Power conversion system for solar cells

Abstract

To provide a power conversion system for a solar cell capable of stabilizing the fluctuation of the output voltage of a power converter and effectively utilizing the generated power of the solar cell even when sunlight intensity or temperature is changed.SOLUTION: A power conversion system (100) connects a solar cell module (102) and a DC distribution network (104), includes a maximum power point tracking controller (10) and a DC / DC converter (12), and changes a step-up ratio of the DC / DC converter (12) according to a change in at least one of solar radiation intensity and temperature when a required power amount for output via the DC / DC converter (12) is smaller than a maximum output power amount of the solar cell module (102).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a power conversion system for solar cells. [Background technology]

[0002] A power conversion system relating to solar cells has been disclosed, including, for example, a solar cell module that converts sunlight irradiating onto the cells into electrical energy, and a control unit that boosts the voltage generated by the cells and stores it in a battery, adjusts the level obtained by boosting the cell voltage depending on the storage state of the battery, the amount of sunlight, and the external temperature, and limits the boost level to a voltage level or lower corresponding to the maximum output point that changes depending on the amount of light and the external temperature (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-027913 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the conventional technology, when the required power output is smaller than the maximum power output of the photovoltaic power generation, the power output corresponding to the output voltage of the photovoltaic system is not uniquely determined but remains indefinite. This means that when balancing the power demand and supply, the voltage may fluctuate and become unstable depending on environmental conditions such as solar radiation conditions.

[0005] Furthermore, it is known that in a DC power grid, it is advantageous to transmit power at the highest possible voltage and the lowest possible current in order to minimize resistance losses, so using conventional techniques to minimize the impact of changes in solar intensity and external temperature on the DC power grid is not desirable. [Means for solving the problem]

[0006] One aspect of the present invention is a power conversion system that connects a solar cell module to a DC power distribution network, comprising a maximum power point tracking controller and a DC / DC converter, and characterized in that when the amount of power required for output via the DC / DC converter is smaller than the maximum output power of the solar cell module, the power conversion system changes the step-up ratio of the DC / DC converter in response to a change in at least one of solar radiation intensity and temperature.

[0007] Here, it is preferable to change the step-up ratio of the DC / DC converter so that the power points are aligned in a straight line in the output current-voltage characteristics.

[0008] It is also preferable that the output voltage of the DC / DC converter does not change when maximum power point tracking control is being performed to track the maximum output power of the solar cell module. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a power conversion system for solar cells that can stabilize fluctuations in the output voltage of a power converter even when the intensity of solar radiation or the outside temperature changes, and can effectively utilize the power generated by the solar cell. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram showing a configuration of a power conversion system for a solar cell according to an embodiment of the present invention; [Figure 2] FIG. 4 is a graph showing the solar radiation intensity dependency of the IV characteristics of the solar cell module according to the embodiment of the present invention. [Figure 3] FIG. 10 is a diagram showing the maximum power point during maximum power point control relative to solar radiation intensity for the solar cell module according to the embodiment of the present invention. [Figure 4] FIG. 1 is a diagram showing a typical output IV characteristic of a conventional power conversion system connected to a DC power distribution network. [Figure 5]FIG. 10 is a diagram showing a typical output IV characteristic of a power conversion system connected to a DC power distribution network according to a conventional embodiment. [Figure 6] FIG. 10 is a diagram showing the relationship between the actual measured value of the DC bus voltage and the actual measured value of the voltage on the output side of the DC / DC converter when the power conversion system according to the conventional embodiment is applied. [Figure 7] 10 is a diagram showing the relationship between the actual measured value of the DC bus voltage and the actual measured value of the current on the output side of the DC / DC converter when the power conversion system according to the conventional embodiment is applied. FIG. [Figure 8] 10 is a diagram showing the relationship between the actual measured value of the DC bus voltage and the actual measured value of the power on the output side of the DC / DC converter when the power conversion system according to the embodiment of the present invention is applied. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0011] As shown in Fig. 1, a power conversion system 100 for solar cells in this embodiment includes a maximum power point tracking controller (MPPT controller) 10, a direct current / direct current converter (DC / DC converter) 12, and a controller 14. The power conversion system 100 is connected between a solar cell module 102 and a DC power distribution network 104. The solar cell module 102 may be a solar cell array in which a plurality of solar cells are arranged.

[0012] The MPPT controller 10 performs maximum power point tracking control, which automatically determines the optimum current and voltage values ​​(maximum power point or optimum operating point) that can maximize the output when the solar cell module 102 generates power. Specific control by the MPPT controller 10 will be described later.

[0013] The DC / DC converter 12 boosts the output from the solar cell module 102 and supplies the boosted output to a DC power distribution network 104 .

[0014] The controller 14 is connected to the MPPT controller 10 and the DC / DC converter 12. The controller 14 comprehensively controls the MPPT controller 10 and the DC / DC converter 12. The controller 14 is configured to include control means such as a microcomputer, and is programmed so that the MPPT controller 10 and the DC / DC converter 12 perform the control described below.

