KR101689824B1 - Modular Multilevel Converter and Submodule of the Converter - Google Patents

Abstract

A modular multilevel converter, and its submodules, are disclosed. The submodule of the modular multilevel converter includes a first bidirectional switch, a second bidirectional switch, a capacitor, a first terminal, a second terminal, and a third bidirectional switch. The first bidirectional switch and the second bidirectional switch each include a switching element and a diode connected in anti-parallel to the switching element, and are connected in series in the same direction. The capacitors are connected in parallel across the first bidirectional switch and the second bidirectional switch connected in series. The first terminal is connected to a node to which the first bidirectional switch and the second bidirectional switch are connected and the second terminal is connected to the node to which the capacitor and the diode positive electrode included in the second bidirectional switch are connected. The third bidirectional switch includes a switching element and a diode connected in anti-parallel to the switching element. When the direct current is applied from the second terminal to the first terminal, the third bidirectional switch is connected in the opposite direction to the second bidirectional switch on the path of the applied direct current do.

Description

Modular Multilevel Converter < RTI ID = 0.0 > Submodule < / RTI &

The present invention relates to a modular multilevel converter used in a voltage type ultra high voltage direct current transmission (HVDC) system, and more particularly to a modular multilevel converter and the form of the module making up the modular multilevel converter.

The ultra high voltage direct current transmission technology converts AC voltage into DC voltage and transmits it. The converter station is referred to as a converter station. The converter station is classified into a current type ultra high voltage direct current transmission system, a voltage type ultra high voltage direct current transmission system, and a hybrid type Hybrid high voltage direct current transmission system.

Among them, the converter station of the voltage type ultra high voltage DC transmission system is classified into a 2-level converter station, a 3-level Neutral Point Clamped (NPC) converter station and a modular multi-level converter station according to the topology used.

Among them, the modular multilevel converter station has fewer harmonics than the converter station of other structure, so it does not need harmonic filter, can reduce the switching loss due to the switching reduction technique, does not have a capacitor connected directly to DC, It is possible to cope with DC short circuit and capacitor failure. It can realize high voltage by implementing more than several hundred levels, and it is widely used because it can use a general transformer because it constitutes a sine wave in the form of a multi-stepped step.

1 is a circuit diagram of a voltage type ultra high voltage DC transmission system having a modular multi-level converter station.

In the figure, AC Grid # 1 and AC Grid # 2 are AC system, Tr 1 and Tr 2 are transformer, black box is initial charging circuit, CBA 1 and CBA 2 are Auxiliary Circuit Breaker, CBM 1, CBM 2 is a main circuit breaker, RIC 1, RIC 2 is an initial charging resistor, SM 1,..., SM N are sub-modules constituting an arm, 1 to N, respectively.

In general, modular multi-level converter stations are classified into three types according to the type of submodule. Fig. 2 shows a structure in which the sub-module SM has a half-bridge shape. Fig. 3 shows a structure in which the sub-module SM has a full bridge. And a structure having a double (double) shape.

When a ground fault occurs on the DC side, all the switches are turned OFF and a large short-circuit current flows in the diode D ₁₂ connected in anti-parallel to the lower switch S ₁₂ of the sub-module. ₁ ) is connected in parallel with the lower switch to protect the diode. In other words, in the event of a DC short, the thyristor is turned on to allow most short-circuit currents to flow through the thyristor to protect the switch.

FIG. 5 shows the flow of short-circuit current (red arrow direction) when the sign of the input voltage is ++ - when a short circuit occurs in the DC system. Using a submodule having the structure as shown in FIG. 2, when a DC short occurs, all the switches are turned off until a general AC circuit breaker whose blocking rate is as low as several tens ms blocks the system, and all thyristors are turned on do.

In other words, if a trip occurs due to a DC short, AC system is disconnected. Therefore, it takes much time to restart, and the reactor capacity and thyristor capacity must be considered in order to cope with short-circuit current. And has the advantage that the conduction loss is the smallest during normal operation.

