requirement for a

symmetri-cal blocking capability results in a special design of the

GCT Therefore, the range of commercially available

devi-ces is limited The typical power range covers 1–7 MW,

and the input voltage is limited to typically 6.6 kV due to

the magnetic power supply of the gate-control circuits

Thyristor Converters

Basically, two thyristor converter topologies have become

prevalent today

Load-Commutated Inverters

LCIs are suitable for driving synchronous motors with a variable output frequency The line- and motor-side recti-fiers are controlled, so that the same dc-link voltage refer-ence value is obtained Hrefer-ence, two dc voltage systems are coupled by a filter choke (Figure 7) This results in motor currents that are approximately square-wave currents On the other hand, the output voltage waveform is close to sinusoidal Because of the reactive power required for the commutation of the thyristors, LCIs can only be used for operating over-excited synchronous motors

LCIs are primarily used for converters (e.g., gas com-pressors, gas liquefaction applications, and main propul-sion drives for ships) that operate continuously The typical power range of these drives is 10–70 MW (Figure 8)

A further application for LCIs is starting converters for synchronous motors Here, gas turbine generators for power generation represent a special application as the starting power is only a fraction of the full generator power Starting the generator not only requires a step-up transformer to increase the output voltage but also a closed-loop control that permits the high field-weakening ratios With this concept, gas turbine generators with a power of 600 MW can be started with a converter power

of approximately 15 MW

Cycloconverters

The CC is the only medium-voltage converter topology that does not use any kind of energy storage device in the

dc link Using three antiparallel three-phase bridges for the three motor phases, the control always connects the appropriate line-voltage sections to the corresponding motor phase by using the thyristor switch matrix How-ever, because the thyristors have to be commutated by the line, the maximum output frequency is limited to half the line frequency On the other hand, because of the high overload capability of the thyristors and the fact that the motor phases are directly connected to the feeding power system, very high-transient overload currents can occur This feature is also used to implement a fast dynamic response of the drive system

The main advantage of the open connected CC (CC-D

in Figure 9) compared with the star-point-connected CC (CC-S in Figure 10) is that it is possible to use a standard-line transformer with low-leakage inductance

These characteristics have resulted in two main applications:

where the rolling process results in high-surge loads

M dc-Link

6

CSI with six-pulse line- and motor-side converters.

Filter

M

7

LCI with 12-pulse line- and motor-side converters.

8

Example of a commercial LCI: 12-pulse motor- and line-side bridge with filter choke of a Siemens Sinamics GL150 converter with six thyristors connected in series per valve

26

(20 MW for 60 s) The frequencies of the directly

driven rolling stands are typically 10–20 Hz

diame-ter of up to 10 m and can be directly operated with

synchronous motors that are just as large (ring

motors) at typically 6–9 Hz Mills such as these

have a continuous rating of approximately 20 MW

Power Semiconductors

Thyristors

Although the basic converter concept of LCIs and CCs has

not changed over the past 20 years, the components that are

used have undergone an astonishing increase in performance

Thyristors for use in converters were already being used in

commercial applications in the early 1960s The

perform-ance of the thyristors has significantly increased This

devel-opment has been accompanied by increasing wafer size,

mainly driven by telecommunications technology This is

because (unlike the finely structured IGBTs, which are

chip) the thyristor function can be realized on wafers of any

size using simple contact masks The increase in the off-state

voltage also requires doped material that is even more pure

and more homogeneous The precise doping is realized

through neutron conversion of silicon to phosphor

Thyris-tor development often follows silicon development

Based on this dynamic development, the number of

individual semiconductors and therefore the costs of the

overall converter have been significantly reduced One

example of this is a 16-MW LCI: today, only 24 individual

semiconductors are required, whereas 20 years ago, 96

thyristors were needed

With the increased silicon surface areas of the individual

thyristors, LCIs and CCs can also be implemented without

fuses without reducing the short-circuit withstand

capabil-ity If a semiconductor fails, thyristors (which are generally

implemented as press packs) have the advantage that they

form a safe short-circuit path for the subcircuit that is

affected This prevents arcing in the case of a fault

Because of the press-pack structure, thyristors have a

practically unlimited service life Many systems have been in

operation for more than 20 years without any fault, which

clearly indicates the high reliability of these components

With the availability of large individual devices, it is

no longer necessary to connect devices in parallel, which

avoids the known problems of static and dynamic

cur-rent distribution

To enhance the converter output voltage and output

power, thyristors must be directly connected in series

Because of the RC snubber and tight tolerances of the reverse

recovery charges, thyristors can be connected in series

with-out having to use individually matched devices (Figure 11)

