Provided is a modular multi-level converter (MMC) including a plurality of sub-modules including switching elements, and a central control unit which assigns an address to each of the plurality of sub-modules for distinguishing each of the plurality of sub-modules, determines switching operation conditions of the plurality of sub-modules based on the assigned addresses, and outputs switching signals corresponding to the determined switching operation conditions. The central control unit determines a switching sequence of the plurality of the sub-modules according to the sequence of the assigned addresses.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. §119(a), this application claims the benefit of earlier filing date and right of priority to Korean Patent Application No. 10-2014-0057356, filed on May 13, 2014, the contents of which are hereby incorporated by reference herein in its entirety.

BACKGROUND

The present disclosure relates to a modular multi-level converter, and more particularly to, a modular multi-level converter capable of effectively controlling a plurality of sub-modules.

High voltage direct current (HVDC) transmission refers to an electric power transmission method in which alternating current (AC) power generated from a power plant is converted into direct current (DC) power and transmitted by a transmission substation, after which the transmitted DC power is converted into AC power again at a receiving substation to supply the power.

HVDC systems are applied to undersea cable transmission, high-capacity long distance transmission, interconnections between AC systems, and the like. Also, HVDC systems make possible interconnections between different frequency system and asynchronous interconnections.

A transmission substation converts AC power into DC power. That is, since the transmission of AC power by using an undersea cable or the like presents a very dangerous situation, the transmission substation converts AC power into DC power to transmit to the receiving substation.

Meanwhile, there are various types of voltage-type converters used in HVDC systems, and modular multi-level voltage-type converters have recently attracted the most interest.

A modular multi-level converter (MMC) is an apparatus which converts DC power into AC power by using a plurality of sub-modules, and operates such that each of the sub-modules are controlled to be in states of charge, discharge, or bypass.

Accordingly, in an MMC, it is most important to control the plurality of sub-modules in the power converting operation, and the control operation of the plurality of sub-modules determines the form and quality of output AC power.

Thus, an MMC capable of efficiently controlling the plurality of sub-modules of the MMC is required.

SUMMARY

Embodiments provide a modular multi-level converter (MMC) capable of efficiently controlling a plurality of sub-modules included in the MMC.

Embodiments also provide an MMC capable of efficiently determining the switching sequence of the plurality of sub-modules included in the MMC.

Embodiments also provide an MMC capable of maintaining the balance of the switching frequency of the plurality of sub-modules included in the MMC.

The objects of the embodiments are not limited to the aforesaid, but other objects not described herein will be clearly understood by those skilled in the art from descriptions below.

In one embodiment, a modular multi-level converter (MMC) includes: a plurality of sub-modules including switching elements; and a central control unit which assigns an address to each of the plurality of sub-modules for distinguishing each of the plurality of sub-modules, determines switching operation conditions of the plurality of sub-modules based on the assigned addresses, and outputs switching signals corresponding to the determined switching operation conditions, wherein the central control unit determines a switching sequence of the plurality of the sub-modules according to a sequence of the assigned addresses.

The central control unit may sequentially assign the addresses from the front according to an arranged sequence of the plurality of sub-modules.

The switching operation conditions may include a charging operation condition, a discharging operation condition, and a bypassing operation condition, and the central control unit may allow the discharging operations to be performed sequentially from the sub-module having a lowest address, based on a target voltage and charged voltages of the plurality of sub-modules.

A sum of the voltages charged in sub-modules performing the discharging operations may correspond to the target voltage, and the central control unit may confirm the charged voltages from the sub-module having the lowest address and determine a switching operation condition of each of the sub-modules to generate an output voltage corresponding to the target voltage.

When sub-modules operating under the discharging operation condition are determined, the central control unit may store information on the sub-module having the last address from among the sub-modules operating under the discharging operation condition.

The central control unit may confirm a sub-module which has the last address and has performed a discharging operation at a previous point in time, and allow a discharging operation to be sequentially performed starting from a sub-module, which has the next address of the confirmed sub-module.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description of embodiments, a detailed description of known functions or configurations incorporated herein will not be provided when it is determined that the detailed description thereof may unnecessarily obscure the subject matter of the inventive concept. The terms which will be described below are terms defined in consideration of the functions in the present disclosure, and may be different according to users, intentions of the users, or customs. Therefore, the definitions of the terms should be determined based on the contents throughout the specification.

