JP2017093039A - Power converter and control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power converter in which increase in the ripple current is suppressed, even when switching is made from three-level operation to two-level operation, and thereby enlargement of a reactor is suppressed or not required, and to provide a control method.SOLUTION: A power converter 1 is constituted of a switching circuit 10 and a controller 15, a DC voltage source 2 is connected to the DC input side via power supply side capacitors 13, 14, and connected with a single phase AC power system 3 via an AC reactor 11, and a filter capacitor 12 on the AC output side. One arm of the switching circuit 10 is constituted of bidirectional switches SW2, SW3 connecting the switching elements in series in the inverse direction. The controller 15 includes a PWM pulse generation circuit 22 for making the switching circuit perform three-level operation and two-level operation, and two-level operation is performed when the absolute value of a voltage command value is larger than a predetermined voltage command threshold, and an effective value IAC of the AC output current is larger than a predetermined threshold.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a power converter and a control method for converting DC power into AC power.

  A power conversion device that converts DC power into AC power is used in many fields of power electronics. In general, a power converter performs an inverter operation using a switching circuit composed of switching elements in order to convert DC power input from DC wiring into AC power and output the AC power to the AC wiring. . The power conversion device performs power conversion by PWM (Pulse Width Modulation) operation by switching the switching element in the switching circuit with a pulse signal. Many power converters are often used by connecting a device having inductance, such as a reactor for filtering or the like, or a transformer to the AC side.

  The switching circuit of the power conversion device is mainly composed of a two-level inverter circuit capable of PWM operation (hereinafter abbreviated as two-level operation) using two potentials. The feature of this two-level switching circuit is that the circuit configuration is the simplest.

  On the other hand, a three-level inverter circuit capable of performing PWM operation (hereinafter referred to as three-level operation) using three potentials is often used. The configuration of the three-level inverter circuit is more complicated than the configuration of the two-level inverter circuit, but has an advantage that the harmonic current and the ripple current can be reduced by the three-level operation.

  FIG. 14 shows an example of a typical three-level inverter circuit. The switching circuit 10 includes three arms: a first arm configured by the switching element SW1, a second arm configured by the switching elements SW2 and SW3, and a third arm configured by the switching element SW4. ing.

  The DC voltage source 2 connected to the switching circuit 10 is composed of two DC voltage sources 91 and 92 connected in series, and provides a DC voltage Edc. The DC voltage source 2 composed of two DC voltage sources 91 and 92 connected in series has three stages of (+ Edc / 2) at the node p and (−Edc / 2) at the node n with the node m as a reference. A voltage is available. In addition, the second arm of the switching circuit 10 is connected to the node m, the first arm of the switching circuit 10 is connected to the node p, and the third arm of the switching circuit 10 is connected to the node n.

  On the AC power system 3 side connected to the switching circuit 10, the wiring AC <b> 1 is connected to the common connection terminals of the three arms of the switching circuit 10, and the wiring AC <b> 2 is connected to the node m of the DC voltage source 2. A reactor 11 is disposed on the wiring AC1, and a filter capacitor 12 is disposed between the wiring AC1 and the wiring AC2.

  By performing the switching operation by three levels by the switching circuit 10 having such a configuration, an alternating voltage and an alternating current are generated and converted in the wiring AC1 and the wiring AC2 by the effect of the current smoothing of the reactor 11 and the filter capacitor 12. The AC power is supplied to the AC power system 3.

  However, when the switching element SW is composed of a field effect transistor (MOSFET), the second arm connected to the node m needs to connect the switching elements SW2 and SW3 in series in order to prevent reverse conduction. When a current is passed through the switching elements SW2 and SW3, approximately twice the conduction loss occurs as compared with the case of passing through the switching elements SW1 and SW4 of the other arms. Since the conduction loss is proportional to the square of the flowing current amount, the conduction loss increase due to the current flowing through the switching elements SW2 and SW3 becomes more remarkable as the current is larger.

  In order to solve this problem, in Patent Document 1, when a current is large, a two-level voltage at nodes p and n is output from a three-level operation that outputs a three-level voltage at nodes p, m, and n. A method of switching to the two-level operation is proposed. By switching to the two-level operation, the switching elements SW2 and SW3 can be turned off so that no current flows through the switching elements SW2 and SW3. doing. Thereby, the loss in the switching elements SW2 and SW3 when the current is large can be reduced, and the power conversion efficiency in the inverter can be increased.

