Grid-connected converter power grid multi-mode active support decision-making method based on power grid impedance identification

Abstract

The invention relates to the technical field of power grid intelligent control, and discloses a grid-connected converter power grid multi-mode active support decision-making method based on power grid impedance identification, which comprises the following steps of: estimating a power grid state through a grid-connected converter, identifying a power grid impedance parameter, and judging a power grid stable state; according to the current signal, the voltage signal and the power grid impedance parameter, selecting a current power grid active support mode, including a voltage regulation control mode, a power grid stability control mode and a harmonic suppression mode; and finally, executing a corresponding control strategy according to the active support mode of the power grid. According to the invention, the power quality problems of voltage imbalance, resonance, line harmonic detection result exceeding allowable distortion rate and the like of the power grid can be improved, and the stable operation capability of the novel power system is enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of power grid intelligent control, and particularly to a multi-mode active support decision method for a grid-connected converter based on grid impedance identification. Background Art

[0002] In recent years, with the gradual expansion of the construction of a new power system with new energy as the main body, a large number of distributed power sources have been connected to the power grid. However, in remote area power grids with long transmission lines or microgrids containing large-scale distributed energy, a weak power grid will be formed, and the grid impedance will increase. Under the condition of a weak power grid, the grid impedance will have a greater impact on the stable operation of the power system and the grid-connected point voltage. Therefore, the large-scale distributed new energy power generation has higher requirements for the new power system. In order to suppress the intermittency and volatility of new energy and maintain the safe and stable operation of the power system, it is necessary to continuously improve the active support ability of new energy power sources.

[0003] Since the power of traditional energy is controllable, while the power of distributed new energy power generation is affected by the external environment. With the increase of the proportion of distributed new energy power generation in the power grid, its impact on the frequency and voltage of the power grid gradually appears, bringing huge impacts and challenges to the safe and stable operation of the power grid, including the volatility of distributed new energy power generation and the low prediction accuracy, which affect the grid balance and economic operation; distributed new energy grid connection does not have the inertia and damping of synchronous generators, the moment of inertia of the power system is low, and the voltage volatility is large; distributed new energy power generation leads to poor system voltage support ability; large-scale new energy power generation access will also have power quality problems such as harmonic resonance. Summary of the Invention

[0004] The present invention proposes a multi-mode active support method for a grid-connected converter based on grid impedance identification. The present invention provides an active support method adapted to the change of grid impedance for grid resonance problems, grid harmonic problems, and grid voltage asymmetry problems. The present invention can improve power quality problems such as voltage imbalance, resonance, and the harmonic detection result of the line exceeding the allowable distortion rate in the power grid, and enhance the stable operation ability of the new power system.

[0005] In order to achieve the above invention purpose, the technical solution of the present invention is as follows:

[0006] A multi-mode active support decision method for a grid-connected converter based on grid impedance identification, the method comprising the following steps:

[0007] Step S1. Estimate the grid state through the grid-connected converter, identify the grid impedance parameters, and judge the grid stable state;

[0008] Step S2. Select the current grid active support mode according to the current signal, voltage signal and grid impedance parameters; wherein, the grid active support modes include:

[0009] When the regional grid voltage is unbalanced, the active support mode of the current grid is the voltage regulation control mode;

[0010] When resonance occurs in the regional grid, the active support mode of the current grid is the grid stability control mode;

[0011] When the harmonic detection result of the regional grid line exceeds the allowable distortion rate, the active support mode of the current grid is the harmonic suppression mode;

[0012] Step S3. Execute the corresponding control strategy according to the grid active support mode.

[0013] Preferably, the priority of the grid active support modes is that the voltage regulation control mode is the first priority, the grid stability control mode is the second priority, and the harmonic suppression mode is the third priority.

[0014] Preferably, when the active support mode is the voltage regulation control mode, in an asymmetric grid, the PI control method is adopted, and based on the calculated reactive current command value, the reactive current output by the grid-connected converter is controlled to be equal to the reactive current command value to support the grid connection point voltage;

[0015] When the active support mode is the grid stability control mode, the PRD control method is adopted to expand the system bandwidth and adjust the voltage and current output by the grid-connected converter;

[0016] When the active support mode is the harmonic suppression mode, increase the system harmonic impedance.

