401

Equivalent circuit of theZ-inverter in the short circuit state

Therefore the diode D

5

is in blocking mode, as the voltage across the two capacitors

combined is at least twiceas high as the input voltage.

in

C

5D

DC

C L

U U2 U

V0 U

U U

− ⋅ =

=

=

During the active and the zero state, the voltage U

DC

appears across the output bridge

as the sum of the input voltage plus the voltage across the inductors L

1

and L

2

. For the

periodT– t

SS

or 1 –D

SS

therefore applies:

in

in

SS

in

L

DC

UB U

D21

1

UU2 U

⋅ =

⋅ −

= + ⋅ =

B is called the boost factor and denominates the ratio between the voltage gain of input

to theoutput side.

For a three-phase output bridge the following relation applies between the DC-bus

voltageU

DC

and the output voltage:

DC

out

U

2

m U

=

m is themodulation factor.

Accordingly it follows fro

n

in

out

UB

2

m U

⋅ ⋅

=

Small input voltages require a large boost factor B, assuming a constant output voltage.

This in turnmeans that the time t

SS

and thus the duty cycleD

SS

have to be chosen long.

Since the shorted condition should only be used for substituting the zero state (to avoid

distortion of the output current) a small modulation factor m is required. A small

modulation factor on the other hand causes the Z-inverter to work as a buck converter.

Accordingly, B must be increased. In consequence, the IGBTs will see high blocking

voltages in comparison with a conventional VSI (plus over-voltages during the current

commutation due to parasitic stray inductances). This means that in a real application,

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