Furthermore, the respective block diagrams may illustrate parts of modules, segments or codes including at least one or more executable instructions for performing specific logic function(s). Moreover, it should be noted that the functions of the blocks may be performed in different order in several modifications. For example, two successive blocks may be performed substantially at the same time, or may be performed in reverse order according to their functions.

FIG. 1illustrates a high voltage direct current (HVDC) transmission system according to an embodiment.

As illustrated inFIG. 1, a HVDC system100according to an embodiment includes a power generation part101, a transmission side alternating current (AC) part110, a transmission side power transformation part103, a direct current (DC) power transmission part140, a customer side power transformation part105, a customer side AC part170, a customer part180, and a control unit190. The transmission side power transformation part103includes a transmission side transformer part120, and a transmission side AC-DC converter part130. The customer side power transformation part105includes a customer side DC-AC converter part150, and a customer side transformer part160.

The power generation part101generates three-phase AC power. The power generation part101may include a plurality of power generating plants.

The transmission side AC part110transmits the three-phase AC power generated by the generation part101to a DC power transformation substation including the transmission side transformer part120and the transmission side AC-DC converter part130.

The transmission side transformer part120isolates the transmission side AC part110from the transmission side AC-DC converter part130and the DC power transmission part140.

The transmission side AC-DC converter part130converts the three-phase AC power corresponding to the output of the transmission side transformer part120into DC power.

The DC power transmission part140transfers the transmission side DC power to the customer side.

The customer side DC-AC converter part150converts the DC power transferred by the DC power transmission part140into three-phase AC power.

The customer side transformer part160isolates the customer side AC part170from the customer side DC-AC converter part150and the DC power transmission part140.

The customer side AC part170provides three-phase AC power corresponding to the output of the customer side transformer part160to the customer part180.

The control unit190controls at least one of the power generation part101, the transmission side AC part110, the transmission side power transformation part103, the DC power transmission part140, the customer side power transformation part105, the customer side AC part170, the customer part180, the control unit190, the transmission side AC-DC converter part130, and the customer side DC-AC converter part150. Particularly, the control unit190may control the turn-on and turn-off timings of a plurality of valves in the transmission side AC-DC converter part130and the customer side DC-AC converter part150. Here, the valves may correspond to a thyristor or an insulated gate bipolar transistor (IGBT).

FIG. 2illustrates a monopolar-type high voltage direct current (HVDC) transmission system.

Particularly,FIG. 2illustrates a system which transmits DC power with a single pole. Hereinafter, the single pole is described on the assumption that it is a positive pole, but is not necessarily limited thereto.

The transmission side AC part110includes an AC power transmission line111and an AC filter113.

The AC power transmission line111transfers the three-phase AC power generated by the generation part101to the transmission side power transformation part103.

The AC filter113removes remaining frequency components other than the frequency component used by the power transformation part103from the transferred three-phase AC power.

The transmission side transformer part120includes one or more transformers121for the positive pole. For the positive pole, the transmission side AC-DC converter part130includes an AC-positive pole DC converter131which generates positive pole DC power, and the AC-positive pole DC converter131includes one or more three-phase valve bridges131arespectively corresponding to the one or more transformers121.

When one three-phase valve bridge131ais used, the AC-positive pole DC converter131may generate positive pole DC power having six pulses by using the AC power. Here, a primary coil and a secondary coil of one of the transformers121may have a Y-Y connection or a Y-delta (Δ) connection.

When two three-phase valve bridges131aare used, the AC-positive pole DC converter131may generate positive pole DC power having 12 pulses by using the AC power. Here, a primary coil and a secondary coil of one of the two transformers121may have a Y-Y connection, and a primary coil and a secondary coil of the other of the two transformers121may have a Y-Δ connection

When three three-phase valve bridges131aare used, the AC-positive pole DC converter131may generate positive pole DC power having 18 pulses by using the AC power. The more the number of the pulses of the positive pole DC power becomes, the lower the price of the filter becomes.

The DC power transmission part140includes a transmission side positive pole DC filter141, a positive pole DC power transmission line143, and a customer side positive pole DC filter145.

The transmission side positive pole DC filter141includes an inductor L1and a capacitor C1and performs DC filtering on the positive pole DC power output by the AC-positive pole DC converter131.

The positive pole DC power transmission line143has a single DC line for transmission of the positive pole DC power, and the earth may be used as a current feedback path. One or more switches may be disposed on the DC line.

