WO2009038336A2

WO2009038336A2 - Apparatus for improving power quality

Info

Publication number: WO2009038336A2
Application number: PCT/KR2008/005483
Authority: WO
Inventors: Hoonyang Park
Original assignee: Enertech Co. Ltd.
Priority date: 2007-09-18
Filing date: 2008-09-17
Publication date: 2009-03-26
Also published as: WO2009038336A3

Abstract

Disclosed is a power quality improving apparatus of a three-phase (R, S, and T phase) four- wire power distribution system. The power quality improving apparatus includes: a load; a power supply device that supplies three-phase power to the load; and an autotransformer that is connected in series between the load and the power supply device in order to reduce harmonics that flow through a neutral line of the three-phase four- wire power distribution system. In the power quality improving apparatus according to the present invention, different from the related art in which harmonic reducing devices need to be individually installed in a plurality of load ends, even though only one harmonic reducing device is installed between a receiving end and a distributing end, it is possible to obtain a harmonic reduction effect that is similar to that in the related art. Therefore, installation costs can be reduced. Since the harmonic reducing device is installed between the receiving end and the distributing end that have a relatively wider space than the load end, the harmonic reducing device can be easily installed in a building, as compared with the related art.

Description

APPARATUS FOR IMPROVING POWER QUALITY

Technical Field

[1] The present invention relates to an apparatus for reducing a harmonic current that flows through a neutral line in a three-phase four-wire power distribution system, and more particularly, to an apparatus for improving power quality that can reduce the amount of current that flows through a neutral line using a zigzag transformer as a type of autotransfomer. Background Art

[2] Currently, a standard power distribution method of a power distribution system that is used at the Korea Electric Power Corporation is a 3-phase, 4-wire, and Y-connection power distribution method, and a conducting winding line, which is connected to a neutral point of a Y-connection in the three-phase four- wire power distribution system and leads therefrom, is called a neutral line.

[3] Theoretically, in the case of the three-phase four- wire power distribution system, in a normal power distribution state, that is, loads connected to individual phases are equilibrated, the amount of current that flows through the neutral line becomes zero.

[4] However, in recent years, in commercial buildings, residential buildings, and factories, non-linear and disequilibrated loads, such as a computer device, an uninterruptible power supply (UPS), a rectification device, an illumination device, and an office device, have been widely used. For this reason, a harmonic current excessively flows through the neutral line of a power distribution system due to the non-linear and disequilibrated loads.

[5] In the case where the harmonic current flows through the neutral line due to the nonlinear and disequilibrated loads, the neutral line may overheat and cause a fire, and a breakdown and an erroneous operation may be generated in various apparatuses. When non-linear loads increase, the harmonic current may flow through a power supply, which may result in causing damage to other facilities in the power distribution system.

[6] Accordingly, in order to resolve the above-described problem, various methods that reduce a harmonic current flowing through a neutral line have been suggested. In the related art, a method that reduces a current flowing through a neutral line uses a method in which a harmonic reducing device using a zigzag transformer is installed on phase lines and a neutral line of a three-phase power supply, the detailed contents of which is disclosed in the following Document 1.

[7] However, in a technology according to the related art that is described in the following Document 1, a harmonic reducing device 3 is installed in parallel between a power supply device 1 functioning as a power receiving end and a load 2 so as to reduce a harmonic component that is generated in the load 2. Therefore, in order to minimize the amount of harmonic current that flows through the neutral line, the harmonic reducing device needs to be installed as close as possible to the load.

[8] Accordingly, in the case of a load distributing end that distributes 3-phase power to a plurality of loads, the harmonic reducing device needs to be installed between each of the loads and the distributing end. There is a problem in that installation costs increase.

[9] Further, when the harmonic reducing device is installed in an existing building, not a new building, it is difficult to install the harmonic reducing device due to a spatial limitation between a load and a distributing end.