[0017] Preferably, when the active support mode is the voltage regulation control mode, in an asymmetric grid, the PI control method is adopted, and based on the calculated reactive current command value, the reactive current output by the grid-connected converter is controlled to be equal to the reactive current command value to support the grid connection point voltage, including:

[0018] Distributed new energy power generation output current I g and the grid-connected converter output voltage U inv , system voltage U g The relationship between them can be expressed by the following formula in the dq coordinate system:

[0019]

[0020] wherein, and are the positive and negative sequence voltages output by the grid-connected converter on the dq axis; and are the positive and negative sequence voltages of the power grid on the dq axes; and are the positive and negative sequence currents of the power grid on the dq axes; R g is the equivalent resistance of the power grid; L g is the equivalent inductance of the power grid;

[0021] The PI controller tracks the system error signal and obtains the system reactive current command value according to the following calculation expression:

[0022]

[0023] where k pv is the proportional coefficient of the PI controller, and k iv is the integral coefficient of the PI controller, is the AC voltage reference value;

[0024] Based on the calculated reactive current command value, control the reactive current output by the grid-connected converter to be equal to the reactive current command value to support the voltage at the grid connection point.

[0025] Preferably, when the active support mode is the power grid stability control mode, the PRD control method is adopted to expand the system bandwidth and adjust the system voltage and current, including: adopting a pole placement control scheme, and calculating the parameters K p 、K r and K d of the PRD controller according to the grid impedance estimated in step S1 and the ideal closed-loop characteristic equation and the actual closed-loop characteristic equation of the current inner loop of the grid-connected converter, and adjusting the voltage and current output by the grid-connected converter.

[0026] Preferably, when the active support mode is the harmonic suppression mode, based on the upper limit value of the parameter of the PRD controller K r , regulate the current parameter K r of the controller to increase the system harmonic impedance.

[0027] Advantages of the present invention:

[0028] 1. The present invention enables distributed new energy generating units to have the active support ability, and when the power system is disturbed, the distributed new energy active support cluster autonomously responds to system changes, provides active support capabilities such as voltage support and harmonic resonance suppression for the system, and enhances the stable operation of the new power system.

[0029] 2. The present invention can achieve good steady-state tracking characteristics and dynamic response performance for the grid impedance, improve the self-adaptability of the system to the time-varying grid, and improve power quality problems such as voltage imbalance, resonance in the grid, and the harmonic detection result of the line exceeding the allowable distortion rate. Description of the Drawings

[0030] The foregoing and following specific descriptions of the present invention will become clearer when read in conjunction with the following drawings, in which:

[0031] Figure 1 is a flowchart of the method of the present invention;

[0032] Figure 2 is a schematic diagram of PI control of the grid-connected converter of the present invention;

[0033] Figure 3 is a schematic diagram of PRD control of the single-phase full-bridge structure at the rear stage of the grid-connected converter of the present invention. Specific embodiments

[0034] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the following will further illustrate the technical solutions for achieving the purpose of the present invention through several specific embodiments. It should be noted that the technical solutions claimed by the present invention include but are not limited to the following embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those skilled in the art without creative efforts shall fall within the protection scope of the present invention.

[0035] The embodiment of the present invention proposes a multi-mode active support decision method for the grid-connected converter based on grid impedance identification. The multi-mode active support strategy based on grid impedance identification generates a control parameter perturbation signal for the grid-connected converter, performs grid state estimation, and identifies the grid impedance parameters; estimates the model and parameters of the current grid; determines the control mode and adjusts the strategy according to the estimation result of the grid model; according to the steady-state operation requirements of the system, when the grid impedance is found to change, it autonomously responds to the system change and actively supports the grid system.

[0036] This embodiment discloses a multi-mode active support decision method for the grid-connected converter based on grid impedance identification. The following will further illustrate the implementation of the present invention in conjunction with the drawings. Figure 1 is a flowchart of the method of the present invention, as Figure 1 shown, the method includes the following steps:

[0037] Step S1. Estimate the grid state through the grid-connected converter, identify the grid impedance parameters, and judge the grid stability state.

