JP2020174465A - Insulation-type power factor improvement device for three-phase alternating current - Google Patents

Abstract

To achieve miniaturization of a transformer, and simple control in a power factor improvement device into which three-phase alternating current is inputted.SOLUTION: An insulation-type power factor improvement device for three-phase alternating current has one transformer for insulating an input side and an output side, and a primary coil and a secondary coil in each phase of the three-phase alternating current are wound around each of three legs of the transformer. Each phase is provided with a switching element serially connected with the primary coil in each phase. The switching element is configured with two FETs serially connected with each other at respective backs, and the switching element in each phase is subjected to on-off control with a same control signal. The secondary coil in each phase is each connected to a rectification bridge configured with four diodes. The control signal has a constant cycle and a constant duty ratio.SELECTED DRAWING: Figure 1

Description

The present invention relates to an insulated power factor improving device that converts three-phase alternating current into direct current.

Conventionally, in a power factor improving device that converts alternating current into direct current, switching processing for improving the power factor is performed after rectifying the alternating current (Patent Documents 1 and 2). Further, in the power factor improving device for three-phase alternating current, a switching control circuit is provided for each phase, and different control is performed for each phase (Patent Documents 3 and 4).

An insulated power factor improving device that insulates the input side and the output side by a transformer is also known. When the power factor improving device for three-phase alternating current is an insulating type, conventionally, a transformer is provided for each phase (Patent Document 5).

JP-A-2002-10632 Japanese Unexamined Patent Publication No. 2005-218224 Japanese Unexamined Patent Publication No. 7-31150 JP-A-2002-44951 JP-A-2017-221074

In the insulated power factor improving device for three-phase alternating current, if a transformer is provided for each phase, there is a problem that the volume of the three transformers becomes bulky.

An object of the present invention is to reduce the size of an isolated power factor improving device to which a three-phase AC is input, and to realize a simple circuit configuration and switching control.

In order to achieve the above object, the present invention provides the following configurations.
-Aspects of the present invention are in an insulated power factor improving device for three-phase alternating current.
It has one transformer to insulate the input side and the output side,
A primary coil and a secondary coil of each phase of three-phase alternating current are wound around each of the three legs included in the transformer.-In the above embodiment, each phase is connected in series with the primary coil. It is preferable that each of the switching elements has a switching element, the switching elements are configured by connecting two FETs in series back to back, and the switching elements of each phase are on / off controlled by the same control signal. is there.
-In the above embodiment, it is preferable that the secondary coil of each phase is connected to a rectifying bridge composed of four diodes.
-In the above embodiment, it is preferable that the control signal has a constant period and a constant duty ratio.

According to the present invention, in an insulated power factor improving device to which a three-phase alternating current is input, the device can be miniaturized, and a simple circuit configuration and control can be realized.

FIG. 1 is a diagram schematically showing a circuit configuration of an embodiment of the power factor improving device of the present invention. FIG. 2A is a cross-sectional view schematically showing a transformer configuration of the power factor improving device shown in FIG. 1, and FIG. 2B is a cross-sectional view showing an example of a transformer configuration of a conventional power factor line device. FIG. 3 is a diagram showing an example of a current flow during the switching on period in the power factor improving device shown in FIG. FIG. 4 is a diagram showing an example of current flow during the switching off period in the power factor improving device shown in FIG.

Hereinafter, embodiments of the power factor improving device according to the present invention will be described with reference to the drawings. The three-phase alternating current is output by, for example, a wind power generator. In the power factor improving device of the present invention, such a three-phase alternating current is input and a direct current is output to the load. This power factor improving device is also a power conversion device that converts three-phase AC power into DC power. Further, when classified from the circuit configuration, it is one of the switching power supplies that converts low-frequency input alternating current into high-frequency alternating current by a switching element and smoothes it again to make direct current. The power factor improving device functions to set the power factor to 1 by making the waveform of the input current a similar waveform whose phase matches with the input voltage. Further, the power factor improving device of the present invention has a function of electrically insulating the input side and the output side. The load connected to the output side of the power factor improving device of the present invention is various devices, an inverter (including a grid interconnection inverter), and the like.

FIG. 1 is a diagram schematically showing a circuit configuration of an embodiment of the power factor improving device of the present invention. This power factor improving device is provided with one transformer TR in order to insulate the input side and the output side. In the transformer TR, three primary coils N1, N2, and N3 and secondary coils N2, N4, and N6 tightly coupled to each primary coil are wound. The detailed configuration of the transformer TR will be described later.

