JP2016220497A - Power converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve power conversion efficiency of a power conditioner which performs step-up operation in two stages of a step-up chopper and a bridge converter.SOLUTION: A power conditioner 1 comprising a step-up circuit 4 which boosts a voltage in two stages by a step-up chopper 41 as a front stage of an input voltage input from a power generation part 2 through an input cable run 12 and a two-way full-bridge converter 42 as a rear stage, and a control part 10 performing step-up operation control over the step-up chopper 41 is enabled to use a low-breakdown-voltage, low-on-resistance element as a primary-side switching element of the step-up chopper 41 and two-way full-bridge converter 42 by setting a step-up ratio of the two-way full-bridge converter 42 larger than a maximum value of a step-up ratio of the step-up chopper 41 under the step-up operation control.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a power converter used in a power generation system such as a fuel cell power generation system.

  In various power generation systems connected to a commercial power system, a power conditioner is used to convert DC power generated by a power generation facility (power generation unit) such as a fuel cell into AC power linked to the system power. Yes. Generally, a power conditioner is configured to boost DC power output from a power generation facility by a booster circuit and then convert the generated power to AC power linked to system power by a DC / AC inverter.

  A booster chopper (non-isolated converter) is often used as a booster circuit for such a power conditioner, but various functions can be achieved by reducing loss during power conversion, reducing noise, and providing a bidirectional power conversion function. For example, a booster chopper and a bidirectional bridge converter (insulated converter) boost the voltage in two stages, which is disclosed in, for example, Patent Document 1 below.

  In the booster circuit described in Patent Document 1, the output voltage of the booster chopper (voltage input to the first converter circuit) is set higher than the output voltage of the bidirectional bridge converter (voltage output from the second converter circuit). By making the voltage higher, the voltage applied to the transformer of the bidirectional bridge converter is increased to relatively reduce the current flowing through the winding of the transformer, thereby reducing the loss caused by the transformer and the power converter. We are trying to improve the overall conversion efficiency.

JP 2014-183634 A

  By the way, for example, when the input voltage of the booster circuit is 100V, it is necessary to boost the output voltage of the bidirectional full bridge converter to 280V for grid connection. Therefore, it is necessary to boost the pressure up to 2.8 times. At this time, since the voltage applied to the switching element of the step-up chopper and the bidirectional full-bridge converter is 280 V or more, a high-breakdown-voltage element is required. The loss becomes very large, and the power MOS-FET having a high withstand voltage and low on-resistance has a problem that the cost is high.

  Therefore, the present invention makes it possible to use a switching element having a low withstand voltage and a low on-resistance by reducing the step-up ratio of the non-insulated converter, which is a switching power supply circuit, as much as possible, thereby reducing the loss due to the on-resistance. An object of the present invention is to provide a power converter capable of reducing and performing highly efficient boosting.

  In order to achieve the above object, the present invention takes the following technical means.

  That is, the present invention includes a booster circuit that boosts an input voltage input from a power generation unit via an input electric circuit in two stages by a non-insulated DC / DC converter that is a front stage and an isolated DC / DC converter that is a rear stage. A power converter including at least a control unit that performs boost operation control of the non-insulated DC / DC converter, wherein the boost ratio of the isolated DC / DC converter is determined by the boost operation control. The step-up ratio is set to be larger than the maximum value (claim 1).

  According to the power converter of the present invention, the step-up ratio of the isolated DC / DC converter is increased, and the voltage is boosted up to a predetermined voltage mainly by the isolated DC / DC converter. As a switching element that constitutes the primary side (non-insulated DC / DC converter side) of the isolated DC / DC converter, as well as adopting a low withstand voltage and low on-resistance product as the switching element constituting the DC / DC converter Can also adopt products with low withstand voltage and low on-resistance, which can greatly improve the efficiency of the booster circuit by reducing power loss when the boost ratio of the entire booster circuit is large, and reduce the cost of parts procurement be able to. Furthermore, according to the above configuration, since the step-up ratio of the isolated DC / DC converter is small, the control allowable range of the duty ratio of the switching element of the isolated DC / DC converter becomes relatively large, and various control methods such as input current control are possible. The duty ratio correction control for the purpose can be allowed.

