WO2017039090A1

WO2017039090A1 - Power conversion system having filters exploiting the sequence of voltage harmonics

Info

Publication number: WO2017039090A1
Application number: PCT/KR2016/001633
Authority: WO
Inventors: Seung-Ki Sul; Sungjae OHN
Original assignee: Seoul National University R&Db Foundation
Priority date: 2015-09-01
Filing date: 2016-02-18
Publication date: 2017-03-09
Also published as: KR20170027178A

Abstract

Disclosed is a power conversion system having filters exploiting the sequence of voltage harmonics, which includes a converter unit having three or its multiple number of three-phase converters connected in parallel, a filter unit, and a load unit, wherein the converter unit includes first to third converters having a phase A output, a phase B output, and a phase C output, respectively, wherein the filter unit includes at least one of a positive-sequence harmonic filter, a negative-sequence harmonic filter, and a zero-sequence harmonic filter, and wherein the filter unit is composed of passive elements.

Description

POWER CONVERSION SYSTEM HAVING FILTERS EXPLOITING THE SEQUENCE OF VOLTAGE HARMONICS

This disclosure relates to a power conversion system having filters for suppressing harmonic components, and more particularly, to a power conversion system having a filter unit exploiting the sequence of voltage harmonics of each of three or its multiple number of three-phase converters operated in parallel.

In an existing technique, various circuit configurations are being considered in order to connect large-capacity energy sources such as renewables or batteries to the grid. A pulse width modulation (PWM) converter is widely used since it can easily handle electric power and power factor, with relatively high efficiency and low current harmonics. Also, the PWM inverter is widely used for high-power motor drive systems.

The PWM voltage waveform contains a lot of voltage harmonics. In order to satisfy harmonic regulations of the grid or suppress the harmonic current introduced to a motor, a filter composed of a passive element is generally connected between a power source (load) and the PWM converter. For power conversion systems of several MVA to several tens of MVA, such a filter, particularly a magnetic core, occupies significant portions in volume, weight, and costs. Thus, various filters are being used to effectively suppress current harmonics with a small volume and a minimum number of passive elements.

Figs. 1a to 1c show prior arts which are widely used for conventional grid-connected power conversion systems. Fig. 1a shows a filter just composed of an inductor. The inductor gives larger impedance at the high frequency, and thus is capable of suppressing current harmonics. However, in order to satisfy stringent harmonic regulations of the grid, the inductor has to be very large in size.

Fig. 1b shows an LCL filter composed of two inductors and one capacitor, which is widely used for a grid-connected converter. The LCL filter may obtain attenuation of -60 dB/dec at frequencies higher than the resonance frequency of the filter.

Therefore, the LCL filter may sufficiently attenuate current harmonics with smaller passive elements, in comparison to the filter using only an inductor as shown in Fig. 1a.

However, the resonance generated by the additional capacitor causes a problem in stability of the power conversion system. Therefore, a delicate design and damping schemes are required.

The structure of an LLCL filter is shown in Fig. 1c, an inductor is added to LCL filter and it is connected to the capacitor in series. The magnitude of current harmonics is greater at the switching frequency band compared to other frequency bands. With the additional inductor, high attenuation can be achieved at this specific frequency band, and thus the grid current harmonics may be suppressed with smaller passive elements compared to LCL filter.

Fig. 2a is a circuit diagram for illustrating an interleaving operation and a phase difference of triangular carrier waves for PWM of converters, and Fig. 2b shows cancellation of current ripple by the phase difference of triangular carrier waves.

Referring to Figs. 2a to 2b, by shifting the phase of the triangular waves of the converters as shown in Fig. 2a, a phase of current harmonics between the paralleled converters may be changed. By doing so, at the output current, the current harmonics with different phases cancel each other as shown in Fig. 2b, which may improve current harmonic characteristics at the power source (load).

However, the interleaving operation does not give any influence on current harmonics of the converter current itself. The current harmonics of the converter current contributes to an additional loss at the passive components of the filter as well as a conduction loss and a switching loss at power semiconductors of the converter. Also the current-rating of the power semiconductor should be increased to handle an increase in peak-value of converter current due to the current harmonics.

