KR101558862B1 - Modular Multilevel converter and DC current accident reduction apparatus - Google Patents

Abstract

Provided is a modular multilevel converter including an accident current reduction structure. A modular multilevel converter according to an embodiment of the present invention includes a normal path which is connected in series to the modular multilevel converter and provides a path when a normal current flows, and a bypass which absorbs an accident current when an accident current flows. On the normal path, a switch for the current flow from AC to DC and a diode for the current flow from DC to AC are connected in parallel. On the bypass, a capacitor for storing the accident current and a diode connected in series to the capacitor is formed.

Description

[0001] MODULAR MULTILVEL CONVERTER AND DC LOAD ACCIDENTAL REDUCTION APPARATUS [0002]

The present invention relates to a modular multi-level converter including an accident current reducing structure and a DC fault current reducing device including the same. More particularly, the present invention relates to a modular multi-level converter including a fault current reducing structure, The present invention relates to a fault current reduction structure of a modular multilevel converter capable of reducing a fault current flowing into the modular multilevel converter by inserting a capacitor and an apparatus using the fault current reduction structure.

Modular Multilevel Converter (MMC) is a kind of multilevel converter, which is composed of several sub modules (SM). These modular multilevel converters can exhibit the high voltage output and high output of multiple converters, and the output voltage can be controlled by the stepped output.

Modular multilevel converters are simpler in structure than conventional multilevel converters and are easier to implement and have the advantage of extending their lifetime by using redundant submodules.

1 is a diagram for explaining a structure of a general modular multi-level converter.

1, the modular multi-level converter 1 includes, for example, three legs (Leg, 13-1, 13-2, 13-3), six arms (Arm 11-1, 2, 11-3, 15-1, 15-2, 15-3), and each arm may include n submodules SM (where n is a natural number). The arm is again composed of upper arms 11-1, 11-2, and 11-3 and lower arms 15-1, 15-2, and 15-3.

The output voltage of the modular multilevel converter 1 varies depending on the on / off state of the sub-module SM included in each arm. That is, when four submodules are included in each of the upper arm and the lower arm, the output voltage can be adjusted to five levels (the number of submodules + 1) according to the number of submodules maintaining the On state. To control the output voltage at this time, the on / off of the sub-module (SM) switch in each arm can be controlled.

2 and 3 show a sub-module (SM) 5 included in an arm of a general modular multi-level converter.

Each sub-module (SM) 5 may include a pair of insulated gate bipolar transistors (IGBTs) 6, a pair of diodes 7, and a capacitor 8. Here, the submodule is switched by the ON / OFF operation of the IGBT 6.

As an example of charging and discharging, the sub module corresponding to On can be charged and discharged according to the current direction of the dark current. For example, if the dark current has a positive value, the submodule having the lowest voltage is selected. If the dark current has a negative value, the submodule having the highest voltage is selected to discharge.

However, the modular multilevel converter (1) has been studied to cope with an AC (alternating current) accident, but a countermeasure against a direct current (DC) accident has been continuously studied as a weak point. In the DC accident of the modular multilevel converter 1, the IGBT 6, the diode 7, and the capacitor 8 in the sub-module (SM) 5 can be damaged due to the flow of a large current. In addition, the transmission line and the cable through which the fault current flows are also greatly damaged. In addition, it takes a long time to restart all of the voltages of the capacitor 8.

U.S. Published Patent Application No. US2013 / 0128636 (published May 23, 2013) U.S. Published Patent Application No. US2014 / 0049230 (published on April 20, 2014) U.S. Published Patent Application No. US2013 / 0279211 (published Oct. 24, 2013)

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems and it is an object of the present invention to provide a modular multilevel converter capable of reducing a fault current in the occurrence of a DC fault and inserting a capacitor in a bypass path of the fault current to reduce a fault current flowing in the modular multilevel converter And an apparatus using the same.

The problems to be solved by the present invention are not limited to the above-mentioned problems, and other matters not mentioned can be clearly understood by those skilled in the art from the following description.

According to an aspect of the present invention, there is provided a modular multilevel converter including a plurality of converter arms to which a plurality of sub-modules are connected in series, the converter arm being protected by bypassing a fault current of the converter arm , A bypass circuit connected in series with the converter arm, a diode installed in the bypass circuit to bypass the fault current, and a capacitor installed in the bypass circuit to store the fault current and connected in series with the diode can do.

