JP6078914B2 - Voltage adjustment circuit for solar cell string - Google Patents

Description

  The present invention can improve the power generation efficiency of the entire system in a photovoltaic power generation system by adjusting variations in output voltage between a plurality of parallel-connected solar cell strings (a series connection of a plurality of solar cell modules). The present invention relates to a voltage adjustment circuit for a solar cell string.

  The photovoltaic power generation system currently in practical use is a centralized type that converts DC power generated by a solar cell array consisting of a plurality of solar cell modules connected in series and parallel to AC power using an inverter of a power conditioner. The inverter method is the mainstream.

  The solar cell array is formed by connecting a plurality of solar cell strings in parallel, and each solar cell string is composed of a plurality of solar cell modules connected in series.

  The power conditioner is composed of an inverter with an MPPT (Maximum Power Point Tracking) circuit that adjusts the load on the input side so that the maximum output can be extracted from the solar cell array. It is provided between.

  It is known that the output voltage of the solar cell module decreases as the temperature rises, and even if the number of solar cell modules of the solar cell strings connected in parallel to each other is the same, depending on the installation conditions The solar cell modules of some of the solar cell strings may become high temperature due to direct sunlight or the like, resulting in a decrease in the output voltage, resulting in variations in the output voltage between the solar cell strings. Moreover, the variation in the output voltage between solar cell strings may be caused by aged deterioration of some solar cell modules.

  FIG. 10 schematically shows a solar cell array in which solar cell strings Str1 and Str2 each having the same number of solar cell modules M connected in series are connected in parallel to an external load via backflow prevention diodes D1 and D2, respectively. FIG.

  In the same figure, when the output voltage is always equal between the two solar cell strings Str1 and the solar cell string Str2, as shown in FIG. 11, a graph of the current-voltage characteristic that changes monotonously according to the increase or decrease of the external load. Is obtained.

  However, when a failure or the like of a part of the solar cell modules M in one solar cell string Str1 occurs, the current-voltage characteristics of the solar cell string Str1 shown in FIG. 12A and the other shown in FIG. Compared with the current-voltage characteristics of the normal solar cell string Str2, the output voltage is lowered. In this case, the graph of the entire current-voltage characteristics is stepped as shown in FIG.

  In FIG. 13, the point P is the maximum output operating point, and the maximum output power is represented by the area of the rectangular region (1) representing the power generated by the solar cell string Str1 and the solar cell string Str2 in FIG. The sum of the areas of the rectangular areas (2) representing the power to be generated, and the power represented by the areas of the rectangular areas (3) that can be generated by the solar cell string S2 does not contribute to the overall output power but is lost.

Moreover, since the currently produced solar cell modules have a low power generation capacity per sheet, it is necessary to secure a light receiving area as large as possible when installing the solar cell array.
For this reason, when installing a solar cell array in a limited installation area such as a roof, the number of solar cell modules in series in the solar cell strings connected in parallel to each other may not be the same. Even in this case, the same power loss problem as described above occurs.

  Therefore, conventionally, as seen in Patent Document 1, for example, a DC-DC converter is incorporated for each solar cell string to individually perform maximum power tracking (MPPT) control and output voltage between solar cell strings. Proposals have been made.

JP-A-8-46231

  However, in the photovoltaic power generation system as described in Patent Document 1 described above, in order to obtain the maximum output power, the MPPT control is performed for each solar cell string, and then the DC-DC conversion is performed to adjust the voltage. Since a conversion circuit is required, there is a problem that conversion loss increases and manufacturing cost increases.

  In addition, if the control circuit stops functioning due to a failure or the like, all the power generated by the solar cell string in which the control circuit is incorporated is lost, which affects the reliability of the photovoltaic power generation system. was there.

  Therefore, the present invention solves the problems in the prior art as described above, and can reduce power loss when solar cell strings having different output voltages are connected in parallel. An object is to provide a voltage regulator circuit.

