Patent Description:
An MMC is a converter that uses a switching element, such as an insulated-gate bipolar transistor (IGBT), etc., which is on/off controllable, and is capable of outputting a voltage equal to or greater than a withstand voltage of the switching element. The MMC is applied in high voltage direct current (HVDC), a STATCOM, a motor drive inverter, etc..

The MMC for HVDC is composed of multiple converter arms, and each converter arm is composed of multiple submodules connected to each other in series. Each submodule includes, for example, an energy storage unit and a power semiconductor. The power semiconductor may include a power semiconductor switch and a freewheeling diode, and may be realized as, for example, an IGBT. In such a submodule, multiple power semiconductors are connected to form a so-called half-bridge or full-bridge circuit.

An HVDC system connected to such an MMC, in order to increase a rate of operation, generally includes a redundancy submodule as a spare in addition to a submodule required at a rating in preparation for a breakdown of the submodule. Thus, when the breakdown of the submodule in operation occurs, the redundancy submodule is applied such that the system may run without interruption. Accordingly, the redundancy submodule operates instead of the submodule which has broken down such that stable operation of the system may be realized. "<NPL>et al. discloses such redundancy techniques among others and also the preamble of claim <NUM>.

However, in the case where there is no more redundancy submodule to be applied due to continuing breakdown of the submodule in operation, when the additional breakdown of the submodule in operation occurs, voltage imbalance between multiple phases causes interruption of the system and affects an electric power system for HVDC.

An object of the present invention is to provide a redundancy control method of an MMC for HVDC, the method lowering a DC link voltage input to each converter arm and lowering an output voltage of each submodule of a converter arm in which a breakdown does not occur so as to continuously operate the MMC when the breakdown of the submodule in operation occurs in the case where there is no more redundancy submodule to be applied in the MMC for HVDC.

The present invention provides a redundancy control method of an MMC for HVDC according to claim.

In the present invention, the multiple submodules in operation included in each of the converter arms may output same voltages within the converter arm.

According to the present invention, in the case where there is no more redundancy module to be applied in the MMC for HVDC, when the breakdown of the submodule of the MMC, which is in operation, occurs, a DC link voltage input to each converter arm of the MMC is lowered and an output voltage of each submodule of a converter arm in which a breakdown does not occur is lowered, whereby the MMC may be continuously operated, damage caused by interruption of a system may be reduced, and replacement and repair schedule for the submodule which has broken down may be flexibly adjusted.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. Also, in describing the present invention, the detailed descriptions of known related constitutions or functions thereof may be omitted if they make the gist of the present invention unclear.

<FIG> is a diagram illustrating a schematic configuration of an MMC for HVDC according to an embodiment of the present invention.

As shown in <FIG>, an MMC <NUM> for HVDC according to the present invention is composed of three converter arms <NUM>, <NUM>, and <NUM>, and each of the converter arms <NUM>, <NUM>, and <NUM> includes multiple submodules <NUM> connected to each other in series, and redundancy modules <NUM> which are spare modules. The submodules <NUM> participate in operation, and when a breakdown of the submodules <NUM> in operation occurs, the redundancy submodule <NUM> is controlled to participate in operation by being applied instead of the submodule.

According to a DC link voltage Vdc input to each of the converter arms <NUM>, <NUM>, and <NUM>, a voltage of each submodule <NUM> is controlled. Also, the same AC voltages with different phases are output through AC voltage terminals A, B, and C, which are load connection terminals, and these output voltages may be connected to a three-phase load, e.g. a three-phase AC power system.

The submodules <NUM> included in each of the converter arms <NUM>, <NUM>, and <NUM> output the same voltages during operation, and the voltages are added by the series connection and phase control of the submodules <NUM> of each of the converter arms <NUM>, <NUM>, and <NUM> and are output as an AC voltage through the AC voltage terminals A, B, and C.

When the breakdown of the submodule <NUM> in operation occurs or when operation thereof is impossible for an arbitrary reason, the redundancy module <NUM> which is a spare module is applied as a replacement module for the submodule <NUM> which has broken down.

