JP2012065441A - Power converter and photovoltaic power generation system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce conduction loss while achieving no necessity for monitoring input capacitor voltages and DC bus capacitor voltages.SOLUTION: A bypass diode 12 is connected to a booster circuit 3 in parallel so as to obtain a forward direction from the positive electrode side of an input capacitor 2 toward the positive electrode side of a DC bus capacitor 4.

Description

  The present invention relates to a power conversion device and a photovoltaic power generation system, and more particularly to a DC / DC converter.

  In power conditioners, the DC voltage generated by solar cells, fuel cells, etc. is boosted to the required voltage, and the boosted voltage is converted to AC voltage for consumption by AC devices or reverse power flow to the power system. ing.

  The boost voltage is determined by the voltage value of the power system and the inverter method. Further, depending on the magnitude of the voltage generated by the solar cell, a condition that does not require boosting by the boosting circuit occurs.

  A general booster circuit includes a booster semiconductor switch, a diode, and a reactor. In this booster circuit, even if the operation of the booster circuit is stopped, current continues to flow through the diode and the reactor, so that conduction loss occurs.

  Further, in Patent Document 1, a solar voltage is boosted and generated by a chopper circuit, and when the solar voltage exceeds a predetermined voltage, the boosting operation of the chopper circuit is stopped and a bypass relay connected in parallel is used. There is disclosed a technique for reducing a loss related to boosting by bypassing a reactor and a diode in a chopper circuit and further eliminating conduction loss of components in the chopper circuit.

JP 2006-238629 A

  However, according to the technique of Patent Document 1, if the difference between the input capacitor voltage and the DC bus capacitor voltage is about 3 V or more when the bypass relay is turned on, a large current flows through the bypass relay, causing a contact failure of the bypass relay. There was a problem that there was something.

  On the other hand, in order to prevent a large current from flowing through the bypass relay, it is necessary to make the DC bus capacitor voltage follow the input capacitor voltage with high accuracy. For this reason, a highly accurate sensor is required to measure the input capacitor voltage and the DC bus capacitor voltage, and there is a problem that the equipment and control for conducting the bypass relay are complicated.

  The present invention has been made in view of the above, and provides a power conversion device and a photovoltaic power generation system that can reduce conduction loss while eliminating the need to monitor the input capacitor voltage and the DC bus capacitor voltage. With the goal.

  In order to solve the above-described problems and achieve the object, a power conversion device of the present invention includes a booster circuit that boosts an input capacitor voltage, an input capacitor that inputs the input capacitor voltage to the booster circuit, and the booster circuit And a bypass diode connected in a forward direction from the positive electrode side of the input capacitor toward the positive electrode side of the DC bus capacitor.

  According to the present invention, there is an effect that it is possible to reduce conduction loss while making monitoring of the input capacitor voltage and the DC bus capacitor voltage unnecessary.

FIG. 1 is a block diagram showing a schematic configuration of a power conversion device according to Embodiment 1 of the present invention. FIG. 2 is a diagram illustrating a relationship between the input capacitor voltage Vin and the DC bus capacitor voltage Vo with respect to time of the power conversion device of FIG. FIG. 3 is a block diagram showing a schematic configuration of the power converter according to the second embodiment of the present invention. FIG. 4 is a diagram illustrating an example of temperature characteristics of the forward voltage and the forward current of the SiC diode of the power conversion device of FIG. 3. FIG. 5 is a block diagram showing a schematic configuration of the power converter according to Embodiment 3 of the present invention. 6A is a diagram showing the bypass current Ix with respect to time of the power converter of FIG. 5, and FIG. 6B is a graph showing the input capacitor voltage Vin and the DC bus capacitor voltage Vo with respect to time of the power converter of FIG. It is a figure which shows the relationship. FIG. 7A is a diagram showing the relationship among the bypass current Ix, the input capacitor voltage Vin, and the DC bus capacitor voltage Vo when the input capacitor voltage Vin of the fourth embodiment of the power conversion device according to the present invention is increased. FIG. 7B is a diagram showing a relationship among the bypass current Ix, the input capacitor voltage Vin, and the DC bus capacitor voltage Vo when the input capacitor voltage Vin is lowered in the power converter according to the fourth embodiment of the present invention. FIG. 8A is a diagram showing the relationship among the bypass current Ix, the input capacitor voltage Vin, and the DC bus capacitor voltage Vo when the input capacitor voltage Vin is increased in the power converter according to the fifth embodiment of the present invention. 8 (b) is a diagram showing a relationship among the bypass current Ix, the input capacitor voltage Vin, and the DC bus capacitor voltage Vo when the input capacitor voltage Vin is lowered in the power conversion device according to the fourth embodiment of the present invention.

