KR102220587B1 - Apparatus for eliminating harmonics of hvdc system and method thereof - Google Patents

Abstract

The present invention relates to an apparatus for removing harmonics of an HVDC system, including a plurality of sub-modules connected in series with each other, and converting AC power to DC power or DC power to AC power and outputting a multi-module converter. , MMC), a control unit for controlling on and off of a plurality of sub-modules based on a reference voltage to be output, and a harmonic removing unit for calculating a control angle for removing harmonics generated as a result of the on and off control, the control unit Is characterized in that switching the plurality of sub-modules based on the control angle calculated through the harmonic removal unit.

Description

Harmonic removal device of HVDC system and its method {APPARATUS FOR ELIMINATING HARMONICS OF HVDC SYSTEM AND METHOD THEREOF}

The present invention relates to an apparatus and method for removing harmonics of an HVDC system, and more particularly, to remove harmonics generated in a multi-module converter (MMC) for transmission and distribution of high-voltage direct current power without a filter. It relates to an apparatus and method for removing harmonics of an HVDC system.

The HVDC (High Voltage Direct Current) system converts AC power generated by an AC generator into DC power through a rectifier, an AC/DC converter, and transmits DC power to the target area for distribution, and then a DC/AC converter. It is a method of supplying power by converting it into AC power through an inverter.

If DC power distribution is used, the size of various electronic products is drastically reduced, all transformers required for various AC devices can be removed, and motors capable of various speeds can also use DC electricity.

The HVDC system is divided into a current type HVDC system using a thyristor valve and a voltage type HVDC system using an IGBT (Insulated Gate Bipolar mode Transistor) element.

In the current HVDC system, a rotating device such as a generator or a synchronous controller is required in the inverter-side system to rectify the thyristor valve, and a capacitor bank for compensation of reactive power must be present on the inverter side or the rectifier side. In particular, since the current-type HVDC system generates harmonics, a harmonic filter to remove them is indispensable.

In order to compensate for the shortcomings of such a current type HVDC system, a voltage type HVDC system using a semiconductor device for IGBT power is used. Compared with the current type HVDC system, the voltage type HVDC system gradually decreases the harmonics by high speed switching, so the size of the harmonic filter can be relatively small.It does not require reactive power supply, as well as active power and invalidity. It has the advantage that power control is possible independently.

In particular, the modularized and standardized design enables power transmission in a short period of time, and it is easy to control voltage and power.

In general, a voltage-type HVDC system is configured to connect two AC buses in the power system to DC, and converts the rectifier as an AC/DC conversion facility, an inverter as a DC/AC conversion facility, and a DC transmission line that connects the rectifier and the inverter. It consists of a transformer.

Specifically, the rectifier side and the inverter side have a symmetrical shape, and each of the rectifier side controller and the inverter side controller can individually control active power/reactive power.

1 is an internal circuit diagram of a voltage-type multi-module converter.

As shown in FIG. 1, the voltage-type multi-module converter is configured by connecting and connecting a plurality of sub-modules in series in order to increase the rated power, and each sub-module is configured in a half-bridge or full-bridge form.

As a prior art related to the present invention, there is Korean Patent Publication No. 2014-0041100 (published on April 4, 2014, title of the invention: a method for suppressing non-characteristic harmonics using an observer PLL of an HVDC system).

In a voltage-type multi-module converter configured to increase the voltage and power rating by connecting a number of submodules in series, when the number of submodules connected in series is 50 or more, the amount of low-order harmonics generated during operation is reduced. Since it is not only within the allowable value, but also has a very small size, the need for a filter to remove it disappears.

However, when the number of submodules connected in series is small, low-order harmonics are generated, and there is a high possibility that the size of the harmonics exceeds the allowable value.

In addition, using a filter to remove such harmonics increases the cost of the entire HVDC system, and there is a problem that additional maintenance costs may be incurred due to the lifetime problem of the capacitor.

The present invention was invented to meet the above-described necessity, and to remove harmonics generated in a multi-module converter (MMC) for transmission and distribution of high-voltage direct current power with minimal switching without a separate filter. An object of the present invention is to provide an apparatus and method for removing harmonics of an HVDC system.