JP 2014-107931 A

  According to Patent Document 1, it has succeeded in suppressing an increase in conduction loss due to a current flowing through the switching elements SW2 and SW3. However, when the two-level operation is performed, another problem that the amplitude of the ripple current included in the alternating current output through the filter reactor 11 increases. Since ripple current is a cause of radiation noise and heat generation, it is often necessary to limit the ripple current to a certain amount in design. Even when switching to the two-level operation, it is necessary to increase the inductance of the filter reactor 11 in order to make it equal to or less than the ripple amount generated only by the three-level operation, thereby increasing the size of the reactor. It will be.

  As described above, the present invention provides a power conversion device and a control method in which an increase in ripple current is suppressed even when switching from a three-level operation to a two-level operation, and an increase in the size of a reactor can be suppressed or not increased. To do.

  As described above, in the present invention, “a switching circuit composed of a plurality of switching elements, a DC voltage source connected to the input side, and an output side connected to the AC power system, and a control circuit for controlling the switching circuit” And a control circuit for converting DC power of a DC voltage source into single-phase AC power of an AC power system, wherein the switching circuit outputs three levels of voltage composed of three arms. The switching element is composed of a unidirectional switch in which a transistor element and a diode element are connected in antiparallel, and at least one of the three arms has a switching element in the reverse direction. The control circuit is composed of bidirectional switches configured in series connection, and the control circuit has a three-level operation and 2 A PWM pulse generation circuit for performing a bell operation is provided, and the PWM pulse generation circuit refers to a voltage command value for controlling an output AC voltage and outputs a gate pulse for controlling ON / OFF to a plurality of transistor elements. When the absolute value of the voltage command value is larger than a predetermined voltage command threshold value and the effective value of the AC output current on the output side of the switching circuit is larger than the predetermined threshold value, the switching circuit The power conversion device is characterized in that it performs a two-level operation and otherwise performs a three-level operation. "

  Further, in the present invention, “a power conversion method for converting DC power of a DC voltage source into single-phase AC power of the AC power system by a switching circuit having a plurality of switching elements, wherein the switching circuit includes a DC voltage source 2 level operation that gives either positive or negative potential and 3 level operation that gives any one of positive, negative, and intermediate potentials of the DC voltage source can be implemented, When the absolute value of the output voltage is larger than a predetermined voltage threshold value and the effective value of the AC output current is larger than the predetermined current threshold value, the two-level operation is performed. A power conversion method characterized by performing level operation. "

  ADVANTAGE OF THE INVENTION According to this invention, the loss of a power converter device can be reduced and power conversion efficiency can be improved. Moreover, the enlargement of the accompanying devices such as the reactor can be suppressed by suppressing the ripple current.

The figure which shows the structural example of the power converter device which concerns on the Example of this invention. The figure which shows the circuit structural example of the PWM pulse generator 22. FIG. The figure which showed the waveform of triangular wave Tri2 which changes between +1 and -1 among the triangular waves which the triangular wave generation circuit 31 generate | occur | produces. The figure which showed the waveform of the triangular waves Tri3a and Tri3b which change between 0 and +1 and 0 and -1 among the triangular waves which the triangular wave generation circuit 31 generate | occur | produces. The figure which shows the operation state of the PWM pulse generator 22 in this invention. The figure which shows the circuit structural example of the operation mode switching determination device. The figure which shows the determination logic of the operation mode switching determination device 23. The figure which shows each part signal waveform in the PWM pulse generator 22 at the time of performing 2 level operation | movement. The figure which shows each part signal waveform in the PWM pulse generator 22 at the time of performing 3 level operation | movement. The figure which showed an example of the voltage command value U in case the effective value Irms of alternating current is larger than the threshold voltage Ith, and the output voltage Vx of the switching circuit 10. FIG. The figure which showed the example of the waveform of a general ripple current. The figure which shows the graph of the ripple current amplitude with respect to the voltage command value U drawn based on (1) Formula and (2) Formula. The figure which shows the graph showing the maximum ripple current amplitude with respect to the voltage command threshold value Uth. The figure which showed the example which applied the Example of this invention to the solar energy power generation system. The figure which showed another structural example of the control apparatus 15 used for the Example of this invention. The figure which showed an example of the circuit of the conventional 3 level inverter.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a configuration example of a power conversion apparatus according to an embodiment of the present invention.

