Control method and system for DC/AC converters

1 . A control method for an electronic DC/AC three-phase converter ( 1 ), characterised in that the method comprises, for each phase of the conversion stage ( 11 , 11 ′) of the electronic DC/AC converter ( 1 ):

calculating, by means of a voltage controller ( 21 ), a first reference voltage (e r,v , e s,v , e t,v ) from predefined voltage setpoints ( 22 ) and a phase voltage (V c r , V c s , V c t );

calculating, by means of a current controller ( 30 ), a second reference voltage

(

e

r

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⁢

s

,

e

s

,

i

p

⁢

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⁢

s

,

e

t

,

i

p

⁢

o

⁢

s

)

based on a difference between a predefined upper current limit (I max ) and the current (i r , i s , i t ) of said at least one phase, and on the phase voltage (V c r , V c s , V c t );

calculating, by means of the current controller ( 30 ), a third reference voltage

(

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r

,

i

n

⁢

e

⁢

g

,

e

s

,

i

n

⁢

e

⁢

g

,

e

t

,

i

n

⁢

e

⁢

g

)

based on a difference between a predefined lower current limit (I max ) and the current (i r , i s , i t ) of said one phase, and on the phase voltage (V c r , V c s , V c t );

comparing the first reference voltage (e r,v , e s,v , e t,v ) with the second reference voltage

(

e

r

,

i

p

⁢

o

⁢

s

,

e

s

,

i

p

⁢

o

⁢

s

,

e

t

,

i

p

⁢

o

⁢

s

)

;

selecting the lowest reference voltage from the first reference voltage (e r,v , e s,v , e t,v ) and the second reference voltage

(

e

r

,

i

p

⁢

o

⁢

s

,

e

s

,

i

p

⁢

o

⁢

s

,

e

t

,

i

p

⁢

o

⁢

s

)

;

comparing the selected reference voltage with the third reference voltage

(

e

r

,

i

n

⁢

e

⁢

g

,

e

s

,

i

n

⁢

e

⁢

g

,

e

t

,

i

n

⁢

e

⁢

g

)

;

selecting as the fourth reference voltage (e r,s , e s,s , e t,s ) that should be generated by the conversion stage ( 11 , 11 ′) of the electronic DC/AC three-phase converter ( 1 ) the highest reference voltage from the selected reference voltage and the third reference voltage

(

e

r

,

i

n

⁢

e

⁢

g

,

e

s

,

i

n

⁢

e

⁢

g

,

e

t

,

i

n

⁢

e

⁢

g

)

;

obtaining ( 53 ) a reference voltage without a non-controlled zero sequence (e r , e s , e t ) for each phase from said selected fourth reference voltages (e r,s , e s,s , e t,s ) to compensate for the effect of a non-controlled zero sequence component contained in the selected fourth reference voltages (e r,s , e s,s , e t,s ),

applying the obtained reference voltage without the non-controlled zero sequence (e r , e s , e t ) to the respective phase of the conversion stage ( 11 , 11 ′) of the electronic DC/AC three-phase converter ( 1 ).

2 . The control method according to claim 1 , wherein the phase voltage (V c r , V c s , V c t ) is a voltage (V c ) measured at terminals of a capacitor of an output filter ( 12 ) of the phase.

3 . The control method according to claim 1 , wherein the phase voltage (V c r , V c s , V c t ) is a voltage (V PDC ) measured at the output of the electronic DC/AC converter ( 1 ).

4 . The control method according to claim 1 , wherein the phase voltage (V c r , V c s , V c t ) is a voltage obtained from a line voltage measurement.

5 . The control method according to claim 1 , wherein the step of calculating the second reference voltage

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r

,

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p

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⁢

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e

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p

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s

,

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⁢

s

)

comprises:

providing as input to a first controller module ( 31 ) the difference between the upper current limit (I max ) and the current (i r , i s , i t ) of the at least one phase; and

adding the phase voltage (V c r , V c s , V c t ) to a voltage

(

v

r

,

i

p

⁢

o

⁢

s

,

v

s

,

i

p

⁢

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⁢

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,

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t

,

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⁢

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⁢

s

)

at the output of the first controller module ( 31 ), wherein the first controller module ( 31 ) is a proportional regulator.

6 . The control method according to claim 5 , comprising, before said phase voltage (V c r , V c s , V c t ) is added to the output voltage

(

v

r

,

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p

⁢

o

⁢

s

,

v

s

,

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p

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⁢

s

,

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t

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⁢

s

)

of the first controller module:

filtering the phase voltage (V c r , V c s , V c t ) by means of a voltage filter ( 71 ); and

providing a phase lead for the filtered phase voltage by means of a phase lead compensator ( 72 ).

7 . The control method according to claim 1 , wherein the step of calculating the third reference voltage

(

e

r

,

i

n

⁢

e

⁢

g

,

e

s

,

i

n

⁢

e

⁢

g

,

e

t

,

i

n

⁢

e

⁢

g

)

comprises:

providing as input to a second controller module ( 32 ) the difference between the lower current limit (−I max ) and the phase current (i r , i s , i t ); and

adding the phase voltage (V c r , V c s , V c t ) to a voltage

(

v

r

,

i

n

⁢

e

⁢

g

,

v

s

,

i

n

⁢

e

⁢

g

,

v

t

,

i

n

⁢

e

⁢

g

)

at the output of the second controller module ( 32 ), wherein the second controller module ( 32 ) i s a proportional regulator.

