WO2004036716A1

WO2004036716A1 - Active eliminating apparatus for harmonic component

Info

Publication number: WO2004036716A1
Application number: PCT/KR2002/002019
Authority: WO
Inventors: Ung-Sik In
Original assignee: C & A Electric Co., Ltd
Priority date: 2002-10-18
Filing date: 2002-10-30
Publication date: 2004-04-29
Also published as: KR100473332B1; AU2002368292A1; KR20030022089A

Abstract

Disclosed is an active harmonic-eliminating apparatus for eliminating harmonics contained in a power line. The apparatus includes a current sensor for sensing current waveform flowing along the power line, a clock generator for generating a clock having a predetermined frequency, phase shifters for consecutively shifting a phase of the clock generated in the clock generator, A/D converters operating in response to the clocks phase-shifted by the phase shifters to convert the current waveform into digital data, harmonic analysis modules for analyzing the harmonics from the digital data, inverse harmonic generation modules for generating inverse harmonics for the analyzed harmonics, modulators for performing a PWM using signals transmitted from the inverse harmonic generation modules, and switching circuits for generating inverse harmonic currents using a modulated wave from the modulators and inputting the inverse harmonic currents to the power line.

Description

ACTIVE ELIMINATING APPARATUS FOR HARMONIC COMPONENT

Technical Field

The present invention relates to an active harmonic- eliminating apparatus that generates inverse harmonic components to eliminate multi-order harmonics, and more particularly, to an active harmonic-eliminating apparatus that can classify multi-order harmonics into a series of harmonic groups and generate inverse harmonic groups corresponding to the harmonic groups to lower a sampling frequency, thereby appropriately generating inverse harmonics for all of the harmonic groups, preventing a power switching element from being damaged, and reducing the heat generation in the power switching element.

Background Art

As an electronic control system has been developed, inverter application products have been widely used as an electric load of an electric power system to save energy. However, such an inverter application product causes a problem that a distortion of a voltage wave becomes more severe due to harmonics and a variety of nonlinear currents . This frequently results in malfunctions or damage of production equipments and facilities. In the case of a single-phase nonlinear load, the harmonic causes a current to increase more than 1.7 times the amount of the rated current. The increased current flows along a neutral line, thus generating an excessive current causing an excessive heat. Since most of the harmonics flow along a surface of a line conductor, the excessive heat caused by the harmonics raises a serious problem. This problem is more serious in a Y-connection transformer.

To solve this problem, an inverse harmonics with respect to the harmonics generated in the electric power system including the inverter load should be generated to maintain power of the power system in a form of a sine waveform.

FIG. 1A shows a circuit diagram illustrating a conventional active harmonic-eliminating principle, FIG. IB shows a current waveform containing a harmonic component generated on a load depicted in FIG. 1A, FIG. 1C shows a current waveform generated in an active harmonic- eliminating apparatus, and FIG. ID shows a current waveform of a power source depicted in FIG. 1A.

When a current is inputted by applying a power voltage Vs to an inverter load Lh, a current containing multi-order harmonics is generated on the inverter load, thereby distorting the current Is. In order to maintain the input current Is with a sine waveform as shown in the drawing, the active harmonic-eliminating apparatus is applied through an ammeter CT with a current flowing along a line to 1) analyze twelve harmonics including third, fifth, seventh, ninth, eleventh, thirteenth, fifteenth, seventh, ninetieth, twenty first, twenty third, and twenty fifth harmonics, and 3) add inverse harmonics to the line. The inverse harmonics added to the line substantially affect a power line formed on the line, thereby eliminating the harmonic components and maintaining the input current Is in a' form of the sine waveform. At this point, the harmonic-eliminating apparatus is generally classified into a mean-wave responsive harmonic- eliminating type and a digital type. The digital type is designed to eliminate the harmonics by generating a proper inverse waveform for each harmonic . A response speed of the mean-wave responsive harmonic-eliminating type is about lms. That is, the responsiveness is fast but the harmonic- eliminating efficiency is low. Furthermore, harmonics of predetermined orders higher than the thirteen or fifteen harmonic may be amplified by stimulating a system parallel resonance point. The digital type has an advantage in that the third to twenty fifth harmonics can be properly eliminated by analyzing each harmonic. However, since it takes much time to analyze each harmonic and generate inverse harmonics, the inverse harmonics for three or four harmonics each having relatively high intensity are selectively generated, making it difficult to eliminate harmonics of a relatively lower orders.

