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State of the Art and Recent 

Developments of Medium Voltage 

Converters 

Steffen Bernet 

Berlin University of Technology 

Power Electronics Lab 

Berlin, Germany 


All rights reserved.Prof. Bernet 9/2003-1

Outline 

1. Market Overview of Industrial Medium Voltage Drives (MVD) 

2. Basic Function and Characteristics of 2L-VSC, 3L-NPC VSC, 

3L-FC VSC and 4L-FC VSC 

3. Comparison of IGBTs and IGCTs in a 3L-NPC VSC 

4. Application Example 

• Medium Voltage Drive ACS 6000 

5. Conclusions 



Two Level – Voltage Source Converter (2L-VSC) 

V dc 

S 1a S 1b S 1c 

a b 

M 

S 2a 

S 2b S 2c 

c 

reference-, carrier signal 

1 

0.5 

0 

-0.5 

-1 

0 0.005 0.01 0.015 0.02 

t [s] 

IGBTs / Diodes : 6 / 6 

Commutation cells per phase leg : 1 

Modulation scheme of 2L-VSC 

(f c = 875 Hz, f o = 50 Hz, m = 1.114) 

Rated commutation voltage: V dc 

(e.g. V ll =2.3kV, V com = V dc,n =3400V → 6500V IGBTs / IGCTs) 




V aM =V dc /2 

S 1a 

v aM 

V dc 

M 

a 

V dc /2 

S 2a 

-V dc /2 




V aM =-V dc /2 

S 1a 

v aM 

V dc 

M 

a 

V dc /2 

S 2a 

-V dc /2 



Converter Voltages and Spectrum of 2L-VSC 

(V ll = 4,16 kV, V dc = 6,1 kV, f c = 875 Hz; m = 1,114; f a = 50 Hz; q = f c /f a = 17,5) 

output volatge u aM [V] 

4000 

2000 

0 

-2000 

-4000 

0 0.01 0.02 0.03 0.04 

u ll /u ll,1 

1 

0.5 

output volatge u ll [V] 

5000 

0 

-5000 

0 0.01 0.02 0.03 0.04 


All rights reserved.Prof. Bernet 9/2003-6 

t [s] 

0 

0 1000 2000 3000 

f [Hz] 

L ll 

= 3 = (2L-1) 

f 1cb =f c =875 Hz

Three Level NPC Voltage Source Converter (3L-NPC VSC) 

V dc 

S 1a 

S 1b 

S 1c 

D 1a D 1b 

D 1c 

S 2a 

a 

S 2b 

b 

S 2c 

c 


1 

0.5 

0 

D 2a 

S 3a D 2b S 3b D 2c 

S 3c 

-0.5 

S 4a 

S 4b 

S 4c 

-1 

0 0.005 0.01 0.015 0.02 

IGBTs / Diodes : 12 / 12 + 6 


Rated commutation voltage: V com =V dc / 2 

(e.g. V ll =2.3kV, V dc,n =3400V, V com = V dc,n / 2 

= 1700V → 3300V IGBTs / IGCTs) 

t [s] 

Modulation scheme of 3L-NPC VSC 

(f c = 875 Hz, f o = 50 Hz, m = 1.114) 



Output Voltage of 3L-NPC VSC 

V aM =V dc /2 

v aM 

V dc 

V dc /2 

M 

D 1 

S 1 

S 2 

a 

V dc /2 

-V dc /2 

V dc /2 

D 2 

S 3 

V aM 

V dc 

/2 

S 1 

on 

S 2 

on 

S 3 

off 

S 4 

off 

S 4 

0 

-V dc 

/2 

off 

off 

on 

off 

on 

on 

off 

on 




V aM =0 

v aM 

V dc 

V dc /2 

M 

D 1 

S 1 

S 2 

a 

V dc /2 

-V dc /2 

V dc /2 

D 2 

S 3 

V DC 

/2 

S 1 

on 

S 2 

on 

S 3 

off 

S 4 

off 

S 4 

0 

-V DC 

/2 

off 

off 

on 

off 

on 

on 

off 

on 




V aM =-V dc /2 

v aM 

V dc 

V dc /2 

M 

D 1 

S 1 

S 2 

a 

V dc /2 

-V dc /2 

V dc /2 

D 2 

S 3 

S 1 

S 2 

S 3 

S 4 

V DC 

/2 

on 

on 

off 

off 

S 4 

0 

off 

on 

on 

off 

-V DC 

/2 

off 

off 

on 

on 



Converter Voltages and Spectrum of 3L-NPC VSC 

(V ll = 4,16 kV, V dc = 6,1 kV, V com = 3,05 kV f c = 875 Hz; m = 1,114; f a = 50 Hz) 


