Improved permanent magnet wind turbine inertia synchronous transient control method

Abstract

This invention relates to an improved transient control method for inertial synchronization of permanent magnet wind turbine generators, belonging to the field of new energy grid connection. The method includes: S1: employing additional DC voltage control, establishing a relationship between active power and DC voltage droop in the active power control loop on the turbine side, and adaptively reducing the reference value of active power output from the turbine-side converter when the DC voltage rises; S2: transient overcurrent suppression, including: reconstructing the equivalent internal potential based on the modulation constraints of the converter output voltage and the current limiting constraints of the converter, controlling the internal potential within the feasible region determined by the dual constraints to suppress the steady-state periodic component of the transient overcurrent; designing an adaptive virtual impedance, the value of which is adaptively adjusted according to the over-limit of the transient current and the sag depth of the grid voltage to suppress the transient DC component of the transient overcurrent. This invention enables fault ride-through in the inertial synchronization control of permanent magnet wind turbine generators.

Description

Technical Field

[0001] This invention belongs to the field of new energy grid connection and relates to an improved inertial synchronization transient control method for permanent magnet wind turbines. Background Technology

[0002] With the increasing global demand for renewable energy, offshore wind power, as a clean and efficient energy source, is experiencing rapid and continuous growth in installed capacity. In offshore wind power systems, the widely used permanent magnet synchronous generator (PMSG) wind turbines need to be connected to the power system via grid-connected converters, resulting in a significant characteristic of a "high proportion of power electronic equipment" in the power transmission system. Therefore, the operating characteristics of the grid-connected converters, especially their control strategies, have become a key factor affecting the stability and security of the entire power system.

[0003] Offshore wind farms are typically located far from land, requiring long-distance cable transmission for power. Coupled with their increasing penetration into the power grid, this leads to a generally low short-circuit ratio (SCR) at the grid connection point, weakening the grid strength. Traditional grid-following (GFL) control strategies rely on phase-locked loops (PLLs) to synchronize with the grid voltage. However, these strategies are poorly adapted to weak grid environments and are prone to problems such as PLL lockout and subsynchronous oscillations, exacerbating system operational risks.

[0004] To address these challenges, grid-forming (GFM) control strategies have emerged. Grid-forming converters can simulate the behavior of synchronous generators, actively providing voltage, frequency, and inertia support to the grid, thereby enhancing grid stability. Inertial Synchronization Control (IGSC) is a promising grid-forming control technology. This technology controls the DC-side voltage by simulating the motion equations of a synchronous generator rotor, thus achieving self-synchronization between the converter and the grid. It eliminates the need for phase-locked loops (PLLs), effectively avoiding the instability issues of PLLs in weak grid conditions. Furthermore, it circumvents the complex coordination issues between generator-side and grid-side control strategies in conventional permanent magnet wind turbines, improving engineering practicality.

[0005] However, despite the many advantages of inertial synchronous control, it still faces two major technical bottlenecks under transient conditions such as power grid failures:

[0006] (1) Transient DC voltage instability and synchronization failure: During severe faults such as grid voltage dips, drastic fluctuations in energy exchange can lead to DC bus capacitor voltage instability. To address this issue, existing technologies propose temporarily switching the control mode from inertial synchronization control to other synchronization methods. One approach is to switch to Virtual Synchronous Generator (VSG), but this requires redesigning and selecting the crowbar resistor to stabilize the DC voltage. This not only increases the design complexity of the crowbar circuit but also introduces the risk of transient shocks during mode switching. Another approach is to switch to a mode with a fixed output grid rated frequency (e.g., 50Hz) during a fault, which also relies on the crowbar circuit to stabilize the DC voltage. While this method can achieve fault ride-through when the grid frequency and phase are constant, actual grid faults are often accompanied by frequency dips and phase jumps. Its fixed-frequency output characteristic makes it unable to perceive and adapt to the actual frequency and phase changes of the grid, limiting its application scenarios.

[0007] (2) Transient Overcurrent Suppression Problem: Inertial synchronous control exhibits voltage source characteristics, making it difficult to effectively limit its output current during grid short-circuit faults, easily leading to overcurrent and damaging power electronic devices such as converters. To address this problem, one approach is to switch the control mode from grid-based to grid-following during a fault, utilizing the current source characteristics of grid-following control to achieve fault current limiting. However, this mode switching may reduce system stability under weak grid conditions due to the introduction of phase-locked loops, and both switching processes (during fault occurrence and fault recovery) may trigger transient impacts. Another approach is to suppress overcurrent by introducing current limiting elements or virtual impedances while maintaining grid-based control characteristics. However, these methods often fail to adequately consider the physical constraints of the converter's modulation circuitry. If only the current reference value is limited in the control algorithm, the calculated modulation signal amplitude may exceed the converter's maximum modulation capability, leading to overmodulation, which in turn causes output voltage waveform distortion and may even endanger equipment safety.

