KR102086530B1 - Method for controlling capacitor voltage of modular multilevel converter submodule and a recording medium having computer readable program for executing the method - Google Patents

Abstract

A method for controlling a capacitor voltage of modular multilevel converter submodules comprises the steps of: determining whether a capacitor voltage of submodules of a modular multilevel converter is within a preset allowable range; opening charge and discharge switches of all the submodules performing charge and discharge when the capacitor voltage is outside the allowable range; comparing the preset reference number of short-circuited switches with the number of switches being currently short-circuited of the charge and discharge switches of the submodules; and short-circuiting switches selected according to preset short circuit criteria corresponding to a difference between the preset reference number of short-circuited switches and the number of switches being currently short-circuited of the currently open charge and discharge switches of the submodules when the preset reference number of short-circuited switches is greater than the number of switches being currently short-circuited, and opening switches selected according to preset opening criteria corresponding to a difference between the preset reference number of short-circuited switches and the number of switches being currently short-circuited of the currently short-circuited charge and discharge switches of the submodules when the preset reference number of short-circuited switches is smaller than the number of switches being currently short-circuited. Therefore, the method can keep the power conversion efficiency of the converter high.

Description

METHOD FOR CONTROLLING CAPACITOR VOLTAGE OF MODULAR MULTILEVEL CONVERTER SUBMODULE AND A RECORDING MEDIUM HAVING COMPUTER READABLE PROGRAM FOR EXECUTING THE METHOD}

The present invention relates to a power conversion system, and more particularly, to a method for effectively adjusting the capacitor voltage of a modular multilevel converter submodule.

Recently, Modular Multilevel Converter (MMC) has been attracting attention in the field of high voltage and large capacity converters. Modular multi-level converters (MMCs) have a large number of sub-modules connected in series for easy voltage level expansion and high output voltages at low switching frequencies. It is used in power compensator (STATCOM) and motor drive applications.

1 is a circuit diagram of a half bridge converter for explaining a submodule of a modular multilevel converter (MMC). As shown in FIG. 1, the half bridge converter consists of two complementary operating IGBT switches and a capacitor for storing energy.

When the dark current direction is positive, when the switch S1 is turned on, the dark current flows through the reflux diode D1 to the capacitor so that the capacitor is charged. The capacitor voltage does not change because switch S2 bypasses the capacitor at turn-on.

On the contrary, when the dark current direction is negative, when the switch S1 is turned on, the dark current flows to the capacitor, and therefore the capacitor is discharged. There is no variation in capacitor voltage. Thus, when the switch S1 is turned on, the charge and discharge of the capacitor are determined according to the direction of the dark current.

2 is a circuit diagram for explaining the configuration of a modular multilevel converter (MMC). As shown in FIG. 2, the N submodules 11 and the female inductor 12 connected in series are connected to form an arm (A node-B node), and two arms are connected based on an AC output terminal to connect one leg. (A node-C node).

The female inductor 12 serves to prevent a sudden rise of the short circuit current in the event of a short circuit. Each submodule capacitor voltage has a size equal to N of the DC link voltage, and the voltage output from one arm is equal to the sum of the output voltages of each submodule constituting the arm.

In such a modular multilevel converter, it is very important to maintain voltage balancing of the submodule capacitors. This is because, when balancing is not maintained and a large voltage is applied to only some of the plurality of capacitors, the capacitor to which the overvoltage is applied may be damaged quickly, which may lead to the failure of the entire converter.

In order to prevent this, the control system of the converter adjusts the voltages of the capacitors of the submodules to perform voltage balancing of the capacitors, and for this purpose, the capacitor voltages are aligned.

3 is a schematic flowchart illustrating an example of a conventional capacitor voltage alignment method for voltage balancing of a submodule capacitor. The method illustrated in FIG. 3 is a full sorting method, in which voltages of all sub-module capacitors are continuously sorted by size, and switching of a switch that performs charging and charging of capacitors according to the sorted order is performed.

However, in the case of continuously switching the switches of the sub-modules, the power loss efficiency of the converter is drastically reduced due to the switching loss. In other words, in order to protect the submodule capacitor of the modular multilevel converter, voltage balancing of the capacitors should be maintained. However, in the case of frequently switching the submodule switch to maintain the voltage balancing of the capacitors, the efficiency of power conversion is reduced due to the switching loss. Falling problem will occur.

US 9473013 B2

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and maintains voltage balancing between modular multilevel converter submodule capacitors to protect expensive capacitors, while maintaining a high power conversion efficiency of the converter. It is an object of the present invention to provide a capacitor voltage adjusting method of a level converter submodule.