As described above, the MMC <NUM> for HVDC according to the present invention generally includes the redundancy submodules <NUM> which are the spare modules in addition to the submodules <NUM> participating in operation, which are required at a rating. Accordingly, even when the breakdown of the submodule <NUM> in operation occurs, the system to which the MMC for HVDC is applied may operate normally without interrupting operation of the system by applying the redundancy submodule <NUM>.

However, when in one converter arm of the multiple converter arms <NUM>, <NUM>, and <NUM>, for example, when in the first converter arm <NUM>, the breakdown of the submodule <NUM> in operation occurs in the case where all redundancy submodules <NUM> are applied, there is no more redundancy submodule <NUM> to be applied. Thus, the output voltages of the submodules <NUM> except for the submodule <NUM> which has broken down are added to be the output voltage of the first converter arm <NUM>.

In this case, the output voltage of the first converter arm <NUM> has a smaller value than the output voltages of the other converter arms <NUM> and <NUM>, and thus imbalance occurs in three-phase AC power. In the case of the MMC <NUM> for HVDC, when imbalance occurs in the AC power which is output, the imbalance may cause a problem with the load, and thus a function of interrupting the system is performed.

In order to prevent interruption in operation of the system, the MMC <NUM> for HVDC adjusts the DC link voltage Vd, which is an input voltage, on the basis of output voltages of submodules of the first converter arm <NUM> except for the submodule which has broken down, and lowers the output voltage of each submodule <NUM> of the other converter arms <NUM> and <NUM> except for the first converter arm <NUM> according to the adjusted DC link voltage, whereby output voltages of respective converter arms <NUM>, <NUM>, and <NUM> are set to be the same and the system operates normally.

Here, the DC link voltage Vdc supplied to each of the converter arms <NUM>, <NUM>, and <NUM> is changed to the sum of output voltages of submodules <NUM> of the first converter arm <NUM> except for the submodule <NUM> which has broken down. Here, the output voltages of submodules <NUM> of the first converter arm <NUM> except for the submodule <NUM> which has broken down are controlled to a rated output voltage.

Next, except for the first converter arm <NUM>, output voltages of the submodules <NUM> of the other converter arms <NUM> and <NUM> are lowered depending on the change in the DC link voltage Vdc supplied to each of the converter arms <NUM>, <NUM>, and <NUM>. The output voltage of each submodule <NUM> of the other converter arms <NUM> and <NUM> is the result of multiplying the original rated output voltage of each submodule <NUM> and a value that is obtained by dividing the adjusted DC link voltage by the original rated Vdc of the other converter arms <NUM> and <NUM>.

For example, it is assumed in the MMC that each of the converter arms <NUM>, <NUM>, and <NUM> includes <NUM> submodules, a rated DC link voltage Vdc input to each of the converter arms <NUM>, <NUM>, and <NUM> is <NUM> V, and the output voltage of each submodule is <NUM> V. Here, when assuming that two of submodules of the first converter arm <NUM> have broken down and there is no more redundancy module to be applied, the number of submodules operable in the first converter arm <NUM> is eight and the DC link voltage Vdc is adjusted to <NUM> V by multiplying eight submodules and <NUM> V which is the output voltage of each submodule.

Also, except for the first converter arm <NUM>, the DC link voltage Vdc of each of the other converter arms <NUM> and <NUM> is also <NUM> V, and thus the output voltage of each submodule of the other converter arms <NUM> and <NUM> is controlled to <NUM> V, which is a value obtained by multiplying the original output voltage, i.e. <NUM> V, and <NUM> V/<NUM> V, according to the Vdc, i.e. <NUM> V. That is, eight submodules of the first converter arm <NUM> of which two submodules have broken down are controlled to <NUM> V equal to the original rated output voltage, and thus the DC link voltage Vdc input to each converter arm is controlled to <NUM> V which is <NUM> V smaller than the original rated voltage <NUM> V. Also, the output voltage of each of <NUM> submodules for each of the other converter arms <NUM> and <NUM> is controlled to <NUM> V according to the DC link voltage Vdc which is <NUM> V.