According to one embodiment, a diode installed in the bypass circuit may be installed so as to flow a current in a reverse direction on a diode and a bypass circuit connected in parallel to the switching device.

According to one embodiment, the fault current may converge to zero by the capacitor.

According to an embodiment, the switching device may be at least one IGBT or a thyristor.

The fault current reduction device of a modular multi-level converter according to another embodiment of the present invention includes a modular multilevel converter including a plurality of converter arms to which a submodule is connected in series, A fault current reduction module including a capacitor for storing a current and a diode installed for a direction of a current output from the capacitor; a fault current detector for detecting a fault current of the modular multilevel converter; And a controller for controlling the fault current to flow through a bypass circuit of the fault current reduction module.

According to one embodiment, the controller may turn off the switching element when the fault current is detected.

Other specific details of the invention are included in the detailed description and drawings.

According to the present invention, the fault current can be reduced by bypassing the fault current in the modular multilevel converter and inserting the capacitor in the bypass path of the fault current.

In addition, the fault current can be reduced to prevent damage to IGBTs, diodes, and capacitors, and the capacitor voltage can be prevented from discharging, allowing quick restart of the modular multilevel converter after accident removal.

Figure 1 is a diagram illustrating one embodiment of a modular multi-level converter.
Fig. 2 is a diagram showing the switching operation of the sub-module SM of the modular multi-level converter of Fig.
3 is a diagram showing a structure of a submodule of the modular multi-level converter of FIG.
4 is a diagram illustrating a modular multi-level converter in accordance with an embodiment of the present invention.
5 is a diagram illustrating a structure of an accident current reducing module of a modular multi-level converter according to an embodiment of the present invention.
6 is a diagram illustrating a fault current reduction structure of a modular multilevel converter connected to an arm according to an embodiment of the present invention.
7 and 8 are diagrams illustrating steady-state current flow in accordance with one embodiment of the present invention.
9 is a diagram illustrating the flow of fault current in the fault current reduction structure of a modular multi-level converter according to an embodiment of the present invention.
10 and 11 are diagrams illustrating steady-state current flow in accordance with another embodiment of the present invention.
12 is a diagram showing the flow of fault current in the fault current reduction structure of the modular multi-level converter according to another embodiment of the present invention
13 is a diagram illustrating an apparatus for reducing the fault current of a modular multi-level converter according to an embodiment of the present invention.
14 is a diagram showing an apparatus for reducing an accident current of a modular multi-level converter according to another embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Is provided to fully convey the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

Although the first, second, etc. are used to describe various elements, components and / or sections, it is needless to say that these elements, components and / or sections are not limited by these terms. These terms are only used to distinguish one element, element or section from another element, element or section. Therefore, it goes without saying that the first element, the first element or the first section mentioned below may be the second element, the second element or the second section within the technical spirit of the present invention.

The terminology used herein is for the purpose of illustrating embodiments and is not intended to be limiting of the present invention. In the present specification, the singular form includes plural forms unless otherwise specified in the specification. &Quot; comprises " and / or "comprising" when used in this specification is taken to specify the presence or absence of one or more other components, steps, operations and / Or additions.

Unless defined otherwise, all terms (including technical and scientific terms) used herein may be used in a sense commonly understood by one of ordinary skill in the art to which this invention belongs. Also, commonly used predefined terms are not ideally or excessively interpreted unless explicitly defined otherwise.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

First, the fault current reduction structure of the modular multilevel converter is intended to reduce the fault current of a modular multilevel converter including a plurality of converter arms (arm) to which the submodule SM is connected in series.

Specifically, the fault current reduction structure of the modular multilevel converter improves the effect of interrupting the fault current by inserting a capacitor in the bypass path of the fault current in the event of a DC fault. The fault current reduction structure of the modular multi-level converter can be connected in the form of a full-bridge or a half-bridge. A more detailed description will be given with reference to the drawings.

4 is a diagram illustrating a modular multi-level converter in accordance with an embodiment of the present invention.

Referring to FIG. 4, unlike FIG. 1, one submodule (SM) 5 is replaced with an accident current reduction module 23 in each arm. The modular multilevel converter 1 according to an embodiment of the present invention is a protection circuit that is connected in series with the submodule SM 5 to protect the submodule SM 5, Can be replaced or added.

