Inverter, parallel inverter system, and inverter control method

Abstract

The present invention is an inverter that is connected in parallel to an AC circuit to which an AC power source is connected, the inverter providing an independent output, wherein the inverter comprises: a power conversion unit that converts DC power to AC power and supplies the AC power to the AC circuit; a voltage sensor that detects the AC voltage of the AC circuit; and a control unit that controls the power conversion unit. The control unit adds a frequency feedback process for applying a frequency correction value to an operation for calculating a frequency detection value in a phase synchronization loop that is executed on the basis of the AC voltage, the frequency correction value being obtained by performing proportional integration on the result of comparing the previous value of the frequency detection value with a frequency reference value.

Description

[Technical Field]

[0001] The present disclosure relates to an inverter, a parallel inverter system, and an inverter control method. This application claims priority to Japanese Patent Application No. 2021-33734 filed on March 3, 2021, and incorporates by reference all of the contents of said Japanese application. [Background technology]

[0002] In recent years, power generation devices based on renewable energy, such as solar power generation, have become popular among power companies and small-scale consumers. Such power generation devices are interconnected with a commercial power grid and operated in parallel. Furthermore, in order to increase private power generation capacity without relying on the commercial power grid, it has been considered to supply AC power to a load by connecting independent outputs of inverters (power conversion devices) in parallel (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-229701 Summary of the Invention

[0004] The present disclosure includes the following inventions, which are defined by the claims.

[0005] (inverter) The inverter of the present disclosure is an inverter that is connected in parallel to an AC power line to which an AC power source is connected and provides an independent output, a power conversion unit that converts DC power into AC power and supplies the power to the AC power circuit; a voltage sensor for detecting an AC voltage of the AC current circuit; a control unit that controls the power conversion unit, The control unit is an inverter that adds frequency feedback processing to a calculation for calculating a frequency detection value in a phase-locked loop that is executed based on the AC voltage, in which a frequency correction value obtained by applying proportional integration to a comparison result between the previous value of the frequency detection value and a frequency reference value is added.

[0006] (Parallel inverter system) The present disclosure also provides a power supply including: a first inverter that provides an isolated output to an AC power line; a second inverter connected to the AC power line in parallel with the first inverter and providing an independent output to the AC power line, Each of the first inverter and the second inverter comprises: a power conversion unit that converts DC power into AC power and supplies the power to the AC power circuit; a voltage sensor for detecting an AC voltage of the AC current circuit; a control unit that controls the power conversion unit, The control unit is a parallel inverter system that adds frequency feedback processing to a calculation for calculating a frequency detection value in a phase-locked loop that is executed based on the AC voltage, in which a frequency correction value obtained by applying proportional integration to a comparison result between the previous value of the frequency detection value and a frequency reference value is added.

[0007] (Inverter control method) From the viewpoint of the method, there is provided a control method for an inverter connected in parallel to an AC power line connected to an AC power source and providing an independent output, the method comprising: a power conversion unit converting DC power into AC power and supplying the power to the AC power circuit; In this inverter control method, a control unit that controls the power conversion unit adds frequency feedback processing to a calculation that calculates a frequency detection value in a phase-locked loop that is executed based on the AC voltage of the AC power line, and adds a frequency correction value obtained by applying proportional integration to a comparison result between the previous value of the frequency detection value and a frequency reference value. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a connection diagram showing an example of a parallel inverter system in which two inverters operating independently are connected in parallel to an AC power line. [Figure 2] FIG. 2 is a control block diagram relating to voltage control in the control unit. [Figure 3] FIG. 3 is a diagram in which the control block diagram of FIG. 2 is expressed as a flowchart. [Figure 4] FIG. 4 shows the simulation results when two inverters, the first and second inverters, are operated in parallel with independent output under the same control as in the case of grid-connected operation. [Figure 5] FIG. 5 shows the simulation results when two inverters, the first and second, are operated in parallel with independent output using a reference voltage signal with a fixed frequency of 60 Hz. [Figure 6] FIG. 6 shows the simulation results when control is performed based on the control block diagram of FIG. [Figure 7] FIG. 7 shows the simulation results when the parallel inverter system of FIG. 1 and a generator of a commercial power system are operated in parallel. [Figure 8] Figure 8(a) is a graph showing an example of the relationship between the inverter output current I and the phase difference φ when inverters having a phase-locked loop with frequency feedback shown in Figure 2 are operated in parallel. Figure 8(b) is a vector diagram showing the relationship between the equivalent induced electromotive force E, the phase difference φ, and the AC voltage V. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Problem to be solved by this disclosure] To operate multiple inverters in parallel to provide independent output, it is necessary to synchronize their phases, and for this purpose a phase-locked loop (PLL) is used. Phase-locked loops used for grid interconnection can synchronize the phase by setting vq, which is obtained by dq transformation (Park transformation) using AC voltage as the reference signal, to zero. However, it has been discovered that when a phase-locked loop is used for parallel-operated independent output inverters, the frequency does not converge to a constant value but continues to change, becoming uncontrollable over time.