The customer side positive pole DC filter145includes an inductor L2and a capacitor C2and performs DC filtering on the positive pole DC power transferred through the positive pole DC power transmission line143.

The customer side DC-AC converter part150includes a positive pole DC-AC converter151and one or more three-phase valve bridges151a.

The customer side transformer part160includes, for the positive pole, one or more transformers161respectively corresponding to one or more three-phase valve bridges151a.

When one three-phase valve bridge151ais used, the positive pole DC-AC converter151may generate AC power having six pulses by using the positive pole DC power. Here, a primary coil and a secondary coil of one of the transformers161may have a Y-Y connection or a Y-delta (Δ) connection.

When two three-phase valve bridges151aare used, the positive pole DC-AC converter151may generate AC power having 12 pulses by using the positive pole DC power. Here, a primary coil and a secondary coil of one of the two transformers161may have a Y-Y connection, and a primary coil and a secondary coil of the other of the two transformers161may also have a Y-Δ connection.

When three three-phase valve bridges151aare used, the positive pole DC-AC converter151may generate AC power having 18 pulses by using the positive pole DC power. The more the number of the pulses of the AC power becomes, the lower the price of the filter becomes.

The customer side AC part170includes an AC filter171and an AC power transmission line173.

The AC filter171removes frequency components other than the frequency component (for example, 60 Hz) used by the customer part180from the AC power generated by the customer side power transformation part105.

The AC power transmission line173transfers the filtered AC power to the customer part180.

FIG. 3illustrates a bipolar type HVDC transmission system according to an embodiment.

Particularly,FIG. 3illustrates a system which transmits DC power with two poles. Hereinafter, the two poles are described assuming a positive pole and a negative pole, but are not necessarily limited thereto.

The transmission side AC part110includes an AC transmission line111and an AC filter113.

The AC power transmission line111transfers the three-phase AC power generated by the generation part101, to the transmission side power transformation part103.

The AC filter113removes frequency components other than the frequency component used by the power transformation part103from the transferred three-phase AC power.

The transmission side transformer part120includes one or more transformers121for the positive pole, and one or more transformers122for the negative pole. The transmission side AC-DC converter part130includes an AC-positive pole DC converter131which generates positive pole DC power and an AC-negative pole DC converter132which generates negative pole DC power. The AC-positive pole DC converter131includes one or more three-phase valve bridges131arespectively corresponding to the one or more transformers121for the positive pole. The AC-negative pole DC converter132includes one or more three-phase valve bridges132arespectively corresponding to the one or more transformers122for the negative pole.

When one three-phase valve bridge131ais used for the positive pole, the AC-positive pole DC converter131may generate positive pole DC power having six pulses by using the AC power. Here, a primary coil and a secondary coil of one of the transformers121may have a Y-Y connection or a Y-delta (Δ) connection.

When two three-phase valve bridges131aare used for the positive pole, the AC-positive pole DC converter131may generate positive pole DC power having 12 pulses by using the AC power. Here, a primary coil and a secondary coil of one of the two transformers121may have a Y-Y connection, and a primary coil and a secondary coil of the other of the two transformers121may have a Y-Δ connection.

When three three-phase valve bridges131aare used for the positive pole, the AC-positive pole DC converter131may generate positive pole DC power having 18 pulses by using the AC power. The more the number of the pulses of the positive pole DC power becomes, the lower the price of the filter becomes.

When one three-phase valve bridge132ais used for the negative pole, the AC-negative pole DC converter132may generate negative pole DC power having six pulses. Here, a primary coil and a secondary coil of one of the transformers122may have a Y-Y connection or a Y-delta (Δ) connection.

When two three-phase valve bridges132aare used for the negative pole, the AC-negative pole DC converter132may generate negative pole DC power having 12 pulses. Here, a primary coil and a secondary coil of one of the two transformers122may have a Y-Y connection, and a primary coil and a secondary coil of the other of the two transformers122may have a Y-Δ connection.

When three three-phase valve bridges132aare used for the negative pole, the AC-negative pole DC converter132may generate negative pole DC power having 18 pulses. The more the number of the pulses of the negative pole DC power becomes, the lower the price of the filter becomes.

The DC power transmission part140includes a transmission side positive pole DC filter141, a transmission side negative pole DC filter142, a positive pole DC power transmission line143, a negative pole DC power transmission line144, a customer side positive pole DC filter145, and a customer side negative pole DC filter146.