The fault current reduction module 23 according to the embodiment of the present invention provides a path through which the current can smoothly flow when the current flows from DC to AC or from AC to DC in the modular multilevel converter 1 The switch 14 is turned on.

However, in the event of a DC fault, the switch is turned off so that the fault current does not flow to the sub-module (SM) 5 in the arm. In the fault current reduction module 23, a path through which a current flows when a switch is on (On) or off (Off) is different, and a current flowing when a DC fault occurs can be defined as a bypass path.

5 is a diagram illustrating a fault current reduction structure 21 of a modular multi-level converter according to an embodiment of the present invention.

5, the fault current reduction structure 21 of the modular multi-level converter according to an embodiment of the present invention can be connected in series with the sub-module SM, and includes a plurality of arms connected in series To reduce the fault current of the modular multilevel converter. The submodule may be in the form of a full-bridge or a half-bridge.

The fault current reduction structure 21 of the modular multilevel converter may include a normal circuit including one switching element and a bypass circuit composed of one capacitor 13 and one diode 12. [ Specifically, the fault current reduction structure 21 of the modular multi-level converter can provide a bypass circuit for the fault current of the converter arm. The bypass circuit is connected between the capacitor 13 and the diode 12 in series so that the capacitor 13 absorbs and absorbs the fault current, thereby reducing the damage of the sub-module due to the fault current.

Here, the diode 12 provided in the bypass circuit provides a bypass path for the fault current, and the fault current through the diode 12 charges the capacitor 13 to reduce the potential difference between AC and DC to reduce the fault current do. The fault current can be converged to zero by the capacitor 13. [

Here, the capacitor 13 provided in the bypass circuit may be charged in advance. In this case, the time required for reducing the fault current is reduced.

The fault current reduction structure 21 of the modular multilevel converter according to an embodiment of the present invention includes a switching device 14 connected to the submodule for interrupting the fault current and a switching device 14 connected to the switching device 14 in parallel And a diode 15 connected in series.

For example, when a steady current flows, the switching element 14 is in an On state, thereby forming a current path through each sub-module. Further, when a fault current flows, the switching element 14 is in an off state, thereby forming a current path that flows to the bypass circuit.

Here, the switching elements 6 and 14 may be IGBTs or the like. Of course, those skilled in the art will appreciate that the switching elements 6 and 14 may be other switching elements than the IGBT elements. For example, the switching element 14 may be a Fast Switch. Also, the switching device 14 may be a thyristor. Further, the switching elements 6 and 14 may be composed of a plurality of IGBTs.

At this time, the diode 12 installed in the bypass circuit and the capacitor 13 connected to the diode can absorb the fault current when the fault current is generated and can be made close to zero. The capacitor (13) provided in such a bypass circuit should have a power capacity sufficient to absorb the fault current when a fault current is generated.

The submodule SM connected to the fault current reduction structure 21 of the modular multilevel converter through the fault current reduction structure 21 of the modular multilevel converter can be initialized through the natural discharge circuit so that all the circuits So that damage to the sub-module (SM) 5 and the fault current reducing structure 21 can be prevented.

In a modular multilevel converter without the fault current reduction structure 21, an excessive current flows continuously in the event of a DC fault. Therefore, all the arms included in the modular multi-level converter must be damaged and must replace the entire modular multi-level converter.

However, by connecting the fault current reduction structure 21 of the modular multilevel converter in series with the modular multilevel converter, it is possible to reduce the cost corresponding to the DC accident of the modular multilevel converter.

6 shows an arm in which a fault current reduction structure 21 of the modular multilevel converter and the modular multilevel converter are connected in series according to an embodiment of the present invention.

According to one embodiment of the present invention, the fault current reduction module 23, in which the fault current reduction structure 21 is connected in series, can be connected in series to the modular multilevel converter. The fault current reduction module 23 is connected to the AC side so that both DC side of the upper arm and lower arm are grounded to prevent DC accident from occurring.

Specifically, the larger the voltage difference between both ends, the larger the current flows. However, when the upper arm of the DC side and the lower arm are grounded, the DC side voltage (VDC) becomes zero. At this time, there is a large difference between the AC side voltage and the DC side voltage, so that excessive current flows.

