Grid-connected VSG low-frequency oscillation suppression strategy for reactive loop forward channel phase correction

Abstract

The invention discloses a grid-connected VSG low-frequency oscillation suppression strategy for reactive loop forward channel phase correction, which is characterized in that a P / Q-omega / U low-frequency small signal model of a grid-connected VSG and a power grid is constructed, and analysis is carried out by adopting an argument principle, so that the system stability depends on a determinant Z sigma (s) of the sum of equivalent P / Q-omega / U power impedance of the grid-connected VSG and the power grid. Through research, under interactive coupling of an active loop and a reactive loop, low-frequency phase lag of a grid-connected VSG reactive loop forward channel enables Zsigma (s) to have a negative resistive component, and low-frequency oscillation may be caused. In combination with an impedance passivity design theory, a feedforward additional branch is introduced into a reactive loop forward channel to carry out lead correction on phase lag of an integration element, so that suppression of low-frequency oscillation is realized. According to the invention, the active loop transient inertia damping support control and the reactive loop steady-state voltage support control of the grid-connected VSG are not changed, the structure is simple, and the method is easy to realize.

Description

Technical Field

[0001] This invention relates to the field of new energy grid-connected power generation technology, and in particular to a grid-connected VSG low-frequency oscillation suppression strategy for reactive power loop forward channel phase correction. Background Technology

[0002] With the introduction of the "dual carbon" target, new energy power generation such as photovoltaics and wind power has developed rapidly, and the power system is showing a "dual high" trend of high proportion of new energy and high proportion of power electronic equipment. Traditional new energy units, when connected to the grid using converters based on phase-locked loop (PLL) grid-following (GFL) control strategies, exhibit good stable operation under strong grid conditions. However, with high penetration rates of power electronic equipment, the system exhibits "low inertia and weak damping" characteristics of a weak grid, making the grid-following control strategy prone to oscillation and instability. To address these issues, the Virtual Synchronous Generator (VSG) control strategy has been introduced. It possesses inertial damping response, can actively support voltage and frequency, and exhibits good stability under weak grid conditions, making it a typical grid-forming (GFM) control strategy.

[0003] However, when grid-connected VSG control is applied to low-voltage power grids, the grid line impedance exhibits high resistivity and inductance. This exacerbates power coupling, potentially worsening the interaction between the converter and the grid and leading to oscillation instability. To address this, scholars both domestically and internationally have proposed strategies such as virtual impedance control and active-reactive power decoupling control to suppress low-frequency oscillations caused by power coupling. However, virtual impedance control reduces the power transmission capacity of the grid-connected VSG, and active-reactive power decoupling control complicates the system control structure, increasing control difficulty. This invention constructs a low-frequency small-signal model of the P / Q-ω / U of the grid-connected VSG and the power grid, and uses the argument principle to determine the determinant Z of the equivalent P / Q-ω / U power impedance sum of the grid-connected VSG and the power grid. ∑ (s) Stability analysis revealed that low-frequency phase lag in the forward path of the reactive power loop could cause low-frequency oscillation divergence. Therefore, a feedforward branch with a proportional gain and low-pass filter was introduced into the reactive power loop to optimize its phase characteristics, ensuring sufficient low-frequency stability margin for the grid-connected VSG and enhancing its low-frequency stability. Summary of the Invention

[0004] This application addresses the shortcomings of existing VSG control techniques in reducing low-frequency oscillations caused by increased coupling in the power control loop under high impedance-to-inductance ratio networks. It proposes a grid-connected VSG low-frequency oscillation suppression strategy based on forward path phase correction of the reactive power control loop. By introducing a feedforward branch control employing a low-pass filter, phase correction is performed on the integral element of the reactive power control loop, improving the low-frequency output phase-frequency characteristics of the reactive power control loop and thus enhancing the low-frequency stability of the VSG. Furthermore, the proposed control strategy does not increase the control order of the VSG's active power output, is simple to apply in engineering, and has significant engineering application value.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0006] The low-frequency oscillation suppression strategy of grid-connected VSG with forward channel phase correction of reactive power loop includes a power control loop and a voltage and current inner loop. The power control loop is divided into an active power loop and a reactive power loop. By constructing a low-frequency small-signal model of grid-connected VSG and grid P / Q-ω / U, a feedforward branch with proportional and low-pass filter is introduced into the reactive power loop to perform phase correction on the integral link of the reactive power loop to eliminate the negative resistive component of the system and achieve low-frequency oscillation suppression.

