JPS624932B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a disconnection detection relay for a power distribution line.

Generally, our country's power distribution system (6.6KV or 3.3KV)
Electricity is distributed in a network of densely populated areas, and due to cost considerations, electricity is distributed over the ground using overhead cables rather than underground cables, except in the center of large cities.

For this reason, accidents often occur in which power distribution lines are cut due to flying obstacles or contact with trees during typhoons, or by retrieval trucks and crane trucks at construction sites.

In the past, most disconnected distribution lines came into contact with the ground, causing a ground fault that could be detected by a ground fault detection relay at the substation, but recently, from a safety standpoint, distribution lines have been coated with insulation. Treated so-called insulated wires have come into widespread use, so in the event that the wire breaks, even if it falls to the ground, it will not turn into a ground fault due to the covering, and will therefore be charged without being detected. It is predicted that the distribution lines will be left in an extremely dangerous condition if left unattended. Although it is said that accidents such as electric shocks have decreased significantly since the introduction of insulated wires, in the event of a wire breakage, the situation is even more dangerous than before.

For this reason, the need for relays for detecting disconnection in power distribution lines has recently increased. Various disconnection detection relays have been proposed and some have been put into practical use, but in the case of distribution lines, only two of the three phases (A phase and C phase) are CT
Currently, the method of detecting wire breakage is severely restricted because the wire breakage detection method is not installed. A method has also been proposed in which a disconnection in other phases is detected by an increase in current detected by a CT installed in two phases, but in a typical three-phase AC system, a disconnection causes the current in the remaining phases to decrease by √3/ It is not suitable for general use because it decreases by a factor of 2. FIG. 9 is a general power distribution system diagram, and it is assumed that line impedance and load are balanced.

The current before disconnection is: ia=V _A /Z _LA +Z _A ib=V _B /Z _LB +Z _B ic=V _C /Z _LC +Z _CHere , if the a phase is disconnected, Therefore, in general, when one phase is disconnected, the current in the other two phases is √3/2 of that before the disconnection, and does not increase.

The present invention provides a disconnection detection relay that can easily and reliably detect a disconnection accident using only CTs for two phases of a distribution line, which is subject to this restriction.

Figure 1 shows the current power distribution system, where 1 is the line disconnector, 2 is the A-phase CT of the line, and 4 is the line. The primary current of line 4 during constant load operation is
If Ia, Ib, and Ic, the current on the CT secondary side is only the A-phase current ia and the C-phase current ic, as shown in Figure 2, but the phase difference between them should be approximately 120°. .

On the other hand, in the event of a disconnection fault, the CT secondary currents ia and ic will be as follows.

(b) If the B phase without CT is disconnected, the 3rd
As shown in the figure, the CT secondary currents ia and ic become antiphase currents.

Therefore, the current phase difference between IA and IC when the system is healthy is 120
By focusing on the fact that a phase difference of 180° occurs when a B-phase wire breaks compared to that of 180°, it is possible to detect a B-phase wire breakage.

(b) If the A or C phase containing the CT is disconnected, the A
When a phase or C phase is disconnected, it can be detected by paying attention to the fact that the current is equivalent to one phase.

In order to make use of the ideas (a) and (b) above, the following points must be taken into consideration.

That is, firstly, determination becomes possible only when a certain constant current exists. In the case of (a) above, it is first necessary to perform phase comparison under the condition that ia and ic are equal to or greater than a certain value (necessary for phase determination).
Now, if the rated current of the line is IN and a certain threshold value is K ₁ IN (K ₁ <1), then ia is as shown in Figure 4.
≧K ₁・IN ic≧K ₁・IN only after both conditions are met
It is sufficient to open the gates of the phase comparison judgment outputs of ia and ic. In Figure 4, 5 and 6 are ia and ic, respectively.
7 is a level detection circuit that outputs an output signal when K is larger than ₁・IN.The phase difference between IA and IC is 120° or more, and in reality, considering the unbalance of IA and IC and other errors, ) is a phase discrimination circuit that outputs an output when there is a phase difference of more than ), and 8 is an AND circuit. Note that α is considered to be approximately 30°, but it may be adjusted according to the actual situation of the distribution line. Of course, as is clear from the above explanation, it goes without saying that α = 0° in theory.

In the case of (b) above, it is necessary to judge when the two-phase current becomes one phase, so ia≧ as shown in Figure 4.
It is necessary to use an OR condition rather than an AND condition of K ₁・IN and ic≧K ₁・IN. Furthermore, as shown in Fig. 5, when the load current approaches the level K ₁ ·IN, if one of IA or IC falls below the level of K ₁ ·IN due to variations, it appears at first glance that there is only one phase current; There is a risk of erroneously determining that the wire is disconnected, so if either ia or ic falls below the level of K ₁・IN, check it at an even lower level of K ₂・IN (K ₂ < K ₁ ). Even so, it is only necessary to determine that the wire is disconnected when there is only one phase current. (FIG. 5 shows an example where ic is slightly lower than ia.) FIG. 6 shows a concrete implementation block diagram for case (b). 9 and 10 are ia<K ₂ IN and ic<K ₂・
11 is an OR circuit, and 12 is an inhibit circuit.

Therefore, if you combine Figures 4 and 6, you will get two phases.
It becomes possible to reliably detect disconnections in distribution lines where only CT exists.

