JPH07337023A - Inverter - Google Patents

Abstract

PURPOSE:To block surge current by feeding an exciting current from an inverter so that the AC voltage approaches the voltage of an AC system at the time of starting an inverter and closing a switch when the exciting current settles. CONSTITUTION:An inverter 2 converts a DC voltage through pulse width modulation into an AC voltage being applied to one winding of a transformer 6 having the other winding connected with an AC system through switches 7c, 7d. When the inverter 2 15 started, an exciting current is outputted therefrom so that an AC voltage induced in the other winding of the transformer 6 approaches the voltage of an AC system. The switches 7c, 7d are closed when the exciting current is settled. Since a high surge current is not generated, the receiving voltage of AC system is not effected adversely.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter device which obtains AC power from a DC power source such as a solar cell and supplies it to another AC power source such as an AC system via a transformer.

[0002]

2. Description of the Related Art As an inverter device of this type, a grid-connected inverter by solar power generation is shown in FIG. In the figure, the DC voltage output from the solar cell 1 is
Is converted into an AC voltage by a filter 2, and a high frequency component associated with pulse width modulation is removed through a filter composed of a reactor 3 and a capacitor 5 and added to one winding of a transformer 6,
The other winding of the transformer 6 is connected to the AC system 8 via the contact 7a, and the power generated by the solar cell is supplied to the AC system. The DC voltage of the solar cell 1 is detected as V _df by the voltage detector 9 and compared with the reference voltage V _d ^* by the voltage control unit 11 to _obtain V _df.
There outputs a signal I _m defining the amplitude of the alternating current reference to be equal to V _d ^*, multiplier 13 is multiplied by the AC voltage V _ac voltage detector 12 AC system 8 detected by the AC current reference i ^* Becomes. The current control unit 14 uses the i ^* and the current detector 4
The output current i _a of the inverter 2 detected by
The PWM signal is output via the WM control unit 15 and the drive unit 16,
The pulse width modulation control is performed so that the output current i _a of the inverter 2 becomes equal to the current reference i ^* , and the control for extracting the maximum power from the solar cell 1 is performed.

Further, the operating circuit 18 judges from the output V _df of the voltage detector 9 that the output voltage of the solar cell 1 exceeds a predetermined value, determines that operation is possible, outputs a signal to the relay drive section 7, and contacts 7a , 7b are closed, the winding of the transformer 6 is connected to the AC system 8, and at the same time, an operating signal is output from the starting circuit 17 to activate the driving unit 16 to start the operation of the apparatus. As a result, the inverter 2 outputs an alternating current having the same phase as the voltage of the alternating current system 8 and the high power factor operation is started.

[0004]

A solar power generation inverter having a capacity of 3 KW to 5 KW installed in a private residence has a relatively high power supply impedance of the AC system 8 and is a device. When the contact 7a is closed and the winding of the transformer 6 is connected to the AC system 8 at the time of starting, the inrush current of 10 to 20 times the rated current flows depending on the closing phase, and the received voltage drops momentarily. There are problems such as flickering of fluorescent lamps.

In recent transformers, the BH saturation characteristic of the iron core is improved, the magnetic flux density is increased, and the size of the transformer is reduced, and the inrush current is also increased. In addition, it is possible that the solar power generation inverter starts up many times a day depending on the change in the intensity of solar radiation, and it is a problem to generate an instantaneous voltage drop each time.

The present invention has been made in order to solve the above problems, and an object thereof is to start the operation of the inverter and connect the transformer outputting the AC voltage to the AC system. An object of the present invention is to provide an inverter device in which inrush current does not flow.

[0007]

In order to achieve the above object, the present invention provides an inverter for converting a DC voltage into an AC voltage by pulse width modulation and applying the same to one winding of a transformer, and the transformer. Switch for connecting the other winding of the transformer to the AC system, and the inverter so that the AC voltage induced in the other winding is close to the voltage of the AC system when the inverter is started. And a control means for outputting the exciting current, and when the exciting current is stabilized, the switch is closed.

