Patent Description:
For converting the voltage of a DC power supply to a single-phase AC voltage, for example, a power conversion device including a boost converter (DC/DC converter) and an inverter circuit is used. In a traditional power conversion device, the voltage of a DC power supply is stepped up by a boost converter to a certain voltage higher than the peak voltage on the AC side, and thereafter, the resultant voltage is converted to an AC voltage by an inverter circuit. In this case, the boost converter and the inverter circuit always perform high-speed switching operations. Therefore, switching loss occurs in each switching element and iron loss occurs in a reactor. These losses become a factor for hampering improvement in conversion efficiency.

Meanwhile, the following control is proposed: while the voltage of the DC power supply and the absolute value of the instantaneous voltage on the AC side are always compared with each other, the boost converter is caused to perform switching operation only during a period in which step-up operation is needed, and the inverter circuit is caused to perform switching operation only during a period in which a step-down operation is needed (see, for example, Patent Literature <NUM>, <NUM>). In such control, the period in which the switching operation is stopped is provided to the boost converter and the inverter circuit. If the stop period is provided, switching loss and reactor iron loss are reduced accordingly, leading to improvement in conversion efficiency.

The invention provides a power conversion device according to claim <NUM> and a control method for a power conversion device according to claim <NUM>.

In the power conversion device of Patent Literature <NUM> or <NUM> described above, not only an active current but also a reactive current flows through the boost converter. The amplitude of the reactive current is equal to the amplitude of the active current, and the frequency of the reactive current is twice the fundamental wave on the AC side (frequency: <NUM> or <NUM>). Therefore, as compared to the traditional power conversion device through which only an active current flows through a boost converter, the peak value of the current flowing through the boost converter is doubled, and also, the effective value thereof becomes (<NUM><NUM>/<NUM>) times greater. Therefore, as the boost converter, the one that can withstand such a current needs to be used. As a result, the size of the boost converter increases.

In addition, in order to prevent a reactive current from flowing through the DC power supply, it is necessary to provide a large-capacity capacitor between the DC power supply and the boost converter, to absorb the reactive current. For example, in a case where the DC power supply is a photovoltaic panel, the output impedance of the panel is great. Therefore, the reactive current can be almost absorbed even by a comparatively small-capacity capacitor.

However, in a case where the DC power supply is a storage battery, the output impedance is smaller as compared to a case of the photovoltaic panel. In this case, it is impossible to absorb the reactive current by only the capacitor. Thus, the reactive current flows through the storage battery, so that losses occurring in the electric path between the storage battery and the power conversion device and inside the storage battery increase. Such losses become a factor for hampering improvement in conversion efficiency.

In view of the above problem, an object of the present disclosure is to provide a power conversion device and a control method therefor, that achieve further compactification and further enhancement of conversion efficiency.

The power conversion device and the control method therefor of the present disclosure can achieve further enhancement of conversion efficiency and further compactification.

Hereinafter, the details of embodiments will be described with reference to the drawings.

First, the basic configuration as a premise of a power conversion device using a minimum switching conversion method will be described.

<FIG> is a single-line connection diagram showing the schematic configuration of a power conversion device <NUM> connected to a photovoltaic panel 3P. In <FIG>, the power conversion device <NUM> performs DC-to-AC power conversion, and includes a DC/DC converter <NUM> as a boost converter, and a DC/AC converter <NUM> as an inverter circuit connected thereto via an intermediate bus (DC bus) <NUM>. The DC/DC converter <NUM> is provided, via a DC-side capacitor <NUM>, between the photovoltaic panel 3P as a DC power supply and the intermediate bus <NUM>. An intermediate capacitor <NUM> is connected to the intermediate bus <NUM>. The DC/AC converter <NUM> is provided, via an AC-side capacitor <NUM>, between the intermediate bus <NUM> and an AC grid.

