Multilevel converter and method for controlling multilevel converter

Abstract

A multilevel converter is disclosed. The multilevel converter of the present invention comprises: a plurality of first side arms connected to a first side cable for direct current transmission; a plurality of second side arms connected to a second side cable for direct current transmission and respectively connected to the first side arms; a transformer connected to each contact point between the first side arms and the second side arms; and a mode switching unit.

Description

Multilevel converter and control method of multilevel converter

[0001] The present invention relates to a multilevel converter and a control method for a multilevel converter.

[0002] A Modular Multilevel Converter (MMC) is a device that controls multiple sub-modules connected in series for a voltage source with non-constant voltage levels to output and transmit a target voltage level.

[0003] Modular multi-level converters are used in DC transmission devices such as LVDC (Low Voltage Direct Current), MVDC (Medium Voltage Direct Current), and HVDC (High Voltage Direct Current).

[0004] To perform DC transmission using a modular multilevel converter, two cables are required. Each cable is connected to +Vdc and -Vdc to perform power transmission. This method is called the monopole method.

[0005] A monopole-type modular multilevel converter is characterized by the DC side potential being floating rather than grounded. In this structure, if one of the DC cables is grounded due to an accident, the voltage of the cable on the other side doubles (+2Vdc or -2Vdc). However, no overcurrent occurs due to the cable ground fault.

[0006] A monopole-type modular multilevel converter cannot operate normally for power transmission if one of the cables in a pair is damaged.

[0007] A bipolar modular multilevel converter does not allow the DC side potential to float but grounds it to zero voltage. A bipolar modular multilevel converter also performs DC transmission by forming +Vdc and -Vdc using two cables.

[0008] In a bipolar modular multilevel converter, even if one of the cables in a pair is damaged, +Vdc and -Vdc are formed from the zero-voltage ground, allowing DC power transmission using only the other cable. In this case, a current return path corresponding to the zero-voltage ground must be established, using a return path based on earth grounding, a return path through a cable, etc.

[0009] In bipolar operation, if one cable is damaged, an overcurrent may occur due to the diode path of the submodule, so a device to protect against this is required, and the use of a special transformer in which the AC side transformer is biased by +1 / 2 Vdc and -1 / 2 Vdc is also required.

[0010] The technical problem that the present invention aims to solve is to provide a multi-level converter and a control method for the multi-level converter that can operate using only healthy lines without generating overcurrent in the event of a single-line fault, while combining the advantages of the monopole method, such as reduced unit cost and facility space (FootPrint), with the advantages of the bipole method.

[0011] To solve the above technical problem, a multi-level converter according to an embodiment of the present invention comprises: a plurality of first side arms connected to a first side cable for DC transmission; a plurality of second side arms connected to a second side cable for DC transmission and each connected to the first side arms; a transformer each connected to the contacts of the first side arms and the second side arms; and a mode switching unit; wherein each of the plurality of arms includes a plurality of submodules connected in series, the transformer is a zigzag transformer, and the mode switching unit may include a switching switch that determines whether the neutral point of the transformer is connected to ground.

[0012] In one embodiment of the present invention, the mode switching unit may further include a blocking resistor connected in parallel with the switching switch and for blocking the incoming current.

[0013] In one embodiment of the present invention, a reactor connected in series to each of the plurality of first side arms and the plurality of second side arms may be further included.

[0014] In one embodiment of the present invention, each of the submodules may include a capacitor connected in a full-bridge manner, a plurality of semiconductor switches connected in parallel to the capacitor, and a bypass switch.

[0015] In one embodiment of the present invention, the zigzag transformer may be a delta zigzag transformer.

[0016] In addition, to solve the above technical problem, a multi-level converter according to an embodiment of the present invention comprises: a plurality of first side arms connected to a first side cable for DC transmission; a plurality of second side arms connected to a second side cable for DC transmission and each connected to the first side arms; a transformer each connected to the contact of the first side arms and the second side arms; and a mode switching unit that determines an operating mode of either a first mode or a second mode by changing the connection relationship between the transformer and ground; wherein the first mode is an operating mode in which the connection between the transformer and ground is cut off, and the second mode is an operating mode in which the transformer and ground are connected.

