Inverter connection method

Abstract

To provide an inverter interconnection method that feedforward-compensates the amplitude and phase of a grid voltage before interconnection according to the fluctuation of an output voltage after filtering.SOLUTION: A trigonometric function including a target value of a phase difference from a grid voltage and a target value of the amplitude ratio, in which the phase and the amplitude of an inverter output voltage after a smoothing filter are adjusted to gradually approach the phase and amplitude of the grid voltage is obtained to obtain a formula for feedforward compensation of the grid voltage, this feedforward compensation causes the phase and amplitude of the filtered inverter output voltage and the phase and amplitude of the grid voltage to match, and the adjustment amount of the target value of the feedforward compensation is gradually decreased after connection with the electric power system.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for connecting an inverter to a power grid, in which the amplitude and phase of the inverter output voltage are matched with those of the grid voltage by feedforward compensation before the inverter is connected to the power grid. [Background technology]

[0002] It has been known for some time that DC power generated by a distributed DC power source such as a solar cell is converted by an inverter into AC power that matches the commercial power grid, and then connected to the power grid. In this case, in order to start up a grid-connected inverter (power conditioner) that is disconnected from the grid and connect to the grid, it is necessary to match the amplitude, phase, and frequency of the inverter output voltage with the grid voltage. If the inverter is connected to the grid in a state where they do not match, an overcurrent (inrush current) may occur.

[0003] As shown in Fig. 1, the power conditioner includes a DC power source 1, an inverter 2, a smoothing filter 3, a grid relay 4, a system (AC power source) 5, and a control unit 6. The grid relay 4 connects the inverter 2 to the grid 5 by turning the relay on, and is not connected (disconnected) by turning the relay off. The grid 5 has, for example, a three-phase AC power source. The inverter 2 converts DC power to AC power, but since the output waveform itself is a square wave, the smoothing filter 3 smooths it into a sine wave similar to the waveform of the grid 5. To connect to the grid, it is necessary to match the amplitude, phase, and frequency of the voltage on both sides of the grid relay 4.

[0004] For this reason, it is known to perform feedforward compensation before grid connection, which compensates for the system voltage so that the amplitude, phase, and frequency of the output voltage at point P1 on the output side of the inverter 2 and the system voltage at point P3 on the system side of the grid connection relay 4 match.

[0005] However, since there is a smoothing filter 3 between point P1 on the output side of the inverter 2 and point P2 on the inverter side of the interconnection relay 4, the amplitude and phase of the output waveform at point P1 on the output side of the inverter 2 and point P2 on the inverter side of the interconnection relay 4 differ before and after passing through the filter 3.

[0006] Therefore, when performing feedforward compensation, it is necessary to adjust the amplitude and phase of the filtered inverter output voltage at point P2 to match the system voltage at point P3. Known methods for this include, for example, a method using coordinate transformation and a method using Laplace transformation with a filter.

[0007] Figure 4 shows a method of feedforward compensation using a conventional bandpass filter. In this method, the secondary transfer function 14 in the control unit 6 (Figure 1) synchronizes the filtered inverter output voltage and the grid voltage using a bandpass filter. Other configuration details will be described later. The secondary transfer function unit 14 contains a bandpass filter that corrects the difference in amplitude and phase between the filtered output voltage and the grid voltage Vgrid, and the formula for the transfer function Gv of this bandpass filter is shown below.

[0008]

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[0009] In the above formula, the Laplace operator s, the parameter a for adjusting the phase angle, the parameter k for adjusting the effective voltage value, the center frequency ωn, and the Q of the Q value (Quality factor) are each given a sign.

[0010] Also, as shown in Fig. 5, a method of performing feedforward compensation using dq transformation and inverse dq transformation is known (for example, Patent Document 1). In this method, first, in a control unit, a three-phase or single-phase system voltage Vgrid is subjected to dq transformation 15 using the system voltage phase. When an inverse dq transformation 16 is performed to obtain a sine wave, a phase Δθ to be adjusted is added. The amplitude ΔA is adjusted by a gain 17. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] JP 2008-211912 A Summary of the Invention [Problem to be solved by the invention]

[0012] However, in the conventional method using a band-pass filter in feedforward compensation, since the filter has the transfer function Gv, it is difficult to remove the filter during operation after grid connection. In addition, since a band-pass filter is used, there are cases where the signal responsiveness in feedback control when calculating the output duty ratio of the inverter is deteriorated.

