1. Field of the Invention
The present invention relates to an apparatus and a method for reducing a neutral current in a three-phase four-wire type power distribution system; and, more particularly, to an apparatus and a method for reducing a neutral current by changing an arrangement of loads connected to each phase of at least one among top and bottom stage power lines according to a level of each phase current of the top and the bottom stage power lines in a three-phase four-wire type two stage electric pole in a power distribution system.
2. Background of the Related Art
Currently, a three-phase four-wire type Y-connection power distribution system is adopted as a standard power distribution method in Korea, USA, Canada, Taiwan, etc. In a partial period, the power distribution system is operated with a one-stage electric pole in combination with a two-stage electric pole. In the three-phase four-wire type power distribution system, a conduction line drawn by being connected to a neutral point of Y-connection is referred as a neutral line, for the case of the two stage electric pole in power distribution system, as shown in FIG. 1, three-phase currents flow through each of a top stage power line 1 and a bottom stage power line 2, individually, and a neutral line(not shown) prepared at the neutral line power line 3 is commonly used by the top stage power line 1 and the bottom stage power line 2.
Theoretically, in case of the three-phase four-wire type two stage electric pole in power distribution system, a normal power distribution state, i.e., when the loads connected to phase lines (not shown) in each of the top stage power line 1 and the bottom stage power line 2 become an equilibrium state to each other, the current flowing through the neutral line becomes 0.
However, as the usages of nonlinear and imbalance loads such as a computer device, an UPS(Uninterruptible Power Supply), a rectifying device, an illumination device and an office device have been rapidly increased recently in a commercial building, a residential building and a factory or the like, the current due to the imbalance of the load flows excessively at the neutral line in a practical power distribution system.
In case that the current flows through the neutral line due to the above-described imbalance load, there occur overheats of the neutral line and fire, and an insulation breakage and erroneous operations of various devices. Further, in case that the non-linear load among the loads increases, there occur problems that the other facilities are damaged in the system by flowing a harmonic wave current into a side of the power supply.
Therefore, in order to solve the above-described problems, various techniques have been proposed for reducing the current flowing into the neutral line. A conventional neutral line current reduction technique applies the method of installing a zigzag transformer or a filter circuit on the phase line and the neutral line of the three-phase power for a one-stage electric pole in a power distribution system. The detailed contents for these are disclosed in detail in the following reference 1 and reference 2 or the like.
However, the conventional techniques disclosed in the following references 1 and 2 or the like do not contain an example which is applied to a two-stage electric pole in a power distribution system. Also there are the problems of the installation being complex and a great deal of cost being consumed since construction of hardware, such as a transformer or a filter circuit, is complex and a large space is required.
And also, although assumes that the conventional techniques disclosed in the following references 1 and 2 or the like are applied to the three-phase four-wire type two stage electric pole power distribution system, such conventional techniques have problems that it is difficult for actively coping with the case that the loads connected to the phases of each of the top stage power line 1 and the bottom stage power line 2 are changed with the lapse of time.
[Reference 1] Korean registration patent No. 557778 (Registered on Feb. 27, 2006)
[Reference 2] Korean Publication patent No. 2004-8610 (Published on Jan. 31, 2004)