The power conversion device <NUM> performs control according to the minimum switching conversion method, in which, while a voltage taken from the photovoltaic panel 3P and the absolute value of the instantaneous voltage on the AC side are always compared with each other, the DC/DC converter <NUM> is caused to perform switching operation during only a period in which step-up operation is needed, and the DC/AC converter <NUM> is caused to perform switching operation during only a period in which step-down operation is needed. In such control, a period in which switching operation is stopped is provided to each of the DC/DC converter <NUM> and the DC/AC converter <NUM>. If the stop period is provided, switching loss and reactor iron loss are reduced accordingly, so that conversion efficiency improves.

By performing the above minimum switching conversion method, a pulsating current containing a reactive current as schematically shown by the waveform at the left in the drawing flows through the DC/DC converter <NUM>. A current outputted from the power conversion device <NUM> to the AC grid has a sine waveform synchronized with a commercial power grid as shown by the waveform at the right in the drawing.

<FIG> is an example of a circuit diagram of the power conversion device <NUM> shown in <FIG>. The parts corresponding to those in <FIG> are denoted by the same reference characters. In <FIG>, the power conversion device <NUM> includes a filter circuit <NUM>, a control unit <NUM>, and measurement sensors described later, as well as the DC-side capacitor <NUM>, the DC/DC converter <NUM>, the intermediate capacitor <NUM>, and the DC/AC converter <NUM> described above.

The DC/DC converter <NUM> is a step-up chopper (step-down is also possible) including a DC reactor <NUM> and a pair of switching elements Q11, Q12. As the switching elements Q11, Q12, for example, IGBTs (Insulated Gate Bipolar Transistors) are used. Diodes d11, d12 are respectively connected in parallel to the switching elements Q11, Q12, in opposite-polarity directions. It is noted that, other than the above, FETs (Field Effect Transistors) may be used as the switching elements Q11, Q12.

The DC/AC converter <NUM> includes four switching elements Q81, Q82, Q83, Q84 forming a full bridge.

The filter circuit <NUM> is composed of an AC reactor <NUM> and the AC-side capacitor <NUM>, and prevents a high-frequency component contained in the AC output of the DC/AC converter <NUM> from leaking to the AC grid <NUM>. The AC grid <NUM> includes an AC load <NUM> and a commercial power grid <NUM>.

As the aforementioned sensors, provided are: a voltage sensor <NUM> for detecting a voltage (Vg) between both ends of the DC-side capacitor <NUM>; a current sensor <NUM> for detecting a current (Iin) flowing through the DC/DC converter <NUM>; a voltage sensor <NUM> for detecting a voltage between both ends of the intermediate capacitor <NUM>, i.e., a voltage (Vo) between two lines of the intermediate bus <NUM>; a current sensor <NUM> for detecting a current (Iinv) flowing on the AC side of the DC/AC converter <NUM>; and a voltage sensor <NUM> for detecting a voltage between both ends of the AC-side capacitor <NUM>. Measurement output signals from all the sensors are sent to the control unit <NUM>. The control unit <NUM> performs switching control for the DC/DC converter <NUM> and the DC/AC converter <NUM>.

The control unit <NUM>, for example, includes a CPU and executes software (computer program) by a computer, thereby realizing necessary control functions. The software is stored in a storage device (not shown) of the control unit <NUM>. It is noted that the control unit <NUM> may be configured from a circuit using only hardware not including a CPU.

Next, the power conversion device <NUM> according to an embodiment of the present invention will be described.

<FIG> is a single-line connection diagram showing the schematic configuration of the power conversion device <NUM> connected to the photovoltaic panel 3P. The same parts as those in <FIG> are denoted by the same reference characters and the description thereof is omitted. The difference from <FIG> is that two systems are provided on the DC side of the intermediate bus <NUM>.

In <FIG>, separately from the first DC/DC converter <NUM>, a second DC/DC converter <NUM> is provided between a DC-side capacitor <NUM> and the intermediate bus <NUM>. A DC power supply is not connected to the second DC/DC converter <NUM>.

