Control method and controller for string inverter, inverter, and inverter system

1. A control method for a string inverter, wherein the string inverter comprises an inverter circuit and N direct current (DC) to DC (DC-DC) converter circuits, wherein N is an integer greater than or equal to 2, wherein an input end of each DC-DC converter circuit is connected to a corresponding photovoltaic string, wherein an output end of each DC-DC converter circuit is connected to an input end of the inverter circuit, wherein the N DC-DC converter circuits comprise a non-current/voltage (non-IV) curve scanning DC-DC circuit and an IV curve scanning DC-DC circuit, and wherein the method comprises:

controlling an output power of the inverter circuit to be a specified power reference value during an IV curve scanning process; and

controlling an output voltage of each non-IV curve scanning DC-DC circuit to be a specified voltage reference value during the IV curve scanning process,

wherein the specified power reference value is less than or equal to a sum of input powers of all non-IV curve scanning DC-DC circuits before the IV curve scanning process, and

wherein the specified voltage reference value is greater than or equal to a preset percentage of a maximum value in open-circuit voltages, before the IV curve scanning process, of photovoltaic strings connected to all IV curve scanning DC-DC circuits.

2. The control method according to claim 1 , wherein before the IV curve scanning process, the method further comprises obtaining the sum of the input powers of all the non-IV curve scanning DC-DC circuits.

3. The control method according to claim 1 , wherein before the IV curve scanning process, the method further comprises obtaining a maximum value in input voltages of all the IV curve scanning DC-DC circuits when input currents of all the IV curve scanning DC-DC circuits are 0, wherein the maximum value in the input voltages is the maximum value in the open-circuit voltages of the photovoltaic strings.

4. The control method according to claim 1 , wherein controlling the output power of the inverter circuit comprises:

obtaining the output power of the inverter circuit;

comparing the output power of the inverter circuit with the specified power reference value to obtain a power comparison result;

obtaining a first current reference value of a d-axis of the inverter circuit via a first proportional-integral regulator based on the power comparison result; and

controlling a current of the d-axis based on the first current reference value of the d-axis.

5. The control method according to claim 1 , wherein controlling the output voltage of each non-IV curve scanning DC-DC circuit comprises:

detecting an output voltage of a DC-DC circuit m, wherein a DC-DC circuit 1 to a DC-DC circuit K are IV curve scanning DC-DC circuits, wherein a DC-DC circuit (K+1) to a DC-DC circuit N are non-IV curve scanning DC-DC circuits, and wherein m=K+1, K+2, . . . , N;

comparing the output voltage of the DC-DC circuit m with the specified voltage reference value to obtain a voltage comparison result;

obtaining a feedback current regulation value of the DC-DC circuit m via a second proportional-integral regulator based on the voltage comparison result;

using a sum of the feedback current regulation value and a feedforward current regulation value as a current reference value of the DC-DC circuit m, wherein the feedforward current regulation value is positively correlated with the specified power reference value, and is negatively correlated with a sum of powers of all the IV curve scanning DC-DC circuits during the IV curve scanning process; and

controlling a current of the DC-DC circuit m based on the current reference value such that the output voltage of the DC-DC circuit m is the specified voltage reference value.

6. The control method according to claim 5 , further comprising obtaining the feedforward current regulation value according to the following formula:

I m_fdb ( t 3 )= K m ·{P ref1 −Σ l=1 K [ U l ( t 3 )· I l ( t 3 )]}/ U m in ( t 3 ),

wherein K m represents a preset feedforward proportion coefficient, wherein 0≤K m ≤1, wherein P ref1 =Σ j=K+1 N P j (t 0 )·α, wherein 0<α≤1, wherein t 0 represents a moment before the IV curve scanning process, wherein P j (t 0 ) represents an input power of a DC-DC circuit before the IV curve scanning process, wherein U l (t 3 ) and I l (t 3 ) respectively represent an input voltage and an input current of the DC-DC circuit during the IV curve scanning process, and wherein U m in (t 3 ) represents an input voltage of the DC-DC circuit m during the IV curve scanning process.

