JP2011193704A - Dc-ac power converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce an input and output current of a smoothing capacitor without mutual restriction in an ON/OFF period of a switching element of a chopper and an ON/OFF period of each phase switching element of an inverter. <P>SOLUTION: The chopper 2 boosts voltages of a DC power supply 1 to a predetermined voltage by PWM control. The smoothing capacitor 3 is charged by the DC output of the chopper. The inverter 4 converts the DC power to a predetermined AC power by the PWM control to supply power to a grid-connected system power supply. A controller 6 for PWM-controlling the chopper and the inverter, synchronizes the frequency of a carrier signal of a chopper carrier forming section 6C, and that of a carrier signal of an inverter carrier forming section 6F, and controls a phase difference of both carrier signals at 90°. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a DC-AC power converter that converts DC power generated by a solar power generator or the like into AC power by a chopper and an inverter and supplies the AC power to a load.

  This type of device uses a solar power generator, wind power generator, fuel cell power generator, etc. as a DC power source, and a system or motor that supplies AC power converted from DC power by a chopper and an inverter to a grid system It is used for a system that supplies power load (see, for example, Patent Document 1 and Patent Document 2).

  Among the DC-AC power converters described above, the inverter reverse-converts DC power into AC power through frequency / voltage control and further phase control, but the output voltage of a DC power source such as a solar power generator varies greatly. For this reason, the DC power supply voltage of the inverter is lowered and the voltage is insufficient. When the inverter's DC power supply voltage drops, the maximum output voltage of the inverter depends on the lower limit voltage of the DC power supply voltage, so the output voltage of the inverter is also insufficient, and the DC power supply voltage is restored until the DC power supply voltage returns to the lower limit voltage or higher. Will be stopped. In addition, when the DC power supply voltage drops to near the lower limit voltage, in order to extract sufficient power from the DC power supply, the current flowing through the inverter switch element increases and the loss at the switch element increases. It also causes a decrease in efficiency.

  Therefore, as shown in FIG. 4, when a chopper is interposed between the DC power source and the three-phase inverter, the chopper can be used even when the output voltage of the DC power source such as a solar power generator is insufficient. By maintaining the DC voltage of the three-phase inverter at a constant value, the grid connection can be continued and the power conversion efficiency can be improved.

  In FIG. 4, the DC power source 1 includes, for example, a power generation device such as a solar power generation device and a small-capacity secondary battery that is charged with DC power generated by the device. The chopper 2 forms a main circuit with the DC reactor L, the semiconductor switch CHP, and the freewheeling diode D, and generates a pulse voltage obtained by boosting the output of the DC power source 1 by ON / OFF operation (chopping) of the semiconductor switch CHP. The smoothing capacitor 3 is charged with a voltage. The three-phase inverter 4 has a main circuit configuration in which the semiconductor switches U to Z are connected in a full bridge, and converts the DC output of the smoothing capacitor 3 and the chopper 2 into AC power synchronized with the frequency / voltage and phase of the grid power supply. The interconnection connection circuit 5 incorporates a filter for removing switching noise and the like included in the AC output of the three-phase inverter 4 and an interconnection switch to link the AC output of the inverter 4 to the system power supply.

  The controller (control device) 6 holds the charging voltage of the smoothing capacitor 3 at a specified value by controlling the conductivity (PWM) of the chopper 2 with respect to the voltage change of the DC power supply 1, and performs the three-phase PWM control of the three-phase inverter 4. The AC output is matched with the frequency, voltage and phase of the system power supply.

JP 2007-221892 A JP 2000-92857 A

  In the DC-AC power converter shown in FIG. 4, waveforms of respective voltages and currents when the chopper 2 and the inverter 4 are subjected to PWM control will be described with reference to FIGS. 5 to 7. FIG. 5 shows the current output timing of the chopper, the ON / OFF (chopper_SW) ratio of the semiconductor switch CHP is determined by comparing the size of the chopper reference value (Chopper_ref) and the chopper carrier wave (Chopper_carrier: triangular wave), and the carrier waveform The semiconductor switch CHP is turned off at the apex, and the chopper output current is output to the smoothing capacitor side.

