Converter for electric power

The present invention relates to a converter for electric power having multiple sub-modules connected in series, the sub-modules having an energy storage unit and multiple power semiconductor circuits connected in parallel to the energy storage unit, and which causes an electric current to bypass a sub-module in case the breakdown of the sub-module occurs. To this end, the converter for electric power according to the present invention has multiple sub-modules connected to each other in series, the sub-modules having an energy storage unit and at least one power semiconductor circuit that is connected in parallel to the energy storage unit and comprises multiple power semiconductor switches and freewheeling diodes, wherein each of the sub-modules comprises a bypass switching unit, which is connected in parallel to any one of said at least one power semiconductor circuit, and bypasses an electric current via the bypass switching unit.

TECHNICAL FIELD

The present invention generally relates to a converter for electric power. More particularly, the present invention relates to an electric power converter in which multiple sub-modules, including an energy storage unit and multiple power semiconductor circuits connected to the energy storage unit, are connected in series. The electric power converter causes current to bypass a sub-module when the sub-module fails.

BACKGROUND ART

Generally, a converter for high voltage uses a power semiconductor, which is turn-on/turn-off controlled for mutual conversion between AC voltage and DC voltage. Because the withstand voltage of the power semiconductor is limited, multiple semiconductor modules having a power semiconductor circuit should be connected in series to process the high voltage. Various semiconductor modules may be connected with each other for a power semiconductor circuit configuration.

As it is known, in the case of a well-known modular multilevel converter (MMC), the power semiconductor circuit includes multiple sub-modules that form two output terminals, and the multiple sub-modules are connected to each other in series. Such sub-modules may be configured to include, for example, an energy storage unit and a power semiconductor circuit, which is connected to the energy storage unit in parallel and comprises multiple power semiconductor switches and free-wheel diodes.

When a certain sub-module fails among these multiple sub-modules, the failing sub-module is shorted to stably operate a system. For a short circuit, a phase current bypasses the failing sub-module by a bypass switch, and the system continuously performs normal operations by other normal sub-modules.

In this case, a technique for quickly bypassing a failing sub-module is very important in terms of a system as well as the corresponding sub-module. If the switching time of the bypass switch is slow, overvoltage and overcurrent are generated in the failing module. Accordingly, the internal components (for example, capacitors, power semiconductors, etc.) of the corresponding sub-module may explode and may lead to a fire. Also, as overvoltage and overcurrent are generated in the whole system, and overvoltage and overcurrent respectively exceeding a rated voltage and a rated current are applied to each sub-module, the reliability of the system is adversely affected. Therefore, the bypass switch must quickly operate and must be stable, but the conventional art may not maintain desired performance due to technological limitations.

Recently, various circuit configurations are proposed for the bypass of a phase current when a sub-module fails. However, these have unilateral circuits and it is difficult to perform an effective bypass operation.

DISCLOSURE

Technical Problem

Accordingly, the present invention has been made keeping in mind the above problems occurring in the conventional art, and an object of the present invention is to provide an electric power converter that causes a current to bypass a sub-module when the corresponding sub-module fails.

Another object of the present invention is to provide an electric power converter capable of reducing the overall bypass operation time required for shorting a failing sub-module even though the bypass switch of which the operation time is slow is used in the electric power converter in which multiple sub-modules are connected in series.

A further object of the present invention is to provide an electric power converter designed to reduce the cost by using a bypass switch of which the operating time is slow but the cost is low.

Yet another object of the present invention is to provide an electric power converter enabling the effective bypass of a current because of a bidirectional bypass switch.

Technical Solution

In order to accomplish the above object, the present invention provides an electric power converter in which multiple sub-modules (10), including an energy storage unit (110) and one or more power semiconductor circuits (120) connected to the energy storage unit (110) in parallel, are connected with each other in series, the power semiconductor circuit including multiple power semiconductor switches (121,123) and free-wheel diodes (122,124). Each of the sub-modules (10) includes a bypass switch unit (130) that is connected to a power semiconductor circuit connected between two output terminals (X1, X2), among the power semiconductor circuits (120), and the electric power converter bypasses a current through the bypass switch unit (130).