  The power conversion device 1 is roughly composed of a switching circuit 10 and a control device 15, and a DC voltage source 2 is connected to a DC input side of the switching circuit 10 via power supply side capacitors 13 and 14. The AC power system 3 is connected to the AC output side of the switching circuit 10 via an AC reactor 11 and a filter capacitor 12. With such a configuration, the power converter 1 receives DC power generated by the DC power supply 2 having the voltage Edc, converts the DC power into single-phase AC power, and supplies the single-phase AC power system 3 with power.

  The power conversion device 1 converts the DC power input through the DC power wirings P and N into AC power by PWM control of the switching elements SW1, SW2, SW3, and SW4 in the switching circuit 10, and the AC reactor 11 , And output from the filter capacitor 12 through the AC power wirings AC1 and AC2.

  The switching circuit 10 basically has the same configuration as the conventional three-level inverter circuit shown in FIG. Specifically, the switching circuit 10 includes a first arm configured by the switching element sw1, a second arm configured by the switching elements SW2 and SW3, and a third arm configured by the switching element SW4. It consists of 3 arms.

  The DC voltage source 2 of the DC voltage Edc connected to the switching circuit 10 is divided by the power supply side capacitors 13 and 14 connected in series and connected to the switching circuit 10. The power supply side capacitors 13 and 14 prepare three-level voltages of (+ Edc / 2) for the wiring P and (−Edc / 2) for the wiring N with the wiring M as a reference. In addition, the second arm of the switching circuit 10 is connected to the wiring M, the first arm of the switching circuit 10 is connected to the wiring P, and the third arm of the switching circuit 10 is connected to the wiring N.

  The switching elements SW1, SW2, SW3, and SW4 of the switching circuit 10 are configured by an anti-parallel connection circuit of a SiC-MOSFET that is a wide gap semiconductor and a diode. There are a case where a PN junction diode inherent in the SiC-MOSFET element is used as the diode and a case where a SiC-SBD (Schottky barrier diode) element is separately prepared, and the present invention may be any of them. .

  The switching element SW1 is connected between the wiring P and the connection node X, and the switching element SW4 is connected between the wiring N and the connection node X. The switching elements SW2 and SW3 are connected in opposite directions to form a bidirectional switch circuit, and both ends of the bidirectional switch circuit are connected to the node M and the connection node X. Capacitors 13 and 14 smooth the DC voltage of wirings P and N, and control the voltage between wirings PM and wiring PN to be Edc / 2 by a three-level operation. The node M is also connected to the wiring AC2.

  Information on various sensors is input to the control device 15. A current sensor 16 is attached to the AC power wiring AC1, and a voltage sensor 17 is attached to both ends of the filter capacitor 12. The current value IAC and the voltage value VAC measured by the sensors are taken into the control device 15, respectively. Although not shown, a plurality of other similar sensors for observing voltages, currents, and temperatures at various places are attached, and the information is sent to the controller 15 and is not the subject of this specification. Used for various controls and protection.

  As described above, the power conversion device according to the present invention shown in FIG. 1 is composed of a plurality of switching elements, connected to the DC voltage source on the input side thereof, and connected to the AC power system via the reactor on the output side. By providing the switching circuit, the DC power of the DC voltage source is converted into single-phase AC power of the AC power system.

  The switching circuit is composed of a three-level inverter circuit that can output three levels of voltage composed of three arms, and the switching element is a unidirectional switch in which a transistor element and a diode element are connected in antiparallel. The at least one arm of the three arms is configured by a bidirectional switch configured by connecting switching elements in series in the reverse direction.

  The control device 15 includes a voltage command generator 21, a PWM pulse generator 22, an operation mode switching determination unit 23, a current effective value measurement unit 24, a gate driver 18, and the like.

  Among these, the voltage command generator 21 supplies an appropriate voltage command value U to the PWM pulse generator 22 in order to cause the switching circuit 10 to perform power stabilization control to the AC power system 3. Moreover, the current effective value measuring device 24 calculates the effective value (RMS value) of the current from the value of the alternating current IAC, and outputs the effective current value Irms. The voltage command value U has a sinusoidal waveform corresponding to the AC voltage in the AC power system 3 to which the power converter 1 is connected, as will be described later with reference to FIG.

