JPH07104741B2 - Power factor adjusting device for three-phase AC circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a power factor adjusting device for a three-phase AC circuit used for preventing voltage flickering, improving power factor, and the like.

[Conventional technology]

It is well known to connect a lead current supply circuit composed of a capacitor, or a parallel circuit of a capacitor and a reactor, between each of the three-phase AC power supply lines in order to prevent flicker and / or improve the power factor. Further, Japanese Laid-Open Patent Publication No. 56-159936 (Japanese Patent Application No. 55-61600) discloses that an instantaneous active current and an instantaneous reactive current are obtained and the power failure compensation circuit is controlled based on the obtained instantaneous active current and instantaneous reactive current.

[Problems to be solved by the invention]

According to the method disclosed in the above publication, it is possible to perform power factor compensation under the condition of three-phase balanced load, but it is possible to accurately and easily perform power factor compensation of three-phase unbalanced load. It was difficult. That is, when the load phase current can be easily detected, the advance current supply circuit connected between the lines can be controlled relatively easily, but it is difficult to detect the load phase current. In such a case, the lead current between the lines must be controlled based on the line current detection, and accurate control is difficult in the case of an unbalanced load.

[Means for solving problems]

The present invention for solving the above-mentioned problems will be described with reference to the reference numerals of the drawings showing an embodiment. First phase of a three-phase unbalanced load (2) connected to a three-phase AC power supply line. , Second
First to detect the line current (I _U , I _V , I _W ) of the phase and the third phase,
The second and third line current detectors (4a) (4b) (4c) and the first, second and third line current detectors (4a) (4b) (4)
c) the first, second and third phase line currents (I _U ,
A first operation circuit (5) for _obtaining the instantaneous reactive currents (I _Uq , I _Vq , I _Wq ) of the first phase, the second phase and the third phase based on I _V , I _W ); Effective value conversion circuit (6) for obtaining effective values (a, b, c) of the first, second and third instantaneous reactive currents (I _Uq , I _Vq , I _Wq ) obtained from the arithmetic circuit (5)
And a second step of converting the effective value (a, b, c) of the instantaneous reactive current of each phase represented by a line current into a phase current (x, y, z)
Arithmetic circuit (7) and three-phase reactive current supply circuits (3a) (3b) (3c) connected between the three-phase AC power supply lines
And the three-phase unbalanced load (2) and the reactive current supply circuits (3a) (3b) based on the phase current (x, y, z) of the instantaneous reactive current obtained from the second arithmetic circuit (7). ) (3c) and a circuit (8) for controlling the reactive current supply circuits (3a) (3b) (3c) so that the power factor of the three-phase AC circuit becomes a predetermined value. Of the power factor adjusting device.

[Work]

According to the above invention, the instantaneous reactive current components (I _Uq , I _Vq , I _Wq ) are calculated by the first arithmetic circuit (5), and then the effective values (a, b, c) is obtained. This effective value corresponds to the line current. By inputting this effective value to the second arithmetic circuit (7), the phase current (x,
y, z) is obtained. That is, despite the unbalanced load, the phase current corresponding to the line current (effective value of the instantaneous reactive current)
Is obtained. The control circuit includes a reactive current supply circuit (3) connected between the lines so that a predetermined power factor (preferably 1) is obtained based on the phase current (effective value of the instantaneous reactive current).
a) Controls (3b) and (3c). As a result, the power factor can be easily adjusted despite the unbalanced load.

〔Example〕

Next, a power factor adjusting device for a three-phase AC circuit according to an embodiment of the present invention will be described with reference to the drawings.

