KR102561610B1 - Hybrid Active Power Filter - Google Patents

Abstract

The present disclosure is a hybrid active power filter that is connected in parallel between a power system and a load to reduce harmonics included in the power system, comprising: a passive power filter having an inductor and a capacitor connected in series; an active power filter including two legs in which two switching elements are connected in series; and a control unit generating a control signal for operating the switching element, wherein the control unit generates the control signal based on a load current supplied to the load.

Description

Hybrid Active Power Filter {Hybrid Active Power Filter}

The present disclosure relates to a hybrid active power filter in which a passive power filter and an active power filter are mixed.

Recently, as various applications driven using DC power increase, Total Harmonics Distortion (THD) introduced into the power system is gradually increasing. These harmonics negatively affect electrical equipment such as transformers, motors, cables, circuit breakers, etc. in various forms.

Harmonics, which account for the largest proportion of factors that impede power quality, refer to the physical amount of electricity corresponding to an integer multiple excluding the basic frequency of 60 [Hz] in Korea.

These harmonics are mainly generated in power electronic components used when converting AC power to DC power.

Harmonics have a variety of adverse effects such as insulation breakdown, overheating, malfunction, etc. to power devices that mainly form a power system, such as cables, transformers, power capacitors, circuit breakers, and the like. In addition, it can be considered that it affects rotating devices and power converters mainly used in industries and consumers, such as shortening of life and malfunction, and adversely affecting the overall area where power flows and is used.

Recently, many studies have been conducted on a hybrid active power filter capable of reducing harmonics by simultaneously using a passive power filter and an active power filter. Such a filter has the advantage of suppressing a resonance phenomenon occurring in a passive filter and simultaneously compensating for harmonics of various frequency bands. Although such a hybrid active power filter has various advantages, it has a disadvantage in that the design is complicated and the price increases due to the increase in the number of elements.

The present invention is to solve the above-mentioned problems, and to control a hybrid active power filter composed of an active power filter composed of two legs and an LC passive power filter connected in series, control of the active power filter using a current method using a hysteresis technique The purpose is to improve response characteristics.

In addition, an object of the present invention is to simultaneously compensate for harmonics and reactive power generated by a nonlinear load and simultaneously improve total harmonic distortion (THD) and power factor.

In addition, an object of the present invention is to efficiently design a passive power filter by presenting a filter capacity calculation formula of a passive power filter in which a capacitor and an inductor are connected in series.

As a means for solving the above problems, the present invention has an embodiment having the following characteristics.

A hybrid active power filter according to an embodiment is a hybrid active power filter connected in parallel between a power system and a load to reduce harmonics included in the power system, comprising: a passive power filter having an inductor and a capacitor connected in series; an active power filter including two legs in which two switching elements are connected in series; and a control unit generating a control signal for operating the switching element, wherein the control unit generates the control signal based on a load current supplied to the load.

The control signal may be generated based on DC components and harmonic components of the reactive current of the load current.

The control signal may be generated based on a harmonic component excluding a direct current component of the effective current of the load current.

The control unit calculates a phase value to be synchronized using the grid voltage supplied to the load by the power system, and then converts the three-phase load current into a two-phase DQ converter using DQ conversion; includes

The control unit may further include a high-pass filter for excluding a direct current component from an effective current among the load currents of the two phases.

The control unit further includes a DQ inverse converter for converting the two-phase current converted by the DQ converter into a three-phase current, wherein the three-phase load current output by the DQ inverse converter and the active filter supply to the passive filter The filter current is continuously compared to generate a PWM waveform in the hysteresis band region.

The controller may change the maximum frequency of the PWM signal by adjusting the size of the hysteresis width.

The inductance L of the inductor is , and the capacitance C of the capacitor is where n is the order of harmonics at which the passive power filter resonates, Qx is the amount of reactive power used by the load, and Vpcc is the RMS voltage at a point where the hybrid active power filter is connected in parallel between the power system and the load , ω is the angular frequency of the grid voltage.

The present invention can improve the control response characteristics of an active power filter by using a current method using a hysteresis technique to control a hybrid active power filter composed of an active power filter composed of two legs and an LC passive power filter connected in series.

In addition, the present invention can simultaneously compensate for harmonics and reactive power generated by nonlinear loads, and can simultaneously improve total harmonic distortion (THD) and power factor.