  The operation mode switching determination unit 23 receives the voltage command value U and the current effective value Irms, and outputs a mode switching signal MODE of the two-level operation and the three-level operation. A circuit configuration example of the operation mode switching determiner 23 will be described later with reference to FIG. 5 and a determination logic of the operation mode switching determiner 23 will be described later with reference to FIG.

  The PWM pulse generator 22 receives the voltage command value U from the voltage command generator 21 and the mode switching signal MODE from the operation mode switching determination unit 23, and receives pulse signals PLS1, PLS2, PLS3 having a carrier frequency fc, PLS4 is generated. The carrier frequency fc is a frequency sufficiently higher than the fundamental frequency f0 of the AC power system 3. For example, in Japan, the fundamental frequency f0 is 50 Hz or 60 Hz, and the carrier frequency fc of a power conversion device of several hundred Hz or higher is sufficiently higher than the fundamental frequency f0. A circuit configuration example of the PWM pulse generator 22 will be described later with reference to FIG. 2 and an operation mode of the PWM pulse generator 22 with reference to FIG.

  The gate driver 18 receives the pulse signals PLS1, PLS2, PLS3, and PLS4, amplifies the current and voltage, and supplies them to the switching elements SW1, SW2, SW3, and SW4, respectively, so that the pulse signals PLS1, PLS2, PLS3, and PLS4 are supplied. The switching elements SW1, SW2, SW3, SW4 are controlled to be turned on / off in accordance with the logical values of

  FIG. 2 shows a diagram of a circuit configuration example of the PWM pulse generator 22. The PWM pulse generator 22 includes a triangular wave generation circuit 31, selectors 32 and 33, comparators 34 and 35, NOT gates 36 and 37, and AND gates 38 and 39.

  Among these, the triangular wave generation circuit 31 outputs three types of triangular wave signals Tri2, Tri3a, Tri3b. Of the three types of triangular wave signals Tri2, Tri3a, Tri3b, the waveform of the triangular wave signal Tri2 is shown in FIG. 3a, and the waveforms of the triangular wave signals Tri3a, Tri3b are shown in FIG. 3b. As shown in FIGS. 3a and 3b, the three types of triangular wave signals Tri2, Tri3a, Tri3b have the same frequency fc but different sizes. The triangular wave signal Tri2 changes between +1 and −1, but the magnitudes of the triangular wave signals Tri3a and Tri3b are ½ of the triangular wave signal Tri2. The triangular wave signal Tri3a changes between +1 and 0, whereas the triangular wave signal Tri3b changes between 0 and -1. That is, the three waveforms are triangular waves with a period of 1 / fc and the same phase, Tri2 is offset = 0, amplitude = 2, Tri3a is offset = + 0.5, amplitude = 1, and Tri3b is offset −0.5. , A triangular wave with amplitude = 1.

  The selectors 32 and 33 select the triangular wave signals Tri2, Tri3a and Tri3b by the mode switching signal MODE from the operation mode switching determination unit 23, and when the MODE = 0 indicating the two-level operation, the triangular wave signal Tri2 is operated by the three-level operation. When MODE = 1 is selected, triangular wave signals Tri3a and Tri3b are selected.

  The mode switching signal MODE is also given to the AND gates 38 and 39 in FIG. When MODE = 0 for instructing the two-level operation, the AND gates 38 and 39 are given a signal of logic level “0”, so that the outputs of the pulse signals PLS2 and PLS3 are continuously “0”, and the result PWM Only pulse signals PLS1 and PLS4 are given as time-varying signals from the pulse generator 22, and a two-level operation is performed by the switching elements SW1 and SW4. Similarly, when MODE = 1 instructing the three-level operation, the AND gates 38 and 39 are continuously supplied with the logic level “1” signal, so that the AND gates 38 and 39 output the other input. As a result, all of the pulse signals PLS1, PLS2, PLS3, and PLS4 are given as time-varying signals from the PWM pulse generator 22, and a three-level operation using all of the switching elements SW1, SW2, SW3, and SW4 is performed. Will be done.