[0010] In view of the above problem, an object of the present disclosure is to stabilize parallel operation of inverters that provide independent outputs to AC power lines.

[0011] [Effects of this disclosure] According to the present disclosure, it is possible to stabilize parallel operation of inverters that provide independent outputs to AC power lines.

[0012] [Description of the embodiments of the present disclosure] The gist of the present disclosure includes at least the following.

[0013] (1) This is an inverter that is connected in parallel to an AC circuit to which an AC power source is connected and provides an independent output, and includes a power conversion unit that converts DC power into AC power and supplies the power to the AC circuit, a voltage sensor that detects the AC voltage of the AC circuit, and a control unit that controls the power conversion unit, and the control unit adds frequency feedback processing to the calculation of a frequency detection value in a phase-locked loop that is executed based on the AC voltage, in which a frequency correction value obtained by applying proportional integration to the comparison result between the previous value of the frequency detection value and a frequency reference value is added.

[0014] In such inverters, the frequency of the independent output gradually approaches and converges to a target value, and therefore, when multiple such inverters are connected in parallel or when connected in parallel to another AC power source, stable parallel operation can be achieved.

[0015] (2) In the inverter of (1), when the frequency correction value is a second frequency correction value, the control unit executes a first calculation to obtain a first frequency correction value based on the deviation between the voltage phase based on the detection output of the voltage sensor and the internal phase in the phase-locked loop, a second calculation to obtain a new frequency detection value by adding the first frequency correction value and the second frequency correction value to the frequency reference value, and a third calculation to generate a control signal to control the output of the power conversion unit based on the new frequency detection value. In this case, in particular, by performing the second calculation, it is possible to prevent the frequency of the independent output from falling into a state where it does not converge.

[0016] (3) In the inverter of (2), the third calculation includes determining a voltage command value based on the new frequency detection value, and determining a voltage correction value by performing a proportional calculation on the deviation between the voltage command value and the AC voltage. In this case, by performing proportional calculation instead of proportional-integral calculation, cross current can be suppressed even if there is a difference in the value detected as AC voltage between the AC power source and another AC power source.

[0017] (4) In the inverter of (2) or (3), the internal phase has a phase difference with respect to the phase of the AC voltage that changes according to the output current of the power conversion unit. The phase difference is proportional to the output current, and as the output current increases, the phase difference also increases. This is similar to the relationship between the output current and the internal phase angle of a synchronous generator. In other words, an inverter with frequency feedback processing added to a phase-locked loop operates as a kind of virtual synchronous generator. Therefore, this inverter can also be operated in parallel with a synchronous generator.

[0018] (5) In any of the inverters (1) to (4), the power conversion unit can also convert AC power to DC power, and the control unit can also control the power conversion unit to convert the AC power of the AC power source into DC power and charge a DC voltage source connected to the DC side of the power conversion unit. When an AC power source (current source) that outputs a current synchronized with the voltage of the AC circuit, such as a grid-connected inverter powered by solar power generation, is connected to the AC circuit, the independent output inverter can convert (AC to DC) the surplus power consumed by the load and charge the DC voltage source (including a storage battery).

