KR20160117974A - Apparatus for protecting Capacitor over-voltage of DC fault blocking modules for sub module - Google Patents

Abstract

Disclosed is a protection device of a submodule (SM) against overvoltage on a capacitor to decrease a DC fault current, which is provided inside an arm of a modular multi-level converter and bypass a fault current when a DC fault of the modular multi-level converter occurs. The protection device comprises: a bypass circuit protecting an converter arm wherein submodules provided in a modular multi-level converter are connected in series by making a fault current of the converter arm bypass, and connected in series to the converter arm; a diode installed in the bypass circuit to make the fault current bypass; capacitors installed in the bypass circuit, protecting the submodules from damage caused by the fault current by storing/absorbing the fault current, and connected in series to the diode; and a resistance installed in the bypass circuit, suppressing overvoltage and voltage imbalance occurring in each phase of the capacitors of the submodules and in each arm, and connected in series to the capacitors.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a capacitor overvoltage protection device,

The present invention relates to a submodule (SM) for reducing DC fault current in an arm of a modular multi-level converter, and more particularly to a capacitor overvoltage protection device of a submodule (SM) for reducing DC fault current .

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

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

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

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

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

2 is a circuit diagram showing a sub-module SM included in an arm of a general modular multi-level converter.

As shown in Fig. 2, each sub-module (SM) 5 includes a pair of insulated gate bipolar transistors (IGBTs) 6, a pair of diodes 7, and a capacitor 8 can do. Here, the submodule is switched by the ON / OFF operation of the IGBT 6.

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

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

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

SUMMARY OF THE INVENTION Accordingly, the present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a modular multi-level converter which is constructed inside an arm of a modular multi- It is an object of the present invention to provide a capacitor overvoltage protection device of a submodule (SM).

Other objects of the present invention are not limited to the above-mentioned objects, and other objects not mentioned can be clearly understood by those skilled in the art from the following description.

According to another aspect of the present invention, there is provided an overvoltage protection apparatus for a DC sub-module for blocking a DC fault, the apparatus comprising: A bypass circuit connected in series with the converter arm for protecting the arm; a diode installed in the bypass circuit so as to bypass the fault current; and a diode installed in the bypass circuit for storing and absorbing the fault current, A capacitor connected in series with the diode and a bypass circuit provided in the bypass circuit to suppress overvoltage and voltage imbalance generated for each phase and each capacitor in each of the capacitors of the submodule, And a resistance to be applied.

Preferably, the modular multilevel converter further includes a switching element connected in parallel to the bypass circuit to prevent a fault current from flowing into the submodule when a fault current is generated, and a diode connected in parallel to the switching element .

Preferably, the diode installed in the bypass circuit is provided so as to flow a current in a reverse direction on the diode and the bypass circuit connected in parallel to the switching element.

Preferably, the switching device comprises at least one IGBT or a thyristor.

According to another aspect of the present invention, there is provided a capacitor overvoltage protection device for a DC module for blocking a DC fault, comprising: a modular multilevel converter including a plurality of converter arms to which a submodule is connected in series; A diode for bypassing a fault current due to a DC accident in case of a DC accident of the converter, a capacitor for storing a fault current, and a resistor for suppressing overvoltage and voltage imbalance generated by each phase and arm of each capacitor of the sub- A capacitor overvoltage protection module of a submodule constituting a bypass circuit that is connected to the converter arm and provides a bypass path for the fault current of the converter arm; an accident detection unit for detecting a fault current or voltage of the modular multilevel converter; The fault current flows into the bypass circuit There comprises a control unit that controls so as to.

Preferably, the accident detection unit is constituted by a current detection sensor for detecting a fault current generated in the modular multilevel converter. At least one fault detection unit is installed in the arm of the modular multilevel converter, or at least one DC transmission line is installed .

Preferably, the accident detection unit comprises a voltage sensing device for sensing a fault current by measuring a voltage across the IGBT or a voltage across the switch.

Preferably, when the fault current is detected, the controller switches the switching element connected to the submodule from an On state to an Off state.

The capacitor overvoltage protection device of the submodule for preventing the DC accident according to the present invention as described above has the following effects.

First, the fault current can be lowered by inverting the fault current in the modular multilevel converter and by inserting the capacitor in the bypass path of the fault current.

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

Third, it is possible to reduce the amount of IGBTs and diodes to be used, compared with a conventional full-bridge type module having a DC fault current reduction function, and the switching losses consumed in IGBTs, etc., It has a decreasing effect.

Fourth, since there is a problem that the capacitor inserted into the sub-module SM generates an unbalance and an over-voltage for each phase and arm, a resistance is inserted in series with the capacitor in the sub-module SM to catch overvoltage and voltage imbalance of the capacitor. There is an effect that can be.

Fifth, since the capacitance can be reduced by inserting the resistor in series with the capacitor in the sub-module (SM), it is advantageous from the economical point of view.