  In the power converter of the present invention, the switch for opening and closing the input electric circuit, the input voltage smoothing capacitor provided between the switch and the booster circuit, and the generated power boosted by the booster circuit A bridge inverter that converts the power into AC power and outputs the power to the power system, wherein the booster circuit and the bridge inverter reverse the system power to the input voltage smoothing capacitor, thereby allowing the input to a voltage corresponding to the system voltage. The voltage smoothing capacitor can be charged, and the step-up ratio of the isolated DC / DC converter can be set so that the charging voltage is lower than the maximum value of the open circuit voltage of the power generation unit ( Claim 2). According to this, when connecting the power generation unit to the power system, the input voltage smoothing capacitor can be charged in advance by the system voltage, and a large inrush current is generated at the moment when the input circuit is closed by the switch. It is possible to suppress the flow from the portion into the input voltage smoothing capacitor. Furthermore, since the step-up ratio of the isolated DC / DC converter is set so that the charging voltage is lower than the maximum value of the open circuit voltage of the power generation unit, the system voltage is charged to exceed the maximum value of the open voltage of the power generation unit. Therefore, current can be prevented from flowing back toward the power generation unit without providing a backflow prevention diode between the input voltage smoothing capacitor and the power generation unit. As a result, when the power generation unit is a fuel cell, it is possible to prevent early deterioration of the fuel cell stack due to instantaneously drawing a large current from the fuel cell or backflow of current to the fuel cell.

  In the power converter of the present invention, the first switch that opens and closes the input electric circuit, the input voltage smoothing capacitor provided between the first switch and the booster circuit, and the booster A voltage-type bridge inverter provided at a subsequent stage of the circuit, wherein the bridge inverter converts the generated power boosted by the booster circuit into AC power and outputs the AC power to the power system, the non-insulated DC The DC / DC converter includes a backflow prevention diode that prevents backflow from the output side to the input side, and a second switch provided in a bypass path that bypasses the backflow prevention diode. The isolated DC / DC converter includes: And a bidirectional converter in which a step-up ratio is set by a winding ratio between the primary side and the secondary side of the transformer. When the control unit starts the power generation operation, Before closing the closing device, the second switch is closed and the isolated DC / DC converter is reverse-converted so that the grid power is supplied to the bridge inverter, the isolated DC / DC converter, and The input voltage smoothing capacitor is configured to be charged to a voltage corresponding to a system voltage by causing a reverse flow to the input voltage smoothing capacitor through the second switch, and the charging voltage is an open voltage of the power generation unit. The winding ratio of the transformer is set to be lower than the maximum value of the above (Claim 3). According to such a configuration, when connecting the power generation unit to the power system, the input voltage smoothing capacitor can be charged in advance by the system voltage, so that a large inrush current is generated at the moment when the input circuit is closed by the switch. Inflow from the power generation unit to the input voltage smoothing capacitor can be suppressed. Furthermore, since the winding ratio of the transformer of the isolated DC / DC converter is set so that the charging voltage is lower than the maximum value of the open circuit voltage of the power generation unit, the system voltage exceeds the maximum value of the open voltage of the power generation unit. Therefore, it is possible to prevent the current from flowing back toward the power generation unit without providing a backflow prevention diode between the input voltage smoothing capacitor and the power generation unit. As a result, when the power generation unit is a fuel cell, it is possible to prevent early deterioration of the fuel cell stack due to instantaneously drawing a large current from the fuel cell or backflow of current to the fuel cell.

  In a power converter such as a power conditioner for connecting the generated power to the power system, when the output power is periodically increased or decreased, for example, when the power system is 60 Hz AC power, the output power of the power converter is also 60 Hz. Therefore, a ripple having a frequency twice the system frequency is generated in the voltage waveform of the output part of the non-insulated DC / DC converter and the isolated DC / DC converter. On the other hand, for example, when the power generation unit is a fuel cell, there is an upper limit for the allowable output current amount depending on the amount of fuel used, and if a current exceeding the upper limit value is drawn, the fuel cell stack will deteriorate, so the input current of the power converter However, if there is a large ripple in the input current, the average value of the input current must be kept low, and power cannot be efficiently extracted from the fuel cell. In order to solve such a problem, in the power converter according to the present invention, a ripple suppression means for suppressing the ripple of the input current based on a ripple component of the input current from the input electric circuit can be provided. ). According to this, when the maximum value of the ripple of the input current is regulated to be in the vicinity of the predetermined upper limit value, the average value of the input current after suppression can be made larger than the average value of the input current before suppression. It is possible to maintain the input power from the power generation unit as large as possible and take out the power with high efficiency while protecting the power generation unit.

