JP2004364493A - Electric power conversion apparatus and control method therefor, as well as solar power generation arrangement - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress, when stopping an inverter in an output destination of a DC/DC converter, abnormal surge in the voltage generated in the output of the converter, in a solar power generation system. <P>SOLUTION: The DC/DC converter stepping up output voltage of a solar battery 1 is constituted by a detector 31 for detecting the input current from the solar battery 1, a drive circuit 46 for supplying drive pulses to switching elements 33 and 34 for switching the output power of the solar battery 1, a transformer 15 for stepping up the power made switching, a rectifier circuit 36 rectifying the power stepped up, as well as, a duty selector circuit 146 decreasing an on-duty of the drive pulse when the input current is small. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a power conversion device, a control method thereof, and a solar power generation device.

  In recent years, problems such as global warming due to emission of carbon dioxide and the like due to the use of fossil fuels, accidents at nuclear power plants and radioactive contamination by radioactive waste have become serious, and interest in the global environment and energy has been increasing. Under such circumstances, photovoltaic power generation using sunlight as an inexhaustible and clean energy source has been put to practical use all over the world.

  As a form of photovoltaic power generation using a solar cell, there are various forms corresponding to an output scale from several W to several thousand kW. As a typical system using a solar cell, DC power generated by the solar cell is converted (orthogonal-converted) into AC power by an inverter or the like, and a load on a customer or a commercial power system (hereinafter, referred to as a “system”). ).

  FIG. 1 is a schematic view of a photovoltaic power generation system, and FIG. 2 is a block diagram showing connection of the photovoltaic power generation system.

  The solar cell module 75 shown in FIGS. 1 and 2 is obtained by connecting a plurality of solar cells having an output voltage of several V in series, and outputs a voltage of about 20 V. Further, a plurality of solar cell modules 75 are connected in series to form a solar cell string 76 that can obtain an output voltage of about 200 V, and an inverter that connects one or more solar cell strings 76 to the system 72 if necessary. By connecting to 71, a grid-connected solar power generation system is built.

  By the way, the photovoltaic cell constituting the photovoltaic power generation system has a characteristic that the voltage changes depending on the load due to its characteristics. The open-circuit voltage under no load is always higher than the optimum operating point voltage at which the output of the solar cell is maximized. In order to use the photovoltaic cell efficiently as a power generation device, the solar cell may be operated at the optimal operating point (that is, an optimal load may be applied), but when the load becomes light (that is, the load resistance increases). As a result, the voltage of the solar cell increases. Therefore, when the inverter 71 is stopped, the voltage of the photovoltaic cells rises to the open-circuit voltage.Therefore, the power conversion device such as the inverter 71 connected to the photovoltaic string 76 in which the photovoltaic cells are connected in series causes the voltage to rise. Must be endurable.

  As a countermeasure against an increase in output voltage such as the open-circuit voltage of a solar cell, as disclosed in JP-A-2000-228529, a method is known in which a voltage suppressor is attached to a solar cell or a module to suppress a voltage increase. . 3 and 4 are diagrams showing a voltage rise suppression method disclosed in Japanese Patent Application Laid-Open No. 2000-228529. A bypass diode 78 and a voltage suppression element 79 are connected in parallel to the solar cell 77, and an increase in the output voltage of the solar cell 7 is suppressed by a forward drop voltage or a Zener voltage of the voltage suppression element 79.

  For such a photovoltaic power generation system, the present inventors have proposed a solar power generation system using a high step-up open-loop DC / DC converter (hereinafter simply referred to as “DC / DC converter”) as an effective form of the photovoltaic power generation system. A photovoltaic power generation system was studied. This solar power generation system does not have a configuration in which a large number of solar cells are serialized and connected to an inverter 71, such as a solar cell module 75 or a solar cell string 76, but serializes a single cell or several cells, An open-loop DC / DC converter that does not perform negative feedback control such as constant voltage control or maximum operating point tracking is arranged in the vicinity, and a solar cell module with a DC / DC converter is configured. Supply. The advantages of such a solar power generation system include the following.

  (1) A general solar cell module has a considerably lower output voltage than the system voltage. Therefore, when a system interconnection type photovoltaic power generation system is formed using a general solar cell module, a required voltage can only be obtained by connecting a large number of solar cell modules in series. As a result, a small-scale solar power generation system cannot be constructed. However, according to the solar cell module with a DC / DC converter, a solar cell module having a high output voltage can be configured, so that a small-capacity solar power generation system can be easily configured.