<FIG> is an example of a circuit diagram of the power conversion device <NUM> shown in <FIG>. The parts corresponding to those in <FIG> and <FIG> are denoted by the same reference characters. In <FIG>, the second DC/DC converter <NUM> is a step-up chopper (step-down is also possible) including a DC reactor <NUM> and a pair of switching elements Q21, Q22. As the switching elements Q21, Q22, for example, IGBTs are used. Diodes d21, d22 are respectively connected in parallel to the switching elements Q21, Q22, in opposite-polarity directions. It is noted that, other than the above, FETs may be used as the switching elements Q21, Q22. A voltage between both ends of the DC-side capacitor <NUM> is detected by a voltage sensor <NUM>, and the measurement signal is sent to the control unit <NUM>. A current flowing through the DC/DC converter <NUM> is detected by a current sensor <NUM>, and the measurement signal is sent to the control unit <NUM>.

If the aforementioned minimum switching conversion method control is expressed from electric power perspective in <FIG>, the control unit <NUM> performs control so that the sum of a power passing through the first DC/DC converter <NUM> and a power passing through the second DC/DC converter <NUM> coincides with the sum of a reactive power for the intermediate capacitor <NUM> and a power arising on the AC side of the DC/AC converter <NUM>.

That is, in this case, the power on the DC side as seen from the intermediate bus <NUM> coincides with the power on the AC side including the intermediate capacitor <NUM>. In other words, the power on the DC side never becomes an excessive power greater than the power on the AC side. Therefore, the first DC/DC converter <NUM> and the second DC/DC converter <NUM> perform minimum necessary switching operations including the stop periods, and the DC/AC converter <NUM> performs minimum necessary switching operation including the stop period.

In addition, the control unit <NUM> performs control so that the sum of a power transferred to the AC grid <NUM> and a power of the AC-side capacitor <NUM> coincides with a power transferred between the AC reactor <NUM> and the DC/AC converter <NUM>. Thus, powers can still be caused to coincide with each other even while considering the filter circuit <NUM>. In other words, the control unit <NUM> performs control considering influence of the filter circuit <NUM>.

In <FIG> and <FIG>, the DC-side capacitor <NUM> serves as an element closing the terminal end circuit on the DC side. Of the current flowing through the intermediate bus <NUM>, the first DC/DC converter <NUM> supplies an active current, and the second DC/DC converter <NUM> supplies a reactive current. The second DC/DC converter <NUM> in this case does not need to supply an active current, but exists only for supplying a reactive current. Such a configuration is a suitable circuit configuration for preventing the first DC/DC converter <NUM> from bearing a reactive current. In the first DC/DC converter <NUM> through which a reactive current does not flow, conversion efficiency can be enhanced as compared to a case where a reactive current flows therethrough. In addition, the peak value and the effective value of the current flowing through the first DC/DC converter <NUM> are reduced, whereby a smaller-sized configuration can be achieved.

It is noted that, in <FIG>, the photovoltaic panel 3P may be replaced with the storage battery 3B. The storage battery 3B serves as a DC power supply when discharging, and serves as a load when being charged.

The power conversion device <NUM> performs the control according to the minimum switching conversion method by the control unit <NUM>. Here, the theory of the minimum switching conversion method will be described. First, various values, including the aforementioned values, will be defined as follows.

Among the above various values, values that vary depending on time t are represented as functions of time in the following expressions. It is noted that, in the following, there is no meaning in difference of character font (upright/italic), and the same character represents the same value (the same applies hereinafter).

First, the current command value I*inv for the DC/AC converter <NUM> is represented as follows.

The voltage command value V*inv for the DC/AC converter <NUM> is represented as follows.

The current command value I*in for the DC/DC converter <NUM> (<NUM>) is represented as follows.