7. The control method according to claim 1 , wherein controlling the output power of the inverter circuit comprises:

obtaining a second sum of input powers of all the IV curve scanning DC-DC circuits during the IV curve scanning process;

subtracting the second sum of the input powers from the specified power reference value to obtain an input power difference; and

controlling the second sum of the input powers to be the input power difference.

8. The control method according to claim 7 , wherein controlling the second sum of the input powers to be the input power difference comprises controlling an input power of a DC-DC circuit m in the non-IV curve scanning DC-DC circuits to be P m_ref (t 3 ) according to the following formula:

P m_ref ( t 3 )= K m_pwr ·{Σ j=K+1 N P j ( t 0 )·α−Σ l=1 K [ U l ( t 3 )· I l ( t 3 )]},

wherein m=K+1, K+2, . . . , N, wherein K m_pwr represents a preset input power allocation factor of the DC-DC circuit m, wherein Σ m=K+1 N K m_pwr =1, wherein P j (t 0 ) represents an input power of a DC-DC circuit before the IV curve scanning process, and wherein U l (t 3 ) and I l (t 3 ) respectively represent an input voltage and an input current of the DC-DC circuit during the IV curve scanning process.

9. The control method according to claim 8 , wherein controlling the input power of the DC-DC circuit m to be P m_ref (t 3 ) comprises:

obtaining an input voltage of the DC-DC circuit m during the IV curve scanning process;

obtaining a current reference value of the DC-DC circuit m based on the input voltage of the DC-DC circuit m and the input power P m_ref (t 3 ) of the DC-DC circuit m; and

controlling a current of the DC-DC circuit m based on the current reference value such that the input power of the DC-DC circuit m is P m_ref (t 3 ).

10. The control method according to claim 1 , wherein controlling the output voltage of each non-IV curve scanning DC-DC circuit comprises:

obtaining an input voltage of the inverter circuit, wherein the input voltage is equal to the output voltage of each non-IV curve scanning DC-DC circuit;

comparing the input voltage with the specified voltage reference value to obtain a comparison result for the input voltage;

obtaining a second current reference value of a d axis of the inverter circuit via a third proportional-integral regulator based on the comparison result; and

controlling a current of the d axis based on the second current reference value such that the output voltage of each non-IV curve scanning DC-DC circuit is the specified voltage reference value.

11. The control method according to claim 1 , wherein the IV curve scanning process comprises:

controlling an input voltage of each IV curve scanning DC-DC circuit to gradually change from an open-circuit voltage to a short-circuit voltage;

recording the input voltage and a corresponding input current of each IV curve scanning DC-DC circuit in a process in which the input voltage of each IV curve scanning DC-DC circuit changes; and

obtaining, based on the recorded input voltage and the corresponding input current, an IV curve corresponding to each IV curve scanning DC-DC circuit.

12. A controller for a string inverter, wherein the string inverter comprises an inverter circuit and N direct current (DC) to DC (DC-DC) converter circuits, wherein N is an integer greater than or equal to 2, wherein an input end of each DC-DC circuit is connected to a corresponding photovoltaic string, wherein an output end of each DC-DC circuit is connected to an input end of the inverter circuit, wherein the N DC-DC circuits comprise a non-current/voltage (non-IV) curve scanning DC-DC circuit and an IV curve scanning DC-DC circuit, and wherein the controller comprises:

a non-transitory storage medium storing computer-readable instructions; and

a processor coupled to the non-transitory storage medium and configured to execute the computer-readable instructions to:

control an output power of the inverter circuit to be a specified power reference value during an IV curve scanning process; and

control an output voltage of each non-IV curve scanning DC-DC circuit to be a specified voltage reference value during the IV curve scanning process,

wherein the specified power reference value is less than or equal to a sum of input powers of all non-IV curve scanning DC-DC circuits before the IV curve scanning process, and

wherein the specified voltage reference value is greater than or equal to a preset percentage of a maximum value in open-circuit voltages of all IV curve scanning DC-DC circuits before the IV curve scanning process.

13. The controller according to claim 12 , wherein the controller is further configured to obtain the sum of the input powers before the IV curve scanning process.