(Explanation of FIG. 1) In FIG. 1, three power supply lines (1u, 1v, 1w) for the first phase (U phase), the second phase (V phase), and the third phase (W phase) are A phase unbalanced load (2) is connected. This three-phase unbalanced load (2) is, for example, a single-phase load (2a) (2b) (2c) of a welding machine or the like having almost the same power factor connected between the lines,
The loads (2a) (2b) (2c) between the lines are not fixed loads, but their magnitudes change. Therefore, if the power factor is not compensated, the fluctuation of the load (2) causes the voltage fluctuation (flicker) of the power line (1u) (1v) (1w),
And the power factor gets worse. (3a) (3b) (3c) are lead reactive current supply circuits for power factor adjustment, and three-phase power line (1
u) (1v) (1w). This proceeds reactive current supply circuit (3a) (3b) (3c ) is a plurality between each line capacitor _{C 1a ~C 1n, C 2a ~C} 2n, thyristors S _1a to S of the C _3a -C _3n as an AC switch _1n , S _{2a to} S _2n , and S _{3a to} S _3n are selectively connected. If the advance currents supplied by the advance reactive current supply circuits (3a), (3b) and (3c) are adjusted according to the change of the load (2), the power factor of the three-phase AC circuit is compensated and the voltage change ( Flickering) is also prevented. In this embodiment, since four capacitors having a capacitance ratio of 1: 2: 4: 8 are provided as the capacitors C _{1a to} C _1n , a combination of these 4 capacitors produces a capacity of 15 stages. can get. The capacitors C _{2a to} C _2n and C _{3a to} C _3n have the same configuration.

In this embodiment, since the loads (2a) (2b) (2c) between the lines as the three-phase load (2) are composed of a large number of loads, it is difficult to directly detect the phase current. Therefore, in order to detect the line currents I _U , I _V , and I _W , each power line (1
u) (1v) (1w) Current detector (4a) (4
b) (4c) are connected respectively.

(5) is a first arithmetic circuit for obtaining the instantaneous reactive current component, which outputs the instantaneous reactive currents I _Uq , I _Vq , and I _Wq of each phase corresponding to the line currents I _U , I _V , and I _W of each phase. To do. The specific configuration of the arithmetic circuit (5) will be described later.

(6) is an effective value conversion circuit, and is a first arithmetic circuit (5)
The respective effective values of the instantaneous reactive currents I _Uq , I _Vq , and I _Wq obtained from the above equation, that is, the line currents a, b, and c of the instantaneous reactive current components are obtained. The RMS value conversion circuit (6) is, for example, an AD536A RMS / DC converter of Analog Devices Co., Ltd.
A DC voltage corresponding to the effective value of the alternating current is sent out, and this converter (6) is configured to obtain the effective value of the sine wave. Therefore, even if the line currents I _U , I _V , I _W and the instantaneous reactive currents I _Uq , I _Vq , I _Wq are sinusoidal waves, even if they are approximate to a sinusoidal wave, they can be effective with extremely small error. It is possible to obtain the line currents a, b, and c indicated by the value. When the instantaneous reactive current is close to a sine wave, a sufficient effect can be obtained even if power factor compensation is performed based on the effective value of this fundamental wave.

(7) is a second arithmetic circuit for converting a line current into a phase current. That is, the line currents a, b, c of each phase of the instantaneous reactive current component
It is a circuit that converts (effective value) into phase current x, y, z (effective value) of each phase. The second arithmetic circuit (7) will be described in detail later.

(8) is a gate control circuit, and a second arithmetic circuit (7)
Phase current x, y of each phase of the instantaneous reactive current component obtained from
Based on z, the load (2) and the reactive current supply circuit (3
a) A circuit for generating a gate signal for controlling the thyristors S ₁ , S ₂ and S ₃ so that the total power factor of (3b) and (3c) becomes 1.

(Explanation of Instantaneous Reactive Current and FIG. 2) Instantaneous reactive current is described in the above-mentioned Japanese Patent Laid-Open No. 56-159936.

The three-phase AC power supply voltages V _U , V _V , and V _W have maximum values V _m and angular frequency ω
If the balanced three-phase AC voltage is expressed by the following equation.

Now, when the three-phase load currents I _U , I _V , and I _W are coordinate-transformed with a transformation matrix that rotates by ω and expressed by I _p and I _q , the following equation is generally established when there is no zero-phase current. To do.

