CN114244173A - Grid voltage feedforward method, electronic equipment and medium for weak grid AC - Google Patents

Abstract

The invention discloses a grid voltage feedforward method for a weak grid AC device, electronic equipment and a medium, wherein the method comprises the following steps: determining an output impedance model of the grid-connected inverter; calculating to obtain an equivalent model of the grid-connected inverter connected with the weak power grid according to an expression of output impedance in an output impedance model of the grid-connected inverter; calculating the grid-connected current of the inverter on an alpha axis according to the equivalent model of the grid-connected inverter connected with the weak power grid; determining a stable operation condition of the grid-connected inverter; a third-order generalized integral filter is added in a power grid voltage feedforward channel. The invention can not only improve the stability of the grid-connected inverter, but also reduce the influence of background harmonic waves at the voltage of a power grid on the quality of grid-connected current, and improve the quality of the output current of the inverter under the condition of weak power grid.

Description

Grid voltage feedforward method, electronic equipment and medium for weak grid AC

technical field

The present invention relates to the technical field of grid-connected inverter control, in particular to a grid voltage feedforward method, electronic equipment, and computer-readable storage medium for a weak grid AC.

Background technique

A grid-tie inverter (GTI for short) is a special inverter. In addition to converting direct current into alternating current, the output alternating current can be synchronized with the frequency and phase of the mains. Can go back to mains. Grid-tied inverters are commonly used in applications where some DC voltage sources (such as solar panels or small wind turbines) are connected to the grid.

As a power conversion device, the grid-connected inverter is a bridge between distributed power and the public grid, and plays a key role in the field of new energy power generation. However, with the continuous improvement of the penetration rate of new energy power generation and the access of nonlinear loads, the power grid sometimes exhibits weak power grid characteristics with increased power grid impedance. In the case of weak grid, the increase of grid impedance will affect the output current of the inverter. Usually, the grid-connected inverter controls the grid voltage feedforward to suppress the influence of the voltage background harmonic and fundamental wave components at the common grid-connected point on its output power quality.

Among the proposed grid-connected inverter control methods, the grid voltage feedforward control method reduces the phase margin of the output impedance of the grid-connected inverter to a certain extent, which is not conducive to the stable operation of the inverter. When the inverter is working under the condition of weak grid, its stability will be affected and it is difficult to ensure the output of high-quality grid-connected current.

Therefore, there is an urgent need to provide a new grid voltage feedforward method for weak grid ACs to solve the above problems.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to provide a grid voltage feedforward method, electronic equipment, and medium for a weak grid AC AC, which only needs to add a high-order generalized integral filter to the grid voltage feedforward channel, thereby improving the grid connection. Robustness of the inverter to grid impedance in weak grid conditions.

In order to solve the above technical problems, the present invention adopts the following technical solutions.

A grid voltage feedforward method for a weak grid AC is characterized by comprising the following steps:

Step 1: Determine the output impedance model of the grid-connected inverter;

Step 2: According to the expression of the output impedance in the output impedance model of the grid-connected inverter in step 1, calculate the equivalent model of the grid-connected inverter connecting to the weak grid;

Step 3: According to the equivalent model of the grid-connected inverter connected to the weak grid in step 2, the grid-connected current i _{2_a} (s) of the inverter on the α axis is calculated;

Step 4: determine the stable operating conditions of the grid-connected inverter;

Step 5: Add a third-order generalized integral filter to the grid voltage feedforward channel.

Preferably, in the step 1, in the output impedance model of the grid-connected inverter, the output impedance Z ₀ (s) of the grid-connected inverter is:

In the above formula (1), Z ₀ (s) represents the output impedance of the grid-connected inverter, and T _m (s) is the product of G _x1 (s), G _x2 (s) and H _i2 ; G _x2 (s) is the equivalent control link on the right side of the forward channel of the inverter α-axis control block diagram, and F _f (s) is the equivalent control link with grid voltage feedforward.