  The voltage adjustment circuit for the solar cell string of the present invention provided for the above-described purpose is respectively incorporated between a plurality of solar cell strings connected in parallel with an external load via a backflow prevention diode. One end is connected to the line connecting the positive electrode side of one adjacent solar cell string and the backflow prevention diode, and the other end is connected to the line connecting the positive electrode side of the other solar cell string and the backflow prevention diode. A positive-side connection line; an inductor incorporated in the positive-side connection line; a first switching element incorporated between one end of the positive-side connection line and the inductor; and a first of the positive-side connection line A second switch incorporated between the switching element and the inductor and the negative connection line connected to the negative electrode side of each solar cell string. A third switching element incorporated between the other end of the positive electrode side connecting line and the inductor, between the third switching element and the inductor of the positive electrode side connecting line, and the negative electrode side connecting line. A fourth switching element built in between the positive electrode side of the one solar cell string and the backflow prevention diode or between one end of the positive electrode side connecting line and the first switching element, The first smoothing circuit having at least one capacitor connected across the negative electrode side connection line and the line connecting the positive electrode side of the other solar cell string and the backflow prevention diode or the positive electrode side connection line Having at least one capacitor connected between the third switching element from the other end of the first switching element and the negative connection line. A smoothing circuit; a first voltage sensor that detects an output voltage V1 of the one solar cell string; a second voltage sensor that detects an output voltage V2 of the other solar cell string; and the switching elements. And a switching control circuit that individually performs ON / OFF control.

  The switching control circuit determines that the voltage V1 detected by the first voltage sensor and the second voltage sensor is higher and lower than the voltage V2, and that the voltage V1 = the voltage V2 in the first step. In this case, when it is determined that the voltage V1 is smaller than the voltage V2 in the second step in which all the switching elements are turned off and re-executed from the first step, the first switching element is determined. Is switched ON, the second switching element is switched OFF, the third switching element and the fourth switching element are complementarily synchronized, and the ON / OFF switching operation is periodically repeated a predetermined number of times. When the voltage V1> the voltage V2 is determined in the third step re-executed from the first step and in the first step, the third switching element is set to O In addition, the fourth switching element is switched OFF, the first switching element and the second switching element are complementarily synchronized, and the ON / OFF switching operation is periodically repeated the predetermined number of times, A fourth step is re-executed from the first step.

  In the voltage adjustment circuit for a solar cell string according to the present invention, the switching control circuit includes a short-circuit prevention period in which, in the third step, the third switching element and the fourth switching element are simultaneously turned off in each cycle. In addition, in the fourth step, the switching timing of each switching element is such that there is a short-circuit prevention period in which the first switching element and the second switching element are simultaneously turned off in each cycle in the fourth step. It is desirable to control. It is also desirable that each of the first smoothing circuit and the second smoothing circuit is constituted by a low-pass filter.

  According to the first aspect of the present invention, by adjusting the difference in output voltage between the solar cell strings connected in parallel, the overall current-voltage characteristics caused by the decrease in the output voltage of some of the solar cell strings Since the step can be smoothed and the power that was originally lost without being taken out as the power generation output can be used, the output power can be improved.

  Further, since the level difference in the current-voltage characteristics of the entire system is smoothed, it becomes easy to detect the maximum output operating point of the power conditioner, and the MPPT mismatch loss can be avoided. As a result, the temperature of some solar cell modules may vary depending on the location of the solar cell array, such as the roof, due to restrictions on the number of solar cell modules that make up each solar cell string. As the output voltage drops and the output voltage drops, there is a solar cell string with a reduced output voltage in the solar cell array. Thus, it is possible to effectively use the power generation efficiency of the photovoltaic power generation system.

  Furthermore, since the voltage adjustment circuit of the solar cell string of the present invention has a simple circuit configuration including a switching element such as a transistor and a relay, an inductor, and a capacitor, MPPT control is performed for each conventional solar cell string. Compared with a complicated circuit that performs DC-DC conversion and voltage adjustment, the energy loss is small and the device can be manufactured at low cost.

  Furthermore, in the voltage regulation circuit of the present invention, even if the circuit function stops due to failure, there is no risk that the solar cell string to which it is connected will not function, and this circuit is not equipped. The power generation function equivalent to the solar power generation system can be maintained as it is.

  According to the second aspect of the present invention, the first switching element and the second switching element, and the third switching element and the fourth switching element are simultaneously turned off for each period. Since the switching timing of each switching element is controlled so that there is a short-circuit prevention period in which a state occurs, the current path can be switched reliably.

  According to the third aspect of the present invention, the first smoothing circuit and the second smoothing circuit are each configured by a low-pass filter, thereby effectively preventing fluctuations in current and noise caused by the switching operation. Can be suppressed.