[0038] Establish the dynamic equation of the grid-connected converter, and its corresponding S-domain expression is as follows:

[0039]

[0040] Among them, U g is the ideal grid voltage; U inv is the converter output voltage; L is the filter inductor; C is the filter capacitor; R gis the equivalent resistance of the power grid; L g is the equivalent inductance of the power grid, I L is the current of the filter inductor; I g is the current at the grid connection point.

[0041] From the above formula, I L and U inv The transfer function in the S domain is as follows:

[0042]

[0043] In the case of a strong power grid, the value of the power grid impedance can be ignored, while in the case of a weak power grid in the regional power grid, the value of the power grid impedance cannot be ignored. Therefore, the present invention uses wavelet transform and Hilbert-Huang transform for signal processing, and then calculates the power grid impedance value according to the corresponding formula. Its basic principle is: injecting a periodic disturbance current signal into the power grid through a converter to obtain the voltage and current responses at the grid connection point. The specific calculation method can refer to the invention patent with the publication number CN118671449A.

[0044] Step S2. Select the current power grid active support mode according to the current signal, voltage signal and the calculated power grid impedance parameters; wherein, the power grid active support modes include:

[0045] Mode a. When the voltage of the regional power grid is unbalanced, the active support mode of the current power grid is the voltage regulation control mode;

[0046] Mode b. When resonance occurs in the regional power grid, the active support mode of the current power grid is the power grid stability control mode;

[0047] Mode c. When the harmonic detection result of the regional power grid line exceeds the allowable distortion rate, the active support mode of the current power grid is the harmonic suppression mode.

[0048] In the embodiment described in the present invention, during the actual operation of the power grid, the above three unstable states may occur simultaneously. When the three unstable states occur simultaneously, first adjust the voltage through the voltage regulation control mode, then control the stability through the power grid stability control mode, and finally solve the harmonic problem. Therefore, the multi-mode active support of the grid-connected converter in the present invention has different priorities for various working modes. Specifically, the voltage regulation control mode is the first priority, the power grid stability control mode is the second priority, and the harmonic suppression mode is the third priority. For different control modes, there are corresponding control methods and strategies, which are as follows.

[0049] In the embodiment described in the present invention, the PI control structure of the grid-connected converter is as Figure 2As shown in the figure, it includes a pre-stage DC / DC part, which is a Boost boost circuit and mainly realizes the MPPT control of distributed new energy batteries; the post-stage DC / AC part is a single-phase full-bridge structure and adopts a double closed-loop control of voltage and current to achieve the two functions of stabilizing the DC bus voltage and controlling the grid-connected current. An ideal voltage source U g and the grid impedance Z g are connected in series to equivalently replace the weak grid. The grid impedance Z g mainly includes a resistive component (R g ) and an inductive component (L g ). The equivalent model between the distributed new energy grid connection point and the grid is represented by an equivalent grid impedance and an infinite grid.

[0050] According to the selected grid active support mode, when the voltage regulation control mode is adopted, under an asymmetric grid, the relationship between the output current I g of the distributed new energy power station, the output voltage U inv of the converter, and the system voltage U g can be expressed by the following formula in the dq coordinate system:

[0051]

[0052] where, and are the positive and negative sequence voltages output by the grid-connected converter on the dq axis; and are the positive and negative sequence voltages of the grid on the dq axis; and are the positive and negative sequence currents of the grid on the dq axis; R g is the equivalent resistance of the grid; L g is the equivalent inductance of the grid.

[0053] In the case of grid asymmetry, decreases, increases, determines the output of the active power of the distributed new energy power station. At this time, it is necessary for the grid-connected converter to send out reactive current to support the voltage at the grid connection point to achieve the effect of regulating the voltage in the grid. And to compensate for the positive sequence voltage and suppress the negative sequence voltage, the reactive current output by the grid-connected converter should be negative. The embodiment of the present invention adopts a PI control method. Based on the calculated reactive current command value, it controls the reactive current output by the grid-connected converter to be equal to the reactive current command value to support the voltage at the grid connection point to regulate the voltage in the grid. The specific process is as follows:

[0054] The PI controller in the system tracks the error signal. At this time, the reactive current command value is:

[0055]

[0056] Among them, and are the positive and negative reactive current command values of the q-axis respectively; k pv is the proportional coefficient of the PI controller, and k iv is the integral coefficient of the PI controller. is the AC voltage reference value.

