Method and apparatus for zero-sequence damping and voltage balancing

1. A method for minimizing common mode voltage in an arrangement having a three-phase, multi-level inverter having a DC-link with a neutral point dividing the DC-link into two halves, wherein an output voltage of the inverter is controlled on the basis of an output voltage reference, an output LCL-filter connecting the inverter to a grid, and a virtual-ground connection between the LCL-filter and the neutral point of the DC-link, the method comprising:

determining a zero-sequence component of an LCL-filter inverter-side current;

calculating a zero-sequence damping and voltage balancing term based on the LCL-filter inverter-side current zero-sequence component and voltages over the two halves of the DC-link; and

adding the zero-sequence damping and voltage balancing term to the output voltage reference.

2. The method according to claim 1 , wherein calculating the zero-sequence damping and voltage balancing term is performed based on the following equation:

e

γ

*

=

-

R

d

⁡

(

x

1

2

⁢

i

1

⁢

γ

-

4

⁢

P

D

⁢

⁢

C

3

⁢

x

2

)

,

where x1 represents a sum of voltages over the DC-link halves, x2 is a voltage difference between the halves, i 1γ is a zero-sequence component of an inverter-side current of the LCL-filter, Rd is a design coefficient, and PDC is a power reference.

3. The method according to claim 2 , wherein calculating the damping and balancing term further comprises

reducing third-harmonic content of the damping and balancing term;

calculating a third-harmonic term based on the inverter output voltage control term; and

adding the third-harmonic term to the damping and balancing term.

4. The method according to claim 1 , wherein calculating the zero-sequence damping and voltage balancing term is performed based on the following equation:

e γ *=−R d1 i 1γ +R d2 x 2 ,

where x2 is a voltage difference between the halves, i1γ is a zero-sequence component of an inverter-side current of the LCL-filter, and Rd1 and Rd2 are design coefficients.

5. The method according to claim 4 , wherein calculating the damping and balancing term further comprises

reducing third-harmonic content of the damping and balancing term;

calculating a third-harmonic term based on the inverter output voltage control term; and

adding the third-harmonic term to the damping and balancing term.

6. The method according to claim 1 , wherein calculating the zero-sequence damping and voltage balancing term is performed by using a PI-controller.

7. The method according to claim 6 , wherein calculating the damping and balancing term further comprises

reducing third-harmonic content of the damping and balancing term;

calculating a third-harmonic term based on the inverter output voltage control term; and

adding the third-harmonic term to the damping and balancing term.

8. The method according to claim 1 , wherein calculating the damping and balancing term further comprises

reducing third-harmonic content of the damping and balancing term;

calculating a third-harmonic term based on the inverter output voltage control term; and

adding the third-harmonic term to the damping and balancing term.

9. The method according to claim 8 , wherein calculating the damping and balancing term further comprises

reducing third-harmonic content of the damping and balancing term;

calculating a third-harmonic term based on the inverter output voltage control term; and

adding the third-harmonic term to the damping and balancing term.

10. The method according to claim 1 , wherein calculating the zero-sequence damping and voltage balancing term comprises:

forming a zero-sequence component model of the LCL filter;

forming a controller for the zero-sequence component model; and

forming a zero-sequence damping and voltage balancing term by using the controller.

11. The method according to claim 10 , wherein calculating the zero-sequence damping and voltage balancing term is performed based on the following equation:

e

γ

*

=

-

R

d

⁡

(

x

1

2

⁢

i

1

⁢

γ

-

4

⁢

P

D

⁢

⁢

C

3

⁢

x

2

)

,

where x1 represents a sum of voltages over the DC-link halves, x2 is a voltage difference between the halves, i1γ is a zero-sequence component of an inverter-side current of the LCL-filter, Rd is a design coefficient, and PDC is a power reference.

12. The method according to claim 10 , wherein calculating the zero-sequence damping and voltage balancing term is performed based on the following equation:

e γ *=−R d1 i 1γ +R d2 x 2 ,

where x2 is a voltage difference between the halves, i1γ is a zero-sequence component of an inverter-side current of the LCL-filter, and Rd1 and Rd2 are design coefficients.

13. The method according to claim 10 , wherein calculating the zero-sequence damping and voltage balancing term is performed by using a PI-controller.

14. The method according to claim 10 , wherein calculating the damping and balancing term further comprises

reducing third-harmonic content of the damping and balancing term;

calculating a third-harmonic term based on the inverter output voltage control term; and

adding the third-harmonic term to the damping and balancing term.

15. An apparatus for an arrangement having a three-phase, multi-level inverter including a DC-link with a neutral point dividing the DC-link into two halves, wherein an output voltage of the inverter is controlled based on an output voltage reference, an output LCL-filter connecting the inverter to a grid, and a virtual-ground connection between the LCL-filter and the neutral point of the DC-link,

the apparatus comprising a controller configured to:

determine a zero-sequence component of an LCL-filter inverter-side current;

calculate a zero-sequence damping and voltage balancing term based on the LCL-filter inverter-side current zero-sequence component and voltages over the two halves of the DC-link; and

add the zero-sequence damping and voltage balancing term to the output voltage reference.