In addition, the present invention can efficiently design a passive power filter by presenting a filter capacity calculation formula of a passive power filter in which a capacitor and an inductor are connected in series.

1 is a circuit diagram of a hybrid active power filter according to an embodiment of the present invention.
2 is a block diagram of a control unit according to an embodiment of the present invention.
3 shows voltage and current waveforms when the control method of the comparative example is used.
4 illustrates voltage and current waveforms when the control method of one embodiment is used.
5 illustrates a process of generating a PWM waveform through hysteresis control.

Advantages and features of the present invention, and methods for achieving them, will become clear with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in a variety of different forms, only the present embodiments make the disclosure of the present invention complete, and those skilled in the art in the art to which the present invention belongs It is provided to fully inform the person of the scope of the invention, and the invention is only defined by the scope of the claims. Like reference numbers designate like elements throughout the specification.

An element is said to be "connected to" or "coupled to" another element when it is directly connected or coupled to another element or intervening with another element. include all cases. On the other hand, when one element is referred to as “directly connected to” or “directly coupled to” another element, it indicates that another element is not intervened. Like reference numbers designate like elements throughout the specification. “And/or” includes each and every combination of one or more of the recited items.

Although first, second, etc. are used to describe various elements, components and/or sections, it is needless to say that these elements, components and/or sections are not limited by these terms. These terms are only used to distinguish one element, component or section from another element, component or section. Accordingly, it goes without saying that the first element, first element, or first section referred to below may also be a second element, second element, or second section within the spirit of the present invention.

Terms used in this specification are for describing embodiments and are not intended to limit the present invention. In this specification, singular forms also include plural forms unless specifically stated otherwise in a phrase. As used herein, "comprises" and/or "comprising" means that a stated component, step, operation, and/or element is present in the presence of one or more other components, steps, operations, and/or elements. or do not rule out additions.

Unless otherwise defined, all terms (including technical and scientific terms) used in this specification may be used in a meaning commonly understood by those of ordinary skill in the art to which the present invention belongs. In addition, terms defined in commonly used dictionaries are not interpreted ideally or excessively unless explicitly specifically defined.

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

1 is a circuit diagram of a hybrid active power filter according to an embodiment of the present invention.

The hybrid active power filter 250 according to the embodiment is connected in parallel between the power system (GRID) 110 and the load 150 to reduce harmonics included in the power system. The hybrid active power filter 250 is composed of a mixture of the passive power filter 210 and the active power filter 250, and includes a controller 300 that controls the active power filter 250. The load current supplied by the load 150 ( ) It may include a first measuring device 130 for measuring. In addition, in the hybrid active power filter, the filter current supplied by the active power filter 250 to the passive power filter 210 ( ) It may include a second measuring device 230 for measuring.

The load 150 generates a load current (from the power system 110 and the hybrid active power filter). ) is supplied. load current ( ) is the system current supplied by the power system 110 ( ) and the filter current supplied by the active power filter ( ) can be expressed as the sum of

The load 150 may include a rectifier, a DC-link capacitance (Cd), and a resistive load (RL), which are types of nonlinear loads.

The passive power filter 210 may be configured by connecting an inductor (L) and a capacitor (C) in series. However, the implementation of the present invention is not limited thereto, and in various embodiments, the inductor L and the capacitor C may be connected in parallel.

The active power filter 250 may be composed of two legs. Each leg may be configured by connecting two switching elements (SW) in series. That is, the active power filter 250 may include four switching elements SW1, SW2, SW3, and SW4. The switching element (SW) is classified as an active element and may be composed of MOSFET, IGBT, and the like. The active power filter 250 may reduce harmonics by using a grid-connected inverter composed of a switching element (SW).

The controller 300 generates a control signal sig for operating the switching element SW. The control unit 300 controls the load current (load 150) supplied from the power system 110 and the hybrid active power filter ( ) to generate a control signal sig. The hybrid active power filter 250 is a device that controls voltage or current using a switching element (SW). That is, in order to normally operate the hybrid active power filter, PWM (Pulse Width Modulation) must be generated based on a signal generated by voltage control or current control to operate switching. A control method of a hybrid active power filter according to an embodiment of the present invention is characterized by generating PWM based on a signal generated by current control through a hysteresis technique. Hereinafter, the control signal generated by the control unit 300 will be described in detail.

2 is a block diagram of a control unit according to an embodiment of the present invention.