[0013] Furthermore, in the method using the dq transform and the inverse dq transform, there is a problem in that the calculation cost in the control unit increases due to the need to perform the dq transform and the inverse dq transform.

[0014] Furthermore, since the feedforward compensation method is originally a method of directly detecting and canceling fluctuations in the output voltage after filtering, there are cases where the method is not as effective at dealing with such fluctuations as the feedback method.

[0015] An object of the present invention is to provide a method for interconnecting an inverter that performs feedforward compensation of the amplitude and phase of the system voltage in response to fluctuations in the filtered output voltage before interconnection, and that can easily eliminate the effects of the feedforward compensation after interconnection. [Means for solving the problem]

[0016] In order to achieve the above object, a method for connecting an inverter according to the present invention connects an inverter that converts DC power of a DC power source into AC power to a power grid via a smoothing filter, the method comprising the steps of: The phase and amplitude of the filtered inverter output voltage are adjusted to gradually approach the phase and amplitude of the grid voltage, respectively, and parameters of a trigonometric function including a target value of the phase difference with respect to the grid voltage and a target value of the amplitude ratio are obtained to determine an equation for performing feedforward compensation of the grid voltage, and the phase and amplitude of the output voltage are made to match the phase and amplitude of the grid voltage by the feedforward compensation, thereby connecting the output voltage to the power grid.

[0017] According to this configuration, the phase and amplitude of the filtered inverter output voltage are adjusted to gradually approach the target values ​​of the phase and amplitude of the grid voltage, respectively, so that even with feedforward compensation, it is possible to deal with fluctuations in the filtered inverter output voltage as much as possible. In addition, the feedforward compensation with trigonometric function parameters makes the phase and amplitude of the filtered output voltage match the phase and amplitude of the grid voltage, so that the inverter can be easily and stably connected to the grid by the feedforward compensation.

[0018] Preferably, after the interconnection, the adjustment amount of the target value of the feedforward compensation is gradually decreased, so that the influence of the feedforward compensation on the operation after the interconnection can be easily removed without abruptly changing the control signal and while the inverter is in a stable state.

[0019] In addition, preferably, when an effective value Vrms_inv of the filtered inverter output voltage Vinv with respect to an effective value Vrms_grid of the grid voltage Vgrid is a voltage amplitude error, an adjustment value based on the voltage amplitude error is denoted as Ashift, and a target value of a phase difference of the filtered inverter output voltage Vinv with respect to the grid voltage Vgrid is denoted as θshift, Vfeedforward(t), which performs feedforward compensation of the system voltage, is expressed by equation (1):

[0020]

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[0021]

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[0022] Preferably, each parameter in the formula (2) is obtained from a signal of the system voltage and calculated using a conversion function, so that each parameter can be easily obtained.

[0023] Preferably, the parameter sin(θgrid) in the equation (2) is obtained from the signal of the system voltage, and the parameter cos(θgrid) is obtained from this sin(θgrid) using the equation (3) (ω0 is the system angular frequency) of the transfer function of the all-pass filter for shifting the phase of the output voltage after the filter.

[0024]

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[0025] The above formulas (1) to (3) are merely examples, and the present invention is not limited to these, and various formulas can be used.

[0026] Preferably, an integrator is used to adjust the phase and amplitude of the filtered output voltage Vinv so that they gradually approach the phase and amplitude of the grid voltage Vgrid, respectively. Therefore, the phase and amplitude of the filtered inverter output voltage can be easily adjusted so that they gradually approach the phase and amplitude of the grid voltage, respectively.