[0057] Furthermore, since the voltage deviation in the grid connection requirements needs to be < ±10%, and the voltage unbalance needs to be < 2%, the reactive current command value is:

[0058]

[0059] Among them, the grid voltage unbalance ε is defined as:

[0060]

[0061] The positive sequence voltage of the d-axis can ensure the output power. The implementation of MPPT is completed by the pre-stage DC / DC Boost module. In the grid-connected converter control, the active current maintains the stability of the DC bus voltage and ensures that the power is normally fed into the grid. The magnitude of the current reflects the active power output by the distributed new energy, and the remaining capacity of the converter is used to compensate the voltage. The outer loop design adopts constant DC voltage control:

[0062]

[0063] The relationship between the positive and negative sequence d-axis currents is:

[0064]

[0065] It can be seen from this that the current command value will not affect the reactive power compensation effect. Therefore, the discharge current command value i ref is used as a reference signal and given to the current inner loop. The PI controller is used to track the error signal. At the same time, considering the voltage feed-forward compensation and decoupling terms, the dynamic equation of the inner loop control is:

[0066]

[0067] Among them, and are the reference voltages of the converter ports, which are used to generate the converter switch trigger signals for the PWM circuit; k ii is k ii The integral coefficient of the current; is the positive sequence component of the d-axis current; is the positive sequence component of the q-axis current.

[0068] In the embodiments described in the present invention, the PI controls the reactive current output by the grid-connected converter to be equal to the reactive current command value. During the control adjustment process, for the adjustment amplitude of the reactive current of the grid-connected converter, it can be calculated through the relationship between the output voltage of the aforementioned converter and the grid current voltage. And when the PI controller tracks the error signal, the error signal refers to the error signal that is and with the difference of.

[0069] In the embodiments described in the present invention, according to the selected grid active support mode, when the grid stability control mode is adopted, an adaptive quasi-proportional resonant differential (PRD) control method is used. On the basis of the proportional resonant (PR) control, differential control is added to compensate the phase margin of the system, expand the bandwidth of the system, solve the deterioration of the dynamic response performance caused by the increase of the grid impedance, shorten the dynamic response time, and at the same time achieve zero steady-state error tracking control for specific frequency signals. The adaptive quasi-proportional resonant differential (PRD) control method is a control method for the current inner loop in the single-phase full-bridge structure of the DC / AC part after the grid-connected converter, specifically as follows:

[0070] The adaptive quasi-proportional resonant differential (PRD) control method contains proportional (P) control, resonant (R) control, and differential control (D). The transfer function of the PRD control is:

[0071]

[0072] Among them, Kp is the proportional coefficient of the controller, Kr is the resonant coefficient of the controller, Kd is the differential coefficient of the controller, ω c is the cut-off frequency, ω 0 is the resonant angular frequency at the resonant point, is the low-pass filter link.

[0073] The proportional control unit in the PRD control method is both a PI controller. The PI controller directly adjusts the output current in the current loop control to match the reference current. The resonant control unit (R) in the PRD control method is an improvement on the basis of the PI controller. By setting closed-loop poles at specific frequencies in the s-plane, the response ability to specific frequency signals is improved. At the resonant frequency, the gain of the PR controller is extremely large, enabling it to accurately track the sinusoidal command signal at the resonant frequency point, thereby completely eliminating the steady-state error and reducing the influence of the fundamental frequency of the grid voltage on the grid-connected current. The high efficiency of the PR controller stems from the precise matching of its internal resonant frequency and the synchronous rotation angular velocity.

[0074] In the PRD control method, the differential control unit (D) can compensate for the phase margin of the system, expand the bandwidth of the system, solve the deterioration of the dynamic response performance caused by the increase in grid impedance, shorten the dynamic response time, and improve the system stability. And The low-pass filter link can further reduce the influence of high-frequency interference, sampling noise, etc. on the differential control unit.