  The comparators 34 and 35 compare the input voltage command value U with the triangular wave signals Tri2, Tri3a and Tri3b selected by the selectors 32 and 33, and output the comparison result of the magnitude as a logical value. The outputs of the comparators 34 and 35 are supplied to the switching elements SW1 and SW4 as pulse signals PLS1 and PLS4, respectively. Further, the results obtained by calculating the outputs of the comparators 34 and 35 by the NOT gates 36 and 37 and the AND gates 38 and 39 are supplied to the switching elements SW2 and SW3 as pulse signals PLS2 and PLS3. The range of the value of the voltage command value U input to the comparators 34 and 35 is a real number in the range of −1 to +1. When U = 1, the voltage value = Edc / 2 and when U = −1. Corresponds to a voltage value = −Edc / 2.

  FIG. 7A is a diagram showing signal waveforms at various parts in the PWM pulse generator 22 when performing a two-level operation, and FIG. 7B is a diagram showing signal waveforms at various parts in the PWM pulse generator 22 when performing a three-level operation. is there.

  As shown in FIG. 7a, when the two-level operation is performed, as a result of the comparison of the magnitude of the triangular wave Tri2 that changes between +1 and −1 and the voltage command value U that changes between +1 and −1, the pulse signal PLS1 A level “1” is given during the period of the command value U ≧ triangular wave Tri2. The other pulse signal PLS4 gives level “0” during the period of voltage command value U ≧ triangular wave Tri2. These pulse signals PLS1 and PLS4 are inverted signals of each other, and the pulse signals PLS2 and PLS3 are blocked by the AND gates 38 and 39 and set to the continuous “0” level.

  As shown in FIG. 7b, when performing the three-level operation, the triangular wave Tri3a that changes between 0 + 1, the triangular wave Tri3b that changes between 0 and −1, and the voltage command value U that changes between +1 and −1. As a result of the size comparison, the pulse signals PLS1 and PLS4 give level “1” during the period of voltage command value U ≧ triangular wave Tri3a or triangular wave Tri3b. The other pulse signals PLS2 and PLS3 give level “0” during the period of voltage command value U ≧ triangular wave 3a or triangular wave Tri3b. These pulse signals PLS1 and PLS2 are mutually inverted signals, and the pulse signals PLS2 and PLS3 are mutually inverted signals.

  The pulse signals PLS1, PLS2, PLS3, and PLS4 are amplified by the gate driver 18 and supplied as the gate voltage of the SiC-MOSFET in the switching elements SW1, SW2, SW3, and SW4. As a result, when the pulse signals PLS1, PLS2, PLS3, and PLS4 are 0, the MOSFETs in the switching elements SW1, SW2, SW3, and SW4 are turned off. On the contrary, when the pulse signals PLS1, PLS2, PLS3, and PLS4 are 1, the switching elements SW1, The MOSFETs in SW2, SW3, and SW4 are related so as to be turned on.

  FIG. 4 shows a relationship table between the ON / OFF states of the SiC-MOSFET with respect to the mode switching signal MODE and the voltage command value U in the present invention. FIG. 4 is a summary of the relationship between FIGS. 7a and 7b. When MODE = 0 designating the two-level operation, the comparators 34 and 35 compare the voltage command value U with the triangular wave Tri2. As a result of the comparison, when the voltage command value U becomes larger than the triangular wave Tri2, only the switching element SW1 among the switching elements SW1, SW2, SW3, and SW4 is turned on. This period is shown as T1 in FIG. 7a. As a result of comparison, when the voltage command value U becomes smaller than the triangular wave Tri2, only the switching element SW4 is turned ON. This period is shown as T2 in FIG. 7a. Thereby, the switching circuit 10 performs a two-level operation according to the voltage command value U when MODE = 0.

  On the other hand, in the case of MODE = 1 designating the three-level operation, the comparators 34 and 35 compare the voltage command value U with the triangular waves Tri3a and Tri3b. As a result of the comparison, when the voltage command value U becomes larger than the triangular wave Tri3a, the switching element SW1 and the switching element SW3 are turned on among the switching elements SW1, SW2, SW3, and SW4. This period is shown as T3 in FIG. As a result of the comparison, when the voltage command value U becomes smaller than the triangular wave Tri3b, the switching element SW2 and the switching element SW4 are turned on. This period is shown as T4 in FIG. 7b. As a result of comparison, when the voltage command value U is smaller than the triangular wave Tri3a and the voltage command value U is larger than the triangular wave Tri3b, the switching element SW2 and the switching element SW3 are turned on. This period is shown as T5 in FIG. Thus, the switching circuit 10 performs a three-level operation according to the voltage command value U when MODE = 1.

