JP7633865B2 - How to connect the inverter - Google Patents

Description

The present invention relates to a method for connecting an inverter to a power grid, which uses feedforward compensation to match the amplitude and phase of the inverter output voltage with the grid voltage before connecting the inverter to the power grid.

It has been known for some time that DC power generated by a distributed DC power source such as a solar cell can be 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 and 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.

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 when the relay is on, and is not connected (disconnected) when the relay is 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, it is necessary to match the amplitude, phase, and frequency of the voltage on both sides of the grid relay 4.

For this reason, it is known that, before interconnection, feedforward compensation is performed to compensate 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 interconnection relay 4 match.

However, because 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 differs before and after passing through the filter 3 at point P1 on the output side of the inverter 2 and point P2 on the inverter side of the interconnection relay 4.

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 a Laplace transform with a filter.

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 system 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 system voltage Vgrid, and the formula for the transfer function Gv of this bandpass filter is shown below.

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 value (Quality factor) Q are each assigned a symbol.

As shown in FIG. 5, a method of performing feedforward compensation using dq transformation and inverse dq transformation is also known (for example, Patent Document 1). In this method, first, in the control unit, a three-phase or single-phase system voltage Vgrid is dq transformed 15 using the system voltage phase. When the dq inverse transformation 16 is performed to a sine wave, the phase Δθ to be adjusted is added. The amplitude ΔA is adjusted by the gain 17.

JP 2008-211912 A

However, in the conventional method using a bandpass filter in feedforward compensation, the filter has the transfer function Gv, which makes it difficult to remove during operation after grid connection. In addition, the use of a bandpass filter can sometimes worsen the signal responsiveness in feedback control when calculating the inverter output duty ratio.

In addition, the method using the dq transform and the inverse dq transform had the problem that the calculation cost in the control unit increased because of the need to perform the dq transform and the inverse dq transform.

Furthermore, since the feedforward compensation method originally directly detects and cancels fluctuations in the output voltage after filtering, it is sometimes unable to adequately respond to such fluctuations compared to the feedback method.

The present invention aims to provide a method for interconnecting an inverter that performs feedforward compensation of the amplitude and phase of the system voltage before interconnection in response to fluctuations in the filtered output voltage, and can easily eliminate the effects of feedforward compensation after interconnection.

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.

With 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 system 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. At the same time, feedforward compensation with trigonometric function parameters matches the phase and amplitude of the filtered output voltage to the phase and amplitude of the system voltage for connection, so that the inverter can be easily and stably connected with the system using feedforward compensation.

Preferably, after the connection, the adjustment amount of the feedforward compensation target value is gradually decreased. In this case, the effect of feedforward compensation on operation after the connection can be easily eliminated without abruptly changing the control signal and while keeping the inverter in a stable state.

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 by 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 by θshift,
Vfeedforward(t) for performing feedforward compensation of the system voltage is expressed by the following equation (1):

式（１）におけるｓｉｎ（θｇｒｉｄ＋θｓｈｉｆｔ）を、式（２）の加法定理から求める。

The sin(θgrid+θshift) in the formula (1) is found from the addition theorem of the formula (2).

したがって、目標値Ａｓｈｉｆｔおよびθｓｈｉｆｔを徐々に０にすることにより、安定した状態で、容易に連系後の運転にフィードフォワード補償の影響を取り除くことができる。

Therefore, by gradually setting the target values Ashift and θshift to 0, the influence of feedforward compensation on operation after grid connection can be easily removed in a stable state.

Preferably, each parameter in the above formula (2) is obtained from the system voltage signal and calculated using a conversion function. Therefore, each parameter can be easily obtained.

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

したがって、オールパスの伝達関数を用いているので、従来のバンドパスフィルタと比べて、インバータの出力デューティ比算出におけるフィードバック制御での信号の応答性を向上させることができる。

Therefore, since an all-pass transfer function is used, it is possible to improve the signal responsiveness in feedback control in calculating the output duty ratio of the inverter, compared to a conventional band-pass filter.

Note that the formulas (1) to (3) above are merely examples and are not limiting; various formulas can be used.

Also 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, it is possible to easily adjust the phase and amplitude of the filtered inverter output voltage so that they gradually approach the phase and amplitude of the grid voltage, respectively.

The distributed power supply system according to the present invention performs the inverter interconnection method, and the DC power supply is a distributed power supply. With this configuration, a distributed power supply system can be obtained that can easily eliminate the effects of feedforward compensation on operation after interconnection in a stable state.

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 with feedforward compensation, it is possible to respond to fluctuations in the filtered inverter output voltage as much as possible.In addition, feedforward compensation with 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 eliminate the effects of feedforward compensation on operation after connection, without abruptly changing the control signal, and with the inverter in a stable state.