Further, a capacitor for removing high frequency is connected to the winding of the transformer, and the exciting current command of the control means is such that when the combined impedance of the exciting inductance of the capacitor and the transformer is inductive. 90 ° delay phase, 90% advance phase when capacitive, so that the AC voltage induced in the other winding has a value close to the voltage of the AC system, the exciting current from the inverter Output.

Further, the difference between the current flowing through the capacitor and the exciting current flowing through the transformer monotonically increases with increasing voltage within a predetermined voltage range including the rated voltage of the transformer. Set the capacitance of the capacitor.

[0010]

When the inverter is started, the control means outputs an exciting current command in synchronism with the voltage of the alternating current system, whereby the exciting current is output from the inverter and is connected to the alternating current system of the transformer. A voltage close to the voltage of the AC system is induced in the winding. When the induced voltage becomes stable, the switch is closed and the inverter output is connected to the AC system without flowing a large inrush current.

Further, the composite impedance is set to a predetermined value or less so as to supply an exciting current of a constant current or more from the inverter, and the impedance is set depending on whether the composite impedance is inductive or capacitive. The phase of the exciting current output from the inverter is determined to be 90 degrees delayed or advanced with respect to the voltage phase of the AC system.

Further, even when the exciting inductance changes non-linearly with respect to the voltage within a predetermined voltage range including the rated voltage when the inverter starts, the exciting current output from the inverter increases in voltage. Since it increases monotonically, a stable induced voltage can be obtained when an exciting current within the predetermined voltage range is supplied.

[0013]

FIG. 1 shows an embodiment of the present invention. In FIG. 1, 21 is an exciting current reference generator that outputs an exciting current reference i _o ^* in synchronization with the phase of the AC voltage V _a of the AC system 8 detected by the voltage detector 12, 22 is a timer, 7c, Reference numeral 7d is a changeover switch that operates according to a command from the relay drive unit 7. Others are the same as the conventional ones and are shown with the same reference numerals as in FIG. 7.

In the above structure, when the apparatus is stopped, the changeover switches 7c and 7d are switched to the B side, and when the output voltage of the solar cell 1 exceeds a predetermined value, the control power source is established and the operating circuit 18 The driving signal is output from the drive unit 16
To operate the inverter 2 to start operation. In this case, the exciting current reference i _o ^{* is} passed through the changeover switch 7d ^.
Since the inverter 2 acts as a current control for outputting only the current of the capacitor 5a and the exciting current of the transformer 6, the winding connected to the AC system of the transformer 6 has a voltage of almost the AC system. Voltages of the same phase are induced. When the time for this control to stabilize elapses, the timer 22 times out and closes the contact 7a via the relay drive unit 7 and at the same time switches the changeover switches 7c and 7d to the A side. Contact 7a
When the winding of the transformer 6 is closed and the winding of the transformer 6 is connected to the AC system, the inrush current hardly flows regardless of the closing phase because the difference voltage is small. Further, the normal current reference i ^* is input via the changeover switch 7d, and the control for supplying the maximum power of the solar cell 1 to the AC system is started. Further, the changeover switch 7c connects the capacitor 5b to establish a function as a filter for removing high frequency components by PWM control. The meaning of providing the changeover switch 7c will be described in detail below.

The equivalent circuit in the unloaded state seen from the output side of the inverter 2 can be shown in FIG. 2 (a). However, c
Is the capacity of the capacitor 5a, and L is the exciting inductance of the transformer 6. Further, the inductance of the reactor 3 is small and can be ignored.

Excitation current i _o from the inverter 2 is applied to this circuit.
Is supplied to the capacitor c and the exciting inductance L, and the exciting current i _l induces a voltage in phase with the AC system voltage in the winding of the transformer 6 on the AC system side.

The magnetic flux density of the transformer 6 increases as the excitation voltage V increases, and the magnetic permeability decreases as the magnetic flux density increases. Therefore, the exciting inductance L of the transformer 6 decreases as the voltage V increases and the exciting current i _l
Increases as shown in FIG. 2 (b). Generally, the rate of increase of i _l in the exciting current starts to increase from 70 to 80% of the rated voltage.
It is designed to increase rapidly from around 110%.