1 is a diagram for explaining the structure of a general modular multi-level converter;
Fig. 2 is a circuit diagram showing a sub-module SM included in an arm of a general modular multi-level converter
3 is a circuit diagram showing a capacitor overvoltage protection device of a submodule for blocking a DC accident according to an embodiment of the present invention.
4 is a circuit diagram illustrating an arm in which a capacitor over-voltage protection device is connected in series to the modular multi-level converter and the modular multi-level converter according to an embodiment of the present invention.
5 and 6 are block diagrams showing a configuration of an overall system of a modular multi-level converter having a capacitor overvoltage protection device of a submodule for DC accident prevention according to an embodiment of the present invention

Other objects, features and advantages of the present invention will become apparent from the detailed description of the embodiments with reference to the accompanying drawings.

A preferred embodiment of a capacitor overvoltage protection device of a submodule for preventing a DC accident according to the present invention will be described with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. It is provided to let you know. Therefore, the embodiments described in the present specification and the configurations shown in the drawings are merely the most preferred embodiments of the present invention and are not intended to represent all of the technical ideas of the present invention. Therefore, various equivalents It should be understood that water and variations may be present.

3 is a circuit diagram showing a capacitor overvoltage protection device of a submodule for blocking a DC accident according to an embodiment of the present invention.

At this time, the capacitor overvoltage protection device of the submodule is connected in series with the sub-module SM, and the modular multi-level converter includes a plurality of arms of the modular multi-level converter connected in series, will be. The sub-module may be a full-bridge type or a half-bridge type.

As shown in FIG. 3, the capacitor overvoltage protection device of the submodule includes a normal circuit including one switching device and a normal circuit in which one diode 12, one capacitor 13 and one resistor 14 are connected in series And a bypass circuit of the fault current of the converter arm can be provided.

Specifically, the diode 12, the capacitor 13, and the resistor are connected to each other in series so that the capacitor 13 stores and absorbs the fault current, thereby reducing the damage of the sub-module due to the fault current.

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

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

Also, the resistor 14 installed in the bypass circuit can suppress the overvoltage and voltage imbalance generated for each phase and arm in the respective capacitors 8 of the submodule, and reduce the generated capacitance.

The normal circuit may further include a switching device 14 composed of an IGBT connected to the submodule and a diode 16 connected in parallel to the switching device 15. [

For example, when a steady current flows, the switching element 15 is in an On state, thereby forming a current path through each sub-module. In addition, when the fault current flows, the switching element 15 is in an off state, thereby forming a current path that flows to the bypass circuit.

Here, although the switching element 15 is composed of an IGBT, it will be apparent to those skilled in the art that the switching element 15 may be a switching element other than the IGBT element. For example, the switching element 15 may be a Fast Switch. In addition, the switching device 15 may be a thyristor. Further, the switching element 15 may be composed of a plurality of IGBTs.

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

The sub module (SM) connected to the capacitor overvoltage protection device of the sub module through the capacitor overvoltage protection device of this sub module can be initialized by the natural discharge circuit, so that all circuits are safely protected even in case of DC accident, It is possible to prevent damage to the capacitor overvoltage protection device of the module.

In a modular multilevel converter without a capacitor overvoltage protector in the submodule, excessive current flows continuously in the event of a DC fault. Therefore, all the arms included in the modular multi-level converter must be damaged and must replace the entire modular multi-level converter.

However, by connecting the capacitor overvoltage protector of the submodule of the modular multilevel converter in series with the modular multilevel converter, the cost of a modular multilevel converter can be reduced.

4 is a circuit diagram showing an arm in which a modular multi-level converter and a capacitor overvoltage protection device are connected in series to the modular multi-level converter according to an embodiment of the present invention.

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

Specifically, the larger the voltage difference between both ends, the larger the current flows. When the upper arm and lower arm of the DC side are grounded, the DC side voltage (VDC) becomes 0 (zero). At this time, there is a large difference between the AC side voltage and the DC side voltage, so that excessive current flows.

In addition, a freewheeling current path is formed due to the voltage formed by the submodule of the upper arm and the lower arm, thereby causing an excessive current to flow.

The capacitor overvoltage protection device 23 of the submodule according to an embodiment of the present invention absorbs a large current like a shield in front of or behind the modular multilevel converter when a large current flows when a DC accident occurs. This role of absorbing is made by the capacitor 13 and the capacitor 13 can be arranged to be different from the capacitor 8 of the submodule SM since it has to include that having a power capacity enough to absorb the fault current .

Thus, by adding a capacitor 13 to the fault current path, the potential difference between the AC and DC connection points is rapidly reduced to prevent the DC fault current from flowing. That is, it is possible to solve this problem by using a submodule of a type called a full-bridge in the past, but in this case, a loss of more than 30% of the existing loss is increased compared to the half-bridge submodule I bring it. However, by adding the capacitor 13 to the fault current path as in the present invention, the same effect can be obtained with a loss increase of less than 10%. It also has a 25% reduction in the amount of IGBTs and diodes used compared to full bridge.