In the field where the reliability of the system is important, such as the interruption of power ripple effect, it is difficult to apply to the submodule having the half bridge structure. Thus, the MMC with the submodule having the full bridge and the clamp double structure has appeared.

Both the full bridge and the double-clamped submodules have the advantage of not having to turn off the AC circuit breaker in the event of a short circuit in the DC system, thus making it possible to quickly prepare for restart without disconnecting the AC system. And the on-state conduction loss is increased.

Disclosure of Invention Technical Problem [8] The present invention has been made in order to solve the conventional problems described above, and it is an object of the present invention to reduce the number of elements constituting sub- And to provide a new submodule structure that is reduced in size.

In order to achieve the above object, a submodule of a modular multilevel converter according to the present invention includes a first bidirectional switch, a second bidirectional switch, a capacitor, a first terminal, a second terminal, and a third bidirectional switch.

The first bidirectional switch and the second bidirectional switch each include a switching element and a diode connected in anti-parallel to the switching element, and are connected in series in the same direction. The capacitors are connected in parallel across the first bidirectional switch and the second bidirectional switch connected in series. The first terminal is connected to a node to which the first bidirectional switch and the second bidirectional switch are connected and the second terminal is connected to the node to which the capacitor and the diode positive electrode included in the second bidirectional switch are connected. The third bidirectional switch includes a switching element and a diode connected in anti-parallel to the switching element. When the direct current is applied from the second terminal to the first terminal, the third bidirectional switch is connected in the opposite direction to the second bidirectional switch on the path of the applied direct current do.

At this time, the third bidirectional switch may be connected to the node where the anode of the included diode is connected to the second bidirectional switch and the capacitor.

Also, the third bidirectional switch may be connected to a node where the cathode of the included diode is connected to the first bidirectional switch and the two bidirectional switches.

Further, a modular multi-level converter according to an aspect of the present invention includes: a first bidirectional switch and a second bidirectional switch, each of which includes a switching element and a diode connected in anti-parallel to the switching element, A first terminal connected to a node to which a first bidirectional switch and a second bidirectional switch are connected, and a second terminal connected to a node to which the capacitor is connected and a diode anode included in the second bidirectional switch, And a third bidirectional switch including a diode connected in parallel to the switching element and the switching element and connected in the opposite direction to the second bidirectional switch on the path of the applied direct current when the direct current is applied from the second terminal to the first terminal, And a submodule that includes the submodule.

Further, a modular multi-level converter according to another aspect of the present invention includes a half bridge submodule portion, and a bi-directional switch portion. The half bridge submodule unit includes a first bidirectional switch and a second bidirectional switch, a first bidirectional switch connected in series and a second bidirectional switch connected in series in parallel in the same direction, respectively, parallel to the switching device and the switching device respectively A plurality of submodules including a connected capacitor, a first terminal connected to a node to which a first bidirectional switch and a second bidirectional switch are connected, and a second terminal connected to a node to which a capacitor is connected and a diode anode included in the second bidirectional switch, And the bidirectional switch unit includes a third bidirectional switch including a switching element and a diode connected in anti-parallel with the switching element, and when the direct current is applied to the half bridge submodule unit, The switch is connected in the opposite direction to the second bi-directional switch And the bridge bridge sub-module unit.

At this time, the bidirectional switch unit may include a plurality of third bidirectional switches connected in series in the same direction.

In addition, the third bidirectional switch may have a plurality of diodes connected in series in the same direction and a plurality of switching elements connected in series in the same direction, which are connected in reverse parallel to each other.

According to the present invention, it is possible to reduce the number of elements constituting the submodule compared with the existing structure while having the same AC / DC blocking capability as the conventional full-bridge and clamp double structure, thereby reducing hardware cost and reducing conduction loss So that the efficiency can be improved.