An apparatus for removing harmonics of an HVDC system according to an aspect of the present invention includes a plurality of sub-modules connected in series with each other, and converts AC power into DC power or converts DC power into AC power and outputs a multi-module converter. Converter, MMC); A controller that controls on and off of the plurality of sub-modules based on a reference voltage to be output; And a harmonic removal unit that calculates a control angle for removing harmonics generated as a result of the on and off control, wherein the control unit switches the plurality of sub-modules based on a control angle calculated through the harmonic removal unit. It is done.

In the present invention, the control unit controls on and off of the plurality of sub-modules using a Nearest Level Control (NLC) technique.

In the present invention, the harmonic removal unit Fourier transform unit for Fourier series conversion of power waveforms having a plurality of levels output from the multi-module converter; And a control angle calculator configured to calculate a control angle for removing harmonics with respect to the Fourier series transformed power waveform.

In the present invention, the control angle calculation unit is characterized in that it calculates a number of control angles corresponding to a plurality of levels output from the multi-module converter.

In the present invention, when the number of sub-modules included in the multi-module converter is less than or equal to a preset reference value, the harmonic removing unit calculates a control angle for removing the harmonics.

A method for removing harmonics of an HVDC system according to an embodiment of the present invention includes the steps of: receiving AC power or DC power by a multi-module converter including a plurality of sub-modules connected in series with each other; Controlling, by a controller, on and off of the plurality of sub-modules based on a reference voltage to be output in response to the AC power or DC power; Calculating, by a harmonic removing unit, a control angle for removing harmonics generated as a result of the on/off control; And switching, by the control unit, the plurality of sub-modules based on a control angle calculated through the harmonic removing unit.

In the step of controlling on and off of the plurality of sub-modules of the present invention, the control unit controls on and off of the plurality of sub-modules using a Nearest Level Control (NLC) technique.

In the present invention, the calculating of a control angle for removing the harmonic may include: a Fourier transform unit converting a power supply waveform having a plurality of levels output from the multi-module converter to a Fourier series; And calculating, by a control angle calculator, a control angle for removing harmonics from the Fourier series-transformed power waveform.

In the step of calculating a control angle for removing the harmonics of the present invention, the control angle calculator calculates a number of control angles corresponding to a plurality of levels output from the multi-module converter.

In the present invention, calculating the control angle for removing the harmonic is performed when the number of sub-modules included in the multi-module converter is less than or equal to a preset reference value.

According to the present invention, harmonics generated in a multi-module converter can be effectively removed with minimal switching for a plurality of sub-modules.

In addition, since the present invention does not need to install a separate filter for removing harmonics as in the prior art, it is possible to save the cost of building an HVDC system.

1 is an internal circuit diagram of a voltage-type multi-module converter.
2 is a functional block diagram of an apparatus for removing harmonics of an HVDC system according to an embodiment of the present invention.
3 is a graph showing power waveforms having a plurality of levels output from a multi-module converter of a harmonic removal device of an HVDC system according to an embodiment of the present invention.
4 is a graph showing a switching control for removing harmonics based on a control angle calculated through a harmonic removal unit of a harmonic removal device of an HVDC system according to an embodiment of the present invention.
5 is a diagram showing the AC system voltage of the HVDC system when only the conventional modulation technique is applied.
6 is a diagram illustrating an AC system voltage to which a harmonic removal technique is applied of a harmonic removal device of an HVDC system according to an embodiment of the present invention.
7 is a flowchart illustrating an implementation process of a method for removing harmonics in an HVDC system according to an embodiment of the present invention.

Hereinafter, an apparatus and method for removing harmonics of an HVDC system according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. In this process, the thickness of the lines or the size of components shown in the drawings may be exaggerated for clarity and convenience of description. In addition, terms to be described later are terms defined in consideration of functions in the present invention, which may vary according to the intention or custom of users or operators. Therefore, definitions of these terms should be made based on the contents throughout the present specification.

2 is a functional block diagram of an apparatus for removing harmonics of an HVDC system according to an embodiment of the present invention.

Referring to FIG. 2, an apparatus for removing harmonics of an HVDC system according to an embodiment of the present invention includes a multi-module converter 100, a control unit 200, and a harmonic removing unit 300.

The multi-module converter (MMC) 100 includes a plurality of sub-modules 10 connected in series to each other, and converts AC power into DC power or DC power into AC power and outputs it.

That is, in a high voltage direct current (HVDC) system, since the converter side that converts AC to DC and the converter side that converts DC to AC have a symmetrical shape, the multi-module converter 100 of the harmonic removal device according to the present embodiment May include any of a converter that converts AC power to DC power or a converter that converts DC power to AC power.