A voltage drop of the DC power supply voltage in the DC/DC converter <NUM> (<NUM>) and a reactive current flowing through the intermediate capacitor <NUM> are small. Therefore, by ignoring these, the following expression (<NUM>) is obtained.

Next, if I*inv and V*inv are sine waves completely synchronized with each other, the following expression (<NUM>) is obtained. Here, ω is 2πf when the frequency of the AC grid is f.

In the expression (<NUM>), I*inv and V*inv without time (t) represent the amplitudes of the sine waves. Expression (<NUM>) can be further deformed into the following expression (<NUM>).

The first term in expression (<NUM>) is a constant value not depending on time, and is an active current. That is, expression (<NUM>) representing an active current I*in_a is as follows.

Here, a notation "< >" indicates an average value of a value in the brackets. In addition, the subscript of I*inv_i represents, for example, the current command value for the DC/DC converter <NUM> as I*inv_1, and the current command value for the DC/DC converter <NUM> as I*inv_2.

As shown by expression (<NUM>), the active current is equal to a value obtained by dividing the effective values of I*inv and V*inv by the DC input voltage Vg, and in a case where there are a plurality of DC/DC converters, the active current can be represented using linear combination of currents I*inv_i for the respective converters.

On the other hand, the second term in expression (<NUM>) is a reactive current having a frequency twice as high as the AC frequency. That is, expression (<NUM>) representing a reactive current I*in_r(t) is as follows.

The effective value of the reactive current is the square root (rms) of mean square of expression (<NUM>), and is represented by the following expression (<NUM>).

This is (<NUM>/√<NUM>) times the active current.

The effective value of a current is represented by the following expression (<NUM>).

This is (<NUM>/<NUM>)<NUM>/<NUM> times the active current.

From the above analysis, if the current command value for the DC/DC converter <NUM> for supplying a reactive current is set as expression (<NUM>) to supply a reactive current to the intermediate bus <NUM>, the DC/DC converter <NUM> can supply only an active current with the current command value therefor set as expression (<NUM>). Thus, as compared to the configuration in <FIG>, the peak value of the current of the DC/DC converter <NUM> is halved. Further, since a low-frequency pulsating current no longer flows through the DC/DC converter <NUM>, the capacitance of the DC-side capacitor <NUM> can be reduced.

The description thus far has been made on the basis of expression (<NUM>) in which voltage drops in the DC/DC converters <NUM>, <NUM> and a reactive current in the intermediate capacitor <NUM> are omitted, for simplification purpose. In practice, it is desirable to perform control based on expression (<NUM>) without such omission. Accordingly, expression (<NUM>) can be replaced with the expression (<NUM>) adapted to a case where a plurality of DC/DC converters <NUM>, <NUM> are provided in parallel as shown in <FIG> and <FIG>.

In expression (<NUM>), in association with the respective systems on the DC side of the intermediate bus <NUM>, the subscript "i" is i = <NUM>, <NUM>, or may be n equal to or greater than <NUM>, and in this case, i = <NUM> to n. The current command value I*inv for the DC/AC converter <NUM> becomes I*inv_i by being divided so as to correspond to the plurality of DC/DC converters. Similarly, the capacitance of the intermediate capacitor <NUM> becomes Co_i by being divided so as to correspond to the plurality of DC/DC converters.

Next, the procedure for determining the current command values for the DC/DC converter <NUM> and the DC/DC converter <NUM> will be described. First, as shown by the following expression (<NUM>), the current command value I*in1 for the DC/DC converter <NUM> calculated by expression (<NUM>) is averaged over a cycle T (half the cycle of AC output from the DC/AC converter <NUM>) of an AC component, thereby obtaining an active current component I*in1_a thereof.

Next, a reactive current component is calculated by the following expression (<NUM>).

A current command value I*inm1 for the DC/DC converter <NUM>, in which the reactive current component is reduced, can be calculated by the following expression (<NUM>), with u set as a value of <NUM> to <NUM>.