[0007] As a further technical solution of the present invention: the active power loop controlled by VSG simulates the characteristics of the rotor motion equation, and the specific expression is as follows:

[0008]

[0009] In the formula, P ref Rated active power; P is output active power; δ and ω are the virtual power angle and virtual output angular frequency, respectively, and ω0 represents the rated angular frequency; J and D p These are virtual inertia and virtual damping, respectively.

[0010] As a further technical solution of the present invention: the traditional reactive power loop controlled by VSG simulates the characteristics of the excitation regulator, and the specific expression is as follows:

[0011]

[0012] In the formula, Q ref Rated reactive power; Q is output reactive power; U ref and U c These are the given voltage and the actual output voltage, respectively; K i It is the reactive power integral coefficient, D q It is the voltage droop coefficient.

[0013] As a further technical solution of the present invention: the grid-connected VSG and the P / Q-ω / U small-signal model of the power grid can obtain the output power at the PCC point, and the specific expression is as follows:

[0014]

[0015] In the formula, ΔP p and ΔQ p These are the small-signal quantities of active and reactive power output at the PCC point, respectively; ΔP ref and ΔQ ref These are small-signal quantities representing given active and reactive power, respectively; Δω g and ΔU g These represent the grid angular frequency and voltage small-signal quantity, respectively; I is the identity matrix; Y b (s), Y g (s), K(s) and K g (s) are all 2×2 transfer function matrices.

[0016] As a further technical solution of the present invention: the closed-loop transfer function representing the stability of the interaction between the grid-connected VSG and the power grid is:

[0017]

[0018] In the formula, Z b (s) and Z g (s) represent the equivalent power impedances on the converter side and the grid side, respectively;

[0019] Assuming that both the grid-connected VSG and the grid can operate independently and stably, the stability of the system depends on the determinant Z of the sum of the equivalent P / Q-ω / U power impedances of the grid-connected VSG and the grid. ∑ (s)=|Z b (s)+Z g (s)|, that is, if and only if Z ∑ (s) The system is stable when there are no zeros in the right half-plane.

[0020] As a further technical solution of the present invention: the determinant Z ∑ (s) When there is no zero point in the right half-plane, according to the argument principle, Z ∑ (s) The Nyquist curve does not wrap around the origin, i.e., Z ∑ The phase frequency response curve of (s) cannot cross -180°. Combined with the passive impedance design theory, Z... ∑ (s) is divided into positive resistive components. With negative resistive component Two parts, positive resistive component This indicates that the phase frequency response of its real part is always within [-90°, 90°], with a negative resistive component. This indicates that the phase frequency response of its real part exists outside the range of [-90°, 90°]. A conservative design is adopted to correct for the negative resistive component. When the phase frequency characteristic is within the range of [-90°, 90°], the system achieves stability, and the negative resistive component is eliminated. The phase frequency characteristic is expressed as:

[0021]

[0022] Where K1 and K2 are both constants; φ P With φ Q These represent the output phases of the active power loop forward channel and the reactive power loop forward channel, respectively; φ cp This indicates the power coupling phase. Due to the integral element in the forward path of the reactive power loop, the output phase φ Q It exhibits a lag characteristic.