It is a figure which shows typically one Embodiment of the voltage adjustment circuit of the solar cell string of this invention. It is a block diagram of a switching control circuit. It is a flowchart which shows operation | movement of a voltage adjustment circuit. It is a figure which shows the process in which electric power is accumulate | stored in the inductor of a voltage adjustment circuit. It is a figure which shows the process in which electric power is discharge | released from the inductor of a voltage adjustment circuit. It is a figure which shows the timing of the complementary switching operation | movement of two switching elements in the case of providing the short circuit prevention period. It is a figure which shows the current voltage characteristic improved by the voltage regulator circuit of this invention. It is a partial circuit diagram which shows the example which comprised the 1st and 2nd smoothing circuit with the low-pass filter. It is a figure which shows typically another embodiment of the voltage adjustment circuit of the solar cell string of this invention. It is the figure which showed typically the conventional solar cell array which consists of two solar cell strings respectively connected in series with the same number of solar cell modules. It is a figure which shows the current-voltage characteristic of a solar cell array in case the output voltage of two solar cell strings is equal. It is a figure which shows the current-voltage characteristic of two solar cell strings from which output voltage differs individually. It is a figure which shows the current-voltage characteristic of a solar cell array in case there exists a difference in the output voltage of two solar cell strings.

FIG. 1 is a diagram showing an embodiment of a voltage adjustment circuit (hereinafter simply referred to as a voltage adjustment circuit) for a solar cell string according to the present invention. The same number of each is connected in series between the solar cell string Str1 and the solar cell string Str2.
These solar cell strings Str1 and Str2 are connected in parallel to an external load, and backflow prevention diodes D1 and D2 are incorporated on the respective positive electrode sides.

  The voltage adjustment circuit 1 includes four switching elements S1, S2, S3, S4, an inductor L, two capacitors C1, C2, two voltage sensors Vd1, Vd2, and a switching control circuit not shown here. Yes.

  Further, the voltage adjustment circuit 1 has a positive-side connection line 2 that connects the switching element S1 (first switching element), the inductor L, and the switching element S3 (third switching element) in series. ing.

  The positive electrode side connection line 2 has one end J1 connected to a line connecting the positive electrode side of the one solar cell string Str1 and the backflow prevention diode D1, and the other end J2 connected to the positive electrode of the other solar cell string Str2. Is connected to a line connecting the side and the backflow prevention diode D2.

Furthermore, the voltage adjustment circuit 1 has a negative electrode side connection line 3 connected to the negative electrode side of each of the solar cell strings Str1, Str2, and between the one end J1 of the positive electrode side connection line 2 and the switching element S1, A capacitor C1 (first smoothing circuit) is connected across the negative-side connection line 3.
Similarly, a capacitor C2 (second smoothing circuit) is connected across the other end J2 of the positive electrode side connecting line 2 and the switching element S3 and between the negative electrode side connecting line 3.

  Furthermore, a switching element S2 (second switching element) is incorporated between the switching element S1 and the inductor L and between the negative electrode side connecting line 3 of the positive electrode side connecting line 2, and similarly, A switching element S4 (fourth switching element) is incorporated between the switching element S3 and the inductor L and between the negative electrode side connecting line 3 in the positive electrode side connecting line 2.

  The voltage sensor Vd1 (first voltage sensor) detects the output voltage V1 of one solar cell string Str1, and the voltage sensor Vd2 (second voltage sensor) detects the output voltage of the other solar cell string Str2. It is provided for detection.

  These four switching elements S1, S2, S3, and S4 are individually ON / OFF controlled by the switching control circuit 4 shown in FIG. In the present embodiment, MOS-FETs are used for the switching elements S1, S2, S3, and S4.

  As shown in the figure, the switching control circuit 4 includes a microcomputer, a driver circuit, and an A / D converter, and the microcomputer is connected to each of the voltage sensors Vd1 and Vd2 through the A / D converter. The output voltage value data of each of the solar cell strings Str1 and Str2 subjected to the / D conversion is taken in, and based on these data, the respective switching elements S1, S2, S3, and S4 are ON / OFF controlled through the driver circuit. Have a role.

  Hereinafter, the operation of the voltage adjustment circuit 1 configured as described above will be described based on the flowchart shown in FIG. When the voltage adjustment circuit 1 is activated, the microcomputer of the switching control circuit 4 detects the output voltages V1 and V2 of the solar cell strings Str1 and Str2 by the two voltage sensors Vd1 and Vd2. (Step 1)

  Here, when the detected two output voltages V1 and V2 are equal, the microcomputer sets all the switching elements S1, S2, S3, and S4 to the open state (OFF) as shown in FIG. The process is executed again from step 1 (step 2).

  When both output voltages V1 and V2 remain equal, the currents output from the two solar cell strings Str1 and Str2 are supplied to the external load through the backflow prevention diodes D1 and D2, respectively. Therefore, the voltage adjustment circuit 1 does not substantially function.

