Direct-current voltage synchronous inverter low-frequency oscillation suppression method based on torque quantization

Abstract

The invention belongs to the field of grid-connected converter control methods, and discloses a torque quantization-based low-frequency oscillation suppression method for a direct-current voltage synchronous inverter, which constructs a grid-connected damping torque model of the inverter, quantifies a damping torque and a synchronous torque of a system and describes oscillation characteristics of the system. When oscillation is unstable and damping or synchronous torque is a negative value, the provided compensation torque stabilizer is introduced through angular frequency deviation, a lag phase of the system at the oscillation frequency is compensated by using a lead-lag corrector, and then the compensation torque stabilizer is fed back to an active power reference value. By designing the compensation torque, it is ensured that the synchronous torque and the damping torque of the system are positive values all the time, and oscillation instability is effectively restrained. The method is suitable for a small-disturbance working condition, the steady-state characteristic of inverter operation is not changed, the grid-connected stability of the grid-forming type photovoltaic inverter is improved, and meanwhile scheduling optimization and energy efficiency evaluation of the system are not affected.

Description

Technical Field

[0001] The invention belongs to the field of grid-connected converter control, and in particular relates to a method for suppressing low-frequency oscillation of a DC voltage synchronous inverter based on torque quantization. Background Art

[0002] Compared with grid-following inverters, grid-building inverters can provide fast voltage and frequency support, and show better adaptability and stabilization capabilities in future renewable energy grid-connected systems with weak grid strength and low physical inertia. However, the output power of renewable energy, mainly photovoltaic / wind power, is intermittent, and it is necessary to achieve maximum power point tracking by adjusting its DC link voltage. In order to meet the needs of grid-building control and maximum power point tracking, DC link voltage synchronous control photovoltaic inverter (PV-DVSC) came into being. This method tracks the dynamic characteristics of the DC link voltage to achieve self-synchronization, and uses the energy of the capacitor to simulate the rotor energy of the synchronous machine. However, there is a mathematical equivalence between the dynamic characteristics of the DC link voltage and the dynamics of the synchronous machine rotor, and its control also has the low-frequency oscillation problem of the traditional synchronous machine (SG). This type of instability problem is caused by the equivalent negative damping of the rotor motion equation, so it is also called synchronous instability.

[0003] In terms of low-frequency oscillation suppression measures for grid-connected inverters, starting from the optimization control strategy, it can be mainly divided into parameter optimization and additional damper design. Parameter optimization improves the damping of new energy units at oscillation frequencies by adjusting control parameters offline. The additional damping controller mainly achieves active damping of oscillations by optimizing control feedback signals, designing state observers and control algorithms, and signal adjustment filtering. Optimizing the adjustment parameter setting value can improve the damping of the dominant characteristic root to a certain extent. However, the scope of this adjustment is usually limited, and may have an adverse effect on the dynamic characteristics of other frequency bands (such as subsynchronous / supersynchronous frequencies). Therefore, it is necessary to coordinate the coordination and coordination of multiple parameters, and its application in actual engineering is limited.

[0004] It is more common to configure additional control devices to compensate for the additional damping required for each control object, and add them to their respective dispatching or control instructions, thereby effectively improving system performance. Some studies have added filter devices to filter out harmonics, which increases the system cost and system power density; it is more common to add damping controllers to inverter control and adjust the system control structure to achieve the control target. This method configures additional damping devices at different locations, resulting in different suppression effects, and it is difficult to select the best signal to ensure the overall optimal performance of the system. Secondly, the traditional additional damping control qualitatively adjusts the controller parameters based on the distribution results of the characteristic roots, and fails to propose quantitative design steps for application in actual engineering. Under changing working conditions, the traditional method needs to repeatedly adjust the appropriate parameters to be applied to specific working conditions, which increases its computational burden and complexity, and restricts its engineering application potential. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides a method for suppressing low-frequency oscillation of a DC voltage synchronous inverter based on torque quantization.