1 is a circuit diagram of a voltage type ultra high voltage DC transmission system having a modular multi-level converter station.
2 is a half-bridge sub-module (HBSM) structure diagram;
3 is a FBSM (Full Bridge Sub-Module) structure diagram.
4 is a schematic diagram of a CDSM (Clamp Double Sub-Module).
FIG. 5 is a diagram showing a short-circuit current flow in a DC short circuit in an MMC having a Half Bridge Sub-Module (HBSM) structure; FIG.
6 illustrates a T-Bridge sub-module structure;
7 shows a T-Bridge 1 for one arm.
8 shows a T-Bridge 2 for one arm.
FIG. 9 is a diagram showing a circuit (T-Bridge 3) in which all S _X3 switches are integrated in T-Bridge 2 of FIG. 8, and FIGS. 9A to 9C are views showing T-Bridge 3 structures for one arm.
10 is a table comparing the devices required for each submodule when the number of submodules per arm is N. FIG.
11 is a table comparing AC / DC blocking capability and loss characteristics according to the submodule structure.
12 is a diagram showing an example of a cut-off path of a short-circuit current during DC short-circuit in an MMC having a T-Bridge 2 structure;
FIG. 13 is a table showing the simulation conditions in a case where 8 submodules are provided per arm in MMC I of FIG. 1, and a smoothing inductance and a resistance are added to the DC-link.
14 is a graph showing a waveform at the time of DC shorting in the structure of T-Bridge 1 with N = 8;
15 and 16 are graphs showing the waveforms of the upper arm and the lower arm in the T-Bridge 1 structure with N = 8, respectively.
FIGS. 17, 18, and 19 are graphs showing the results of simulations of the DC short circuit in the T-Bridge 2 structure with N = 8, respectively.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

FIGS. 6A and 6B are views showing two types of structures of the T-Bridge sub-module 100 devised in the present invention. The two module types have the same characteristics when connected in a cascade form as shown in FIG.

The T-Bridge sub-module 100 includes a first bidirectional switch 110, a second bidirectional switch 120, a capacitor 130, a first terminal 140, a second terminal 150, (160).

A first two-way switch 110, and a second two-way switch 120 comprises a switching device (S _11, S12) and switching elements (S _11, S ₁₂₎ and a diode (D _11, D ₁₂₎ anti-parallel connected to each And are connected in series in the same direction.

The switches S ₁₁ and S ₁₂ perform a complementary on-off operation as in the conventional case. S ₁₃ maintains the turn-on state at all times in the normal state and turns off at the occurrence of the abnormal state including the DC short circuit.

The capacitor 130 is connected in parallel to the first bidirectional switch 110 and the second bidirectional switch 120 connected in series.

The first terminal 140 is connected to the node to which the first bidirectional switch 110 and the second bidirectional switch 120 are connected and the second terminal 150 is connected to the anode of the diode D ₁₂ included in the second bidirectional switch The capacitor 130 is connected to the node to which it is connected.

The third bidirectional switch 160 includes a diode D ₁₃ connected in parallel to the switching element S ₁₃ and the switching element S ₁₃ and is connected in series to the first terminal 140 via a direct current Directional switch 120 on the path of the applied direct current when the current is applied.

FIG. 7 shows a circuit (T-Bridge 1) in which one arm is formed by using N T-bridge sub-module structures 100, FIG. 8 shows N / 2 (T-Bridge 2) which consists of one arm using N / 2 of a pure half-bridge sub-module structure.

Generally, the number of TBSM and HBSM is N / 2 because twice the AC peak voltage is used as DC total voltage. However, the number of correct TBSM and HBSM is different from AC power and TBSM circuit . That is, how many switches S _X3 are needed in the TBSM to block the maximum peak size of the AC voltage.

9A to 9C show a circuit (T-Bridge 3) in which all of the switches S _X3 in the T-Bridge 2 of FIG. 8 are collected. The T-Bridge 3 circuit includes a half bridge submodule unit 200, and a bidirectional switch unit 300.