On the other hand, the current i _{c of the} capacitor c is proportional to the exciting voltage V. In addition, the exciting current i _o supplied from the inverter 2 is the exciting current i _l of the transformer near the rated voltage.
The exciting current reference i _o ^* is output from the exciting current reference generator 21 so as to be a difference current between the capacitor current i _c and the current i _c . In this case, when i _l > i _c , the delay phase is 90 °, and i _l
When <i _c , by setting the lead phase to 90 °, the voltage phase induced in the winding of the transformer 6 can be matched with the voltage phase of the AC system. However, the exciting current i _l of the transformer
And the current i _{c of the} capacitor have the relationship shown in FIG. 2C, the following problem occurs. For example, the excitation voltage V is 70%
As the inverter - if supplying the exciting current i _o from motor 2, the flow advances the phase of the current in the magnitude of the i _o = i _c1 -i _l1. However, when the relationship between i _l and i _c is as shown in FIG. 2 (c), the difference current i _c2 −i _l2 when the excitation voltage V is 100%.
For almost the same reason as above, when the differential current i _c −i _l exceeds the maximum excitation voltage (voltage near 90% in the case of FIG. 2 (c)), i _l rapidly increases and i _c − i _l decreases, and conversely i _l becomes larger than i _c .

Since the output voltage (excitation voltage) is not controlled at the beginning of the operation of the inverter 2, the voltage is in an unstable state, and if the difference current exceeds the maximum excitation voltage for some reason or other. The required differential current is reduced, an excessive exciting current is supplied, and the exciting voltage rapidly increases. Then, the difference current of i _l -i _c is balanced at the point where it matches the current i _o supplied from the inverter 2 (i _l3 -i _c3 of 140% excitation voltage in FIG. 2 (c)). In this case, since the exciting current i _o of 90 ° lead phase is supplied to the inductive equivalent circuit, the voltage phase induced in the winding of the transformer 6 is opposite to the voltage phase of the AC system (180 ° or so). Phase difference)
It becomes very dangerous.

The changeover switch 7c is for changing the capacity of the capacitor c only at the beginning of operation in order to avoid the above-mentioned dangerous state. FIG. 3A is a diagram showing the relationship between the exciting current i _l and the capacitor current i _c when the capacitance of the capacitor is changed to a small value. When a transformer with a relatively large exciting current is used, the capacitance of the capacitor is changed so that the capacitor current i _c becomes smaller than the exciting current i _{l as} shown in FIG. In this case, the combined impedance of the equivalent circuit becomes inductive, and the required exciting current also increases as the exciting voltage increases. Then, i _l -i _c in the excitation voltage 100% exciting current reference generator 21
The excitation current reference i _o ^* having a delay phase of 90 ° with respect to the voltage phase of the AC system and having a magnitude corresponding to the difference current is output from the inverter 2 and the excitation current i _o shown in FIG. Is output. As a result, an exciting voltage of 100% in phase with the voltage of the AC system is stably induced in the winding of the transformer 6.

Further, even if the transformer 6 is replaced and the exciting current characteristic changes like i _l1 and i _l2 , the induced voltage remains as shown in FIG.
As shown by V ₁ and V ₂ in (a), an extremely stable induced voltage can be obtained by only slightly changing.

Since the switch 7a is closed in this state,
A large inrush current does not flow through the transformer regardless of the closing phase. The changeover switch 7c can be omitted when the function of the filter is satisfied by the capacity of the capacitor at the time of starting in this way.

When the exciting current of the transformer 6 is relatively small, the capacitor current i _c can be started so as to be larger than the exciting current i _l as shown in FIG. 3 (b). In this case, the changeover switch 7c is shown by A in FIG.
The side and the B side are reversed and the capacitor 5b is connected in parallel with the capacitor 5a at the time of starting. And capacitors 5a and 5
The combined capacitance of b is such that, within the range of possible excitation voltage (about 120%), the change di _c / dV of the capacitor current i _c with respect to the change of the voltage V is the exciting current i _{l with} respect to the change of the voltage V.
Change di _l / dV is determined to be larger than the maximum value. In this case, the combined impedance of the equivalent circuit becomes capacitive, and the required exciting current i _o increases as the exciting voltage V increases. Then, the excitation current reference generator 21
From the inverter 2, the excitation current reference i _o ^* is output with a magnitude corresponding to the difference current of i _c −i _l at 80% of the excitation voltage and with a lead phase of 90 ° with respect to the voltage phase of the AC system. The exciting current i _o shown in 3 (b) is output. As a result, the exciting voltage of the same phase as the AC system voltage is applied to the winding of transformer 6.
% Stably induces. It goes without saying that the exciting current supplied from the inverter can be slightly increased to i _o1 to induce a 100% exciting voltage.