In addition, a freewheeling current path is formed due to the voltage formed by the submodule of the upper arm and the lower arm, thereby causing an excessive current to flow. The fault current reduction module 23 according to an embodiment of the present invention absorbs a large current like a shield in front of or behind the modular multilevel converter when a large current flows in the event of a DC accident. Such a function of absorbing is made by the capacitor 13 and the capacitor 13 can be arranged to be different from the capacitor 8 of the submodule SM since the capacitor 13 should include the power capacity enough to absorb the fault current .

Let's look at the current path in a modular multilevel converter.

First, let's look at the current path in a modular multilevel converter when steady current flows.

FIGS. 7 and 8 are diagrams showing the steady current flow in the fault current reduction structure installed in the arm of the modular multi-level converter of FIG. 6, respectively.

FIG. 7 illustrates current flow from AC to DC in the fault current reduction structure of a modular multi-level converter according to an embodiment of the present invention. 8 illustrates current flow from DC to AC in the fault current reduction structure of a modular multi-level converter according to an embodiment of the present invention.

7 and 8, when the steady current flows, the switching element 14 is in the ON state, so current does not flow to the diode 12 and the capacitor 13 of the bypass circuit, and current flows to the switching element.

As shown in Fig. 7, the DC voltage can be changed stepwise from the AC voltage by the number of times the switches of the IGBT 6 in the submodule are turned on. As shown in FIG. 7, when three sub-modules 5 are connected in series, the number of AC voltages that can be generated is four. On / off switching of the IGBT 6 is performed according to the voltage command of AC, and the DC voltage is changed to the AC voltage through the switching.

In FIG. 7, only the switches of the two sub-modules are in the On state and the switches of the lower sub-module are in the Off state. Therefore, only the voltages of the two capacitors are generated as the upper arm voltage. Therefore, the switch of the submodule is switched to the on / off state according to the AC voltage to be generated, which is changed by the controller of the arm. Also, the sum of the number of submodules (SM) on the upper arm of the same leg and on the lower arm of the same leg is always constant. Therefore, the generated DC voltage is always constant.

8, in the fault current reducing module 23 according to the embodiment of the present invention, when a current flows from DC to AC, the current is supplied to the diode 15 connected in parallel with the switch 14, Flows.

As shown in FIG. 8, when a current flows from the DC to the AC side, the state of the sub-module SM is determined according to the operation of the switch 6 under the sub-module SM. In this case, when the lower switch 6 is turned on, the sub-module SM does not form the voltage of the capacitor, and thus the sub-module SM is turned off. In FIG. 8, since the states of the three sub-modules SM are on, the voltages of the three capacitors 8 are generated as the upper arm voltage. Therefore, the switch 6 of the submodule SM is switched to the on / off state according to the AC voltage to be generated, which is changed by the controller of the arm.

As shown in Fig. 8, a current flowing from the DC to the AC side flows toward the diode 15 of the fault current reducing structure 23. Fig. The current flowing to the diode (15) flows to the AC side. At this time, no current can flow through the bypass path due to the resistance of the diode 12. [

Next, when fault current flows, let's look at the current path in the modular multilevel converter.

Fig. 9 is a diagram showing the flow of fault current in the fault current reducing structure of the modular multi-level converter of Fig. 6; Fig.

An excessive current flows to the arm including the fault current reduction module 23 and the sub module 5 at a time in the direction of the upper arm from the lower arm. At this time, the fault current reduction structure may further include an overcurrent sensing unit, so that the protection operation can be operated by the signal of the overcurrent sensing unit. However, the overcurrent detection part may be installed not only inside the fault current reduction structure but also at both ends of the sub module (SM). The fault current reduction structure installed at both ends of the sub module (SM) can switch off the fault current reduction structure for the operation of the protection circuit by the fault current reduction structure when the fault current is detected.

Further, the protection operation can be operated by the DC fault signal from the overcurrent detection unit flowing in the DC transmission line. Further, the protection operation can be activated by a signal from the control unit and the arm control unit of the modular multi-level converter.

Here, the protection operation is to turn off all the switch elements 6 and 14. By turning off the elements of all switches in the event of a DC fault, it is possible to create a bypass path to be made in the present invention.

The capacitor 13 on the bypass path serves to absorb the excessive current. The diode 12 connected behind the capacitor 13 allows the current to flow in one direction. Since the capacitor 13 on the bypass path has absorbed an excessive current, excessive current does not flow to the switch element 6 and the diode 7 on the modular multilevel converter.