[0006] The present invention is implemented as follows: a method for suppressing low-frequency oscillation of a DC voltage synchronous inverter based on torque quantization, including: quantizing the total synchronous torque and total damping torque of the system, and ensuring that the total torque of the system after compensation is always positive by designing the compensation torque. The compensation torque is introduced from the angular frequency deviation, and the lead-lag corrector is used to compensate the lagging phase of the system at the oscillation frequency, and then fed back to the reference value of the active power. At the same time, in order to prevent the stabilizer from affecting the steady-state voltage of the system, a DC isolation link is required to be connected in series, so that the torque-based phase compensator (TOPC) is in the form of:

[0007]

[0008] In the formula, K p is the stabilizer gain coefficient, T w is the time constant of the DC isolation link, θ p is the phase to be compensated, a is the lag compensation intensity, ω d is the low-frequency oscillation frequency of the system. The specific steps for quantitatively designing the controller parameters are: according to the oscillation frequency ω d , get the total torque phasor ΔT of the system e ; Set the total damping torque as the inherent damping torque, set the target damping ratio ξ(0~1), and then s =(M d ω b / 2ξ) 2 / (ω b J) Calculate the total synchronous torque of the system after compensation; obtain the compensation torque ΔT corresponding to TOPC p , then we can determine T p (jω d ) has an amplitude of M p , the phase angle is θ p ; Get θ p After that, a can be calculated to obtain M p After that, K can be calculated p .

[0009] Further, according to the solution steps of the above formula, it is necessary to construct the damping torque model of the system, so as to obtain the analytical formula of the total torque of the system. Using the space vector and complex transfer function representation method, the output voltage and current dynamics of the PV-DVSC can be obtained based on the circuit relationship:

[0010]

[0011] Among them, R g , L g are the grid side resistance and grid side inductance, U g is the grid voltage, C f is the machine side filter capacitor, f ac (s) is the PI controller of the voltage loop, ω0 and ω pu is the nominal value and per unit value of the rated angular frequency, U ref is the voltage loop reference value, δ is the difference between the phase angle of the DC voltage synchronous control output and the grid phase angle, also known as the power angle. All quantities in the above formula are per unit values ​​and are in the control coordinate system. Combining the above formula, the output current ΔI g The expression is:

[0012]

[0013] ΔI g The dq component of can be expressed as:

[0014]

[0015] Furthermore, the DC voltage synchronous control (DVSC) is expressed as:

[0016]

[0017] Among them, ω dvsc is the output angular frequency of DVSC, K D and K J are the damping equivalent coefficient and inertia equivalent coefficient, K T is the DC link voltage tracking coefficient, U dc and U dcref is the DC capacitor voltage and its reference value. Combined with the dynamic characteristics of the DC capacitor, the above formula can be expanded as:

[0018]

[0019] Among them, P s is the active power reference value, P e is the inverter output active power. The left side of the above equation represents the equivalent power synchronization equation. Therefore, PV-DVSC has a power-frequency characteristic similar to SG. In contrast, the right side introduces an additional component to track the DC link voltage. Further, linearizing the above equation, we can get:

[0020]

[0021] Analogous to the power synchronization equation of SG, K J C dc / 2 is equivalent to inertia, expressed in J; K D C dc / 2 is equivalent to the damping coefficient, denoted by D.

[0022] Furthermore, combined with the output voltage equation, the active and reactive power input from the inverter to the grid side can be obtained as:

[0023]

[0024] in,

[0025]

[0026] Furthermore, by combining the expressions of active power, reactive power and current components, the relationship between active power and reactive power with respect to power angle and voltage reference value can be constructed:

[0027] ΔP e =K1Re(G 11 (s))Δδ+K1Re(G 12 (s))ΔU ref +K2Im(G 11 (s))Δδ+K2Im(G 12 (s))ΔU ref +K s Δδ

[0028] =[K1Re(G 11 (s))+K2Im(G 11 (s))+K s ]Δδ+[K1Re(G 12 (s))+K2Im(G 12 (s))]ΔU ref

[0029] =X 11 (s)Δδ+X 12 (s)ΔU ref ΔQ e =K3Re(G 11 (s))Δδ+K3Re(G 12 (s))ΔU ref +K4Im(G 11 (s))Δδ+K4Im(G 12 (s))ΔU ref +K s1 Δδ