Therefore, even with the interleaving operation, a filter with considerable size is required to reduce current harmonics of the converter itself.

Fig. 3a shows a coupled inductor which is widely used along with the interleaving operation in many applications, and Fig. 3b is a circuit diagram representing the coupled inductor. Fig. 3c is a circuit diagram which shows the connection among three coupled inductors, when they are used for three three-phase converters connected in parallel. The connection here is drawn only for one phase.

The coupled inductor is exploiting the phase difference of current-harmonics among interleaved converters. Its structure is originated from a DC-DC converter. Two windings are connected to a single magnetic core as shown in Fig. 3a. Magnetic flux by the fundamental-frequency current cancels each other in the magnetic core, but the magnetic flux generated by the harmonic currents with different phases overlaps each other inside the core. By doing so, very small impedance may be provided to the fundamental current, but large impedance may be provided to the phase-shifted harmonic current through interleaving. Therefore, the phase-shifted current harmonics can also be suppressed on the converter-side.

When N number of converters is connected in parallel, the magnetic core for the coupled inductor may have various structures. A combinatorial cascade structure as shown in Fig. 3c is one example. This structure may solve a problem that the magnetic core is saturated by fundamental current, but the number of cores required for connecting N-paralleled converters is increased to N(N-1)/2. For example, when three converters are operated in parallel, three magnetic cores are required for a single-phase as shown in Fig. 3c, and a three-phase system requires nine magnetic cores in total.

Fig. 4 shows a structure of common-mode filter. The common-mode filters are widely used to suppress a circulating current of a zero-sequence component in paralleled converters or to attenuate common-mode noise generated by PWM. The common-mode filter exploits phase-difference between three phases. Three-windings are connected to phase A, phase B and phase C, respectively. By the phase difference, the magnetic flux formed by the fundamental current is canceled in the magnetic core, while the magnetic flux of the zero-sequence currents which have the same magnitude and phase among three phases, reinforce each other in the core. Consequently, the zero-sequence current may be effectively suppressed just with a relatively small magnetic core in comparison to a general inductor. However, the common-mode filter for three-phase converter may not attenuate any current harmonics other than the zero-sequence current.

Fig. 5 shows a frequency spectrum of output voltage of a three-phase converter. Each component at different frequency is labeled with its sequence. Here, the term "sequence" indicates positive-sequence, negative-sequence and zero-sequence, depending on the phase-rotation among phase A, phase B and phase C. Referring to Fig. 5, a phase of output voltage of a converter at a predetermined output frequency 51 is determined by the control of a control unit, and a zero-sequence voltage harmonics 52 is present at a multiple times of the output frequency.

In addition, in case of voltage harmonics 53 appearing near an integer-multiple frequency of the switching frequency of the converter, there exist positive-sequence components, negative-sequence components and zero-sequence components. In other words, the conventional common-mode filter only attenuates zero-sequence components among three sequence components.

In summary, the coupled inductor utilizes the phase difference between interleaved converters. If the coupled inductor as shown in Fig. 3 is connected to power conversion systems with phase-shifted triangular waves as shown in Fig. 2, the phase-shifted current harmonics may be reduced.

However, if the coupled inductors are to be used with three or more paralleled converters, a great number of magnetic cores would be required depending on the structure of the coupled inductor, and actual implementation of such filter may be difficult. Thus, in general, two-parallel converters are preferred when the coupled inductor is used for a three-phase system.

On the other hand, the common-mode inductor as shown in Fig. 4, utilize the phase-difference between three-phases. The zero-sequence current harmonics may be effectively reduced with a small magnetic core, but neither positive-sequence harmonics nor negative-sequence current harmonics can be suppressed with the common-mode filter.

This disclosure is directed to providing a passive filter, which may greatly attenuate current harmonics with small-size magnetic cores by exploiting the sequence of voltage harmonics in a three-phase system and a phase-shift of voltage harmonics between interleaved converters. Based on the phase difference between phase A, B and C, each component of the voltage-harmonics is classified into a positive-sequence component, a negative-sequence component and a zero-sequence component. According to an amount of phase-shift of the triangular carriers, the phase of voltage-harmonics can be shifted between paralleled converters. By exploiting these two factors influencing the phase of voltage harmonics, the present disclosure is directed to reducing a size and cost of passive elements for suppressing current harmonics in a power conversion system using three or its multiple three-phase PWM converter. In this way, it is possible to suppress current harmonics flowing to a three-phase AC load/power source and also to greatly reduce current harmonics flowing in each converter.