[0023] As a further technical solution of the present invention: the suppression strategy is specifically controlled as follows:

[0024] In the reactive power loop, a feedforward control branch using a proportional gain and a low-pass filter is introduced to correct the phase lag characteristic of the integrator in the low-frequency band. The reactive power loop expression then becomes:

[0025]

[0026] In the formula, K q ω is the proportional adjustment coefficient for the phase correction branch. c This is the cutoff frequency of the low-pass filter.

[0027] As a further technical solution of the present invention, the design requirements for the feedforward control branch parameters of the low-pass filter are as follows:

[0028] Cutoff frequency ω c Take the low-frequency dominant frequency or the low-frequency band boundary value, and the proportional adjustment coefficient K. q Satisfy constraints:

[0029]

[0030] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages:

[0031] The present invention discloses a low-frequency oscillation suppression strategy for grid-connected VSG with reactive power loop forward channel phase correction. This strategy addresses the shortcomings of existing grid-connected VSG control technologies in addressing the low-frequency oscillation problem caused by increased coupling in the power control loop under high impedance-inductance ratio networks. By improving the output phase frequency characteristics of the reactive power control loop in the low-frequency band, the low-frequency stability of the grid-connected VSG is improved.

[0032] Unlike existing technologies, this invention only adds control to the reactive power loop, without affecting the active power loop's inertial response and transient support. It does not increase the control order of the VSG output active power, nor does it introduce additional nonlinear power coupling into the proposed control strategy. This invention requires fewer design parameters, is simple to apply in engineering, and has good engineering application value. Attached Figure Description

[0033] Figure 1 This is a topology diagram of grid-connected VSG control with forward channel phase correction including reactive power loop;

[0034] Figure 2 This is a block diagram for a dual closed-loop control system for voltage and current.

[0035] Figure 3 Diagram of the P / Q-ω / U small-signal model for a grid-connected VSG system;

[0036] Figure 4 Power waveforms of a traditional VSG under different impedance-to-inductance ratios;

[0037] Figure 5 The graph shows a comparison of the power waveforms of the traditional VSG and the proposed strategy. Detailed Implementation

[0038] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0039] Example 1:

[0040] Figure 1 This is a grid-connected VSG control topology diagram with forward phase correction for reactive power loop, including the main circuit and control module. The VSG of the main circuit is connected to the infinite grid via an LC filter, line impedance, and grid impedance, where U... dc R is the ideal DC voltage source for the DC side of the inverter; fi For the filter resistor, L fi For the filter inductance, corresponding to the virtual stator armature resistance and virtual synchronization inductance respectively; C fi For filtering capacitors; PCC is the point of common coupling (PCC); L li R li These are the line inductance and line resistance, respectively, L g R g These are the equivalent inductance and resistance of the power grid, respectively; S i S p and S gThese represent the apparent power on the output side, at the PCC point, and on the grid side, respectively. It is the space vector of the VSG output voltage. and These represent the space vectors of PCC voltage and grid voltage, respectively. The control module consists of a power outer loop and a voltage-current inner loop. The power outer loop simulates the rotor mechanical model and the excitation regulator model, providing frequency and voltage support for the VSG control system. The voltage-current inner loop controls the output voltage and output current of the VSG control system, improving the accuracy and stability of the VSG control system. Figure 2 This is the inner loop control block diagram for the voltage and current dual closed loop.

[0041] like Figure 1 As shown, the grid-connected VSG low-frequency oscillation suppression strategy for reactive power loop forward channel phase correction according to the present invention includes the following steps:

[0042] S1: Construct a mathematical model of the VSG power control loop, which includes an active power control loop and a reactive power control loop;

[0043] S2: Calculate the output power, construct the P / Q-ω / U small-signal model of the grid-connected VSG and the power grid, and obtain the small-signal expression of the output power;

[0044] S3: Analysis using the argument principle shows that system stability depends on the determinant Z of the sum of the grid-connected VSG and the grid's equivalent P / Q-ω / U power impedance. ∑ (s), and the low-frequency phase lag of the reactive power loop forward channel will cause Z ∑ (s) has a negative resistive component, which may cause low-frequency oscillations;

[0045] S4: Combining the passive impedance design theory, a feedforward branch with a proportional low-pass filter is introduced into the reactive power control loop to perform phase correction on the integral element of the reactive power control loop, thereby enhancing the low-frequency stability of the system.