[0030] =[K3Re(G 11 (s))+K4Im(G 11 (s))+K s1 ]+[K3Re(G 12 (s))+K4Im(G 12 (s))]ΔU ref

[0031] =X 21 (s)Δδ+X 22 (s)ΔU ref

[0032] Furthermore, the relationship between the power angle and the voltage reference value is solved through the reactive loop. The voltage-reactive equation can be expressed as:

[0033]

[0034] Combined with the expression of reactive power, we can get:

[0035]

[0036] Furthermore, we can get the equation of active power with respect to power angle:

[0037]

[0038] Substituting the above equation into the DC voltage synchronization equation, we can obtain the rotor motion equation considering the DC capacitor dynamics:

[0039]

[0040] Furthermore, we can get ΔP s The closed-loop transfer function with is the input and Δδ is the output:

[0041]

[0042] Where, T(s) = T1(s) + T2(s) + T3(s). Since the LFO mode studied has weak damping characteristics, its real part is close to the imaginary axis. Therefore, σ can be approximately equal to 0, and the corresponding characteristic root s≈jω d . T(s) can be further simplified to

[0043] T(s)≈T(jω d )=ΔM s +jω d ΔM d =ΔM s +ΔM d s

[0044] Among them, ΔM s =Re[T(jω d )],ΔM d =Im[T(jω d )] / ω d .

[0045] Furthermore, the total synchronous torque and total damping torque coefficients of the PV-DVSC are expressed as:

[0046]

[0047] Among them, K s is the inherent synchronization coefficient of the system, and D is the inherent damping coefficient of the system.

[0048] One aspect of the present invention provides a compensating torque stabilizer for executing the method for suppressing low-frequency oscillation of a DC voltage synchronous inverter as described above.

[0049] In view of the technical problems existing in the above-mentioned prior art, some creative technical effects are brought about after solving the problems. The specific description is as follows:

[0050] First, the present invention comprehensively considers the dynamics of the DC bus and studies the synchronous stability of the DC voltage synchronous photovoltaic grid-connected system. The proposed damping torque model decouples the action paths of each loop. In addition, the torque is used to quantify the influence of the inverter control dynamics on the synchronous stability, and a rapid evaluation of the synchronous stability is achieved. This method can intuitively analyze the influence mechanism of the control loop on the inverter synchronous stability from the perspective of damping.

[0051] Existing stability analysis uses graphical methods such as characteristic roots, Bode diagrams, and Nyquist diagrams to qualitatively describe the stability trend under parameter changes, while the damping torque model provided by the present invention uses torque as a performance indicator to quantify the stability margin of the system and intuitively analyze the stability boundary of the control parameters and line parameters. On this basis, a safe operation domain for the system facing all working conditions can be constructed to guide the system parameter design, which helps to enhance the stability of the system in the entire working range and its adaptability to the operating environment.

[0052] The existing additional damping control requires the selection of control feedback signals, and the effects achieved at different positions are different, and it is impossible to determine the optimal signal selection and configuration. When the target characteristic roots are configured with poles, improper parameter adjustment will worsen the distribution of other characteristic roots, and the introduced controller will indirectly affect the stability of the system. The oscillation suppression method proposed in the present invention, on the basis of torque quantization, sets the optimal damping ratio to ensure that the synchronous torque and damping torque of the system are always positive under any working conditions. In addition, the implementation process of the present invention is simple, the control path is clear, and it is easy to expand, and there is potential for development in the synchronous stability suppression strategy of multiple machines.