In the above formula (1), the output impedance of the grid-connected inverter is Z ₀ (s) obtained according to the schematic diagram of the three-phase grid-connected inverter system in the two-phase static coordinate system in Fig. 1 .

Preferably, in the step 3, the grid-connected current i _{2_a} (s) of the inverter on the α axis is:

In the above formula (2), Z ₀ (s) represents the output impedance of the grid-connected inverter, Z _g (s) is the equivalent impedance of the grid, i ₀ (s) is the inverter output current, and V _{g_a} (s ) is the grid voltage component on the α axis.

Preferably, in the step 4, the stable operating conditions of the grid-connected inverter are:

When the amplitude-frequency characteristic curves of the two transfer functions of the output impedance Z ₀ (s) of the grid-connected inverter and the equivalent impedance Z _g (s) of the grid do not have an intersection point, the grid-connected inverter runs stably; When there is an intersection between the amplitude-frequency characteristic curves of the two, the phase difference between the intersection of the amplitude-frequency characteristic curves between the two should be within ±180° to ensure the stable operation of the inverter.

Preferably, in the third-order generalized integral filter, the transfer functions of the three output voltage signals u ₁ (t), u ₂ (t) and u ₃ (t) to the input voltage signal u(t) are respectively G ₁ (s), G ₂ (s) and G ₃ (s).

Preferably, the G ₁ (s) is:

In the above formula (4), G ₁ (s) outputs the transfer function of the voltage signal u ₁ (t), k is the gain coefficient, and ω is the angular frequency; U ₁ (s) represents the S domain of the voltage signal u ₁ (t) output, U(s) represents the S-domain input of the voltage signal u ₁ (t).

Preferably, the G ₂ (s) is:

In the above formula (5), G ₂ (s) outputs the transfer function of the voltage signal u ₂ (t), k is the gain coefficient, and ω is the angular frequency; U ₂ (s) represents the S domain of the voltage signal u ₂ (t) output, U(s) represents the S-domain input of the voltage signal u ₂ (t).

Preferably, the G ₃ (s) is:

In the above formula (6), G ₃ (s) outputs the transfer function of the voltage signal u ₃ (t), k is the gain coefficient, and ω is the angular frequency; U ₃ (s) represents the S domain of the voltage signal u ₃ (t) The output, U(s) represents the S-domain input of the voltage signal _u3 (t).

The present invention also provides an electronic device, comprising: at least one processor; and a memory communicatively connected to the at least one processor; wherein,

The memory stores instructions executable by the at least one processor, the instructions being executed by the at least one processor to enable the at least one processor to perform the execution of any of claims 1 to 8 The described grid voltage feedforward method for weak grid AC.

The present invention also provides a computer-readable storage medium storing a computer program, when the computer program is executed by a processor, the grid voltage front-end for a weak grid AC AC according to any one of claims 1 to 8 is realized. feed method.

The beneficial effects of the present invention are:

The invention discloses a grid voltage feedforward method, electronic equipment, and medium for a weak grid AC. The method includes the following steps: determining an output impedance model of a grid-connected inverter; according to the output impedance model of the grid-connected inverter The equivalent model of the grid-connected inverter connected to the weak grid is obtained by calculating the expression of the output impedance in current; determining the stable operating conditions of the grid-connected inverter; adding a third-order generalized integral filter to the grid voltage feedforward channel.

1. The present invention enables the output impedance of the grid-connected inverter to have a higher amplitude in the entire frequency band, effectively suppressing the harmonics of the grid-connected current.

2. The present invention only needs to add a high-order generalized integral filter to the grid voltage feedforward channel, which improves the robustness of the grid-connected inverter to grid impedance under weak grid conditions, and the implementation is simple and efficient.

The grid voltage feedforward method, electronic equipment, and medium for the weak grid AC of the present invention can not only improve the stability of the grid-connected inverter, but also reduce the influence of the background harmonics at the grid voltage on the grid-connected current quality. Improve the quality of the output current of the inverter in the case of weak grid.

Description of drawings

FIG. 1 is a schematic diagram of a three-phase grid-connected inverter system in a two-phase static coordinate system of the present invention.