The transmission side positive pole DC filter141includes an inductor L1and a capacitor C1and performs DC filtering on the positive pole DC power output by the AC-positive pole DC converter131.

The transmission side negative pole DC filter142includes an inductor L3and a capacitor C3and performs DC filtering on the negative pole DC power output by the AC-negative pole DC converter132.

The positive pole DC power transmission line143has a single DC line for transmission of the positive pole DC power, and the earth may be used as a current feedback path. One or more switches may be disposed on the DC line.

The negative pole DC power transmission line144has a single DC line for transmission of the negative pole DC power, and the earth may be used as a current feedback path. One or more switches may be disposed on the DC line.

The customer side positive pole DC filter145includes an inductor L2and a capacitor C2and performs DC filtering on the positive pole DC power transferred through the positive pole DC power transmission line143.

The customer side negative pole DC filter146includes an inductor L4and a capacitor C4and performs DC filtering on the negative pole DC power transferred through the negative pole DC power transmission line144.

The customer side DC-AC converter part150includes a positive pole DC-AC converter151and a negative pole DC-AC converter152. The positive pole DC-AC converter151includes one or more three-phase valve bridges151aand the negative pole DC-AC converter152includes one or more three-phase valve bridges152a.

The customer side transformer part160includes, for the positive pole, one or more transformers161respectively corresponding to one or more three-phase valve bridges151a, and for the negative pole, one or more transformers162respectively corresponding to one or more three-phase valve bridges152a.

When one three-phase valve bridge151ais used for the positive pole, the positive pole DC-AC converter151may generate AC power having six pulses by using the positive pole DC power. Here, a primary coil and a secondary coil of one of the transformers161may have a Y-Y connection or a Y-delta (Δ) connection.

When two three-phase valve bridges151aare used for the positive pole, the positive pole DC-AC converter151may generate AC power having 12 pulses by using the positive pole DC power. Here, a primary coil and a secondary coil of one of the two transformers161may have a Y-Y connection, and a primary coil and a secondary coil of the other of the two transformers161may have a Y-Δ connection.

When three three-phase valve bridges151aare used for the positive pole, the positive pole DC-AC converter151may generate AC power having 18 pulses by using the positive pole DC power. The more the number of the pulses of the AC power becomes, the lower the price of the filter becomes.

When one three-phase valve bridge152ais used for the negative pole, the negative pole DC-AC converter152may generate AC power having six pulses by using the negative pole DC power. Here, a primary coil and a secondary coil of one of the transformers162may have a Y-Y connection or a Y-delta (Δ) connection.

When two three-phase valve bridges152aare used for the negative pole, the negative pole DC-AC converter152may generate AC power having 12 pulses by using the negative pole DC power. Here, a primary coil and a secondary coil of one of the two transformers162may have a Y-Y connection, and a primary coil and a secondary coil of the other of the two transformers162may have a Y-Δ connection.

When three three-phase valve bridges152aare used for the negative pole, the negative pole DC-AC converter152may generate AC power having 18 pulses by using the negative pole DC power. The more the number of the pulses of the AC power become, the lower the price of the filter becomes.

The customer side AC part170includes an AC filter171and an AC power transmission line173.

The AC filter171removes frequency components other than the frequency component (for example, 60 Hz) used by the customer part180from the AC power generated by the customer side power transformation part105.

The AC power transmission line173transfers the filtered AC power to the customer part180.

FIG. 4illustrates a connection between a transformer and a three-phase valve bridge according to an embodiment.

Particularly,FIG. 4illustrates the connection between the two transformers121for the positive pole and the two three-phase valve bridges131afor the positive pole. Since the connection between the two transformers122for the negative pole and the two three-phase valve bridges132afor the negative pole, the connection between the two transformers161for the positive pole and the two three-phase valve bridges151afor the positive pole, the connection between the two transformers162for the negative pole and the two three-phase valve bridges152afor the negative pole, the connection between the one transformer121for the positive pole and the one three-phase valve bridge131afor the positive pole, the connection between the one transformer161for the positive pole and the one three-phase valve bridge151afor the positive pole, etc., could be easily derived from the embodiment ofFIG. 4, drawings and descriptions thereof will not be provided herein.