FIG. 2 shows . a block diagram illustrating a conventional digital type active harmonic-eliminating apparatus .

An ammeter detects a current flowing along a power line and transmits the same to a current sensor 21. The current sensor 21 generates a current waveform, and an A/D converter 22 converts the current waveform into digital data. At this point, a clock generator 28 generates a sampling frequency of 18Khz so that the A/D converter 22 can generate the' digital data of 18 hz. The digital data are transmitted to a harmonic analysis module 23 and then consecutively analyzed by harmonic orders. The analyzed data are transmitted to an intensity measuring module 24 and an inverse harmonic generation module 25. The intensity measuring module 24 extracts a harmonic of a predetermined order having predetermined intensity from a harmonic of the highest order. The extracted harmonic is outputted to the inverse harmonic generating module 25 so that the inverse harmonic generating module 25 can generate an inverse harmonic corresponding to the extracted harmonic . The inverse harmonic is transmitted to a modulator 26 and then modulated in a pulse width modulation (PWM). The inverse harmonic modulated in the PWM turns on/off an insulated gate bipolar transistor (IGBT) switching part 27 that is a switching circuit. The IGBT switching part 27 amplifies the inverse harmonic and inputs the same into the power line.

In the above-described conventional harmonic- eliminating apparatus, since each harmonic should be extracted and an inverse harmonic for a specific harmonic should be generated by detecting intensity of each harmonic, a very high sampling frequency should be maintained to improve the responsiveness.

That is, to eliminate a harmonic of the highest order

(the twenty fifth harmonic), a sampling frequency (60hz x25th*12 (sampling rate) = 18 kHz) should be maintained. At this point, the "60hz" is a power frequency of a domestic power line, and the "12" is a test value for increasing the harmonic analysis extent by sampling a frequency 12 times as high as a harmonic of a target order to be eliminated. Accordingly, in order to analyze the twenty fifth harmonic and input an inverse harmonic to the power line, a sampling is performed with the 18Khz and the IGBT switching part 27 should be switched by the inverse harmonic that is modulated in the PWM at the 18Khz.

Therefore, when the switching part 27 is switched at the 18Khz, the IGBT switching part 27 along which a large amount of current flows during amplification generates a large amount of heat. The large amount of heat causes the distortion of a waveform, thereby generating a new harmonic and not performing the generation of the inverse harmonic for eliminating the harmonic.

Disclosure of the Invention Therefore, the present invention has been made in an effort to solve the above described problems .

An object of the present invention is to provide an active harmonic-eliminating apparatus that can eliminate a harmonic of a target order. to be eliminated, increasing the elimination speed.

Another object of the present invention is to provide an active harmonic-eliminating apparatus that can perform an accurate switching operation by reducing heat generated in a switching element, prolonging the life of a switching element.

To accomplish the above objects and advantages, there 'is provided an active harmonic-eliminating apparatus for eliminating harmonics contained in a power line, the apparatus comprising a current sensor for sensing current waveform flowing along the power line; a clock generator for generating a clock having a predetermined frequency; phase shifters for consecutively shifting a phase of the clock generated in the clock generator; A/D converters operating in response to the clocks phase-shifted by the phase shifters to convert the current waveform into digital data; harmonic analysis modules for analyzing the harmonics from the digital data; inverse harmonic generation modules for generating inverse harmonics for the analyzed harmonics; modulators for performing a PWM using signals transmitted from the inverse harmonic generation modules ; and switching circuits for generating inverse harmonic currents using a modulated wave from the modulators and inputting the inverse harmonic currents to the power line.