  Further, the non-insulated DC / DC converter is constituted by a boost chopper, and the control unit performs the boost operation control by controlling a duty ratio of a PWM pulse for driving a switching element constituting the boost chopper. The ripple suppression means includes a ripple extraction circuit that extracts a ripple component of an input current from the input circuit, and the PWM pulse increases as the value of the ripple component increases during a period in which the value of the ripple component is greater than a reference value. A duty correction circuit that corrects the duty ratio of the PWM pulse to be smaller as the value of the ripple component is smaller during a period in which the value of the ripple component is smaller than a reference value. 5). In this case, when a ripple of a predetermined frequency is generated in the voltage of the output unit of the boost chopper, the input current of the boost chopper decreases and the voltage of the output unit decreases as the voltage of the output unit increases due to the principle of the boost chopper. As the input current of the step-up chopper increases, ripples also occur in the input current of the step-up chopper, but as the value of the ripple component of the input current increases, the duty ratio of the PWM pulse that drives the switching element of the step-up chopper increases. By correcting the duty ratio to be smaller as the ripple component value is smaller, the amplitude of the ripple of the input current can be reduced.

  As described above, according to the power converter of the first aspect of the present invention, the step-up ratio of the isolated DC / DC converter is increased, and the boost to a predetermined voltage is mainly performed by the isolated DC / DC converter. By doing so, a product having a low withstand voltage and a low on-resistance can be adopted as a switching element constituting the non-insulated DC / DC converter, and the primary side (non-insulated DC / DC / DC converter) The DC converter side) can also be used as a switching element that has a low withstand voltage and low on-resistance, which reduces power loss and increases the efficiency of the boost circuit when the boost ratio of the entire boost circuit is large. It is possible to improve and reduce the parts procurement cost. Furthermore, according to the above configuration, since the step-up ratio of the isolated DC / DC converter is small, the control allowable range of the duty ratio of the switching element of the isolated DC / DC converter becomes relatively large, and various control methods such as input current control are possible. The duty ratio correction control for the purpose can be allowed.

  Further, according to the power converter of claim 2 of the present invention, the input voltage smoothing capacitor can be charged with the system voltage in advance when the power generation unit is connected to the power system, and thereby the switch It is possible to suppress a large inrush current from flowing into the input voltage smoothing capacitor from the power generation unit at the moment when the input electric circuit is closed. Furthermore, since the step-up ratio of the isolated DC / DC converter is set so that the charging voltage is lower than the maximum value of the open circuit voltage of the power generation unit, the system voltage is charged to exceed the maximum value of the open voltage of the power generation unit. Therefore, current can be prevented from flowing back toward the power generation unit without providing a backflow prevention diode between the input voltage smoothing capacitor and the power generation unit. As a result, when the power generation unit is a fuel cell, it is possible to prevent early deterioration of the fuel cell stack due to instantaneously drawing a large current from the fuel cell or backflow of current to the fuel cell.

  According to the power converter of the present invention, the input voltage smoothing capacitor can be charged with the system voltage in advance when the power generation unit is connected to the power system. It is possible to suppress a large inrush current from flowing into the input voltage smoothing capacitor from the power generation unit at the moment when the input electric circuit is closed. Furthermore, since the winding ratio of the transformer of the isolated DC / DC converter is set so that the charging voltage is lower than the maximum value of the open circuit voltage of the power generation unit, the system voltage exceeds the maximum value of the open voltage of the power generation unit. Therefore, it is possible to prevent the current from flowing back toward the power generation unit without providing a backflow prevention diode between the input voltage smoothing capacitor and the power generation unit. As a result, when the power generation unit is a fuel cell, it is possible to prevent early deterioration of the fuel cell stack due to instantaneously drawing a large current from the fuel cell or backflow of current to the fuel cell.