When the value of u is <NUM>, I*inm1 becomes equal to I*in1_a, and the reactive current component is completely eliminated from the current command value for the DC/DC converter <NUM>, so that only an active current remains.

On the other hand, a current command value I*inm2 for the DC/DC converter <NUM> to bear the reactive current is obtained by adding u·I*in1_r to I*in2 calculated by expression (<NUM>), as shown by the following expression (<NUM>).

When a DC power supply is not connected to the DC/DC converter <NUM>, I*in2 becomes <NUM>. Further, when the value of u is <NUM>, I*inm2 becomes I*in1_r, so that only the DC/DC converter <NUM> supplies a reactive current component. The value of u is determined in consideration of the size, cost, conversion efficiency, and the like of the power conversion device <NUM>. In addition, the value of u may be changed depending on the operation condition.

In any case, by expressions (<NUM>), (<NUM>), mainly the DC/DC converter <NUM> is to supply a reactive current flowing through the intermediate bus <NUM>.

That is, in such a power conversion device <NUM>, mainly the DC/DC converter <NUM> is to take on the reactive current, and therefore, conversely, the DC/DC converter <NUM> can mainly supply an active current while a reactive current is reduced. Thus, the peak value of the current of the DC/DC converter <NUM> is reduced, conversion efficiency is enhanced, and further compactification can be achieved.

In addition, if the DC/DC converter <NUM> is caused to bear the entire reactive current, only an active current flows through the DC/DC converter <NUM>. Thus, the peak value of the DC/DC converter <NUM> is maximally reduced, conversion efficiency is enhanced, and compactification can be achieved.

In addition, it is also possible to control reactive currents flowing through the DC/DC converter <NUM> and the DC/DC converter <NUM> so that the peak values of currents flowing through the DC/DC converter <NUM> and the DC/DC converter <NUM> are minimized. In this case, the current capacities of the switching elements Q11, Q12, Q21, Q22 and the DC reactors <NUM>, <NUM> of the respective converters can be minimized, whereby the power conversion device <NUM> can be downsized.

In addition, it is also possible to control reactive currents flowing through the DC/DC converter <NUM> and the DC/DC converter <NUM> so that the mean squares of currents flowing through the DC/DC converter <NUM> and the DC/DC converter <NUM> are minimized. In this case, resistance losses occurring in the DC/DC converter <NUM> and the DC/DC converter <NUM> can be minimized, whereby the efficiency of the power conversion device <NUM> can be enhanced.

It is preferable that the control unit <NUM> controls the reactive current of the DC/DC converter <NUM> so that the voltage between both ends of the DC-side capacitor <NUM> coincides with the voltage of the DC power supply, i.e., the voltage of the DC-side capacitor <NUM>.

In this case, the DC/DC converter <NUM> to which a DC power supply is not connected can be caused to perform switching operation at the same timing as the DC/DC converter <NUM>. Therefore, the switching operation period of the DC/DC converter <NUM> can be minimized.

<FIG> is a single-line connection diagram showing the schematic configuration of the power conversion device <NUM> connected to the storage battery 3B. The difference from <FIG> is that, instead of the photovoltaic panel, the storage battery 3B is connected to the DC/DC converter <NUM>.

In this case, reactive currents flowing through the DC/DC converter <NUM> and the DC/DC converter <NUM> are controlled so that the reactive current flowing through the DC/DC converter <NUM> becomes zero. Thus, a reactive current can be prevented from flowing through the storage battery 3B.

<FIG> is a single-line connection diagram showing the schematic configuration of the power conversion device <NUM> connected to the photovoltaic panel 3P and the storage battery 3B. The differences from <FIG> are that, in a system separate from the photovoltaic panel 3P, the storage battery 3B is connected to the DC/DC converter <NUM> and that switches <NUM>, <NUM> are provided. The switch <NUM> is provided between the photovoltaic panel 3P and the DC/DC converter <NUM>. The switch <NUM> is provided between the storage battery 3B and the DC/DC converter <NUM>.