In one general aspect, there is provided a power conversion system having filters exploiting the sequence of voltage harmonics, which includes a converter unit having three or its multiple number of three-phase converters connected in parallel, a filter unit, and a load unit, wherein the converter unit includes first to third converters having a phase A output, a phase B output, and a phase C output, respectively, wherein the filter unit includes at least one of a positive-sequence harmonic filter, a negative-sequence harmonic filter, and a zero-sequence harmonic filter, and wherein the filter unit is composed of a passive element.

In the power conversion system having filters exploiting the sequence of voltage harmonics according to an embodiment, the positive-sequence harmonic filter may include: a first positive-sequence filter configured to receive a phase A output of the first converter, a phase C output of the second converter, and a phase B output of the third converter; a second positive-sequence filter configured to receive a phase B output of the first converter, a phase A output of the second converter, a phase C output of the third converter; and a third positive-sequence filter configured to receive a phase C output of the first converter, a phase B output of the second converter, and a phase A output of the third converter.

In the power conversion system having filters exploiting the sequence of voltage harmonics according to an embodiment, the negative-sequence harmonic filter may include: a first negative-sequence filter configured to receive a phase A output of the first converter, a phase B output of the second converter, and a phase C output of the third converter; a second negative-sequence filter configured to receive a phase C output of the first converter, a phase A output of the second converter, and a phase B output of the third converter; and a third negative-sequence filter configured to receive a phase B output of the first converter, a phase C output of the second converter, and a phase A output of the third converter.

In the power conversion system having filters exploiting the sequence of voltage harmonics according to an embodiment, the zero-sequence harmonic filter may include: a first zero-sequence filter configured to receive a phase A output, a phase B output, and a phase C output of the first converter; a second zero-sequence filter configured to receive a phase A output, a phase B output, and a phase C output of the second converter; and a third zero-sequence filter configured to receive a phase A output, a phase B output, and a phase C output of the third converter.

The phase connection order of each converter may be changed in various ways remaining the same functionality.

In the power conversion system having filters exploiting the sequence of voltage harmonics according to an embodiment of the present invention, the filter unit may include a positive-sequence harmonic filter, a negative-sequence harmonic filter, and a zero-sequence harmonic filter. In the power conversion system having filters exploiting the sequence of voltage harmonics according to an embodiment, the positive-sequence harmonic filter, the negative-sequence harmonic filter, and the zero-sequence harmonic filter may be configured to have at least one of a ring core(circular core), an EI core, a CI core, a cylindrical core, and a core where three EI cores for a positive-sequence harmonic filter, a negative-sequence harmonic filter and a zero-sequence harmonic filter are integrated into a single magnetic core. However, in the present disclosure, the type of core to be integrated into a single magnetic core is not limited to the EI core.

In the power conversion system having filters exploiting the sequence of voltage harmonics according to an embodiment, the second converter may have a phase of a triangular wave behind the first converter by 120°, and the third converter may have a phase of a triangular wave ahead of the first converter by 120˚.

In the power conversion system having filters exploiting the sequence of voltage harmonics according to an embodiment, the second converter has a triangular wave which has 120° phase difference with the triangular carrier wave of the first and the third converters, and wherein the third converter has a triangular wave which has 120° phase difference with the triangular carrier wave of the first and the second converter.

The power conversion system having filters exploiting the sequence of voltage harmonics according to an embodiment may further include a control unit configured to control output power of the converters and a phase of a triangular wave of the first to third converter.

The power conversion system having filters exploiting the sequence of voltage harmonics according to an embodiment may further include an additional filter unit, wherein the additional filter unit may include at least one inductor or capacitor.