InFIG. 4, the transformer121having the Y-Y connection is referred to as an upper transformer, the transformer121having the Y-Δ connection is referred to as a lower transformer, the three-phase valve bridge131aconnected to the upper transformer is referred to as upper three-phase valve bridge, and the three-phase valve bridge131aconnected to the lower transformer is referred to as lower three-phase valve bridge.

The upper three-phase valve bridge and the lower three-phase valve bridge have two output terminals outputting DC power, i.e., a first output terminal OUT1and a second output terminal OUT2.

The upper three-phase valve bridge includes six valves D1to D6, and the lower three-phase valve bridges include six valves D7to D12.

The valve D1has a cathode connected to the first output terminal OUT1and an anode connected to a first terminal of the secondary coil of the upper transformer.

The valve D2has a cathode connected to the anode of the valve D5and an anode connected to the anode of the valve D6.

The valve D3has a cathode connected to the first output terminal OUT1and an anode connected to a second terminal of the secondary coil of the upper transformer.

The valve D4has a cathode connected to the anode of the valve D1and an anode connected to the anode of the valve D6.

The valve D5has a cathode connected to the first output terminal OUT1and an anode connected to a third terminal of the secondary coil of the upper transformer.

The valve D6has a cathode connected to the anode of the valve D3.

The valve D7has a cathode connected to the anode of the valve D6and an anode connected to a first terminal of the secondary coil of the lower transformer.

The valve D8has a cathode connected to the anode of the valve D11and an anode connected to a second output terminal OUT2.

The valve D9has a cathode connected to the anode of the valve D6and an anode connected to a second terminal of the secondary coil of the lower transformer.

The valve D10has a cathode connected to the anode of the valve D7and an anode connected to the second output terminal OUT2.

The valve D11has a cathode connected to the anode of the valve D6and an anode connected to a third terminal of the secondary coil of the lower transformer.

The valve D12has a cathode connected to the anode of the valve D9and an anode connected to the second output terminal OUT2.

Meanwhile, the customer side DC-AC converter part150may be configured as a modular multi-level converter200.

The modular multi-level converter200may convert DC power into AC power by using a plurality of sub-modules210.

Referring toFIGS. 5 and 6, the configuration of the modular multi-level converter200will be described.

The modular multi-level converter200includes a central control unit250, a plurality of sub-control units230and a plurality of sub-modules210.

The central control unit250controls the plurality of sub-control units230, and the sub-control units230may respectively control the sub-modules210connected thereto.

Here, as illustrated inFIG. 5, one sub-control unit230is connected to one sub-module210, and accordingly, may control the switching operation of the one sub-module210connected thereto based on a control signal transferred through the central control unit250.

Also, alternatively, as shown inFIG. 6, one sub-control unit230is connected to a plurality of sub-modules210, and accordingly, may confirm each of the control signals for the plurality of sub-modules210connected thereto based on a plurality of control signals transferred through the central control unit250; each of the plurality of sub-modules210may be controlled based on the confirmed control signal.

The central control unit250determines the operation condition of the plurality of sub-modules210, and generates a control signal to control the operation of the plurality of sub-modules210according to the determined operation condition.

The operation condition may include a discharging operation, a charging operation, and a bypassing operation.

Here, different addresses are assigned to the plurality of sub-modules210, respectively.

Preferably, the addresses, which sequentially increase from the front according to the arranged sequence of the sub-modules, are assigned to the plurality of sub-modules210, respectively.

That is, the sub-module210may perform any one of the discharging operation, the charging operation, and the bypassing operation after receiving DC power.

The sub-module210includes a switching element having a diode, and accordingly, may perform any one of the discharging operation, the charging operation, and the bypassing operation of the sub-module210by a switching operation and the rectifying operation of the diode.

Each of the sub-control unit230receives a switching signal for controlling the plurality of sub-modules210through the central control unit250, and controls the switching operation of the sub-module210according to the received switching signal.

That is, the central control unit250may control the overall operations of the modular multi-level converter200.

The central control unit250may measure the current and voltage of the AC parts110and170and Dc power transmission part140, which are interconnected thereto.

Also, the central control unit250may calculate an overall control value.

Here, the overall control value may be a target value for the voltage, current, frequency of the output AC power of the modular multi-level converter200.

The central control unit250may calculate an overall control value based on one or more of the current and the voltage of the AC parts110and170which are interconnected with the modular multi-level converter200and the current and the voltage of the DC power transmission part140.