Preferably, when assuming that the composed harmonic current input to the power line is "N" KHz and the number of A/D converters is "n," both of a clock frequency of the

, clock generator and a switching frequency of the switching circuits is "N/n" KHz.

Preferably, one of the phase shifters performs a phase shift by 0° so that the clock generated by the clock generator is transmitted to one of the A/D converter without a phase shift.

Preferably, each switching element of the switching circuits is formed of IGBT.

According to another aspect of the present ' invention, there is provided an active harmonic-eliminating apparatus for eliminating harmonics contained in a power line, the apparatus comprising a current sensor for sensing current waveform flowing along the power line; a clock generator for generating a clock having a predetermined frequency; at least one phase shifter for consecutively shifting a phase of the clock generated in the clock generator; A/D converters operating in response to the clocks phase- shifted by the phase shifters to convert the current waveform into digital data; harmonic analysis modules connected to the respective A/D converters to analyze the harmonics of predetermined orders; inverse harmonic generation modules for generating inverse harmonics for the harmonics output from the harmonic analysis modules; PWM modulators for receiving inverse harmonic data from the inverse harmonic generation modules and performing a PWM of the inverse harmonic data, the number of modulators is identical to that of the A/D converters; and switching circuits connected to the respective PWM modulators to generate inverse harmonic currents in accordance with signals outputted from the PWM modulators and transmit the inverse harmonic current to the power line.

Preferably, the number of harmonic analysis modules connected to one of the A/D converters is less than 4. Preferably, one of the phase shifters performs a phase shift by 0° so that one of the A/D converter is operated by a clock having a phase identical to the clock generator . According to still another aspect of the present invention, there is provided a method for actively eliminating harmonics contained in a power line, the method comprising the steps of sensing a current waveform flowing along the power line; converting the current waveform into digital data using a clock that is consecutively phase- shifted to more than one shifting step; analyzing the harmonics in accordance with the digital data and generating inverse harmonics corresponding to the analyzed harmonics; receiving the inverse harmonics and performing a PWM; and generating inverse harmonic currents in accordance with PWM signals and inputting the inverse harmonic currents to the power line.

Brief Description of the Drawings The above object, other features and advantages of the present invention will become more apparent by describing the preferred embodiment thereof with reference to the accompanying drawings, in which:

FIG. 1A is .a circuit diagram illustrating a conventional active harmonic-eliminating principle;

FIG. IB is a current waveform containing a harmonic generated on a load depicted in FIG. 1A;

FIG. 1C is a current waveform generated in an active harmonic-eliminating apparatus;

FIG. ID is a current waveform of a power source depicted in FIG. 1A;

FIG. 2 is a circuit block diagram illustrating a conventional digital type active harmonic-eliminating apparatus ;

FIG. 3 is a circuit block diagram illustrating an active harmonic-eliminating apparatus according to a first embodiment of the present invention; FIG. 4 is a waveform illustrating the first embodiment of the present invention; and

FIG. 5 is a circuit block diagram illustrating an active harmonic-eliminating apparatus according to a second embodiment of the present invention.

Best Mode for Carrying Out the Invention

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts . It is intended that the following description covers the modifications and variations of this invention. Therefore, it will be apparent to those skilled .in the art that various modifications and variations can be made in the present invention. In addition, in the description of the present invention, when it is determined that related art or a well-known constitution may inferior the feature of the present invention, the description thereof will be omitted herein.

FIG. 3 shows a circuit block diagram illustrating an active harmonic-eliminating apparatus according to a first embodiment of the present invention.

