JPH0576585B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a current detection device for detecting phase current of a three-phase unbalanced load circuit including a welding machine or the like. BACKGROUND OF THE INVENTION Voltage flickers occur in power distribution lines that include welding machines, so to prevent this, capacitors are connected between the lines. In this case, the compensation capacity must be determined based on the phase current. Therefore, conventionally, when many single-phase welding machines with different capacities are connected to a three-phase AC circuit, the load current of each welding machine is detected by a current transformer, and the phase currents of the same phase are added. ,
The total phase current was determined, and the capacitor capacity of the flicker compensator was determined based on this total phase current. Problems to be Solved by the Invention If the phase current of each load is detected as described above, current transformers equal to the number of loads are required, which inevitably increases the cost of the device. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a current detection device that can easily approximate phase current. Means for Solving the Problems In order to achieve the above object, the present invention provides the following features: first, second, and third line current detectors provided on the first, second, and third lines to detect line currents a, b, and c of the two-phase and third-phase lines; , are connected to the first, second, and third line current detectors, and the phase currents x, y, and z of the first, second, and third phases of the unbalanced load are expressed as x=A/ 3+a ² +b ² -2c ² /3A y=A/3+b ² +c ² -2a ² /3A z=A/3+c ² +a ² -2b ² /3A (However, A is

It is [ ]. ) was provided. Operation In the above invention, the line current of each phase obtained from the line current detector of each phase is sent to the line current-phase current conversion circuit and converted into a phase current based on the above equation. Therefore, the phase current can be approximately known without directly detecting the phase current. Embodiment Next, a current detection device in a welding machine circuit equipped with a flicker compensator according to an embodiment of the present invention will be described with reference to FIGS. 1 and 2. In FIG. 1, 1, 2, and 3 are the first, second, and third lines 1, 2, and 3 of a three-phase AC circuit,
Connected to transformer 4. 5, 6, 7, 8,
Reference numerals 9, 10, 11, 12, and 13 are single-phase loads having almost the same power factor, and in this embodiment, they are welding machines that generate flicker. Reference numerals 14, 15, and 16 indicate first, second, and third current detectors each consisting of a current transformer, and detect the line currents a, b, and c of the lines 1, 2, and 3, respectively. 17 is a line current to phase current conversion circuit, which converts the line currents a, b, and c of each phase detected by the current detectors 14, 15, and 16 into phase currents x,
This is a circuit that converts into y and z. 18, 19, 20
is a flicker compensator, for example, a capacitor is connected to each line 1, 2, 3 via a thyristor switch.
It is configured to be connected between. Note that these flicker compensators 18, 19, 20
is configured to determine the flicker compensation capacity based on each phase current x, y, z obtained from the line current-phase current conversion circuit 17. FIG. 2 shows the line current-phase current conversion circuit 17. This conversion circuit 17 converts line currents a, b, c into phase currents x, y, z based on the following equations (1), (2), and (3).
is configured to convert to . x=A/3+a ² +b ² -2c ² /3A ...(1) y=A/3+b ² +c ² -2a ² /3A ...(2) z=A/3+c ² +a ² -2b ² /3A ... …(3) However,

[Image Omitted] Next, it will be explained that the phase currents x, y, and z can be approximately obtained by equations (1), (2), and (3). Assuming that each power factor of each load 5 to 13 in Fig. 1 is the same, the summed phase currents x, y, and z of each phase have a phase difference of 120 degrees, and the vector diagram in Fig. 3 It can be expressed.
Then, each phase current x, y, z and each line current a, b,
Since the relationship with c is as shown in FIG. 3, the following equation (4) holds between the two. a ² =z ² +x ² −2zxcosθ b ² =x ² +y ² −2xycosθ c ² =y ² +z ² −2yzcosθ ...(4) Since θ=2/3π, x ² +y ² +xy=b ² y ² +z ² +yz=c ² z ² +x ² +zx=a ² ...(5) From this equation (5), the following equation (6) is established. 2(x+y+z) ² −3(xy+yz+zx)=b ² +c ² +a ²
...(6) On the other hand, the following formula (7) holds true according to the formula. 3 (xy + yz + zx) ² = 2 (b ² c ² + c ² a ² + a ² b ² ) - (b ⁴ + c
⁴ + a ⁴ )...(7) From equation (7), the following equation is obtained.

[ka] Substituting equation (8) into equation (6) yields the following equation.