4000 

2000 

0 

-2000 

-4000 

0 0.01 0.02 0.03 0.04 

u ll /u ll,1 

1 

0.5 


5000 

0 

-5000 

0 0.01 0.02 0.03 0.04 

t [s] 



0 

0 1000 2000 3000 

f [Hz] 

L ll = 5 = (2L-1) 

f 1cb =f c =875 Hz

Multi-Level VSC (e.g. 3L-FC VSC) 

S 1 

C 1 

S 

1 2 

V dc 

M 

V 

2 dc 

S 4 


Flying Caps: 3 

S 3 



(e.g. V ll =2.3kV, V dc,n =3400V, V com = V dc,n / 2 

= 1700V → 3300V IGBTs ) 

a 


1 

0.5 

0 

-0.5 

-1 

0 0.005 0.01 0.015 0.02 

t [s] 

Modulation scheme of 3L-FLC VSC 

(f c = 875 Hz, f o = 50 Hz, m = 1.114) 




v aM 

V dc /2 

V 

aM 

= 

1 

V 

2 

dc 

S 1 

C 1 

S 

1 2 

V dc 

M V 

2 dc 

S 4 

S 3 

a 

-V dc /2 

V aM 

S 1 

S 2 

S 3 

S 4 

V dc 

/2 

on 

on 

off 

off 

0 

on 

off 

on 

off 

-V dc 

/2 

off 

off 

on 

on 




v aM 

1 1 1 

V = V − V = V − V = 

aM dc C1 

dc dc 

2 2 2 

0 

V dc /2 

S 1 

C 1 

S 

1 2 

V dc 

M 

V 

2 dc 

S 4 

S 3 

a 

-V dc /2 

V aM 

S 1 

S 2 

S 3 

S 4 

V dc 

/2 

on 

on 

off 

off 

0 

on 

off 

on 

off 

-V dc 

/2 

off 

off 

on 

on 




v aM 

V 

aM 

=− 

1 

V 

2 

dc 

V dc /2 

S 1 

C 1 

S 

1 2 

V dc 

M 

V 

2 dc 

S 4 

S 3 

a 

-V dc /2 

V aM 

S 1 

S 2 

S 3 

S 4 

V dc 

/2 

on 

on 

off 

off 

0 

on 

off 

on 

off 

-V dc 

/2 

off 

off 

on 

on 





Output Voltage of 3L-FLC VSC 

4000 

2000 

0 

-2000 


-4000 

0 0.01 0.02 0.03 0.04 

5000 

0 

-5000 

0 0.01 0.02 0.03 0.04 

t [s] 



u ll /u ll,1 

1 

0.5 

0 

0 1000 2000 3000 

f [Hz] 

L ll = 5 = (2L-1) 

f 1cb =2·f c =1750 Hz


S 1 

S 2 S 3 

1 

V 

V 

3 dc 

3 dc 

S 6 

2 

a 

V dc 

M C 1 C 2 


Flying Caps: 6 

S 5 

S 4 



(e.g. V ll =2300 V, V dc,n =3400V, V com = V dc,n / 

3 = 1133V → 2500V IGBTs ) 


1 

0.5 

0 

-0.5 

-1 

0 0.005 0.01 0.015 0.02 

t [s] 

Modulation scheme of 4L-FLC VSC 

(f c = 875 Hz, f o = 50 Hz, m = 1.114) 

FC VSC 

• V C =V DC / (L-1) 