[10] [Document 1] Korean Patent No. 428459 (registered on April 10, 2004, in the

Korean Intellectual Property Office). Disclosure of Invention Technical Problem

[11] Accordingly, the present invention has been made to solve the above-described problems, and it is an object of the present invention to provide a power quality improving apparatus that can achieve a harmonic reduction effect similar to that of the related art using only one harmonic reducing device installed in a power supply device (that is, receiving end), different from the related art in which harmonic reducing devices need to be individually installed in a plurality of load and distributing ends, thereby decreasing costs and the amount of work required when the harmonic reducing device is installed. Technical Solution

[12] In order to achieve the above-described object, a power quality improving apparatus according to the present invention is a power quality improving apparatus of a three- phase (R, S, and T phase) four- wire power distribution system. The power quality improving apparatus includes: a load; a power supply device that supplies three-phase power to the load; and an autotransformer that is connected in series between the power supply device and the load in order to reduce harmonics that flow through a neutral line of the three-phase four- wire power distribution system.

[13] The load is composed of at least one load module, the power supply device includes a receiving module and a distributing module that distributes the three-phase power supplied from the receiving module to at least one load module, and the autotransformer is installed between the receiving module and the distributing module.

[14] The autotransformer is a zigzag transformer that includes a core module having first, second, and third legs and a winding module having first, second, and third winding lines wound around the core module. The first, second, and third winding lines each include an input line that is connected to one of R, S, and T phases of the receiving module and an output line that leads from a tap of each winding line and is connected to the distributing module.

[15] The first winding line is wound in the order of the first leg, the second leg, and the first leg and connected to the neutral line, in a state where a winding direction of the second leg is opposite to a winding direction of the first leg. The second winding line is wound in the order of the second leg, the third leg, and the second leg and connected to the neutral line, in a state where a winding direction of the third leg is opposite to a winding direction of the second leg. The third winding line is wound in the order of the third leg, the first leg, and the third leg and connected to the neutral line, in a state where a winding direction of the first leg is opposite to a winding direction of the third leg.

Advantageous Effects

[16] In the power quality improving apparatus according to the present invention, different from the related art in which harmonic reducing devices need to be individually installed in a plurality of load ends, even though only one harmonic reducing device is installed between a receiving end and a distributing end 1, it is possible to obtain a harmonic reduction effect that is similar to that in the related art. Therefore, installation costs can be reduced.

[17] Since the harmonic reducing device is installed between the receiving end and the distributing end that have a relatively wider space than the load end, the harmonic reducing device can be easily installed in a building, as compared with the related art. Brief Description of Drawings

[18] FIG. 1 is a block diagram illustrating the structure of a power quality improving apparatus of a three-phase four- wire power distribution system according to the related art.

[19] FIG. 2 is a graph illustrating a phenomenon of a third harmonic current being expanded in a neutral line of a three-phase four- wire power distribution system.

[20] FIG. 3 is a block diagram illustrating the structure of a power quality improving apparatus of a three-phase four- wire power distribution system according to a first embodiment of the present invention.

[21] FIG. 4 is a diagram illustrating the winding structure of a zigzag transformer that is used in a power quality improving apparatus according to a first embodiment of the present invention.

[22] FIG. 5 is a block diagram illustrating the structure of a power quality improving apparatus of a three-phase four- wire power distribution system according to a second embodiment of the present invention. Best Mode for Carrying out the Invention

[23] Before specifically describing the preferred embodiments of the present invention, first, the theoretical background in which a flow of a harmonic current is generated in a neutral line of a three-phase four- wire power distribution system will be described using third harmonics that are zero sequence harmonics.

[24] FIG. 2 is a graph illustrating a phenomenon of a third harmonic current being expanded in a neutral line of a three-phase four- wire power distribution system. In FIG. 2, slant-printed symbols denote vectors.

[25] As shown in FIG. 2, currents of three phases (R, S, and T phases) in an equilibrium state have a phase difference of 120°as represented by the following Expression 1, and the amount of current that flows through the neutral line N is a sum of vectors of the three-phase currents (1 , 1 , and I ), and becomes zero as represented by the following

R S T

Expression 2. [26] [Expression 1]

[27] IRl = Imsinωt, ISl = Imsin(ωt-120°), ITl = Imsin(ωt-240°)

[28] [Expression 2]

[29] I +1 +1 = 1 sinωt + I sin(ωt-120°)+I sin(ωt-240°)= 0

Rl Sl Tl m m m

[30] However, in the cases of the third harmonics that flow through an R phase, the third harmonics that flow through an S phase, and the third harmonics that flow through a T phase, the phases thereof are the same. Therefore, as represented by the following Expression 3, the amount of current that flows through the neutral line is not zero but is expanded to a current larger than a phase current, which causes the damage by harmonics.