Meanwhile, the central control unit250may also control the operation of the modular multi-level converter200based on one or more from the reference active power, the reference reactive power, the reference current, the reference voltage received from an upper layer control unit (not shown) through a communications apparatus (not shown).

The central control unit250may transmit and receive data to/from the sub-control unit230.

Here, the central control unit250described herein assigns addresses according to the arranged sequence of the plurality of sub-modules210, and determines the switching sequence of the plurality of sub-modules210by using the assigned addresses.

That is, in general, all the sub-modules210do not operate under the same switching conditions, but a certain sub-module performs a charging operation or a bypassing operation according to the present required voltage, and the remaining sub-modules perform a discharging operation.

Accordingly, the central control unit250should firstly determine the sub-module which will perform the discharging operation.

Here, as the discharging operation is performed, the service life of the plurality of sub-modules210may be increased only if the plurality of sub-modules210perform the discharging operations within balanced frequencies with each other.

In other words, when a discharging operation frequency of a certain sub-module is high, the service life of the sub-module is turned out to be lower than that of other sub-modules having a low discharging operation frequencies.

Accordingly, it is very important to more rapidly determine the switching conditions of the plurality of sub-modules210while the balance of the switching frequencies of the plurality of sub-modules210is maintained.

Thus, in the embodiments, the switching sequence of the plurality of sub-modules210is determined according to the sequence of the addresses which are sequentially assigned.

For example, when there are sub-modules which are assigned with addresses 1 to 5 respectively, the central control unit250allows the discharging operations to be performed from the address 1. Here, the number of the sub-modules, in which the discharging operations are performed, is determined on the basis of a charged voltage value and a target value of each of the plurality of sub-modules.

That is, the central control unit250determines the switching conditions such that the sum of the charged voltage values of the plurality of sub-modules reach the target value. In other words, if power corresponding to the target value may be output by discharging even when the sub-modules assigned with address 1 and 2 are discharged, the central control unit250allows only the sub-modules assigned with addresses 1 and 2 to perform the discharging operations.

In addition, when determining the next switching condition, the central control unit250determines that a discharge operation is performed starting from a sub-module next to the sub-module having the latest address among the sub-modules previously performing discharging operations.

This will be described below in more detail.

Referring toFIG. 7, description will be given of connections of the plurality of sub-modules210included in the modular multi-level converter200.

FIG. 7illustrates connections of the plurality of sub-modules210included in the modular multi-level converter200.

Referring toFIG. 7, the plurality of sub-modules210may be serially connected, and the plurality of sub-modules210connected to a positive pole or negative pole of one phase may constitute one arm.

The three-phase modular multi-level converter200may normally include six arms, and include a positive pole and a negative pole for each of the three-phases A, B, and C to form the six arms.

Accordingly, the three-phase modular multi-level converter200may include: a first arm221including a plurality of sub-modules for a positive pole of phase A; a second arm222including a plurality of sub-modules for a negative pole of phase A; a third arm223including a plurality of sub-modules for a positive pole of phase B; a fourth arm224including a plurality of sub-modules for a negative pole of phase B; a fifth arm225including a plurality of sub-modules for a positive pole of phase C; and a sixth arm226including a plurality of sub-modules for a negative pole of phase C.

Also, the plurality of sub-modules210for one phase may constitute a leg.

Accordingly, the three-phase modular multi-level converter200may include a phase A leg227including a plurality of sub-modules210for phase A; a phase B leg228including a plurality of sub-modules210for phase B; and a phase C leg229including a plurality of sub-modules210for phase C.

Therefore, the first to six arms221to226are respectively included in the phase A leg227, the phase B leg228, and phase C leg229.

Specifically, in the phase A leg227, the first arm221, which is the positive pole arm of phase A, and the second arm222, which is the negative pole arm of phase A, are included; and in the phase B leg228, the third arm223, which is the positive pole arm of phase B, and the fourth arm224, which is the negative pole arm of phase B, are included. Also, in the phase C leg229, the fifth arm225, which is the positive pole arm of phase C, and the sixth arm226, which is the negative pole arm of phase C, are included

Also, the plurality of sub-modules210may constitute a positive pole arm227and a negative pole arm228according to polarity.

Specifically, referring toFIG. 7, the plurality of sub-modules210included in the modular multi-level converter200may be classified, with respect to a neutral line n, into a plurality of sub-modules210corresponding to the positive pole and a plurality of sub-modules210corresponding to the negative pole.