In the present embodiment, an example of removing harmonics generated when DC power is converted into AC power and output will be described.

The controller 200 controls on and off of the plurality of sub-modules 10 based on the reference voltage to be output.

Specifically, the controller 200 controls on and off of the plurality of sub-modules 10 using a Nearest Level Control (NLC) technique.

The NLC technique is a modulation technique applied to the voltage-type multi-module converter 100 composed of a plurality of sub-modules 10, and is turned on based on _{a sinusoidal reference voltage (V ref) to be generated when an AC voltage is to be generated.} This is a technique to follow the sine wave reference voltage according to the number of the sub-modules 10 to be turned on by determining the number of sub-modules 10 to be turned on or bypassed.

That is, the controller 200 sets power waveforms having a plurality of levels to follow the reference voltage, and controls on and off of the plurality of sub-modules 10 to output voltages corresponding to the corresponding power waveforms.

In this case, the level of the power waveform may be set equal to the number of sub-modules 10.

_{Assuming that the voltage (V dc} /N) output from each sub-module 10 in the multi-module converter 100 including N sub-modules 10 is constant, The converter arm (not shown) may generate a discrete output voltage having _{N+1 (0, V dc} /N, 2V _dc /N, ..., V _{dc) levels.}

Specifically, the number of sub-modules 10 to be turned on or bypassed is calculated as in Equation 1 below.

(At this time, N _on,u means the number of submodules to be turned on from the upper arm of the converter, N _on,l means the number of submodules to be turned on from the lower arm of the converter, and round is a rounding function. Means)

Accordingly, when the sub-module 10 is controlled to be turned on or off with the value determined through Equation 1, the average value of the output voltage follows _{the reference voltage V ref of the sine wave.}

3 is a graph showing power waveforms having a plurality of levels output from a multi-module converter of a harmonic removal device of an HVDC system according to an embodiment of the present invention.

Referring to FIG. 3, the controller 200 determines a power waveform having a plurality of levels to follow a reference voltage, and turns on and off the plurality of sub-modules 10, respectively, to output voltages corresponding to the corresponding power waveform. .

Since this NLC modulation technique is a technique of following the reference voltage of the sine wave by adjusting the turn-on or the bypass of the sub-module 10, it is possible to accurately follow the reference voltage of the sine wave as the number of converters of the sub-module 10 increases. .

That is, as the number of sub-modules 10 increases, the spacing between a plurality of levels output from each sub-module 10 decreases, so that the output AC voltage approaches a sine wave, and the switching frequency decreases, so that loss may be reduced. As the number of modules 10 increases, the multi-module converter 100 can output a voltage that accurately follows the reference voltage of the sine wave without harmonics.

However, the multi-module converter 100 having a small number of sub-modules 10, for example, the multi-module converter 100 including less than 20 sub-modules 10, generates low-order harmonics within 20 orders of magnitude during operation. In this case, the magnitude of the harmonic exceeds the allowable value.

Accordingly, in the present embodiment, the harmonic removal unit 300 calculates a control angle for removing harmonics generated as a result of on/off control of the submodule 10 of the control unit 200.

In particular, the harmonic removal unit 300 calculates a control angle for removing harmonics when the number of sub-modules 10 included in the multi-module converter 100 is less than or equal to a preset reference value.

In this case, the reference value refers to the number of sub-modules 10 capable of generating harmonics exceeding the allowable harmonics of the HVDC system, and may be freely set according to theoretical values or experimental experience values.

That is, in this embodiment, the control angle for removing the harmonics of the power supply waveform having a plurality of levels modulated by the NLC technique is calculated so that the harmonics can be selected and removed with minimal switching.

In this case, the harmonic removal unit 300 removes the Fourier transform unit 320 for converting a power waveform having a plurality of levels output from the multi-module converter 100 to a Fourier series and a harmonic to the power waveform converted from the Fourier series. It includes a control angle calculation unit 340 for calculating a control angle for.

Specifically, looking at the case where the Fourier transform unit 320 performs Fourier series conversion of the power waveform having a plurality of levels shown in FIG. 4, the corresponding waveform does not have a dc component and is an odd function. The corresponding voltage is calculated as in Equation 2 below.

In this case, the coefficient b _n may be calculated through Equation 3 below.