However, in such a configuration, overvoltage and voltage imbalance occur in the capacitor 8 in the blocking module in each phase and arm. In the present invention, the side effect is suppressed by inserting a resistor in the bypass circuit.

5 is a block diagram showing the configuration of an overall system of a modular multilevel converter having a capacitor overvoltage protection device of a submodule for blocking a DC accident according to an embodiment of the present invention.

As shown in FIG. 5, the capacitor overvoltage protection device includes a plurality of converter arms to which a submodule SM (not shown) is connected in series, and includes a diode 12 for bypassing the fault current, A resistor 14 for suppressing overvoltage and voltage unbalance generated for each phase and each arm of each capacitor 8 of the submodule is connected in series to provide a bypass path for the fault current of the converter arm (110) for detecting a fault current of the modular multilevel converter (110); and a fault current detector (120) for detecting a fault current of the modular multilevel converter (110) And a controller 130 for controlling the fault current to flow.

At this time, the modular multi-level converter 110 includes a plurality of converter arms to which submodules are connected in series. In addition, the converter arm may further include a switching element and a diode connected to the submodule for interrupting the fault current.

Here, a bypass circuit for providing a bypass path for the fault current of the converter arm, and a diode, a capacitor, and a resistor provided in the bypass circuit for bypassing the fault current have the structure described above with reference to FIG. 3 The configuration of the capacitor overvoltage protection device of the submodule according to the embodiment is the same as that of the capacitor overvoltage protection device, and a detailed description thereof will be omitted.

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

The fault current detection unit 120 may be provided in the arm of the modular multilevel converter, or may be provided in a plurality of DC transmission lines.

6, the controller 130 may recognize the fault current through the voltage sensing device 140 instead of the fault current sensing unit 120 to operate the fault current reduction structure.

That is, when the current flows through the IGBT in the steady state, the voltage across both ends becomes O (zero), but when the fault current flows, the current flows, but the voltage across both ends does not become zero . Therefore, it is possible to detect the fault current by measuring the voltage across the IGBT. In addition, the voltage sensing device 140 may sense a fault current by measuring a voltage across the switch.

When the fault current is detected, the controller 130 switches the switching element connected to the sub-module from the On state to the Off state.

Thereby, the fault current flows along the bypass circuit without passing through the switching elements of the respective submodules of the modular multilevel converter, and the switching elements of each submodule can be protected. In addition, the potential difference between AC and DC is eliminated by the capacitor 13 in each bypass path, so that the fault current can be reduced to zero, and the diode and the capacitor of each submodule can be protected.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. It will be apparent to those skilled in the art that various modifications may be made without departing from the scope of the present invention. Accordingly, the true scope of the present invention should be determined by the technical idea of the appended claims.

Claims (8)

A bypass circuit connected in series with the converter arm to protect the converter arm by bypassing a fault current of a converter arm connected in series with the submodule constituted in the modular multilevel converter,
A capacitor connected in series with the diode, and a capacitor connected in series with the diode, the diode being installed in the bypass circuit so as to bypass the fault current; a capacitor installed in the bypass circuit to prevent damage to the sub- And a resistor connected in series with the capacitor for suppressing an overvoltage and a voltage unbalance generated in each of the capacitors of the submodule, Overvoltage protection of module capacitor. 2. The apparatus of claim 1, wherein the modular multi-level converter
Further comprising a switching element connected in parallel with the bypass circuit to prevent a fault current from flowing into the sub-module when a fault current is generated, and a diode connected in parallel to the switching element. Overvoltage protection equipment. 3. The method of claim 2,
The diode installed in the bypass circuit may include:
And a diode connected in parallel with the switching device and a bypass current flowing in a reverse direction on the bypass circuit. The method according to claim 1,
Wherein the switching element is composed of at least one IGBT or a thyristor. ≪ RTI ID = 0.0 > [10] < / RTI > A modular multilevel converter including a plurality of converter arms to which the submodules are connected in series,
A diode for bypassing a fault current due to a DC accident when a DC accident occurs in the modular multilevel converter, a capacitor for storing a fault current, and an overvoltage and voltage imbalance generated for each phase and arm of each capacitor of the submodule A capacitor overvoltage protection module of the submodule constituting a bypass circuit connected in series to provide a bypass path for the fault current of the converter arm,
An accident detection unit for detecting a fault current or voltage of the modular multilevel converter;
And a controller for controlling the fault current to flow to the bypass circuit when the fault current is sensed. 6. The method of claim 5,
The abnormality sensing unit may include a current sensing sensor for sensing a fault current generated in the modular multilevel converter. At least one or more of the fault sensing unit may be installed in the arm of the modular multilevel converter, or at least one DC transmission line may be provided. Capacitor overvoltage protection device of submodule for DC accident prevention. 6. The method of claim 5,
Wherein the accident detection unit comprises a voltage sensing device for sensing a fault current through a voltage across the IGBT or a voltage across the switch. 6. The method of claim 5,
Wherein the controller switches the switching element connected to the submodule from an On state to an Off state when a fault current is sensed.

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