JPH07255132A - Inverter for system interconnection - Google Patents

Abstract

PURPOSE:To prevent the rush current into a transformer, when such a DC power as the ones of solar batteries or fuel cells whose output state is changed largely is converted into an AC power by an inverter and the output of the inverter which is insulated from the inverter by the transformer is interconnected to a power system, by making the breaker provided on the side of a power system when the interconnection condition is satisfied. CONSTITUTION:The output power of such a DC power supply 1 as a solar batteries and fuel cells whose variation is large is converted into an AC power by an inverter bridge 2, and the output power of the inverter bridge 2 is fed to an insulation transformer 6 via an inductor 3 and capacitor 4. Further, the output of a DC voltage sensor 10 and a power- system-voltage sensor 11 are inputted to an excitation-current-reference generator 24. Also, a current reference I2*sinwt associated with power factor equal to 1 and an excitation current reference I1*sin(wt-pi/2) of the transformer 6 which is sensed by a CT 12 are added to each other and are inputted to a current amplifier 22, and further, the output of the amplifier 22 is inputted to a PWM generator 23, and by the output of the PWM generator 23, the inverter bridge 2 is controlled. When an output voltage EV of the excitation-current-reference generator 24 and an output voltage VX of the power-system-voltage sensor 11 are confined within predetermined ranges respectively, a breaker driver 8 is turned on, and the inverter bridge 2 is operated so as to interconnected to a power system. Thereby, the rush current into the transformer 6 is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter for system interconnection, and more particularly to an inverter for system interconnection used in solar cell power generation equipment where total efficiency such as conversion efficiency / operation efficiency is important. Is.

[0002]

2. Description of the Related Art In recent years, a grid interconnection system has been used in which DC power obtained from a solar cell, a fuel cell or the like is converted into AC power having a commercial frequency to supply power to a commercial AC system. An example of such a system interconnection system is shown in FIG. As shown in the figure, electric power from a DC power supply 1 such as a solar cell is converted into AC power by an inverter bridge 2, and a filter including a reactor 3 and a capacitor 4 removes a high frequency component by PWM control and cuts it off. It is connected to the system power supply 7 through the contacts of the device 5 and the commercial frequency transformer 6.

The DC voltage detector 10 detects the voltage of the DC power source supplied to the inverter bridge 2, and outputs the DC voltage feedback signal E _FB and the signal E output when the voltage exceeds a predetermined level.
Output X. The system voltage detector 11 detects the voltage on the inverter side of the commercial frequency transformer 6 and outputs the system voltage feedback signal V
_FB (Vsinωt) and a signal VX that is output when the system voltage is within a predetermined range are output. In addition, the current detector 1
2 detects the output current of the inverter bridge 2 and outputs a current feedback signal I _FB .

If the system voltage is normal and within a predetermined range, the DC voltage is above a predetermined level, and it is determined that power can be generated in the system, that is, if both the signal EX and the signal VX are established, the circuit breaker is activated. The driver 8 of No. 5 is driven, and the circuit breaker 5 is closed.

Further, the DC voltage command E ^* for driving the DC power source 1 such as a solar cell at the maximum output and the DC voltage feedback signal E _FB are input to the control amplifier 20, and the AC current peak value command I is output from the control amplifier 20. ^* Is output. This AC current peak value command I ^* is applied to the system voltage feedback signal V by the reference generator 21.
_It is converted into an inverter output current reference I ^* sinωt synchronized with _FB (Vsinωt), and is input to the current control amplifier 22 together with the inverter output current feedback signal I _FB.
The inverter bridge 2 is controlled via the generator 23.

As described above, the control power supply of the inverter device is taken from the DC portion of the inverter, and when the DC power supply starts power generation and the DC voltage is established to some extent, the control power supply of the inverter device is established. As described above, in the conventional control of the system interconnection system, the output current is controlled with a power factor of 1 in synchronization with the system voltage phase with an amplitude according to the generation amount of the DC power supply.

[0007]

Since the solar cell is very expensive, not only the conversion efficiency during operation but also the overall operation efficiency in consideration of operating time and no-load loss during stop are important points. By the way, as described above, in the conventional technology,
Since the inverter performs the power factor 1 control of the output current, the exciting current of the commercial frequency transformer 6 is supplied from the system side, and the commercial frequency transformer 6 is always connected to the system power supply. This causes the system power supply to continue supplying the exciting current to the transformer even at night when the solar cell is not generating power, which causes a decrease in overall operation efficiency.