  FIG. 5 shows a configuration diagram of the operation mode switching determination unit 23. The operation mode switching determination unit 23 includes an absolute value circuit 40, comparators 41 and 42, a NAND gate 43, and latch circuits 44 and 45.

  Of these, the comparator 41 compares the effective value Irms of the alternating current with the current threshold value Ith. The comparison result output from the comparator 41 is latched by the latch circuit 44 and updated every fundamental wave period 1 / f0 of the AC output current. On the other hand, the comparator 42 compares the absolute value | U | of the voltage command value U converted into the absolute value through the absolute value circuit 40 with the voltage command threshold value Uth. The comparison result output from the comparator 42 is logically calculated by the NAND gate 43 and then latched in the latch circuit 45 and updated every carrier cycle 1 / fc.

  The output of the latch circuit 45 becomes the mode switching signal MODE and is supplied to the PWM pulse generator 22 to determine the 2-level operation and the 3-level operation of the switching circuit 10.

  FIG. 6 shows a map of the operation mode of the switching circuit 10 with respect to the effective value Irms of the alternating current and the absolute value | U | of the voltage command value. When the effective value Irms of the alternating current is larger than the current threshold value Ith and when the absolute value | U | of the voltage command value is larger than the threshold value Uth, MODE = 0, and the switching circuit 10 performs the two-level inverter operation. Do. Otherwise, MODE = 1 and the switching circuit 10 performs a three-level inverter operation.

  FIG. 8 shows an example of the voltage command value U and the output voltage Vx of the switching circuit 10 when the effective value Irms of the alternating current is larger than the threshold voltage Ith. In FIG. 8, the time from time t0 to t5 represents the time (= 1 / f0) of one cycle of the voltage command value U corresponding to the AC voltage of the AC power system 3.

At time t0, voltage command value U is 0, voltage command value U then increases, and switching circuit 10 performs a three-level operation until U <Uth. After time t1, when U> Uth, the switching circuit 10 switches to the two-level operation. Thereafter, when the voltage command value U decreases past the peak and U <Uth after time t2, the switching circuit switches to the three-level operation. Thereafter, the voltage command value U decreases to a negative value, and when the time U <−Uth is reached, the switching circuit 10 switches to the two-level operation again. After that, when the voltage command value U rises past the bottom and U> −Uth after time t4, the switching circuit 10 switches to the three-level operation. As is apparent from this waveform, the AC output voltage controlled by the control circuit in the power converter of the present invention switches between the two-level operation and the three-level operation at least four times during the basic period.
The control circuit in the above-described power conversion device of the present invention includes a PWM pulse generation circuit for causing the switching circuit to perform a three-level operation and a two-level operation, and this PWM pulse generation circuit controls the output AC voltage. A gate pulse for controlling ON / OFF is supplied to a plurality of transistor elements with reference to a voltage command value for controlling the voltage command value so that the absolute value of the voltage command value is larger than a predetermined voltage command threshold value and the switching circuit When the effective value of the AC output current on the output side is larger than a predetermined threshold value, the switching circuit is controlled to perform a two-level operation, and otherwise it is controlled to perform a three-level operation.

  FIG. 9 shows an example of a general ripple current waveform. The upper waveform is an example of the output voltage Vx when the two-level inverter operates, and the lower waveform is a current waveform of the alternating current IAC at that time. The alternating current IAC can be expressed as a sum of an average current Iave controlled by the PWM operation and a ripple current having an amplitude I_ripple (pp value).

  In the two-level operation, the ripple current amplitude I_ripple can be expressed by I_ripple2 shown in the equation (1). Here, Edc is a DC voltage (= voltage between wirings PN), fc is a carrier frequency, L is an inductance of the reactor 11, and U is a voltage command value (−1 ≦ U ≦ + 1).

  On the other hand, in the three-level operation, the ripple current amplitude I_ripple can be expressed by I_ripple3 shown in the equation (2).