[0017] In one embodiment of the present invention, the device further includes an abnormal current detection unit that detects an abnormal current of the converter unit and generates detection information; and when the abnormal current is detected, the operating mode may be determined in the second mode.

[0018] In one embodiment of the present invention, the arm on the side where the abnormal current is detected can be controlled to be bypassed.

[0019] In addition, to solve the above technical problem, a control method for a multi-level converter according to an embodiment of the present invention may include: a step in which the mode switching unit determines an operating mode in the first mode; a step in which the abnormal current detection unit receives an abnormal current detection signal; and a step in which the mode switching unit determines an operating mode in the second mode.

[0020] The present invention has the advantage of reducing unit cost and facility space (FootPrint), which is the advantage of the monopole method, and the advantage of the bipole method, which allows operation with only healthy lines without the occurrence of overcurrent in the event of a single line failure.

[0021] Figure 1 shows a first form of a multilevel converter.

[0022] Figure 2 shows a second form of a multilevel converter.

[0023] Figure 3 shows a third form of a multilevel converter.

[0024] FIG. 4 shows a multilevel converter according to an embodiment of the present invention.

[0025] FIG. 5 shows a multilevel converter according to an embodiment of the present invention in detail.

[0026] FIG. 6 shows a multilevel converter according to an embodiment of the present invention in detail.

[0027] FIG. 7 shows a submodule of a multilevel converter according to an embodiment of the present invention.

[0028] FIG. 8 illustrates a control method for a multilevel converter according to an embodiment of the present invention.

[0029] Information regarding the national research and development projects that supported this invention is as follows.

[0030] [Project ID] 1415187628

[0031] [Assignment No.] 20225500000090

[0032] [Ministry Name] Ministry of Trade, Industry and Energy

[0033] [Project Management (Specialized) Agency Name] Korea Institute of Energy Technology Evaluation and Planning

[0034] [Research Project Name] Next-Generation AC / DC Hybrid Distribution Network Technology Development Project

[0035] [Research Project Title] Advanced Multi-Terminal Control System for Extra-High Voltage DC Distribution and Development of Converter Station Engineering Technology

[0036] [Contribution Rate] 1 / 1

[0037] [Name of Project Performing Organization] Korea Electrotechnology Research Institute

[0038] [Research Period] 2024.01.01 ~ 2024.12.31

[0039] The present invention is capable of various modifications and may have various embodiments, and specific embodiments are illustrated in the drawings and described in detail. However, this is not intended to limit the present invention to specific embodiments, and it should be understood that it includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the invention.

[0040] In describing the present invention, if it is determined that a detailed description of related known technology may obscure the essence of the present invention, such detailed description is omitted.

[0041] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0042] Figure 1 shows a first form of a multilevel converter.

[0043] Referring to FIG. 1, a first form of a multilevel converter includes a plurality of first side arms, a plurality of second side arms, a first side cable (C1), a second side cable (C2), a transformer (13), and a grounding resistor (14).

[0044] In the description of the present invention, the first side may be the upper side in the drawing and the positive polarity direction, and the second side opposite to the first side may be the lower side in the drawing and the negative polarity direction.

[0045] A plurality of first-side arms may be composed of three arms, and the three arms can convert three-phase AC voltage into DC voltage or convert DC voltage into three-phase AC voltage.

[0046] Multiple second-side arms may be composed of three arms, and the three arms can convert three-phase AC voltage into DC voltage or convert DC voltage into three-phase AC voltage.

[0047] The contacts of the first side arm and the second side arm are each connected to a transformer (13).

[0048] Each arm includes a plurality of submodules (11) and reactors (12) connected in series.

[0049] The submodule (11) may include a semiconductor switch, a diode, a capacitor, and a bypass switch, and these may form a circuit in a half bridge or full bridge manner.

[0050] Multiple submodules (11) can distribute high voltage and respond to voltage magnitude variability. Each of the multiple submodules (11) can generate an AC signal by controlling a semiconductor switch and can determine the magnitude of the total voltage by controlling a bypass switch.

[0051] To control multiple submodules (11), the system may include at least one processor.

[0052] The reactor (12) prevents overcurrent caused by momentary control imbalance between the arms.

[0053] The transformer is a Y-delta transformer in which the primary side (left side in the drawing) is Y-connected and the secondary side (right side in the drawing) is Delta-connected.