[0027] The distributed power supply system according to the present invention performs the above-mentioned inverter interconnection method, and the DC power supply is a distributed power supply. With this configuration, it is possible to obtain a distributed power supply system that can easily eliminate the influence of feedforward compensation on operation after interconnection in a stable state. Effect of the Invention

[0028] In the present invention, the phase and amplitude of the filtered inverter output voltage are adjusted to gradually approach the target values ​​of the phase and amplitude of the system voltage, respectively, so that even feedforward compensation can respond to fluctuations in the filtered inverter output voltage as much as possible. In addition, feedforward compensation having trigonometric function parameters is used to match the phase and amplitude of the filtered output voltage to the phase and amplitude of the system voltage, and after connection is established, the adjustment amount of the feedforward compensation target value is gradually reduced. This makes it possible to easily remove the effects of feedforward compensation on operation after connection, without suddenly changing the control signal, and while keeping the inverter in a stable state. [Brief description of the drawings]

[0029] [Figure 1] 1 is an overall configuration diagram showing a power conditioner used in a grid-connection method for an inverter according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a control block diagram showing a method of interconnecting the inverter. [Diagram 3] FIG. 11 is a calculation block diagram included in a synchronizer in a control unit. [Figure 4] FIG. 1 is a control block diagram showing a conventional method for interconnecting inverters. [Diagram 5] FIG. 1 is a control block diagram showing a conventional method for interconnecting inverters. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Fig. 1 is an overall configuration diagram showing a power conditioner (PCS) used in a grid-connection method of an inverter according to an embodiment of the present invention. This power conditioner converts DC power from a DC power source 1, such as a solar cell, which is a distributed power source, into AC power and connects it to a commercial power system. This overall configuration diagram is the same as the conventional diagram, but the configuration of a synchronizer 10 in a control unit 6 in Fig. 2, which will be described later, is different.

[0031] As shown in Fig. 1, the power conditioner includes a DC power source 1, an inverter 2, a smoothing filter 3, an interconnection relay 4, a system (AC power source) 5, and a control unit 6. The interconnection relay 4 interconnects the inverter 2 and the system 5 when the relay is on, and is not interconnected (disconnected) when the relay is off. The system 5 has, for example, a three-phase AC power source, but may also be a single-phase AC power source.

[0032] The inverter 2 has, for example, four switching elements and a bridge circuit that converts DC power to AC power by switching. A smoothing filter 3 smoothes the rectangular wave output from the inverter 2 into a sine wave similar to the system voltage. This filter 3 changes the amplitude and phase of the inverter output. A grid-connection relay 4 is provided between the filter 3 and an AC power source 5, and switches the connection with the power grid by switching the relay on (conducting) and the relay off (cutting off).

[0033] The control unit 6 is configured using, for example, a microcomputer, and mainly controls the operation of the inverter 2. The amplitude and phase of the filtered output voltage Vinv and the grid voltage Vgrid are detected by, for example, a PLL (Phase Locked Loop) algorithm (not shown).

[0034] Fig. 2 shows a control block diagram of the control unit 6. Fig. 2 mainly shows a control block related to feedforward correction based on the grid voltage vgrid.

[0035] In Fig. 2, the grid current command value i*grid is set and output by current feedback of the grid current igrid through the DC voltage control at the DC bus, the AC voltage control and power control of the AC output, and the current is compensated by the current control unit 7, by the external control loop based on these controls. After that, the mode selection switch 8 is set to the "0" side in the preparation mode, and is switched to the "1" side in the grid-connected mode.

[0036] The PWM output control unit generates a PWM signal having a pulse width corresponding to the output duty ratio received from the output current control unit, and drives and controls each switching element of the bridge circuit in the inverter 2 based on the PWM signal.

[0037] 3 shows a calculation block diagram for synchronizing the output voltage after the output filter of the inverter 2 with the system voltage in the synchronizer 10 in the control unit 6, which performs feedforward compensation. An equation for feedforward compensation of the signal of the system voltage is obtained by obtaining the parameters of a trigonometric function including a target value of the phase difference with the system voltage and a target value of the amplitude ratio, which are obtained by gradually adjusting the phase and amplitude of the voltage after the output filter of the inverter 2 to be closer to the phase and amplitude of the system voltage, respectively.