In the present invention, the bypass switch unit (130) includes a first switch element (131) connected to one among the power semiconductor circuits (120), a diode (132) connected to the first switch element (131) in anti-parallel, and a second switch element (133) connected to the diode (132) in parallel; a cathode side of the first switch element (131) is connected to an emitter side of the power semiconductor switch (123) of the power semiconductor circuit (120); and the current flows through the diode (132) and the first switch element (131) when the sub-module (10) fails, and the current is bypassed through the second switch element (133) after a switching operation of the second switch element (133).

In the present invention, the first switch element (131) includes a semiconductor switch or a power semiconductor switch, and the second switch element (133) includes a mechanical switch.

In the present invention, the first switch element (131) includes an SCR.

In the present invention, the bypass switch unit (130) includes a first switch element (131) connected in parallel to a power semiconductor circuit connected between two output terminals, among the power semiconductor circuits (120), a third switch element (132′) connected to the first switch element (131) in anti-parallel, and a second switch element (133) connected to the third switch element (132′) in parallel; a cathode side of the first switch element (131) is connected to an emitter side of the power semiconductor switch (123) of the power semiconductor circuit (120); and the current flows through the third switch element (132′) and the first switch element (131) when the sub-module (10) fails, and the current is bypassed through the second switch element (133) after a switch operation of the second switch element (133).

In the present invention, the first switch element (131) and the third switch element (132′) include an SCR element.

In the present invention, an anode side of the third switch element (132′) is connected to a cathode side of the first switch element (131).

In the present invention, a switching operation time of the second switch element (133) is 6 to 10 milliseconds.

Advantageous Effects

According to the present invention as described above, the electric power converter has the following effects.

According to the present invention, though an electric power converter in which multiple sub-modules are connected in series does not reduce an operating time of the bypass switch for quickly shorting a failing sub-module, the overall bypass operation time may be reduced by preferentially operating a fast semiconductor element for electric power.

Also, according to the present invention, as a relatively slow bypass switch is used compared to the conventional art, the cost may be reduced.

Furthermore, according to the present invention, because the bypass switch is a bidirectional switch, effective bypass is possible when a sub-module fails, and thus a system may operate stably.

BEST MODE

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, it is to be noted that, when the functions of conventional elements and the detailed description of elements related with the present invention may make the gist of the present invention unclear, a detailed description of those elements will be omitted.

The present invention provides an electric power converter. In the electric power converter according to the present invention, multiple sub-modules are connected in series. Especially, when a certain sub-module fails, the electric power converter is operated by other normal sub-modules by making a phase current bypass the failing sub-module to prevent stopping of the operation of the converter, an explosion, a fire, the generation of overvoltage and overcurrent in the system, and the like.

FIG. 1is an equivalent circuit diagram of an electric power converter according to an embodiment of the present invention.

Referring toFIG. 1, an electric power converter according to the present invention includes one or more phase modules1, and in each of the phase modules1, multiple sub-modules10are connected in series. Also, each of the phase modules1connects the DC voltage sides to positive and negative DC voltage bus-bars P0and N0. DC voltage, though not illustrated inFIG. 1, is present between the DC voltage bus-bars P0and N0. Each of the phase modules1has an intermediate AC voltage terminal and two outer DC voltage terminals as a load connection terminal. One phase module arm (z) is formed between the intermediate AC voltage terminal and each of the outer DC voltage terminals, and the phase module arm (z) becomes the series circuit of a sub-module10. Each of the sub-modules10, connected in series in each of the phase module1, forms two connection terminals, that is, a first connection terminal X1and a second connection terminal X2.

The electric power converter configured as the above-description may be formed as a part of equipment for transmitting high-voltage current, and serves to connect AC voltage power systems with each other to transmit high power between the systems. Also, the electric power converter may be a part of FACTS equipment that stabilizes the system or guarantees desired voltage quality. Furthermore, the converter illustrated inFIG. 1may be used in the power driving technology.