However, the modularization regarding the number of

devices connected in series requires a suitable control

method In the past, simple magnetic couplers were

usually used to provide the necessary galvanic isolation

However, this requires individually adapted transformers

that meet the requirements of the insulation system

This is the reason that manufacturers changed over to

module LCI and CC concepts with thyristor control

electronics, including power supplies fed directly from the

M

9

Cycloconverter in an open connection in a six-pulse design.

10

M

Cycloconverter with common star point (CC-S) in a six-pulse design.

TABLE 2 DEVELOPMENT OF THE MAIN THYRISTOR CHARACTERISTIC VALUES [2].

27

RC circuit This technique has been

used in high-voltage dc transmission

systems for a long time Because of the

progress made in digital electronics,

state-of-the-art gate drivers are able to

transmit additional information about

the state of the thyristor to the higher

level control Using a complex

program-mable logic device (CPLD), numerous

coded feedback signals (thyristor

volt-age, correct firing) can be signaled back

to the control electronics

Communica-tion is implemented using standard

du-plex fiber-optic cables This allows

individual thyristors to be monitored

using the control electronics

This technique allows simple

mod-ular structures to be realized up to very high power ratings

Taking everything into account, this explains why the

thyr-istors still play a significant role in medium-voltage power

electronics even after 45 years of industrial use Of course,

the lower and medium power range is now addressed using

components that can be turned off (IGBTs) At higher

power ratings, a special thyristor version is used: the IGCT

In the highest power range, the thyristor still remains

unbeatable when it comes to performance, reliability, and

low equipment costs

IGCTs

The IGCT is a further development of the gate turn-off thyristor (GTO) The fine structure of the silicon pellet with more than ten single thyristors per square centimeter allows the anode– cathode current to be commutated into

an anode-gate current This means that, for several microseconds, the complete load current has to be conducted by the gate However, as a result of the progress made in MOS power transistors and electrolytic capacitor technology, today, this is possible up to currents of 6,000 A The only disadvantage today

is the relatively high power require-ment of the gate-control circuit Figure 12 shows an example of a typical IGCT power electronic building block (PEBB) for a 3L-NPC converter

It consists of four IGCTs, including gate drivers and the cor-responding free-wheeling diodes, the water cooling system, RCD snubbers, and di/dt-chokes Using 4 kA/4.5 kV IGCT devices, the maximum converter power in a 3L-NPC topol-ogy is 9–10 MVA

IGBTs

IGBTs now completely dominate the low-voltage converter sector and are also being increasingly used for medium-voltage converters Because of the low cell size of the MOS structure, only relatively small chip sizes of maximum 15

are currently in industrial production Because each of these chips has to be capable of blocking the maxi-mum device voltage, an individual edge termination has to

be integrated However, the edge width on the chip increases nearly linearly with the required blocking capa-bility and results in a significant reduction of the active semiconductor surface While a 1,700-V chip still has approximately 75% active surface, for a 6,500-V chip, the active surface is reduced to 50%

The main advantages of IGBTs, especially for small and medium power ratings, are the controllability of the switching behavior as well as the short-circuit capability

L1 L2 L3

P

N

11

Siemens LCI thyristor module with 18 thyristors, including drivers

and RC snubbers and the corresponding circuit arrangement.

12

MULTILEVEL CONVERTERS ARE FORMED BY CONNECTING

A NUMBER OF SINGLE CONVERTERS

IN SERIES.