[C] Hereinafter, equation (9) is referred to as A. Furthermore, the following equation holds true from equation (5). b ² −c ² =x ² −z ² +y(x−y)−(x−z)(x+y
+z) c ² -a ² = (y-x) (x+y+z) a ² -b ² = (z-y) (x+y+z) ...(10) x+y+z=b ² -c ² /x-z=c ² - a ² /y−z=a ² −b ²
/z-y=A x-z= ^b2 - ^c2 /Ay-x= ^c2 - ^a2 /Az-y= ^a2 - ^b2 /A...(11) Equation (11) and x+y+z From the relationship =A, the following equation holds true. 3x=A+ ^b2 - ^c2 /A- ^c2 - ^{a2 /A3y=A+b2-a2/A-a2-c2/A3z=A+c2-b2 / A - b2 - a2 /} A...(12) From this equation (12), the above-mentioned equations (1), (2), and (3) can be obtained.
Therefore, equations (1), (2), and (3) represent the phase current of each phase. The line current-phase current conversion circuit 17 in FIG.
In order to execute the calculation of equation (3), the line current a detected by the current detectors 14, 15, 16 of each phase in FIG.
Square calculators 24, 25 for calculating the square values of b and c,
26 in the current detection lines 21, 22, and 23 of each phase. ^At the output ^stage of each square calculator 24 ^{, 25 ,} 26, there is ^a Adders 27, 28, 29
Further, in order to obtain 2c ² , 2a ² , and 2b ² of equations (1), (2), and (3), double value calculators 30, 31, and 32 are provided. 33, 34, and 35 are subtracters, which subtract the outputs of the arithmetic units 30, 31, and 32 from the outputs of the adders 27, 28, and 29 in the previous stage, and obtain (1)(2)(3).
This is to obtain the equations a ² +b ² -2c ² , b ² +c ² -2a ² , c ² +a ² -2b ² . 36 is an adder; adder 29
The output b 2 of the squaring calculator 25 is added to the output c ² +a ^{2 of} , to obtain a ² +b ² +c ² included in A in equations (1), (2), and (3). 37, 38, and 39 are dividers, and the outputs of subtractors 33, 34, and 35 in the previous stage were obtained separately.
3A to find the second term in equations (1), (2), and (3). 40, 41, and 42 are adders, which are obtained separately from the outputs of the previous stage dividers 37, 38, and 39 (1) (2)
The value A/3 of the first term of equation (3) is added to output the phase currents x, y, and z of equations (1), (2), and (3). The adders 43, 44, 45 connected to the line current detection lines 21, 22, 23 are
This is to find a+b, b+c, and c+a in the expression. The adder 46 outputs the output a of the adder 43.
(a+b+c) in the equation representing A is obtained by adding c on line 23 to +b. Subtractor 4
7 subtracts the output of line 23 from the output of adder 43 to obtain a+b−c of the equation representing A. The subtracter 48 subtracts the output of the line 22 from the output of the adder 44 to obtain (-a+b
This is a circuit that obtains +c). The subtracter 49 is a circuit that subtracts the output of the line 22 from the output of the adder 45 to obtain (ab+c) of the equation representing A. The multiplier 50 multiplies the output of the adder 46 and the output of the subtracter 47 to obtain the formula (a+b+c)(a+b
-c). Multiplier 51 is subtracter 4
This circuit multiplies the output of 8 and the output of the subtracter 49 to obtain (-a+b+c)(ab+c) of the equation representing A. The multiplier 52 is the two multipliers 5 in the previous stage.
Multiply the two outputs of 0 and 51 to obtain the formula (a+b+c)(-a+b+c)(a-b+c)(a
+b−c) (hereinafter referred to as B). The multiplier 53 receives the output B of the multiplier 52 in the previous stage.
This is a circuit that obtains B×3 based on . The square root calculator 54 is a circuit that obtains the square root √3 of the output 3B of the multiplier 53 at the previous stage. The adder 55 is a circuit that adds the output of the arithmetic unit 54 in the preceding stage and the output of the adder 36 to obtain the equation representing A, a ² +b ² +c ² +√3. The 1/2 divider 56 is a circuit that obtains 1/2 of the output of the adder 55 at the previous stage. Square root calculator 57
is the square root of the output of the 1/2 divider 56 in the previous stage, that is, A
This is to obtain the output of the formula that shows . The multiplier 58 multiplies the output A of the arithmetic unit 57 in the previous stage by 3 to obtain 3A, and supplies this to the dividers 37, 38, and 39 of each phase. The divider 69 is the square root operator 57
Find 1/3 of the output A of , and add this to adders 40, 41,
42. The accuracy of the method using the line current-to-phase current conversion circuit 17 shown in FIG. 2 is shown below. In the circuit shown in Figure 1, the phase current flowing through loads 5 to 13 was directly measured using a current transformer, and the total phase current of the first, second, and third phases was determined to be 0.3 (A) and 0.6 (A). ), 0.9(A).
When the line currents of the first, second, and third phases in this case were measured using current detectors 14, 15, and 16, they were found to be 1.06
(A), 0.8 (A), and 1.38 (A). The phase currents x, y, and z of the first, second, and third phases of the output stage of the conversion circuit 17 are 0.31 (A), 0.6 (A), and 0.89 (A).
It was hot. Therefore, the total phase current can be determined by the conversion circuit 17 with an error within ±1%. Effects of the Invention As is clear from the above, according to the present invention, a large number of single-phase loads having substantially the same power factor are connected between each line,
Even if the load is unbalanced as a whole, the total phase current of each phase can be determined by simply detecting the line current of each phase without directly detecting the phase current of each phase. . Therefore, the total phase current can be easily known with a simple configuration. It is also possible to know the phase current of a three-phase unbalanced load, which cannot be directly measured, such as in an electric furnace.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a current detection device according to an embodiment of the present invention, and FIG. 2 is a block diagram showing the line current-phase current conversion circuit of FIG. FIG. 3 is a vector diagram showing the relationship between phase current and line current. 1...first track, 2...second track, 3...
Third track, 5, 6, 7, 8, 9, 10, 11,
12, 13... Welding machine load, 14... First current detector, 15... Second current detector, 16... Third current detector, 17... Line current-phase current conversion circuit.

Claims (1)

[Claims] 1. Line currents a, b of the first, second, and third phases of a three-phase AC circuit to which an unbalanced load consisting of a plurality of single-phase loads with substantially the same power factor is connected. , c, first, second, and third line current detectors provided on the first, second, and third lines to detect the first, second, and third lines; the first, second, and third unbalanced loads connected to a current detector;
^The phase ^currents x, y ^{, z of the phases are :} A current detection device for a three-phase AC circuit consisting of a line current-to-phase current conversion circuit calculated according to the formula 3A (where A is [C]).