<FIG> is an example of a circuit diagram of the power conversion device <NUM> corresponding to <FIG>. The differences from <FIG> are that the switches <NUM>, <NUM> are provided and that the storage battery 3B is connected to the DC/DC converter <NUM>. The switches <NUM>, <NUM> can be opened or closed by the control unit <NUM>. As the switches <NUM>, <NUM>, for example, relay contacts may be used.

Returning to <FIG>, in a case where the photovoltaic panel 3P is generating a power and the storage battery 3B is not in operation, the switch <NUM> is closed and the switch <NUM> is opened. In this case, the DC/DC converter <NUM> can be used for supplying a reactive current. By causing a reactive current to flow through the DC/DC converter <NUM> without causing the reactive current to flow to the storage battery 3B, the peak values of currents flowing through the DC/DC converters <NUM>, <NUM> or the mean squares of these currents can be minimized.

<FIG> is a single-line connection diagram showing the schematic configuration of the power conversion device <NUM> connected to the photovoltaic panel 3P and the storage battery 3B. The difference from <FIG> is that the open/close states of the switches <NUM>, <NUM> are reversed. In a case where the photovoltaic panel 3P is not generating a power, for example, during the night and the storage battery 3B is being charged or discharged, the switch <NUM> is opened and the switch <NUM> is closed as shown in the drawing. Thus, the solar battery is prevented from being energized by the voltage of the DC-side capacitor <NUM> and at the same time, a reactive current is caused to flow through the DC/DC converter <NUM>, whereby a reactive current can be prevented from flowing through the DC/DC converter <NUM> and the storage battery 3B.

<FIG> is a single-line connection diagram showing the schematic configuration of the power conversion device <NUM> connected to the photovoltaic panel 3P and the storage battery 3B. The difference from <FIG> and <FIG> is that both switches <NUM>, <NUM> are closed. In a case where the photovoltaic panel 3P is generating a power and the storage battery 3B is being charged or discharged, both switches <NUM>, <NUM> are closed as shown in the drawing. Then, a reactive current of the DC/DC converter <NUM> is controlled so that a reactive current flowing through the DC/DC converter <NUM> becomes zero. Thus, a reactive current can be prevented from flowing through the storage battery 3B.

It is noted that, in a case where the photovoltaic panel 3P is generating a power and the storage battery 3B is being charged, reactive currents flowing through the DC/DC converter <NUM> and the DC/DC converter <NUM> are cancelled with each other, so that the peak values of currents flowing through the respective converter <NUM>, <NUM> are reduced. Therefore, when the photovoltaic panel 3P is generating a power, if only charging is performed for the storage battery 3B without performing discharging, the current capacities of the DC/DC converter <NUM> and the DC/DC converter <NUM> can be reduced. Thus, the size and the weight of the power conversion device <NUM> can be reduced.

It is noted that the first to fifth examples may be, at least partially, optionally combined with each other. In addition, the number of systems on the DC side is not limited to two, but may be three or more.

As shown in the third to fifth examples, when the switch <NUM> is opened, the DC/DC converter <NUM> can be used only for supplying a reactive current, and when the switch <NUM> is closed, the DC/DC converter <NUM> can supply not only a reactive current but also an active current.

Also for the DC/DC converter <NUM>, similarly, when the switch <NUM> is opened, the DC/DC converter <NUM> can be used only for supplying a reactive current, and when the switch <NUM> is closed, the DC/DC converter <NUM> can supply not only a reactive current but also an active current.

In the above examples, the power conversion device <NUM> has been described as a device for performing DC-to-AC power conversion. However, on the basis of the same control theory, reverse-direction power conversion is also applicable by changing signs as appropriate considering the current direction.