An ammeter CT detects a current flowing along a power line, and a current sensor 31 senses a current waveform of the current. The clock generator 32 generates a clock having a frequency of 1.5Khz. The clock generated in the clock generator 32 is inputted to an A/D converter-3. The A/D converter-3 converts the current waveform into digital data having a frequency of 1.5Khz. A third harmonic of the current waveform that is converted into the digital data is analyzed by a third harmonic analysis module and is then transmitted to a third inverse harmonic generation module, thereby generating a third inverse harmonic for compensating for the third harmonic. The third inverse harmonic is transmitted to a PWM module-3, thereby generating a PWM modulation signal corresponding to the third inverse harmonic. The modulated third inverse harmonic controls a conductive time of a switching circuit SW3 to generate a third inverse harmonic current. The third inverse harmonic current is then inputted to the power line. Meanwhile, the clock generated in the clock generator

32 is phase-shifted by a phase shifter-5 by one clock, and is then inputted to an A/D converter-5. The A/D converter-

5 converts the current waveform into digital data using a frequency of 1.5Khz having the shifted phase. An inverse harmonic corresponding to a fifth harmonic is generated through a process identical to that for the third harmonic, i.e.,' through a fifth harmonic analysis module, a PWM module-5, and a switching circuit SW5. The inverse harmonic corresponding to the fifth harmonic is then inputted to the power line.

In addition, the clock phase-shifted in the phase shifter-5 is further phase-shifted by one clock and is then inputted to an A/D converter-7. Then, an inverse harmonic corresponding to a seventh harmonic is generated through a process identical to those for the third and fifth harmonics, i.e., through a seventh harmonic analysis module, a PWM module-7, and a switching circuit SW7. The inverse harmonic corresponding to the seventh harmonic is then inputted to the power line.

Likewise, the clock is consecutively phase-shifted by one clock (30°) by a phase shifter-9, a phase shifter-11, a phase shifter-13, a phase shifter-15, a phase shifter-17, a phase shifter-19, a phase shifter-21, a phase shifter-23, and a phase shifter-25. The digital data are generated by the shifted phases. A harmonic analysis is realized for the corresponding digital data. The inverse harmonic generation modules generate inverse harmonics corresponding to the analyzed harmonics. The inverse harmonics are modulated by the PWM modules, and the switching circuits input the inverse harmonic currents to the power line using the modulated signals. It will be obvious to obtain identical results when assuming that individual phase shifter is connected to each A/D converter to consecutively perform a phase shift and a phase shifter that can p'hase-shift a clock outputted from the clock generator 32 by 0° is connected to the A/D converter-3.

FIG. 4 shows a waveform illustrating the first embodiment of the present invention.

In the convention art, the clock generator generates a clock waveform of an 18Khz in frequency (see FIG. 4A) , and obtains the digital data corresponding to a current waveform having the frequency. However, in the first embodiment of the present invention, as shown in FIG. 4B, a clock of a 1.5Khz in frequency is generated, and as shown in FIG. 4C, a clock waveform that is not phase-shifted is inputted to the A/D converter-3. In addition, as shown in FIG. 4D, a clock waveform that is phase-shifted from the clock waveform shown in FIG. 4C by one clock is inputted to the A/D converter-5. Likewise, clock waveforms that are consecutively phase-shifted by one clock are inputted to the respective A/D converter-7, A/D converter-9, , and

A/D converter-25.

As described above, the A/D converters generate digital data having a frequency of 1.5Khz that are phase- shifted by one clock, and the switching circuits perform the switching operation using an identical frequency.

However, the frequencies are composed in the power line, the phases are composed to generate an inverse harmonic current having a 18Khz in frequency.

As described with reference to FIGs. 3 and 4, in order to generate the inverse harmonic current having the frequency of 18Khz in the power lihe, each of the switching circuits performs its switching operation using the switching frequency of 1.5Khz. Therefore, the IGBT that is a switching element of the switching circuit generates a small amount of heat, thereby eliminating the harmonics without the waveform distortion. In addition, since inverse harmonics corresponding to the third to twenty fifth harmonics are simultaneously generated, the responsiveness for generating the inverse harmonics can be improved.

FIG. 5 shows a block diagram illustrating an active harmonic-eliminating apparatus according to a second embodiment of the present invention.

The second embodiment differs from the first embodiment in that plural harmonic analysis modules and plural inverse harmonic generation modules corresponding to the harmonic analysis modules are connected to a single A/D converter to reduce the number of parts and simplify the circuit.