[0019] (6) Also disclosed is a parallel inverter system including a first inverter that provides an independent output to an AC electric circuit, and a second inverter that is connected to the AC electric circuit in parallel with the first inverter and provides an independent output to the AC electric circuit, wherein each of the first inverter and the second inverter includes a power conversion unit that converts DC power to AC power and supplies the power to the AC electric circuit, a voltage sensor that detects the AC voltage of the AC electric circuit, and a control unit that controls the power conversion unit, and the control unit adds a frequency feedback process to a calculation of a frequency detection value in a phase-locked loop that is executed based on the AC voltage, the calculation adding a frequency correction value obtained by performing proportional integration on a comparison result between a previous value of the frequency detection value and a frequency reference value.

[0020] In such a parallel inverter system, the frequency of the independent output of each inverter asymptotically converges to a target value, and therefore the parallel inverter system can perform stable parallel operation.

[0021] (7) From the viewpoint of a method, this is a control method for an inverter that is connected in parallel to an AC circuit to which an AC power source is connected and provides an independent output, wherein a power conversion unit converts DC power into AC power and supplies the power to the AC circuit, and a control unit that controls the power conversion unit adds frequency feedback processing to a calculation of a frequency detection value in a phase-locked loop that is executed based on the AC voltage of the AC circuit, in which the control unit adds a frequency correction value obtained by applying proportional integration to a comparison result between the previous value of the frequency detection value and a frequency reference value.

[0022] According to this inverter control method, the frequency of the independent output asymptotically converges to a target value, and therefore, when multiple inverters of this type are connected in parallel or when connected in parallel to another AC power source, stable parallel operation can be achieved.

[0023] [Details of the embodiments of the present disclosure] Hereinafter, an inverter, a parallel inverter system, and an inverter control method according to the present disclosure will be described with reference to the drawings.

[0024] Parallel inverter system 1 is a connection diagram showing an example of a parallel inverter system 100 in which two inverters 1 and 2 operating independently are connected in parallel to an AC power line 5. Note that although each of the inverters 1 and 2 can also be operated in grid-connected relation to a commercial power grid, this disclosure only shows the state during independent operation. The use of two inverters is an example of the minimum configuration of "multiple," and three or more inverters may also be used.

[0025] In Fig. 1, a DC voltage source 3 is connected to a first inverter 1. The DC voltage source 3 may be a power source itself, such as a solar panel or a storage battery, or may be a circuit in which a DC / DC converter is connected to a power source. Similarly, a DC voltage source 4 is connected to a second inverter 2. The first inverter 1 and the second inverter 2 are connected in parallel to an AC current circuit 5. A load (AC load) 6 is connected to the AC current circuit 5.

[0026] The first inverter 1 is composed of a control unit 1A and a power conversion unit 1B. The power conversion unit 1B includes a full-bridge circuit 10 connected to a DC voltage source 3, an AC reactor 15 provided on two wires on the AC side of the full-bridge circuit 10, an AC-side capacitor 16, a voltage sensor 17, a current sensor 18, and a voltage sensor 19, which are connected as shown in the figure.

[0027] The full-bridge circuit 10 is composed of switching elements 11, 12, 13, and 14 and diodes 11d, 12d, 13d, and 14d connected in antiparallel to the switching elements 11, 12, 13, and 14. The illustrated switching elements 11, 12, 13, and 14 are IGBTs (Insulated Gate Bipolar Transistors), but MOS-FETs (Metal-Oxide-Semiconductor Field Effect Transistors) may be used instead.

[0028] Full-bridge circuit 10 is connected to both ends of DC voltage source 3. Voltage sensor 17 detects the DC voltage input to full-bridge circuit 10 from DC voltage source 3 and sends the detected output to control unit 1A. Current sensor 18 detects the current flowing through AC reactor 15 and sends the detected output to control unit 1A. Voltage sensor 19 detects the AC voltage output to AC circuit 5 and sends the detected output to control unit 1A.