  (2) When a general solar cell module is connected in series to increase the voltage, the solar cell element in the solar cell module necessarily has a high voltage applied portion. Under such circumstances, the cost of the covering material (insulating material) of a general solar cell module increases in order to cope with a breakage of the covering. However, a solar cell module with a DC / DC converter can make the coating (insulation) of the solar cell element thinner, and as a result, a significant cost reduction can be achieved.

  (3) When a single cell is used, a serialization step is not required, and the active area is wide because there is no need to keep a distance between cells in series.

  In the process of studying a photovoltaic power generation system using a photovoltaic module with a DC / DC converter, when the inverter at the output destination of the DC / DC converter was stopped, the inventors added the aforementioned output to the DC / DC converter. Abnormal voltage rise that cannot be explained by the voltage rise of the photovoltaic cells at light load alone has occurred, and a problem has arisen that the inverter must be designed to have a higher input voltage than necessary.

  This phenomenon will be described in more detail. For example, it is assumed that the open voltage of the solar cell is Voc, the optimum operating point voltage is Vpm, and the boost ratio of the open-loop DC / DC converter operating at full duty is N. In this case, the output voltage of the DC / DC converter should be approximately Vpm × N [V] while the inverter is operating, and theoretically Voc × N [V] when the inverter stops. However, actually, when the output voltage when the inverter is stopped is measured, the output voltage is much higher than Voc × N [V]. The inventors have named this phenomenon "abnormal voltage rise".

JP 2000-228529 A

  The present invention solves the above-mentioned problems individually or collectively, and aims at suppressing an abnormal voltage rise.

  The present invention has the following configuration as one means for achieving the above object.

  A power converter for boosting an output voltage of a solar cell according to the present invention includes a detector that detects an input value from the solar cell, a driving circuit that supplies a driving pulse to a switching circuit that switches output power of the solar cell, And a control circuit for controlling the on-duty of the drive pulse in accordance with the detection value of the detector.

  Further, a control method of the present invention is the control method of the power conversion device, wherein the on-duty of the drive pulse is controlled according to a detection value of the detector.

  Further, a solar power generation device according to the present invention includes a solar cell, the above-described power conversion device, and an inverter that converts DC power output from the power conversion device into AC power.

  According to the present invention, an abnormal voltage rise can be suppressed. Therefore, there is no need to design the inverter to which the DC / DC converter is connected to an input voltage that is higher than necessary, which contributes to a reduction in the total cost of the photovoltaic power generation device (system) including the inverter.

  Hereinafter, a photovoltaic power generation system (apparatus) according to an embodiment of the present invention will be described in detail with reference to the drawings.

  The inventors have developed a “high step-up open-loop DC / DC converter” as one effective form of a photovoltaic power generation system. As a result, when the inverter connected as a load of the DC / DC converter does not input a sufficient current or stops, an abnormal voltage rise occurs, and the inverter is required to have a higher withstand voltage than necessary. Encountered. Hereinafter, the system using the DC / DC converter, the configuration and operation of the DC / DC converter, and the abnormal voltage rise and its countermeasures will be described step by step.

[Configuration of solar power generation system]
FIG. 5 is a block diagram showing a configuration example of a photovoltaic power generation system using a DC / DC converter.

  The power generated by the solar cell 1 is boosted by the DC / DC converter 2, converted into AC power by the inverter 3, and connected to the grid 4. In the system shown in FIG. 5, a plurality of sets of a solar cell 1 and a DC / DC converter 2 are connected in parallel and connected to an inverter 3 to constitute a system interconnection system in which power flows backward from the inverter 3 to a system 4. I do.

● Solar cell The solar cell 1 is installed outdoors such as a roof where sunlight is not blocked, and is configured so that the output voltage is low by limiting the number of series connections. As the solar cell 1, an amorphous silicon-based, polycrystalline silicon-based, or crystalline silicon-based solar cell can be used. In the examples, a stacked solar cell in which three layers of amorphous silicon were stacked was used for the solar cell 1. The characteristics are shown below. FIG. 6 is a diagram showing output characteristics of the solar cell 1.
Rated solar radiation 1kW / m ² Ambient temperature 25 ℃
Short circuit current Isc = 12.45A
Open circuit voltage Voc = 1.4V
Maximum operating point current Ipm = 10.0A
Maximum operating point voltage Vpm = 1.0V
Maximum output power Pmax = 10W

● DC / DC converter An open-loop converter 2 is provided near the solar cell 1 for boosting the output of the solar cell 1 having a low voltage and a large current and converting the output to a high voltage and a small current. This is for reducing the power transmission loss in the power transmission path up to the inverter 3 placed indoors, for example. It should be noted that the solar cell 1 and the DC / DC converter 2 may be configured as an integrated (laminated) module. By doing so, the wiring between the solar cell 1 and the DC / DC converter 2 can be shortened, and loss due to wiring resistance can be effectively reduced.