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. FIG. 2 is a control block diagram showing a method of interconnecting the inverter. FIG. 11 is a calculation block diagram included in a synchronizer in a control unit. FIG. 1 is a control block diagram showing a conventional method for interconnecting inverters. FIG. 1 is a control block diagram showing a conventional method for interconnecting inverters.

Figure 1 is an overall configuration diagram showing a power conditioner (PCS) used in a method for interconnecting an inverter according to one 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 interconnects it to a commercial power system. This overall configuration diagram is the same as the conventional diagram, but the configuration of the synchronizer 10 in the control unit 6 in Figure 2, which will be described later, is different.

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 and the grid 5 when the relay is on, and is not connected (disconnected) when the relay is off. The grid 5 has, for example, a three-phase AC power source, but may also be a single-phase AC power source.

The inverter 2 has, for example, four switching elements and a bridge circuit that converts DC power to AC power by switching. The 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. The grid-connection relay 4 is provided between the filter 3 and the AC power source 5, and switches the connection to the power grid, switching the relay on (conducting) and the relay off (cutting off).

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, for example, by a PLL (Phase Locked Loop) algorithm (not shown).

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

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

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

Figure 3 shows a calculation block diagram for synchronizing the output voltage after the output filter of inverter 2 with the system voltage in synchronizer 10 in control unit 6, which performs feedforward compensation. By obtaining the parameters of a trigonometric function including the target value of the phase difference with the system voltage and the target value of the amplitude ratio, which are adjusted so that the phase and amplitude of the output filtered voltage of inverter 2 are gradually closer to the phase and amplitude of the system voltage, respectively, an equation for feedforward compensation of the system voltage signal is obtained.

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 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 θshift,
Vfeedforward(t) for performing feedforward compensation of the system voltage is expressed by the following equation (1).

Calculate sin(θgrid+θshift) in equation (1) using the addition theorem in equation (2).

The calculation block diagram in Figure 3 will now be explained. 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.

The above formulas (1) to (3) are merely examples of formulas related to the processing that realizes the technology of the present invention, and various formulas can be used. For example, formula (3) is a model formula that converts 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.

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

In FIG. 3, the phase difference Δθ of the filtered inverter output voltage Vinv, which is adjusted relative to the phase of the grid voltage Vgrid, is expressed by the following equation (5):

The phase difference Δθ passes through the "integrator K1/s" 21 and becomes the adjustment value θshift based on the phase difference.

The "sin(θshift)" 23 and "cos(θshift)" 24 in FIG. 3 are obtained from the adjustment value θshift based on the obtained phase difference. "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 added in adder 28.

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

The parameter sin(θgrid) in the above formula (2) is obtained from the system voltage signal, and the parameter cos(θgrid) can be obtained from this sin(θgrid) using formula (3) for the transfer function Gap(s) of the all-pass filter. In other words, when a signal with the phase of the input signal advanced by 90° is output using formula (3), an input of sin(ωt+θ) results in an output of cos(ωt+θ).

The amplitude error ratio ΔA passes through the "integrator K2/s" 22 in Figure 3 to become the adjustment value Ashift based on the voltage amplitude error, and this is added to "1.0" by the adder 29.

Due to the nature of the calculation process, the above ΔA and Δθ are each 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 adjustments before and after the relay when connecting to the grid voltage are optimized.

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 feedforward compensation described above is obtained.

This feedforward compensation causes the phase and amplitude of the output voltage to match the phase and amplitude of the grid voltage, and the interconnection relay 4 turns on to connect to grid 5.

After the connection, 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, making it possible to easily eliminate the effects of feedforward compensation on operation after connection without abruptly changing the control signal and keeping the inverter in a stable state.

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 feedforward compensation can respond to fluctuations in the filtered inverter output voltage as much as possible. At the same time, feedforward compensation with 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 after connection is established, the adjustment amount of the feedforward compensation target value is gradually reduced, so that the effects of feedforward compensation on operation after connection can be easily eliminated without abruptly changing the control signal and while keeping the inverter in a stable state.

In the above embodiments, solar cells are used as distributed power sources, but fuel cells, etc. may also be used.

In this embodiment, the phase of the filtered output voltage is shifted using an all-pass filter, but this is not limiting and the phase may be shifted using, for example, a three-phase αβ transform or a Hilbert transform.

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 phase of the system voltage and the output voltage after filtering are detected using a PLL, but this is not limited to this and zero-cross detection, for example, may also be used.

This method can be applied not only to inverters but also to converters such as matrix converters.

The present invention is not limited to the above-described embodiments, and various additions, modifications, and deletions are possible without departing from the spirit of the present invention. Therefore, such additions, modifications, and deletions are also included within the scope of the present invention.

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

Claims (7)

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; 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; 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):

A method of interconnecting an inverter in which sin(θgrid+θshift) in equation (1) is calculated from the addition theorem of equation (2).

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. 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.

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. A distributed power supply system that performs the inverter interconnection method according to any one of claims 1 to 6, in which the DC power supply is a distributed power supply.

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