In the power conversion system having filters exploiting the sequence of voltage harmonics according to an embodiment, the additional filter unit may be formed between a point of common coupling (PCC) and the load unit or formed at each output end of the converter unit.

According to an embodiment of the present disclosure, a power conversion system having a harmonic filter exploiting a sequence of voltage harmonics includes three converters connected in parallel, and a magnetic core to which AC terminals of the converters are appropriately connected, respectively.

In the present disclosure, according to the phase, magnetic flux formed by harmonic current components are overlapped in the core and most current harmonics are effectively suppressed. On the other hand, magnetic flux formed by fundamental current components cancels each other in the magnetic core. Considering that the magnitude of the current harmonics is generally much smaller than that of the fundamental current, and the magnetic flux is only generated by harmonic currents, the size of the magnetic core may be designed to be only capable of the flux by harmonic current. This allows the magnetic core to have a very small size.

In the present disclosure, the structure of magnetic cores widely used in the industry may be applied, which allows easy implementations of the present disclosure.

In the present disclosure, when a capacitor as shown in Figs. 1b and 1c is used in the same manner, the harmonic current may be further attenuated.

Figs. 1a to 1c show three examples of filters widely used in conventional grid-connected power conversion systems.

Fig. 2a is a circuit diagram illustrating an interleaving operation and a phase difference of two triangular waves, and Fig. 2b shows cancellation of current ripple due to the phase difference.

Fig. 3a shows a coupled inductor used along with the interleaving operation in many applications, Fig. 3b is a circuit diagram showing the coupled inductor, and Fig. 3c is a circuit diagram showing that coupled inductors are connected to three paralleled converters with a combinatorial cascade structure.

Fig. 4 shows a common-mode filter widely used for EMI (Electro-Magnetic Interference) filtering and suppression of a zero-sequence circulating-current in paralleled converters.

Fig. 5 shows a frequency spectrum of three-phase converter output voltage of a power conversion system, with each component classified according to their sequence.

Fig. 6 is a block diagram showing a power conversion system 1000 having filters exploiting the sequence of voltage harmonics according to an embodiment of the present disclosure.

Fig. 7 is a block diagram showing a filter unit 200 of the power conversion system 1000.

Figs. 8(a) to 8(c) are phasor diagrams respectively showing positive-sequence voltage harmonics in a switching frequency band of a first converter 110, a second converter 120 and a third converter 130.

Figs. 9(a) to 9(c) show an example of first to third zero-sequence filters 211-213, when a zero-sequence harmonic filter 210 is a common-mode filter using ring cores.

Figs. 10(a) to 10(c) show an example of first to third positive-sequence filters 221-223, when a positive-sequence harmonic filter 220 is a common-mode filter using ring cores.

Figs. 11(a) to 11(c) show an example of first to third negative-sequence filters 231-233, when a negative-sequence harmonic filter 230 is a common-mode filter using ring cores.

Figs. 12(a) to 12(c) show an example of the positive-sequence harmonic filter 210, the negative-sequence harmonic filter 220 and the zero-sequence harmonic filter 230 of the filter unit 200 composed of an EI core.

Figs. 13(a) and 13(b) show experiment results in which the filter unit 200 is implemented using an EI core.

The embodiments described in the specification may be implemented as hardware entirely, hardware partially and software partially. In the specification, the term "unit", "module", "device", "system" or the like indicates a computer-related entity like hardware, a combination of hardware and software, or software. For example, the term "unit", "module", "device", "system" or the like used in the specification may be a process, a processor, an object, an executable file, a thread of execution, a program, and/or a computer, without being limited thereto. For example, both a computer and an application executed in the computer may correspond to the term "unit", "module", "device", "system" or the like in the specification.

Hereinafter, configurations and features of the present disclosure are described based on embodiments, but these embodiments are just for better understanding of the present disclosure and not intended to limit the present disclosure. If like reference symbols are used in several figures, these like reference symbols designate similar or identical functions in various embodiments.

In the following description, the term "converter" is an element replaceable with an inverter, and the "converter" may include a converter or an inverter.

Fig. 6 is a block diagram showing a power conversion system 1000 having filters exploiting the sequence of voltage harmonics according to an embodiment of the present disclosure, and Fig. 7 is a block diagram showing a filter unit 200 of the power conversion system 1000.