Figure 2 is an equivalent model of the shown grid-connected inverter connected to a weak grid.

FIG. 3 is a control block diagram of the high-order generalized integral filter of the present invention.

Figure 4 is a system control block diagram after adding a high-order generalized integral filter.

FIG. 5 is a simplified control model of the output impedance of the grid-connected inverter derived from the schematic diagram of the three-phase grid-connected inverter system of FIG. 1 .

FIG. 6 is a Bode diagram of the output impedance Z ₀ (s) of the grid-connected inverter using the control method of the present invention.

The explanation of each parameter in the context and in Figures 1-6 of the specification is as follows.

H _i1 is the capacitance current feedback coefficient

H _i2 is the grid-connected current feedback coefficient

H _v is the grid voltage sampling coefficient

G _ff (s) is the grid voltage feedforward control link

G _i (s) is a PR-controlled current regulator

θ represents the phase-locked loop output phase angle

I* represents the grid-connected current reference value

i ₀ (s) is the output current of the grid-connected inverter,

Z ₀ (s) is the output impedance of the grid-connected inverter;

i _{2_a} (s) is the grid-connected current of the inverter on the α axis;

Z _g (s) is the equivalent impedance of the connected weak grid

V _{g_a} is the grid voltage component on the α axis,

ω is the angular frequency of the phase-locked loop output

i* _{2_a} (s) is the grid-connected current reference value of the inverter on the α axis;

G _inv (s) is the transfer function of the inverter bridge

Z _L1 (s) is the equivalent impedance of the inverter side inductor

Z _L2 (s) is the equivalent impedance of the grid-side inductor

Z _c (s) is the equivalent impedance of the filter capacitor

T _m (s) is the product of G _x1 (s), G _x2 (s) and G _i2 ;

G _x1 (s) is the equivalent control link on the left side of the forward channel of the inverter α-axis control block diagram

G _x2 (s) is the equivalent control link on the right side of the forward channel of the inverter α-axis control block diagram,

F _f (s) is the equivalent control link with grid voltage feedforward

Z _g (s) is the equivalent impedance of the connected weak grid,

Filter represents a higher-order generalized integral filter.

Z ₀₁ represents the output impedance of the inverter when the grid voltage feedforward is not used;

Z ₀₂ represents the inverter output impedance with grid voltage feedforward when PR control is used,

L _fc is the lower limit of the frequency band in the Bode diagram of Z ₀₁ and Z ₀₂

H _fc is the upper limit of the frequency band in the Bode diagram of Z ₀₁ and Z ₀₂ .

Detailed ways

The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, and the protection scope of the present invention can be more clearly defined.

1-6, a grid voltage feedforward method for a weak grid AC AC according to the present invention includes the following steps:

Step 1: Determine the output impedance model of the grid-connected inverter;

Step 2: According to the expression of the output impedance in the output impedance model of the grid-connected inverter in step 1, calculate the equivalent model of the grid-connected inverter connecting to the weak grid;

Step 3: According to the equivalent model of the grid-connected inverter connected to the weak grid in step 2, the grid-connected current i _{2_a} (s) of the inverter on the α axis is calculated;

Step 4: determine the stable operating conditions of the grid-connected inverter;

Step 5: Add a third-order generalized integral filter to the grid voltage feedforward channel.

Further, in the step 1, in the output impedance model of the grid-connected inverter, the output impedance Z ₀ (s) of the grid-connected inverter is:

In the above formula (1), Z ₀ (s) represents the output impedance of the grid-connected inverter, and T _m (s) is the product of G _x1 (s), G _x2 (s) and H _i2 ; G _x2 (s) is the equivalent control link on the right side of the forward channel of the inverter α-axis control block diagram, and F _f (s) is the equivalent control link with grid voltage feedforward.

In the above formula (1), the output impedance of the grid-connected inverter is Z ₀ (s) obtained according to the schematic diagram of the three-phase grid-connected inverter system in the two-phase static coordinate system in Fig. 1 .

