JPH0213230A - Overcurrent preventing method for neutral line in three-phase four-wire distribution system - Google Patents

Abstract

PURPOSE:To prevent overcurrent in the neutral line by connecting a three-phase active filter between a power source and a load and eliminating three-phase a harmonic current flowing through the active filter. CONSTITUTION:An active filter AF is arranged for each trunk line of distribution system or the output bus from a transformer Tr. The gate turn-OFF thyristor in the three-phase bridge circuit of the active filter AF is controlled to maintain the current flowing through a DC reactor DL constant so as to feed a harmonic current IM to the distribution system. Consequently, the harmonic current is eliminated from the neutral line thus eliminating the unbalanced current in the neutral line. As a result, it appears as if only a balanced load is connected to a three-phase four-wire distribution system thus causing no overheat due to the overcurrent flowing through the neutral line nor burning due to overheat.

Description

DETAILED DESCRIPTION OF THE INVENTION A2m Commercial Application Field The present invention relates to a method for preventing overcurrent in a neutral line in a three-phase four-wire power distribution system employed in factory power distribution, in-building power distribution, and the like.

39 Summary of the Invention The present invention connects a single-phase load between the neutral wire and another phase of a three-phase, four-wire distribution line, and connects a three-phase active filter between the load and the power supply. The neutral line current is suppressed by preventing three-phase harmonic currents.

C1 Prior Art FIG. 3 shows an example of load connection in a three-phase, four-wire power distribution system, which is increasingly being used in normal factory power distribution and in-building power distribution.

As shown in the figure, three-phase load L receives three-phase four-wire power distribution.
s, a single-phase load is connected, but in this case, the single-phase load is distributed as evenly as possible between the neutral wire N and the other phases U, V, and W so that the negative 6:j has been done. i'r is a transformer.

01 Problems to be Solved by the Invention No matter how evenly a single-phase load is distributed to each phase, if there is a nonlinear load such as a single-phase transformer or a single-phase rectifier in the single-phase load, third-order harmonics will occur. Generates wave current. Particularly; in recent years, the number of electronic devices such as OA and FA has increased, and since switching regulators are used in their power supply units, there is a tendency for a large amount of third harmonic current to flow. Unlike the fundamental wave current, the harmonic current does not flow in a well-balanced manner between the U phase, ■ phase, and W phase, but always flows through the neutral wire N as shown in FIG. Of course, it is well known that if each single-phase load L3 centered on the N phase and connected to the U, V, and W phases is in an unbalanced state, the unbalanced current flows through the N phase even though it is a fundamental wave current. be. Considering these, three-phase four-phase
Line distribution voltage transformation f! ! The thickness of the neutral line N of the Tr is set to 1/of each phase.
It is selected so that an unbalanced current of about 3 may flow.

In addition, in a current example for a system with a three-phase 41;I power distribution system and a relatively large number of single-phase rectifiers as loads,
Values such as 418 for phase, 69A for W phase, and 110A for N phase are shown. In this way, more current flows in the N phase than in the other phases, and when looking at the component meter of this current, it can be seen that it is a third harmonic current, as shown in FIG. As a result, the neutral phase winding of the transformer W'rr and the neutral phase T1 wire become overcurrent, creating the risk of overheating and sintering. This overcurrent cannot be detected by ordinary two-phase or three-phase overcurrent relays alone, and it is thought that the danger will increase in the future with the normal level of protection for distribution lines.

Accordingly, it is an object of the present invention to provide a method for preventing overcurrent flowing through a neutral conductor.

Means for Solving Problem E9 The present invention provides a method for connecting a single-phase load between a neutral wire and another phase in a three-phase, four-wire power distribution system, in which a single-phase load is connected to a three-phase distribution line between the load and the power source. An active filter is provided.

28 Flow to U, V, W phase by active filter? :&
The harmonic current is removed and the current is improved to only the fundamental wave, so even if an unbalanced current flows due to the unbalance of each single-phase load, a balanced current flows from the active filter to the power supply side, and the harmonics flowing to the neutral line are eliminated. The current disappears.

G. Example FIG. 1 shows an embodiment of the present invention.

In this figure, the same parts as in FIG. 3 are given the same reference numerals and their explanations are omitted. An active filter AP is installed on the bus bar.

For example, as shown in FIG. 2, this active filter AP is composed of a three-phase bridge circuit B made of a self-extinguishing element such as a GTO (gate turn-off circuit), and a DC reactor DL, and has a U-phase.

Connected to phase ■ and W phase.

The load current IL containing harmonics is the fundamental wave current I3 and the harmonic current ■. Those that have been multiplied by h4, that is! .

=Is+1. It can be expressed as. If a current IN is injected into the distribution line from the active filter AP connected in parallel with the load, the load current iL remains the same, but the power supply current can be reduced to only 3.

For this purpose, the GTO of the three-phase hybrid circuit B of the active filter AP is controlled, and the DC reactor DL is controlled.

While keeping the current flowing through constant, record the harmonic current Ix? 1! feed the line. In this case, the pulse width modulation method is used as a method for converting the DC current into an AC component commensurate with the harmonic current generated by the load. That is, the active filter AP basically operates as follows.

The first is hai? ! ! The harmonics flowing in the U, V, and W phases of the line are removed to improve the current to only the fundamental wave.

Second, even if an unbalanced current flows due to the unbalance of each single-phase load, the m source side (left side in FIG. 1) of the active filter AP is improved so that the current is balanced.

Therefore, due to these actions, the harmonic current flowing through the neutral wire N disappears, and the unbalanced current flowing through the neutral tlAN also disappears. As a result, it seems as if only the balanced linear load is arranged in the three-phase 41a? The i of the neutral phase 7If line is connected to the U line, which is close to the ideal state close to electrical theory.
! J? There is no need to worry about overheating due to the Ii flow and burning due to overheating.

Note that the effect would be ideal if the active filters ΔF were distributed and arranged in the vicinity of the loads that generate harmonics, but this would cause a problem in terms of cost, so in the present invention, they are installed separately for each branch main line. It is set up.

Further, the embodiment disposed at the position indicated by the dotted line is the most advantageous in terms of cost, and is an example in which the focus is solely on saving the transformer Tr, which is susceptible to thermal effects.

H. More than the effects of the invention - According to the present invention, the installation position of the active filter is 1! By flowing a balanced fundamental current to the source side, harmonic currents and unbalanced currents flowing to the neutral phase wiring are prevented, so there is no risk of the neutral phase wiring burning out. .

Further, in addition to the problem of harmonic current flowing through the neutral wire of the wiring Tr1, problems specific to transformers can also be solved. In other words, when a 3rd harmonic current is flowing, if there is a Δ connection in the transformer winding, this current circulates through the Δ connection and rises to the magnitude of the 3rd harmonic current. However, this problem can be solved by eliminating the third harmonic current.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a block diagram of an active filter used in the present invention, and Fig. 3 is a conventional three-phase 4! FIG. 4 is a current flow state diagram for explanation, and FIG. 5 is a current waveform diagram for explanation. Tr...Transformer, L...Single phase load,...Three phase load, AP...Active filter. According to the present invention, Fig. 1 Kutebhui Fig. 2 + main phase IL Bizawa form Fig. 5

Claims (1)

[Claims] In a system in which a single-phase load is connected between the neutral wire of a three-phase, four-wire distribution line and another phase, an active filter connected to the three-phase distribution line is provided between the load and the power supply, and the current flows through the neutral line. A neutral line overcurrent prevention method for three-phase four-wire power distribution characterized by suppressing current.