[0053] Second, the present invention provides a method for suppressing low-frequency oscillation of a DC voltage synchronous inverter based on torque quantification, including model construction and suppression methods. When the parameter design is inappropriate, the proposed damping torque model can quantify the stability margin of the system with torque as the performance indicator and characterize the stability boundary under parameter changes. On the basis of torque quantification, the proposed compensating torque stabilizer takes the optimal synchronous torque and damping torque as the target to achieve the synchronous stability of the system under all working conditions. The proposed control method does not require the addition of additional filtering devices, which reduces the system cost and system power density, and there is no need to consider the impact of the adjustment characteristics of the additional devices themselves on the system; in addition, the corresponding synchronous torque and damping torque can be accurately characterized under various working conditions, and the compensating torque stabilizer can be quantitatively designed based on this, so the proposed method can damp the unstable state of various working conditions, including oscillation divergence or sliding out of step; since the oscillation suppression control method disclosed in the present invention does not change the control structure of the original inverter, it only needs to add a path in the synchronization link to change the active command value to achieve it, which greatly reduces the difficulty of implementation in actual engineering, and the proposed method is suitable for small disturbance conditions, does not change the steady-state characteristics of the inverter during operation, and does not affect the scheduling optimization and energy efficiency evaluation of the original system.

[0054] The method for suppressing low-frequency oscillation of a DC voltage synchronous inverter provided by the present invention provides a smarter, more reliable and more efficient way to stabilize a grid-connected inverter. It can effectively suppress the low-frequency oscillation of the inverter grid-connected under all operating conditions and significantly improve the synchronization stability of the inverter.

[0055] Third, the present invention solves the low-frequency oscillation suppression method of the DC voltage synchronous inverter based on torque quantization, which solves the low-frequency oscillation problem existing in the prior art, especially for the application of weak power grid environment and distributed energy system, and proposes a method for stabilizing the system by quantizing damping torque and synchronous torque. The following are the technical problems solved by the present invention in terms of parameters, algorithms and mathematical models, as well as the significant technical progress achieved:

[0056] 1. Existing technical problems solved

[0057] Low-frequency oscillation problem: In DC voltage synchronous inverters, low-frequency oscillation (generally between 0.1 and 2 Hz) is a common problem. Especially when the grid damping is insufficient or the system inertia is low, low-frequency oscillation can cause system instability and affect the power quality.

[0058] Stability issues in weak grid environments: With the massive access of renewable energy, especially in weak grid environments, the volatility of grid frequency increases, causing inverter power output fluctuations and easily triggering low-frequency oscillations, affecting grid stability.

[0059] Lag of traditional control methods: Traditional PI controllers have deficiencies in dynamic response, especially in compensating for system lag in complex power grid conditions, resulting in the system being unable to suppress low-frequency oscillations in a timely manner.

[0060] Lack of accurate torque compensation methods: In the existing technology, the control strategies for suppressing low-frequency oscillations are mostly based on empirical formulas and rough damping designs, which cannot provide accurate torque compensation, resulting in poor system performance.

[0061] 2. Advances in parameters, algorithms and mathematical models of the present invention

[0062] 2.1. Parameterized design and torque quantification

[0063] The present invention quantifies the damping torque and the synchronous torque, uses a clear mathematical model (such as the damping torque model, the rotor motion equation, etc.) to calculate the target torque and the original torque of the system, and designs a lead-lag corrector at the oscillation frequency to compensate for the phase lag of the system. The introduction of this quantification method can accurately control the torque change of the system, thereby effectively suppressing the low-frequency oscillation problem.

[0064] Quantification of damping torque and synchronous torque: The phase and amplitude of the system torque are calculated through clear formulas, and combined with the target torque to compensate for the insufficient part, thus achieving accurate oscillation suppression.

[0065] Stabilizer parameter design: The stabilizer in the present invention uses an algorithm to optimize the gain parameters (such as the stabilizer gain coefficient K p , time constant T w etc.), so that the control system has good phase compensation capability at the low-frequency oscillation point, ensuring the stability and performance of the entire system.

[0066] 2.2. Modeling and optimization based on dynamic equations

[0067] By combining the dynamic equations of the transmission line and the inverter (including the line dynamic equation and the voltage loop dynamic equation), the present invention eliminates the nonlinear factors of the output voltage, optimizes the relationship model between current and power angle, and enables the controller to maintain good dynamic response characteristics under different working conditions.

[0068] Combination of line dynamics and voltage loop dynamics equations: By eliminating the voltage loop output variables and combining the reactive loop dynamics, the relationship between current and power angle is solved, greatly improving the stability of the system.