  Similarly, the output current of the inverter is at the timing shown in FIG. The ON / OFF signal (U / V / W / X / Y / Z_SW) of each switch of U to Z is obtained by comparing the magnitude of the inverter reference value (U / V / W_ref) and the inverter carrier wave (Inverter_carry: triangular wave). The inverter output current is determined when any of the XYZ switching is on when any of the UVW switches are on. In this case, current does not flow in the peak and valley of the carrier, but current flows in the slope portion.

  Next, FIG. 7 shows the current that flows through the smoothing capacitor when the carrier of the chopper and the inverter are synchronized. (A) of the figure shows a case where the duty ratios of the chopper and the inverter are approximately the same, and (b) and (c) show cases where the duty ratios of the chopper and the inverter are different. Thus, when the chopper and the carrier of the inverter are synchronized, the current output from the chopper is stored in the smoothing capacitor and then flows to the inverter.

  As shown in FIGS. 5 to 7, in the DC-AC power converter shown in FIG. 4, a large charging current flows from the chopper 2 to the smoothing capacitor 3 at a high frequency, and a large discharging current flows from the smoothing capacitor 3 to the inverter 4. Flows. In order to absorb this charging / discharging current (ripple current) with certainty, the smoothing capacitor 3 needs to have a large capacity. Thereby, the cost and volume of the smoothing capacitor 3 are increased in the apparatus, and the duty for a large charge / discharge current at a high frequency is increased, so that the life of the smoothing capacitor is shortened.

  As a countermeasure, in Patent Document 1, the chopper PWM signal and the inverter complementary PWM signal are synchronized, and the U phase and the V phase of the inverter circuit are driven by complementary PWM, thereby smoothing the ON period of the chopper switching element. The current (ripple current) flowing from the capacitor to the U phase or V phase of the inverter circuit is reduced.

  On the other hand, in Patent Document 2, the on / off control frequency of the converter (boost chopper) is matched with the PWM control frequency of the inverter circuit, and the switching element of the converter is turned off during the period when the switching element of the inverter circuit is on. This reduces the ripple current flowing into the smoothing capacitor.

  However, in the methods of Patent Documents 1 and 2, the ON / OFF period of the switching element for chopper and the ON / OFF period of the U to W phase of the inverter circuit are mutually restricted, and the input / output current of the smoothing capacitor depends on the operation mode. Reduction may be difficult. Further, it is assumed that the required power conversion performance cannot be obtained due to restrictions on the ON / OFF period.

  An object of the present invention is to provide a DC-AC power conversion capable of reducing the input / output current of a smoothing capacitor without being mutually restricted by the ON / OFF period of the switching element of the chopper and the ON / OFF period of each phase switching element of the inverter. To provide an apparatus.

  In order to solve the above-mentioned problem, the present invention synchronizes the frequencies of both carrier signals for PWM control of the chopper and the inverter, and controls the phase difference between the two carrier signals to 90 degrees. It is characterized by the configuration of

(1) A chopper that boosts the voltage of a DC power source to a specified voltage by PWM control, a smoothing capacitor that is charged by a DC output of the chopper, PWM power using the output power of the chopper and the charging power of the smoothing capacitor as a DC power source A DC-AC power conversion device including an inverter that converts power into predetermined AC power by control and supplies power to a load;
The controller that controls the chopper and the inverter synchronizes the frequency of the carrier signal for PWM control of the chopper and the frequency of the carrier signal for PWM control of the inverter, and controls the phase difference of both carrier signals to 90 degrees. Carrier synchronization control means is provided.