  Hereinafter, an embodiment of a power converter according to the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a schematic configuration of a power conversion device according to Embodiment 1 of the present invention. In FIG. 1, the power converter is provided with a booster circuit 3 that boosts the input capacitor voltage Vin. An input capacitor 2 is connected to the input side of the booster circuit 3, and a DC bus capacitor 4 is connected to the output side of the booster circuit 3.

  A solar cell 1 is connected as a variable DC power source before the input capacitor 2, and an AC load 6 is connected via an inverter circuit 5 after the DC bus capacitor 4. The AC load 6 may be a power system.

  The booster circuit 3 is provided with a boost semiconductor switch 7, a boost reactor 8, and a boost diode 9. The step-up reactor 8 and the step-up diode 9 are connected in series, and this series circuit is configured so that the step-up diode 9 is in the forward direction from the positive side of the input capacitor 2 toward the positive side of the DC bus capacitor 4. Are connected between the positive electrode side and the positive electrode side of the DC bus capacitor 4. A step-up semiconductor switch 7 is connected to a connection point between the step-up reactor 8 and the step-up diode 9.

  A bypass diode 12 is connected in parallel to the booster circuit 3 so as to be forward from the positive side of the input capacitor 2 toward the positive side of the DC bus capacitor 4.

  The voltage of the solar cell 1 is charged to the input capacitor 2, and the input capacitor voltage Vin is input to the booster circuit 3. Here, in the power generation means using natural energy such as the solar cell 1, the fluctuation of the DC voltage is large, and the voltage of the input capacitor 2 is stably increased to a predetermined voltage or higher, or the DC voltage generated in the solar cell 1 is increased. In some cases, the boosting operation becomes unnecessary.

  When the input capacitor voltage Vin is boosted to a desired value, the boost semiconductor switch 7 is controlled to be turned on / off. When the step-up semiconductor switch 7 is on, a current flows through the step-up reactor 8 via the step-up semiconductor switch 7, and when the step-up semiconductor switch 7 is off, the current flowing through the step-up reactor 8 charges the DC bus capacitor 4. As a result, the input capacitor voltage Vin is boosted, and the DC bus capacitor voltage Vo is generated.

  The DC bus capacitor voltage Vo is converted from DC to AC by the inverter circuit 5 and supplied to the AC load 6. For example, when the AC load 6 is a power system, the voltage output from the inverter circuit 5 is determined by the power system voltage. The DC bus capacitor voltage Vo is also determined by the system voltage and the inverter circuit system. Further, the system voltage may fluctuate, and the target value of the DC bus capacitor voltage Vo is changed to cope with this.

  When boosting the input capacitor voltage Vin to a desired value, the booster circuit 3 turns off the booster semiconductor switch 7 and stops the boosting operation. At this time, even if the switching of the boosting semiconductor switch 7 is stopped, if the input capacitor voltage Vin is larger than the DC bus capacitor voltage Vo + the forward voltage of the boosting diode 9, a current flows through the boosting reactor 6 and the boosting diode 9. When a current flows through the boosting reactor 6 and the boosting diode 9, conduction loss occurs.

  Here, when the input capacitor voltage Vin is larger than the DC bus capacitor voltage Vo + the forward voltage of the bypass diode 12, the bypass diode 12 becomes conductive, and when the input capacitor voltage Vin is smaller than the DC bus capacitor voltage Vo, the bypass diode 12 is turned off. To do.

  When the bypass diode 12 is conductive, the booster circuit 3 is stopped. For this reason, the booster circuit 3 performs a boosting operation when the bypass diode 12 is off.