[0015] FIG. 2 shows the solar radiation intensity dependency of the current-voltage characteristics (IV characteristics) of the solar cell module 102. The range of change in solar radiation intensity is assumed to be from A sun to D sun. In FIG. 2, the solar radiation intensity is assumed to be A sun > B sun > C sun > D sun. The solar radiation intensity can be obtained by measuring it using a solar radiation intensity sensor connected to the controller 14, for example. Furthermore, the solar radiation intensity can be obtained as solar radiation intensity information from an external device connected to the controller 14 via a communication network or the like, for example.

[0016] For each of these solar radiation intensities, maximum power point tracking control is performed by the MPPT controller 10. FIG. 3 shows the maximum power point P MA , P MB , P MC , P MD Shows.

[0017] Figure 4 shows a typical output IV characteristic of a power conversion system connected to a conventional DC power distribution network. The output characteristic of a power conversion system is determined by the characteristics of the DC / DC converter. In a power conversion system, the upper limit voltage V SPC is generally determined to be constant regardless of the solar radiation intensity or temperature. Also, when the maximum power point of the solar cell is maintained, the maximum power point voltage V is determined for each of the solar radiation intensities A sun, B sun, C sun, and D sun. MA , V MB , V MC , V MD The DC / DC converter operates at the maximum power point voltage V MA , V MB , V MC , V MD The output upper limit voltage VSPC However, in terms of the output IV characteristics of the solar cell, the open-circuit voltage V OA , V OB , V OC , V OD are the maximum power point voltages V MA , V MB , V MC , V MD Since the open circuit voltage V OA , V OB , V OC , V OD is the upper limit output voltage V SPC In many cases, the voltage is boosted to the same level as the

[0018] When the MPPT control of the solar cell is effective and tracking the maximum power point is achieved, if the input and output power are the same and loss is not taken into account, the output voltage V of the power conversion system that deviates from the DC power grid voltage is PCO and the output current I of the power conversion system PCO is inversely proportional.

[0019] Under these conditions, the maximum power point P for each of the solar radiation intensities A sun, B sun, C sun, and D sun is MA , P MB , P MC , P MD The power conversion system output I PCO -V PCO Characteristic power point P DCMA , P DCMB , P DCMC , P DCMD The voltage and current at this time are V DCMA , V DCMB , V DCMC , V DCMD , and current I DCMA , I DCMB , I DCMC , I DCMD The maximum power point P MA = power point P DCMA = voltage V DCMA ×Current I VDMAis.

[0020] In a conventional power conversion system connected to a DC power grid, the output I PCO -V PCO The characteristics are inversely proportional. In addition, in the range where MPPT control is not effective, the output I PCO -V PCO The output voltage V at each of the solar radiation intensities A sun, B sun, C sun, and D sun is PCO Voltage V DCMA , V DCMB , V DCMC , V DCMD and the output voltage V PCO Between the output voltage V PCO is the upper limit output voltage V SPC As the output current approaches I PCO are the currents I DCMA , I DCMB , I DCMC , I DCMD To summarize, in a conventional power conversion system, the output I PCO -V PCO It becomes a characteristic.

[0021] Output I PCO -V PCO When using a conventional power conversion system that exhibits these characteristics, if the solar radiation intensity fluctuates while trying to maintain a constant set voltage on the DC power grid under conditions where MPPT control is not applied, the output I PCO -V PCO Current fluctuations occur that move the operating point up and down along the vertical axis of the characteristic, making the behavior of the DC power distribution network unstable. Also, if the solar radiation intensity fluctuates while trying to keep the set current of the DC power distribution network constant, the output I PCO -V PCO Voltage fluctuations occur that move the operating point up and down along the horizontal axis, which again makes the behavior of the DC power distribution network unstable.

[0022] In particular, in voltage droop control, which is expected to be used in DC distribution networks, the voltage of the DC distribution network must be changed using solar radiation intensity as a parameter, so such unstable behavior could be a fatal flaw in ensuring a stable power supply.

[0023] Therefore, in this embodiment, as shown in FIG. 5, for example, the output I PCO -V PCO That is, the output I PCO -V PCO The power conversion system 100 is configured so that the characteristics do not become unstable depending on the solar radiation intensity. PCO -V PCO Power point P in the characteristic DCMA , P DCMB , P DCMC , P DCMD For example, the DC / DC converter 12 is controlled so that the output I of the power conversion system 100 is linearly aligned. PCO -V PCO Power point P in the characteristic DCMA , P DCMB , P DCMC , P DCMD The characteristics are such that the lines are arranged in a straight line.

[0024] Output I PCO -V PCO Characteristic power point P DCMA , P DCMB , P DCMC , P DCMD to the output upper limit voltage V SPC The linear slope of the voltage droop control (voltage drop control) can be set arbitrarily. However, since the slope affects the stability and responsiveness of the voltage droop control (voltage drop control) in the DC power distribution network, it is advantageous to select a slope that is suitable for the stability and responsiveness of the voltage droop control (voltage drop control).

