High-voltage direct current converter including a 12-pulse diode recitifier connected in series with a voltage-source converter

1. A converter for high voltage direct current transmission, comprising:

a 12-pulse diode rectifier including two three-phase full bridge diode rectifiers connected in series and rectifying AC power input from an offshore point of common coupling (PCC) into 12-pulse power; and

a voltage-source converter connected in series to a lower end of the 12-pulse diode rectifier,

wherein the voltage-source converter is a multi-phase converter in which multiple phases are coupled in parallel with each other at the lower end of the 12-pulse diode rectifier, and

wherein the voltage-source converter controls voltage of AC power input from the offshore PCC and DC link voltage of the voltage-source converter.

2. The converter for high voltage direct current transmission according to claim 1 , further comprising:

a Y/Y/Δ 3-winding transformer connected to an input end of the 12-pulse diode rectifier and inputting AC power to each of the three-phase full bridge diode rectifiers of the 12-pulse diode rectifier.

3. The converter for high voltage direct current transmission according to claim 1 , wherein the voltage-source converter has a rated voltage corresponding to ⅓ of rated voltage of an HVDC system.

4. The converter for high voltage direct current transmission according to claim 1 , further comprising:

a filter inductor connected in series to an input side of each of the three-phase full bridge diode rectifiers,

wherein each of two secondary sides of the Y/Y/Δ 3-winding transformer is connected in series to the filter inductor.

5. The converter for high voltage direct current transmission according to claim 4 , further comprising:

an AC filter connected in parallel to a primary side of the Y/Y/Δ 3-winding transformer and removing higher order harmonics.

6. The converter for high voltage direct current transmission according to claim 5 , wherein the AC filter comprises a resistor, an inductor and a capacitor, and removes higher order harmonic components of grid current, including at least 23rd and 25th harmonic components.

7. The converter for high voltage direct current transmission according to claim 1 , further comprising:

a control unit for controlling the voltage-source converter,

wherein the control unit comprising:

an offshore PCC voltage controller receiving an AC voltage command value and a measured value of AC voltage at the offshore PCC as a feedback signal and outputting a Q-axis command signal to control reactive power;

a DC link voltage controller receiving a DC link voltage command value and a measured value of DC link voltage of the voltage-source converter as a feedback signal and outputting a D-axis command signal to control active power;

a d-q converter converting a two-phase signal of the Q-axis command signal and the D-axis command signal output from the offshore PCC voltage controller and the DC link voltage controller, respectively, into a three-phase signal;

a proportional-resonant controller removing 11th and 13th harmonic current components through band pass filtering, the harmonic current components being generated at the offshore PCC by the 12-pulse diode rectifier; and

a space-vector PWM generator receiving a command value obtained by adding output of the proportional-resonant controller to output of the d-q converter and controlling each of IGBT devices constituting the voltage-source converter in a space-vector PWM manner.

8. The converter for high voltage direct current transmission according to claim 7 , wherein each of the offshore PCC voltage controller and the DC link voltage controller is a proportional-integral (PI) controller.

9. The converter for high voltage direct current transmission according to claim 1 , wherein the voltage-source converter comprises at least one of a 2-level voltage-source converter, a multilevel NPC converter, and a modular multilevel converter.

10. A converter for high voltage direct current transmission, comprising:

a 12-pulse diode rectifier including two three-phase full bridge diode rectifiers connected in series and rectifying AC power input from an offshore point of common coupling (PCC) into 12-pulse power;

a voltage-source converter connected in series to a lower end of the 12-pulse diode rectifier; and

a Y/Y/Δ 3-winding transformer connected to an input end of the 12-pulse diode rectifier and inputting AC power to each of the three-phase full bridge diode rectifiers of the 12-pulse diode rectifier,

wherein the voltage-source converter controls voltage of AC power input from the offshore PCC and DC link voltage of the voltage-source converter, and

wherein the Y/Y/Δ 3-winding transformer removes 5th and 7th harmonic current components generated by each of the three-phase full bridge diode rectifiers by virtue of 30° phase difference between two secondary side phase voltages of the Y/Y/Δ three-winding transformer.

11. A converter for high voltage direct current transmission, comprising:

a 12-pulse diode rectifier including two three-phase full bridge diode rectifiers connected in series and rectifying AC power input from an offshore point of common coupling (PCC) into 12-pulse power;

a voltage-source converter connected in series to a lower end of the 12-pulse diode rectifier; and

a boost inductor connected in series to an input side of the voltage-source converter,

wherein the voltage-source converter controls voltage of AC power input from the offshore PCC and DC link voltage of the voltage-source converter, and

wherein the boost inductor filters out harmonic components generated in current due to switching of the voltage-source inverter, and a secondary side of a converter transformer is connected in series to the boost inductor.

12. The converter for high voltage direct current transmission according to claim 11 , further comprising:

an AC filter connected in parallel between the secondary side of the converter transformer and the boost inductor and removing higher order harmonics of grid current.