Next, operation of the power conversion device <NUM> (<FIG>, <FIG>, or <FIG>) with the photovoltaic panel 3P and the storage battery 3B connected in two systems on the DC side will be verified under various conditions.

In each of <FIG>, waveforms at the first to fifth stages from the top represent the following.

<FIG> and <FIG> are waveform diagrams of the power conversion device <NUM> that is charging the storage battery 3B in a state in which there is no power generated by the photovoltaic panel 3P (including a case where the photovoltaic panel 3P is not connected). That is, the generated power is <NUM> kW. In addition, here, the charge power is <NUM> kW, the received power from the AC grid <NUM> is <NUM> kW, and the voltage of the storage battery 3B is <NUM> V. <FIG> shows a case of not performing the above control relevant to reactive current, and <FIG> shows a case of performing the control.

If a reactive current that would flow through the DC/DC converter <NUM> is borne by the DC/DC converter <NUM> which receives no output from the photovoltaic panel 3P, the current of the DC/DC converter <NUM> is smoothed (flat line at the third stage in <FIG>). At this time, if control is performed so that the voltage of the DC-side capacitor <NUM> of the DC/DC converter <NUM> keeps <NUM> V which is the same as that of the storage battery 3B, the switching period of the DC/DC converter <NUM> hardly varies, and the condition in which the DC/DC converter <NUM> stops switching during the period in which the DC/AC converter <NUM> performs switching, is kept. Switching of the DC/DC converter <NUM> is also performed during the same period as the DC/DC converter <NUM>, and is stopped during the period in which the DC/AC converter <NUM> operates.

The AC current Ia and a total harmonic distortion (THD) before current smoothing (<FIG>) and after current smoothing (<FIG>) are as follows.

It is noted that THD is calculated from a waveform that has passed through a low-pass filter having a cut-off frequency of <NUM>, in order to eliminate a ripple of a switching frequency of <NUM> or higher.

<FIG> and <FIG> are waveform diagrams of the power conversion device <NUM> that is discharging the storage battery 3B in a state in which there is no power generated by the photovoltaic panel 3P (including a case where the photovoltaic panel 3P is not connected). The voltage of the storage battery 3B is <NUM> V, and the discharge power is <NUM> kW. <FIG> shows a case of not performing the control relevant to reactive current, and <FIG> shows a case of performing the control.

The AC current Ia and the total harmonic distortion THD before current smoothing (<FIG>) and after current smoothing (<FIG>) are as follows.

In this case, the DC/DC converter <NUM> is caused to bear a reactive current, and the voltage of the DC-side capacitor <NUM> of the DC/DC converter <NUM> is controlled to keep <NUM> V which is the same as that of the storage battery 3B. Also in this case, the switching period of the DC/DC converter <NUM> hardly varies by the smoothing, and the condition in which the DC/DC converter <NUM> stops switching during the period in which the DC/AC converter <NUM> performs switching, is kept. Switching of the DC/DC converter <NUM> is also performed during the same period as the DC/DC converter <NUM>, and is stopped during the period in which the DC/AC converter <NUM> operates. At a stage before smoothing, dip occurs on the AC current immediately after shifting from the switching period of the DC/AC converter <NUM> to the switching period of the DC/DC converters <NUM>, <NUM>, and therefore the total harmonic distortion is as great as <NUM>%. On the other hand, by the smoothing, dip of the AC current is eliminated and the total harmonic distortion reduces to <NUM>%.

<FIG> and <FIG> are waveform diagrams of the power conversion device <NUM> that is charging the storage battery 3B in a state in which there is a power generated by the photovoltaic panel 3P. The voltage of the storage battery 3B is <NUM> V which is lower than the optimum operation voltage of the photovoltaic panel 3P. In addition, the generated power is <NUM> kW, the charge power is <NUM> kW, and the reverse-flow power is <NUM> kW. <FIG> shows a case of not performing the control relevant to reactive current, and <FIG> shows a case of performing the control.