On the primary side of the transformer TR, each phase of three-phase alternating current is input from the three input terminals R, S, and T, respectively. In this specification, each phase of three-phase alternating current is referred to as R phase, S phase, and T phase. The waveform of each phase is a sine wave, and the phases differ by 2π / 3 (120 °). The frequency of the input voltage is, for example, about several Hz to several tens of Hz.

Switching elements S1, S2, and S3 are inserted between the input ends R, S, and T and the first ends of the primary coils N1, N3, and N5, respectively. Each of the switching elements S1, S2, and S3 is composed of two switching elements (Q1 and Q2, Q3 and Q4, Q5 and Q6) connected in series back to back. Each switching element is here a FET (Field Effect Transistor), particularly an n-channel MOSFET. As another example, it can be configured by a p-channel FET.

In the switching element S1, the gates and sources of the switching element Q1 and the switching element Q2 are connected to each other. Each gate is connected to the emitter of the output transistor of the photocoupler PC1. Each source is connected to the negative end of a given power source Vg. The positive end of the power supply Vg is connected to the collector of the output transistor of the photocoupler PC1. Therefore, the switching element Q1 and the switching element Q2 are turned on and off at the same time, and the on / off is controlled by the photocoupler PC1. When the output transistor of the photocoupler PC1 is turned on, a voltage across the bias resistor connected between the gate and source is generated by the power supply Vg, and is applied to the switching elements Q1 and Q2 as a voltage between the gate and source. As a result, the switching elements Q1 and Q2 are turned on. When the output transistor of the photocoupler PC1 is turned off, the switching elements Q1 and Q2 are turned off. The drain of the switching element Q1 is connected to the input end R, and the drain of the switching element Q2 is connected to the first end of the primary coil N1. The switching element S1 switches the input voltage of the R phase.

Each of the switching elements S2 and S3 has the same configuration as the switching element S1. The switching elements S2 and S3 are on / off controlled by the photocouplers PC2 and PC3, respectively, and switch the input voltages of the S phase and the T phase, respectively.

The other ends of each of the primary coils N1, N3, and N5 are connected, and this common end serves as the reference potential end on the primary side.

Although not shown, as another example, the switching elements S1, S2, and S3 may be inserted between the other ends of the primary coils N1, N3, and N5 and the reference potential end on the primary side, respectively. That is, the switching elements S1, S2, and S3 may be connected in series with each of the primary coils N1, N3, and N5, respectively.

The input diodes of the photocouplers PC1, PC2, and PC3 are all connected in series, and are further connected in series with the input diodes of the photocoupler PC4. The anode side of these four series-connected diodes is connected to the first output end of the control unit 10, and the cathode side is connected to the second output end.

The control unit 10 generates and outputs one control signal for switching. The control signal is a PWM signal in a preferred example, and is a pulse voltage signal having a constant period and a predetermined duty ratio. The frequency of the switching control signal is, for example, about several kHz to several MHz, which is larger than the frequency of the input voltage.

Next, the configuration of the secondary side of the transformer TR will be described. The configuration on the secondary side of the transformer TR shown in FIG. 1 is an example. As a general isolated switching power supply, there are a forward method, a flyback method, a method combining these, and the like, and various circuits are known. In the power factor improving device, a method capable of transmitting electric power from the primary side to the secondary side is adopted regardless of the magnitude of the input voltage. Therefore, a configuration that includes at least a flyback method is usually selected.

The circuit on the secondary side shown in the figure will be described by taking the R phase as an example. Each end of the secondary coil N2 is connected to two intermediate terminals of a rectifying bridge B1 composed of four diodes. One end of the reactor L1 is connected to the positive terminal of the rectifying bridge B1. The other end of the reactor L1 is connected to the anode of the output diode D1. The cathode of the output diode D1 is connected to the positive output end p. The negative terminal of the rectifying bridge B1 is connected to the negative output end n. The negative output end n is the reference potential end on the secondary side. A smoothing capacitor C is connected between the positive output end p and the negative output end n.

Since each end of the secondary coil N2 is connected to the rectifying bridge B1, the current in the subsequent stage of the rectifying bridge B1 regardless of whether the winding of the secondary coil N2 has the opposite polarity or the same polarity as the winding of the primary coil N1. The flow will be the same. Providing a rectifying element on the secondary side is more power efficient than providing a rectifying element on the primary side.