  For example, when the input capacitor voltage Vin is smaller than the DC bus capacitor voltage Vo and the booster circuit 3 is performing a boost operation, the bypass diode 12 is in a non-conducting state, so that the booster circuit 3 can operate normally.

  On the other hand, when the input capacitor voltage Vin is larger than the DC bus capacitor voltage Vo, the booster circuit 3 does not need to boost and the booster circuit 3 stops the boosting operation. At this time, the bypass diode 12 becomes conductive, the bypass current Ix flows through the bypass diode 12, and the input capacitor voltage Vin becomes the DC bus capacitor voltage Vo + the forward voltage of the bypass diode 12.

  In the bypass diode 12, the switching operation is performed according to the nature of the input capacitor voltage Vin and the DC bus capacitor voltage Vo depending on the nature of the input capacitor voltage Vin and the DC bus capacitor voltage Vo. Therefore, the current flowing through the boost reactor 8 and the boost diode 9 can be bypassed, and the conduction loss between the boost reactor 8 and the boost diode 9 can be reduced.

  When the bypass diode 12 is used, conduction loss due to the forward voltage of the bypass diode 12 occurs, but the conduction loss can be reduced by the amount of current flowing through the boost reactor 8. Furthermore, by making the forward voltage of bypass diode 12 smaller than the forward voltage of boost diode 9, the conduction loss of bypass diode 12 can be made smaller than the conduction loss of boost diode 9. Here, since the bypass diode 12 does not require a switching operation, the recovery loss may be large, and a diode having a forward voltage lower than that of the boost diode 9 can be selected.

  FIG. 2 is a diagram illustrating a relationship between the input capacitor voltage Vin and the DC bus capacitor voltage Vo with respect to time of the power conversion device of FIG. In FIG. 2, it is assumed that the input capacitor voltage Vin increases with time. Since the input capacitor voltage Vin is lower than the necessary DC bus capacitor voltage Vo between time A and time B, the booster circuit 3 of FIG. 1 operates and increases the input capacitor voltage Vin to a predetermined value. In this state, since the input capacitor voltage Vin is smaller than the DC bus capacitor voltage Vo, the bypass diode 12 is turned off and the bypass current Ix does not flow.

  Between time B and time C, the input capacitor voltage Vin exceeds a predetermined voltage. Therefore, the booster circuit 3 does not need to be boosted and stops. In this state, the input capacitor voltage Vin becomes larger than the DC bus capacitor voltage Vo + the forward voltage of the boost diode 9, so that the bypass diode 12 becomes conductive and the bypass current Ix flows.

  The bypass current Ix can be passed through the bypass diode 12 while the booster circuit 3 is stopped. Therefore, the conduction loss when the operation of the booster circuit 3 is stopped can be reduced from the conduction loss of the boosting reactor 8 and the boosting diode 9 to the conduction loss of the bypass diode 12. Furthermore, since the bypass diode 12 does not require switching at a high repetition frequency, a diode having a smaller conduction loss than the boost diode 9 can be selected, and the loss in the bypass diode 12 can be reduced.

Embodiment 2. FIG.
FIG. 3 is a block diagram showing a schematic configuration of the power converter according to the second embodiment of the present invention. 3, in this power converter, wide gap semiconductor diodes 12a and 12b are provided as the bypass diode 12 of FIG. These wide gap semiconductor diodes 12a and 12b are connected in parallel to each other. As a material for the wide gap semiconductor, for example, SiC, GaN, or diamond can be used.
By using SiC diodes in parallel, conduction loss can be reduced.

  Here, the wide-gap semiconductor diodes 12a and 12b have lower on-resistance than the Si diode. For this reason, by using the wide gap semiconductor diodes 12a and 12b as the bypass diode 12, the conduction loss can be reduced, and the efficiency of the power converter can be improved.