  The gate drive circuit 46 of the DC / DC converter 2 supplies two gate drive signals having opposite phases to the gates of the switching elements 33 and 34 constituting the push-pull switching circuit. Although the duty is fixed because of the gate drive signal (rectangular wave), 50% which is the maximum duty of the push-pull switching circuit is preferable because the conversion efficiency is high. The reason why the duty of the gate drive signal is fixed is to reduce the cost and improve the reliability by simplifying the switching control circuit by eliminating the feedback circuit. The switching frequency is set between 20 kHz and several hundred kHz due to a trade-off between power loss due to switching and miniaturization of the transformer. In this embodiment, in order to obtain the highest conversion efficiency, the switching frequency is set as low as possible, and the switching frequency is set to 20 kHz.

  The switching elements 33 and 34 are preferably MOSFETs having low on-resistance because the output voltage of the solar cell 1 is low. At present, MOSFETs, which are unipolar elements, are particularly excellent in terms of on-resistance.

  As the input capacitor 32, it is preferable to use a capacitor having a small equivalent series resistance (ESR) and an excellent high-frequency characteristic so that the power supply source of the DC / DC converter 2 can be regarded as a voltage source. ), Low ESR capacitors such as multilayer ceramic capacitors and tantalum electrolytic capacitors can be used. With the input capacitor 32, the power supply source of the DC / DC converter 2 can be regarded as a voltage source, and the DC / DC converter 2 becomes what is called a voltage type converter.

  The transformation ratio (turn ratio) of the transformer 15 is such that when the operating voltage of the solar cell 1 is minimum, a DC voltage (for example, 150 V or more) necessary for the inverter 3 to output an AC voltage of 100 V is supplied to the inverter 3. Set to. Note that the output voltage of the DC / DC converter 2 is a value obtained by multiplying the input voltage by the transformation ratio of the transformer 15 because the switching circuit is always operated at the maximum duty.

The optimal operating point of a solar cell has the characteristic that the voltage becomes low at high temperature and low solar radiation. In the embodiment, the minimum operation voltage of the solar cell is set to 0.75 V, and at that time, the winding ratio that allows the output voltage to be raised to 150 V is set in the transformer 15.
Turn ratio 0.75: 150
Primary winding 2 turns x 2
Secondary winding 400 turns

  A rectifier circuit 36 for rectifying the high-frequency AC output of the transformer 15 and a smoothing circuit 37 for smoothing are provided at the subsequent stage of the transformer 15. Note that if the capacitance of the input capacitor 51 of the inverter 3 that is the output destination of the DC / DC converter 2 is sufficiently large, the capacitor of the smoothing circuit 37 may not be provided.

  DC power is input from the solar cell 1 to the DC / DC converter 2. The input DC power is boosted by a booster circuit including the gate drive circuit 46, the switching elements 33 and 34, and the transformer 15. The high-frequency AC power output from the secondary winding of the transformer 15 is rectified by the diode bridge 36, smoothed by the smoothing circuit 37, and supplied to the inverter 3.

● Inverter The control circuit 53 of the inverter 3 monitors the voltage and current input from the DC / DC converter 2 by the input voltage detector 54 and the input current detector 55, and performs PWM control of the switching element of the inverter bridge 52. . By this control, the output voltage and output current of DC / DC converter 2 are controlled, and as a result, the operating point voltage and current of solar cell 1 can be controlled. In other words, the control circuit 53 performs the maximum power tracking control (MPPT control) of the DC / DC converter 2 and the solar cell 1 together, so that the power generated by the solar cell 1 is effectively used.

  The inverter 3 of the present embodiment is operated in an input voltage range of 150 V to 250 V by MPPT control. This input voltage range is determined by the change in the maximum power point due to the solar radiation and temperature of the solar cell 1, the transformation ratio of the DC / DC converter 2, and the switching duty.

  The DC power input to the inverter 3 is converted into AC power by the filter 58 including the inverter bridge 52 and the interconnection reactor, and is supplied to the grid 4. As a method of monitoring the input voltage and the input current of the inverter 3 and performing PWM control of the inverter bridge 52, a number of publicly-known and publicly-known methods are known, and therefore description thereof is omitted.

  By such PWM control of the inverter 3, while extracting maximum power from the solar cell 1 and the DC / DC converter 2 (MPPT control), AC power having the same current phase as that of the system 4 and having a power factor of 1 is output.