Further, the switching element S4 is connected between the other end of the reactor L1 and the negative output end n. The switching element S4 is an n-channel MOSFET as an example. The gate of the switching element S4 is on / off controlled by the output transistor of the photocoupler PC4.

The secondary side of the S phase and the T phase also has the same configuration as the R phase. In the illustrated circuit, switching elements S4, S5, and S6 are provided on the secondary side, and on / off control is performed by the output transistor of the photocoupler PC4.

Each switching element in the circuit of FIG. 1 is insulated from the control unit 10 by the photocouplers PC1 to PC4. In the circuit of FIG. 1, all the switching elements are controlled by one control signal output from the control unit 10, and therefore all the switching elements are turned on and off at the same time. This realizes extremely simple control.

FIG. 2A is a cross-sectional view schematically showing the configuration of the transformer TR in the circuit of FIG. The transformer TR has a core composed of three legs in the vertical direction, an upper arm connecting the upper ends of the three legs, and a lower arm connecting the lower ends. The three legs are evenly spaced from each other and have the same width. That is, the cross-sectional areas of the three legs are the same. R-phase, S-phase, and T-phase primary coils and secondary coils are wound around each leg in an overlapping manner. In the figure, the R-phase primary coil N1 and the secondary coil N2 are wound on the left leg, the S-phase primary coil N3 and the secondary coil N4 are wound on the center leg, and the T-phase primary coil N5 and the secondary coil N6 are wound on the right leg. It has been done.

FIG. 2B is a schematic cross-sectional view showing the configuration of a conventional isolation transformer for three-phase alternating current for reference. Conventionally, one transformer is provided for each of the three phases, and the primary coil and the secondary coil are wound respectively. Therefore, three transformers Tr, Ts, and Tt are required, and the overall size is bulky. In the present invention, the size of the isolation transformer for three-phase alternating current can be significantly reduced by winding three sets of coils around one core.

The operation of the circuit of FIG. 1 will be described with reference to FIGS. 3 and 4. 3 and 4 are examples of currents at a certain temporary point in the temporal change of three-phase alternating current. The three input voltages of three-phase AC have either one phase positive potential and the other two phases negative potential with respect to the reference potential end on the primary side at any time point in the time change. One phase has a negative potential and the other two phases have a positive potential. In the former case, the input current flows from the positive potential of one phase to the negative potential of two phases, and in the latter case, the input current flows from the positive potential of two phases to the negative potential of one phase. 3 and 4 are examples of the former case. In the latter case as well, the circuit operation is substantially the same.

FIG. 3 schematically shows a current flow when the switching elements S1, S2, and S3 are turned on when the R phase is a positive potential and the S phase and the T phase are negative potentials among the three-phase input voltages. Shown. The current flow is indicated by a white arrow. The switching elements S4, S5, and S6 on the secondary side are also turned on at the same time.

When the switching elements S1, S2, and S3 are turned on, the current i1 flows in the following two paths.
・ R phase input end → primary coil N1 (one end to the other end) → primary coil N3 (one end to one end) → S phase input end ・ R phase input end → primary coil N1 (one end to the other end) → primary Coil N5 (one end from the other end) → T-phase input end

Since the switching elements S1, S2, and S3 on the primary side are bidirectional switches, a current that causes a loss can flow in any direction.

The current i1 increases with a constant slope in proportion to time during the on period. In the present invention, since the ON period of one cycle of the control signal is constant, the average value of the current i1 in one cycle is proportional to the slope of the increasing current i1. The slope of the current i1 is Vin / Lω, where Vin is the instantaneous value of the input voltage at that time, the inductance L on the path of the current i1, and the frequency of the control signal is ω. Since L and ω are constants, the average value of the current i1 in one cycle is proportional to the input voltage Vin. This means that the input current i1 changes in proportion to the input voltage Vin. That is, it means that the power factor is 1.

On the secondary side of the R phase, the current i2 flows in the following path due to the electromotive force generated in the secondary coil N2 by mutual induction.
・ Secondary coil N2 (one end from the other end) → Rectifier bridge B1 → Reactor L1 → Switching element S4
As a result, magnetic energy is stored in the reactor L1.

On the secondary side of the S phase, the current i3 flows in the following path due to the electromotive force generated in the secondary coil N4 by mutual induction.
・ Secondary coil N4 (from one end to the other end) → Rectifier bridge B2 → Reactor L2 → Switching element S5
As a result, magnetic energy is stored in the reactor L2.