Further, if the three-phase currents I _U , I _V , and I _W are represented by I _p and I _q , the following equation is obtained.

Here, since only the reactive current I _q is calculated, (2) and (3)
If I _p = 0 in the equation, the following equation is obtained.

The reactive current components I _Uq , I _Vq , and I _Wq of the load currents I _U , I _V , and I _W are expressed by the following formulas from the formulas (4) and (5).

FIG. 2 is a block diagram showing the first arithmetic circuit (5) for calculating the equation (7) in matrix display. (9u)
(9v) and (9w) are lines that give line currents I _U , I _V , and I _W , (1
(0) is a line giving information on ωt, (11) is a sin ωt generation circuit, (12) is a sin (ωt-2π / 3) generation circuit, and (1
3) is a sin (ωt-4π / 3) generation circuit, (14) (15) (1
6) is a multiplier of the line currents I _U , I _V , I _W and the outputs of the circuits (11) (12) (13), (17) is three multipliers (14) (15)
(16) output adder, (18) (19) (20) are multipliers of adder (16) output and circuit (11) (12) (13) output, from which instant invalid Currents I _Uq , I _Vq , and I _Wq are output. The multipliers (18) (19) (20) add a coefficient to the multiplication result. It is configured to multiply and output. For example, the first
The instantaneous reactive current I _Uq of the phase is obtained based on the following equation.

The second- and third-phase instantaneous reactive currents I _Vq and I _Wq are calculated in the same manner. These instantaneous reactive currents I _Uq , I _Vq , and I _Wq are line currents I
Since it is determined based on _U , _IV , and _IW , it is the instantaneous reactive current component of the line current. This instantaneous reactive current I _Uq , I _Vq , I
_Wq is converted into the effective value, that is, the line currents a, b, and c of the instantaneous reactive current component by the effective value conversion circuit (6) in FIG.

(Description of Line Current-Phase Current Conversion) Next, referring to FIGS. 3 and 4, the line currents (instantaneous reactive currents) a, b, and c are converted into phase currents (instantaneous reactive currents) x, y, and z. The conversion method will be described.

Second arithmetic circuit (7) for converting line current to phase current in FIG.
Is the line current a, based on the following equations (8), (9) and (10).
It is configured to convert b, c into phase currents x, y, z.

However, A is shown by the following equation.

It will be described that the phase currents x, y, and z are approximately obtained by the equations (8), (9), and (10). Each load in Figure 1 (2a)
If the power factors of (2b) and (2c) are the same, the summed phase currents x, y, z of each phase have a phase difference of 120 degrees, and
It can be represented by the vector diagram of the figure. Since the relationship between the phase currents x, y, z and the line currents a, b, c is also as shown in FIG. 4, the following equation (11) is established between them.

The following expression (13) is established by the expression (12).

Meanwhile ^{2 (x + y + z)} 2 -3 (xy + yz + zx) = b 2 + c 2 + a 2 ...... (13), the following equation (14) holds the official.

^{3 (xy + yz + zx)} 2 = 2 (b 2 c 2 + c 2 a 2 + a 2 b 2) - (b 4 + c 4 + a 4) ...... (14) (14) the following equation from the equation are obtained.

Substituting equation (15) into equation (13), the following equation is obtained.

Hereinafter, the expression (16) is referred to as A. Further, the following equation is established from the equation (12).

The following equation is established from the equation (18) and the relation of x + y + z = A.

From the equation (19), the above equations (8), (9) and (10) are obtained. Therefore, the equations (8), (9) and (10) represent the phase current of each phase.