[0069] Multi-objective optimization algorithm: The present invention sets the target damping torque and synchronous torque through a multi-objective optimization algorithm, so that the system can be adjusted adaptively, thereby improving the anti-disturbance capability of the inverter under weak grid conditions.

[0070] 2.3. Lead-lag corrector design

[0071] The stabilizer design in the present invention uses a lead-lag corrector to compensate for the lag of the system phase, so that the system can maintain sufficient stability margin at the low-frequency oscillation point. Through the optimization of mathematical models and parameters, the total torque of the system is ensured to be always positive, avoiding the oscillation out-of-control problem caused by insufficient phase compensation in traditional systems.

[0072] Improved stability: The algorithm enables the system to respond quickly through precise phase compensation, avoiding system oscillation caused by lag. At the same time, the series DC isolation link ensures that the steady-state voltage is not affected.

[0073] 3. Significant technical advancement of the present invention

[0074] Improve system anti-interference and stability: Through precise torque quantization design, the present invention greatly improves the stability of the inverter in a weak power grid environment, allowing the system to maintain efficient operation in the face of frequency fluctuations and low inertia conditions.

[0075] Improve system response speed and accuracy: Compared with traditional PI control, the present invention significantly improves the dynamic response speed and control accuracy of the system through the design of lead-lag corrector and torque quantization, reduces the oscillation caused by lag, and ensures the steady-state and dynamic performance of the system.

[0076] Enhance the grid-connected stability of renewable energy systems: In the grid-connected applications of renewable energy such as wind power and photovoltaics, the present invention effectively suppresses the low-frequency oscillations caused by power fluctuations, improves the grid-connected stability and reliability of renewable energy systems, and provides important technical support for large-scale renewable energy access.

[0077] Reduce system energy consumption and failure rate: By effectively suppressing the low-frequency oscillation of the inverter, the present invention reduces excessive use and wear of the equipment, reduces the energy consumption and failure rate of the system, and extends the service life of the equipment.

[0078] The present invention solves the low-frequency oscillation problem in the prior art through precise torque quantization design and algorithm optimization, improves the dynamic response and anti-interference ability of the system, and has significant technological progress and practical industrial value, especially in the application of weak power grids and renewable energy grid connection. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments of the present invention. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0080] Figure 1 It is a control block diagram of a DC voltage synchronous control inverter grid-connected system and a structure diagram of a low-frequency oscillation suppression method provided by an embodiment of the present invention;

[0081] Figure 2 is the damping torque model of the system under study provided by an embodiment of the present invention;

[0082] Figure 3 is an action path diagram of the oscillation suppression method provided by an embodiment of the present invention;

[0083] Figure 4 is a phasor diagram of the oscillation suppression method provided by an embodiment of the present invention;

[0084] Figure 5 is a Bode diagram before and after adding the oscillation suppression method provided by an embodiment of the present invention;

[0085] Figure 6 are the synchronization and damping torques under different grid strengths provided by the embodiments of the present invention;

[0086] Fig. 7A is a diagram showing the hardware-in-the-loop test results of the oscillation suppression method provided by an embodiment of the present invention in working condition a;

[0087] Figure 7B is a diagram showing the hardware-in-the-loop test results of the oscillation suppression method provided by an embodiment of the present invention in working condition b;

[0088] Figure 7C is a diagram showing the hardware-in-the-loop test results of the oscillation suppression method provided by an embodiment of the present invention in working condition c;

[0089] Fig.7D is a diagram showing the hardware-in-the-loop test results of the oscillation suppression method provided by an embodiment of the present invention in working condition d;

[0090] Figure 8 This is the hardware-in-the-loop experimental result diagram of the traditional additional damping control method in working condition d. DETAILED DESCRIPTION

[0091] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0092] The following is a specific embodiment, which relates to the application of a method for suppressing low-frequency oscillation of a DC voltage synchronous inverter based on torque quantization:

[0093] The present invention constructs a damping torque model for the grid-connected DC voltage synchronous inverter, quantifies the synchronous torque and damping torque of the system, helps to intuitively analyze the stable operation limit, and issues an early warning when there is a synchronous torque or the damping torque is negative, so as to take the proposed oscillation suppression measures to keep the system within the safe operation domain. In addition, the quantitative index can guide the design of system parameters, which helps to enhance the stability of the system in the entire working range and its adaptability to the operating environment. On this basis, the proposed low-frequency oscillation suppression method directly acts on the synchronous control, adjusts the power angle output of the system to ensure that the photovoltaic inverter maintains synchronous stability with the power grid, and significantly improves the low-frequency oscillation control performance. Under the condition of changing working conditions, its torque quantization design has good adaptability and reflects strong engineering application potential. The method is universal and universal for other types of inverters, helping the inverter to suppress oscillation phenomena under various operating conditions and provide stable phase output to meet the requirements of the power grid.

[0094] In view of the problems existing in the prior art, the present invention provides a method for suppressing low-frequency oscillation of a DC voltage synchronous inverter based on torque quantization. The present invention is described in detail below in conjunction with the accompanying drawings.

[0095] like Figure 1 As shown, the method for suppressing low-frequency oscillation of a DC voltage synchronous inverter based on torque quantization provided by an embodiment of the present invention includes a photovoltaic inverter module based on DC voltage synchronous control and the proposed oscillation suppression method module. The inverter of the main circuit part is connected to the infinite AC power grid through the LCL filter and the transmission line, wherein the grid-side filter inductor can be combined with the line inductor (collectively referred to as L g The outer control loop generates the phase δ and voltage amplitude U by the DC voltage synchronization equation and the reactive-voltage control link. ref The command is then input into the voltage and current dual loop to improve the voltage regulation and current limiting characteristics.

[0096] according to Figure 1 The circuit part of the inverter transmission line dynamic equation can be obtained as follows:

[0097]

[0098] Further linearization yields

[0099]

[0100] The active and reactive power input to the grid from the generator side is

[0101]

[0102] in,

[0103]

[0104] Using space vector and complex transfer function representation methods, the output voltage and current dynamics of the system can be obtained based on the circuit relationship:

[0105]

[0106] Eliminating the output voltage variable, the dynamics of the output current can be obtained as:

[0107]

[0108] in,

[0109]

[0110] Furthermore, the voltage-reactive power equation and DC voltage synchronization equation can be expressed as:

[0111]

[0112] Furthermore, combining the dynamic equations of current and active power, the relationship between active power and power angle can be obtained as follows:

[0113]

[0114] Therefore, the equivalent rotor motion equation of the studied system is:

[0115]

[0116] According to the above formula, the damping torque model of the photovoltaic inverter grid-connected based on DC voltage synchronous control is as follows: Figure 2 As shown. Further, we can get ΔP s The closed-loop transfer function with is the input and Δδ is the output:

[0117]

[0118] Where, T(s) = T1(s) + T2(s) + T3(s). Since the LFO mode studied has weak damping characteristics, its real part is close to the imaginary axis. Therefore, σ can be approximately equal to 0, and the corresponding characteristic root s≈jω d . T(s) can be further simplified to

[0119] T(s)≈T(jω d )=ΔM s +jω d ΔM d =ΔM s +ΔM d s

[0120] Among them, ΔM s =Re[T(jω d )],ΔM d=Im[T(jω d )] / ω d .

[0121] Therefore, the control link introduces additional damping and synchronous torque. Therefore, the total synchronous torque and total damping torque coefficient of the system are expressed as:

[0122]

[0123] Among them, K s is the inherent synchronization coefficient of the system, and D is the inherent damping coefficient of the system. The system can only be stable when the total synchronous torque and the total damping torque are both positive. When the synchronous torque is negative, the system will slip out of step, and when the damping torque is negative, the system will oscillate and diverge.