On the secondary side of the T phase, the current i5 flows in the following path due to the electromotive force generated in the secondary coil N6 by mutual induction.
・ Secondary coil N6 (from one end to the other end) → Rectifier bridge B3 → Reactor L3 → Switching element S6
As a result, magnetic energy is stored in the reactor L3.

FIG. 4 schematically shows the current flow when the switching elements S1, S2, and S3 are turned off after the on period of FIG. The switching elements S4, S5, and S6 on the secondary side are also turned off at the same time.

On the primary side, the current path is cut off, so the current becomes zero. Since the body diodes of the two FETs of the switching elements S1, S2, and S3 on the primary side are arranged in opposite directions, current cannot flow in either direction and the switching elements are completely turned off.

On the secondary side of the R phase, the current i5 flows in the following path due to the counter electromotive voltage generated in the secondary coil N2.
・ Secondary coil N2 (from one end to the other end) → Rectifier bridge B1 → Reactor L1 → Diode D1 → Output end p

On the secondary side of the S phase, the current i6 flows in the following path due to the counter electromotive voltage generated in the secondary coil N4.
・ Secondary coil N4 (one end from the other end) → Rectifier bridge B2 → Reactor L2 → Diode D2 → Output end p

On the secondary side of the T phase, the current i7 flows in the following path due to the counter electromotive voltage generated in the secondary coil N6.
・ Secondary coil N6 (one end from the other end) → Rectifier bridge B3 → Reactor L3 → Diode D3 → Output end p

The currents i5, i6, and i7 flow from the output end p to a load (not shown) and return to the output end n. As a result, the magnetic energy stored in each of the secondary coils N2, N4, and N6 is released.

Further, on the secondary side of the R phase, the current i8 flows in the following path due to the counter electromotive voltage generated in the reactor L1.
・ Output end n → Rectifier bridge B1 → Reactor L1 → Diode D1 → Output end p

Further, on the secondary side of the S phase, the current i9 flows in the following path due to the counter electromotive voltage generated in the reactor L2.
・ Output end n → Rectifier bridge B2 → Reactor L2 → Diode D2 → Output end p

Further, on the secondary side of the T phase, the current i0 flows in the following path due to the counter electromotive voltage generated in the reactor L3.
・ Output end n → Rectifier bridge B3 → Reactor L3 → Diode D3 → Output end p

The currents i8, i9, and i10 flow from the output end p to a load (not shown) and return to the output end n. As a result, the magnetic energy stored in the reactors L1, L2, and L3 is released.

Whether the currents i8, i9, and i10 pass through the upper two diodes and the lower two diodes in the rectifying bridges B1, B2, and B3 depends on the magnitude of the currents i5, i6, and i7, respectively. It is decided.

The feature of the control method in the power factor improving device of the present invention is that in switching control, only one control signal having a constant period, a constant phase, and a constant duty ratio with respect to the input voltage of each phase of three-phase alternating current is transmitted. Is to control using. That is, all phases are turned on and off at the same timing, and the on time and the off time are constant. Therefore, the control unit need only determine the predetermined duty ratio. The predetermined duty ratio is determined, for example, based on the detected input voltage, input current and / or output voltage.

In many conventional power factor improving devices for three-phase alternating current, a control signal for changing the duty ratio is given by PWM processing, or a switch control is performed for each phase at different timings. The control method of the present invention is extremely simple as compared with these.

R, S, T Input end p Positive output end n Negative output end TR Transformer N1, N3, N5 Primary coil N2, N4, N6 Secondary coil S1, S2, S3 Switching element S4, S5, S6 Switching element PC1, PC2, PC3, PC4 Photocoupler L1, L2, L3 Reactor D1, D2, D3 Output diode C smoothing capacitor

Claims (4)

In an insulated power factor improving device for three-phase AC
It has one transformer to insulate the input side and the output side,
An insulated power factor improving device for three-phase alternating current, characterized in that a primary coil and a secondary coil of each phase of three-phase alternating current are wound around each of the three legs included in the transformer. Each phase has a switching element connected in series with the primary coil, the switching element is configured by connecting two FETs in series back to back, and the switching element of each phase has the same control. The insulated power factor improving device for three-phase alternating current according to claim 1, wherein the on / off control is performed by a signal. The insulated power factor improving device for three-phase alternating current according to claim 1 or 2, wherein the secondary coil of each phase is connected to a rectifying bridge composed of four diodes. The insulated power factor improving device for three-phase alternating current according to any one of claims 1 to 3, wherein the control signal has a constant period and a constant duty ratio.

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