  In the power converter according to claim 4 of the present invention, when the maximum value of the ripple of the input current is regulated to be in the vicinity of the predetermined upper limit value, the average value of the input current before the suppression is suppressed. The average value of the subsequent input current can be increased, the input power from the power generation unit can be maintained as large as possible, and the power can be extracted with high efficiency while protecting the power generation unit.

  According to the power converter of claim 5 of the present invention, when a ripple having a predetermined frequency occurs in the voltage of the output unit of the boost chopper, according to the principle of the boost chopper, the voltage of the boost chopper increases as the voltage of the output unit increases. As the input current decreases and the output voltage decreases, the input current of the boost chopper increases, and this causes ripples in the input current of the boost chopper, but the boost chopper increases as the ripple component of the input current increases. The amplitude of the ripple of the input current can be suppressed small by correcting the duty ratio to be smaller as the duty ratio of the PWM pulse for driving the switching element is larger and the value of the ripple component is smaller.

1 is a schematic circuit diagram of a booster circuit and a boost chopper control circuit of a power converter according to an embodiment of the present invention. It is a whole schematic block diagram of the power converter. It is a wave form diagram of the ripple component of input current. It is a wave form diagram which shows the correction example of the PWM pulse which drives a pressure | voltage rise chopper. FIG. 6 is a waveform diagram of an input current, an input voltage, a boost chopper output unit voltage, and a DC link unit voltage when input current ripple suppression control is performed. FIG. 6 is a waveform diagram of an input current, an input voltage, a boost chopper output unit voltage, and a DC link unit voltage when input current ripple suppression control is not performed.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.

  FIG. 2 shows a schematic configuration of a power conditioner 1 (power converter) according to an embodiment of the present invention. The power conditioner 1 according to the present embodiment converts the DC generated power output from the power generation unit 2 into AC power connected to the commercial power system 3 and outputs it to the commercial power system 3. The booster circuit 4 that boosts the generated power input from the unit 2 to a predetermined voltage (eg, 320 V) corresponding to the maximum value of the system voltage (280 V in the case of a 200 V power system), and the DC power generated after the boost are connected to the system power. A DC / AC inverter 5 for converting to AC power to be connected, a DC link unit 6 comprising a DC link capacitor provided between the booster circuit 4 and the inverter 5, an output unit of the inverter 5 and the commercial power system 3. Provided between the interconnection relay 7 provided between them, the control unit 10 for controlling the operation of the booster circuit 4 / inverter 5 and the interconnection relay 7, and the generator unit 2 through the booster circuit 4 and the DC link unit 6. And an auxiliary power supply unit 8 which generated power or on the basis of the power system 3 to the system power supplied via a diode bridge 9 generates a control power supply voltage to the control unit 10 is. Note that the output of the generated power toward the power system 3 may be anything that links the generated power to the system power in, for example, a home or on-site lead-in wiring. There is no need to reverse flow.

  The power generation unit 2 is, for example, a solid oxide fuel cell unit (SOFC), and the power generation operation is controlled by a built-in control unit 2a. Fuel cell power generation operation control is carried out to continue power generation as much as possible.However, if the power generation operation cannot be continued due to the amount of hot water stored in the hot water storage tank or the amount of electricity stored in the storage battery, When the improvement in energy efficiency cannot be expected, the power generation operation is stopped. The control unit 2a of the power generation unit 2 is communicatively connected to the control unit 10 of the power conditioner 1, and various information such as an output current limit value is transmitted from the power generation unit 2 to the control unit 10 of the power conditioner 1. Thus, the operation control of the power conditioner 1 can be accurately performed according to the operation state of the power generation unit 2.