Referring to Fig. 6 a power conversion system 1000 having filters exploiting the sequence of voltage harmonics may include a converter unit 100 having a plurality of three-phase converters connected in parallel, a filter unit 200, and a load unit 300. The load unit 300 is an AC load, for example a motor or AC source.

The converter unit 100 may include first to third converters 110-130 connected in parallel. In addition, each converter has a phase A output, a phase B output, and a phase C output. In the following specification and drawings, for clear explanation, the phase A output of the first converter is expressed as a1, and the phase C output of the second converter is expressed as c2. Therefore, b3 represents a phase B output of the third converter.

In addition, a control unit 102 may control so that a triangular wave of the second converter 120 has a phase behind a triangular wave of the first converter 110 by 120°, and a triangular wave of the third converter 130 has a phase ahead of a triangular wave of the first converter 110 by 120˚. For example, the control unit 102 may adjust a switching timing of each converter 110-120 to control a phase of voltage harmonics of each converter.

Figs. 8(a), 8(b) and 8(c) are respectively phasor diagrams showing positive-sequence voltage harmonics in a switching frequency

band of a first converter 110, a second converter 120 and a third converter 130. In Figs. 8(a), 8(b) and 8(c), the second converter 120 is controlled to have a phase behind the first converter 110 by 120°, and the third converter 130 is controlled to have a phase ahead of the first converter 110 by 120˚. Such phase control by 120° may be changed between converters, and accordingly, a connection order of each converter to a positive-sequence filter, a negative-sequence filter and a zero-sequence filter may also be changed.

The converters 110-130 connected in parallel generate three-phase voltage and a zero-sequence voltage from a DC power source 101 according to a control signal for controlling each converter current. Each of the converters 110-130 divides and outputs a load power equally, and may have a control unit in common or have control units separately for an interleaving operation. If each converter has a control unit individually, a communication link may be provided for synchronization among control units. The converters 110-130 are circuits for three-phase AC/DC power conversion, and may be implemented with various circuit configurations for three-phase AC-DC power conversion such as a multi-level converter.

Even though Fig. 6 shows three converters 110-130, this is just an example of the present disclosure, and a multiple number of three of converters may be further included.

Referring to Fig. 7, the filter unit 200 may include at least one of a zero-sequence harmonic filter 210, a positive-sequence harmonic filter 220, and a negative-sequence harmonic filter 230. In an embodiment, the filter unit 200 may be configured just with passive elements.

The zero-sequence harmonic filter 210, the positive-sequence harmonic filter 220, and the negative-sequence harmonic filter 230 may be connected in series, and a connection order of each of the filter units 210-230 are not limited to Fig. 7.

The zero-sequence harmonic filter 210 may include a first zero-sequence filter 211 for receiving a phase A output, a phase B output and a phase C output of the first converter 110, a second zero-sequence filter 212 for receiving a phase A output, a phase B output and a phase C output of the second converter 120, and a third zero-sequence filter 213 for receiving a phase A output, a phase B output and a phase C output of the third converter.

Each of the filters 211-213 may reduce zero-sequence harmonic components of AC signals transmitted from the converters 110-130.

Figs. 9(a) to 9(c) show a case where the zero-sequence harmonic filter 210 is a common-mode filter using a ring core. Figs. 9(a) to 9(c) show an example of the first to third zero-sequence filters 211-213, respectively. The positive-sequence harmonic filter 220 may include a first positive-sequence filter 221 for receiving a phase A output of the first converter, a phase C output of the second converter, and a phase B output of the third converter; a second positive-sequence filter 222 for receiving a phase B output of the first converter, a phase A output of the second converter, and a phase C output of the third converter; and a third positive-sequence filter 223 for receiving a phase C output of the first converter, a phase B output of the second converter, and a phase A output of the third converter.

Fig. 10 show a case where the positive-sequence harmonic filter 220 is a common-mode filter using a ring core, and Figs. 10(a) to 10(c) show an example of the first to third positive-sequence filters 221-223, respectively. In other words, the filters depicted in Figs. 10(a) to 10(c) may reduce positive-sequence current harmonics in a switching frequency

, among current harmonics by the voltage harmonics depicted in Fig. 8.