In order to achieve the above object, a capacitor voltage adjusting method of a submodule performed by a capacitor voltage adjusting system of a modular multilevel converter submodule according to the present invention may include a capacitor voltage of a submodule of a modular multilevel converter within a preset allowable range. Determining whether the capacitor voltage is outside the allowable range, opening the charge / discharge switch of all the sub-modules performing charging / discharging, the current switch short-circuit among the preset number of reference switch short circuits and the charge / discharge switch of the submodules. Comparing the number. And shorting the selected switches according to a preset short-circuit criterion by the difference between the reference switch short-circuit number and the current switch short-circuit number among the charge / discharge switches of the currently open submodule, if the number of short-circuit switches of the reference switch is greater than the current switch short-circuit number. If the number of short circuits is smaller than the number of current switch shorts, opening the selected switches according to a preset opening criterion by the difference of the reference switch short circuit number from the current switch short circuit number among the charge / discharge switches of the currently shorted submodule.

According to such a configuration, while setting the allowable range for the capacitor voltage to limit the magnitude of the voltage applied to the capacitor, the voltage adjustment is performed only for some capacitors corresponding to a predetermined condition, not the whole. As a result, it is possible to protect the expensive capacitors while reducing the overall number of switching to maintain a high power conversion efficiency of the converter.

In this case, the preset short-circuit reference may be a sequence in which the capacitor voltage is small when the dark current of the modular multilevel converter is a direction for charging the capacitor, and may be in a sequence in which the capacitor voltage is large when the dark current of the modular multilevel converter is a direction for discharging the capacitor. . In addition, the preset open reference may be a sequence in which the capacitor voltage is larger when the dark current of the modular multilevel converter is in the direction of charging the capacitor, and in a sequence in which the capacitor voltage is small when the dark current of the modular multilevel converter is in the direction of discharging the capacitor. .

According to such a configuration, when opening or shorting of a switch for charging and discharging of a capacitor is necessary, voltage balancing can be performed even within an allowable range by opening or shorting an appropriate switch in the dark current direction.

In addition, the allowable range may be set using the magnitude of the maximum value of the capacitor voltage, and in particular, may be set by further using the magnitude of the minimum value of the capacitor voltage.

In addition, it may be set using a difference between a maximum value of the capacitor voltage and a preset comparison value, wherein the comparison value may be an average value of the capacitor voltages.

In addition, the reference switch short circuit number may be a value calculated by the control system of the modular multilevel converter to control the output voltage of the modular multilevel converter.

According to the present invention, according to this configuration, while setting the allowable range for the capacitor voltage to limit the voltage applied to the capacitor, the voltage adjustment is performed only for some capacitors corresponding to a predetermined condition, not the whole. As a result, it is possible to protect the expensive capacitors while reducing the overall number of switching to maintain a high power conversion efficiency of the converter.

In addition, when opening or shorting the switch for charging and discharging the capacitor is necessary, by opening or shorting the appropriate switch according to the dark current direction, voltage balancing can be performed even within the allowable range.

1 is a circuit diagram of a half bridge converter for explaining a submodule of a modular multilevel converter (MMC).
2 is a circuit diagram for explaining the configuration of a modular multilevel converter (MMC).
3 is a schematic flowchart illustrating an example of a conventional capacitor voltage alignment method for voltage balancing of a submodule capacitor.
4 is a schematic flowchart for performing a capacitor voltage adjusting method of a modular multilevel converter submodule according to an embodiment of the present invention;
5 is a diagram illustrating an example in which the number of reference switch short circuits for power conversion in a converter is transmitted.
Figures 6 to 13 are diagrams showing the experiments implemented by implementing the conventional ATB method and the method of the present invention in an MMC HILS (Hardware In the Loop Simulation) system having 432 submodules per arm.
14 is a graph showing an average switching frequency according to% band for the ATB scheme and the scheme proposed by the present invention.

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

4 is a schematic flowchart of a method of adjusting a capacitor voltage of a modular multilevel converter submodule according to an embodiment of the present invention. In FIG. 4, capacitor voltage regulation may be performed by a capacitor voltage regulation system included in a modular multilevel converter control system. In this case, the capacitor voltage regulation system may be implemented only in hardware, or may be implemented together in hardware and software operating on the hardware.

In Figure 4, V _cap _max the Maximum Voltage of SM (Sub-Module ) Capacitors, V cap _avr is Average Voltage of SM Capacitors,% band is Percentage of Tolerance Band, N is a Number of SM, N _on the Number of SM for Switching on, N _{on_old} means Previous number of SM for Switching on, i _arm (t) means Current of Arm, respectively.