  FIG. 10 shows a graph of the ripple current amplitude with respect to the voltage command value U drawn based on the equations (1) and (2). The ripple current during the two-level operation is I_ripple2, and the ripple current during the three-level operation is I_ripple3. The ripple current amplitude I_ripple2 during the two-level operation is maximum when U = 0, and the ripple current amplitude I_ripple3 during the three-level operation is maximum when U = ± 0.5. Further, the maximum value of the ripple current amplitude I_ripple2 during the two-level operation is twice the maximum value of the ripple current amplitude I_ripple3 at the three-level operation.

  Here, the power conversion device of the present invention switches between the two-level operation and the three-level operation at the voltage command threshold value Uth. The power conversion device of the present invention operates in two levels when the voltage command value U is in the range of U <−Uth or U> + Uth, and the ripple current amplitude at that time is I_ripple2. On the other hand, when the voltage command value U is in the range of −Uth <U <+ Uth, the power conversion device of the present invention operates at three levels, and the ripple current amplitude at that time is I_ripple3. For this reason, in the power conversion device that switches between the two-level operation and the three-level operation according to the present invention, the ripple current shown by the dashed line D in the figure is generated within the range of the voltage command value in FIG. become.

  Further, when the two-level operation and the three-level operation are switched in the voltage command threshold value Uth of FIG. 10, the maximum ripple current amplitude with respect to the voltage command threshold value Uth has the relationship of the graph of FIG. Uth = 0 always corresponds to 2-level operation, and Uth = 1 always corresponds to 3-level operation. The maximum ripple current amplitude when Uth = 0 is twice the ripple current when Uth = 1, but gradually decreases by increasing Uth, and Uth = 1 / √2 (≈0.7). , The maximum ripple current amplitude is the same as when Uth = 1, and Uth is constant in the range of 1 / √2 (≈0.7) <Uth <1.

  Therefore, when the voltage command threshold value Uth is set in the range of 1 / √2 <Uth <1, the maximum ripple current amplitude is the same as the maximum ripple current amplitude of the three-level inverter, so the inductance value L of the reactor 11 is There is no need to change and the same size reactor can be used. In particular, when the voltage command threshold value Uth is set to 1 / √2 (≈0.7), the time ratio of the two-level operation can be maximized, which is effective in reducing loss. Thus, the predetermined voltage command threshold value Uth is desirably about 0.7.

  Even when the voltage command threshold value Uth is set in the range of 0 <Uth <1 / √2, the required inductance value L is less than twice, so the required reactor size is suppressed. can do.

  FIG. 12 shows an example in which the power conversion device according to the embodiment of the present invention is applied to a solar power generation system. DC power is supplied to the power converter 1 by connecting the DC wirings P and N to the solar cell module array 52. Further, the AC wirings AC1 and AC2 are connected to the step-up transformer 54, and the output power of the power conversion device 1 is converted into high-voltage AC power and supplied to the AC power system 53.

  Since switching elements SW1, SW2, SW3, and SW4 use SiC-MOSFET and SiC-SBD, which are wide gap semiconductors, a high voltage close to 1 kV generated by the solar module array can be input. Switching loss of the switching element can be reduced.

  FIG. 13 shows another configuration example of the control device 15 used in the embodiment of the present invention. 13 includes a voltage command generator 21, a PWM pulse generator 22, an operation mode switching determination unit 23, a current effective value measurement unit 24, and a voltage normalization circuit 65. The configuration of the control device 15 in FIG. 1 is different only in that a voltage normalization circuit 65 is provided.

  First, the voltage command generator 21 supplies an appropriate voltage command value U to the PWM pulse generator 22 in order to cause the switching circuit 10 to perform power stabilization control for the AC power system 3.

  The current effective value measuring device 24 calculates the RMS value of the current from the value of the current measured value IAC and outputs the current effective value Irms.

  The voltage normalization circuit 65 normalizes the voltage measurement value VAC by dividing it by the DC voltage Vdc, and supplies the normalized voltage value U ′ to the operation mode switching determination unit. The normalized voltage value U ′ becomes +1 when VAC = (1/2) Vdc, and becomes −1 when VAC = (− ½) Vdc. Note that U ′ is 0.7 when VAC = 0.35 · Vdc.

  The operation mode switching determination unit 23 receives the voltage measurement value VAC and the current effective value Irms, and outputs a mode switching signal MODE for two-level operation and three-level operation.

  The PWM pulse generator receives the voltage command value U and the mode switching signal MODE, and generates pulse signals PLS1, PLS2, PLS3, and PLS4 having a carrier frequency fc.