[0046] The active power control loop of the VSG control described in step S1 simulates the characteristics of the rotor motion equation, and the specific expression is as follows:

[0047]

[0048] In the formula, P ref Rated active power; P is output active power; δ and ω are the virtual power angle and virtual output angular frequency, respectively, and ω0 represents the rated angular frequency; J and D p These are virtual inertia and virtual damping, respectively.

[0049] The conventional reactive power control loop of VSG control described in step S1 simulates the characteristics of the excitation regulator, and its specific expression is as follows:

[0050]

[0051] In the formula, Q ref Rated reactive power; Q is output reactive power; U ref and U c These are the given voltage and the actual output voltage, respectively; K i It is the reactive power integral coefficient, D q It is the voltage droop coefficient.

[0052] The small-signal expression for the output power obtained by constructing the P / Q-ω / U small-signal model of the grid-connected VSG and the power grid in step S2 is as follows:

[0053]

[0054] In the formula, ΔP p and ΔQ p These are the small-signal quantities of active and reactive power output at the PCC point, respectively; ΔP ref and ΔQ ref These are small-signal quantities representing given active and reactive power, respectively; Δω g and ΔU g These represent the grid angular frequency and voltage small-signal quantity, respectively; I is the identity matrix; Y b (s), Y g (s), K(s) and K g (s) are all 2×2 transfer function matrices.

[0055] The closed-loop transfer function for the stability of the interaction between the grid-connected VSG and the power grid is as follows:

[0056]

[0057] In the formula, Z b (s) and Z g (s) represent the equivalent power impedances on the converter side and the grid side, respectively.

[0058] Assuming that both the grid-connected VSG and the grid can operate independently and stably, the stability of the system depends on the determinant Z of the sum of the equivalent P / Q-ω / U power impedances of the grid-connected VSG and the grid. ∑ (s)=|Z b (s)+Z g (s)|, that is, if and only if Z ∑ (s) The system is stable when there are no zeros in the right half-plane.

[0059] According to the argument principle, as stated in step S3, Z ∑ (s) When there is no zero point in the right half-plane, Z ∑ (s) The Nyquist curve does not wrap around the origin, i.e., Z ∑The phase frequency response curve of (s) cannot cross -180°. This can be addressed by combining impedance passive design theory with Z... ∑ (s) is divided into positive resistive components. With negative resistive component Two parts, positive resistive component This indicates that the phase frequency response of its real part is always within [-90°, 90°], with a negative resistive component. This indicates that the phase frequency response of its real part exists outside the range of [-90°, 90°]. A conservative design is adopted, requiring only the correction of the negative resistive component. The system is stable when its phase frequency response is within the range of [-90°, 90°]. Negative resistive component. The phase frequency response can be approximated as:

[0060]

[0061] Where K1 and K2 are both constants; φ P With φ Q These represent the output phases of the active power loop forward channel and the reactive power loop forward channel, respectively; φ cp This indicates the power coupling phase. Due to the integral element in the forward path of the reactive power loop, the output phase φ... Q It exhibits a lag characteristic.

[0062] Step S4 is as follows: Based on small-signal analysis, it is known that improving the forward channel phase characteristics of the reactive power loop can reduce the influence of the negative resistive component. A feedforward control branch using a low-pass filter is introduced into the reactive power loop to correct the low-frequency phase characteristics of the integrator, thereby reducing the negative phase characteristics of the reactive power loop output forward channel. This makes the reactive power loop expression become:

[0063]

[0064] In the formula, K q ω is the proportional adjustment coefficient for the phase correction branch. c This is the cutoff frequency of the low-pass filter.