[Operation of DC / DC converter]
In the above configuration, a case where a drive signal with a duty of 50% (or slightly less than 50%, for example, 49% to less than 50%) is always supplied to switching elements 33 and 34 of DC / DC converter 2 will be described. The reason for setting the duty to 50% is to maximize the conversion efficiency of the DC / DC converter 2. In other words, if the duty is reduced to transmit the same power, the current value at the time of on increases in inverse proportion to the duty, and the conduction loss represented by I ² R × duty due to the on-resistance of the printed circuit board and FET increases. . Therefore, the duty is fixed to 50%, the current value at the time of ON is suppressed, and the conduction loss is suppressed. However, if the duty is always fixed to 50%, the above-described abnormal voltage rise occurs.

For example, when the inverter 3 which is the output destination of the DC / DC converter 2 is stopped under the condition that the solar cell 1 is irradiated with the rated solar radiation of 1 kW / m ² , the DC / DC converter 2 and the solar cell 1 are almost unloaded ( The output current becomes zero), and the solar cell 1 operates at the point A in FIG. In other words, the DC / DC converter 2 inputs the voltage at the point A (about 1.4 V) which is 1.4 times higher than the rated time.

Here, since the switching circuit of the DC / DC converter 2 is driven with a fixed duty, the ratio between the input voltage and the output voltage becomes almost constant during normal load operation. Therefore, the output voltage of the DC / DC converter 2 is represented by the following equation, and the operating point voltage of the solar cell 1 and the output voltage of the DC / DC converter 2 fluctuate in proportion.
Vd = Tr x Vop
Where Vd is the output voltage of DC / DC converter 2.
Vop is the operating voltage of solar cell 1
Tr is the boost ratio (turn ratio)

However, when there is no load or when the load current is small, an abnormal voltage rise occurs. As a result, the IV curve characteristics (at the rated solar radiation of 1 kW / m ² ) of the DC / DC converter 2 that is always operated with the duty fixed at 50% are as shown in FIG. Nevertheless, the no-load voltage is 450V (point B).

FIG. 8 shows this relationship in terms of the relationship with the output power of the DC / DC converter 2. The graph shown in FIG. 8, in the configuration of FIG. 3, while irradiating the rated solar radiation 1 kW / m ² to the solar cell 1, by changing the output power of the DC / DC converter 2 by changing the current inverter 3 consumes , At which time the output voltage of the DC / DC converter 2 was measured. The output voltage at rated power output (input power 10 W) is 200 V, but when the output current is zero, the output voltage is as high as 450 V. For this reason, for example, 450 V is applied to the input terminal of the inverter 3 when, for example, the inverter 3 that detects the power failure of the system 4 stops while the solar cell 1 is irradiated with 1 kW / m ² .

  FIG. 9 is a diagram for explaining the operation of the DC / DC converter 2, showing the operation of the DC / DC converter 2 when the inverter 3 stops at the timing t1 and resumes operation at the timing t2.

  When the inverter 3 stops and the output power of the DC / DC converter 2 becomes zero, the input voltage of the DC / DC converter 2 becomes about 450 V as shown in FIG. Thereafter, the operation of the inverter 3 is restarted, and the abnormal voltage rise is continued until the timing t2 at which the input current of the DC / DC converter 2 starts to rise.

  While studying measures for abnormal voltage rise, the inventor found that abnormal voltage rise occurred when the input / output current of the DC / DC converter 2 was small, such as when the inverter stopped or did not emit low solar radiation, which did not contribute to the integrated power generation of the system. It was found that in such a state, the abnormal voltage rise was effectively suppressed by slightly reducing the duty of the pulse output from the gate drive circuit 46 from 50%.

  FIG. 10 is a diagram showing measurement results showing that reducing the duty is effective in suppressing abnormal voltage rise.The duty of the drive pulse supplied to the switching elements 33 and 34 was reduced from 50% to 48% and 46%. 3 shows the relationship between the output power and the output voltage of the DC / DC converter 2 at the time.

  As shown in FIG. 10, it can be seen that the output voltage at the time of light load where the output power is 1 W or less and at the time of no load where the output power is zero is greatly suppressed as compared with the case of the 50% duty. From this, when the output of the DC / DC converter 2 becomes low, for example, when the input current of the DC / DC converter 2 is equal to or less than a predetermined value, the duty of the gate drive pulse is slightly reduced from the 50% duty, and the input current is reduced to a predetermined value. It has been found that it is preferable to set the gate drive pulse to 50% duty when the value exceeds the value.