The controller 300 controls the load current ( ) generates a control signal based on the DC component and harmonic component of the reactive current. That is, the hybrid active power filter proposed by the present invention can compensate for harmonics and reactive power components together in order to compensate for the disadvantages caused by using the passive power filter 210 and the active power filter 250 independently.

The control unit 300 includes a DQ converter 310, a high pass filter 330, a DQ inverse converter 320 and a hysteresis controller 340.

in Figure 2 is the voltage measured in the grid. load current ( ) is the current measured by the current flowing through the nonlinear load 150. In other words, the load current supplied by the load 150 from the power system 110 and the hybrid active power filter ( ) is the current measured by the first measuring device 130 measuring .

DQ converter 310 to synchronize using After calculating the value, use it to load current ( ) to the two-phase current , generate The conversion equation of the DQ converter 310 used here is the same as Equation 1.

is the reactive current and is the effective current. and Distinguishing into direct current and harmonic components can be expressed as Equations 2 and 3.

here is the direct current component of the reactive current, is the harmonic component of the reactive current. is the direct current component of the effective current, is the harmonic component of the effective current.

Accordingly, if the measured current is not a pure sine wave, and When a current is generated in the component and the reactive component is measured current is generated in

Two-phase current generated by the DQ converter 310 , is passed to the DQ inverse transformer 320. where the effective current is transmitted to the DQ inverse transformer 320 through the high pass filter 330. and reactive current is passed to the DQ inverse transformer 320 as it is. As a result, the active current in the DQ inverse converter 320 Only the harmonic components of are delivered, and the load current ( ) of the reactive current is delivered to the DQ inverse transformer 320 with both a direct current component and harmonic components. Since the load 150 includes a nonlinear load, it generates power of ineffective components as well as harmonic components. Therefore, compensation control for the ineffective component is required. To this end, the control unit 300 according to the embodiment includes a load current ( ), a control signal sig is generated based on the DC component and the harmonic component of the invalid component.

3 shows voltage and current waveforms when the control method of the comparative example is used.

Comparative example is reactive current This is also the case when it is transmitted to the DQ inverse transformer 320 through the high pass filter 330. That is, the control unit 300 is a load current ( ) is a case where a control signal is generated based only on the harmonic components of the invalid component. load current ( ), the system current ( ) is restored in the form of a fundamental sine wave (Fundamental Frequency). At this time, the THD of each phase voltage was measured as 5.2 [%], and the total harmonic distortion (hereinafter referred to as THD) of each phase current was measured as 4.6 [%]. Since the control method of the comparative example cannot compensate for reactive power, it can be seen that a phase difference between voltage and current occurs as shown in FIG. 3 . As a result of measuring the power factor accordingly, 93.2[%] was measured.

4 illustrates voltage and current waveforms when the control method of one embodiment is used.

load current ( ), the system current ( ) is almost completely restored to the form of a fundamental sine wave (Fundamental Frequency). At this time, the THD of each phase voltage was measured as 4.9 [%] and the THD of each phase current was measured as 3 [%].

As the control method according to the embodiment can compensate for reactive power, it can be seen that a phase difference between voltage and current does not occur as shown in FIG. 4 . As a result of measuring the power factor accordingly, 99.7[%] was measured.

That is, it was verified that the control scheme according to an embodiment of the present invention can efficiently compensate for reactive power and harmonics generated from the nonlinear load 150 .

The two-leg active hybrid active power filter according to the embodiment has the advantage of reducing cost by reducing the number of switching elements (SW) in the form of removing two switches corresponding to phase C as shown in FIG. 1, but is more controllable than the three-leg method It has the disadvantage of low responsiveness. In particular, when control of a switch is performed by applying a sine wave PWM (hereinafter referred to as SPWM), a control delay may occur and voltage distortion may occur.

In order to solve this problem, the present invention is characterized in that the control unit 300 continuously compares the current reference and the filter current to generate a PWM waveform in the hysteresis band region.

Specifically, with reference to FIG. 2, the generation of the PWM waveform in the hysteresis band region will be described.

The hysteresis controller 340 is a three-phase current output by the DQ inverse converter 320 As a current reference, the filter current supplied by the active power filter to the passive power filter ( ) is continuously compared to a current reference to generate a PWM waveform in the hysteresis band region.

5 illustrates a process of generating a PWM waveform through hysteresis control.