[0065] The design requirements for feedforward control branch parameters are as follows:

[0066] Because the low-frequency band is relatively narrow, the cutoff frequency ω c The dominant low-frequency frequency or the low-frequency band boundary value can be selected. When the phase effect of power coupling is significant, the negative phase effect of the power loop should be completely eliminated to maximize the system stability margin. Therefore, the proportional adjustment coefficient K... q Constraints must be met:

[0067]

[0068] In the actual design process, it is necessary to combine the steady-state and dynamic requirements of the system and then further adjust the parameters.

[0069] Example 2:

[0070] Based on Example 1, a simulation model of a grid-connected VSG control system was built on the Matlab / Simulink platform, and simulation experiments were conducted to verify the effectiveness and correctness of the grid-connected VSG low-frequency oscillation suppression strategy with reactive power loop forward channel phase correction provided in Example 1. The main simulation parameters are shown in Table 1. Due to the narrow low-frequency band, the low-pass filter cutoff frequency ω... c 2.5Hz is acceptable.

[0071] Figure 4 The graph shows the power waveforms of a traditional VSG control system with impedance ratios of 0.5, 1.0, 1.5, and 2. As the R / X ratio increases, the low-frequency oscillations gradually increase and even diverge. When R / X is less than 1, the system exhibits slight oscillations and reaches steady state after a relatively short time. When R / X is 1.5, the system exhibits prolonged low-frequency oscillations and gradually converges. When R / X is 2, the system oscillates and diverges. Therefore, in grid-connected systems with low R / X values, low-frequency oscillations are not severe. However, in grid-connected systems with high R / X values, low-frequency oscillations become more pronounced and may lead to instability.

[0072] Comparison of power waveforms between traditional VSG and reactive power loop forward path, as follows: Figure 5 As shown, under high impedance-to-inductance ratio conditions, when J = 1.2pu, the VSG system exhibits low-frequency divergence, with an oscillation divergence frequency of approximately 2Hz. When the proposed reactive power feedforward compensation strategy is adopted, the system recovers stable operation within 0.5s. Simulation results verify the effectiveness of the proposed strategy.

[0073] Table 1: Main Parameters of Grid-Connected VSG System

[0074]

[0075]

[0076] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0077] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment have been appropriately combined to form other embodiments that are easy for those skilled in the art to understand.

Claims

1. A grid-connected VSG low-frequency oscillation suppression strategy with reactive power loop forward channel phase correction, characterized in that, Includes the following steps: S1. Construct a mathematical model of the VSG power control loop, which includes an active power control loop and a reactive power control loop; S2. Calculate the output power: Construct the P / Q-ω / U small-signal model of the grid-connected VSG and the power grid to obtain the small-signal expression for the output power; S3. System Stability Analysis: Analysis using the argument principle reveals that system stability depends on the determinant Z of the sum of the grid-connected VSG and the equivalent P / Q-ω / U power impedance of the grid. ∑ (s), and the low-frequency phase lag of the reactive power loop forward channel will cause Z ∑ (s) has a negative resistive component, which will cause low-frequency oscillation; S4. Combining the passive impedance design theory, a feedforward branch with a proportional low-pass filter is introduced into the reactive power control loop to perform phase correction on the integral element of the reactive power control loop, thereby enhancing the low-frequency stability of the system.

2. The grid-connected VSG low-frequency oscillation suppression strategy for reactive power loop forward channel phase correction according to claim 1, characterized in that, The active power control loop of the VSG control described in step S1 simulates the characteristics of the rotor motion equation, and the specific expression is as follows: In the formula, P ref Rated active power; P is output active power; δ and ω are the virtual power angle and virtual output angular frequency, respectively, and ω0 represents the rated angular frequency; J and D p These are virtual inertia and virtual damping, respectively.