  The inverter 5 is a full-bridge type voltage-type bridge inverter, and is configured by connecting a plurality of switching elements 5a such as MOS-FETs in an H-bridge type and connecting a feedback diode 5b in parallel to each switching element 5a. The interconnection reactor 5c is provided on the output side. The inverter 5 controls the output current by performing so-called current mode control (current control of the voltage source inverter) by the control unit 10, and the DC power from the DC link unit 6 is output when power is output to the system 3. Is converted into AC power by PWM control or PAM control and output to the grid 3. On the other hand, at the start of power generation operation or the like, by closing the interconnection relay 7 while stopping the switching operation control of the bridge converter as necessary, a plurality of feedback diodes 5b connected in a bridge function as a diode bridge, The AC system power can be rectified and supplied to the DC link unit 6. In FIG. 2, a two-level inverter is exemplified as the inverter 5, but a three-level inverter can also be used.

  The DC link unit 6 is connected to the power input unit of the auxiliary power supply unit 8 via the backflow prevention diode 11, and the booster circuit 4 and the DC link unit 6 are used while the power generation unit 2 outputs the generated power. The generated power is supplied to the auxiliary power supply unit 8. Even if the booster circuit 4 is stopped, the input voltage of the booster circuit 4 is applied to the DC link unit 6 so that the DC link voltage becomes substantially equal to the input voltage of the booster circuit 4, and the control unit 10 is based on the voltage. Auxiliary power supply unit 8 is configured to generate a control power supply voltage.

  The control unit 10 controls each power conversion operation in the booster circuit 4 and the inverter 5 and the opening / closing operation of the interconnection relay 7, and also includes a display unit (for example, a liquid crystal display attached to the casing of the power conditioner 1). (Not shown), various abnormality detection controls in the power conditioner 1, and various controls such as integration of the amount of generated power and the amount of power consumed in the home can be performed. The control unit 10 typically includes a microcomputer as a central processing unit, and includes a drive circuit for generating and outputting a drive signal for each control object based on a control signal from the microcomputer for each control object. .

  As shown in FIG. 1, the booster circuit 4 receives an input voltage input from the power generation unit 2 via the input electric circuit 12 and a booster chopper 41 (non-insulated DC / DC converter) as a previous stage and a bidirectional as a subsequent stage. A full bridge converter 42 (insulated DC / DC converter) is used to boost the voltage in two stages. The step-up chopper 41 and the full-bridge converter 42 are respectively subjected to step-up operation control by the control unit 10.

  An input relay 43 (first switch) whose switching operation is controlled by the control unit 10 is provided in the input electric circuit 12 to the boost chopper 41, and the booster circuit is opened by opening the input relay 43 during standby operation. 4 can be cut off from the power generation unit 2. An input current detection sensor 44 and an input voltage detection sensor 45 are provided on the input side of the boost chopper 41, and the detected input current value and input voltage value are input to the control unit 10 for various controls. Used. The DC link voltage of the DC link unit 6 is also input to the control unit 10 via the voltage divider 47 and the insulating circuit 48. Note that the input current detection sensor 44 uses a sensor that outputs a voltage corresponding to the input current as a detection signal.

  An input voltage smoothing capacitor 46 is provided between the input relay 43 and the boost chopper 41.

  The step-up chopper 41 includes a step-up reactor 41a, a backflow prevention diode 41b connected in series to the output side of the step-up reactor 41a, a switching element 41c such as a MOS-FET for controlling the stored energy of the step-up reactor 41a, and a backflow prevention. A smoothing capacitor 41d provided at the output of the diode 41b and an inrush current prevention relay 41e (second switch) for opening and closing a bypass path bypassing the backflow prevention diode 41b are provided. The switching element 41c and the relay 41e are The drive is controlled by the control unit 10. In the present embodiment, the inductance of the step-up reactor 41a, the frequency of the PWM pulse that drives the switching element 41c, and the like are designed so that the step-up chopper 41 operates in a continuous mode.

  When the boost chopper 41 is in the discontinuous mode, the output voltage equation becomes a non-linear function, so that the optimal correction gain changes in a complex manner depending on the boost ratio. May oscillate. In addition, ringing may occur in the DC link unit 6 due to the slow response speed, and the input current may cause hunting because the PWM correction amount by the control unit 10 and the correction amount by the ripple suppression control do not match. is there. In any of these cases, an excessive current flows in the fuel cell and causes deterioration of the module. However, in the present embodiment, by operating the boost chopper 41 in the continuous mode with high response speed and high stability, The problem has been avoided.