[0029] During stand-alone operation, the inverter 1 configured as described above converts DC power input from the DC voltage source 3 into voltage-controlled AC power using a full-bridge circuit 10 that is PWM (Pulse Width Modulation) controlled. High-frequency ripple contained in the AC waveform is suppressed by an AC reactor 15 and an AC-side capacitor 16. The AC output of the inverter 1 is provided to an AC power circuit 5 and supplied to a load 6.

[0030] Similarly, the second inverter 2 is composed of a control unit 2A and a power conversion unit 2B. The power conversion unit 2B includes a full-bridge circuit 20 connected to a DC voltage source 4, an AC reactor 25 provided on two wires on the AC side of the full-bridge circuit 20, an AC-side capacitor 26, a voltage sensor 27, a current sensor 28, and a voltage sensor 29, which are connected as shown in the figure.

[0031] The full-bridge circuit 20 is configured by switching elements 21, 22, 23, and 24, and diodes 21d, 22d, 23d, and 24d connected in antiparallel to the switching elements, respectively.

[0032] Full-bridge circuit 20 is connected to both ends of DC voltage source 4. Voltage sensor 27 detects the DC voltage input to full-bridge circuit 20 from DC voltage source 4 and sends the detected output to control unit 2A. Current sensor 28 detects the current flowing through AC reactor 25 and sends the detected output to control unit 2A. Voltage sensor 29 detects the AC voltage output to AC circuit 5 and sends the detected output to control unit 2A.

[0033] During stand-alone operation, the inverter 2 configured as described above converts DC power input from the DC voltage source 4 into voltage-controlled AC power using the PWM-controlled full-bridge circuit 20. High-frequency ripple contained in the AC waveform is suppressed by the AC reactor 25 and the AC-side capacitor 26. The AC output of the inverter 2 is provided to an AC power circuit 5 and supplied to a load 6.

[0034] The control units 1A and 2A of the first and second inverters 1 and 2A each include a computer, and the computer executes software (computer programs) to realize the necessary control functions. The software is stored in a storage device (not shown) of the control unit.

[0035] <<Function of the control unit>> Next, the function of the control unit 1A (the same applies to the control unit 2A, and hereinafter simply referred to as the "control unit") will be described. Figure 2 is a control block diagram related to voltage control in the control unit. For simplicity of explanation, this diagram shows the simplest example that does not include a current control loop. Figure 3 is a flowchart of the control block diagram of Figure 2.

[0036] The following description will be made with reference to FIG. 2 and FIG. First, in FIG. 3, the control unit obtains a phase difference detection value Δθ (step S1). In FIG. 2, an AC voltage va and an internal phase (previous value) θ are input to a phase difference detector B1, and a phase difference detection value Δθ is obtained.

[0037] Next, in FIG. 3, the control unit calculates a first frequency correction value Δf1 (step S2). In FIG. 2, the phase difference detection value Δθ passes through a first proportional integrator B2 and becomes a frequency correction value Δf1.

[0038] Next, in FIG. 3, the control unit calculates a second frequency correction value Δf2 (step S3). In FIG. 2, an adder B5 compares the frequency reference value fo with the previous value of the frequency detection value f, and the difference between them is passed through a second proportional integrator B6 to become a frequency correction value Δf2.

[0039] Next, in FIG. 3, the control unit obtains a frequency detection value f (step S4). 2, the frequency correction value Δf1, the frequency correction value Δf2, and the frequency reference value fo are added together by an adder B3 to obtain a new frequency detection value f. When first determining a new frequency detection value f, the previous frequency detection value f is used.

[0040] Next, in FIG. 3, the control unit obtains the internal phase θ (step S5). In Figure 2, the frequency detection value f passes through integrator B4 and becomes the internal phase θ. The internal phase θ is the output of phase-locked loop L, which incorporates a frequency feedback loop F as an internal loop.

[0041] Next, in FIG. 3, the control unit calculates the voltage command value va * is calculated (step S6). In FIG. 2, the voltage command unit B7 determines the voltage command value va based on the internal phase θ and the voltage amplitude reference value E0. * Output.