3. The grid-connected VSG low-frequency oscillation suppression strategy for reactive power loop forward channel phase correction according to claim 2, characterized in that, In step S1, the traditional reactive power loop controlled by VSG simulates the characteristics of the excitation regulator, and the specific expression is as follows: In the formula, Q ref Rated reactive power; Q is output reactive power; U ref and U c These are the given voltage and the actual output voltage, respectively; K i It is the reactive power integral coefficient, D q It is the voltage droop coefficient.

4. The grid-connected VSG low-frequency oscillation suppression strategy for reactive power loop forward channel phase correction according to claim 3, characterized in that, In step S2, constructing the P / Q-ω / U small-signal model of the grid-connected VSG and the power grid yields the output power at the PCC point, specifically expressed as: In the formula, ΔP p and ΔQ p These are the small-signal quantities of active and reactive power output at the PCC point, respectively; ΔP ref and ΔQ ref These are small-signal quantities representing given active and reactive power, respectively; Δω g and ΔU g These represent the grid angular frequency and voltage small-signal quantity, respectively; I is the identity matrix; Y b (s), Y g (s), K(s) and K g (s) are all 2×2 transfer function matrices.

5. The grid-connected VSG low-frequency oscillation suppression strategy for reactive power loop forward channel phase correction according to claim 4, characterized in that, The closed-loop transfer function representing the stability of the interaction between the grid-connected VSG and the power grid is: In the formula, Z b (s) and Z g (s) represent the equivalent power impedances on the converter side and the grid side, respectively; Assuming that both the grid-connected VSG and the grid can operate independently and stably, the stability of the system depends on the determinant Z of the sum of the equivalent P / Q-ω / U power impedances of the grid-connected VSG and the grid. ∑ (s)=|Z b (s)+Z g (s)|, that is, if and only if Z ∑ (s) The system is stable when there are no zeros in the right half-plane.

6. The grid-connected VSG low-frequency oscillation suppression strategy for reactive power loop forward channel phase correction according to claim 5, characterized in that, The determinant Z ∑ (s) When there is no zero point in the right half-plane, according to the argument principle, Z ∑ (s) The Nyquist curve does not wrap around the origin, i.e., Z ∑ The phase frequency response curve of (s) cannot cross -180°. Combined with the passive impedance design theory, Z... ∑ (s) is divided into positive resistive components. With negative resistive component Two parts, positive resistive component This indicates that the phase frequency response of its real part is always within [-90°, 90°], with a negative resistive component. This indicates that the phase frequency response of its real part exists outside the range of [-90°, 90°]. A conservative design is adopted to correct for the negative resistive component. When the phase frequency characteristic is within the range of [-90°, 90°], the system achieves stability, and the negative resistive component is eliminated. The phase frequency characteristic is expressed as: Where K1 and K2 are both constants; φ P With φ Q These represent the output phases of the active power loop forward channel and the reactive power loop forward channel, respectively; φ cp This indicates the power coupling phase. Due to the integral element in the forward path of the reactive power loop, the output phase φ Q It exhibits a lag characteristic.

7. The grid-connected VSG low-frequency oscillation suppression strategy for reactive power loop forward channel phase correction according to claim 5, characterized in that, The specific control of the suppression strategy is as follows: In the reactive power loop, a feedforward control branch using a proportional gain and a low-pass filter is introduced to correct the phase lag characteristic of the integrator in the low-frequency band. The reactive power loop expression then becomes: In the formula, K q ω is the proportional adjustment coefficient for the phase correction branch. c This is the cutoff frequency of the low-pass filter.

8. The grid-connected VSG low-frequency oscillation suppression strategy for reactive power loop forward channel phase correction according to claim 1, characterized in that, The design requirements for the feedforward control branch parameters of the low-pass filter are as follows: Cutoff frequency ω c Take the low-frequency dominant frequency or the low-frequency band boundary value, and the proportional adjustment coefficient K. q Satisfy constraints:

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