(In this case, D means _{V dc} , the voltage of the DC transmission line)

Since the method of calculating the Fourier series is a known technique, a more detailed description of the operation method will be omitted.

In this case, the Fourier coefficient b ₁ is a coefficient that controls the magnitude of the fundamental wave, and the magnitude becomes a modulation index (M).

And, in order to remove a specific harmonic, the coefficient b _k = 0 at the order k must be satisfied.

Based on this, the control angle calculator 340 calculates a control angle for removing harmonics through Equation 4 below.

(N ₀ = 0, α _i means the control angle that satisfies the modulation index and harmonic removal)

In this case, the range of the control angle and the modulation index for the switching transition should satisfy Equation 5 below.

Accordingly, the control angle calculation unit 340 calculates the control angle of Equation 4 based on the number of the sub-modules 10 to be turned on determined through Equation (1).

Specifically, the control angle calculator 340 calculates a number of control angles corresponding to a plurality of levels output from the multi-module converter 100.

That is, since the number of levels of voltage output from the multi-module converter 100 varies according to the number of sub-modules 10, each control operation unit 340 corresponds to a plurality of levels output from the multi-module converter 100. Calculate the number of control angles.

Specifically, the control angle calculator 340 calculates a control angle corresponding to twice the number of levels (the number of sub-modules) output from the multi-module converter 100.

Accordingly, as shown in FIG. 3, the control angle calculation unit 340 calculates eight control angles for the power waveform having four levels.

In addition, the control unit 200 may remove harmonics by switching the plurality of sub-modules 10 based on the control angle calculated by the harmonic removal unit 300.

4 is a graph showing a switching control for removing harmonics based on a control angle calculated through a harmonic removal unit of a harmonic removal device of an HVDC system according to an embodiment of the present invention.

As described above, the control unit 200 _{switched the plurality of sub-modules 10 based on the eight control angles corresponding to α 1} to α ₈ , and accordingly, it is confirmed that the output voltage is formed as shown in FIG. 4. I can.

In particular, in the case of a balanced load, a harmonic corresponding to a multiple of 3 due to the periodicity of 120° and a harmonic corresponding to a multiple of 2 due to the symmetry of 180° do not appear in the switching of the three-phase multi-module converter 100.

Therefore, in this embodiment, 6n±1st harmonics that do not correspond to both a multiple of 2 and a multiple of 3 are removed.

5 is a diagram showing the AC system voltage of the HVDC system when only the conventional modulation technique is applied.

6 is a diagram illustrating an AC system voltage to which a harmonic removal technique is applied of a harmonic removal device of an HVDC system according to an embodiment of the present invention.

5A shows the phase voltage of the AC system voltage of the HVDC system when only the conventional modulation technique is applied, and (B) shows the line voltage.

6A shows the phase voltage of the AC system voltage to which the harmonic removal technique of the harmonic removal device of the HVDC system according to an embodiment of the present invention is applied, and (B) shows the line voltage.

Referring to FIG. 5, when a conventional modulation technique is applied, it can be seen that the size of a harmonic corresponding to 6n±1st order is high. As shown in FIG. 6, the harmonic removal technique according to the present embodiment is applied. It can be seen that the harmonics corresponding to the ±1st order have been removed and the size of the harmonics is only 0.2% at the maximum.

As described above, it can be seen that the low-order harmonics are effectively removed through minimal switching at the optimum control angle by the harmonic removal technique according to the present embodiment.

7 is a flowchart illustrating an implementation process of a method for removing harmonics in an HVDC system according to an embodiment of the present invention.

Referring to FIG. 7, looking at the implementation process of the method for removing harmonics of the HVDC system according to an embodiment of the present invention, first, a multi-module converter 100 including a plurality of sub-modules 10 connected in series with each other is exchanged. Receives power or DC power (S10).

That is, in a high voltage direct current (HVDC) system, since the converter side that converts AC to DC and the converter side that converts DC to AC have a symmetrical shape, the multi-module converter 100 of the harmonic removal device according to the present embodiment May include any of a converter that converts AC power to DC power or a converter that converts DC power to AC power.

In the present embodiment, an example of removing harmonics generated when DC power is converted into AC power and output will be described.

Subsequently, the controller 200 controls on and off of the plurality of sub-modules 10 based on the reference voltage to be output (S20).