  The bidirectional full bridge converter 42 includes first and second bridge circuits connected via a transformer 42a. Each bridge circuit includes switching elements 42b and 42c such as a plurality of H-bridge connected MOS-FETs. And feedback diodes 42d and 42e connected in parallel to the switching elements 42b and 42c. During the power generation operation, the control unit 10 alternately switches the plurality of switching elements 42b of the first bridge circuit at a predetermined frequency to convert the output of the step-up chopper 41 into an alternating current so that the primary side winding (20T) of the transformer 42a. Is applied to the secondary side winding (52T), the AC power boosted in accordance with the winding ratio is output, and this is converted to DC by the feedback diode 42e of the second bridge circuit and output to the DC link unit 6. To do. Further, when the power is caused to flow backward from the DC link unit 6 side to the boost chopper 41 side, the control unit 10 alternately switches the plurality of switching elements 42c of the second bridge circuit at a predetermined frequency to thereby change the DC link voltage. When AC conversion is applied to the secondary winding (52T) of the transformer 42a, AC power that is stepped down according to the winding ratio is output from the primary winding (20T), and this is output to the first bridge circuit. DC conversion is performed by the feedback diode 42d and the result is output to the smoothing capacitor 41d.

  In the present embodiment, the ratio of the number of secondary side windings to the number of primary side windings is 52T / 20T = 2.6. Therefore, the step-up ratio during power generation operation is about 2.6 times, and during reverse flow The step-down ratio is about 0.4 times. Note that the boost ratio can be further increased by using a forward converter, a flyback converter, or the like.

  On the other hand, the boost ratio of the boost chopper 41 is determined as a result of the boost operation control of the boost chopper 41 by the input voltage of the boost chopper 41, the target voltage of the DC link voltage, and the boost ratio of the bidirectional full bridge converter 42. .

  The step-up chopper 41 and the inverter 5 can perform various operation controls. For example, the rated output 700 W (output voltage 100 V, maximum output current 7 A) of the power generation unit 2 is connected to the output side of the power conditioner 1. When the power consumption of the load is 700 W or more, or when there is a power storage means for surplus power, the power conditioner 1 can be operated at the maximum output. Input current control is performed to feedback control the duty of the PWM pulse that drives the switching element 41c of the step-up chopper 41 so that the input current according to the output current limit command value from the control unit 2a is obtained, and the DC link voltage is a predetermined voltage. Thus, the output control of the inverter 5 can be performed. It is also possible to feedback control the duty of the PWM pulse that drives the switching element 41c of the step-up chopper 41 so that the DC link voltage becomes the target voltage based on the DC link voltage.

  In this embodiment, feedback control is performed so that the DC link voltage is boosted to a predetermined voltage required to link the system 3 to 320 V, for example, 320 V. However, the rated output voltage of the power generation unit 2 is 100 V (the maximum voltage at the time of opening is 120V), the output voltage of the step-up chopper 41 is (320V / 2.6 times) = 123V in a steady state, and the step-up ratio of the step-up chopper 41 is about 1.23 times. At the time of grid connection, a ripple having a frequency twice as high as the frequency of the system power 3 is generated in the DC link voltage, and the output voltage of the power generation unit 2 varies due to various factors. Are not always constant, but various control target values, circuit constants, and the like are set so that the maximum value of the boost ratio of the boost chopper 41 during normal operation is smaller than the boost ratio of the bidirectional full bridge converter 42.

  In the present embodiment, a ripple suppression unit that suppresses the ripple of the input current of the boost chopper 41 is provided. As shown in FIG. 1, the ripple suppression means drives a ripple extraction circuit 13 that extracts and amplifies a ripple component of the input current based on an output signal of the input current sensor 44, and a switching element 41c of the boost chopper 41. For this purpose, the control unit 10 mainly includes a duty correction circuit 14 that corrects the duty of the PWM pulse output.

  The ripple extraction circuit 13 includes a coupling capacitor 13a that cuts off the direct current component of the ripple component of the input current, and an amplifier 13b that amplifies the ripple component and shifts the reference potential to 2.5V according to the amplitude of the PWM pulse. I have.