[0054] The Y connection part is connected to ground (G), and a grounding resistor (14) is placed between them.

[0055] The first type of multilevel converter exemplified in FIG. 1 is a monopole structure. In the monopole structure, the DC cable (C1, C2) side is floating. This structure can be used for LVDC, MVDC, and HVDC. The DC side voltage is controlled to +Vdc and -Vdc. In the first type of multilevel converter, the voltage across the cables (C1, C2) structurally creates only a potential difference and effectively becomes an ungrounded system.

[0056] Figure 2 shows a second form of a multilevel converter.

[0057] The second type of multilevel converter has a structure in which two first-type multilevel converters are placed on the first side and the second side and connected. However, the difference from the first type in the structure of the multilevel converter included in the second type is that the negative potential side cable of the first-side multilevel converter and the positive potential side cable of the first-side multilevel converter are both connected to ground (G).

[0058] The second type of multilevel converter has a bipolar structure. This structure can also be used for LVDC, MVDC, and HVDC. This structure is a directly grounded system, and the first side transformer (13) may be a special transformer capable of DC biasing by +1 / 2 Vdc and the second side transformer (13') by -1 / 2 Vdc.

[0059] Figure 3 shows a third form of a multilevel converter.

[0060] The third type of multilevel converter also has a modified connection relationship with the transformer (13) in the bipolar structure of the second type. In the third type, a three-winding transformer (13) with a Y-delta-delta connection method is used to connect the delta connection transformer on the secondary side to the first-side multilevel converter and the second-side multilevel converter, respectively.

[0061] The third type of multilevel converter reduces the number of transformers, thereby decreasing the footprint. The third type of multilevel converter is connected to a three-winding transformer. Although the third type of multilevel converter reduces the number of transformers compared to the second type, it has the disadvantage that the size of the three-winding transformer itself increases and the entire system must be shut down in the event of a transformer failure.

[0062] FIG. 4 shows a multilevel converter according to an embodiment of the present invention.

[0063] Referring to FIG. 4, a multi-level converter according to one embodiment of the present invention includes a converter unit (100), a control unit (200), and an abnormal current detection unit (300).

[0064] FIG. 5 shows in detail the converter section (100) of a multi-level converter according to an embodiment of the present invention.

[0065] Referring to FIG. 5, the converter unit (100) includes a plurality of first side arms, a plurality of second side arms, a first side cable (C1), a second side cable (C2), a transformer (130), and a mode switching unit (140).

[0066] In the description of the present invention, the first side may be the upper side in the drawing and the positive polarity direction, and the second side opposite to the first side may be the lower side in the drawing and the negative polarity direction.

[0067] A plurality of first side arms may be composed of three arms, and the three arms can convert a three-phase AC voltage into a DC voltage or convert a DC voltage into a three-phase AC voltage. The first side end of each of the plurality of first side arms forms a contact with the first side cable (C1).

[0068] A plurality of second-side arms may be composed of three arms, and the three arms can convert a three-phase AC voltage into a DC voltage or convert a DC voltage into a three-phase AC voltage. The second-side ends of the plurality of second-side arms each form a contact with the second-side cable (C1).

[0069] The contacts of the first side arm and the second side arm are each connected to a transformer (130).

[0070] Each arm includes a plurality of submodules (110) and reactors (120) connected in series.

[0071] The submodule (110) may include a semiconductor switch (112), a diode (114), a capacitor (116), and a bypass switch (118), and these may form a circuit in a half bridge or full bridge manner.

[0072] Multiple submodules (110) can distribute high voltage and respond to variations in voltage magnitude. Each of the multiple submodules (110) can generate an AC signal by controlling a semiconductor switch (112) and can determine the magnitude of the total voltage by controlling a bypass switch (118).

[0073] The semiconductor switches (112) and bypass switches (118) of the plurality of submodules (110) can each be controlled to be ON / OFF by the control unit (200).

[0074] FIG. 7 shows a submodule (110) of a multilevel converter according to an embodiment of the present invention.

[0075] Referring to FIG. 7, the submodule (110) may include a semiconductor switch (112), a diode (114), a capacitor (116), and a bypass switch (118), and these may be connected in a half bridge or full bridge manner to form a circuit.