If the converters connected in parallel share power equally, a magnetic flux by the fundamental current is canceled in the core by means of the connection as shown in Fig. 10 since the magnitude of a fundamental current is identical among the converters. Meanwhile, in case a phase of the current harmonics is shifted by means of triangular wave phase-shift, the magnetic fluxes are overlapped in the core so that the magnetic core may give an large impedance. Therefore, in this embodiment, since a magnetic flux by the fundamental current is not formed in the core, the magnetic core may be implemented with a small size. In addition, since magnetic fluxes by the current harmonics are overlapped with each other, the current harmonics may be effectively suppressed.

In this same way, each filter of the negative-sequence harmonic filter 230 may be configured as follows.

The negative-sequence harmonic filter 230 may include a first negative-sequence filter 231 for receiving a phase A output of the first converter, a phase B output of the second converter, and a phase C output of the third converter; a second negative-sequence filter 232 for receiving a phase C output of the first converter, a phase A output of the second converter, and a phase B output of the third converter; and a third negative-sequence filter 233 for receiving a phase C output of the first converter, a phase A output of the second converter, and a phase B output of the third converter.

Fig. 11 shows a case where the negative-sequence harmonic filter 230 is a common-mode filter using a ring core, and Figs. 11(a) to 11(c) show an example of the first to third negative-sequence filters 231-233, respectively. In other words, the filters depicted in Figs. 11(a) to 11(c) may reduce negative-sequence current harmonics in a switching frequency

, among current harmonics by the voltage harmonics depicted in Fig. 8.

Accordingly, the positive-sequence harmonic filter 220 and the negative-sequence harmonic filter 230 may reduce positive-sequence and negative-sequence current harmonics in the switching frequency

band.

For example, the filter unit 200 may include all of the positive-sequence harmonic filter 220, the negative-sequence harmonic filter 230, and the zero-sequence harmonic filter 210, described above. In this case, the zero sequence, positive sequence and negative-sequence harmonics may be reduced. If the positive-sequence harmonic filter 220, the negative-sequence harmonic filter 230 and the zero-sequence harmonic filter 210 are formed with ring cores, zero-sequence harmonics, positive-sequence harmonics and negative-sequence harmonics may be reduced using nine ring cores in total.

Even though it has been described above that the positive-sequence harmonic filter 220, the negative-sequence harmonic filter 230 and the zero-sequence harmonic filter 210 are configured with ring cores, in another embodiment, the positive sequence, negative sequence and zero-sequence filter units 210-230 may also be configured with EI cores. In addition, the present disclosure is not limited to the ring core and the EI core.

Figs. 12(a) to 12(c) show an example of the filter unit 200 configured using EI cores. In Figs. 12(a) to 12(c), a positive-sequence harmonic filter (Fig. 12(a)), a negative-sequence harmonic filter (Fig. 12(b)) and a zero-sequence harmonic filter (Fig. 12(c)) are described when each of three filters is configured with an EI core.

In an embodiment, the power conversion system 1000 having filters exploiting the sequence of voltage harmonics may further include an additional filter unit 400. The additional filter unit 400 may be configured to include at least one inductor or capacitor. For example, the additional filter unit 400 may include existing filters described above with reference to Figs. 1(a) to 1(c). In particular, the additional filter unit 400 may reduce current harmonics at multiple of third switching frequency band (3fsw, 6fsw, 9fsw...).

In addition, the additional filter unit 400 may distributed and connected between a point of common coupling (PCC) 201 and the load unit 300, as shown in Fig. 6, or at each converter. For example, the additional filter unit 400 may be connected to output portions of the filters 211-213 or 221-223 or 231-233.

Figs. 13(a) and 13(b) show experiment results implementing the filter unit 200 using three EI cores. Fig. 13(a) shows a load current waveform at PCC 201, and Fig. 13(b) shows a current waveform of the first converter 110. Referring to Fig. 13(a), at the load-side, most current harmonics are canceled by means of 120° interleaving, thereby exhibiting low total harmonic distortion (THD).