• f 1cb =(L-1)f c 




4-Level-FLC Converter 

v aM 

V 

aM 

= 

1 

V 

2 

dc 

V dc /2 

V dc /6 

S 1 

S 2 S 

2 

3 

1 

V dc 

M C 1 V C 2 

3 dc 

V 

3 dc 

S 6 S 5 

S 4 

a 

-V dc /6 

-V dc /2 

S 1 

S 2 

S 3 

S 4 

S 5 

S 6 

V dc 

/2 

on 

on 

on 

off 

off 

off 

V dc 

/6 

on 

on 

off 

off 

off 

on 

-V dc 

/6 

on 

off 

off 

on 

on 

off 

-V dc 

/2 

off 

off 

off 

on 

on 

on 





v aM 

1 1 1 1 

V = V − V = V − V = V 

aM dc C 2 dc dc dc 

2 2 3 6 

V dc /2 

V dc /6 

S 1 

S 2 S 

2 

3 

1 

V dc 

M C 1 V C 2 

3 dc 

V 

3 dc 

S 6 S 5 

S 4 

a 

-V dc /6 

-V dc /2 

S 1 

S 2 

S 3 

S 4 

S 5 

S 6 

V dc 

/2 

on 

on 

on 

off 

off 

off 

V dc 

/6 

on 

on 

off 

off 

off 

on 

-V dc 

/6 

on 

off 

off 

on 

on 

off 

-V dc 

/2 

off 

off 

off 

on 

on 

on 





1 1 2 1 

V = V − V = V − V = − V 

aM dc C1 

dc dc dc 

2 2 3 6 

v aM 

V dc /2 

V dc /6 

S 1 

S 2 S 

2 

3 

1 

V dc 

M C 1 V C 2 

3 dc 

V 

3 dc 

S 6 S 5 

S 4 

a 

-V dc /6 

-V dc /2 

S 1 

S 2 

S 3 

S 4 

S 5 

S 6 

V dc 

/2 

on 

on 

on 

off 

off 

off 

V dc 

/6 

on 

on 

off 

off 

off 

on 

-V dc 

/6 

on 

off 

off 

on 

on 

off 

-V dc 

/2 

off 

off 

off 

on 

on 

on 





V aM 

V 

aM 

=− 

1 

V 

2 

dc 

V dc /2 

V dc /6 

S 1 

S 2 S 

2 

3 

1 

a 

V dc 

M C 1 V C 2 

3 dc 

V 

3 dc 

S 6 S 5 

S 4 

-V dc /6 

-V dc /2 

S 1 

S 2 

S 3 

S 4 

S 5 

S 6 

V dc 

/2 

on 

on 

on 

off 

off 

off 

V dc 

/6 

on 

on 

off 

off 

off 

on 

-V dc 

/6 

on 

off 

off 

on 

on 

off 

-V dc 

/2 

off 

off 

off 

on 

on 

on 




Converter Voltages and Spectrum of 4L-FLC VSC 

4000 

2000 


0 

-2000 

-4000 

0 0.01 0.02 0.03 0.04 

5000 

0 

-5000 

0 0.01 0.02 0.03 0.04 



t [s] 

1 

0.5 

u ll /u ll,1 


0 

0 1000 2000 3000 

f [Hz] 

L ll = 7 = (2L-1) 

f 1cb =3·f c =2625 Hz

Converter Comparison - f c = 750 Hz 

(V ll,ν=1 = 2,3 kV, V dc =3400V, f c =750Hz, 2L-VSI: 6.5kV IGBTs, 3L-NPC / FC VSI: 

3.3kV IGBTs, 4L-FC VSC: 2.5kV IGBTs, S S = 38,61 MVA = const.) 