  (2) The carrier synchronization control means reads the carrier wave data of the chopper and the inverter, checks whether or not the phase difference between the two carrier waves is equal to or less than a predetermined value, and ends the phase control if it is equal to or less than the predetermined value. If it exceeds the default value, it checks whether the carrier wave of the inverter is ahead or behind the chopper carrier wave. If it is on the advance side, the frequency of the inverter carrier wave is reduced. Means is provided for periodically performing processing for increasing the frequency of the carrier wave of the inverter.

  As described above, according to the present invention, the frequencies of both carrier signals for PWM control of the chopper and the inverter are synchronized and the phase difference between the two carrier signals is controlled to 90 degrees. Therefore, it is possible to reduce the input / output current of the smoothing capacitor, thereby reducing the size of the smoothing capacitor or extending the service life by reducing the duty.

The apparatus block diagram of the DC-AC power converter device which shows embodiment of this invention. The flowchart of the phase control by the carrier synchronous control part 6G. The waveform diagram of each part voltage and current in the embodiment. The apparatus block diagram of the conventional DC-AC power converter. The waveform diagram of each part voltage and current of a conventional chopper. The waveform diagram of each part voltage and current of a conventional inverter. The conventional waveform diagram of each part voltage and current.

  FIG. 1 is a device configuration diagram of a DC-AC power converter showing an embodiment of the present invention. The main circuit configuration of the figure is the same as that of FIG.

  In FIG. 1, a controller (control device) 6 implements all hardware configurations or computations by software processing using a microprocessor, performs PWM control of the chopper 2 and the three-phase inverter 4 respectively, and performs both PWM control. The frequency of the carrier signal is synchronized, and the phase difference between both carrier signals is controlled to 90 degrees.

  Among the controllers 6, the chopper 2 is PWM-controlled by the chopper controller 6A, the chopper PWM controller 6B, and the chopper carrier generator 6C. The chopper controller 6A sets the voltage command value of the smoothing capacitor 3, and performs a proportional integral (PI) calculation of the deviation between this voltage command value and the detected voltage of the smoothing capacitor 3 (detector not shown), and the calculation result Is a chopper voltage command value (chopper reference value: Chopper_ref) of the chopper PWM controller 6B. The chopper PWM control unit 6B determines the ON / OFF (chopper_SW) ratio of the semiconductor switch CHP of the chopper 2 by comparing the chopper voltage command value with the chopper carrier wave (Chopper_carrier: triangular wave) generated by the chopper carrier generation unit 6C. The semiconductor switch CHP is controlled with this ON / OFF ratio.

  Among the controllers 6, the inverter control unit 6D, the inverter PWM control unit 6E, and the inverter carrier generation unit 6F perform PWM control on the output current of the inverter 4. In the inverter control unit 6D, for example, the output current command value of the inverter 4 is set based on the current generated power that can be output from the DC power source 1, and this current command value and the detected output current of the inverter 4 (the detector is not shown). Is calculated as the output voltage command value of the inverter PWM control unit 6E. The inverter PWM control unit 6E controls the amplitude of the sine wave signal matched with the frequency / phase of the interconnection system with the output voltage command value, and the controlled sine wave signal (inverter reference value: U / V / W_ref) The ON / OFF signal (U / V / W / X / Y / Z_SW) of each phase switch U to Z of the inverter 4 is determined by comparing with the inverter carrier wave (Inverter_carry: triangle wave) generated by the inverter carrier generation unit 6F. The phase switches U to Z are controlled by the ON / OFF signal.

  Of the controller 6, the carrier synchronization control unit 6G synchronizes the inverter carrier wave generated by the inverter carrier generation unit 6F with the same frequency as the chopper carrier wave generated by the chopper carrier generation unit 6C, and The phase difference is controlled to 90 degrees.