Further, in the step 3, the grid-connected current i _{2_a} (s) of the inverter on the α axis is:

In the above formula (2), Z ₀ (s) represents the output impedance of the grid-connected inverter, Z _g (s) is the equivalent impedance of the grid, i ₀ (s) is the inverter output current, and V _{g_a} (s ) is the grid voltage component on the α axis.

According to the formula (1) of the output impedance Z ₀ (s) of the grid-connected inverter, the equivalent model of the grid-connected inverter connected to the weak grid is obtained as shown in Figure 2. According to Fig. 2, the above equation (2) of the grid-connected current i _{2_a} (s) of the grid-connected inverter on the α axis is obtained from the equivalent model of the grid-connected inverter connected to the weak grid.

The grid-connected inverter is in the ideal grid condition (Z _g (s)=0) the system is stable, but in the weak grid condition, the stability of the grid-connected inverter system will be determined by the right bracket term Z _g (s)/Z ₀ (s) OK.

According to the Nyquist stability criterion, when the output impedance Z ₀ (s) of the grid-connected inverter and the equivalent impedance Z _g (s) of the grid do not have an intersection point between the amplitude-frequency characteristic curves of the two transfer functions, the grid-connected reverse The inverter can run stably; and when there is an intersection between the amplitude-frequency characteristic curves of the two, the phase difference between the intersection of the amplitude-frequency characteristic curves between the two should be within ±180° to ensure the stable operation of the inverter;

According to the formula (1) of the output impedance Z ₀ (s) of the grid-connected inverter, and according to the schematic diagram of the three-phase grid-connected inverter system in the two-phase static coordinate system in Fig. 1, the output impedance of the grid-connected inverter is obtained. The simplified control model of Z ₀ (s) is shown in Fig. 5. According to Fig. 5, the equivalent model of the grid-connected inverter connected to the weak grid can obtain the grid-connected current i _{2_a} (s) of the grid-connected inverter on the α axis as follows (3).

In formula (3), T _m (s) is the loop gain of the simplified control model in Fig. 5, and T _m (s) is the product of G _x1 (s), G _x2 (s) and H _i2 .

Further, in the step 4, the stable operating conditions of the grid-connected inverter are:

When the amplitude-frequency characteristic curves of the two transfer functions of the output impedance Z ₀ (s) of the grid-connected inverter and the equivalent impedance Z _g (s) of the grid do not have an intersection point, the grid-connected inverter runs stably; When there is an intersection between the amplitude-frequency characteristic curves of the two, the phase difference between the intersection of the amplitude-frequency characteristic curves between the two should be within ±180° to ensure the stable operation of the inverter.

Further, in the third-order generalized integral filter, the transfer functions of the three output voltage signals u ₁ (t), u ₂ (t) and u ₃ (t) to the input voltage signal u(t) are respectively G ₁ (s), G ₂ (s) and G ₃ (s).

Further, the G ₁ (s) is:

In the above formula (4), G ₁ (s) outputs the transfer function of the voltage signal u ₁ (t), k is the gain coefficient, and ω is the angular frequency; U ₁ (s) represents the S domain of the voltage signal u ₁ (t) output, U(s) represents the S-domain input of the voltage signal u ₁ (t).

Further, the G ₂ (s) is:

In the above formula (5), G ₂ (s) outputs the transfer function of the voltage signal u ₂ (t), k is the gain coefficient, and ω is the angular frequency; U ₂ (s) represents the S domain of the voltage signal u ₂ (t) output, U(s) represents the S-domain input of the voltage signal u ₂ (t).

Further, the G ₃ (s) is:

In the above formula (6), G ₃ (s) outputs the transfer function of the voltage signal u ₃ (t), k is the gain coefficient, and ω is the angular frequency; U ₃ (s) represents the S domain of the voltage signal u ₃ (t) The output, U(s) represents the S-domain input of the voltage signal _u3 (t).