Thus, the modular multi-level converter200may include a positive arm227including the plurality of sub-modules210corresponding to the positive pole, and a negative arm228including the plurality of sub-modules210corresponding to the negative pole.

Accordingly, the positive pole arm227may include the first arm221, the third arm223, and the fifth arm225; and the negative pole arm228may include the second arm222, the fourth arm224, and the sixth arm226.

Next, referring toFIG. 8, the configuration of the sub-module210is described.

FIG. 8is an exemplary view illustrating a configuration of the sub-module210.

Referring toFIG. 8, the sub-module210includes two switches, two diodes, and a capacitor. Such a shape of the sub-module210is also referred to as a half-bridge shape or a half bridge inverter.

In addition, the switch included in a switching part217may include a power semiconductor.

Here, the power semiconductor refers to a semiconductor element for a power apparatus, and may be optimized for the conversion or control of electric power. Also, the power semiconductor is referred to as a valve unit.

Accordingly, the switch included in the switching part217may include a power semiconductor, for example, may include an insulated gate bipolar transistor (IGBT), a gate turn-off thyristor, an integrated gate commutated thyristor, etc.

The storage part219includes the capacitor, and thus may charge or discharge energy. Meanwhile, the sub-module210may be represented as an equivalent model based on the configuration and the operation of the sub-module210.

FIG. 9illustrates an equivalent model of the sub-module210, and referring toFIG. 9, the sub-module210may be illustrated as an energy charge and discharge unit including a switch and a capacitor.

Accordingly, it may be turned out that the sub-module210is the same as an energy charge and discharge unit having an output voltage of Vsm.

Next, referring toFIGS. 10 to 13, the operation of the sub-module210will be described.

The switch part217of the sub-module210ofFIGS. 10 to 13includes a plurality of switches T1and T2, and each of the switches is connected to each of diodes D1and D2. Also, the storage part219of the sub-module210includes a capacitor.

Referring toFIGS. 10 and 11, the charging and discharging operations of the sub-module210will be described.

FIGS. 10 and 11illustrate formation of the capacitor voltage Vsm of the sub-module210.

FIGS. 10 and 11illustrate a state in which the switch T1of the switching part217is turned on and the switch T2is turned off. Accordingly, the sub-module210may form the capacitor voltage according to each of the switching operations.

Specifically, referring toFIG. 10, the current introduced into the sub-module210is transferred to the capacitor via the diode D1and thus forms the capacitor voltage. Then, the formed capacitor voltage may charge energy into the capacitor.

Also, the sub-module210may perform discharging operation of discharging the charged energy.

Specifically, referring toFIG. 11, the stored energy of the capacitor, which is energy charged into the sub-module210, is discharged via the switch T1. Accordingly, the sub-module210may discharge the stored energy.

Referring toFIGS. 12 and 13, the bypassing operation of the sub-module210will be described.

FIGS. 12 and 13illustrate the formation of a zero voltage of the sub-module210.

FIGS. 12 and 13illustrate a state in which the switch T1of the switching part217is turned off and the switch T2is turned-on. Accordingly, current does not flow to the capacitor of the sub-module210, and the sub-module210may form a zero voltage.

Specifically, referring toFIG. 12, the current introduced into the sub-module210is output through the switch T2and the sub-module may form a zero voltage.

Also, referring toFIG. 13, the current introduced into the sub-module210is output through the diode D2and the sub-module210may form a zero voltage.

In this way, the sub-module210may form the zero voltage, and thus perform the bypassing operation in which the current does not flow into the sub-module210but bypasses the sub-module210.

FIGS. 14 to 16are views illustrating an operation of determining a switching sequence of a modular multi-level converter according to an embodiment.

Referring toFIG. 14, when the plurality of sub-modules210include a sub-module 1, a sub-module 2, a sub-module 3, a sub-module 4, a sub-module 5, a sub-module 6, and a sub-module 7, the central control unit250sequentially assigns addresses from the sub-module 1.

That is, address 1 may be assigned to the sub-module 1, address 2 may be assigned to the sub-module 2, address 3 may be assigned to the sub-module 3, address 4 may be assigned to the sub-module 4, address 5 may be assigned to the sub-module 5, address 6 may be assigned to the sub-module 6, and address 7 may be assigned to the sub-module 7.

Also, the central control unit250sequentially determines the switching sequence from the address 1. Here, the switching sequence is determined based on the charged voltage, which each of the sub-modules has, and a target voltage.