In other words, in the case of the T-Bridge 3, the structure is composed of a bidirectional switch unit 300, which is a set of switches located in a direction opposite to the half bridge submodule unit 200 of a pure Half-Bridge sub- As shown in FIG. 9B, the structure of the bidirectional switch unit 300 in which the diode and the IGBT are separated and connected in series and the structure of the bidirectional switch unit 300 composed of one bidirectional switch 300 as shown in FIG. 9C are possible.

Here, the switches are not limited to IGBTs but all controllable switches that can be turned off. For example, when the IGCT or GTO switch has less conduction loss than the IGBT, on-state conduction loss can be further reduced. The switch constituting the square box of FIG. 9C serves as an arm switch composed of power semiconductor elements.

FIG. 10 is a table comparing elements required for each submodule when the number of submodules per arm is N. FIG. In the table, Full-Bridge 1 represents a form in which all submodules are configured in a full-bridge structure, and Full-Bridge 2 represents a half-bridge structure in which half of all submodules are constituted by a half-bridge structure .

Although T-Bridge 2 devised in accordance with the present invention shows that it can be configured with the fewest elements, using the T-Bridge 3 structure requires fewer elements than the T-Bridge 2 depending on the voltage rating of the device used You can also have it.

11 shows a table comparing AC / DC blocking capability and loss characteristics according to the submodule structure. It is shown that the T-Bridge 3 structure has the smallest conduction loss among AC / DC blocking capacitors.

12 is a circuit diagram showing an example in which the DC current is cut off during a DC short circuit in the T-Bridge 2 structure. When DC is shorted, all switches are all OFF, which means that current can not flow to S _X3 switch and D _X3 diode connected in anti-parallel is also reverse biased and current can not flow. From this it can be seen that it has AC / DC blocking capability.

In order to verify the feasibility of the present invention, a simulation was performed. FIG. 13 shows simulation conditions. The simulation circuit has 8 submodules per arm in MMC I of FIG. 1 and adds a smoothing inductance and a resistor to the DC-link. A DC short was generated in 0.5 seconds during normal operation.

FIG. 14 is a graph showing a waveform at the DC short circuit in the T-Bridge 1 structure with N = 8. In FIG. 14, when a short circuit occurs in 0.5 second and the DC link current is increased to reach the limit value, All of the switches are turned off. At this time, the DC current and the AC current are reduced to zero to indicate that they are shut off.

15 and 16 show the waveforms of the upper arm and the lower arm in the T-Bridge 1 structure with N = 8, respectively. The voltage across the eight IGBTs (V _CE , collector and emitter voltage) and the voltage across both ends of the upper and lower submodules are distributed to a voltage less than 2500 V at steady state, .

FIGS. 17, 18 and 19 are graphs showing the results of simulation of the DC short circuit in the T-Bridge 2 structure with N = 8. Even if the number of T-Bridge submodules is reduced by half, it can be confirmed that the voltage is less than 2500V at the steady state and the voltage is cut off stably.

Although the present invention has been described in terms of some preferred embodiments, the scope of the present invention should not be limited thereby but should be modified and improved in accordance with the above-described embodiments.

100: T-Bridge Sub-Module
110: first bidirectional switch
120: second bi-directional switch
130: Capacitor
140: first terminal
150: second terminal
160: Third bidirectional switch
200: Half bridge submodule part
300: bidirectional switch section

Claims (11)