[31] [Expression 3]

[32] I Rl +1 Sl +1 Tl = 1 m sin3ωt + I m sin3(ωt-120°)+I m sin3(ωt-240°)

[33] = 31 sin3ωt

[34] That is, the thickness of the neutral line is generally smaller than or equal to the thickness of the other phases. If a large amount of current flows through the neutral line due to zero sequence harmonics, a cable may overheat. Further, since the third harmonics have a frequency component of 180 Hz that is triple the frequency component of a fundamental wave as represented by Expression 3, an effective cross section of a cable is reduced due to a skin effect. As a result, overheating is amplified due to an increase in resistance.

[35] As an example, in a transformer that is responsible for a load that generates harmonics, such as a personal computer, Joule's heat is increased because of an increase in resistance due to overlapping of harmonic currents and a skin effect. For this reason, the transformer that is responsible for the load that generates harmonics needs to have a capacity that is 2 to 2.5 times larger than a capacity of a transformer that is responsible for a load that does not generate harmonics.

[36] Next, the preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[37] FIG. 3 is a block diagram illustrating the structure of a power quality improving apparatus of a three-phase four- wire power distribution system according to a first embodiment of the present invention. The power quality improving apparatus shown in FIG. 3 includes a power supply device 10 that outputs three-phase power supplied from a power- transmission site (not shown), such as a power station, a load 20 that operates using the three-phase power supplied from the power supply device, and a harmonic reducing device 30 that is connected in series between the power supply device 10 and the load 20.

[38] As described above, the harmonic reducing device 30 shown in FIG. 3 is connected in series between the power supply device 10 and the load 20 and reduces a harmonic component that is included in the three-phase power output from the power supply device 10. That is, the harmonic reducing device 30 primarily reduces generation of a harmonic current due to the load 20.

[39] Accordingly, in the power quality improving device according to this embodiment, different from the related art, even when the harmonic reducing device 30 is installed close to the power supply device 10 without being installed close to the load 20, it is possible to obtain a reduction effect of harmonic current similar to that in the related art.

[40] In this embodiment, a zigzag transformer that is a type of autotransformer is used as an example of the harmonic reducing device 30, but the present invention is not limited thereto. It is possible to use any type of transformers that can be implemented as a type of autotransformer among known transformers for harmonic reduction.

[41] FIG. 4 is a diagram illustrating the winding structure of a zigzag transformer that is used in a power quality improving apparatus according to a first embodiment of the present invention. The zigzag transformer shown in FIG. 4 includes an iron core 50 that has a first leg 51, a second leg 52, and a third leg 53, and first, second, and third winding lines 61, 62, and 63 that are wound around the iron core 50.

[42] The first winding line 61 includes an input line R that is connected to an R phase of the power supply device 10 and an output line r that leads from a tap (not shown) of the first winding line 61. Like this, the second winding line 62 includes an input line S that is connected to an S phase of the power supply device 10 and an output line s that leads from a tap (not shown) of the second winding line 62. The third winding line 63 includes an input line T that is connected to a T phase of the power supply device 10 and an output line t that leads from a tap (not shown) of the third winding line 63. [43] At this time, the tap of each of the winding lines 61, 62, and 63 may be differently installed depending on the capacity of each load. In this embodiment, in each of the winding lines 61, 62, and 63, the tap may be installed at a point at which a secondary voltage of the autotransformer becomes 96% of a primary voltage.

[44] Meanwhile, the first winding line 61 is wound in the order of the first leg 51, the second leg 52 and the first leg 51 and then connected to the neutral line N. At this time, the wounding direction of the second leg 52 is opposite to that of the first leg 51.