In summary, a third-order generalized integral filter is added to the grid voltage feedforward channel. The third-order generalized integral filter is a system with two inputs and three outputs. The input of the system is the angular frequency ω and the voltage signal u(t), and the output is u. ₁ (t), u ₂ (t), and u ₃ (t). The transfer functions G ₁ (s), G ₂ (s) and G ₃ (s) of the three output voltage signals to the input voltage signals are respectively as shown in the above equations (4) to (6).

The present invention also provides an electronic device, comprising: at least one processor; and a memory communicatively connected to the at least one processor; wherein,

The memory stores instructions executable by the at least one processor, the instructions being executed by the at least one processor to enable the at least one processor to perform the execution of any of claims 1 to 8 The described grid voltage feedforward method for weak grid AC.

The present invention also provides a computer-readable storage medium storing a computer program, when the computer program is executed by a processor, the grid voltage front-end for a weak grid AC AC according to any one of claims 1 to 8 is realized. feed method.

The grid voltage feedforward method, electronic equipment, and medium for a weak grid AC converter of the present invention can solve the problem that the phase margin of the inverter output impedance is reduced in the case of a weak grid in the existing control technology, thereby affecting the stability of the grid-connected inverter. sexual issues. The invention can not only improve the stability of the grid-connected inverter, but also reduce the influence of the background harmonics at the grid voltage on the grid-connected current quality, thereby improving the output current quality of the inverter in a weak grid condition.

According to the characteristics of grid-connected inverter and grid impedance under weak grid conditions, a high-order generalized integral filter is added to the grid voltage feedforward channel, and the system control block diagram is shown in Figure 4.

FIG. 6 is a Bode diagram of the output impedance of the grid-connected inverter using the control strategy of the present invention. In the figure, Z ₀ (s) represents the output impedance of the present invention, Z ₀₁ represents the output impedance of the inverter without grid voltage feedforward, Z ₀₂ represents the inverter output impedance with grid voltage feedforward when PR control is used, Z _g represents the grid equivalent impedance. It can be seen from Fig. 6 that before the present invention is adopted, the phase margin of the output impedance of the inverter is relatively low in a certain frequency band, which causes the phase difference between the output impedance of the inverter and the equivalent resistance of the power grid to exceed ±180°, and the inverter cannot be stabilized. Operation; after adopting the invention, the output impedance of the inverter has a higher amplitude in the whole frequency band of the Bode diagram, and the phase margin is improved, which ensures that the grid-connected inverter can still operate stably even in the case of a weak grid. . The control method of adding a high-order generalized integral filter to the grid voltage feedforward channel proposed by the invention can improve the stability of the grid-connected inverter in the weak grid condition, and effectively improve the grid-connected current quality.

Aiming at the problem that the output current quality of the grid voltage feedforward inverter is low in the case of a weak grid, the present invention proposes a grid voltage feedforward control method using a high-order generalized integral filter. The high-order generalized integral filter realizes that the output impedance of the inverter has a high amplitude in the entire frequency range, so that the grid-connected inverter can run stably in the weak grid with large grid impedance, improving the inverter's performance. Stability and grid adaptability.

It will be apparent to those skilled in the art that the present invention is not limited to the details of the above-described exemplary embodiments, but that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments are to be regarded in all respects as illustrative and not restrictive, and the scope of the invention is to be defined by the appended claims rather than the foregoing description, which are therefore intended to fall within the scope of the claims. All changes within the meaning and scope of the equivalents of , are included in the present invention. Any reference signs in the claims shall not be construed as limiting the involved claim.

In addition, it should be understood that although this specification is described in terms of embodiments, not each embodiment only includes an independent technical solution, and this description in the specification is only for the sake of clarity, and those skilled in the art should take the specification as a whole , the technical solutions in each embodiment can also be appropriately combined to form other implementations that can be understood by those skilled in the art.