[0042] Next, in FIG. 3, the control unit calculates a voltage correction value Δva (step S7). In Figure 2, the voltage command value va *and the AC voltage va are compared by an adder B8, and are proportionally processed by a proportionalizer B9 to obtain a voltage correction value Δva. Note that the AC current ia can also be used in the calculation to obtain the voltage correction value Δva.

[0043] If proportional-integral control were used to calculate the voltage correction value Δva, the greater the deviation in the AC voltage va between the two inverters, the greater the reactive power due to the cross current flowing between them. This would result in a deviation in the output current, a DC component, and a phase difference. However, if proportional control (proportional regulator B9) were used, even if the deviation in the AC voltage va between the two inverters was around 1%, almost no cross current would flow, and the outputs of the two inverters would be uniform.

[0044] Then, in FIG. 3, the control unit obtains a control signal for the inverter (step S8). In Figure 2, the voltage command value va * A PWM signal generator B10 generates a pulse-width modulated inverter control signal based on the voltage correction value Δva and the DC voltage vi, and a gate drive signal for the inverter is obtained based on the pulse-width modulated inverter control signal.

[0045] The process shown in the flowchart of FIG. 3 is repeatedly executed during the independent operation of the inverters 1 and 2. In FIG. 2, a phase difference detector B1, a first proportional integrator B2, an adder B3, and an integrator B4 form a phase locked loop L. In addition, the adder B3, the adder B5, and the second proportional integrator B6 form a frequency feedback loop F. In an inverter that performs grid-connected operation rather than stand-alone operation, such a frequency feedback loop F does not need to be used.

[0046] "verification" First, a reference example of control that does not use a frequency feedback loop as described above will be verified by simulation.

[0047] (Reference example 1) Figure 4 shows the simulation results when two inverters, the first and second, are operated in parallel with independent output using the same control as when grid-connected operation is performed. The horizontal axis in (a), (b), (c), and (d) is the same and represents time (0 to 2 seconds). The values ​​on the vertical axis represent voltage, current, or frequency. In this control, the first inverter is operated first, and the second inverter begins operation one second later.

[0048] 1(a) is a waveform diagram showing the AC voltage va and the AC current output by the first inverter superimposed on each other. The larger amplitude is the AC voltage va, and the smaller amplitude is the AC current. (b) is a waveform diagram of the two-phase voltages vd (top) and vq (bottom) after dq transformation. (c) is the output frequency of the phase-locked loop. (d) shows the output currents of the first inverter and the second inverter. From 0 to approximately 1.1 seconds, the larger amplitude is from the first inverter, and the smaller amplitude is from the second inverter. From approximately 1.1 seconds to approximately 1.5 seconds, the two are overlapping.

[0049] As shown in Figure 4(d), immediately after the second inverter begins output, the two inverters share the output current equally, with almost no cross current. However, as shown in (b), the voltage vq does not reach 0 but remains near -20 V. As shown in (c), the frequency continues to drop from its initial value and becomes uncontrollable after 1.5 seconds. When there is a grid voltage, the phase of the AC voltage input to the phase-locked loop is constant regardless of the internal frequency of the phase-locked loop. Therefore, the voltage vq obtained by dq conversion can be used as a phase comparator. However, in an isolated output, the phase of the AC voltage changes depending on the internal frequency, so the frequency does not converge if the AC voltage is used as the reference phase.

[0050] (Reference example 2) Figure 5 shows the simulation results for two inverters, the first and second, operating in parallel with independent output using a reference voltage signal with a fixed frequency of 60 Hz. The horizontal axis in (a), (b), (c), and (d) is the same and represents time (0 to 10 seconds). The values ​​on the vertical axis represent voltage, current, or frequency. The rightmost graph shows an enlarged view of the horizontal axis (time axis) just before 10 seconds, and the vertical axis in (c) and (d) is also enlarged.