  FIG. 11 is a diagram showing the relationship between the output power of the DC / DC converter 2 and the conversion efficiency when the duty of the drive pulse supplied to the switching elements 33 and 34 is reduced from 50% to 48% and 46%. Specifically, under the condition that the inverter 3 always performs the MPPT operation, the conversion efficiency characteristic was measured by changing the amount of solar radiation applied to the solar cell 1. According to this, when the output power is small, there is almost no difference in the conversion efficiency due to the duty. Of course, the difference between the conversion efficiencies of the DC / DC converter 2 during low solar radiation has a very small effect on the integrated conversion efficiency throughout the day.

[Abnormal voltage rise suppression means]
A DC / DC converter 2 provided with means for suppressing the abnormal voltage rise by reducing the duty of the drive pulse will be described. FIG. 12 is a diagram showing a configuration example of the DC / DC converter 2 having a means for suppressing an abnormal voltage rise.

● Current detector
A current detector 31 is provided between the input terminal of the DC / DC converter 2 and the smoothing capacitor 32. When the inverter 3, which is the load of the DC / DC converter 2, stops, only a current (excitation current) corresponding to the excitation of the core of the transformer 15 flows through the current detector 31. The current detector 31 measures the current value without contact with the current path by inserting a shunt resistor into the current path and measuring the voltage between both ends, or by measuring the strength of the magnetic field generated by the current. It can be configured in various ways. The current detector 31 converts the detected current value into a voltage value and outputs it to the comparator 101.

Comparator The comparator 101 compares the detection value (output voltage value) of the current detector 31 with a predetermined value (threshold) for switching the duty. Is output. The reference voltage source 102 supplies the comparator 101 with a voltage (threshold voltage) obtained by converting a current value (reference current value) at which the duty is to be switched (voltage).

  In the present embodiment, 1 A is selected as a reference current value for switching the duty which is equal to or more than the above-described excitation current and has a value that hardly affects the conversion efficiency of the DC / DC converter 2. Therefore, when the detection value of the current detector 31 becomes 1 A or more, the comparator 101 sets the switching signal to the H level.

FIG. 13 is a diagram showing a configuration example of a duty switching circuit 146 for one gate drive signal. Inverters 143 and 145, capacitors and resistors constituting a delay circuit 142, a diode 144, and a delay circuit 142 are shown. It is configured by a bypass switch 141 for bypassing, receives a pulse of 50% duty from the gate drive circuit 46, and supplies a gate drive signal to the switching element 33 or.

  The bypass switch 141 is in the open state when the switching signal is at the L level, and the inter-pulse dead time (the time from the fall of the gate pulse G1 to the rise of G2, or G1 from the fall of the gate pulse G2, (Time until the rise of the signal). Therefore, by adjusting the values of the capacitor and the resistor constituting the delay circuit 142, the duty switching circuit 146 outputs, for example, a drive pulse having a 46% duty. When the switching signal is at the H level, the bypass switch 141 is closed, the delay circuit 142 is bypassed, and the duty switching circuit 146 outputs a 50% duty driving pulse.

  In addition to the circuit for generating the dead time, there are many publicly known methods other than those shown in FIG. 13, and any of them may be used.

[Operation of DC / DC Converter] FIGS. 14 and 15 are diagrams for explaining the operation of the DC / DC converter 2 having a means for suppressing an abnormal voltage rise, when the inverter 3 stops at the timing t1 and resumes operation at the timing t2. The operation of the DC / DC converter 2 will be described.

  At the time of the MPPT operation of the inverter 3 before the timing t1, the DC / DC converter 2 operates at the MPPT operation voltage. In this embodiment, the output voltage of the DC / DC converter 2 is 200V. When the inverter 3 stops at the timing t1, the output current of the DC / DC converter 2 becomes 0 A, and the input current becomes a minute current of about the exciting current of the transformer 15. At this time, the detection value of the current detector 31 falls below the threshold value 1A, and the output (switching signal) of the comparator 101 transitions from the H level to the L level.

  When the L-level switching signal is input, the duty switching circuit 146 outputs, for example, a drive pulse having a duty of 46%. Therefore, as shown in FIG. 15, the output voltage of the DC / DC converter 2 is suppressed, and becomes about 270 V even when there is no load.

  Next, when the operation of the inverter 3 is restarted at the timing t2, the input current of the DC / DC converter 2 gradually increases and exceeds the threshold value of 1 A at the timing t3. At this time, the output (switching signal) of the comparator 101 changes from the L level to the H level, and the duty switching circuit 146 outputs a driving pulse with a duty of 50%. That is, when the inverter 3 is operating (during power generation by the photovoltaic power generation system), the DC / DC converter 2 enters a full-duty operating state with high conversion efficiency.