Especially, in the electric power converter according to the present invention, when a sub-module10fails, the failing sub-module10is shorted to prevent an open circuit of the phase module1. The phase current bypasses the failing sub-module10by the short circuit, and the phase module1is normally operated by a phase current that flows through other normal sub-modules10. In this case, the present invention includes a bypass switch unit (reference numeral130inFIG. 2) in the sub-module10, which serves as a short-circuiting device to short the sub-module10. The bypass switch unit130is controlled by the control signal of a control unit to short the sub-module within a few milliseconds after the fail. Therefore, a normal phase current flows through multiple sub-modules10during the normal operation, but when a certain sub-module10fails, the failing sub-module is shorted by the operation of the bypass switch unit200and the phase current is bypassed through the bypass switch unit130. Accordingly, the phase module1is protected.

FIG. 2is an equivalent circuit diagram of a sub-module in an electric power converter according to the present invention.

Referring toFIG. 2, in the electric power converter according to the present invention, each of the sub-modules10comprises an energy storage unit110and one or more power semiconductor circuits120connected to the energy storage unit110in parallel. The power semiconductor circuits120are connected with each other in series. Each of the power semiconductor circuits120includes power semiconductor switches121and123that are turn-on/turn-off controlled, and free-wheel diodes122and124respectively connected to the power semiconductor switches121and123in anti-parallel. In another embodiment, each of the sub-modules10may be variously implemented by arranging the energy storage unit110and the one or more power semiconductor circuits120, differently fromFIG. 2.

Also, in the sub-module10according to the present invention, the bypass switch unit130is connected in parallel to any one among the power semiconductor circuits120.FIG. 2illustrates an example in which the bypass switch unit is connected to the power semiconductor circuit in the lower side in parallel among the two power semiconductor circuits120, but it may be connected to the power semiconductor circuit in the upper side. In this case, desirably, the bypass switch unit is connected in parallel to the power semiconductor circuit that is connected between the two output terminals X1and X2of the sub-module10. For example, when the two output terminals X1and X2are linked to the both ends of the upper power semiconductor circuit, the bypass switch unit may be connected to the upper power semiconductor in parallel. The bypass switch unit130causes the phase current to bypass a sub-module when the corresponding sub-module10fails. In this case, the bypass operation of the bypass switch unit130should be performed within a short duration. Otherwise, overvoltage and overcurrent are generated in the failing sub-module10, and the internal components (capacitors and power semiconductors) of the corresponding sub-module10may explode and a fire may be caused. Also, as overvoltage and overcurrent are generated in the whole system, and overvoltage and overcurrent respectively exceeding a rated voltage and a rated current are applied to each of the sub-modules10, the reliability of a system may be adversely affected. Therefore, it is desirable that the bypass operation is performed within a few milliseconds after the fail.

To this end, the bypass switch unit130of the sub-module10according to the present invention comprises a first switch element131connected to any one of power semiconductor circuits120in parallel, a diode132connected to the first switch element in anti-parallel, and a second switch element133connected to the diode132in parallel. Desirably, the bypass switch unit130is connected in parallel to the power semiconductor circuit120that is connected between the two output terminals X1and X2. In the embodiment, the first switch element131includes, for example, a silicon controller rectifier (SCR) element. The SCR element, as a thyristor, is turned on by the signal of a control unit140. Specifically, in the example ofFIG. 2, the first switch element131, which is connected to a certain power semiconductor circuit120in parallel, is turned on when a signal is input from the control unit140, and sends the current. The cathode side of the first switch unit131is connected to the emitter side of the power semiconductor switch123of the power semiconductor circuit120. Also, the diode132is connected to the first switch element131in anti-parallel. Generally, the second switch element133is implemented as a well-known mechanical configuration of switch.