In this case, current does not flow through the DC/DC converter <NUM> unless the voltage thereof is raised to the same voltage as output of the DC/DC converter <NUM>, and therefore the DC/DC converter <NUM> always performs switching. After current smoothing, the current of the DC/DC converter <NUM> is almost smoothed, and the amplitude of a pulsating current of the DC/DC converter <NUM> is also reduced. Even after current smoothing, the switching period of the DC/DC converter <NUM> is separated from the switching period of the DC/AC converter <NUM> without overlapping, and the number of times of switching thereof is not increased.

<FIG> and <FIG> are waveform diagrams of the power conversion device <NUM> that is charging the storage battery 3B in a state in which there is a power generated by the photovoltaic panel 3P. It is noted that the voltage of the storage battery 3B is <NUM> V which is higher than the optimum operation voltage of the photovoltaic panel 3P. In addition, the generated power is <NUM> kW, the charge power is <NUM> kW, and the reverse-flow power is <NUM> kW. <FIG> shows a case of not performing the control relevant to reactive current, and <FIG> shows a case of performing the control. In this case, the DC/DC converter <NUM> always performs switching. It is found that it is possible to perform current smoothing even when the voltage of the storage battery 3B is higher than that of the photovoltaic panel 3P.

<FIG> and <FIG> are waveform diagrams of the power conversion device <NUM> that is discharging the storage battery 3B in a state in which there is a power generated by the photovoltaic panel 3P. The voltage of the storage battery 3B is <NUM> V. In addition, the generated power is <NUM> kW, the discharge power is <NUM> kW, and the reverse-flow power is <NUM> kW. <FIG> shows a case of not performing the control relevant to reactive current, and <FIG> shows a case of performing the control. Also in this case, current smoothing for the DC/DC converter <NUM> is performed without any problem. The DC/DC converter <NUM> has a switching stop period, and even after smoothing, the original operation in which the DC/DC converter <NUM> and the DC/AC converter <NUM> alternately perform switching, is maintained.

The upper stage, the middle stage, and the lower stage in <FIG> show waveform diagrams of the AC current Ia, the output current Ip of the photovoltaic panel 3P, and the output current Ib of the storage battery 3B, respectively, under the condition in <FIG>.

The current flowing through the DC/DC converter <NUM> to which the photovoltaic panel 3P is connected contains a reactive current, but it is found that, through smoothing by the DC-side capacitor <NUM>, the output current Ip becomes almost a constant value. In this case, the values of Ia and THD after current smoothing are as follows. Ia: <NUM> Arms, THD: <NUM>%.

It is noted that the embodiments disclosed herein are merely illustrative in all aspects and should not be recognized as being restrictive. The scope of the present invention is defined by the scope of the claims, and is intended to include meaning equivalent to the scope of the claims and all modifications within the scope.

As a further example, the power conversion device can also be expressed as follows.

Claim 1:
A power conversion device (<NUM>) configured to perform DC/AC power conversion via an intermediate bus (<NUM>), the power conversion device (<NUM>) comprising:
a first DC/DC converter (<NUM> or <NUM>) provided between a first DC power supply or a load, and the intermediate bus (<NUM>);
a second DC/DC converter (<NUM> or <NUM>) provided between a DC-side capacitor (<NUM> or <NUM>) and the intermediate bus (<NUM>);
an intermediate capacitor (<NUM>) connected to the intermediate bus (<NUM>);
a DC/AC converter (<NUM>) provided between the intermediate bus (<NUM>) and an AC grid; and
a control unit (<NUM>) configured to control the first DC/DC converter (<NUM> or <NUM>), the second DC/DC converter (<NUM> or <NUM>), and the DC/AC converter (<NUM>),
characterized in that:
the control unit (<NUM>) is configured to make such current command value setting that mainly the second DC/DC converter (<NUM> or <NUM>) supplies a reactive current flowing through the intermediate bus (<NUM>).