  Immediately after the duty becomes 50%, the output voltage slightly increases as shown in FIGS. 14 and 15, but this voltage increase is within a range in which there is no problem. Of course, the output voltage of the DC / DC converter 2 does not rise sharply due to the influence of the smoothing capacitor and the like. Thereafter, the inverter 3 performs the MPPT operation, and the DC / DC converter 2 continues the high-efficiency operation at the MPPT operating voltage and 50% duty.

  Thus, when the inverter 3 returns to the normal operation, the DC / DC converter 2 also automatically shifts to the normal operation. Further, while the operation of the inverter 3 is stopped due to a power failure or the like, the DC / DC converter 2 maintains an output voltage of about 270 V (in the case of a 46% duty). As a result, the start-up preparation of the inverter 3 is ready at any time, and when the power failure is resolved, the inverter 3 can promptly restart the system interconnection operation.

  In this embodiment, since the input power supply is the solar cell 1, the input current increases as the solar radiation increases, and the duty switches from, for example, 46% to 50% under a certain solar radiation condition. At this time, the apparent load seen from the solar cell 1 also increases, and the input current of the DC / DC converter 2 further increases (the input voltage decreases). Therefore, chattering is less likely to occur when the duty is switched, and there is no need to provide hysteresis in the reference value (threshold). However, it is naturally possible to provide a hysteresis such as, for example, a duty of 50% above 1A and a duty of 46% below 0.8A.

  In the above description, an example in which the duty is switched based on the input current of the DC / DC converter 2 has been described. However, this configuration is simpler than providing a detection circuit and a control circuit on the secondary side of the DC / DC converter 2. Because it becomes.

  Of course, a configuration may be used in which a switching signal or the like is transmitted from the secondary side of the DC / DC converter 2 to the primary side using an insulating transformer or a photo plastic. Similarly, a simple configuration can be realized even when the primary side and the secondary side of the DC / DC converter 2 are not insulated and the duty is switched based on the output current of the DC / DC converter 2. In these cases, the duty may be increased so that the duty increases when the output current increases, and the duty decreases when the output current decreases.

  In the above description, an example in which two duties are switched has been described, but the number of duties may be any number. Increasing the number of duties increases the efficiency of the DC / DC converter or makes the voltage suppression more remarkable, though the configuration becomes complicated. That is, although the cost of the DC / DC converter 2 increases, more detailed control can be performed in consideration of the balance between the loss during power generation and the suppression of the abnormal voltage rise.

  Hereinafter, a solar power generation system according to a second embodiment of the present invention will be described. In the second embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof will be omitted.

  In the first embodiment, the example in which the input current of the DC / DC converter 2 is detected to switch the duty is described.In the second embodiment, the input power of the DC / DC converter 2 is detected, and the detected value is An example will be described in which the duty is increased when the value exceeds a predetermined value, and the duty is decreased when the value falls below the predetermined value.

  FIG. 16 is a diagram illustrating a configuration example of the DC / DC converter 2 according to the second embodiment.In comparison with the DC / DC converter 2 according to the first embodiment illustrated in FIG. 12, the current detector 31 is replaced with a power detector 41. A comparator 103 and a reference voltage source 104 are added.

  In the second embodiment, when the input power exceeds the reference value, the duty increases, and when the input power falls below the reference value, the duty decreases. However, unlike the case of detecting the input current, the input power hardly changes before and after the duty switching. That is, there is a concern that chattering will occur. Therefore, in the present embodiment, two reference voltage sources 102 and 104 are provided as shown in FIG. 16 in order to provide hysteresis for duty switching. For example, when the detection value of the power detector 41 increases and exceeds 1 W, the duty The duty is reduced when it decreases below 0.8W.

  With such a configuration, it operates in the same manner as the DC / DC converter 2 of the first embodiment, and can prevent an abnormal voltage rise when the inverter 3 stops or the like.

  Hereinafter, a solar power generation system according to a third embodiment of the present invention will be described. In the third embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and the detailed description thereof will be omitted.

  In the third embodiment, the input current of the DC / DC converter 2 is detected simply and at low cost. Specifically, as shown in FIG. 17, a voltage drop in wiring 30 from solar cell 1 to DC / DC converter 2 is detected, thereby detecting an input current of DC / DC converter 2. In general, the voltage drop in the wiring 30 is small (for example, 10 mV at 10 A) and needs to be amplified by the amplifier 103 in order to ensure comparison with a threshold. Further, it is preferable to provide a noise filter, an integration circuit, and the like in the amplifier 103 to prevent erroneous detection due to noise.

  As another method for detecting the input current of the DC / DC converter 2, there is a method using a voltage drop due to the on-resistance of the switching elements 33 and 34. In other words, the input current is detected by measuring the drain-source voltage when the switching element is turned on.