The twenty fifth harmonic component has a frequency of "60hzx25 = 1.5Khz" at an alternate current of 60hz and a period of 0.64m/s. When a harmonic is generated, it is preferable that the harmonic-eliminating apparatus analyzes the harmonic and generates an inverse harmonic in a 1/4 period (in a period of 0.16m/s) of the harmonic. When a harmonic analysis and inverse harmonic generation for four harmonics with respect to a digital current waveform for the single A/D converter is realized, it takes about "0.16ms/4 = 0.04ms = 40 s" to generate the inverse harmonic for each harmonic. Since it is a minimum time for analyzing the current waveform and generating the inverse harmonic when a high-performance microprocessor is used, the each number of the harmonic analysis modules and inverse harmonic generation modules are limited to four. A waveform sensed by a current sensor coupled to an ammeter CT is converted into digital data by an A/D converter-1 operated by a clock generated in a clock generator 52. At this point, a frequency of the clock generated in the clock generator 52 becomes 6 KHz when a frequency of an inverse harmonic supplied from the power line is 18Khz and the number of A/D converters is three. The third, fifth, seventh, and ninth harmonic analysis modules are connected to the A/D converter-1, and the third, fifth, seventh and ninth inverse harmonic generation modules are connected to the respective harmonic analysis modules . A digital waveform of the inverse harmonic generated by the inverse harmonic generation module is transmitted to a PWM module-1 and is then modulated. The switching circuit SW1 is operated by the modulated digital waveform, thereby generating an inverse harmonic current.

The inverse harmonic current is then inputted to the power line. At this point, an operation frequency of the switching circuit SWl is identical to a clock frequency of 6khz of the clock generator 52. At this point, the clock generated in the clock generator 52 is transmitted to the A/D converter-1 via a phase shifter (not shown) by which the clock is phase-shifted by 0°. In addition, the clock generated in the clock generator 52 is transmitted to an A/D converter 2 via a phase shifter-1 in which the clock is phase-shifted by one clock. The A/D converter-2 converts a current waveform outputted from the current sensor into digital data by the phase-shifted clock. The eleventh, thirteenth, fifteenth, and seventeenth harmonic analysis modules are connected to the A/D converter-2, and eleventh, thirteenth, fifteenth, and seventeenth inverse harmonic generation modules are connected to the respective eleventh, thirteenth, fifteenth, and seventeenth inverse harmonic generation modules . Output signals from each of the inverse harmonic generation modules are transmitted to a PWM module-2 and are then modulated. The switching circuit SW2 is operated by the modulated signals, thereby generating the eleventh, thirteenth, fifteenth, seventeenth harmonic inverse harmonic currents . The inverse harmonic currents are then inputted to the power line.

In addition, the clock phase-shifted in the phase shifter-1 is further phase-shifted and is then transmitted to an A/D converter 3. The A/D converter-3 converts a current waveform outputted from the current sensor 51 into digital data by the phase-shifted clock. The nineteenth, twenty first, twenty third, and twenty fifth harmonic analysis modules are connected to the A/D converter-3, and nineteenth, twenty first, twenty third, and twenty fifth inverse harmonic generation modules are connected to the respective nineteenth, twe'nty first, twenty third, and twenty fifth harmonic analysis modules. Output signals from each of the inverse harmonic generation modules are transmitted to a PWM module-3 and are then modulated. The switching circuit SW3 is operated by the modulated signals, thereby generating the nineteenth, twenty first, twenty third, and twenty fifth inverse harmonic currents. The inverse harmonic currents are then inputted to the power line.

At this point, since switching frequencies of the switching circuits SW1, SW2 and SW3 are identical to a clock frequency of 6khz of the clock generator 52, the amount of heat generated is lower than that of a case where the conventional switching circuit is operated with a frequency of 18Khz. Therefore, since it is possible to operate the switching element with a high voltage, sufficient inverse harmonics for harmonics having relatively high intensity can be inputted to the power line.