[0051] 1(a) is a waveform diagram showing the AC voltage va and the AC current output by the first inverter superimposed on each other. The AC voltage va has a larger amplitude, and the AC current oscillates around 0. (b) is a waveform diagram of the two-phase voltages vd (top) and vq (bottom) after dq transformation. (c) is the output frequency of the phase-locked loop. (d) shows the output currents of the first inverter and the second inverter. The second inverter starts operating one second later than the first inverter.

[0052] According to Figure 5(b), the voltage Vq becomes 0, and the output frequency of the phase-locked loop converges to 60 Hz, as shown in (c). However, this control requires a signal line to share a common reference voltage signal between the two inverters, which makes the signal line more susceptible to noise.

[0053] (Control based on the control block diagram in Figure 2) Figure 6 shows the simulation results when control is performed based on the control block diagram of Figure 2. The horizontal axis in (a), (b), (c), and (d) is the same and represents time (0 to 10 seconds). The values ​​on the vertical axis represent voltage, current, or frequency. The rightmost graph shows an enlarged view of the horizontal axis (time axis) just before 10 seconds, and the vertical axis in (c) and (d) is also enlarged. The proportional gain of the second proportional integrator B6 in the frequency feedback loop F was set to 0.2, and the integral time constant was set to 0.01 seconds.

[0054] 1(a) is a waveform diagram showing the AC voltage va and the AC current output by the first inverter superimposed on each other. The AC voltage va has a larger amplitude, and the AC current oscillates around 0. (b) is a waveform diagram of the two-phase voltages vd (top) and vq (bottom) after dq transformation. (c) is the output frequency of the phase-locked loop. (d) shows the output currents of the first inverter and the second inverter. The second inverter starts operating one second later than the first inverter.

[0055] According to Figure 6, as shown in (b), the voltage vq does not become 0, but converges to a constant negative value. As shown in (c), the frequency of the phase-locked loop L converges to a constant value roughly as targeted. Also, as shown in (d), the output current after the second inverter starts operating is shared equally between the two inverters, with a power factor of 1 and no DC component. No loss of control occurs, and the frequency is stable. In this way, by incorporating a frequency feedback loop F into the frequency control of the phase-locked loop L, it is possible to stabilize the parallel operation of inverters that provide independent output to AC power circuits.

[0056] 《Parallel operation with a generator》 Figure 8(a) is a graph showing an example of the relationship between the output current I of the inverter (power conversion unit) and the internal phase difference φ when the inverters having the phase-locked loop with frequency feedback shown in Figure 2 are operated in parallel. The phase difference φ is proportional to the output current I, and as the output current I increases, the phase difference φ also increases. This is similar to the relationship between the output current and the internal phase angle of a synchronous generator.

[0057] FIG. 8(b) is a vector diagram showing the relationship between the equivalent induced electromotive force E, the phase difference φ, and the AC voltage V. Here, the inverter is connected to the synchronous reactance x s If we consider this to be a synchronous generator, and the phase difference φ is sinφ≒φ, then the following relationship holds: φ≒(jx s I) / E That is, an inverter with frequency feedback processing added to a phase-locked loop operates as a kind of virtual synchronous generator (VSG), and therefore, the inverter can also be operated in parallel with a synchronous generator.

[0058] 7 shows the simulation results when the parallel inverter system 100 of FIG. 1 is operated in parallel with a generator on a commercial power system. The horizontal axis in (a), (b), (c), and (d) is the same and represents time (9.9 to 10.1 seconds). The values ​​on the vertical axis represent voltage, current, or frequency.

[0059] 7(a) is a waveform diagram showing the AC voltage va and the AC current output by the first inverter superimposed on each other. The AC voltage va has a larger amplitude, and the AC current oscillates around 0. (b) is a waveform diagram of the two-phase voltages vd (top) and vq (bottom) after dq transformation. (c) is the output frequency of the phase-locked loop. (d) shows the output current of the first inverter and the second inverter. The generator is disconnected at 10 seconds.

[0060] As shown in Figure 7, parallel operation is possible even when two inverters and generators are connected in parallel, and the output currents of the two inverters are the same. However, the output current contains a DC component that is just over 50% of the effective value, and the power factor drops to 0.824. No DC component flows through the load, and the DC current component output by the inverter is absorbed by the AC power supply.