Referring toFIG. 15, when the target voltage is 60, and voltages of about 20 are identically charged into the sub-modules 1 to 7, the central control unit250determines the switching sequence of the sub-modules to meet the target voltage.

Here, since the voltage of about 20 is charged in each of the sub-modules, it is enough that only the three sub-modules from the front perform the discharging operations to meet the target voltage.

Accordingly, the central control unit250allows, according to the address sequence, only the sub-module 1, the sub-module 2, and the sub-module 3 to perform the discharging operations, and allows the remaining sub-modules to perform the bypassing operation or charging operation.

Here, when the above-mentioned switching condition is determined, the central control unit250remembers the sub-module which has the latest address among the sub-modules performing the discharging operations.

Also, referring toFIG. 16, when determining the next switching condition, the switching sequence is determined from the address next to the address of the remembered sub-module.

That is, since the discharging operations have been performed up to the sub-module 3, the discharging operation will be performed from the sub-module 4 at the next switching.

Accordingly, when the target voltage is 40, only the two sub-modules from the sub-module next to the remembered sub-module are allowed to perform the discharging operations.

Accordingly, the central control unit250allows only the sub-module 4, and the sub-module 5 to perform the discharging operations, and allows the remaining sub-modules to perform the bypassing operation or charging operation. Also, as described above, the central control unit250remembers information regarding the sub-module 5 which is assigned with the latest address among the sub-modules performing the discharging operations, and applies the remembered information when determining the switching condition later.

According to an embodiment, the switching sequence of the plurality of sub-modules is determined according to the assigned addresses, so that the time required to determine the operation condition of the sub-modules may be reduced.

Also, according to an embodiment, a plurality of sub-modules are switched according to the address sequence to maintain a balance of the switching frequencies of the plurality of sub-modules, so that a situation in which only a certain sub-module is continuously switched may be prevented in advance, and a situation in which the service life of the certain sub-module is reduced may also be prevented.

FIGS. 17 and 18are flowcharts illustrating, step by step, a method of determining a switching sequence of a modular multi-level converter according to an embodiment.

First, referring toFIG. 17, the central control unit250assigns addresses according to the arranged sequence of the sub-modules (operation S100). That is, the lowest address is assigned to the frontmost one of the sub-modules, and the highest address is assigned to the sub-module which is disposed at the last.

Next, the central control unit250confirms a target voltage and the charged voltages of the plurality of sub-modules (operation S110).

Next, the central control unit250sequentially determines a switching sequence from the sub-module having the lowest address based on the target voltage and the charged voltages (operation S120).

That is, discharging operations are performed from the sub-module having the lowest address so that the target voltage may be output based on the charged voltages (operation S130).

Next, the output voltage corresponding to the target voltage is generated through the sequential discharging operations (operation S130).

Here, the central control unit250remembers the information on the sub-module which is assigned with the last address among the sub-modules performing the discharging operations, and later, determines the switching conditions of the sub-modules using the remembered information.

That is, referring toFIG. 18, the central control unit250confirms the target voltage and the charged voltages of the sub-modules (operation S200).

Next, the central control unit250confirms the sub-module which has the last address among the sub-modules which performed discharging operations at the previous time (operation S210).

Next, the central control unit250determines a switching sequence to output the target voltage from the sub-module having the address next to the confirmed sub-module (operation S220).

For example, when discharging operations were performed at the previous time up to the sub-module having address 3, discharging operations are performed from the sub-module having address 4 at the current time.

Next, as the discharging operations of the sub-modules are performed according to the determined switching sequence, the output voltage corresponding to the target voltage is generated (operation S230).

According to an embodiment, the switching sequence of the plurality of sub-modules is determined according to the assigned addresses, so that the time required to determine the operation condition of the sub-modules may be reduced.

Also, according to an embodiment, a plurality of sub-modules are switched according to the address sequence to maintain a balance of the switching frequencies of the plurality of sub-modules, so that a situation in which only a certain sub-module is continuously switched may be prevented in advance, and a situation in which the service life of the certain sub-module is reduced may also be prevented.

Furthermore, although preferred embodiments are illustrated and described above, the specification is not limited to a specific embodiment mentioned above. Various modifications are possible by those skilled in the art without departing from the spirit and scope of the claims. Also, such modifications should not be understood separately from the spirit and scope of the inventive concept.