  The duty correction circuit 14 includes an integration circuit 14 a that integrates the PWM pulse waveform, and a comparator 14 b that compares the PWM pulse waveform after integration with the output waveform of the ripple extraction circuit 13.

  FIG. 3 shows a waveform of a ripple component of the input current, and the ripple component pulsates at a frequency of 120 Hz or 100 Hz which is twice the system frequency. On the other hand, as shown in FIG. 4, the frequency of the PWM pulse is set to 60 kHz, and by integration, a gentle curve is drawn as the rise time elapses immediately after the rise of the pulse, and similarly, immediately after the fall of the pulse The waveform has a gentle curve as the fall time elapses.

  Therefore, when the ripple component of the input current is larger than the reference potential (ripple peak), as shown in the corrected (crest period), the PWM pulse waveform whose duty is corrected to be smaller as the ripple component is larger is output from the comparator 14b. Is output. On the other hand, when the ripple component of the input current is smaller than the reference potential (ripple valley), as shown in the corrected (valley period), the PWM pulse waveform whose duty is corrected to be larger as the ripple component is smaller is output from the comparator 14b. Is output.

  The corrected PWM pulse output from the comparator 14b is input to the drive circuit 15 of the switching element 41c, and the switching element 41c is on / off controlled based on the corrected PWM pulse. Here, as shown in FIG. 5, the peak period of the ripple of the input current corresponds to the valley period of the ripple of the DC link voltage, and the valley period of the ripple of the input current corresponds to the peak period of the ripple of the DC link voltage. Therefore, when the DC link voltage decreases due to the boost chopper principle, the reactance current, that is, the input current tends to increase, but the input current increases by correcting the duty of the PWM pulse to be small in the peak period of the input current. It suppresses the rise, and when the DC link voltage rises, the input current tends to fall, but during the valley period of the input current, the PWM pulse duty is largely corrected to prevent the input current from drastically dropping. This suppresses the ripple component of the input current to be small.

  For reference, FIG. 6 shows the ripple state of the input current when the duty of the PWM pulse is not corrected by the ripple suppression means. As is clear from the figure, a very large ripple component is included in the input current. Appears. At this time, the amplitude of the ripple component is about 5A, and the maximum value of the amplitude is about 12A. Since the instantaneous maximum output current of the fuel cell unit is 7A, it is necessary to control the average value of the input current so that the maximum value of the ripple of the input current is 7A when ripple suppression is not performed. Although it becomes impossible to take out electric power, according to this embodiment, generated electric power can be taken out to 7 A by the average value of input current.

  As described above, the smaller the boost ratio of the boost chopper 41, the higher the efficiency. However, if the boost ratio is too small, the on-duty becomes 0% when correction by the ripple suppression control is performed, and the ripple suppression control is performed. Since it cannot be performed, it is preferable to perform DC link voltage control by the control unit 10 so that the on-duty is equal to or higher than the PWM duty fluctuation value by correction.

  Further, in the present embodiment, when starting the power generation operation, the control unit 10 closes the inrush current prevention relay 41e and reversely converts the bidirectional bridge converter 42 before closing the input relay 43. Thus, in the case of the 200V power system, the DC link voltage is charged to 280V, and the smoothing capacitor 41d and the input voltage smoothing capacitor 46 are charged to 280V / 2.6 times = about 107V by the bidirectional bridge converter. Thus, the step-up ratio of the bidirectional bridge converter 42, that is, the winding ratio of the primary side and the secondary side of the transformer 42a is set so that the charging voltage 107V is lower than 120V that is the open circuit voltage of the power generation unit 2. Thus, the inrush current when the input relay 43 is closed can be suppressed and the backflow to the fuel cell module can be prevented, thereby preventing the deterioration of the fuel cell module.