[0025] Thus, the output I PCO-V PCO Power point P in the characteristic DCMA , P DCMB , P DCMC , P DCMD By configuring the power conversion system 100 so that the operating points are arranged in a straight line, even if the solar radiation intensity fluctuates when attempting to keep the set voltage of the DC power distribution network constant under conditions in which MPPT control is not applied, the operating point moves along a linear slope, and unstable current fluctuations do not occur, making it possible to stably control the behavior of the DC power distribution network.

[0026] 6 to 8 show the relationship between the measured values ​​of the DC bus voltage and the measured values ​​of the voltage, current, and power on the output side of the DC / DC converter 12 when sunlight with intensities of 1 sun and 0.2 sun is irradiated onto the solar cell module 102 in the conventional power conversion system 100.

[0027] The scope of application of the present invention is not limited to the solar radiation intensity, and similar control can be applied to temperature. That is, the output I PCO -V PCO The power conversion system 100 is configured so that the characteristics do not become unstable depending on the temperature. PCO -V PCO Power point P in the characteristic DCMA , P DCMB , P DCMC , P DCMD For example, the DC / DC converter 12 is controlled so that the output I of the power conversion system 100 is linearly aligned by changing the step-up ratio of the DC / DC converter 12 according to the temperature. PCO -V PCO Power point P in the characteristic DCMA , P DCMB , P DCMC , P DCMD The characteristics are such that the lines are arranged in a straight line.

[0028] The temperature of the solar cell module 102 can be obtained by measuring the temperature of the solar cell module 102 using a temperature sensor connected to the controller 14, for example.

[0029] Thus, the output I PCO -V PCO Power point P in the characteristic DCMA , P DCMB , P DCMC , P DCMD By configuring the power conversion system 100 so that the operating points are arranged in a straight line, even if the temperature fluctuates when attempting to keep the set voltage of the DC power distribution network constant under conditions in which MPPT control is not applied, the operating point moves along a linear slope, and unstable current fluctuations do not occur, making it possible to stably control the behavior of the DC power distribution network.

[0030] In addition, when both the solar radiation intensity and the temperature change simultaneously, for example, the output I of the power conversion system 100 can be increased by changing the step-up ratio of the DC / DC converter 12 according to the combination of the solar radiation intensity and the temperature. PCO -V PCO Power point P in the characteristic DCMA , P DCMB , P DCMC , P DCMD The characteristics are such that the lines are arranged in a straight line.

[0031] Thus, the output I PCO -V PCO Power point P in the characteristic DCMA , P DCMB , P DCMC , P DCMD By configuring the power conversion system 100 so that the operating points are arranged in a straight line, even if the solar radiation intensity and temperature fluctuate when attempting to keep the set voltage of the DC power distribution network constant under conditions in which MPPT control is not applied, the operating point moves along a linear slope, and unstable current fluctuations do not occur, making it possible to stably control the behavior of the DC power distribution network.

[0032] [Configuration of the present invention] [Configuration 1] A power conversion system that connects a solar cell module to a DC power distribution network, A maximum power point tracking controller and a DC / DC converter are provided, A power conversion system characterized by changing the step-up ratio of the DC / DC converter in response to a change in at least one of solar radiation intensity and temperature when the amount of power required for output via the DC / DC converter is smaller than the maximum output power of the solar cell module. [Configuration 2] The power conversion system according to configuration 1, A power conversion system characterized in that the step-up ratio of the DC / DC converter is changed so that power points are aligned linearly in the output current-voltage characteristics. [Configuration 3] The power conversion system according to the first or second aspect, A power conversion system characterized in that the output voltage of the DC / DC converter does not change when maximum power point tracking control is being performed to track the maximum output power of the solar cell module. [Explanation of symbols]

[0033] 10 Maximum power point tracking controller (MPPT controller), 12 Direct current / Direct current converter (DC / DC converter), 14 Controller, 100 Power conversion system, 102 Solar cell module, 104 DC power distribution network.

Claims

1. A power conversion system that connects a solar cell module to a DC power distribution network, A maximum power point tracking controller and a DC / DC converter are provided, A power conversion system characterized by changing the step-up ratio of the DC / DC converter in response to a change in at least one of solar radiation intensity and temperature when the amount of power required for output via the DC / DC converter is smaller than the maximum output power of the solar cell module.

2. 2. The power conversion system of claim 1, A power conversion system characterized in that the step-up ratio of the DC / DC converter is changed so that power points are aligned linearly in the output current-voltage characteristic.

3. 3. The power conversion system according to claim 1 or 2, A power conversion system characterized in that the output voltage of the DC / DC converter does not change when maximum power point tracking control is being performed to track the maximum output power of the solar cell module.

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