The arithmetic circuit (7) shown in FIG. 3 is connected to the RMS conversion circuit (6) shown in FIG. 1 in order to execute the equations (8), (9) and (10). ) (22) (23) Squared calculator (24) (25) for obtaining the squared value of the line current a, b, c
It has (26). At the output stage of each of the square calculators (24) (25) (26), a ² + b ² and b ² + in the equations (8), (9) and (10)
In order to perform the operation of c ² and c ² + a ² , the adder (27) (2
8) and (29) are provided, and 2 in the formulas (8), (9) and (10)
Double value calculator (30) (31) to obtain c ² , 2a ² and 2b ²
(32) is 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) of the preceding stage, and 8)
(9) (10) of ^{a 2 + b 2 -2c 2,} b 2 + c 2 -2a 2, c 2 + a 2 -
You get 2b ² . (36) is an adder, and the output b ² of the square calculator (25) is added to the output c ² + a ² of the adder (29) and included in A of the equations (8), (9) and (10). A ² + b ² + c ²
Is what you get. (37) (38) (39) are dividers, which divide the output of the subtractors (33) (34) (35) at the previous stage by 3A, which is calculated separately, and then the formulas (8) (9) (10) The second term of is calculated. (40) (41) (42) are adders, which are obtained separately from the outputs of the dividers (37) (38) (39) in the previous stage (8)
(9) Add the value A / 3 of the first term in equation (10) to (8)
(9) The phase currents x, y, and z of the equations (10) are output.

The adders (43), (44) and (45) connected to the line current input lines (21), (22) and (23) are a + b in the formulas (8), (9) and (10) showing A. , B + c, c + a. The adder (46) adds (c) of the line (23) to the output a + b of the adder (43) to obtain (a + b
+ C). The subtractor (47) is the adder (4
The output of line (23) is subtracted from the output of 3) to obtain a + bc of the expression indicating A. The subtractor (48) subtracts the output of the line (21) from the output of the adder (44),
This is a circuit for obtaining (-a + b + c) in the formula showing A. 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 expression indicating A. The multiplier (50) is connected to the output of the adder (46) and the subtractor (4
(A + b + c) of the formula showing A by multiplying with the output of 7)
This is a circuit for obtaining (a + b-c). The multiplier (51) multiplies the output of the subtractor (48) and the output of the subtractor (49) to obtain A
Is a circuit that obtains (-a + b + c) (a-b + c) in the equation. The multiplier (52) is the former two multipliers (50) (5
The two outputs of 1) are multiplied, and (a + b +
c) (-a + b + c) (a-b + c) (a + b-c)
(Hereinafter, referred to as B). The multiplier (53) is B × 3 based on the output B of the multiplier (52) in the previous stage.
Is a circuit to obtain. The square root calculator (54) is the square root of the output 3B of the multiplier (53) in the previous stage. Is a circuit to obtain. The adder (55) is the previous-stage arithmetic unit (54)
Of the formula showing A by adding the output of and the output of the adder (36) Is a circuit to obtain. The 1/2 divider (56) is the adder (5
This is a circuit that obtains 1/2 the output of 5). The square root calculator (57) obtains the square root of the output of the 1/2 divider (56) in the preceding stage, that is, the output of the expression indicating A. The multiplier (58) multiplies the output A of the previous stage arithmetic unit (57) by 3 to obtain 3A, and supplies this to the dividers (37) (38) (39) of the respective phases. The divider (69) is 1/3 of the output A of the square root calculator (57).
Is obtained and is supplied to the adders (40) (41) (42).

(Function and effect) In FIG. 1, a lead reactive current supply circuit (3a) (3b)
With all the capacitors in (3b) disconnected, the line currents I _U , I _V , and I _W are 1.78A, 1.65A, 1.2A, the power factor of each phase is 0.6, 0.7, 0.8, and the three phases of the input stage. In a three-phase circuit connected with an unbalanced load (2) such that the total power factor of 0.595 becomes 0.595, the phase currents x, y, z of the instantaneous reactive current are calculated based on the line currents I _U , I _V , and I _W. Then, the capacitor was selected so as to cancel it out, and when the capacitance ratio of the capacitors between U-V, V-W, and W-U was set to 7: 6: 5, the power factor became about 1.0. . That is, the power factor could be satisfactorily compensated based on the line current.