<FIG> is a flowchart illustrating a redundancy control method of an MMC for HVDC according to an embodiment of the present invention.

Referring to <FIG>, in the MMC <NUM> for HVDC, wherein the MMC is connected to HVDC and is composed of multiple converter arms <NUM>, <NUM>, and <NUM>, and each of the converter arms <NUM>, <NUM>, and <NUM> includes multiple submodules <NUM> in operation and redundancy submodules <NUM> which are spare module, when the breakdown of the submodule <NUM> of the first converter arm <NUM> occurs among the multiple converter arms <NUM>, <NUM>, and <NUM> at step S100, whether there is a redundancy submodule <NUM> to replace the submodule <NUM> in the first converter arm <NUM> is checked at step S110.

In the case where all redundancy submodules <NUM> of the first converter arm <NUM> are applied in operation, when there is no more redundancy submodule <NUM> to be applied for replacement, the DC link voltage Vdc supplied to the multiple converter arms <NUM>, <NUM>, and <NUM> is adjusted to be equal to the sum of output voltages of normal submodules <NUM> of the first converter arm <NUM> except for the submodule <NUM> which has broken down, and the output voltage of each submodule <NUM> of the other converter arms <NUM> and <NUM> is lowered according to the adjusted DC link voltage at step S120, thereby keep operating the MMC <NUM> for HVDC at step S140.

Here, the output voltages of remaining submodules <NUM>, in which the breakdown does not occur, of the first converter arm <NUM> remain the same, and except for the first converter arm <NUM>, the output voltage of each submodule <NUM> of the other converter arms <NUM> and <NUM> is obtained by multiplying the original output voltage and a value that is obtained by dividing the adjusted DC link voltage Vd by each submodule <NUM> of the first converter arm <NUM> by the rated DC link voltage of the other converter arms <NUM> and <NUM>.

The output voltage of each submodule <NUM> of the other converter arms <NUM> and <NUM> is reduced than the original rated output voltage, and as the DC link voltage Vd which is input decreases, the total active power output from each converter arm decreases. However, interruption in operation of the system may be delayed by a desired time, and replacement and repair schedule for the module which has broken down may be flexibly adjusted.

On the contrary, when there is a redundancy module <NUM> to replace the submodule <NUM> which has broken down, the redundancy module <NUM> is applied at step S130, and the MMC <NUM> for HVDC keeps operating at step S140.

Claim 1:
A redundancy control method of a modular multilevel converter (<NUM>) for high voltage direct current, wherein the modular multilevel converter has multiple converter arms (<NUM>, <NUM>, <NUM>) each including multiple submodules (<NUM>) and redundancy submodules (<NUM>), the method comprising:
checking whether a breakdown of a submodule in operation of a first converter arm among the multiple converter arms occurs in a case where all of the redundancy submodules of the first converter arm are applied and in operation; and
controlling a DC link voltage (Vdc) supplied to the multiple converter arms to be equal to a sum of output voltages of remaining submodules of the first converter arm except for the submodule which has broken down
characterized in that the method further comprises:
lowering an output voltage of each submodule of the other converter arms according to the DC link voltage when the breakdown of the submodule in operation of the first converter arm occurs,
wherein a number of submodules operable in the first converter arm except for the submodule which has broken down is different from a number of submodules operable in each of the other converter arms,
wherein each of the output voltages of the operable submodules of the first converter arm is different from each of the output voltages of the operable submodules of each of the other converter arms,
wherein each of the remaining submodules of the first converter arm outputs a rated output voltage such that the DC link voltage is adjusted to a voltage by multiplying the remaining submodules of the first converter arm and the rated output voltage, and
wherein the output voltage of each submodule of the other converter arms is adjusted to be equal to a value obtained by multiplying the rated output voltage and a value obtained by dividing the adjusted DC link voltage by a sum of a rated output voltage of each submodule of the other converter arms, such that a sum of the output voltage of submodules of the other converter arms is equal to the adjusted DC link voltage.