  Both methods are greatly affected by the temperature dependence of the resistivity of copper and the on-resistance of the FET, and the accuracy of detecting the current value is low. However, even with such a level of detection accuracy, it can be used sufficiently for the current detection of this embodiment. For example, in consideration of the influence of temperature, a threshold (current value or power value) for increasing the duty may be set higher as shown in an example in FIG. 18, and the abnormal voltage rise is suppressed by a simple current detection method. can do.

  According to each of the embodiments described above, it is possible to suppress the abnormal voltage rise at the time of light load and no load while maintaining the high conversion efficiency of the DC / DC converter.

  Also, by suppressing the abnormal voltage rise of the open-loop converter that inputs the power generated by the solar cell, the withstand voltage (allowable input voltage) of the inverter, which is the output destination of the converter, can be reduced, and the sunlight including the inverter can be reduced. Contribute to reducing the total cost of the power generation system.

  In addition, since a circuit that suppresses abnormal voltage rise can be configured only on the primary side of the DC / DC converter transformer, (i) the secondary side circuit is unnecessary and the circuit configuration is simplified, and (ii) the secondary side Therefore, there is an advantage that a configuration (insulating parts or the like) for transmitting a signal from the first side to the primary side is unnecessary.

  In addition, the DC / DC converter maintains the output voltage even when the operation of the inverter is stopped due to a power failure, so that the inverter is ready to start. Driving can be started.

  In the above description, an example in which the duty is reduced from 50% (or slightly less than 50%) to 46% when the load is light or no load is described, but the duty is not limited to 46%. The point is that the output voltage of the DC / DC converter can be suppressed to the input voltage allowed by the inverter, and may be 48%, 47 or 45%, or a value between those.

  Hereinafter, a solar power generation system according to a fourth embodiment of the present invention will be described. In the fourth embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and the detailed description thereof will be omitted. In the fourth embodiment, it is described that the power conversion efficiency of the DC / DC converter of the embodiment is further improved by the characteristics of the transformer.

  In order to reduce the size and thickness of a DC / DC converter, it is necessary to make the core of the transformer thinner, which reduces the magnetic flux that causes the core to be saturated (hereinafter referred to as “saturated magnetic flux amount”). The only way to compensate for the decrease in the amount of saturation magnetic flux is to increase the number of turns or increase the switching frequency.However, if the switching frequency is increased, the switching loss increases, and the conversion efficiency of the power converter decreases. Need to be increased.

  Generally, when the number of turns is increased in the same core and window area (winding winding area of the core), the winding length becomes longer, and it is necessary to accommodate the winding in the window area. The conduction loss (copper loss) increases. On the other hand, when the number of turns is increased, the magnetic flux density of the core is reduced, and the core loss (fixed loss) is reduced. In other words, if the optimal number of turns is set with a compact and thin transformer, the number of turns will increase, and the conversion efficiency will decrease at high power when conduction loss is dominant, and at low power when fixed loss is dominant. The conversion efficiency is improved.

  It should be noted that the weather is not always clear, and in the case of photovoltaic power generators, in addition to high power (power generation in clear weather), low power (power generation in cloudy weather) may occur for a long time. Conceivable. That is, in a photovoltaic power generator, optimization only at the time of high power is insufficient, and a DC / DC converter with good overall power conversion efficiency for each input power value of high power and low power is required. It can be said. Embodiment 4 presents a DC / DC converter optimized from such a viewpoint.

[Configuration of solar power generation system]
In the fourth embodiment, the configurations of the solar cell, the inverter 3, and the like to be used are the same as those in the first embodiment. The difference is that the size of the DC / DC converter 2 is reduced in size and thickness compared to the first embodiment, and as a result, the number of turns of the transformer 15 is increased. The specifications of the transformer 15 of the fourth embodiment are shown below.
Primary winding: 3 turns x 2
Secondary winding: 600 turns

  The turns ratio is the same as in Example 1, and the number of turns of the primary and secondary windings is both increased by 1.5 times. FIG. 19 is a graph illustrating the relationship between the input current of the DC / DC converter 2 and the power conversion efficiency when the on-duty is used as a parameter in the fourth embodiment. In general, the input current is plotted on the horizontal axis because the output voltage of the solar cell is almost constant and the current value is almost proportional to the power value, except for dim conditions such as dawn. This is because the input current and the input power of converter 2 can be regarded as the same.

  As is clear from FIG. 19, the transformer 15 of the fourth embodiment has a smaller power conversion efficiency (46%) when the on-duty is large (50%) than when the on-duty is large (50%). Is good.