First, the capacitor voltage adjusting system of the modular multilevel converter submodule arranges the capacitor voltage of the submodule according to its size (S110), and determines whether the capacitor voltage is within a preset allowable range (S120).

In this case, the allowable range may be set using the magnitude of the maximum value of the capacitor voltage, and in particular, may be set by further using the magnitude of the minimum value of the capacitor voltage. For example, the capacitor voltage may be set in a range larger than a preset minimum value and smaller than a preset maximum value.

In addition, it may be set using a difference between a maximum value of the capacitor voltage and a preset comparison value, wherein the comparison value may be an average value of the capacitor voltages. That is, as in the example shown in FIG. 4, the allowable range may use a band using an average voltage, but may use a band having another specific value.

As a result of the determination, when the capacitor voltage is outside the allowable range, the charge / discharge switches of all the sub-modules performing charge / discharge are opened (S130), and the number of currently shorted switches is set to '0' (S140). Subsequently, the preset reference switch short circuit number and the current switch short circuit number of current short-circuit among the charge / discharge switches of the submodules are compared (S150), and it is determined which one is larger (S160).

Charge-discharge switch is a switch for manually performing the charging and inversion of the switches located in the sub-module, S means _one of the two switches S ₁ and S ₂ which also operates in the case of the complementary one.

In this case, the number of reference switch short circuits may be a value calculated by the control system of the modular multilevel converter to control the output voltage of the modular multilevel converter. 5 is a diagram illustrating an example in which the number of reference switch short circuits for power conversion in a converter is transmitted.

5 shows an example in which the control system of the modular multilevel converter calculates the number of reference switch short circuits and transmits them to the capacitor voltage adjusting system, and the capacitor voltage adjusting system generates the gating signals of the sub-module switches using the transferred information. It is.

More specifically, when a voltage reference for power conversion is delivered, a reference-switched number generation module (Phase-Shifted Carrier or NLC) of the control system of the converter generates a reference switch short-number (N _on ) and An example in which a selection algorithm of the capacitor voltage adjusting system received this selects a switch of a submodule to be shorted or opened to transmit a gating signal.

As a result of comparison, when the number of reference switch short-circuits is larger than the current switch short-circuit number, the selected switches are shorted according to a preset short-circuit criterion by the difference between the reference switch short-circuit number and the current switch short-circuit number among the charge / discharge switches of the currently open submodule. .

To this end, the direction of the dark current of the modular multi-level converter is determined (S170), when the dark current of the modular multi-level converter is a direction to charge the capacitor, the switch is short-circuited in the order of the capacitor voltage (S180), the modular multi-level converter If the dark current of the direction to discharge the capacitor short-circuit the switch in the order of the capacitor voltage is large (S190).

Also, as a result of comparison, when the number of reference switch short circuits is smaller than the number of current switch short circuits, the switches selected according to a preset opening criterion by the difference of the reference switch short circuit number from the current switch short circuit number among the charge / discharge switches of the currently shorted submodule. Open.

To this end, the dark current direction of the modular multilevel converter is determined (S200). When the dark current of the modular multilevel converter is a direction for charging the capacitor, the switches are opened in the order of the capacitor voltages being larger (S210). When the dark current is in the direction of discharging the capacitor, the switch is opened in the order of the capacitor voltage is small (S220).

Finally, as a result of the comparison, when the number of reference switch short circuits and the number of current switch short circuits are the same, the current switching state of the submodule is maintained (S230).

6 to 13 are diagrams illustrating waveforms implemented by implementing the conventional ATB scheme and the inventive scheme in an MMC HILS (Hardware In the Loop Simulation) system having 432 submodules per arm.

More specifically, FIGS. 6 and 7 show sub-module capacitor voltage waveforms according to the ATB method and the method of the present invention, respectively, when the tolerance band is 4%, and FIGS. 8 and 9 respectively show the cyclic current waveforms. The figure is shown.

10 and 11 show sub-module capacitor voltage waveforms according to the ATB method and the method of the present invention, respectively, when the tolerance band is 8%, and FIGS. 12 and 13 show the cyclic current waveforms, respectively. Drawing.

ATB (Average Tolerance) method is also proposed to improve the conventional Full Sorting Method, which sorts when the sub-module capacitor voltage is out of a specific band, switches using a new sorting list, and does not go out of a specific band. Otherwise, switching is done using the existing sorting list.

 The ATB method has the advantage of reducing the switching frequency while limiting the ripple of the capacitor voltage by setting a tolerance band, but using the existing sorting list when the capacitor voltage is in the allowable band, thus reflecting the current voltage value. As a result, the number of times that the capacitor voltage of the sub-module that is switched on reaches the allowable band is relatively increased.