In addition, referring to Fig. 13(b), it can be found that the phase-shifted positive sequence, negative-sequence current harmonics and the zero-sequence current are effectively suppressed by means of the proposed filters, so that the converter current has a waveform close to a sine wave, which is similar to a load current.

The power conversion system according to the present disclosure may be widely utilized for a large-capacity power conversion system requiring parallel operation of an AC-DC power conversion system, or in various applications requiring a small design of the entire power conversion system having an EMI filter by means of triangular wave phase shifting.

The present disclosure can be changed or modified in various ways without departing from the scope of the present disclosure by those having ordinary skill in the art, and thus is not limited to the above embodiments and the accompanying drawings.

Claims

A power conversion system having filters exploiting the sequence of voltage harmonics, which includes a converter unit having three or its multiple number of three-phase converters connected in parallel, a filter unit, and a load unit,

wherein the converter unit includes first to third converters having a phase A output, a phase B output, and a phase C output, respectively,

wherein the filter unit includes at least one of a positive-sequence harmonic filter, a negative-sequence harmonic filter, and a zero-sequence harmonic filter, and

wherein the filter unit is composed of passive elements.
The power conversion system having filters exploiting the sequence of voltage harmonics according to claim 1, wherein the positive-sequence harmonic filter includes:

a first positive-sequence filter configured to receive a phase A output of the first converter, a phase C output of the second converter, and a phase B output of the third converter;

a second positive-sequence filter configured to receive a phase B output of the first converter, a phase A output of the second converter, a phase C output of the third converter; and

a third positive-sequence filter configured to receive a phase C output of the first converter, a phase B output of the second converter, and a phase A output of the third converter.
The power conversion system having filters exploiting the sequence of voltage harmonics according to claim 1, wherein the negative-sequence harmonic filter includes:

a first negative-sequence filter configured to receive a phase A output of the first converter, a phase B output of the second converter, and a phase C output of the third converter;

a second negative-sequence filter configured to receive a phase C output of the first converter, a phase A output of the second converter, and a phase B output of the third converter; and

a third negative-sequence filter configured to receive a phase B output of the first converter, a phase C output of the second converter, and a phase A output of the third converter.
The power conversion system having filters exploiting the sequence of voltage harmonics according to claim 1, wherein the zero-sequence harmonic filter includes:

a first zero-sequence filter configured to receive a phase A output, a phase B output, and a phase C output of the first converter;

a second zero-sequence filter configured to receive a phase A output, a phase B output, and a phase C output of the second converter; and

a third zero-sequence filter configured to receive a phase A output, a phase B output, and a phase C output of the third converter.
The power conversion system having filters exploiting the sequence of voltage harmonics according to claim 1,

wherein the filter unit includes a positive-sequence harmonic filter, a negative-sequence harmonic filter, and a zero-sequence harmonic filter.
The power conversion system having filters exploiting the sequence of voltage harmonics according to claim 1,

wherein the positive-sequence harmonic filter, the negative-sequence harmonic filter, and the zero-sequence harmonic filter are configured to have at least one of a ring core, an EI core, a CI core, a cylindrical core, and a core where three EI cores for a positive-sequence harmonic filter, a negative-sequence harmonic filter and a zero-sequence harmonic filter are integrated into a single magnetic core.
The power conversion system having filters exploiting the sequence of voltage harmonics according to claim 1,

wherein the second converter has a triangular wave which has 120° phase difference with the triangular carrier wave of the first and the third converters and

wherein the third converter has a triangular wave which has 120° phase difference with the triangular carrier wave of the first and the second converters.
The power conversion system having filters exploiting the sequence of voltage harmonics according to claim 1, further comprising:

a control unit configured to control a phase of a triangular wave of the first to third converters.
The power conversion system having filters exploiting the sequence of voltage harmonics according to claim 1, further comprising:

an additional filter unit,

wherein the additional filter unit includes at least one inductor or capacitor.
The power conversion system having filters exploiting the sequence of voltage harmonics according to claim 8,

wherein the additional filter unit is formed between a point of common coupling (PCC) and the load unit or formed at each output end of the converter unit.