A bypass operation in the bypass switch unit130configured as the above-description is described. When a certain sub-module10fails, the second switch element133is switched and the current is bypassed through the second switch element133. However, as mentioned in the conventional art, because it is difficult to quicken the switching operation time of the second switch element133, high instantaneous current may be provided to the sub-module10. Technical development has been made in reducing the switching operation time of the second switch element133, but high technical skill is required and much time and expenses are spent achieving such a goal. Accordingly, the present invention uses the second switch element133of which the switching operation time is relatively long, but additionally includes the first switch element131and the diode132to reduce the overall bypass operation time. Therefore, when a sub-module10fails, the fault current preferentially flows through the diode132and the first switch element131. Then, the second switch element133is switched and the current is bypassed.

As described above, though the present invention uses the second switch element133of which the switching time is relatively long, the bypass operation may be quickly performed in the bypass switch unit130. The current flow through the second switch element133is substituted with the current flow through the combination of the diode132and the first switch element131, and then the current is bypassed by sending the current through the second switch element133depending on the switching operation of the second switch element133.

Practically, in the case of the electric power converter according to the conventional art, 3 to 4 milliseconds switching operation time of the second switch element133is required for the stable system operation. However, in the present invention, though the second switch element133of which the switching operation time is 6 to 10 milliseconds is used, the quicker bypass operation time, which is 1 to 2 milliseconds, may be implemented by the combination of the first switch element131and the diode132. This is because the second switch element133is generally formed by a mechanical configuration, whereas the combination of the first switch element131and the diode133is formed by an electrical configuration. Therefore, the limitations in the bypass operation time, caused by the mechanical operation, may be reduced.

The switching operation time of the second switch element133is very important because it determines the spread of the fail of the sub-module10and the magnitude of the current flowing after a certain operation time. In other words, as the switching operation time of the switch133is longer, the possibility of a fire increases in the failing sub-module10, and the fire leads to a secondary accident in the surrounding sub-modules.

Also, as the switching operation time of the second switch element133is longer, the magnitude of the dark current flowing after the completion of the switching operation of the second switch unit133increases. Because the magnitude of the current may exceed the rated current of the power semiconductor element of the sub-module10, all the sub-modules10may be damaged by a fire. Consequently, it is important to reduce the switching operation time of the second switch element133.

FIG. 3is an equivalent circuit diagram according to another embodiment of the present invention.

Referring toFIG. 3, the sub-module10according to another embodiment of the present invention has a bypass switch unit130, which is differently configured in some parts compared to the sub-module10according to the embodiment illustrated inFIG. 2. Specifically, in another embodiment of the present invention, the bypass switch unit130comprises a first switch unit131, a third switch unit132′, and a second switch unit133. In this case, it is desirable that the first and the third switch elements131and132′ have the same configuration, and may be implemented as a SCR element. The third switch element132′ is also turned on by the signal from the control unit140. The anode side of the third switch element132is connected to the cathode side of the first switch element131. In the operation of such a bypass switch unit130, the instantaneous current flows to the third switch element132′ and the first switch element131, same asFIG. 2, and then the current is bypassed through the second switch element133after the second switch element133is switched.

As described above, the bypass switch unit130according to another embodiment of the present invention is connected to any one among the power semiconductor circuits120. Desirably, the bypass switch unit130is connected to the power semiconductor circuit that is connected between two output terminals X1and X2of the sub-module10.

FIGS. 4 to 8are graphs showing the pattern of a dark current depending on the operation time of a bypass switch unit according to an embodiment of the present invention.

FIGS. 4, 5, 6, 7, and 8illustrate a dark current pattern when the operating time of the bypass switch unit is 10 ms, 8 ms, 6 ms, 4 ms, and 2 ms, respectively. As shown inFIGS. 4 to 8, as the bypass operation time in the bypass switch unit130is shorter, the dark current comes near to a sine wave. This means that the voltage accumulated in the energy storage unit in the phase module comes close to a sine wave as the bypass operation time is shorter.

As described above, optimal embodiments of the present invention have been disclosed in the drawings and the specification. Although specific terms have been used in the present specification, these are merely intended to describe the present invention, and are not intended to limit the meanings thereof or the scope of the present invention described in the accompanying claims. Therefore, those skilled in the art will appreciate that various modifications and other equivalent embodiments are possible from the embodiments. Therefore, the technical scope of the present invention should be defined by the technical spirit of the claims.