[0061] A high-pass filter to suppress the DC component and a low-pass filter to suppress oscillation are installed in the calculation of the current command value. As a result, the DC component is reduced and the power factor is improved to 0.993. When the generator is disconnected after 10 seconds, the AC voltage va remains almost unchanged, and the output current of the two inverters increases, compensating for the current that the generator had been carrying. After disconnection, the voltage vq and the output frequency of the phase-locked loop decrease slightly, but the two inverters each bear half of the 20A load current equally.

[0062] To control the ratio of the current shared by the two inverters, you can change the control target value according to the inverter output. If you advance the voltage phase, the active power increases, and if you increase the voltage amplitude, the reactive power increases. Therefore, the simplest method is to change the voltage command value v * of, v * =E * cos2πf * t Then, the voltage amplitude target value E * decreases, and the frequency target value f * It is conceivable to give the material a drooping characteristic that reduces the amount of

[0063] The output phase of the phase-locked loop leads the AC voltage, just like a synchronous generator, and the phase difference increases in proportion to the inverter output current. Inverters with independent output controlled by such a phase-locked loop can be operated not only in parallel with multiple units, but also in parallel with other AC power sources, and can be treated as virtual synchronous generators.

[0064] "summary" The above disclosure can be generalized and expressed as follows: The control unit in each of the first inverter (1) and the second inverter (2) shown in FIG. 1 adds a frequency feedback process (frequency feedback loop (F)) to the calculation of a frequency detection value in a phase-locked loop (L) executed based on an AC voltage (Va), in which a frequency correction value (Δf2) obtained by applying proportional integration to a comparison result between the previous value of the frequency detection value and a frequency reference value is added. In such inverters, the frequency of the independent output gradually approaches and converges to a target value, and therefore, when multiple such inverters are connected in parallel or when connected in parallel to another AC power source, stable parallel operation can be achieved.

[0065] More specifically, when the frequency correction value (Δf2) is set as a second frequency correction value, the control unit executes a first calculation to obtain a first frequency correction value (Δf1) based on the deviation between the voltage phase based on the detection output of the voltage sensors (19, 29) and the internal phase in the phase-locked loop (L); a second calculation to obtain a new frequency detection value (f) by adding the first frequency correction value (Δf1) and the second frequency correction value (Δf2) to the frequency reference value (fo); and a third calculation to generate, based on the new frequency detection value (f), a control signal for controlling the output of the power conversion units (1B, 2B) based on the new frequency detection value. By performing the second calculation described above, it is possible to prevent the frequency of the independent output from falling into a state where it does not converge.

[0066] In addition, the third calculation involves calculating the voltage command value (va) based on the new frequency detection value (f). * ) and the voltage command value (va * ) and the AC voltage, and then a proportional operation is performed to obtain a voltage correction value (Δva). In this case, by performing proportional calculation instead of proportional-integral calculation, cross current can be suppressed even if there is a difference in the value detected as AC voltage between the AC power source and another AC power source.

[0067] <<Additional information about forward conversion (charging)>> Although the inverters 1 and 2 in FIG. 1 have been described as independent output inverters, if the DC voltage source is based on a storage battery, the inverters 1 and 2 (power conversion units 1B and 2B) may also convert AC power to DC power. For example, consider a case where an AC power source (current source) that outputs a current synchronized with the voltage of the AC power circuit 5, such as a grid-connected inverter powered by solar power generation, is connected to the AC power circuit 5. In this case, the control units 1A and 2A convert the AC power from the AC power source to DC power and control it to charge the storage batteries connected to the DC side of the power conversion units 1B and 2B. The inverters 1 and 2 can forward convert (AC to DC) the surplus power consumed by the load 6 to charge the storage batteries included in the DC voltage sources 3 and 4. In this case, the internal phase of the inverter's phase-locked loop lags the phase of the AC voltage.