  In the DC / DC converter 2 having such characteristics, the on-duty is reduced when the output of the current detector 31 is equal to or lower than a certain level in order to suppress the abnormal voltage rise with the same configuration as that of the first embodiment shown in FIG. Assume that control is performed. For example, operate the DC / DC converter 2 with an on-duty of 50% for a detected value of input current exceeding 1A, and operate the DC / DC converter 2 with an on-duty of 46% for a detected value of input current of 1A or less. In this case, as shown in FIG. 19, the power conversion efficiency can be improved in a region where the input current is 1 A or less as compared with the case where the DC / DC converter 2 is operated with the on-duty fixed at 50%. That is, in the fourth embodiment, not only the abnormal voltage rise is suppressed but also a special effect of improving the power conversion efficiency of the solar power generation device can be obtained.

  Note that the transformer 15 of the fourth embodiment is not limited to the above, and the transformer and the transformer having the following characteristics can achieve the above-described effects. In other words, the effect on the power conversion efficiency of the DC / DC converter 2 is dominated by the copper loss of the transformer in the output power of the solar cell during sunny sunlight, and conversely, by the transformer in the output power of the solar cell during cloudy weather. Any transformer can be used as long as the iron loss becomes dominant. In addition, the output power at the time of the clear sun is 100 days (there is no cloud at all) during the daytime, and the output power at the time of the cloudy day is 0 (the cloud and clear sky area that can be made into the sky) Cloud is 100%).

  Further, in the fourth embodiment, in addition to the configuration for controlling the on-duty by detecting the input current, a configuration for detecting the input power and controlling the on-duty may be employed similarly to FIG. 16 of the second embodiment.

Overview of the photovoltaic system, Block diagram showing connection of the solar power generation system, Diagram showing a voltage rise suppression method, Diagram showing a voltage rise suppression method, Block diagram showing a configuration example of a photovoltaic power generation system using a DC / DC converter, Diagram showing output characteristics of the solar cell, A diagram showing IV curve characteristics of a DC / DC converter, A diagram showing the relationship between output power and output voltage of the DC / DC converter, Diagram for explaining the operation of the DC / DC converter, A diagram showing measurement results showing that reducing the duty is effective in suppressing abnormal voltage rise, A diagram showing the relationship between the output power and conversion efficiency of the DC / DC converter when the duty of the drive pulse supplied to the switching element is reduced from 50% to 48% and 46%. A diagram showing a configuration example of a DC / DC converter having an abnormal voltage rise suppression means, FIG. 3 is a diagram illustrating a configuration example of a duty switching circuit; FIG. 7 is a diagram illustrating the operation of a DC / DC converter having an abnormal voltage rise suppression unit. FIG. 7 is a diagram illustrating the operation of a DC / DC converter having an abnormal voltage rise suppression unit. A diagram showing a configuration example of a DC / DC converter of the second embodiment, A diagram showing a configuration example of a DC / DC converter of Embodiment 3, FIG. 7 is a diagram for explaining setting a higher threshold for duty increase. 13 is a graph showing the relationship between the input current of the DC / DC converter and the power conversion efficiency in Example 4.

Claims (8)

A power converter for boosting an output voltage of a solar cell,
A detector for detecting an input value from the solar cell,
A drive circuit that supplies a drive pulse to a switching circuit that switches output power of the solar cell,
A transformer that boosts the switched power,
A rectifier circuit for rectifying the boosted power;
A control circuit for controlling an on-duty of the drive pulse according to a detection value of the detector.   The power converter according to claim 1, wherein the control circuit reduces the on-duty when the detection value is smaller than a predetermined value.   2. The power converter according to claim 1, wherein the control circuit switches the on-duty to 50% or less than 50% based on a comparison result of the detection value and a predetermined value.   The power converter according to claim 1, wherein the control circuit reduces the on-duty so as to satisfy an input allowable voltage of a device to which power is supplied when the detection value is lower than a predetermined value. apparatus.   5. The power conversion device according to claim 1, wherein the detector detects the input value from a voltage drop between the solar cell and the power conversion device. A detector that detects an input value from the solar cell, a driving circuit that supplies a driving pulse to a switching circuit that switches output power of the solar cell, a transformer that boosts the switched power, and rectifies the boosted power. A control method of a power conversion device having a rectifier circuit and boosting an output voltage of the solar cell,
A control method, wherein an on-duty of the drive pulse is controlled according to a detection value of the detector. Solar cells,
The power converter according to any one of claims 1 to 5,
An inverter for converting DC power output from the power converter to AC power. Multiple solar cells,
Connected to each of the plurality of solar cells, a plurality of the power conversion device according to any one of claims 1 to 5,
An inverter for converting DC power output from the plurality of power converters into AC power.

Images