The purpose of the present invention is not to simply combine FIG. 4 and FIG. 6, but to simplify FIG. 6 in particular to obtain a disconnection detection relay with a simple structure as a whole.

That is, in FIG. 6, the output from the level detection circuit 5 is A, the output from the level detection circuit 6 is B,
If the output from the level detection circuit 9 is C and the output from the level detection circuit 10 is D, then A has ia at the reference value.
Since K indicates that ia is larger than K ₁・IN, and C indicates that ia is smaller than the reference value K ₂・IN (here, K ₁・IN > K ₂・IN), between A and C. The 7th
The relationship shown in Figure b holds true. Similarly, the relationship shown in FIG. 7c holds between B and D.

That is, A.C=0 and B.D=0.

Furthermore, between A and B, there are four skills shown in Figure 7a.

Now, in Fig. 6, the output of 8a is expressed as A,
Using B, C, and D, and showing the logical sum (+) and logical product (・), the output from 8a = (A+B)・(A・)・(C+
D), which is expressed as the following formula. That is, (A+B)・(A・)・(C+D) =(A・A・+B・A・)・(C+D) =A・(C+D) (〓B・=0) =A・・D (〓A・C=0) =A・D (〓・D=D) Similarly, output from 8b=(A+B)・(・B)・(C+
D) =(A..B+B..B).(C+D) =B(C+D)=B.C Therefore, the logic operation circuit in Figure 6 consists of two
This means that it can be constructed using an AND operation.

From the above, the principle of wire breakage detection according to the present invention can be expressed as a block diagram as shown in FIG.

Next, a calculation method for calculating the block diagram of FIG. 8 by digital processing will be described.
That is, an example will be described in which analog current information obtained from a current transformer is sampled at certain time intervals, converted into digital quantities, and then digitally calculated.

When ia = Ia sin wt ic = Ic sin (wt + θ) (a) As a method for calculating the magnitude of ia, ic, let's take the case of sampling at an electrical angle of 30° as an example. As a method (addition method), a method using the following formula as the calculation principle can be considered.

That is, this method is based on the principle that if six amounts (or even 12 amounts) of data sampled at 30° intervals are successively added, a certain amount will be obtained.

Next, as a second method (multiplication method), one can be considered that uses the following formula as the calculation principle: 1=sin ² φ+cos ² φ. That is, this method is Ia ² = Ia ² sin ² (wt) + Ia ² cos ² (wt) = Ia ² sin ² (wt) + Ia ² sin ² (wt−π/2) (Iasinwt) ² + [Iasin {w( t-Δt)}] ² . Here, Δt is a time corresponding to an electrical angle of 90°, and when sampling is performed at 30°, Iasinw (t−Δt) corresponds to data three times before Iasinwt.

(b) Method to see the phase difference between ia and ic Iasinwt・Icsin(wt+θ)+Iasinw(t−Δt)・Icsin{w(t−Δt)+θ} = Iasinwt・Icsin(wt+θ)+Iacoswt・Iccos(wt+θ)=Ia・Ic cosθ Therefore, it can be calculated as cosθ.

In actual application, when calculating Ia and Ic using the above addition formula, The magnitude relationship between θ and (120° + ^α ) can be calculated by comparing the ^magnitude with Ia and Ib can be found by
² and [(Iasinwt) ² + {Iasinw (t-Δt)} ² ] ・[{Icsin{wt+θ)} ² + {Icsin (wt-wΔt)+θ} ² ]・cos ² (120°+
By comparing the magnitude with α), it is possible to compare the magnitude of θ and (120° + α).

As described above, according to the present invention, a disconnection detection relay that can reliably respond to inputs from CTs for two phases has an extremely simple configuration of four level detection circuits, one phase discrimination circuit, and three AND circuits. can be obtained.

[Brief explanation of the drawing]

Figure 1 is the current standard distribution system diagram, Figure 2 is 2
Current vector diagram of phase CT, Figure 3 is a current vector diagram of A phase and C phase when B phase is disconnected, Figure 4 is a block diagram for explaining the principle of B phase disconnection detection, and Figure 5 is A phase current vector diagram. 6 is a block diagram for explaining the principle of A-phase or C-phase wire breakage detection; FIG. 7 is a diagram for explaining the idea that led to this invention; FIG. The figure is a block diagram showing one embodiment of the present invention, and FIG. 9 is a diagram showing voltage, current, and impedance in a general power distribution system. In the figure, 5, 6, 9, and 10 are level detection circuits, and 7 is a In the phase discrimination circuit, 8 is an AND circuit serving as an output circuit. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] 1 A-phase current ia, c-phase current ic of the 3-phase distribution line
a device for deriving the above A-phase current and C-phase current, first and second level detection circuits that respectively output an output when the above-mentioned A-phase current and C-phase current are above a first predetermined value, and a phase difference between the above-mentioned A-phase current and C-phase current of 120°. A phase discrimination circuit which produces an output in the above cases, a first output circuit which produces an output when the first level detection circuit, second level detection circuit and phase discrimination circuit all produce outputs, a phase current and c
Third and fourth level detection circuits each output an output when the phase current is equal to or less than a second predetermined value smaller than the first predetermined value, and the second level detection circuit and the third level detection circuit all output an output. A disconnection detection relay comprising a second output circuit that outputs an output when the output is output, and a third output circuit that outputs an output when the first level detection circuit and the fourth level detection circuit all output the output. .