[0038] When the effective value Vrms_inv of the filtered inverter output voltage Vinv with respect to the effective value Vrms_grid of the grid voltage Vgrid is a voltage amplitude error, an adjustment value based on the voltage amplitude error is denoted as Ashift, and an adjustment value based on the phase difference of the filtered inverter output voltage Vinv with respect to the grid voltage Vgrid is denoted as θshift, Vfeedforward(t), which performs feedforward compensation of the system voltage, is expressed by equation (1).

[0039]

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[0040] The sin(θgrid+θshift) in equation (1) is calculated from the addition theorem of equation (2).

[0041]

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[0042] The calculation block diagram in Figure 3 will be explained below. The all-pass filter used in this example passes inputs of any frequency without changing their amplitude. The system voltage Vgrid shifts the phase of the filtered output voltage Vinv according to equation (3) of the transfer function Gap(s)20 of this all-pass filter. In other words, since the smoothing filter 3 causes a phase delay in the output, the phase can be advanced by that amount. Note that ω0 is the system angular frequency.

[0043]

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[0044] The above formulas (1) to (3) are merely examples of formulas related to the processing for realizing the technology of the present invention, and various formulas can be used. For example, formula (3) is a model formula for converting sine to cosine, but it is not limited to this, and can be replaced with a time delay, an α-β conversion calculation method in a three-phase system, etc.

[0045] In FIG. 1, the amplitude error ratio ΔA of the inverter output effective value Vrms_inv (P2) adjusted with respect to the grid voltage effective value Vrms_grid (P3) is expressed by the following equation (4).

[0046]

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[0047] In FIG. 3, the phase difference Δθ of the filtered inverter output voltage Vinv adjusted with respect to the phase of the grid voltage Vgrid is expressed by the following equation (5).

[0048]

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[0049] The phase difference Δθ passes through an “integrator K1 / s” 21 and becomes an adjustment value θshift based on the phase difference.

[0050] 3 are obtained from the adjustment value θshift based on the obtained phase difference. “sin(θshift)” 23 and “cos(θshift)” 24 in Fig. 3 are obtained. “sin(θshift)” is multiplied by the output of “Gap(s)” 20 in multiplier 25, and “cos(θshift)” is multiplied by Vgrid in multiplier 26 and the results are added in adder 28.

[0051] Thus, sin(θgrid+θshift) in equation (1) can be obtained from the addition theorem of equation (2).

[0052] The parameter sin(θgrid) in the above formula (2) is obtained from the signal of the system voltage, and the parameter cos(θgrid) can be obtained from this sin(θgrid) using formula (3) of the transfer function Gap(s) of the all-pass filter. That is, when a signal in which the phase of the input signal is advanced by 90° is output using formula (3), cos(ωt+θ) is output when sin(ωt+θ) is input.

[0053] The amplitude error ratio ΔA passes through the “integrator K2 / s” 22 in FIG. 3 to become an adjustment value Ashift based on the voltage amplitude error, and this is added to “1.0” by the adder 29 .

[0054] Due to the nature of the calculation process, the above ΔA and Δθ are calculated using integral calculations based on the elements A shift and θ shift. Therefore, the result values ​​of ΔA and Δθ are optimized and stabilized by accumulating the necessary amount of elements. As a result, the phase and amplitude adjustment before and after the relay when connecting to the grid voltage is optimized.

[0055] The output of adder 28 and the output of adder 29 are multiplied by multiplier 27 to calculate Vfeedforward. In this way, equation (1) for Vfeedforward(t) that performs the above-mentioned feedforward compensation is obtained.

[0056] By this feedforward compensation, the phase and amplitude of the output voltage are made to match the phase and amplitude of the system voltage, and the interconnection relay 4 is turned on to interconnect with the system 5 .

[0057] After the interconnection, the adjustment amounts of the target value Ashift of the amplitude ratio of the feedforward compensation and the target value θshift of the phase difference are gradually reduced to 0, thereby making it possible to easily remove the influence of the feedforward compensation on operation after the interconnection, without causing a sudden change in the control signal, and keeping the inverter in a stable state.