  In addition, the forward voltage of the wide gap semiconductor diodes 12a and 12b has a positive temperature coefficient. When the temperature of the wide gap semiconductor diodes 12a and 12b increases, the forward voltage increases and current does not easily flow. For this reason, when the current is concentrated on one of the wide gap semiconductor diodes 12a and 12b, the temperature of the wide gap semiconductor diode 12a and 12b on which the current is concentrated rises, and the current is suppressed. As a result, by connecting the wide gap semiconductor diodes 12a and 12b in parallel to each other, the current flowing through the wide gap semiconductor diodes 12a and 12b can be equalized, and the loss in the wide gap semiconductor diodes 12a and 12b is reduced. be able to.

  FIG. 4 is a diagram illustrating an example of temperature characteristics of the forward voltage and the forward current of the SiC diode of the power conversion device of FIG. 3. In FIG. 4, since the forward voltage of the wide gap semiconductor diodes 12a and 12b has a positive temperature coefficient, the forward voltage becomes higher and the current hardly flows when the temperature becomes higher.

  For this reason, by connecting the wide gap semiconductor diodes 12a and 12b in parallel, the current flowing through the wide gap semiconductor diodes 12a and 12b can be equalized, and the loss in the wide gap semiconductor diodes 12a and 12b can be reduced. Can do.

  On the other hand, the forward voltage of the silicon diode has a negative temperature coefficient. When the temperature of the silicon diode increases, the forward voltage decreases and current easily flows. For this reason, when current concentrates on a specific silicon diode, the silicon diode generates heat, and the forward voltage is further lowered.This further promotes current concentration, and conduction loss even when silicon diodes are connected in parallel. Cannot be improved.

Embodiment 3 FIG.
FIG. 5 is a block diagram showing a schematic configuration of the power converter according to Embodiment 3 of the present invention. 5, in this power converter, a bypass auxiliary relay 13 is connected in parallel to the bypass diode 12 of FIG.

  Further, an ammeter 14 for measuring the bypass current Ix flowing through the bypass diode 12 and a relay control device 16 for controlling the conduction of the bypass auxiliary relay 13 are provided.

  In the example of FIG. 5, the method of connecting the ammeter 14 between the connection point of the bypass auxiliary relay 13 and the positive electrode side of the input capacitor 2 has been described. However, the connection point of the bypass auxiliary relay 13 and the bypass diode 12 are described. It is also possible to connect between the anode terminals of the bypass diode 12 so that the current of only the bypass diode 12 can be measured. Examples of the ammeter 14 include a shunt resistance, a sensor for measuring a magnetic field, or a current transformer. The relay control device 16 may be configured by a microcomputer or a DSP, or may be configured by a digital circuit such as a comparator.

  Next, the operation of the power conversion device in FIG. 5 will be described.

  When the booster circuit 3 is stopped and the bypass current Ix begins to flow through the bypass diode 12, the bypass current Ix is measured by the ammeter 14, and the current measurement value 15 is sent to the relay control device 16.

  When the current measurement value 15 is sent, the relay control device 16 outputs a bypass auxiliary relay control signal 17 and controls the bypass auxiliary relay 13 to be conductive. Here, when the bypass current Ix flows, the input capacitor voltage Vin is the DC bus capacitor voltage Vo + the forward voltage of the bypass diode 13.

  That is, the forward voltage of the bypass diode 12 is applied to both ends of the bypass auxiliary relay 13. At this time, if the potential difference between both ends of the bypass auxiliary relay 13 is 1 V or less, the contact of the bypass auxiliary relay 13 is hardly damaged.

  On the other hand, the forward voltage of the bypass diode 12 is generally around 0.6V. For this reason, when the bypass auxiliary relay 13 is turned on after the bypass diode 12 is turned on, the potential difference between both ends of the bypass auxiliary relay 13 becomes 1 V or less, and damage to the contact of the bypass auxiliary relay 13 can be prevented.

  Here, when a forward current flows through the bypass diode 12 from the anode to the cathode, a forward voltage is generated in the bypass diode 12 and conduction loss occurs. On the other hand, in the bypass auxiliary relay 13, the on-resistance can be reduced as compared with the bypass diode 12, and the conduction loss can be reduced.

  For this reason, by conducting the bypass auxiliary relay 13 after the bypass diode 12 is conducted, the conduction loss can be further reduced while preventing the contact of the bypass auxiliary relay 13 from being damaged.