NPC-Diode Powercard

IGBT-Powercard

T1

2

T3

T4

T ′1

D′1

D2 D′2

T ′2

T ′3

T ′4

13

Siemens Sinamics GM150 IGBT 3L-NPC phase design with IGBT- and NPC-Diode-Powercards for 3.3–7.2 kV converters 28

of the components This makes it

pos-sible to operate the IGBTs without a

snubber, enabling a simple, low-cost

converter design A further advantage

results from the simple series

connect-ability of the IGBTs, which now

per-mits converters with output voltages

up to 7.2 kV rms to be implemented

in 3L-NPC technology

The design of 3L-NPC phase of the

converter is shown in Figure 13 The

modular phase design consists of four

identical IGBT-Powercards and one

NPC-Diode-Powercard Each

IGBT-Powercard comprises two IGBT

mod-ules (including gate drivers), whereas

two SC IGBTs are located on

neigh-boring Powercards The

NPC-Diode-Powercard consists of two double-diode modules and the

corresponding RC snubbers for passive voltage balancing

in the series connection This design is used for all

Sie-mens Sinamics GM150 IGBT converters with output

vol-tages from 3.3 to 7.2 kV using either 3.3 or 6.5

kV-IGBTs A simplified structure without SC

semiconduc-tors is used for the 2.3-kV inverter

The converter cabinet design of a Siemens Sinamics

GM150 IGBT converter using the phase design in Figure

13 is shown in Figure 14 It consists of a standard 12-pulse

diode rectifier, the three-phase 3L-NPC-inverter, and the

control cabinet The output voltage range of this design is

3.3–7.2 kV The maximum output power is 5.8 MVA for

a single water-cooled unit or 4.3 MVA for the air-cooled

version The output power can be extended up to 10 MVA

for the water-cooled version when units are connected in

parallel

Load-Cycling Capability

While the load-cycling capability of

the press-pack components currently

used for all applications offers a

suffi-cient margin, IGBT modules with

their soldering and bonding

techni-ques require a precise analysis of the

load-cycling requirements and the

resulting temperature changes that

occur in the application The

load-cycling capability of the connection

bond wire/chip can be significantly

increased by applying an additional

polyimide coating to the bond wires

For the necessary resistance to cyclic

temperature stress, which determines

the reliability at low operating

fre-quencies (typically <15 Hz) or

tempo-rary overload conditions (in rolling

mills typically 60 s load, 300 s no

load), further measures have to be

taken in the module structure The

aim is to adapt the thermal and

mechanical expansion coefficients to

those of silicon to avoid cracks in the

soldered joints This adaptation is achieved using Al-N ceramic substrates and a base plate made of Al-SiC (alumi-num-filled Si-C)

Diodes

Diodes are used in all of the VSIs For IGBT modules, diode chips with the same production technique are used However, in this case, developers have

no possibility of selecting a specific silicon surface of the diode chips The diode surface is usually about half that

of the IGBT surface This limits the converter design regarding the regen-erative power capability and the short-circuit protection

Rectifier 12-Pulse

Inverter (3L-NPC)

Control Cabinet

NPC-Diode Powercard

IGBT-Powercard

2.4 m

1.2 m 2.2 m

14

Air-cooled Sinamics GM150 IGBT converter (Siemens).

THYRISTORS ARE STILL IMPORTANT DEVICES FOR HIGH-POWER AND LOW-FREQUENCY DRIVES IN SPECIAL APPLICATIONS.

1 10 100

2.3 kV 3.3 kV4.16 kV 7.2 kV

Siemens Perfect Harmony SC-HB (2L) 1.7 kV-IGBT

Siemens Perfect Harmony SC-HB (2L) 3.3 kV-IGBT

Simovert GL1503 LCI Thyristor

1 Up to 2 × Parallel Connection

2 Up to 3 × Parallel Connection

3 Output Six-Pulse for Starting Converter or 12-Pulse for Continuous-Duty Converter

4 Implemented Drive with Four Motor Winding Systems

VA

[kVeff ]