  On the other hand, in step 1, when the output voltage V2 of the solar cell string Str2 is higher than the output voltage V1 of the solar cell string Str1, the microcomputer turns on the switching element S1 and turns off the switching element S2. At the same time, the switching element S3 and the switching element 4 are complementarily synchronized, and the ON / OFF switching operation is periodically repeated a predetermined number of times programmed in advance in the microcomputer. Step 3 is re-executed (Step 3)

  Here, as shown in FIG. 4, when the switching element S1 is ON and the switching element S2 is OFF, when the switching element S3 is OFF and the switching element S4 is ON, the positive electrode of the solar cell string Str1 Current flows from the positive electrode side through the negative electrode side communication line 2, the switching element S1, the inductor L, and the switching element S4 to the negative electrode side connection line 3, and as the current flowing through the inductor L increases, the solar cell string Str1 side Is generated as magnetic energy in the inductor L.

  At this time, the output current of the solar cell string Str2 is supplied to the external load through the backflow prevention diode D2. On the other hand, since the output voltage V1 of the solar cell string Str1 is lower than the output voltage V2 of the solar cell string Str2, no current flows from the solar cell string Str1 to the external load through the backflow prevention diode D1.

  Next, as shown in FIG. 5, when the switching element S3 is turned on and the switching element S4 is turned off while the switching element S1 is kept on and the switching element S2 is kept off, the switching element S4 is accumulated in the inductor L. At the other end J2 of the positive electrode side connection line 2, the voltage generated by the inductor L is added to the output voltage V1 of the solar cell string Str1, and the output current of the solar cell string Str1 Combined with the output current of the string Str2, it is supplied to the external load through the backflow prevention diode D2.

  While the switching element S3 and the switching element S4 are performing the complementary and periodic ON / OFF switching operation between FIG. 4 and FIG. 5 described above, the current generated by the switching operation is changed. The fluctuation is smoothed by two capacitors C1 and C2 operating as a smoothing circuit and supplied to an external load.

  In step 3 described above, the complementary switching operation between the switching element 3 and the switching element 4 is performed at a switching timing so that there is a short-circuit prevention period a in which both are simultaneously turned off, as shown in FIG. By controlling, the certainty of the switching operation can be increased.

  FIG. 7 is a diagram showing the current-voltage characteristics of the system improved by the operation of the voltage regulator circuit 1 as described above. The output voltage V1 of the solar cell string Str1 is boosted by the inductor L, so that As shown in FIG. 13, the step in the current-voltage characteristic is smoothed, and at the maximum output operating point P, it is possible to make maximum use of the power generated by both the solar cell strings Str1, Str2.

  On the other hand, when the output voltage V1 of the solar cell string Str1 is higher than the output voltage V2 of the solar cell string Str2 in step 1 described above, the microcomputer switches the switching element S3 shown in FIG. After switching element S4 to OFF and switching element S1 and switching element S2 in a complementary manner, the ON / OFF switching operation is periodically repeated a predetermined number of times as in the third step described above. Then, the above-described step 1 is executed again (step 4).

  As a result, even when the output voltage of the solar cell string Str2 is lowered, the current-voltage characteristics with respect to the external load are improved as in the case shown in FIG. 7, and the electric power generated by both the solar cell strings Str1 and Str2 is improved. Can be used to the maximum.

  In step 4, as in step 3 described above, the switching timing is controlled so that there is a short-circuit prevention period that is simultaneously OFF in the complementary switching operations between switching element 1 and switching element 2. Thus, the certainty of the switching operation can be improved.

  Moreover, in the thing of embodiment mentioned above, the capacitor | condenser C1 used as a 1st smoothing circuit and the capacitor | condenser C2 used as a 2nd smoothing circuit are respectively the positive electrode side connection line 2, the negative electrode side connection line 3, and The capacitor C1 is connected across the line connecting the positive electrode side of one solar cell string Str1 and the backflow prevention diode D1 and the negative electrode side connection line 3. Similarly, the capacitor C <b> 2 may be connected across the line connecting the positive electrode side of the other solar cell string Str <b> 2 and the backflow prevention diode D <b> 2 and the negative electrode side connection line 3.

  Note that the first and second smoothing circuits are not limited to the capacitors C1 and C2. For example, as shown in FIG. 8, the coil La incorporated in the positive electrode side connection line 2 and the coil La It may be constituted by a low-pass filter LPF1 (LPF2) composed of two capacitors Ca and Cb connected across the positive-side connecting line 2 and the negative-side connecting line 3 on both sides. By configuring the second smoothing circuit, it is possible to more effectively suppress fluctuations in current and noise generation associated with the switching operations of the switching elements S1, S2, S3, and S4.