2L-VSC 

3L-NPC VSC 

3L-FLC VSC 

4L-FLC VSC 

Rated IGBT current I C,n 

990 A 

975 A 

975 A 

858 A 

Max. phase current I ph,max 

@ T j,max 

= 125° C, f c 

= 750 Hz 

600 A 

897 A 

871 A 

726 A 

Maximum apparent converter 

output power S C,max 

2.390 MVA 

3.573 MVA 

3.47 MVA 

2.892 MVA 

4 

output power [MVA] 

3 

2 

1 

0 

149,5% 145% 

121% 

100% 

2L-VSI 3L-NPC 3L-FLC 4L-FLC 

⇒ 3L–NPC VSC enables the 

highest converter output 

power 






40 

30 

total losses Ploss Vll = 2.3 kV 

2L-VSI f c=750Hz 

3L-NPC f c=750Hz 

3L-FLC f c=750Hz 


40 

30 

loss distribution Vll=2.3kV, fc=750Hz 

PconT 

PconD 

PonT 

PoffT 

PoffD 

Ploss [kW] 

20 

Pv [kW] 

20 

10 

10 

0 

0 200 400 600 800 1000 

I ph h [A] [A] 

0 


Total converter losses as a 

function of phase current 

( f c = 750 Hz, f o = 50 Hz, m = 1.11, 

cos (ϕ) = 0.9) 

Loss distribution 

(I ph = 600A, f c = 750 Hz, f o = 50 Hz, 

m = 1.11, cos (ϕ) = 0.9) 




harmonic content of line-to-line voltage 

(V ll,ν=1 = 2,3 kV; S S = 38,61 MVA; m = 1.11) 

2L-VSC, f C = 750 Hz, f 1cb = 750 Hz 

10 0 harmonic content of line-to-line voltage 2L VSC,fc=750Hz 

3L-NPC, f C = 750 Hz, f 1cb = 750 Hz 

10 0 harmonic content of line-to-line voltage 3L-NPC VSC,fc=750Hz 

Vll,n/Vll,1 

10 -1 

Vll,n/Vll,1 

10 -1 

10 -2 

0 500 1000 1500 2000 2500 

f [Hz] 

10 0 harmonic content of line-to-line voltage 3L-FLC VSC,fc=750Hz 

10 -2 

0 500 1000 1500 2000 2500 

f [Hz] 

3L-FLC, f C = 750 Hz, f 1cb = 1500 Hz harmonic 4L-FLC, content fof C line-to-line = 750 voltage Hz, 4L-FLC f 1cb VSC,fc=750Hz = 2250 

10 0 Hz 

Vll,n/Vll,1 

10 -1 

Vll,n/Vll,1 

10 -1 

10 -2 

0 500 1000 1500 2000 2500 

f [Hz] 

10 -2 

0 500 1000 1500 2000 2500 

f [Hz] 




THD and WTHD of line-to-line voltage 

(V ll,ν=1 = 2,3 kV; S S = 38,61 MVA; m = 1.11) 

60 

50 

∞ 

∞ ⎛V 

⎞ 

2 

ll,n 

∑V 

∑ 

⎜ 

ll,n 

⎟ 

= 

n= 

2⎝ 

n 

n 2 

⎠ 

THD = WTHD = 

n : 

V 

V 

ll,1 

ll,1 

2 

order of harmonics 

THD 

WTHD 

3 

57% 

2.5 2,8% 

THD [%] 

40 

30 

20 

10 

29% 

36% 

21% 

WTHD [%] 

2 

1.5 

1 

0.5 

1,5% 

0,8% 

0 


0 

0,3% 




Converter Comparison – f 1cb = 750 Hz 



2L-VSC 

3L-NPC VSC 

3L-FLC VSC 

4L-FLC VSC 

Rated IGBT current I C,n 

990 A 

975 A 

975 A 

858 A 

Carrier frequency f c 

750 Hz 

750 Hz 

375 Hz 

250 Hz 

Max. phase current 

I ph,max 

@ T j,max 

= 125° C 

600 A 

897 A 

1009 A 

840 A 

Maximum apparent converter 

output power S C,max 

2.390 MVA 

3.573 MVA 

4.02 MVA 

3.346 MVA 

5 

output power [MVA] 

4 

3 

2 

1 

100% 

149,5% 168% 140% 

⇒ 3L – FLC VSC enables the highest 

converter output power S C,max 

0 




(V ll,ν=1 = 2,3 kV; S S = 38,61 MVA)




Ploss [kW] 

40 

30 

20 

10 

total losses Ploss Vll = 2.3 kV 

2L-VSI f c=750Hz 

3L-NPC f c=750Hz 



Pv [kW] 