6 to 13, it can be seen that the cyclic current ripple decreases in the case of the scheme proposed by the present invention. 14 is a graph showing an average switching frequency according to% band for the ATB scheme and the scheme proposed by the present invention. 14, in the scheme of the present invention, it can be seen that the switching frequency is reduced by approximately 20% around the conventional ATB scheme.

Conventional ATB technology does not update the sorting list when the capacitor voltage is in the allowable band, but the proposed scheme updates the sorting list even when it is in the allowable band to continue switching according to the current direction and switching state. By reducing the number of times, the average switching frequency is reduced and circulating current ripple and DC-link voltage ripple are reduced.

In summary, in the present invention, when the band is out of band, the capacitor voltage is switched from the smallest SM or the largest SM through sorting, and when the band is not out of the band, the capacitor voltage is not sorted. Compared to the existing ATB method of switching, the proposed method continuously sorts the capacitor voltage among SMs that should be turned on or off even when leaving the band, so that the smallest submodule (SM) or the largest submodule (SM) Switch from

Therefore, since voltage balancing is more evenly compared to the conventional method, the size of the circulating current and the DC-link ripple are reduced, and the switching frequency is reduced because the speed of reaching the band is relatively slow. In addition, the switching frequency can be reduced, thereby reducing the loss of the system.

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 construed as a modification or improvement of the above embodiments supported by the claims.

Claims (9)

A capacitor voltage adjusting method of a submodule performed by a capacitor voltage adjusting system of a modular multilevel converter submodule,
Determining whether the capacitor voltages of all individual submodules of the modular multilevel converter are within a preset allowable range;
Opening charge / discharge switches of all the sub-modules for performing charge / discharge when the capacitor voltage is outside the allowable range;
Comparing a preset reference switch short circuit number with a current switch short circuit number of charge / discharge switches of the sub-modules; And
When the reference switch short-circuit number is larger than the current switch short-circuit number, the switches selected according to a predetermined short-circuit criterion equal to the difference between the reference switch short-circuit number and the current switch short-circuit number among the charge / discharge switches of the submodule currently open. and,
When the reference switch short circuit number is smaller than the current switch short circuit number, the switches selected according to a predetermined opening criterion by the difference of the reference switch short circuit number from the current switch short circuit number among the charge / discharge switches of the submodule currently shorted are opened. Capacitor voltage adjustment method of the modular multi-level converter submodule, characterized in that it comprises a.
The method according to claim 1,
The predetermined short-circuit reference is the order of the capacitor voltage is small when the dark current of the modular multi-level converter is the direction to charge the capacitor, the capacitor voltage is the direction when the dark current of the modular multi-level converter is the direction to discharge the capacitor A capacitor voltage adjusting method of a modular multilevel converter submodule, characterized in that in large order.
The method according to claim 1,
The predetermined open reference may be the order of the capacitor voltage when the dark current of the modular multi-level converter is the direction to charge the capacitor, the capacitor voltage is the order when the dark current of the modular multi-level converter is the direction to discharge the capacitor A capacitor voltage adjusting method of a modular multilevel converter submodule, characterized in that the order is small.
The method according to claim 1,
And the allowable range is set using a magnitude of a preset maximum value.
The method according to claim 4,
And the allowable range is set by further using a preset minimum value.
A capacitor voltage adjusting method of a submodule performed by a capacitor voltage adjusting system of a modular multilevel converter submodule,
Determining whether a difference between a maximum value of the capacitor voltages of the sub-modules of the modular multilevel converter and a preset comparison value is within a preset allowable range;
Opening charge / discharge switches of all the sub-modules that perform charge / discharge when the difference between the maximum value and the comparison value is outside the allowable range;
Comparing a preset reference switch short circuit number with a current switch short circuit number of charge / discharge switches of the sub-modules; And
When the reference switch short-circuit number is larger than the current switch short-circuit number, the switches selected according to a predetermined short-circuit criterion equal to the difference between the reference switch short-circuit number and the current switch short-circuit number among the charge / discharge switches of the submodule currently open. and,
When the reference switch short circuit number is smaller than the current switch short circuit number, the switches selected according to a predetermined opening criterion by the difference of the reference switch short circuit number from the current switch short circuit number among the charge / discharge switches of the submodule currently shorted are opened. Capacitor voltage adjusting method of a modular multilevel converter submodule, characterized in that it comprises a step of.
The method according to claim 6,
And wherein the comparison value is an average value of the capacitor voltages.
The method according to claim 1,
And the reference switch short circuit number is a value calculated by a control system of the modular multilevel converter to control an output voltage of the modular multilevel converter.
A recording medium having recorded thereon a computer readable program for executing the method of any one of claims 1 to 8.

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