[45] Similarly, the second winding line 62 is wound in the order of the second leg 52, the third leg 53, and the second leg 52 and then connected to the neutral line N. At this time, the wounding direction of the third leg 53 is opposite to that of the second leg 52.

[46] Further, the third winding line 63 is wound in the order of the third leg 53, the first leg 51, and the third leg 53 and then connected to the neutral line N. At this time, the wounding direction of the first leg 51 is opposite to that of the third leg 53.

[47] Like this, in the case where magnetic flux is the same but the winding directions are opposite to each other in the individual winding lines 61, 62, and 63, phases of zero sequence harmonics that are generated in the load become opposite to each other and the magnetic flux is offset. As a result, the zero sequence harmonics are automatically reduced.

[48] The zigzag transformer that has the above-described structure primarily reduces the harmonic component that is supplied from the power supply device 10, as described above. Then, when the zero sequence harmonic current is generated from the side of the load 20, the zero sequence harmonic current is made to flow through the load 20 due to the zigzag transformer having low image impedance, such that only a positive- phase- sequence component current and a negative-phase- sequence component current flow through the neutral line. As a result, it is possible to secondarily prevent a large amount of zero sequence harmonics from flowing through the neutral line of the three- phase four- wire power distribution system.

[49] FIG. 5 is a block diagram illustrating the structure of a power quality improving apparatus of a three-phase four- wire power distribution system according to a second embodiment of the present invention. In this case, the same components as those in the first embodiment are denoted by the same reference numerals.

[50] The power quality improving apparatus shown in FIG. 5 includes a receiving end 10a that outputs three-phase power supplied from a power-transmission site (not shown), such as a power station, a distributing end 10b that distributes the three-phase power supplied from the receiving end 10a to a plurality of loads 20a and 20b, and a harmonic reducing device 30 that is connected in series between the receiving end 10a and the distributing end 10b.

[51] At this time, since the structure of the harmonic reducing device 30 is the same as that of the first embodiment, the detailed description thereof will be omitted.

[52] The power quality improving apparatus according to this embodiment is used for a large-capacity distribution line where a plurality of loads are connected to each other. Different from the related art in which the harmonic reducing devices 30 need to be individually installed in the plurality of load ends, even though only one harmonic reducing device 30 is installed between the receiving end 10a and the distributing end 10b, it is possible to obtain a harmonic reduction effect that is similar to that in the related art. Therefore, installation costs can be reduced.

[53] Since the harmonic reducing device 30 is installed between the receiving end 10a and the distributing end 10b that have a relatively wider space than the load end, the harmonic reducing device 30 can be easily installed in a building, as compared with the related art.

[54] The power quality improving apparatus according to the present invention may be used to reduce a harmonic current that flows through the neutral line of the three-phase four-wire power distribution system. In particular, the power quality improving apparatus may be used for a large-capacity distribution line that includes a plurality of load ends.

Claims

[1] A power quality improving apparatus of a three-phase (R, S, and T phase) four- wire power distribution system, comprising: at least one load; a power supply device that supplies three-phase power; a distributing unit that distributes the three-phase power supplied from the power supply device to at least one load; and an autotransformer that is connected in series between the power supply device and the distributing unit in order to reduce harmonics that flow through a neutral line of the three-phase four- wire power distribution system.

[2] The power quality improving apparatus of claim 1, wherein the autotransformer is a zigzag transformer that includes a core module having first, second, and third legs and a winding module having first, second, and third winding lines wound around the core module, and the first, second, and third winding lines each include an input line that is connected to one of R, S, and T phases of a receiving module and an output line that leads from a tap of each winding line and is connected to a distributing module.

[3] The power quality improving apparatus of claim 2, wherein the first winding line is wound in the order of the first leg, the second leg, and the first leg and connected to the neutral line, in a state where a winding direction of the second leg is opposite to a winding direction of the first leg, the second winding line is wound in the order of the second leg, the third leg, and the second leg and connected to the neutral line, in a state where a winding direction of the third leg is opposite to a winding direction of the second leg, and the third winding line is wound in the order of the third leg, the first leg, and the third leg and connected to the neutral line, in a state where a winding direction of the first leg is opposite to a winding direction of the third leg.