  FIG. 2 shows a flowchart of phase control by the carrier synchronization control unit 6G. The carrier synchronization control unit 6G reads the carrier wave data of the carrier generation units 6C and 6F of the chopper and the inverter (S1), and determines whether or not the phase difference between the two carrier waves is equal to or less than a predetermined value (for example, 90 ° ± 5 °). Check (S2), if it is less than the predetermined value, the phase control is terminated, and if it exceeds the predetermined value, it is checked whether the carrier wave of the inverter is ahead or behind the chopper carrier wave (S3), When it is on the advance side, the frequency of the carrier wave of the inverter is decreased (S4), and when it is on the delay side, the frequency of the carrier wave of the inverter is increased (S5). The carrier synchronization control unit 6G periodically performs the series of processes S1 to S5, synchronizes the inverter carrier wave with the same frequency as the chopper carrier wave, and controls the phase difference between the two carrier waves to 90 degrees. .

  FIG. 3 shows waveforms of voltages and currents when the controller 6 performs PWM control of the chopper 2 and the inverter 4. (A) of the figure shows a case where the duty ratios of the chopper and the inverter are approximately the same, and (b) and (c) show cases where the duty ratios of the chopper and the inverter are different.

  In FIG. 3, when the current flows through the chopper 2 and the inverter 4, the current is output at the peak of the carrier wave in the chopper, and the current is output at 90 and −90 degrees with respect to the peak of the carrier wave in the inverter. Will be input. Therefore, by shifting the carrier of the chopper and the inverter by 90 degrees, it becomes possible to cause one of the inverter currents flowing twice per one carrier cycle to flow to the inverter 4 without passing through the smoothing capacitor 3, which is shown in FIG. As compared with the above, the current flowing into the smoothing capacitor 3 can be reduced. In other words, the current flowing from the smoothing capacitor 3 to the inverter 4 can be reduced.

  Therefore, the DC-AC power conversion according to the present embodiment eliminates the disadvantages caused by the restrictions on the ON / OFF period of the conventional chopper and inverter, and controls the phase of the carrier signal in the chopper and inverter PWM control to 90 degrees. As a result, the input / output current of the smoothing capacitor can be reduced, and along with this, the smoothing capacitor can be expected to be downsized or have a longer life due to reduced duties.

  In addition, although embodiment shows the case where it applies to the system which supplies the alternating current power converted from the DC power source used for a solar power generation device etc. to a connection system, it applies to the system which supplies electric power loads, such as a motor Equivalent effects can be obtained.

1 DC power supply 2 Chopper 3 Smoothing capacitor 4 Three-phase inverter 5 Interconnection connection circuit 6 Controller (control device)
6A Chopper control unit 6B Chopper PWM control unit 6C Chopper carrier generation unit 6D Inverter control unit 6E Inverter PWM control unit 6F Inverter carrier generation unit 6G Carrier synchronization control unit

Claims (2)

A chopper that boosts the voltage of the DC power supply to a specified voltage by PWM control, a smoothing capacitor that is charged by the DC output of this chopper, and a predetermined power supply by PWM control using the output power of the chopper and the charging power of the smoothing capacitor as DC power. A DC-AC power converter comprising an inverter that converts the AC power into a load and supplies power to the load,
The controller that controls the chopper and the inverter synchronizes the frequency of the carrier signal for PWM control of the chopper and the frequency of the carrier signal for PWM control of the inverter, and controls the phase difference of both carrier signals to 90 degrees. DC-AC power converter characterized by comprising carrier synchronization control means for performing   The carrier synchronization control means reads the carrier wave data of the chopper and the inverter, checks whether or not the phase difference between the two carrier waves is equal to or less than a predetermined value, and if not equal to the predetermined value, terminates the phase control and sets the default value. If it exceeds, check whether the carrier wave of the inverter is on the leading side or the lag side of the carrier wave of the chopper.If it is on the leading side, the frequency of the carrier wave of the inverter is reduced. The DC-AC power converter according to claim 1, further comprising means for periodically performing a process of increasing the frequency of the wave.

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