EP-4742485-A1 - HVDC SYSTEM

Abstract

Proposed is a High Voltage Direct Current (HVDC) system in which a DC terminal of a Modular Multilevel Converter (MMC) is formed into a bipolar structure. The HVDC system includes a first transformer having a primary side connected to an AC grid side, a power transmission line connected to a secondary side of the first transformer, an MMC connected to the secondary side of the first transformer through the power transmission line, a positive terminal (P) and a negative terminal (N) formed at a DC terminal of the MMC, a second transformer connected to the power transmission line, and a ground terminal (G) connected to a neutral point of the second transformer and to a ground (GND). Furthermore, the DC terminal of the MMC forms a bipolar structure by the positive terminal (P), the negative terminal (N), and the ground terminal (G).

Inventors

• KU, Nam Joon
• PARK, Hong Bean
• LEE, DOO YOUNG
• JUNG, HONG JU
• PARK, SEONG WON

Assignees

• Hyosung Heavy Industries Corporation

Dates

Publication Date

20260513

Application Date

20240701

Claims (5)

1. an HVDC system comprising: a first transformer having a primary side connected to an AC grid side; a power transmission line connected to a secondary side of the first transformer; an MMC connected to the secondary side of the first transformer through the power transmission line; a positive terminal and a negative terminal formed at a DC terminal of the MMC; a second transformer connected to the power transmission line; and a ground terminal connected to a neutral point of the second transformer and to a ground (GND), wherein the DC terminal of the MMC forms a bipolar structure by the positive terminal, the negative terminal, and the ground terminal.
2. The HVDC system of claim 1, wherein the second transformer comprises a zig-zag transformer.
3. The HVDC system of claim 1, wherein the MMC comprises an upper arm and a lower arm, and each of the upper arm and the lower arm comprises a plurality of submodules connected in series.
4. The HVDC system of claim 3, wherein an on/off duty ratio of each switching unit of the submodules of the upper arm and an on/off duty ratio of each switching unit of the submodules of the lower arm are independently adjusted.
5. The HVDC system of claim 1, wherein the neutral point of the second transformer is directly connected to the ground (GND).

Description

Technical Field The present disclosure relates to a High Voltage Direct Current (HVDC) system. More particularly, the present disclosure relates to an HVDC system in which a DC terminal of a Modular Multilevel Converter (MMC) constituting the HVDC system has a bipolar structure. Background Art Generally, a High Voltage Direct Current (HVDC) system converts an alternating current produced at a power plant into a direct current and transmits the direct current, and reconverts the direct current into an alternating current at a power reception terminal to supply power to a load. Such an HVDC system can efficiently and economically transmit power, connect different systems, and transmit power a long distance with high efficiency by stepping up a voltage. In the HVDC system, a Modular Multilevel Converter (MMC) may be connected for power transmission and reactive power compensation. The MMC includes a plurality of submodules connected in series, and is a power converter capable of outputting a direct current voltage from an alternating current voltage according to switching operations of a plurality of switches (e.g., IGBTs) provided in the submodules. In a conventional general HVDC MMC, a method in which a secondary side of a transformer is delta-connected (Δ) so as to separate an AC side and a DC side is applied. However, in this case, there is a problem that voltage imbalance may occur on the DC side, and there is a problem that an overvoltage occurs at a DC terminal when a ground fault occurs on the DC side. In order to solve such problems, conventionally, a secondary side of the transformer and an AC side of a converter are grounded. For example, a zig-zag transformer is used between the AC side and the MMC. As an example, in Korean Patent Application Publication No. 10-2020-0007164, an HVDC system in which a zig-zag transformer is connected to a delta-connection (Δ) at a secondary side of an HVDC transformer so as to form a neutral point is proposed. As another example, in Korean Patent No. 10-2502391, an HVDC system in which a zig-zag transformer connected such that a neutral point is formed on a power transmission line between an AC side and an MMC is proposed. In the above prior patents and related art, a DC terminal of an MMC has a monopolar structure. Such a monopolar structure supplies a common direct current voltage to a positive pole and a negative pole, and has a simple structure, thereby facilitating control. However, since entire power is blocked when an accident occurs at a DC terminal, there is a disadvantage in that power supply reliability is low. Disclosure Technical Problem Accordingly, an objective of the present disclosure is to provide a High Voltage Direct Current (HVDC) system in which a DC terminal of a Modular Multilevel Converter (MMC) connected to the HVDC system is implemented in a bipolar structure. Another objective of the present disclosure is to provide an HVDC system in which a zig-zag transformer is connected to a power transmission line between an AC side and an MMC in the HVDC system such that outputs of two poles at the DC terminal of the MMC are capable of being independently controlled. Technical Solution According to an aspect of the present disclosure, there is provided an HVDC system including: a first transformer having a primary side connected to an AC grid side; a power transmission line connected to a secondary side of the first transformer; an MMC connected to the secondary side of the first transformer through the power transmission line; a positive terminal (P) and a negative terminal (N) formed at a DC terminal of the MMC; a second transformer connected to the power transmission line; and a ground terminal (G) connected to a neutral point of the second transformer and to a ground (GND), wherein the DC terminal of the MMC forms a bipolar structure by the positive terminal (P), the negative terminal (N), and the ground terminal (G). In the present disclosure, the second transformer may include a zig-zag transformer. In the present disclosure, the MMC may include an upper arm and a lower arm, and each of the upper arm and the lower arm may include a plurality of submodules connected in series. In the present disclosure, an on/off duty ratio of each switching unit of the submodules of the upper arm and an on/off duty ratio of each switching unit of the submodules of the lower arm may be independently adjusted. In the present disclosure, the neutral point of the second transformer may be directly connected to the ground (GND). Advantageous Effects According to the HVDC system according to an embodiment of the present disclosure, there are effects as follows. According to the present disclosure, by forming the DC terminal of the MMC connected to the HVDC system in the bipolar structure, outputs of the two poles are capable of being independently controlled. According to the present disclosure, since the DC terminal of the MMC is capable of being independently cont