Wind farm, high voltage ride through control method therefor, system, MMC and machine-side inverter

1. A method for controlling high voltage ride through of a wind farm, the method being applied in a Modular Multilevel Converter (MMC) connected between Direct Current (DC) buses and a power grid of the wind farm, and the method comprising:

determining an amplitude of a voltage of the power grid;

determining that a high voltage ride through event is occurred under a condition that the amplitude of the voltage of the power grid exceeds a first threshold;

increasing a set value for a voltage across the DC buses to raise the voltage across the DC buses;

acquiring a fundamental frequency modulation wave of the MMC;

adjusting the fundamental frequency modulation wave based on the increased set value for the voltage across the DC buses and a measured value of the voltage across the DC buses;

superimposing a triple harmonic on the adjusted fundamental frequency modulation wave to obtain a superimposed modulation wave; and

controlling the MMC to operate based on the superimposed modulation wave.

2. The method according to claim 1 , wherein:

the fundamental frequency modulation wave is determined based on a degree of modulation, a rotation angle and an initial phase angle; and

the triple harmonic is determined based on a coefficient for the triple harmonic, the degree of modulation, the rotation angle and the initial phase angle.

3. A Modular Multilevel Converter (MMC), comprising modules for performing the steps in the method according to claim 1 .

4. A system for controlling high voltage ride through of a wind farm having a plurality of wind turbines, comprising:

the MMC according to claim 3 ; and

a plurality of machine-side converters comprising a machine-side converter which is configured to:

determine an amplitude of a voltage across the DC buses;

determine that a high voltage ride through event is occurred under a condition that the amplitude of the voltage across the DC buses exceeds a second threshold;

calculate, based on the amplitude of the voltage across the DC buses, a minimum reactive power that the MMC with capability of high voltage ride through needs to absorb;

calculate, based on the minimum reactive power and an apparent power of the MMC, a minimum duty ratio allowed by a braking circuit of the machine-side converter with capability of high voltage ride through; and

control the braking circuit to operate based on a preset duty ratio, wherein the preset duty ratio is greater than or equal to the minimum duty ratio.

5. A wind farm, comprising:

the MMC according to claim 3 ; and

a plurality of wind turbines, wherein at least one of the plurality of wind turbines comprises a machine-side converter which is configured to:

determine an amplitude of a voltage across the DC buses;

determine that a high voltage ride through event is occurred under a condition that the amplitude of the voltage across the DC buses exceeds a second threshold;

calculate, based on the amplitude of the voltage across the DC buses, a minimum reactive power that the MMC with capability of high voltage ride through needs to absorb;

calculate, based on the minimum reactive power and an apparent power of the MMC, a minimum duty ratio allowed by a braking circuit of the machine-side converter with capability of high voltage ride through; and

control the braking circuit to operate based on a preset duty ratio, wherein the preset duty ratio is greater than or equal to the minimum duty ratio.

6. A computer-readable storage medium having programs stored thereon, wherein the programs comprises operation instructions for performing a method for controlling high voltage ride through according to claim 1 .

7. A method for controlling high voltage ride through of a wind farm, the wind farm including a Modular Multilevel Converter (MMC) and a machine-side converter of a wind turbine, wherein the machine-side converter is connected to the MMC through Direct Current (DC) buses, and the method comprising:

determining, by the MMC, an amplitude of a voltage of the power grid;

determining, by the MMC, that a high voltage ride through event is occurred under a condition that the amplitude of the voltage of the power grid exceeds a first threshold;

increasing, by the MMC, a set value for a voltage across the DC buses to raise the voltage across the DC buses;

acquiring, by the MMC, a fundamental frequency modulation wave of the MMC;

adjusting, by the MMC, the fundamental frequency modulation wave based on the increased set value for the voltage across the DC buses and a measured value of the voltage across the DC buses;

superimposing, by the MMC, a triple harmonic on the adjusted fundamental frequency modulation wave to obtain a superimposed modulation wave; and,

controlling, by the MMC, the MMC to operate based on the superimposed modulation wave;

determining, by the machine-side converter, an amplitude of a voltage across the DC buses;

determining, by the machine-side converter, that a high voltage ride through event is occurred under a condition that the amplitude of the voltage across the DC buses exceeds a second threshold;

calculating, by the machine-side converter, based on the amplitude of the voltage across the DC buses, a minimum reactive power that the MMC with capability of high voltage ride through needs to absorb through;

calculating, by the machine-side converter, based on the minimum reactive power and an apparent power of the MMC, a minimum duty ratio allowed by a braking circuit of the machine-side converter with capability of high voltage ride through; and

controlling, by the machine-side converter, the braking circuit to operate based on a preset duty ratio, wherein the preset duty ratio is greater than or equal to the minimum duty ratio.

8. The method according to claim 4 , wherein the calculating, according to the amplitude of the voltage across the DC buses, the minimum reactive power that the MMC with capability of high voltage ride through needs to absorb comprises:

acquiring a minimum reactive current that the MMC with capability of high voltage ride through needs to absorb;

calculating a product of the amplitude of the voltage across the DC buses, the minimum reactive current and the apparent power of the MMC; and

determining the product as the minimum reactive power that the MMC with capability of high voltage ride through needs to absorb.

9. The method according to claim 7 , wherein the calculating, based on the minimum reactive power and the apparent power of the MMC, the minimum duty ratio allowed by the braking circuit of the machine-side converter with capability of high voltage ride through comprises:

calculating, based on the minimum reactive power and the apparent power of the MMC, a maximum active power allowed to be output by the machine-side converter of the wind turbine with capability of high voltage ride through;

calculating a ratio of the maximum active power to the apparent power of the MMC; and

determining, based on the ratio, the minimum duty ratio allowed by the braking circuit of the machine-side converter with capability of high voltage ride through.

10. A machine-side converter of a wind turbine, comprising modules for performing the steps in the method according to claim 7 .

11. A computer-readable storage medium having programs stored thereon, wherein the programs comprises operation instructions for performing a method for controlling high voltage ride through according to claim 7 .