H is the current reference filter current ( )to It can be set to be H. The maximum frequency of the PWM signal can be changed by adjusting the size of H, that is, the size of the hysteresis width. SPWM has low response to the current reference and can distort the current waveform. However, if the hysteresis control is applied to the 2-leg hybrid active power filter control as in the embodiment, control response can be improved.

Referring to FIG. 1 again, the design method of the passive power filter 210 will be described in detail.

As described above, the hybrid active power filter can simultaneously compensate for reactive power and harmonic components of a power system by using the passive power filter 210 and the active power filter 250 at the same time.

Passive power filter 210 may be configured by connecting a capacitor (C) and an inductor (L) in series. 1 can be seen that the passive power filter 210 is configured in the form of an LC series filter. If Z is the impedance when C and L are connected in series, it can be expressed by Equation 4 as follows.

Here, since the capacitor and the inductor (L) are in series, the reactance (X) is as shown in Equations 5 and 6 below.

If R = 0, transforming like Equation 4 using Equation 6 is equivalent to Equation 7.

In order to select the capacities of C and L, two major considerations can be made as follows.

Resonate in the frequency domain corresponding to the desired harmonics. (mainly 5th harmonic)

Design an LC filter with the desired capacity of the passive power filter 210. (The amount of reactive power used mainly by non-linear loads)

In order to design an LC filter with a desired capacity of the passive power filter 210 while resonating in a frequency domain corresponding to a desired harmonic, it is necessary to first look at the resonance frequency of the LC series filter.

The resonant frequency of the LC series filter is shown in Equation 8.

Equation 8 is transformed into Equations 9 to 11 below.

here represents the frequency of the grid voltage If denotes the order of harmonics can be expressed as in Equation 9, Therefore, Equation 11 is obtained by rearranging the equation.

Capacity of 3-Phase Series Filter Can be expressed as in Equations 12 to 16 below.

here, Means the central RMS voltage of the system, the nonlinear load, and the HAPF, and when Equation 11 is modified, the following Equations 17 to 18 are obtained.

Using Equations 15 and 17, it can be developed as in Equation 19.

Equation 19 is rearranged to Equation 20.

Using Equations 16 and 18, Equation 21 is obtained.

When Equation 21 is rearranged, Equation 22 is obtained.

The final capacity calculation of C and L is shown in Equations 23 and 24.

It should be understood that the embodiments described above are illustrative in all respects and not limiting. The scope of the present invention is indicated by the following claims rather than the detailed description above, and all changes or modifications derived from the meaning and scope of the claims and equivalent concepts should be construed as being included in the claims of the present invention. .

Claims (8)

A hybrid active power filter that is connected in parallel between the power system and the load to reduce harmonics included in the power system,
A passive power filter in which an inductor and a capacitor are connected in series;
an active power filter including two legs in which two switching elements are connected in series; and
Including; a control unit for generating a control signal for operating the switching element;
The control unit,
a DQ converter that calculates a phase value to be synchronized using a grid voltage supplied to a load by the power system and then converts a 3-phase load current supplied to the load into a 2-phase using DQ conversion;
a DQ inverse converter for converting the 2-phase current converted by the DQ converter into 3-phase current; and
A hysteresis controller generating a PWM waveform in a hysteresis band region by continuously comparing the three-phase load current output by the DQ inverse converter and the filter current supplied to the passive power filter by the active power filter.
Hybrid active power filter.
According to claim 1,
The control unit,
generating the control signal based on a direct current component and a harmonic component of the reactive current of the load current;
Hybrid active power filter.
According to claim 1,
The control unit,
Generating the control signal based on a harmonic component excluding a direct current component of the effective current of the load current,
Hybrid active power filter.
delete According to claim 1,
The control unit
A high-pass filter for excluding a direct current component from the effective current of the two-phase load current; further comprising,
Hybrid active power filter.
delete According to claim 1,
The control unit
Characterized in that the maximum frequency of the signal of the PWM waveform is changed by adjusting the size of the hysteresis width,
Hybrid active power filter.
According to claim 1
The inductance L of the inductor is ego,
The capacitance C of the capacitor is is,
Here, n is the order of harmonics at which the passive power filter resonates, Qx is the amount of reactive power used by the load, Vpcc is the RMS voltage at the point where the hybrid active power filter is connected in parallel between the power system and the load, and ω Is the angular frequency of the grid voltage
Hybrid active power filter.

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