Claims (10)

1. a grid voltage feedforward method for weak grid alternator, is characterized in that, comprises the following steps: Step 1: Determine the output impedance model of the grid-connected inverter; Step 2: According to the expression of the output impedance in the output impedance model of the grid-connected inverter in step 1, calculate the equivalent model of the grid-connected inverter connecting to the weak grid; Step 3: According to the equivalent model of the grid-connected inverter connected to the weak grid in step 2, the grid-connected current i _{2_a} (s) of the inverter on the α axis is calculated; Step 4: determine the stable operating conditions of the grid-connected inverter; Step 5: Add a third-order generalized integral filter to the grid voltage feedforward channel. 2 . The grid voltage feedforward method for a weak grid AC AC according to claim 1 , wherein in the step 1, in the output impedance model of the grid-connected inverter, the output impedance of the grid-connected inverter The impedance Z ₀ (s) is:

In the above formula (1), Z ₀ (s) represents the output impedance of the grid-connected inverter, and T _m (s) is the product of G _x1 (s), G _x2 (s) and H _i2 ; G _x2 (s) is the equivalent control link on the right side of the forward channel of the inverter α-axis control block diagram, and F _f (s) is the equivalent control link with grid voltage feedforward. 3. The grid voltage feedforward method for a weak grid alternator according to claim 1, wherein in the step 3, the grid-connected current i _{2_a} (s) of the inverter on the α axis is:

In the above formula (2), Z ₀ (s) represents the output impedance of the grid-connected inverter, Z _g (s) is the equivalent impedance of the grid, i ₀ (s) is the inverter output current, and V _{g_a} (s ) is the grid voltage component on the α axis. 4. The grid voltage feedforward method for a weak grid AC AC according to claim 1, wherein in the step 4, the stable operation conditions of the grid-connected inverter are: When the amplitude-frequency characteristic curves of the two transfer functions of the output impedance Z ₀ (s) of the grid-connected inverter and the equivalent impedance Z _g (s) of the grid do not have an intersection point, the grid-connected inverter runs stably; When there is an intersection between the amplitude-frequency characteristic curves of the two, the phase difference between the intersection of the amplitude-frequency characteristic curves between the two should be within ±180° to ensure the stable operation of the inverter. 5. The grid voltage feedforward method for a weak grid alternator according to claim 1, characterized in that, in the third-order generalized integral filter, three output voltage signals u ₁ (t), u ₂ ( The transfer functions of t) and u ₃ (t) to the input voltage signal u(t) are G ₁ (s), G ₂ (s) and G ₃ (s), respectively. 6. The grid voltage feedforward method for a weak grid alternator according to claim 5, wherein the G ₁ (s) is:

In the above formula (4), G ₁ (s) outputs the transfer function of the voltage signal u ₁ (t), k is the gain coefficient, and ω is the angular frequency; U ₁ (s) represents the S domain of the voltage signal u ₁ (t) output, U(s) represents the S-domain input of the voltage signal u ₁ (t). 7. The grid voltage feedforward method for a weak grid alternator according to claim 5, wherein the G ₂ (s) is:

In the above formula (5), G ₂ (s) outputs the transfer function of the voltage signal u ₂ (t), k is the gain coefficient, and ω is the angular frequency; U ₂ (s) represents the S domain of the voltage signal u ₂ (t) output, U(s) represents the S-domain input of the voltage signal u ₂ (t). 8. The grid voltage feedforward method for a weak grid alternator according to claim 5, wherein the G ₃ (s) is:

In the above formula (6), G ₃ (s) outputs the transfer function of the voltage signal u ₃ (t), k is the gain coefficient, and ω is the angular frequency; U ₃ (s) represents the S domain of the voltage signal u ₃ (t) The output, U(s) represents the S-domain input of the voltage signal _u3 (t). 9. An electronic device, characterized in that, comprising: at least one processor; and a memory communicatively coupled to the at least one processor; wherein, The memory stores instructions executable by the at least one processor, the instructions being executed by the at least one processor to enable the at least one processor to perform the execution of any of claims 1 to 8 The described grid voltage feedforward method for weak grid AC. 10. A computer-readable storage medium storing a computer program, characterized in that, when the computer program is executed by a processor, the grid voltage for a weak grid alternator according to any one of claims 1 to 8 is realized Feedforward method.

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