It will be apparent to those skilled in the art that various modifications and variations can . be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents . Industrial Applicability

As described above, the active harmonic-eliminating apparatus according to the present invention has a variety of advantages, as follows: (1) Harmonics of target orders to be eliminated can be simultaneously eliminated. Inverse harmonics for multi- order harmonics can be quickly generated without time delay, the stable power system can be realized, thus improving the power quality. (2) Since the switching circuit is switched by a frequency lower than that of the prior art, an amount of the heat generated in the switching element can be reduced, thereby prolonging the life of the switching element and inputting accurate inverse harmonics into the power line. In addition, since a driving voltage for the switching circuit is increased compared with the prior art in which the driving voltage is limited due to the heat generated in the switching circuit, the inverse harmonics can be easily generated, thereby sufficiently eliminating the harmonics generated in the power line.

(3) As shown in FIG. 5, since it is possible to connect plural harmonic analysis modules and plural inverse harmonic generation modules to a single A/D converter, the ■switching frequency is to be lowered, sufficiently and the number of parts can be reduced, thereby reducing the manufacturing costs.

Claims

Claims:

1. An active harmonic-eliminating apparatus for eliminating multi-order harmonics contained in a' power line, the apparatus comprising: a current sensor for sensing current waveform flowing along the power line; a clock generator for generating a clock having a predetermined frequency; phase shifters for consecutively shifting a phase of the clock generated in the clock generator;

A/D converters operating in response to the clocks phase-shifted by the phase shifters to convert the current waveform into digital data; harmonic analysis modules for analyzing the harmonics from the digital data; inverse harmonic generation modules for generating inverse harmonics for the analyzed harmonics; modulators for performing a PWM using signals transmitted from the inverse harmonic generation modules; and switching circuits for generating inverse harmonic currents using a modulated wave from the modulators and inputting the inverse harmonic currents to the power line.

2. The active harmonic-eliminating apparatus of claim

1, wherein when assuming that the composed harmonic current inputted to the power line is "N" KHz and the number of A/D converters is "n," both of a clock frequency of the clock generator and a switching frequency of the switching circuits is "N/n" KHz.-

3. The active ' harmonic-eliminating apparatus of ariy one of claims 1 and 2, wherein one of the phase shifters performs a phase shift by 0° so that the clock generated by the clock generator is transmitted to one of the A/D converter without a phase shift.

4. The active harmonic-eliminating apparatus of claim 1, wherein. each switching element of the switching circuits is formed of IGBT.

5. An active harmonic-eliminating apparatus for eliminating multi-order harmonics contained in a power line, the apparatus comprising: a current sensor for sensing current waveform flowing along the power line; a clock generator for generating a clock having a predetermined frequency; at least one phase shifter for consecutively shifting a phase of the clock generated in the clock generator;

A/D converters operating in . response to the clocks phase-shifted by the phase shifters to convert the current waveform into digital data; harmonic analysis modules connected to the respective

A/D converters to analyze the harmonics of predetermined orders; inverse harmonic generation modules for generating inverse harmonics for the harmonics output from the harmonic analysis modules;

PWM modulators for receiving inverse harmonic data from the inverse harmonic generation modules and performing a PWM of the inverse harmonic data, the number of modulators is identical to that of the A/D converters; and switching circuits connected to the respective PWM modulators to generate inverse harmonic currents in accordance with signals outputted from the PWM modulators and transmit the inverse harmonic current to the power line.

6. The active harmonic-eliminating apparatus of claim 5, wherein the number of harmonic analysis modules connected to one of the A/D converters is less than 4.

7. The active harmonic-eliminating apparatus of one of claims 5 and 6, wherein one of the phase shifters performs a phase shift by 0° so that one of the A/D converter is operated by a clock having a phase identical to the clock generator.

8. A method for actively eliminating multi-order harmonics contained in a power line, the method comprising the steps of: sensing a current waveform flowing along the power line; converting the current waveform into digital data using a clock that is consecutively phase-shifted to more than one shifting step; analyzing the harmonics in accordance with the digital data and generating inverse harmonics corresponding to the analyzed harmonics; receiving the inverse harmonics and performing a PWM; and generating inverse harmonic currents in accordance with PWM signals and inputting the inverse harmonic currents to the power line.