[0076] Two semiconductor switches (112) are connected in series in pairs, and two pairs of semiconductor switches (112) are connected in parallel. One end of an input / output terminal to the positive pole is connected between one pair of semiconductor switches (112), and the other end of an input / output terminal to the negative pole is connected between the other pair.

[0077] The semiconductor switch (112) may include any one of a metal oxide semiconductor field effect transistor (MOSFET), a bipolar transistor (BJT), a field effect transistor (FET), a bipolar transistor (IGBT / NPN) having an insulating gate electrode and an npn structure, or an n-channel bipolar transistor (IGBT / N-channel) having an insulating gate electrode.

[0078] The semiconductor switch (112) includes a diode (114) connected in antiparallel.

[0079] The capacitor (116) is connected in parallel with the semiconductor switch (112).

[0080] The bypass switch (118) is connected to the input / output terminal and can be installed on the path that bypasses the current. The bypass switch (118) can be turned on in the event of a DC fault to prevent current from flowing into other components of the submodule (110).

[0081] The bypass switch (118) included in each submodule (110) can be partially turned on or off to control the magnitude of the output voltage.

[0082] The reactor (120) is connected in series with the submodule (110) in each arm.

[0083] The reactor (120) prevents overcurrent caused by momentary control imbalance between the arms.

[0084] The transformer (130) is a delta-zigzag transformer in which the primary side (left side in the drawing) is delta-connected and the secondary side (right side in the drawing) is zig-zag-connected.

[0085] The primary side of the transformer (130) may use a wye connection method, but the secondary side must be connected in a zigzag manner.

[0086] The zigzag transformer includes a neutral point (N), and the neutral point (N) is connected to a mode switching unit (140).

[0087] The mode switching unit (140) can be placed between the transformer (130) and ground (G).

[0088] The mode switching unit (140) may be any one of a disconnect switch, a vacuum circuit breaker (VCB), and an air circuit breaker (ACB), with a configuration including a large resistor and a switch.

[0089] The mode switching unit (140) may include a blocking resistor (142) and a switching switch (144).

[0090] The blocking resistor (142) may be an insulation resistor having a resistance value that is not destroyed by the incoming voltage of the system when the switching switch (144) is in the off state.

[0091] The blocking resistor (142) is connected in parallel with the switching switch (144).

[0092] The switching switch (144) is configured such that when it is on (Fig. 6), the neutral point of the transformer (130) is directly connected to ground (G) and grounded, and when it is off (Fig. 5), the neutral point of the transformer (130) is resistance grounded by a blocking resistor (142).

[0093] In the description of the present invention, the case where the switching switch (144) is off is referred to as the first mode. The first mode is operation in a normal state where no accident occurs. In this case, it becomes an ungrounded system with a structure similar to the first type of monopole method. In this case, the secondary side of the transformer (130) and the DC network become an ungrounded system. In this state, even if one of the cables (C1, C2) is damaged, no overcurrent occurs.

[0094] In the description of the present invention, the case where the switching switch (144) is off is referred to as the second mode. The second mode is a mode of operation that can be performed in the event of an accident. In this case, a return path for the DC current can be secured through a zigzag transformer. Among the multiple arms of the first side and the second side, three arms of one side that are burned out due to an accident are switched to a bypass state, and DC transmission operation can be performed using the remaining three arms of the other side that are normal.

[0095] The control unit (200) may include at least one processor.

[0096] The control unit (200) can control the submodules (110) of the converter unit (100) to control the AC / DC conversion and determine the submodules (110) to turn on the bypass switch (118).

[0097] The control unit (200) can determine one of the first mode and the second mode by controlling the switching switch (144) of the mode switching unit (140).

[0098] The abnormal current detection unit (300) is connected to the converter unit (100) and the control unit (200) and detects the abnormal current of the converter unit (100).

[0099] The abnormal current detection unit (300) can generate information regarding whether an abnormal current is detected and which side cable is associated with the abnormal current detection, and provide this information to the control unit (200).

[0100] The control unit (200) can determine the operating mode to a second mode when an abnormal current is detected. Determining the operating mode to a second mode may mean controlling the mode switching unit (140) to turn on the switching switch (144).