Further, when the circuit breaker 5 is connected to the system side of the commercial frequency transformer 6 and opened / closed by the operation of the inverter, an inrush current of about 20 times the rated value is supplied to the transformer 6 each time the circuit is opened / closed. Induces frequent disturbances to the flow and system.
In order to prevent this, it was necessary to add an initial charging resistance.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an inverter for system interconnection which has a large fluctuation in conversion efficiency / operation efficiency and is required to have a wide operation range. Another object of the present invention is to provide a grid interconnection inverter that improves the overall operation efficiency of the inverter by supplying the exciting current of the transformer from the inverter side. Still another object of the present invention is to realize a grid interconnection inverter having high overall operation efficiency and no protruding current to the transformer.

[0010]

To achieve the above object, the invention according to claim 1 uses an inverter to convert a direct current power source, which has a large fluctuation in generated power / output voltage, etc., depending on various conditions such as a solar cell and a fuel cell. In a system interconnection inverter that converts to electric power, isolates it with a transformer and connects it to the power system, a circuit breaker is provided on the system side of the transformer, and the circuit breaker is opened when the DC power source is not generating power. The circuit breaker is closed when the DC power source is capable of generating sufficient power for grid interconnection.

According to a second aspect of the present invention, in the grid interconnection inverter according to the first aspect, the control power source of the inverter is supplied from the DC power source, the DC power source starts power generation, and the breaker. The system voltage on the system side is detected by the detector, and the circuit breaker is closed while the excitation current of the transformer corresponding to the system AC voltage can be supplied from the inverter, and the excitation current of the transformer cannot be supplied. In this state, the circuit breaker is opened.

According to a third aspect of the present invention, in the grid interconnection inverter according to the second aspect, the current control reference of the inverter is the excitation current reference of the transformer and the power supply current reference to the system from the DC power source. Vector sum, in accordance with the power supply establishment of the DC power supply side, the excitation current reference of the transformer is soft-started at a predetermined rate, the circuit breaker on the condition that the excitation of the transformer according to the system voltage is established. The circuit breaker is closed, and the circuit breaker is opened when the amount of power generated by the DC power supply is reduced and the power supply current reference becomes 0 and the exciting current cannot be maintained.

[0013]

According to the present invention, the exciting current of the transformer is vector-added to the output current reference of the inverter, and the exciting current of the transformer is supplied from the inverter, and at the same time, the inverter including the transformer is provided on condition that the excitation of the transformer is established. Since the device is connected to the grid, it is possible to provide a grid interconnection inverter with high overall operation efficiency and no protruding current to the transformer. Further, it is possible to provide an inverter for system interconnection, which is required to have a wide operation range with large fluctuations in conversion efficiency / operation efficiency.

[0014]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a block diagram of an embodiment of the present invention. The present embodiment is different from the conventional system interconnection system of FIG. 3 described above in the following three points.

(1) A point at which the circuit breaker 5 and the system voltage detector 11 move to the system side of the commercial frequency transformer 6 side. (2) the exciting current reference generator 24 are added, the current reference power factor 1 current reference is determined from a conventional DC voltage control I ₂ ^*
sinωt and the excitation current reference I ₁ ^* sin of the transformer 6
A point that is the sum of (ωt−π / 2).

(3) The circuit closing condition of the circuit breaker 5 is that the system voltage is normal and the transformer excitation is established. Since the configuration other than the above is the same as that of the conventional system interconnection system of FIG. 3, the same parts are designated by the same reference numerals and described.

Next, the operation of this embodiment will be described.
Excitation current reference generator 24, the excitation characteristics of the commercial frequency transformer 6 previously given as Parameta gives excitation commensurate with excitation by the system voltage depending on the system voltage feedback signal V _FB (V _AC sinωt) excitation current reference I ₁ ^* sin (ω
t-π / 2) is generated. The detailed functional block is shown in FIG.

In the functional block of FIG. 2, the system voltage feedback signal V _FB is the transformer excitation amplitude reference generator 30, and the function f (V _AC , ω) is determined by the excitation characteristic of the transformer.
Excitation current amplitude command I according to system voltage / frequency
₀ ^* is generated and output to the exciting current soft start circuit 31.

In the soft start circuit 31, the DC voltage feedback signal E _FB is equal to or more than the lower limit value E _{L of the} operating range, that is, the excitation current amplitude reference I ₁ ^* is soft started under the condition of E _FB ≧ E _L Launch towards. By taking a load with the exciting current, the solar cell supplies electric power for the exciting loss, so that the DC voltage decreases. If lower than the lower limit of the operating range as in the middle under the following conditions of the soft start, i.e., if the E _FB <E _L, reduce the excitation amplitude reference. When the soft start is completed and the excitation current amplitude command I ₀ ^* and the excitation current amplitude reference I ₁ ^* match, it is determined that the transformer has been excited, and the excitation establishment signal EY is output.