Specifically, the controller 200 controls on and off of the plurality of sub-modules 10 using a Nearest Level Control (NLC) technique. That is, the controller 200 sets power waveforms having a plurality of levels to follow the reference voltage, and controls on and off of the plurality of sub-modules 10 to output voltages corresponding to the corresponding power waveforms.

Further, the Fourier transform unit 320 of the harmonic removal unit 300 converts the power waveform having a plurality of levels output from the multi-module converter 100 to a Fourier series (S30), and the control angle calculator 340 converts the Fourier series. A control angle for removing harmonics to the generated power waveform is calculated (S40).

In particular, the control angle calculation unit 340 calculates a control angle for removing harmonics when the number of sub-modules 10 included in the multi-module converter 100 is less than or equal to a preset reference value.

That is, in the present embodiment, by performing the above-described steps (S30 and S40) only when the number of sub-modules 10 is small enough to generate harmonics that are out of tolerance, it is possible to output the converted power without unnecessary control process. .

Next, the control unit 200 switches the plurality of sub-modules 10 based on the control angle calculated by the harmonic removal unit 300 (S50) to reduce the low-order harmonics included in the output voltage of the multi-module converter 100. Can be removed.

According to the present embodiment, it is possible to effectively remove harmonics generated in a multi-module converter with minimal switching for a plurality of sub-modules.

In addition, in the present embodiment, since there is no need to install a separate filter for removing harmonics as in the prior art, the cost of constructing an HVDC system can be saved.

Although the present invention has been described with reference to the embodiments shown in the drawings, this is only exemplary, and those of ordinary skill in the art to which the present technology pertains, various modifications and other equivalent embodiments are possible. I will understand. Therefore, the true technical protection scope of the present invention should be determined by the following claims.

10: sub module
100: multi-module converter
200: control unit
300: harmonic removal unit
320: Fourier transform unit
340: control angle operation unit

Claims (10)

A multi-module converter (MMC) converting AC power to DC power or converting DC power to AC power and outputting a plurality of sub-modules connected in series with each other;
A controller that controls on and off of the plurality of sub-modules based on a reference voltage to be output; And
A harmonic removal unit for calculating a control angle for removing a harmonic generated as a result of the on and off control, wherein the control unit switches the plurality of sub-modules based on a control angle calculated through the harmonic removal unit,
The harmonic removing unit calculates a control angle for removing the harmonic when the number of sub-modules included in the multi-module converter is less than or equal to a preset reference value,
The harmonic removing unit calculates a control angle corresponding to twice the number of sub-modules output from the multi-module converter. The method of claim 1,
The control unit controls on and off of the plurality of sub-modules using a Nearest Level Control (NLC) technique. The method of claim 2,
The harmonic removing unit may include a Fourier transform unit for converting a power waveform having a plurality of levels output from the multi-module converter to a Fourier series; And
A control angle calculator that calculates a control angle for removing harmonics from the Fourier series-converted power waveform
Harmonic removal device of the HVDC system comprising a. The method of claim 3,
The control angle calculation unit calculates a number of control angles corresponding to a plurality of levels output from the multi-module converter. delete Receiving an AC power or DC power by a multi-module converter including a plurality of sub-modules connected in series with each other;
Controlling, by a controller, on and off of the plurality of sub-modules based on a reference voltage to be output in response to the AC power or DC power;
Calculating, by a harmonic removing unit, a control angle for removing harmonics generated as a result of the on/off control; And
Including the step of switching the plurality of sub-modules based on a control angle calculated by the control unit through the harmonic removal unit,
The step of calculating the control angle for removing the harmonics is performed when the number of sub-modules included in the multi-module converter is less than or equal to a preset reference value,
The harmonic removing unit calculates a control angle corresponding to twice the number of sub-modules output from the multi-module converter. The method of claim 6,
In the step of controlling on and off of the plurality of sub-modules, the control unit controls on and off of the plurality of sub-modules using a Nearest Level Control (NLC) technique. How to remove. The method of claim 7,
The calculating of the control angle for removing the harmonic may include: a Fourier transform unit converting a power waveform having a plurality of levels output from the multi-module converter to a Fourier series; And
Computing a control angle for removing harmonics from the Fourier series-converted power waveform by a control angle calculator
Harmonic removal method of the HVDC system comprising a. The method of claim 8,
In the step of calculating a control angle for removing the harmonics, the control angle calculation unit calculates a number of control angles corresponding to a plurality of levels output from the multi-module converter. delete

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