  Next, FIG. 9 is a diagram schematically showing another embodiment of the voltage adjustment circuit for the solar cell string according to the present invention, which shows three solar cell strings Str1 with respect to an external load. , Str2, and Str3 are systems connected in parallel via backflow prevention diodes D1, D2, and D3, respectively, and the voltage adjusting circuit 1 described above is interposed between these solar cell strings Str1, Str2, and Str3, respectively. Is incorporated.

  In the present embodiment, when the output voltage varies between the solar cell strings Str1 to Str3, the voltage adjustment circuit 1 independently outputs the output voltage between adjacent solar cell strings. Although the variation is adjusted, finally, the output voltages of all the solar cell strings are averaged, and the power generated by all the solar cell strings Str1 to Str3 can be used to the maximum extent.

  The voltage adjustment circuit of the solar cell string of the present invention is limited to the installation location of the solar cell array such as a roof, and when the number of solar cell modules constituting each solar cell string cannot be equal, For example, when the temperature of some of the solar cell modules rises and the output voltage decreases, it can be widely used as an effective means for improving the power generation efficiency of the solar power generation system.

DESCRIPTION OF SYMBOLS 1 Voltage adjustment circuit 2 Positive side connection line 3 Negative side connection line 4 Switching control circuit
C1 capacitor (first smoothing circuit)
C2 capacitor (second smoothing circuit)
D1, D2, D3 Backflow prevention diode J1 One end of positive side connection line J2 The other end of positive side connection line L Inductor M Solar cell module Str1, Str2, Str3 Solar cell string
S1 switching element (first switching element)
S2 switching element (second switching element)
S3 switching element (third switching element)
S4 switching element (fourth switching element)
V1, V2 output voltage Vd1 voltage sensor (first voltage sensor)
Vd2 voltage sensor (second voltage sensor)

Claims (3)

Each of these is a voltage adjustment circuit incorporated between a plurality of solar cell strings connected in parallel with an external load via a backflow prevention diode,
One end connected to the line connecting the positive electrode side of one adjacent solar cell string and the backflow prevention diode, and the other end connected to the line connecting the positive electrode side of the other solar cell string and the backflow prevention diode Tracks,
An inductor incorporated in the positive connection line;
A first switching element incorporated between one end of the positive-side connection line and the inductor;
A second switching element incorporated between the first switching element and the inductor of the positive electrode side connecting line, and between the negative electrode side connecting line connected to the negative electrode side of each solar cell string;
A third switching element incorporated between the other end of the positive-side connection line and the inductor;
A fourth switching element incorporated between the third switching element and the inductor of the positive side connection line and between the negative side connection line;
A line connecting between the positive electrode side of the one solar cell string and the backflow prevention diode or between one end of the positive electrode side connection line and the first switching element, and at least connected between the negative electrode side connection line A first smoothing circuit having one capacitor;
A line connecting between the positive electrode side of the other solar cell string and the backflow prevention diode or the other end of the positive electrode side connection line, the third switching element, and a connection between the negative electrode side connection line. A second smoothing circuit having at least one capacitor;
A first voltage sensor for detecting an output voltage V1 of the one solar cell string;
A second voltage sensor for detecting an output voltage V2 of the other solar cell string;
A switching control circuit for individually controlling the ON / OFF of each switching element;
The switching control circuit includes a first step of comparing and determining the level of the voltage V1 and the voltage V2 detected by the first voltage sensor and the second voltage sensor, respectively.
A second step of turning off all the switching elements and re-executing from the first step when it is determined that the voltage V1 = the voltage V2 in the first step;
When it is determined in the first step that the voltage V1 <the voltage V2, the first switching element is turned on, the second switching element is turned off, and the third switching element and the fourth switching element are complemented. And a third step of re-execution from the first step after repeating the ON / OFF switching operation periodically a predetermined number of times,
When it is determined in the first step that the voltage V1> the voltage V2, the third switching element is turned on, the fourth switching element is turned off, and the first switching element and the second switching element are complemented. Adjusting the voltage of the solar cell string, wherein a fourth step re-executed from the first step is performed after the ON / OFF switching operation is periodically repeated for the predetermined number of times. circuit.   In the third step, the switching control circuit has a short-circuit prevention period in which the third switching element and the fourth switching element are simultaneously turned off in each cycle, and in the fourth step, the first switching element 2. The switching timing of each switching element is controlled so that there is a short-circuit prevention period in which the switching element and the second switching element are simultaneously turned off in each cycle. Voltage adjustment circuit for solar cell string.   The voltage adjustment circuit for a solar cell string according to claim 1 or 2, wherein each of the first smoothing circuit and the second smoothing circuit is configured by a low-pass filter.

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