  At this time, the bypass auxiliary relay 13 can be conducted when the bypass current Ix flows through the bypass diode 12 and the potential difference between the anode and the cathode of the bypass diode 12 becomes 1V or less.

  Therefore, by measuring the bypass current Ix with the ammeter 14, the conduction of the bypass diode 12 can be accurately monitored, and the bypass auxiliary relay 13 can set the conduction timing with high accuracy. For this reason, it is possible to prevent an overcurrent from flowing through the bypass auxiliary relay 13, and damage to the bypass auxiliary relay 13 can be avoided.

  6A is a diagram showing the bypass current Ix with respect to time of the power converter of FIG. 5, and FIG. 6B is a graph showing the input capacitor voltage Vin and the DC bus capacitor voltage Vo with respect to time of the power converter of FIG. It is a figure which shows the relationship.

  In FIG. 6, when the potential difference between the anode and the cathode of the bypass diode 12 is 1 V or less, the current peak of the bypass current Ix can be suppressed, and damage to the bypass auxiliary relay 13 can be avoided.

  In the above-described embodiment, a method has been described in which the ammeter 14 for measuring the bypass current Ix flowing through the bypass diode 12 is provided and the bypass auxiliary relay 13 is turned on when the ammeter 14 detects the bypass current Ix. However, a voltmeter for measuring the input capacitor voltage Vin and the DC bus capacitor voltage Vo is provided so that the bypass auxiliary relay 13 is turned on when the difference between the input capacitor voltage Vin and the DC bus capacitor voltage Vo is about 1 V or less. May be.

Embodiment 4 FIG.
FIG. 7A is a diagram showing the relationship among the bypass current Ix, the input capacitor voltage Vin, and the DC bus capacitor voltage Vo when the input capacitor voltage Vin of the fourth embodiment of the power conversion device according to the present invention is increased. FIG. 7B is a diagram showing a relationship among the bypass current Ix, the input capacitor voltage Vin, and the DC bus capacitor voltage Vo when the input capacitor voltage Vin is lowered in the power converter according to the fourth embodiment of the present invention.

  7 (a) and 7 (b), assuming that the input capacitor voltage Vin has risen in time, the input capacitor voltage Vin and the DC bus capacitor voltage Vo are equal, and the input capacitor voltage Vin has further increased. At a point, a bypass relay conducting voltage 18 and a bypass relay disconnecting voltage 19 are set.

  When the input capacitor voltage Vin and the DC bus capacitor voltage Vo become equal, the bypass current Ix flows. Further, when the input capacitor voltage Vin exceeds the bypass relay conducting voltage 18, a bypass auxiliary relay control signal 17 is output from the relay control device 16 of FIG. 5, and the bypass auxiliary relay 13 is conducted.

  Further, as shown in FIG. 7B, when the input capacitor voltage Vin decreases and the input capacitor voltage Vin falls below the bypass relay disconnection voltage 19, the bypass auxiliary relay control signal 17 is cut off, and the bypass auxiliary relay 13 is turned on. Disconnect.

  As a result, the bypass auxiliary relay 13 can be conducted in a state where current flows through the bypass diode 12, an inrush current that can destroy the bypass auxiliary relay 13 can be prevented, and a power conversion device with low conduction loss is expected. it can.

  That is, the state where the input capacitor voltage Vin is higher than the DC bus capacitor voltage Vo is a state where current flows through the bypass diode 12. By confirming this state with the relay control device 16 and making the bypass auxiliary relay 13 conductive, an overcurrent does not occur in the bypass auxiliary relay 13 and damage to the bypass auxiliary relay 13 can be avoided.

  Further, by comparing the input capacitor voltage Vin with the bypass relay conducting voltage 18 and the bypass relay disconnecting voltage 19, it is not necessary to directly measure the DC bus capacitor voltage Vo, and the bypass auxiliary relay 13 can be easily controlled. it can.