10 shows an accident current reduction structure 24 according to another embodiment of the present invention.

The fault current reduction structure 24 according to another embodiment of the present invention includes one capacitor 13 and one diode 12 as bypass paths. This is because the number of the capacitors 13 and the diodes 12 is different from that of the current reduction structure 24 according to the embodiment of the present invention.

However, in the embodiment according to FIG. 10, the capacitor 13 used in the fault current reducing structure 23 may be different from the capacitor 13 in the embodiment according to FIG. 10 shows that the number of the capacitors 13 and the number of the diodes 12 are reduced compared with the embodiment according to FIG. 6, but the use of the capacitor 13 having a large capacity is similar to the use of the two capacitors 13 The advantage of reducing the number of capacitors 13 in the embodiment 10 is not significant.

In the embodiment according to FIG. 6, the fault current reduction structure 23 is made into a module type, and the same two fault current reduction modules 21 are connected in series. By doing this, unlike the embodiment according to FIG. 10, the expansion and maintenance are easy, and if the module is broken, the replacement is easy. However, the fault current reduction structure 24 in the embodiment according to FIG. 10 must replace the entire fault current reduction structure 24 when the capacitor 13 fails.

The fault current reduction structure 24 is shown as being located on the AC side of the modular multilevel converter 1, but is not limited thereto. Since the current is one flow, it does not matter where it is located in the modular multilevel converter 1, but if it is connected in series with the modular multilevel converter 1, the overcurrent can be absorbed to prevent the accident.

10, the capacitor 13 is placed in the opposite position to the capacitor 8 of the modular multilevel converter 1, as in the fault current reducing structure 23 according to another embodiment of the present invention.

In the fault current reducing structure 23 according to the embodiment of the present invention shown in Fig. 10, the switch 14 is always on. However, the switch 7 in the modular multi-level converter 1 controls the operation of the switch in the control section.

11 is a diagram showing a current flow from DC to AC in the fault current reducing structure 24 according to an embodiment of the present invention.

The current flowing out of the modular multilevel converter 1 flows into the diode 15 of the fault current reduction structure 24. The current flowing to the diode (15) flows to the AC side. At this time, no current can flow through the bypass path due to the resistance of the diode 12. [

FIG. 12 shows a flow of current when an accident current is generated in an accident current reducing structure 24 according to an embodiment of the present invention.

When the fault current is generated, the potential difference between the AC side and the DC side is increased. The overcurrent flowing from the AC side flows to the switch 14 included in the fault current reducing structure 24 so that the switch 14 is turned off due to the overcurrent. The switch 14 is turned off due to the overcurrent. The switch 14 in which the overcurrent flows may need to be replaced. The switch 14 can be turned off by the signals of the overcurrent sensing unit, the system control unit, and the arm control unit.

When the switch 14 is turned off due to the overcurrent, the overcurrent flows from the fault current reducing structure 24 to the bypass path. The capacitor 13 is provided in the bypass path so as to absorb the overcurrent. The capacitor 13 is charged while absorbing the overcurrent, so that the voltage becomes high. When the voltage of the capacitor 13 is increased, the potential difference between AC and DC is reduced and the current is reduced. The capacitor 13 used in the fault current reducing structure 24 according to another embodiment of the present invention can use a capacitor large enough to absorb all the current at the time of a DC fault. If the capacity of the capacitor 13 is so large that it has absorbed all of the DC fault current, no current may flow to the modular multilevel converter 1 any more.

In order to reduce the fault current, the fault current reduction structure of the above-described full-bridge and half-bridge type modular multi-level converters can be appropriately used and accordingly, a full bridge ) And / or the number of half-bridge type connection structures can be reduced.

13 and 14 are diagrams showing an apparatus 100 for reducing an accident current of a modular multi-level converter according to an embodiment of the present invention.

Referring to FIG. 13, the fault current reducing apparatus 100 of the modular multi-level converter according to the embodiment of the present invention includes a plurality of converter arms to which the sub-module SM is connected in series, A modulo multilevel converter (110) including a bypass circuit providing a bypass path of the fault current, a diode installed in the bypass circuit to bypass the fault current, and a capacitor installed in the bypass circuit to store the fault current, A fault current detector 120 for detecting a fault current of the modular multilevel converter 110 and a controller 130 for controlling the fault current to flow to the bypass circuit when the fault current is detected .