30 

25 

20 

15 

10 

loss distribution Vll=2.3kV 

PconT 

PconD 

PonT 

PoffT 

PoffD 

5 

0 

0 200 400 600 800 1000 

II ph h [A] 

Total converter losses as a 

function of phase current 

(f o 

= 50 Hz, m = 1.11, cos (ϕ) = 0.9) 

0 


Loss distribution (I ph 

= 600A, 

f c,2L-VSC 

= 750 Hz, f c,3L-NPC VSC 

= 750 Hz, 

f c,3L-FLC VSC 

= 375 Hz, f c,4L-FLC VSC 

= 250 Hz 

f o 

= 50 Hz, m = 1.11, cos (ϕ) = 0.9) 




10 0 harmonic content of line-to-line voltage 2L VSC,fc=750Hz 

Harmonic Content of line-to-line Voltage 

(V ll,ν=1 = 2,3 kV; S S = 38,61 MVA; m = 1.11) 

2L-VSC, f C = 750 Hz, f 1cb = 750 

Hz 

3L-NPC, f C = 750 Hz, f 1cb = 750 

Hz 

harmonic content line-to-line voltage voltage 3L-NPC VSC,fc=750Hz 

0 

10 

Vll,n/Vll,1 

10 -1 

Vll,n/Vll,1 

10 -1 

10 -2 

0 500 1000 1500 2000 2500 

f [Hz] 

3L-FLC, f C = 375 Hz, f 1cb = 750 Hz 


10 -2 

0 500 1000 1500 2000 2500 

f [Hz] 

4L-FLC, f C = 250 Hz, f 1cb = 750 Hz 


Vll,n/Vll,1 

10 -1 

Vll,n/Vll,1 

10 -1 

10 -2 

0 500 1000 1500 2000 2500 

f [Hz] 

10 -2 

0 500 1000 1500 2000 2500 

f [Hz] 



! 3L-NPC VSC: 

Summary - Topologies 

! Minimum losses for a given expense of semiconductors 

! Maximum S C for f c =const 

! 3L-FLC VSC 

! Maximum converter output power for a constant frequency of the first 

carrier band 

! Identical losses like 3L-NPC VSC for identical spectrum 



Summary - Topologies 

! 4L-FLC VSC 

! attractive if very high switching frequency, low harmonic distortion, a small 

output filter or a high output voltage (V ll >= 4.16 kV) is required 

! 2L-VSC 

! poorest utilisation of semiconductors, lowest maximum carrier frequency 

and worst harmonic distortion 

! attractive for fs

PWM 3L-NPC VSC with Active Front-End 

Medium Voltage 

Mains (e.g. 20kV) 

Mains 

Transformer 

3.3kV 

Mains 

Rectifier 

(Active Front End) 

3L-NPC-VSR 

DC Voltage Link 

Inverter 

3L-NPC-VSI 

3.3kV 

Synchronous 

Machine 

SM 



Data of 3L-NPC-VSI 

•V ll =3300V, I o =1050A, S=6MVA 

•V dc =4840V, f c =840Hz 

• Sinusoidal PWM with added third Harmonics 

•IGBT: 

Eupec (Series connection, V CES =3300V, I C =1200A, 

A Si =100%, R G+ =1.8Ω / R G- =3.3Ω) 

• 91mm RCIGCT: 

ABB (V DRM =4500V, I tgqm =2190A, A Si =69% / 34.5%) 



3.3kV 3L-NPC-VSI with IGCTs 

L R1i T 11i 

T 21i T 31i 

D C1i 

V dc 

/2 

V dc 

/2 

R S1i 

T 

C 12i 

T 22i 

T 32i 

i o1 

... i o3 

V l1 .. l 3 

C1i 

T 

C 13i T 23i 

T 33i 

C2i 

R S2i 

L R2i 

D C2i 

T 14i 

T 24i 

T 34i 

(91mm, 4.5kV IGCT (2700V; 2190A), A Si =34.5%, V dc =4840V, I o =1050A, S=6MVA) 