On the other hand, the system voltage feedback signal V _FB (V _AC sin
ωt) is input to the phase shift circuit 32, and the normalized sine wave reference sin (ωt−π / 2) is delayed by π / 2.
Is generated, the excitation amplitude reference I ₁ ^* is multiplied, and the excitation current reference I ₁ ^* sin (ωt−π / 2) is output.

Now, in this embodiment, when the solar cell starts power generation, the DC voltage starts rising, and the control power supply is first established. Then, after the control power supply is established, when the DC voltage becomes higher than the lower limit E _{L of the} operating range, the exciting current command starts to rise by soft start and the transformer starts to be excited from the inverter side. At this time, the inverter current reference I ₂ ^* sin ωt = 0 with a power factor of 1, so I ₁ ^* si of the excitation current reference
Only n (ωt−π / 2).

When the soft start is completed and the transformer excitation is established in this way, the circuit breaker 5 is turned on and the grid interconnection operation is started. If the power generation capacity is equal to or more than the exciting component, the DC voltage further rises, and the current reference I ₂ ^* sin with a power factor of 1 is obtained by voltage control for maximum power control of the solar cell.
ωt starts to rise, and the current reference of the inverter is the sum of them, that is, I ₁ ^* sin (ωt−π / 2) + I ₂ ^*
sinωt is reached, and power supply to the grid is started.

As the power generated by the solar cell decreases, the current reference I ₂ ^* sinωt with a power factor of 1 begins to decrease, and E _FB <
E ^* become the I ₂ ^* sinωt = 0, and the and interconnection only by the excitation current to open the circuit breaker 5 at the stage of a E _FB <E _L.

As described above, in this embodiment, the output current of the inverter including the transformer exciting current is controlled according to the power generation capacity of the solar cell, and the inverter including the transformer is established in the state where the excitation of the transformer is established and the power can be generated. Since the system is connected to the system, no transformer excitation loss occurs when power is not being generated, and inrush current does not flow because the transformer excitation is established in synchronization with the system power supply even when the transformer is turned on.

[0025]

As described above, according to the present invention, the excitation current of the transformer is vector-added to the output current reference of the inverter, and the excitation current of the transformer is supplied from the inverter, and the excitation of the transformer is established at the same time. Since the inverter device including the transformer is connected to the system under the condition, it is possible to provide a system interconnection inverter with high overall operation efficiency and no protruding current to the transformer. Therefore, it is possible to provide a commercial-frequency-insulated system interconnection inverter that does not cause system disturbance.

[Brief description of drawings]

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

FIG. 2 is a functional block diagram of the excitation current reference generator shown in FIG.

FIG. 3 is a block configuration diagram of a conventional commercial frequency isolated type grid-connected inverter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... DC power supply, 2 ... Inverter bridge, 3 ... Reactor, 4 ... Capacitor, 5 ... Circuit breaker, 6 ... Commercial frequency isolation transformer, 7 ... System power supply, 8 ... Circuit breaker driver, 10 ... DC voltage detector, 11 ... System voltage detector, 12 ... Current detector, 20 ... DC voltage control amplifier, 21 ... Current reference generator, 22 ... Current control amplifier, 23 ... PWM generator, 24
... excitation current reference generator, 30 ... transformer excitation amplitude command generator, 31 ... excitation current reference soft start circuit, 32 ...
Phase shift circuit.

Claims (3)

[Claims] 1. A grid interconnection system in which a DC power source, which has a large fluctuation in generated power / output voltage, etc. depending on various conditions such as a solar cell and a fuel cell, is converted into AC power by an inverter and insulated by a transformer to be connected to a power system. In the inverter for inverter, a circuit breaker is provided on the system side of the transformer, the circuit breaker is opened when the DC power supply is not generating power, and the circuit breaker is generated when the DC power supply is capable of generating sufficient power for grid connection. An inverter for system interconnection characterized by closing a circuit breaker. 2. The system interconnection inverter according to claim 1, wherein the control power supply of the inverter is supplied from the DC power supply, the DC power supply starts power generation, and a system voltage on the system side of the circuit breaker is supplied. Detected by the detector, closing the circuit breaker in a state where the exciting current of the transformer according to the system AC voltage can be fed from the inverter,
An inverter for grid interconnection, wherein the circuit breaker is opened in a state where the exciting current of the transformer cannot supply power. 3. The system interconnection inverter according to claim 2, wherein the current control reference of the inverter is a vector sum of an excitation current reference of the transformer and a power supply current reference to the system from the DC power source, and the DC According to the power supply on the power supply side, the excitation current reference of the transformer is soft-started at a predetermined rate, and the circuit breaker is closed on the condition that the excitation of the transformer according to the system voltage is established. An inverter for system interconnection, characterized in that the circuit breaker is opened when the amount of power generation of the power source decreases and the power supply current reference becomes 0 and the exciting current cannot be maintained.