8 . The control method according to claim 7 , comprising, before said phase voltage (V c r , V c s , V c t ) is added to the output voltage

(

v

r

,

i

n

⁢

e

⁢

g

,

v

s

,

i

n

⁢

e

⁢

g

,

v

t

,

i

n

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e

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)

of the second controller module ( 32 ):

filtering the phase voltage (V c r , V c s , V c t ) by means of a voltage filter ( 71 ); and

providing a phase lead for the filtered phase voltage by means of a phase lead compensator ( 72 ).

9 . The control method according to claim 1 , comprising calculating the setpoint voltage values ( 22 ) by the grid-forming method.

10 . The control method according to claim 1 , wherein if none of the three selected fourth reference voltages (e r,s , e s,s , e t,s ) is imposed by the current controller ( 30 ), then the obtained reference voltages without a non-controlled zero sequence (e r , e s , e t ) coincide with the selected fourth reference voltages (e r,s , e s,s , e t,s ), i.e.:

e

r

=

e

r

,

s

;

e

s

=

e

s

,

s

;

e

t

=

e

t

,

s

.

11 . The control method according to claim 1 , wherein if one of the three selected fourth reference voltages (e r,s , e s,s , e t,s ) is imposed by the current controller ( 30 ), and therefore the other two selected control voltages are imposed by the voltage controller ( 21 ), then the obtained reference voltages without a non-controlled zero sequence (e r , e s , e t ) are calculated as follows:

if the voltage of phase r i s the one imposed by the current controller ( 30 ):

e

r

=

e

r

,

s

;

e

s

=

e

s

,

s

-

e

r

,

s

-

e

t

,

s

2

;

e

t

=

e

t

,

s

-

e

r

,

s

-

e

s

,

s

2

if the voltage of phase s i s the one imposed by the current controller ( 30 ):

e

s

=

e

s

,

s

;

e

r

=

e

r

,

s

-

e

s

,

s

-

e

t

,

s

2

;

e

t

=

e

t

,

s

-

e

r

,

s

-

e

s

,

s

2

if the voltage of phase t is the one imposed by the current controller ( 30 ):

e

t

=

e

t

,

s

;

e

r

=

e

r

,

s

-

e

s

,

s

-

e

t

,

s

2

;

e

s

=

e

s

,

s

-

e

r

,

s

-

e

t

,

s

2

.

12 . The control method according to claim 1 , wherein if two of the three selected fourth reference voltages (e r,s , e s,s , e t,s ) are imposed by the current controller ( 30 ), and therefore the other selected control voltage i s imposed by the voltage controller ( 21 ), then the obtained reference voltages without a non-controlled zero sequence (e r , e s , e t ) are calculated as follows:

if the voltages of phases r and s are the ones imposed by the current controller ( 30 ):

e

r

=

e

r

,

s

;

e

s

=

e

s

,

s

;

e

t

=

-

(

e

r

,

s

-

e

s

,

s

)

if the voltages of phases r and t are the ones imposed by the current controller ( 30 ):

e

r

=

e

r

,

s

;

e

t

=

e

t

,

s

;

e

s

=

-

(

e

r

,

s

-

e

t

,

s

)

if the voltages of phases s and t are the ones imposed by the current controller ( 30 ):

e

s

=

e

s

,

s

;

e

t

=

e

t

,

s

;

e

r

=

-

(

e

s

,

s

-

e

t

,

s

)

.

13 . The control method according to claim 1 , wherein if the three selected fourth reference voltages (e r,s , e s,s , e t,s ) are imposed by the current controller ( 30 ), then the two control voltages corresponding to the two phases with the highest current are imposed, i.e.:

if phases r and s are the two phases with the highest current:

e

r

=

e

r

,

s

;

e

s

=

e

s

,

s

;

e

t

=

-

(

e

r

,

s

-

e

s

,

s

)

if phases r and t are the two phases with the highest current:

e

r

=

e

r

,

s

;

e

t

=

e

t

,

s

;

e

s

=

-

(

e

r

,

s

-

e

t

,

s

)

if the two phases s and t are the two phases with the highest current:

e

s

=

e

s

,

s

;

e

t

=

e

t

,

s

;

e

r

=

-

(

e

s

,

s

-

e

t

,

s

)

.

14 . The control method according to claim 1 , comprising:

calculating modulation components (m r,d , m s,d , m t,d ) from the obtained reference voltages without a non-controlled zero sequence (e r , e s , e t ),

controlling switches of the conversion stage of the electronic DC/AC converter ( 1 ) from said modulation components (m r,d , m s,d , m t,d ).

15 . The control method according to claim 14 , wherein for controlling said switches, a pulse width modulator uses said modulation components (m r,d , m s,d , m t,d ) or modulation components with a zero sequence component (m r , m s , m t ) for generating the firing orders of the switches.

16 . The control method according to claim 1 , comprising:

calculating modulation components (m r,d , m s,d , m t,d ) from the obtained reference voltages without a non-controlled zero sequence (e r , e s , e t ),

introducing a desired zero sequence component (mor desired) in said modulation components (m r,d , m s,d , m t,d ), thus obtaining modulation components with a zero sequence component (m r , m s , m t ),

controlling switches of the conversion stage of the electronic DC/AC converter ( 1 ) from said modulation components with a zero sequence component (m r , m s , m t ).

17 . The control method according to claim 1 , wherein the method is repeated at set time instants according to a clock frequency.

18 . The control method according to claim 1 , further comprising reducing the predefined voltage setpoints ( 22 ) if one of the control voltages is imposed by the current controller ( 30 ).

19 . A control system for an electronic DC/AC three-phase converter ( 1 ) which carries out the method described in claim 1 .

20 . The control system according to claim 19 , wherein the voltage controller ( 21 ) further comprises a control loop for reducing the predefined voltage setpoints ( 22 ) if one of the control voltages is imposed by the current controller ( 30 ).