[0101] In the embodiments of the present invention, the limitations of conventional operation using only a monopole method or only a bipole method are overcome. That is, in normal conditions where no accident occurs, operation is performed with a topology substantially identical to that of a monopole method to prevent overcurrent in the event of an accident, and when an accident is detected, the fault-side arm is switched to a bypass mode and the neutral point of the zigzag transformer is directly grounded, thereby switching to a topology similar to that of a bipole method in terms of the grounding system, while enabling power transmission with only one cable.

[0102] FIG. 8 illustrates a control method for a multilevel converter according to an embodiment of the present invention.

[0103] The control method of the multilevel converter is exemplified as being performed by the multilevel converter described above, but the performing entity is not necessarily limited to the exemplified embodiment.

[0104] Referring to FIG. 8, at step S100, the control unit (200) operates the system in a first mode.

[0105] The first mode has the advantage of a monopole method in that it does not generate overcurrent in the event of an accident, as it is a power transmission when the switching switch (144) is off.

[0106] In step S200, the abnormal current detection unit (300) detects an abnormal current. The abnormal current may be a current flowing into the converter unit (100) when either of the cables (C1, C2) is damaged due to an accident, such as a ground fault.

[0107] The abnormal current detection unit (300) can provide information regarding the abnormal current and the fault cable to the control unit (200).

[0108] In step S300, the control unit (200) controls the bypass switches (118) of the multiple submodules (110) included in each arm of the accident side to the ON state, thereby switching all arms of the accident side to the bypass state.

[0109] In step S400, the control unit (200) controls the mode switching unit (140) to operate the system in the second mode.

[0110] The control unit (200) controls the off-state switching switch (144) to switch it to on. When switched to the second mode, the system can transmit power through one cable using only three normal arms that are not faulty.

[0111] The terms used in this application are used merely to describe specific embodiments and are not intended to limit the invention. In this application, terms such as “comprising” or “having” are intended to indicate the presence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

Claims

1. A plurality of first side arms connected to a first side cable for direct current transmission; A plurality of second side arms connected to a second side cable for direct current transmission and each connected to the first side arm; A transformer respectively connected to the contacts of the first side arm and the second side arm; and Includes a mode switching unit; but, Each of the above plurality of arms includes a plurality of submodules connected in series, and The above transformer is a zigzag transformer, and A multi-level converter characterized by the above-described mode switching unit including a switching switch that determines whether the neutral point of the transformer is connected to ground.

2. In Paragraph 1, A multilevel converter characterized by the above mode switching unit further including a blocking resistor connected in parallel with the switching switch to block the incoming current.

3. In Paragraph 1, A multilevel converter characterized by further including a reactor connected in series to each of a plurality of first side arms and a plurality of second side arms.

4. In Paragraph 1, Each of the above submodules is, A multilevel converter characterized by comprising: a capacitor connected in a full-bridge manner; a plurality of semiconductor switches connected in parallel to the capacitor; and a bypass switch.

5. In Paragraph 1, A multilevel converter characterized in that the above zigzag transformer is a delta zigzag transformer.

6. A plurality of first side arms connected to a first side cable for DC transmission; A plurality of second side arms connected to a second side cable for direct current transmission and each connected to the first side arm; A transformer respectively connected to the contacts of the first side arm and the second side arm; and A mode switching unit that determines an operating mode of either a first mode or a second mode by changing the connection relationship between the transformer and the ground; wherein The first mode above is an operating mode in which the connection between the transformer and ground is disconnected, and A multi-level converter characterized in that the second mode is an operating mode in which the transformer and ground are connected.

7. In Paragraph 6, It further includes an abnormal current detection unit that detects the abnormal current of the converter unit and generates detection information; A multilevel converter characterized by determining the operating mode to the second mode when the above abnormal current is detected.

8. In Paragraph 7, A multilevel converter characterized by controlling the arm on the side where the above-mentioned abnormal current is detected to be bypassed.

9. A method for controlling a multilevel converter using the multilevel converter of claim 7, Step of the above mode switching unit determining the driving mode in the first mode; The step of the above abnormal current detection unit receiving an abnormal current detection signal; A control method for a multilevel converter characterized by including the step of the mode switching unit determining the driving mode to the second mode.

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