The modular multi-level converter 110 includes a plurality of converter arms to which the submodules are connected in series. In addition, the converter arm may further include a switching element and a diode connected to the submodule for interrupting the fault current. Here, a bypass circuit for providing a bypass path for the fault current of the converter arm, a diode installed in the bypass circuit for bypassing the fault current, and a capacitor installed in the bypass circuit for storing the fault current The modular multilevel converter 110 having a structure according to an embodiment of the present invention described above or a fault current reduction structure 23 of a modular multilevel converter according to another embodiment of the present invention Reduction structure 24. Therefore, a detailed description thereof will be omitted.

The fault current detection unit 120 detects a fault current generated in the modular multi-level converter 110, and can use a current detection sensor or the like.

The fault current detection unit may be provided in the arm of the modular multilevel converter, or a plurality of fault current detection units may be provided in the DC transmission line.

In FIG. 14, the controller 130 recognizes the fault current through the voltage sensing device 140 instead of the fault current detector 120 to operate the fault current reduction structure.

In the voltage sensing device 140, when a current flows through the IGBT in a steady state, the voltage across both ends becomes 0. However, when the fault current is generated, the voltage is zero, but the voltage across the IGBT does not become zero. Therefore, it is possible to detect the fault current by measuring the voltage across the IGBT.

In addition, the voltage sensing device 140 may sense the fault current by measuring the voltage across the switch.

When the fault current is detected, the controller 130 switches the switching element connected to the sub module from the On state to the Off state. Thereby, the fault current flows along the bypass circuit without passing through the switching elements of the respective submodules of the modular multilevel converter, and the switching elements of each submodule can be protected. In addition, the potential difference between AC and DC is canceled by the capacitor 13 in each bypass path, so that the fault current can be reduced to zero, and the diode and the capacitor of each submodule can be protected.

The fault current reduction structure of the modular multi-level converter according to an embodiment of the present invention and the device using the same can reduce the fault current of the modular multi-level converter, thereby further increasing the life of the modular multi-level converter . Therefore, as the demand for high capacity converters such as medium and large HVDC (High Voltage Direct Current) and multi-terminal HVDC is increasing, the technology for reducing the accident current can be used in connection with future offshore wind power generation. It is expected that the technology of reducing the fault current of the modular multilevel converter will become more important.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

23, 24: Fault Current Reduction Structure of Modular Multilevel Converter
7, 12, 15: Diodes
8, 13: Capacitor
6,14: Switching element
100: Fault Current Reduction Device for Modular Multilevel Converter
110: Modular multilevel converter
120: fault current detection unit
130:
140: Voltage sensing device

Claims (7)

A modular multilevel converter including a plurality of converter arms to which a submodule is connected in series,
A bypass circuit installed inside the converter arm and connected in series with each converter arm, bypassing the fault current of the converter arm to protect the converter arm;
A diode installed in the bypass circuit to bypass the fault current; And
And a capacitor installed in the bypass circuit for storing the fault current and being connected in series with the diode. The method according to claim 1,
The modular multi-level converter includes:
Further comprising: a switching device connected in parallel with the bypass circuit to prevent a fault current from flowing to the submodule when an accident current occurs; and a diode connected in parallel to the switching device. 3. The method of claim 2,
The diode installed in the bypass circuit may include:
A diode connected in parallel to the switching device and a modular multilevel converter installed in a reverse direction on the bypass circuit. The method according to claim 1,
Wherein the fault current converges to zero by the capacitor. 3. The method of claim 2,
Wherein the switching element comprises at least one IGBT or a thyristor. A fault current reducing apparatus for a modular multilevel converter including a plurality of converter arms to which submodules are connected in series,
A modular multilevel converter including a plurality of converter arms to which submodules are connected in series;
And a diode installed for a direction of a current outputted from the capacitor and storing a fault current caused by a DC accident when a DC accident occurs in the modular multilevel converter, An accident current reduction module connected in series with the fault current reduction module;
A fault current detector for detecting a fault current of the modular multilevel converter; And
And a control section for controlling the fault current to flow to a bypass circuit of the fault current reduction module when the fault current is detected.
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