[0058] In this way, the phase and amplitude of the filtered inverter output voltage are adjusted to gradually approach the target values ​​of the phase and amplitude of the grid voltage, respectively, so that even with feedforward compensation, it is possible to deal with fluctuations in the filtered inverter output voltage as much as possible.In addition, the feedforward compensation having trigonometric function parameters is used to match the phase and amplitude of the filtered output voltage to the phase and amplitude of the grid voltage, and then the adjustment amount of the feedforward compensation target value is gradually reduced after connection, so that the influence of feedforward compensation on operation after connection can be easily eliminated without abruptly changing the control signal and while the inverter is in a stable state.

[0059] In each of the above embodiments, solar cells are used as distributed power sources, but fuel cells or the like may also be used.

[0060] In this embodiment, the phase of the filtered output voltage is shifted using an all-pass filter, but this is not limited to this, and the phase may be shifted using, for example, three-phase αβ transformation or Hilbert transformation.

[0061] In this method, the value required for compensation can be calculated from the signal for one of the three phases of the system voltage. In this example, the phases of the system voltage and the output voltage after filtering are detected by PLL, but this is not limiting and may be detected by zero-crossing detection, for example.

[0062] This method may be applied not only to inverters but also to converters such as matrix converters.

[0063] The present invention is not limited to the above-described embodiment, and various additions, modifications, and deletions are possible without departing from the scope of the present invention. Therefore, such modifications are also included in the scope of the present invention. [Explanation of symbols]

[0064] 1:DC power supply 2: Inverter 3: Filter 4: Grid-connected relay 5: Grid (AC power supply) 6: Control unit 10: Synchronizer 20: All-pass filter 21: Integrator 22: Integrator

Claims

1. 1. A method for connecting an inverter that converts DC power of a DC power source into AC power to a power grid via a smoothing filter, comprising: a phase and an amplitude of the filtered inverter output voltage are adjusted to gradually approach the phase and the amplitude of the grid voltage, respectively, to obtain a trigonometric function parameter including a target value for a phase difference with respect to the grid voltage and a target value for an amplitude ratio, thereby obtaining an equation for performing feedforward compensation of the grid voltage; a phase and amplitude of the output voltage being matched to a phase and amplitude of a grid voltage by the feedforward compensation, thereby connecting the inverter to the power grid;

2. In claim 1, a control unit for controlling the inverter to be connected to the grid, the control unit gradually decreasing an adjustment amount of the target value of the feedforward compensation after the connection is made;

3. In claim 1, The effective value Vrms_inv of the filtered inverter output voltage Vinv with respect to the effective value Vrms_grid of the grid voltage Vgrid is defined as a voltage amplitude error, The adjustment value based on the voltage amplitude error is represented as Ashift, When an adjustment value based on a phase difference of the filtered inverter output voltage Vinv with respect to the grid voltage Vgrid is denoted by θshift, Vfeedforward(t) for performing feedforward compensation of the system voltage is expressed by the following equation (1): [0010] A method of interconnecting an inverter in which sin(θgrid+θshift) in equation (1) is calculated from the addition theorem of equation (2). [0025]

4. In claim 3, The inverter interconnection method according to claim 2, wherein each parameter in the formula (2) is obtained from a system voltage signal and calculated using a conversion function.

5. In claim 3, The parameter sin(θgrid) in the above formula (2) is obtained from a system voltage signal, and a parameter cos(θgrid) is obtained from this sin(θgrid) by using formula (3) (ω0 is a system angular frequency) of a transfer function Gap of an all-pass filter for shifting the phase of the filtered output voltage Vinv. [0030]

6. In claim 3, A method of interconnecting an inverter, comprising: using an integrator to gradually adjust the phase and amplitude of the filtered output voltage Vinv closer to the phase and amplitude of the grid voltage Vgrid, respectively.

7. A distributed power supply system that performs the inverter interconnection method according to claim 1 , wherein the DC power supplies are distributed power supplies.

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