Embodiment 5 FIG.
FIG. 8A is a diagram showing the relationship among the bypass current Ix, the input capacitor voltage Vin, and the DC bus capacitor voltage Vo when the input capacitor voltage Vin is increased in the power converter according to the fifth embodiment of the present invention. 8 (b) is a diagram showing a relationship among the bypass current Ix, the input capacitor voltage Vin, and the DC bus capacitor voltage Vo when the input capacitor voltage Vin is lowered in the power conversion device according to the fourth embodiment of the present invention.

  In FIG. 8, a bypass relay conducting voltage 18 ′ and a bypass relay disconnecting voltage 19 ′ are set instead of the bypass relay conducting voltage 18 and the bypass relay disconnecting voltage 19 shown in FIG. The bypass relay conducting voltage 18 'and the bypass relay disconnecting voltage 19' are different from each other so that a change hysteresis operation of the input capacitor voltage Vin is possible.

  As a result, the bypass auxiliary relay 13 can be turned on after a sufficient amount of current flows through the bypass diode 12, and the bypass auxiliary relay 13 can be disconnected before the current flowing through the bypass diode 12 runs out. For this reason, chattering of the bypass auxiliary relay 13 due to a change in the input capacitor voltage Vin can be prevented, and a stable operation becomes possible.

  As described above, the power conversion device according to the present invention makes it possible to reduce conduction loss while eliminating the need to monitor the input capacitor voltage and the DC bus capacitor voltage, and to reduce the DC voltage generated in solar cells, fuel cells, and the like. Suitable for power converters and solar power generation systems that boost the voltage to the required voltage.

DESCRIPTION OF SYMBOLS 1 Solar cell 2 Input capacitor 3 Booster circuit 4 DC bus capacitor 5 Inverter circuit 6 AC load (electric power system)
7 Boosting Semiconductor Switch 8 Boosting Reactor 9 Boosting Diode 12 Bypass Diode 13 Bypass Auxiliary Relay 14 Ammeter 15 Current Measurement Value 16 Relay Controller 17 Bypass Auxiliary Relay Control Signal 18, 18 ′ Bypass Relay Conducting Voltage 19, 19 ′ Bypass Relay Disconnection Voltage

Claims (11)

A booster circuit for boosting the input capacitor voltage;
An input capacitor for inputting the input capacitor voltage to the booster circuit;
A DC bus capacitor connected to the output side of the booster circuit;
And a bypass diode connected in a forward direction from the positive electrode side of the input capacitor toward the positive electrode side of the DC bus capacitor.   The power converter according to claim 1, wherein the bypass diode is a wide gap semiconductor diode.   The power converter according to claim 2, wherein a plurality of wide gap semiconductor diodes are connected in parallel. A variable DC power supply connected to the previous stage of the input capacitor;
4. The power conversion device according to claim 1, further comprising: an inverter circuit connected to a subsequent stage of the DC bus capacitor. 5.   5. The power conversion device according to claim 1, further comprising a bypass auxiliary relay connected in parallel to the bypass diode. 6.   The power conversion device according to claim 5, further comprising a relay control device that turns on the bypass auxiliary relay after the bypass diode is turned on. Further comprising an ammeter for measuring the current flowing through the bypass diode;
The power conversion device according to claim 6, wherein the relay control device makes the bypass auxiliary relay conductive based on a current flowing through the bypass diode. It further includes a voltmeter that measures the input capacitor voltage and the DC bus capacitor voltage,
The power conversion device according to claim 6 or 7, wherein the relay control device makes the bypass auxiliary relay conductive based on a difference between the input capacitor voltage and the DC bus capacitor voltage. A voltmeter for measuring the input capacitor voltage;
The relay control device compares the input capacitor voltage with a bypass relay conducting voltage and a bypass relay disconnecting voltage, and when the input capacitor voltage is higher than the bypass relay conducting voltage, causes the bypass auxiliary relay to conduct, The power converter according to claim 6 or 7, wherein when the voltage is lower than the bypass relay disconnection voltage, the bypass auxiliary relay is disconnected.   The power converter according to claim 9, wherein the bypass relay conduction voltage and the bypass relay disconnection voltage are different from each other.   A solar power generation system comprising a power conditioner having the power conversion device according to any one of claims 1 to 10.

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