[0124] Figure 3 Based on the damping torque model, the path of the proposed oscillation suppression method is given by e An equivalent torque is introduced into the path to compensate for the lagging phase of the original torque. Figure 4 The effect diagram of the compensation torque is given and intuitively represented by a phasor diagram. With Δδ as the horizontal coordinate and Δω as the vertical coordinate, the projection of the torque on the horizontal axis is the synchronous torque, and the projection on the vertical axis is the damping torque. Therefore, the torque is in the first quadrant is a necessary and sufficient condition for the stability of the system. When the system oscillates, diverges or is out of sync, the original torque is in the fourth or third quadrant, which are ΔT e1 and ΔT e2 When the total torque after the system compensation is set to be in the first quadrant, the compensation torques are ΔT p1 and ΔT p2 , the leading phase generated by it compensates the negative phase of the original torque of the system. At the same time, in order to prevent the stabilizer from affecting the steady-state voltage of the system, a DC isolation link needs to be connected in series, so that the torque-based phase compensator (TOPC) is in the form of:

[0125]

[0126] When the damping torque model is used to quantify the original total torque ΔT of the system e After that, the inherent damping torque is kept unchanged, and the total synchronous torque after compensation is calculated after setting ξ. ξ is set to 0.707, and the concept of the optimal second-order system is used to achieve fast response and small overshoot. After determining the original torque and the compensated torque, the phase and amplitude of the compensated torque can be calculated. The various parameters of TOPC are calculated according to their expressions.

[0127] Figure 5 Shows T e (s) and T Σ The Bode diagram of (s) corresponds to the total torque before and after compensation, where Te (s) = T e (s)+T p (s). It can be seen that before joining TOPC, T e (s) There will be a resonance peak in the frequency band under study, causing the phase to drop suddenly to a negative value, and the total torque is in the third or fourth quadrant. After adding TOPC, T e The phase of (s) increases and moves to between 0 and 90°, and finally the torque is pulled to the first quadrant, and the system regains stability.

[0128] The present invention provides two embodiments of the invention: first, hardware-in-the-loop experimental results under multiple working conditions using the method disclosed in the present invention; second, hardware-in-the-loop experimental results of a traditional phase compensator under the worst working conditions. Figure 6 The total synchronous torque and total damping torque of the studied system under different grid strengths are quantified. It can be seen from the figure that as the grid strength increases, the system falls into the unstable region. Points a, b, c, and d in the figure fully cover the three unstable regions of torque distribution. Hardware-in-the-loop experiments are carried out under these four working conditions.

[0129] The Typhoon simulation platform was built as follows Figure 1 The system shown in FIG. 1 has parameters as shown in Table 1. In the first embodiment, Fig. 7A -D correspond to the above four working conditions. When the disturbance occurs, the system will oscillate, and the amplitude will gradually diverge. When running under working condition d, the dynamic response of the system after the disturbance is the most intense. Due to the excessive negative synchronous torque, sliding out of step will occur. After starting TOPC under all working conditions, the system returns to stability.

[0130] In the second embodiment, under the worst working condition mentioned above, the phase compensation introduced by the traditional method is insufficient, resulting in the inability to damp the oscillation, and the system continues to oscillate and diverge until it becomes unstable.

[0131] Table 1 Parameters of DC voltage synchronous inverter grid-connected system

[0132]

[0133]

[0134] The following are two industrial application examples of the method for suppressing low-frequency oscillation of a DC voltage synchronous inverter based on torque quantization:

[0135] 1. Grid-connected application of renewable energy (wind farms and photovoltaic power stations)

[0136] In wind farms and photovoltaic power plants, grid-connected inverters are essential for synchronizing renewable energy generation with the grid. However, these systems often face low-frequency oscillation problems due to power input fluctuations and weak grid conditions. By using the low-frequency oscillation suppression method in the present invention, the torque of the system is quantified and compensated, and the method can be applied to wind turbines or photovoltaic inverters. The method can ensure stable power output, improve the stability of the grid and increase the utilization of renewable energy. By adopting this method, the risk of grid disturbances, equipment damage or energy loss can be reduced, thereby enhancing the reliability of renewable energy systems.

[0137] 2. Microgrid Application of Distributed Energy Resources (DER)

[0138] In microgrid systems, distributed energy resources such as battery storage systems, small wind and photovoltaic power generation rely on inverters to interact with the grid. When energy resources frequently switch between island mode and grid-connected mode, especially in weak grid environments or low inertia, low-frequency oscillation problems may occur. This method can be integrated in the inverter control system to effectively suppress these oscillations by stabilizing the interaction between the distributed generation equipment of the microgrid and the main grid. By ensuring good low-frequency damping, this method can improve the operational stability of the microgrid, especially in smart grid applications, and help achieve better energy balance and system reliability.