3.3kV 3L-NPC VSI with 3.3kV IGBTs 

V dc 

/2 

T 11i 

T 21i 

T 31i 

(3300V, 1200A) IGBTs, 

(A Si 

=100%), V dc 

=4840V, 

I o 

=1050A, S=6MVA 

T 12i 

T 22i 

T 32i 

V l1 .. l3 

i o1 

... i o3 

V dc 

/2 

T 13i 

T 23i 

T 33i 

T 14i 

T 24i 

T 34i 



VSC Losses versus Output Current 

140000 

P LOSS 

120000 

[W] 

100000 

80000 

60000 

40000 

20000 

0 

Psw_IGCT+Pclamp 

Psw_IGBT 

Pcon_IGBT 

Ptotal_IGCT 

Ptotal_IGBT 

Pcon_IGCT 

0 200 400 600 800 I1000 o [A] 1200 

((3300V, 1200A)-IGBT module (A Si =100%), (4500V, 2190A)-IGCT(A Si =34.5%), 

V ll =3008V, V dc =4840V, cosφ o =1, cosφ i =-1, f s =840Hz, T j =125°C) 



Circuit Configuration of ACS 6000 

RECTIFIER 

DC VOLTAGE LINK 

INVERTER 

T 11r 

T 21r T 31r 

D C1r 

L R1r 

L R1i T 11i 

T 21i T 31i 

D C1i 

R S1r 

R S1i 

V m1 .. m3 

T 12r 

T 22r 

T 32r 

C C1r 

V dc 

/2 

C C1i 

T 12i 

T 22i 

T 32i 

i o1 

... i o3 

V l1 .. l 3 

T 13r T 23r 

T 33r 

C C2r 

V dc 

/2 

C C2i 

T 13i T 23i 

T 33i 

R S2r 

R S2i 

T 14r 

T 24r 

T 34r 

D C2r 

L R2r 

L R2i 

D C2i 

T 14i 

T 24i 

T 34i 

(3.3kV PWM Drive for SM and IM) 



Characteristics of MV Drive ACS 6000 

• Power range: 3 MVA-27 MVA 

• Base Units: 5MVA / 7 MVA / 9 MVA 

• Voltage: V ll =3.3 kV (4.5kV IGCTs) 

• Rectifier: 

• Active front end (3L-NPC-VSI) 

• Diode front end (6p, 12p, 24p) 

• Common DC Voltage Bus 

• Inverter: 3L-NPC-VSI 

• Protection: Internal shoot through 

• Output filter: RLC-filter limits dv/dt ≤ 3kV/µs 

• Control / Modulation: 

• Inverter: Direct Torque Control 

• Rectifier: Vector Control + Optimized Pulse Patterns 

• Information: http://www.abb.com/drives / http://www.abb.com/powerelectronics 



ACS 6000 - the compact high power drive 

4.9 m 

LSU 

TEU&COU 

INU 

CBU 

WCU 

Line Supply 

Unit (6p- or 

12p-diode 

supply) 



-Terminals 

behind the 

control 

swing frame 

Inverter Unit 

- 3 phase modules 

S = 9 MVA 

DC-Link 

Capacitor 

Bank Unit 

Water 

Cooling 

Unit

Conclusions 

State of the Art 

- Increasing importance of PWM-VSIs 

- Industry: Replacement of Cycloconverter and LCI 

- Energy Systems: Windparks, HVDC, STATCOMs, 

Active Filters, High Power UPS 

- Traction: Line Side Converter, Machine Side Converter 

- PWM -Converter Topologies / Semiconductors 

- Industry: 3L-NPC VSC, CSI, Multi-Level Converter 

(e.g. 4L-FC VSC, Isolated Cell VSC) – IGCT, IGBT 

- Energy Systems: 2L VSC, 3L -NPC VSC 

(e.g. HVDC, High Power UPS) – IGCT, IGBT 

- Traction: 2L VSC, (3L-NPC / FC VSC) – IGBT, GTO 



Conclusions 

! Characteristics of future AC Drives: 

! Line friendly 

! Wide range of converter voltage (2.3kV....13.8kV) and power 

(500kW....>27 MW) 

! Motor friendly 

! Retrofit applications 

! High reliability 

! High efficiency 

! High power density 

! Low costs

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