In this way, since the switch 7a is closed while the excitation voltage is stable, a large inrush current does not flow. Further, at the same time when the switch 7a is closed, the changeover switch 7c operates to disconnect the capacitor 5b from the circuit. The changeover switch 7c can be omitted when the above conditions are satisfied only by the capacitor 5a.

In the embodiment shown in FIG. 1, the switching circuit for the filter and the capacitor is shown as an example provided on the inverter 2 side of the transformer 6, but it may be provided on the contact 7a side. In this case, the reactor 3 can be omitted by utilizing the leakage inductance of the transformer 6. Also,
Although the example of using the solar cell as the DC power source is shown, the present invention can be applied to the case of using another DC power source such as a fuel cell.

[0026]

According to the present invention, in a device for supplying the AC power of the inverter output through the transformer to the AC system, a voltage close to the voltage of the AC system is generated when the inverter is started. Since the winding of the transformer is connected to the AC system, a large inrush current does not occur, and an inverter device that does not adversely affect the received voltage of the AC system can be provided.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention.

2A and 2B are diagrams for explaining the operation of the present invention, in which FIG. 2A is an equivalent circuit diagram, FIG. 2B is an exciting current characteristic diagram of a transformer, and FIG.
Is a characteristic diagram of i _l and i _c when a problem occurs.

3A and 3B are diagrams for explaining the operation of the present invention, in which FIG. 3A is a characteristic diagram in the case of i ₁ > i _c , and FIG. 3B is a characteristic diagram in the case of i ₁ <i _c .

FIG. 4 is a block diagram of a conventional device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... DC power supply (solar cell) 2 ... Inverter 3 ... Reactor 4 ... Current detector 5a, 5b ... Capacitor 6 ... Transformer 7 ... Relay drive part 7a ... Contact 7c, 7d ... Changeover switch 8 ... AC system 9 , 12 ... Voltage detector 11 ... Voltage control unit 13 ... Multiplier 14 ... Current control unit 15 ... PWM control unit 16 ... Driving unit 18 ... Driving circuit 21 ... Excitation current reference generating unit 22 ... Timer-

Front page continuation (72) Inventor Takuo Itami No. 1 in Toshiba Fuchu factory, Fuchu-shi, Tokyo (72) Inventor Kenichi Kimoto No. 1 Toshiba-machi in Fuchu, Tokyo Inside Fuchu factory, Toshiba

Claims (4)

[Claims] 1. An inverter for converting a DC voltage into an AC voltage by pulse width modulation and applying the same to one winding of a transformer, and a switch for connecting the other winding of the transformer to an AC system. At the time of starting the inverter, a control means for outputting an exciting current from the inverter so that the AC voltage induced in the other winding has a value close to the voltage of the AC system is provided, and the exciting current is An inverter device, characterized in that the switch is closed at a stable time. 2. The inverter device according to claim 1, wherein a capacitor for removing high frequencies is connected to the winding of the transformer, and the exciting current command of the control means is to excite the capacitor and the transformer. If the combined impedance of the inductance is inductive, it is delayed by 90 °, and if it is capacitive,
An inverter device which outputs an exciting current from the inverter so that the AC voltage induced in the other winding has a value close to the voltage of the AC system as a lead phase of 90 °. . 3. The inverter device according to claim 2, wherein a difference between a current flowing through the capacitor and an exciting current flowing through the transformer is within a predetermined voltage range including a rated voltage of the transformer. The inverter device is characterized in that the capacitance of the capacitor is set so as to increase monotonically with the increase of. 4. The inverter device according to claim 3, wherein the capacity of the capacitor is changed for a predetermined period from the time of activation of the inverter.