[0139] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for suppressing low-frequency oscillation of a DC voltage synchronous inverter based on torque quantization, characterized in that: The damping torque and synchronous torque of the system are quantified, and the stabilizer is quantitatively designed to compensate for the lagging phase of the original torque, ensuring that the total torque of the system after compensation is always positive.

2. The method for suppressing low-frequency oscillation of a DC voltage synchronous inverter according to claim 1, characterized in that: The stabilizer uses a lead-lag corrector to compensate for the lag phase of the system at the oscillation frequency. In order to prevent the stabilizer from affecting the steady-state voltage of the system, a DC isolation link must be connected in series. The expression is: In the formula, K p is the stabilizer gain coefficient, T w is the time constant of the DC isolation link, θ p is the phase to be compensated, a is the lag compensation intensity, ω d is the low frequency oscillation frequency of the system.

3. The method for suppressing low-frequency oscillation of a DC voltage synchronous inverter according to claim 2, characterized in that: The phase and amplitude of the torque to be compensated are determined by the original torque phasor and the target torque phasor, and the relationship is: In the formula, θ p is the phase of the compensation torque, M p is the magnitude of the compensation torque, M pd and M ps denote the damping and synchronization components of the compensation torque, ΔT Σ represents the target torque, Represents the angle between the target torque and Δδ, ΔT e Represents the original torque, Represents the angle between the original torque and Δδ.

4. The method for suppressing low-frequency oscillation of a DC voltage synchronous inverter according to claim 3, characterized in that: The damping torque model based on the rotor motion equation is constructed to obtain the analytical expression of the total torque of the system and quantify its damping and synchronous torque. The equation expression is:

5. The method for suppressing low frequency oscillation of a DC voltage synchronous inverter according to claim 4, characterized in that: Combine the line dynamic equation and the power dynamic equation, eliminate the voltage variable, and solve the relationship between power, power angle, and current. Among them, the transmission line dynamics is: Where U t is the filter capacitor voltage, I g is the inverter output current, R g , L g are the grid side resistance and grid side inductance, U g is the grid voltage, δ is the difference between the phase angle of the DC voltage synchronous control output and the grid phase angle, also known as the power angle. The active and reactive power input from the inverter to the grid side are: in, 6. The method for suppressing low frequency oscillation of a DC voltage synchronous inverter according to claim 4, characterized in that: Combining the line dynamics and voltage loop dynamics equations, eliminating the output voltage variable, the relationship between current and power angle is solved. Among them, the voltage loop dynamics equation is: In the formula, f ac (s) is the PI controller of the voltage loop, U ref is the reference value of the reactive loop input to the voltage loop. The dynamics of the output current can be obtained as: in, 7. The method for suppressing low frequency oscillation of a DC voltage synchronous inverter according to claim 6, characterized in that: Combined with the dynamic equation of the reactive loop, the relationship between the voltage loop reference value dynamics and the power angle can be solved. Among them, the dynamic equation of the reactive loop is: Then the voltage loop reference value dynamics is:

8. The method for suppressing low-frequency oscillation of a DC voltage synchronous inverter according to claims 5 to 7, characterized in that: Combining the control loop dynamics and line dynamics, the relationship between active power and power angle is: The rotor motion equation based on the above formula is expressed as:

9. The method for suppressing low-frequency oscillation of a DC voltage synchronous inverter according to claim 8, characterized in that: For the weakly damped low-frequency oscillation mode, the synchronous torque and damping torque coefficients of the system are expressed as: Among them, ΔM s =Re[T(jω d )],ΔM d =Im[T(jω d )] / ω d For the setting of target torque, the inherent damping torque can be set as the target damping torque, and the target synchronous torque is obtained by the optimal damping ratio constraint, then: Among them, ω b is the reference angular frequency, and J is the equivalent inertia.

10. A controller, characterized in that: Used to execute the low-frequency oscillation suppression method of a DC voltage synchronous inverter based on torque quantization as described in any one of claims 1-9.

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