delete A first bidirectional switch and a second bidirectional switch each including a switching element and a diode connected in anti-parallel with the switching element, the first bidirectional switch and the second bidirectional switch being connected in series in the same direction;
A capacitor connected in parallel to the first bidirectional switch and the second bidirectional switch connected in series;
A third bidirectional switch including a switching element and a diode connected in anti-parallel to the switching element, the anode of the diode included being connected to a node to which the second bidirectional switch and the capacitor are connected;
A first terminal connected to a node to which the first bidirectional switch and the second bidirectional switch are connected; And
And a second terminal connected to the diode cathode included in the third bidirectional switch. A first bidirectional switch and a second bidirectional switch each including a switching element and a diode connected in anti-parallel with the switching element, the first bidirectional switch and the second bidirectional switch being connected in series in the same direction;
A capacitor connected in parallel to the first bidirectional switch and the second bidirectional switch connected in series;
A third bidirectional switch including a switching element and a diode connected in anti-parallel to the switching element, wherein the diode cathode is connected to a node to which the first bidirectional switch and the second bidirectional switch are connected;
A first terminal coupled to the diode anode of the third bidirectional switch; And
And a second terminal connected to a node to which the second bidirectional switch and the capacitor are connected. delete A first bidirectional switch and a second bidirectional switch each including a switching element and a diode connected in anti-parallel with the switching element, the first bidirectional switch and the second bidirectional switch being connected in series in the same direction;
A capacitor connected in parallel to the first bidirectional switch and the second bidirectional switch connected in series;
A third bidirectional switch including a switching element and a diode connected in anti-parallel to the switching element, the anode of the diode included being connected to a node to which the second bidirectional switch and the capacitor are connected;
A first terminal connected to a node to which the first bidirectional switch and the second bidirectional switch are connected; And
And a second terminal coupled to the diode cathode of the third bidirectional switch. A first bidirectional switch and a second bidirectional switch each including a switching element and a diode connected in anti-parallel with the switching element, the first bidirectional switch and the second bidirectional switch being connected in series in the same direction;
A capacitor connected in parallel to the first bidirectional switch and the second bidirectional switch connected in series;
A third bidirectional switch including a switching element and a diode connected in anti-parallel to the switching element, wherein the diode cathode is connected to a node to which the first bidirectional switch and the second bidirectional switch are connected;
A first terminal coupled to the diode anode of the third bidirectional switch; And
And a second terminal coupled to the second bidirectional switch and a node to which the capacitor is connected. A first bidirectional switch and a second bidirectional switch respectively connected in series in the same direction, a capacitor connected in parallel to all of the first bidirectional switch and the second bidirectional switch connected in series, and a diode connected in parallel with the switching element and the switching element, A plurality of submodules including a first terminal connected to a node to which a first bidirectional switch and a second bidirectional switch are connected and a second terminal connected to a node to which the capacitor is connected and a diode anode included in the second bidirectional switch are connected in series A half bridge submodule portion; And
A modular multilevel converter including a third bidirectional switch including a switching element and a diode connected in anti-parallel with the switching element, and a bidirectional switch portion connected in series with the harbridge submodule portion,
Wherein the bidirectional switch unit is connected to the harp bridge sub-module unit such that a diode cathode of the third bidirectional switch is connected to a first terminal of one of the plurality of submodules. 8. The method of claim 7,
Wherein the bidirectional switch unit includes a plurality of third bidirectional switches connected in series in the same direction. 8. The method of claim 7,
Wherein the third bidirectional switch has a plurality of diodes connected in series in the same direction and a plurality of switching elements connected in series in the same direction are connected in reverse parallel to each other. A first bidirectional switch and a second bidirectional switch respectively connected in series in the same direction, a capacitor connected in parallel to all of the first bidirectional switch and the second bidirectional switch connected in series, and a diode connected in parallel with the switching element and the switching element, A plurality of submodules including a first terminal connected to a node to which a first bidirectional switch and a second bidirectional switch are connected and a second terminal connected to a node to which the capacitor is connected and a diode anode included in the second bidirectional switch are connected in series A half bridge submodule portion; And
A modular multilevel converter including a third bidirectional switch including a switching element and a diode connected in anti-parallel with the switching element, and a bidirectional switch portion connected in series with the harbridge submodule portion,
Wherein the bidirectional switch unit is connected to the harp bridge sub-module unit such that a diode anode of the third bidirectional switch is connected to a second terminal of one of the plurality of submodules. 11. The method of claim 10,
Wherein the third bidirectional switch has a plurality of diodes connected in series in the same direction and a plurality of switching elements connected in series in the same direction are connected in reverse parallel to each other.

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