[0075] Adopt an adaptive control scheme based on pole placement. According to control theory, the control performance of the system is closely related to the position of the closed-loop characteristic roots in the S-plane. Since the PRD control sets the closed-loop poles at a specific frequency in the s-plane, its transfer function is Equation (17). Discretize the controlled object and the controller, and the discretization formula is:

[0076]

[0077] Perform Taylor expansion on the discretization formula:

[0078]

[0079] Among them, T sw is the carrier period;

[0080] Ignore the high-order terms and only retain the first-order terms, which is simplified to

[0081]

[0082] Substitute it into the PRD control transfer function, and the actual closed-loop characteristic equation of the current inner loop in the single-phase full-bridge structure of the DC / AC part at the rear stage of the grid-connected converter can be obtained:

[0083]

[0084] There are 4 closed-loop poles in the system under weak grid conditions. Since only the dominant poles have the greatest influence on the dynamic performance of the system, and the farther the dominant poles are from the imaginary axis, the better the dynamics of the system. Therefore, the transfer function is reduced in order.

[0085] Assume that the two closed-loop dominant poles of the system are:

[0086]

[0087] Assume that the non-dominant poles of the system are:

[0088]

[0089] Substitute into:

[0090]

[0091] Among them, T sis the sampling time; m and n are integers; ξ is a real number; ω n is the natural frequency of the system;

[0092] Then, the discretized form of the ideal closed-loop characteristic equation of the current inner loop in the single-phase full-bridge structure of the DC / AC part at the rear stage of the grid-connected converter is as follows:

[0093] A m (z -1 ) = (1 - z 1 z -1 )(1 - z 2 z -1 )(1 - z 3 z -1 )(1 - z 4 z -1 ) Equation (26);

[0094] By comparing the discretized actual closed-loop characteristic equation, the relationships between Kp, Kr, and Kd and the fixed parameters of the system as well as the grid impedance can be obtained. By setting the closed-loop dominant poles of the transfer function as the root locus to a stable state through the PRD controller, the problems of resonance in the regional power grid can be solved by adjusting the output current and voltage of the grid-connected converter.

[0095] In summary, for the grid stability control mode, the specific control strategy is as follows: According to the system grid impedance R estimated in step S1 g and the ideal closed-loop characteristic equation and the actual closed-loop characteristic equation, when the ideal closed-loop characteristic equation is equal to the actual closed-loop characteristic equation, solve the Kp, Kr, and Kd parameters of the controller, and use the obtained parameters as inputs to adjust the voltage and current output by the grid-connected converter to avoid resonance.

[0096] In the embodiment described in the present invention, according to the selected grid active support mode, when the harmonic suppression mode is adopted, since the larger the system harmonic impedance, the smaller the output harmonic current caused, and the better the anti-interference performance of the system.

[0097] The system harmonic impedance is as follows:

[0098]

[0099] Among them,

[0100] A = Kk p + 2Lω c + R;

[0101] B = Lω 0 2 + 2Kk p ω c + 2Rω c + 2Kk r ωc Equation (29);

[0102] The system harmonic impedance is affected by Kp. As Kp increases, the harmonic impedance of the system becomes larger and the system's anti-interference performance is better. Therefore, to achieve the purpose of harmonic suppression, increasing the harmonic impedance within an appropriate range will result in better anti-interference performance of the system. Since the controller bandwidth ω c and the proportional coefficient Kp of the controller have stable values in the control scheme, the upper limit value of the gain coefficient Kr is determined while ensuring that the system has a certain stability margin, the gain required by the controller is determined, and an appropriate Kr is selected to optimize the steady-state performance and anti-interference performance of the system.

[0103] According to the output impedance amplitude / frequency variation diagram when Kr increases, as Kr increases, the offset of the resonant frequency increases and the amplitude increase of the output impedance slows down. Therefore, by plotting the root locus of the system parameters when the gain coefficient Kr changes, the system stability is ensured and the upper limit value of the gain coefficient Kr is determined. As the system parameter root locus changes with the increase of the gain coefficient Kr, a pair of conjugate poles approach the imaginary axis. When Kr exceeds a certain amplitude, the conjugate poles are in the right half-plane of the zero-pole diagram and the system becomes unstable. At this time, it is the limit value of Kr. Therefore, in the harmonic suppression mode, on the basis that Kr does not exceed the upper limit, the Kr coefficient value of the controller is increased as much as possible to improve the harmonic suppression effect while ensuring the stability of the converter.