(Modification) The present invention is not limited to the above-described embodiments, and the following modifications are possible, for example.

(A) Figure 5 shows the advanced reactive current supply circuit (3a) (3b)
A modified example of (3c) is shown. In this circuit the reactor L ₁ in parallel with the capacitor C _1, C _2, C ₃ between each line, L _2, L ₃ is connected, the reactor L _1, L _2, thyristors in series with L ₃ S _1, S _2,
S ₃ is connected. Thyristors S ₁ , S ₂ and S ₃ have a power factor of 1
The phase is controlled (conduction angle control) so that Therefore,
An effect equivalent to continuously controlling the capacitance of the capacitor can be obtained. The phase control angles of the thyristors S ₁ , S ₂ and S ₃ are determined based on the phase currents x, y and z of the instantaneous reactive current.

(B) Instead of the thyristors S _{1a to} S _3n shown in Fig. 1, connect a mechanical switch such as an electromagnetic contactor to delay the control response speed, and simply use the three-phase unbalanced load and unbalanced power factor force. It may be a rate adjusting device.

(C) The arithmetic circuits (5) and (7) may be digital arithmetic circuits using a micro computer instead of the analog arithmetic units.

(D) Although the effective value conversion circuit (6) is configured to determine the effective value of the fundamental wave component, it may be configured to determine the effective value of all the instantaneous reactive current components.

(E) The thyristors S _{1a to} S _3n and S _{1 to} S ₃ are not limited to triacs, but may be unidirectional controlled thyristors (SCRs) in antiparallel.

〔The invention's effect〕

As apparent from the above, according to the present invention, the power factor improvement of the three-phase unbalanced load can be easily achieved based on the line current detection.

[Brief description of drawings]

1 is a block diagram showing a three-phase power factor adjusting device according to an embodiment of the present invention, FIG. 2 is a block diagram showing a first arithmetic circuit of FIG. 1, and FIG. 3 is a second diagram of FIG. 4 is a block diagram showing the arithmetic circuit of FIG. 4, FIG. 4 is a block diagram showing the relationship between the line current and the phase current of a three-phase unbalanced load, and FIG. 5 is a circuit diagram showing a power factor adjusting circuit of a modified example. (1u) (1v) (1w) ... power line, (2) ... unbalanced load, (3a) (3b) (3c) ... advanced reactive current supply circuit,
(4a) (4b) (4c) ... current detector, (5) ... first arithmetic circuit, (6) ... effective value conversion circuit, (7) ... second
Operation circuit of (8) ... Gate control circuit.

Claims (1)

[Claims] 1. A line current (I _U , I _V , I _W ) of a first phase, a second phase and a third phase of a three-phase unbalanced load (2) connected to a three-phase AC power line is detected. First, second and third line current detectors (4a) (4b) (4c), and the first, second and third line current detectors (4a) (4b)
The first, second and third phase line currents (I _U , detected in (4c))
A first arithmetic circuit (5) for _obtaining instantaneous reactive currents (I _Uq , I _Vq , I _Wq ) of the first phase, the second phase and the third phase based on I _V , I _W ); The effective value (a, a) of the first, second and third instantaneous reactive currents (I _Uq , I _Vq , I _Wq ) obtained from the arithmetic circuit (5)
b, c) effective value conversion circuit (6), and converts the effective value (a, b, c) of the instantaneous reactive current of each phase represented by line current into phase current (x, y, z) Second
Obtained from the arithmetic circuit (7), the three-phase reactive current supply circuits (3a) (3b) (3c) connected between the three-phase AC power supply lines, and the second arithmetic circuit (7). Of a three-phase AC circuit combining the three-phase unbalanced load (2) and the reactive current supply circuits (3a) (3b) (3c) based on the phase current (x, y, z) of the instantaneous reactive current A power factor adjusting device for a three-phase AC circuit, comprising a circuit (8) for controlling the reactive current supply circuits (3a) (3b) (3c) so that the power factor becomes a predetermined value.