  Since the voltage value of the AC output is generally a value close to the voltage command value, the measured value VAC of the output voltage can be used instead of the voltage command U.

SW1, SW2, SW3, SW4: switching element P, N: DC wiring AC1, AC2: AC wiring IAC: AC current measurement value VAC: AC voltage measurement value Edc: DC power supply voltage 1: power converter 2: DC power supply 3: AC power system 10: switching circuit 11: reactor 12: filter capacitor 13, 14: capacitor 15: control device 21: voltage command generator 22: PWM pulse generator 23: operation mode switching determination unit 24: current effective value measuring unit 31 : Triangular wave generation circuit 32, 33: Selector 34, 35: Comparator 36, 37: NOT gate 38, 39: AND gate 40: Absolute value circuit 41, 42: Comparator 43: NAND gate 44, 45: Latch circuit 51: Power conversion Device 52: Solar cell module array 53: AC power system 54: Step-up transformer 6 : Voltage normalizing circuit 91, 92: DC voltage source

Claims (7)

The DC voltage source comprises a switching circuit composed of a plurality of switching elements, a DC voltage source connected to the input side thereof, and an output side connected to an AC power system, and a control circuit for controlling the switching circuit. A power converter that converts the direct current power into single-phase alternating current power of the alternating current power system,
The switching circuit is composed of a three-level inverter circuit that can output three levels of voltage composed of three arms, and the switching element is a unidirectional switch in which a transistor element and a diode element are connected in antiparallel. And at least one of the three arms is composed of a bidirectional switch configured by connecting switching elements in series in the reverse direction,
The control circuit includes a PWM pulse generation circuit for causing the switching circuit to perform a three-level operation and a two-level operation,
The PWM pulse generation circuit refers to a voltage command value for controlling the output AC voltage, supplies a gate pulse for controlling ON / OFF to the plurality of transistor elements, and an absolute value of the voltage command value is predetermined. If the effective value of the AC output current on the output side of the switching circuit is greater than a predetermined threshold value, the switching circuit performs a two-level operation, otherwise A power conversion device that performs a three-level operation.   2. The power converter according to claim 1, wherein the predetermined voltage command threshold is about 0.7.   2. The power conversion device according to claim 1, wherein the two-level operation and the three-level operation are switched at least four times during a basic period of the AC output voltage.   2. The power conversion device according to claim 1, wherein the switching element is made of a MOSFET.   2. The power converter according to claim 1, wherein the switching element is formed of a wide gap semiconductor. A switching circuit comprising a plurality of switching elements, a DC voltage source connected to an input side thereof, and an output side connected to an AC power system is provided, and the DC power of the DC voltage source is converted to a single-phase AC power of the AC power system. A power conversion device for converting to
The switching circuit includes a first switching element having one end connected to a common terminal and the other end connected to a positive potential terminal of the DC voltage source, one end connected to the common terminal, and the other end connected to the DC voltage source. A second switching element connected to the negative potential terminal, a third end connected to the common terminal, one end connected to the intermediate potential terminal of the DC voltage source, and a third connected in series in opposite directions. It is composed of a switching element and a fourth switching element, and the intermediate potential terminal of the common terminal and the DC voltage source is an AC terminal of the single-phase AC power system,
The first, second, third, and fourth switching elements connect diodes in antiparallel,
When the absolute value of the AC output voltage of the AC power system is greater than a predetermined voltage threshold value and the effective value of the AC output current is greater than a predetermined current threshold value, A two-level operation is performed to control conduction of the first and fourth switching elements by making the third switching element non-conducting. Otherwise, the first, second, third, and fourth switching elements A power converter characterized by performing a three-level operation for controlling conduction. A control method for a power converter that converts DC power of a DC voltage source into single-phase AC power of the AC power system by a switching circuit having a plurality of switching elements,
The switching circuit performs both a two-level operation that gives either a positive or negative potential of the DC voltage source and a three-level operation that gives a positive, negative, or intermediate potential of the DC voltage source. Is possible,
When the absolute value of the AC output voltage of the AC power system is greater than a predetermined voltage threshold and the effective value of the AC output current is greater than a predetermined current threshold, the two-level operation is performed. In other cases, the control method of the power conversion device, wherein the three-level operation is performed.

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