[0104] Step S3. According to the power grid active support mode, execute the corresponding control strategy.

[0105] The above are only the preferred embodiments of the present invention and do not constitute any form of obstruction to the present invention. Any simple modification or equivalent change made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A grid-connected converter grid multi-mode active support decision method based on grid impedance identification, characterized in that: The method comprises the following steps: Step S1. Estimate the grid state through the grid-connected converter, identify the grid impedance parameters, and determine the grid stability state; Step S2. Select the current grid active support mode according to the current signal, the voltage signal and the grid impedance parameter; wherein the grid active support mode includes: When the regional power grid voltage is unbalanced, the active support mode of the current power grid is the voltage regulation control mode; The regional power grid resonates, and the current active support mode of the power grid is the power grid stability control mode; The harmonic detection results of the regional power grid lines exceed the allowable distortion rate, and the active support mode of the current power grid is the harmonic suppression mode; Step S3: Execute the corresponding control strategy according to the active support mode of the power grid.

2. The grid-connected converter grid multi-mode active support decision method based on grid impedance identification according to claim 1 is characterized in that: The priority of the active grid support mode is that the voltage regulation control mode is the first priority, the grid stability control mode is the second priority, and the harmonic suppression mode is the third priority.

3. The grid-connected converter grid multi-mode active support decision method based on grid impedance identification according to claim 1 is characterized in that: When the active support mode is the voltage regulation control mode, under an asymmetric power grid, a PI control method is adopted to control the reactive current output by the grid-connected converter to be equal to the reactive current command value based on the calculated reactive current command value, so as to support the grid connection point voltage; When the active support mode is the grid stability control mode, the PRD control method is used to expand the system bandwidth and adjust the voltage and current output by the grid-connected converter; When the active support mode is the harmonic suppression mode, the system harmonic impedance is increased.

4. The grid-connected converter grid multi-mode active support decision method based on grid impedance identification according to claim 3 is characterized in that: The reactive current output by the grid-connected converter is equal to the negative reactive current command value.

5. The grid-connected converter grid multi-mode active support decision method based on grid impedance identification according to claim 3 is characterized in that: When the active support mode is the voltage regulation control mode, under an asymmetric power grid, a PI control method is adopted to control the reactive current output by the grid-connected converter to be equal to the reactive current command value based on the calculated reactive current command value, so as to support the grid connection point voltage, including: Distributed new energy power station output current I g The output voltage U inv 、System voltage U g The relationship between can be expressed in the dq coordinate system using the following formula: in, and is the positive and negative sequence voltage output by the grid-connected converter on the dq axis; and is the positive and negative sequence voltage of the dq axis of the power grid; and is the positive and negative sequence current of the dq axis of the power grid; R g is the equivalent resistance of the power grid; L g is the equivalent inductance of the power grid; The PI controller tracks the system error signal and obtains the system reactive current command value according to the following calculation expression: Among them, k pv is the proportional coefficient of the PI controller, k iv is the integral coefficient of the PI controller, is the AC voltage reference value; Based on the calculated reactive current command value, the reactive current output by the grid-connected converter is controlled to be equal to the reactive current command value, so as to support the grid-connected point voltage.

6. The grid-connected converter grid multi-mode active support decision method based on grid impedance identification according to claim 3 is characterized in that: When the active support mode is the grid stability control mode, the PRD control method is used to expand the system bandwidth and adjust the system voltage and current, including: using a pole configuration-based control scheme, according to the grid impedance estimated in step S1 and the ideal closed-loop characteristic equation and the actual closed-loop characteristic equation of the grid-connected converter current inner loop, calculate the parameter K of the PRD controller p , K r and K d , regulate the voltage and current output by the grid-connected converter.

7. The grid-connected converter grid multi-mode active support decision method based on grid impedance identification according to claim 3 is characterized in that: When the active support mode is the harmonic suppression mode, based on the PRD controller K r The upper limit of the parameter, which regulates the current K of the controller r Parameters to increase the system harmonic impedance.

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