14. The controller according to claim 13 , wherein the controller controls the output voltage of each non-IV curve scanning DC-DC circuit to be the specified voltage reference value by:

obtaining an input voltage of the inverter circuit, where the input voltage is equal to the output voltage of each non-IV curve scanning DC-DC circuit;

comparing the input voltage with the specified voltage reference value to obtain a comparison result for the input voltage;

obtaining a second current reference value of a d axis of the inverter circuit via a third proportional-integral regulator based on the comparison result; and

controlling a current of the d axis based on the second current reference value such that the output voltage of each non-IV curve scanning DC-DC circuit is the specified voltage reference value.

15. The controller according to claim 12 , wherein the controller is further configured to obtain, before the IV curve scanning process, a maximum value in input voltages of all the IV curve scanning DC-DC circuits when input currents of all the IV curve scanning DC-DC circuits are 0, wherein the maximum value in the input voltages is a maximum value in open-circuit voltages of photovoltaic strings connected to all the IV curve scanning DC-DC circuits.

16. The controller according to claim 12 , wherein the controller controls the output power of the inverter circuit to be the specified power reference value by:

obtaining the output power of the inverter circuit;

comparing the output power of the inverter circuit with the specified power reference value to obtain a power comparison result;

obtaining a first current reference value of a d axis of the inverter circuit via a first proportional-integral regulator based on the power comparison result; and

controlling a current of the d axis based on the first current reference value of the d axis.

17. The controller according to claim 12 , wherein the controller controls the output voltage of each non-IV curve scanning DC-DC circuit to be the specified voltage reference value by:

detecting an output voltage of a DC-DC circuit m, wherein a DC-DC circuit 1 to a DC-DC circuit K are IV curve scanning DC-DC circuits, wherein a DC-DC circuit (K+1) to a DC-DC circuit N are non-IV curve scanning DC-DC circuits, and wherein m=K+1, K+2, . . . , N;

comparing the output voltage of the DC-DC circuit m with the specified voltage reference value to obtain a voltage comparison result;

obtaining a feedback current regulation value of the DC-DC circuit m via a second proportional-integral regulator based on the voltage comparison result;

using a sum of the feedback current regulation value and a feedforward current regulation value as a current reference value of the DC-DC circuit m, wherein the feedforward current regulation value is positively correlated with the specified power reference value, and is negatively correlated with a sum of powers of all the IV curve scanning DC-DC circuits in the IV curve scanning process; and

controlling a current of the DC-DC circuit m based on the current reference value such that the output voltage of the DC-DC circuit m is the specified voltage reference value.

18. The controller according to claim 12 , wherein the controller controls the output power of the inverter circuit to be the specified power reference value by:

obtaining a second sum of input powers of all the IV curve scanning DC-DC circuits;

subtracting the second sum of the input powers from the specified power reference value to obtain an input power difference; and

controlling the second sum of the input powers to be the input power difference.

19. The controller according to claim 18 , wherein the controller controls the second sum of the input powers to be the input power difference by controlling an input power of a DC-DC circuit m in the non-IV curve scanning DC-DC circuits to be P m_ref (t 3 ) according to the following formula:

P m_ref ( t 3 )= K m_pwr ·{Σ j=K+1 N P j ( t 0 )·α−Σ l=1 K [ U l ( t 3 )· I l ( t 3 )]},

wherein m=K+1, K+2, . . . , N, wherein K m_pwr represents a preset input power allocation factor of the DC-DC circuit m, wherein Σ m=K+1 N K m_pwr =1, wherein P j (t 0 ) represents an input power of a DC-DC circuit before the IV curve scanning process, and wherein U l (t 3 ) and I l (t 3 ) respectively represent an input voltage and an input current of the DC-DC circuit during the IV curve scanning process.

20. The controller according to claim 19 , wherein the controller controls the input power of the DC-DC circuit m to be P m_ref (t 3 ) by:

obtaining an input voltage of the DC-DC circuit m;

obtaining a current reference value of the DC-DC circuit m based on the input voltage of the DC-DC circuit m and the input power P m_ref (t 3 ) of the DC-DC circuit m; and

controlling a current of the DC-DC circuit m based on the current reference value such that the input power of the DC-DC circuit m is P m_ref (t 3 ).