[0068] "others" Although the full-bridge circuits 10 and 20 in FIG. 1 are two-leg single-phase inverter circuits, the processes in FIGS. 2 and 3 can be similarly applied to three-leg three-phase inverter circuits.

[0069] 《Addendum》 The present invention is intended to cover a wide range of applications, including those related to the present invention, including those related to the present invention. [Explanation of symbols]

[0070] 1 First inverter 1A Control section 1B Power conversion section 2 Second inverter 2A control section 2B Power conversion section 3,4 DC voltage source 5 AC line 6 Load 10 Full-bridge circuit 11, 12, 13, 14 Switching elements 11d, 12d, 13d, 14d Diodes 15 AC reactor 16 AC side capacitor 17 Voltage Sensor 18 Current Sensor 19 Voltage Sensor 20 Full-bridge circuit 21, 22, 23, 24 Switching elements 21d, 22d, 23d, 24d Diodes 25 AC reactor 26 AC side capacitor 27 Voltage Sensor 28 Current Sensor 29 Voltage Sensor 100 parallel inverter systems B1 Phase difference detector B2 1st proportional integrator B3 Adder B4 integrator B5 Adder B6 2nd proportional integrator B7 Voltage command unit B8 Adder B9 Proportionator B10 PWM signal generation section F Frequency Feedback Loop L phase locked loop

Claims

1. An inverter that is connected in parallel to an AC circuit to which an AC power source is connected and provides an independent output, a power conversion unit that converts DC power into AC power and supplies the power to the AC circuit; a voltage sensor for detecting an AC voltage of the AC current path; A control unit that controls the power conversion unit, The control unit adds a frequency feedback process to a calculation for calculating a frequency detection value in a phase-locked loop executed based on the AC voltage, the frequency correction value being obtained by applying proportional integration to a comparison result between the previous value of the frequency detection value and a frequency reference value.

2. When the frequency correction value is a second frequency correction value, the control unit a first calculation for determining a first frequency correction value based on a deviation between a voltage phase based on a detection output of the voltage sensor and an internal phase in the phase locked loop; a second calculation of adding the first frequency correction value and the second frequency correction value to the frequency reference value to obtain a new frequency detection value; a third calculation for generating a control signal for controlling an output of the power conversion unit based on the new frequency detection value; 2. The inverter according to claim 1, wherein the inverter performs

3. The third calculation includes: determining a voltage command value based on the new frequency detection value; and determining a voltage correction value by performing a proportional operation on the deviation between the voltage command value and the AC voltage; 3. The inverter of claim 2 including:

4. 4. The inverter according to claim 2, wherein the internal phase has a phase difference with respect to a phase of the AC voltage that changes according to an output current of the power conversion unit.

5. The power conversion unit is also capable of converting AC power to DC power, 5. The inverter according to claim 1, wherein the control unit controls the power conversion unit to convert AC power from the AC power source into DC power and charge a DC voltage source connected to a DC side of the power conversion unit.

6. a first inverter for providing an isolated output to the AC power line; A parallel inverter system including: a second inverter connected in parallel with the first inverter to the AC power line and providing an independent output to the AC power line; Each of the first inverter and the second inverter has a power conversion unit that converts DC power into AC power and supplies the power to the AC circuit; a voltage sensor for detecting an AC voltage of the AC current path; A control unit that controls the power conversion unit, The control unit adds a frequency feedback process to a calculation for calculating a frequency detection value in a phase-locked loop executed based on the AC voltage, the frequency correction value being obtained by applying proportional integration to a comparison result between a previous value of the frequency detection value and a frequency reference value.

7. A method for controlling an inverter that is connected in parallel to an AC circuit to which an AC power source is connected and provides an independent output, comprising the steps of: A power conversion unit converts DC power into AC power and supplies the power to the AC circuit; The control unit that controls the power conversion unit adds a frequency feedback process to a calculation of a frequency detection value in a phase-locked loop that is executed based on the AC voltage of the AC power line, the frequency correction value being obtained by applying proportional integral to a comparison result between a previous value of the frequency detection value and a frequency reference value. Inverter control method.

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