SA

15

Sinamics SM1502 3L-NPC 4.5 kV-IGCT

Sinamics GM150 1

3L-NPC 3.3 kV-IGBT

Perfect Harmony4 SC-HB (2L) 1.7 kV-IGBT

A greater degree of freedom is offered by converters

with press-pack components in which the diode function

is implemented in separate components In this case, the

diode area can be adapted to the specific requirements of

the application

Summary

The typical converter topologies over the power and output

voltage range are shown in Figure 15 using examples from

the Siemens product spectrum in the medium-voltage

range CCs have been omitted Voltage-source converters

(3L-NPC and SC-HB) cover the complete power range up

to 25 MVA and motor voltages up to 7.2 kV In some cases,

converters with a voltage dc link (SC-HB) up to 60 MW

have already been implemented by connecting single

con-verters in parallel In the power range up to approximately

20 MW, IGBTs (off-state voltage between 1.7 and 6.5 kV)

and IGCTs (off-state voltage 4.5 kV) are used

IGCTs (and IGBTs in a press-pack package) are preferred

wherever the application demands a very high load-cycling

capability, e.g., in rolling mills A further advantage when

using IGCTs is the fact that the actual semiconductor

switches are separated from the free-wheeling diode so that

the diode surface and the diode properties can be adapted to

the requirements of the application Examples are an

improved regenerative feedback capability or a higher surge

current capability when compared with a module diode

Despite the wide overlap between the different converter

systems (Figure 15), each topology has its own preferred

fields of application The selection of a converter concept

that is suitable for a specific application largely depends on

the drive system costs The semiconductors make up a

considerable portion of the costs, especially for VSIs The silicon area used per megawatt of output power is therefore

an indicator of the costs of each converter concept Figure 16 shows the silicon area per megavoltampere of converter output power for the various converter concepts explained earlier For comparison, the corresponding silicon area for a 690-V motor-side converter in 2L technology (pink bar) is also shown

It is clearly apparent that by using high-voltage IGBT

the silicon area and therefore the semiconductor costs are considerably higher This is because of the higher switch-ing and on-state losses of the high-voltage IGBTs requiring a larger silicon area per megavolt ampere compared with IGCT or thyristor-based solutions However, the higher semiconductor costs are nearly com-pensated by the simpler mechanical design and the consid-erably lower snubber costs compared with the drives using press-pack technology A comparably small semicon-ductor surface can be achieved in the SC-HB(2L) topol-ogy using 1.7-kV components The smaller nonactive edge termination area in 1.7 kV devices enables a higher utilization of the installed silicon area In conjunction with the relatively small switching and on-state losses of low-voltage IGBTs, this leads to a substantially lower silicon area per megavoltampere compared with topologies requiring high-voltage IGBT devices (e.g., 3L-NPC) However, in the SC-HB topology, the very high costs of passive components (dc-link capacitors and line transform-ers) also have to be taken into account

LCIs have clearly the smallest silicon surface of all the concepts The LCI will therefore remain an important drive technology, especially for high power applications and because of its high reliability (lower number of switching components and control electronics) Comparing the CC with the VSC topologies, the silicon area is comparable Therefore, the CC will remain an attractive solution for special applications (e.g., rolling and ball mills)

In the future, the drive market up to 30 MW will be dominated by VSIs with IGBTs and IGCTs At higher powers and for special applications, thyristor converters will still be important

References

[1] S S Fazel, D Krug, T Taleb, and S Bernet, ‘‘Comparison of power semiconductor utilization, losses and harmonic spectra of state-of-the-art 4.16 kV multi-level voltage source converters,’’ in Proc 2005 Euro-pean Conf on Power Electronics and Applications (EPE’05), Dresden, Germany.

[2] A Hoffmann and K Stocker, ‘‘Thyristor-Handbuch,’’ 4 Auflage, 1976.

[3] B Wu, High-Power Converters and AC Drives New York: Wiley, 2006 [4] M Ruff, R Sommer, and G Zaiser, ‘‘Spannungszwischenkreisum-richter im Mittelspannungsbereich,’’ ETG-Fachtagung Bauelemente der Leistungs elektronik und ihre Anwendungen, Bad Nauheim, Ger-many, 2002.

[5] C Keller, ‘‘Low power converters for high output voltages,’’ in Proc.

2005 European Conf on Power Electronics and Applications (EPE’05), Dresden, Germany, 2005.

Marc Hiller (marc.hiller@siemens.com), Rainer Sommer, and Max Beuermann are with Siemens AG in Nuremberg, Germany This article first appeared as ‘‘Converter Topologies and Power Semiconductors for Industrial Medium Voltage Converters’’ at the

2008 IEEE Industry Applications Society Annual Meeting

0 50

100

150

200

250

300

350

400

-Module 3L-NPC

-Module 3L-NPC

1) CC All Semiconductors Considered Because Line Side and Motor Side Are Not Separable

2) Converters with High Load Cycle Capability

16

Silicon surface for various topologies and semiconductors

with reference to the output apparent power (motor-side

converters only).

30