  The present invention is not limited to the above-described embodiment, and can be appropriately changed in design. For example, the insulation type DC / DC converter 42 does not have to be controlled by the control unit 10. For example, even if it is a self-excited type, it has a low breakdown voltage as the switching element on the primary side of the transformer or the switching element of the step-up chopper. -There is a merit that low on-resistance can be adopted. The ripple suppression control is particularly useful in a power conditioner for a fuel cell power generation system. Even in the case of a power converter that outputs DC power to a load, the power consumption of the load fluctuates at a predetermined frequency. However, ripples may occur in the input current of the power converter, and the ripple suppression control can be performed in order to suppress such ripples. Further, the method or path for charging the input voltage smoothing capacitor 46 by causing the system power to flow backward is not limited to the inrush current prevention relay 41e, but is a bidirectional converter as described in Patent Document 1, for example. Thus, a booster circuit can be configured.

DESCRIPTION OF SYMBOLS 1 Power conditioner 2 Power generation part 3 Electric power system 4 Booster circuit 41 Boost chopper (non-insulation type DC / DC converter)
41b Backflow prevention diode 41c Switching element 41e Second switch 42 Bidirectional full-bridge converter (insulated DC / DC converter)
42a transformer 43 first switch 46 input voltage smoothing capacitor 5 bridge inverter 6 DC link unit 10 control unit 12 input circuit 13 input current ripple component extraction circuit 14 PWM pulse duty correction circuit

Claims (5)

A booster circuit that boosts an input voltage input from a power generation unit via an input electric circuit in two stages by a non-insulated DC / DC converter in a preceding stage and an isolated DC / DC converter in a subsequent stage, and at least the non-insulated DC A power converter including a control unit that performs step-up operation control of the DC converter,
The power converter, wherein the step-up ratio of the isolated DC / DC converter is set to be larger than the maximum value of the step-up ratio of the non-insulated DC / DC converter by the step-up operation control. The power converter according to claim 1, wherein
A switch that opens and closes the input electric circuit, an input voltage smoothing capacitor provided between the switch and the booster circuit, and converts the generated power boosted by the booster circuit into alternating current power into an electric power system. An output bridge inverter, and the booster circuit and the bridge inverter are configured to charge the input voltage smoothing capacitor to a voltage corresponding to the system voltage by causing the system power to flow backward to the input voltage smoothing capacitor. The step-up ratio of the isolated DC / DC converter is set such that the charging voltage is lower than the maximum value of the open circuit voltage of the power generation unit. The power converter according to claim 1, wherein
A first switch for opening and closing the input circuit; an input voltage smoothing capacitor provided between the first switch and the booster circuit; and a voltage-type bridge inverter provided at a subsequent stage of the booster circuit. And the bridge inverter converts the generated power boosted by the booster circuit into AC power and outputs it to the power system,
The non-insulated DC / DC converter includes a backflow prevention diode that prevents backflow from the output side to the input side, and a second switch provided in a bypass path that bypasses the backflow prevention diode,
The isolated DC / DC converter is composed of a bidirectional converter in which a step-up ratio is set by a winding ratio between the primary side and the secondary side of the transformer.
When the power generation operation is started, the control unit closes the second switch and reverse-converts the insulated DC / DC converter before closing the first switch. The input voltage smoothing capacitor is reduced to a voltage corresponding to the system voltage by causing the system power to flow back to the input voltage smoothing capacitor via the bridge inverter, the isolated DC / DC converter, and the second switch. Configured to charge,
The power converter, wherein the transformer turns ratio is set so that the charging voltage is lower than the maximum value of the open circuit voltage of the power generation unit.   The power converter according to claim 1, 2, or 3, further comprising a ripple suppression means for suppressing a ripple of the input current based on a ripple component of the input current from the input electric circuit. .   5. The power converter according to claim 4, wherein the non-insulated DC / DC converter includes a step-up chopper, and the control unit controls a duty ratio of a PWM pulse that drives a switching element that forms the step-up chopper. The ripple suppression means includes a ripple extraction circuit that extracts a ripple component of an input current from the input electric circuit, and the ripple during a period in which the value of the ripple component is greater than a reference value. A duty correction circuit that corrects the duty ratio of the PWM pulse to be larger as the component value is larger and corrects the duty ratio of the PWM pulse to be smaller as the ripple component value is smaller during a period when the value of the ripple component is smaller than a reference value. And a power converter.

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