[0008] In summary, existing inertial synchronization control strategies are not perfect in terms of fault ride-through, and it is difficult to simultaneously ensure DC voltage stability and effective overcurrent suppression during transient processes. Therefore, there is an urgent need to propose an improved transient control strategy to address these issues. Summary of the Invention

[0009] In view of this, the purpose of this invention is to provide an improved transient control method for inertial synchronization of permanent magnet wind turbines. By using DC voltage regulation control and an adaptive virtual impedance method that considers both modulation and converter current limiting constraints, the method achieves safe and reliable fault ride-through of inertial synchronization control of permanent magnet wind turbines, while maintaining DC voltage stability and effectively suppressing overcurrent.

[0010] To achieve the above objectives, the present invention provides the following technical solution:

[0011] An improved transient control method for inertial synchronization of permanent magnet wind turbine generators specifically includes the following steps:

[0012] S1: Additional DC voltage control;

[0013] An additional DC voltage control is adopted to establish a relationship between active power and DC voltage droop in the active power control loop on the machine side. When the DC voltage rises, the active power reference value output by the machine side converter is adaptively reduced to maintain power balance and DC voltage stability during the transient process.

[0014] S2: Transient overcurrent suppression, including steady-state periodic component suppression and transient DC component suppression;

[0015] Steady-state periodic component suppression: Based on the modulation constraint of the converter output voltage and the current limiting constraint of the converter, the equivalent internal potential is reconstructed and controlled within the feasible region determined by the dual constraints to suppress the steady-state periodic component of transient overcurrent.

[0016] Transient DC component suppression: An adaptive virtual impedance is designed, the value of which is adaptively adjusted according to the over-limit of transient current and the drop depth of grid voltage, in order to suppress the transient DC component of transient overcurrent.

[0017] Furthermore, in step S1, the established relationship between active power and DC voltage droop is as follows:

[0018] P ref =P mref -k·ΔU dc

[0019] Among them, U dc P is a DC voltage. ref For additional U dc Control the adjusted active power reference value, P mref The reference power for maximum power point tracking; ΔU dc This refers to the DC voltage fluctuation.

[0020] k is an additional U dc The control adjustment factor, or droop factor for short, is set by combining the maximum U. dc Fluctuation limits are determined by the power angle curve characteristics of the converter output, specifically satisfying the following conditions:

[0021] k≤(P mref -P g ) / ΔU dcmax

[0022] In the formula, ΔUdcmax For the maximum allowed U dc Fluctuation.

[0023] Furthermore, in step S2, the modulation constraint specifically refers to: the radius of the inscribed circle of the converter output voltage is... The area covered by the inscribed circle is the feasible region of the converter's output internal potential and voltage. Exceeding this region will cause overmodulation.

[0024] Furthermore, in step S2, the current limiting constraint specifically means that when the grid voltage drops, the transient overcurrent will cause a voltage drop I across the impedance. t X g The constraint under converter current limiting is I. t X g ≤mI N X g , among which, I t For the grid-side inverter output current, I N Where m is the rated current, m is the converter overcurrent limit, and X is the rated current. g This represents the total reactance between the grid-side converter and the power grid.

[0025] Furthermore, in step S2, the reconstructed equivalent internal potential refers to controlling the operating point of the internal potential within the overlapping portion of the current-limiting constraint current feasible region and the voltage modulation feasible region of the converter.

[0026] Furthermore, in step S2, the designed adaptive virtual impedance sets the virtual reactance value to zero and uses only the virtual resistance for current limiting.

[0027] Furthermore, in step S2, the formula for calculating the virtual resistance is:

[0028]

[0029] In the formula, I F I is the current at the moment of the fault. Flimit For steady-state periodic overcurrent limiting, i.e., I Flimit =min(I max ,mI N ), where I N Where m is the rated current, I is the converter overcurrent limit, and m is the rated current. max U represents the maximum current under modulation constraints. t0 U is the rated voltage of the power grid. s R is the voltage drop value of the power grid. v This is a virtual resistor.

[0030] The beneficial effects of this invention are as follows:

[0031] (1) This invention achieves autonomous stabilization of DC voltage under transient operating conditions, ensuring the continuous effectiveness of inertial synchronization control. This invention dynamically couples DC voltage with active power output by adding DC voltage control to the machine-side control. When a grid fault causes a rise in DC voltage, this method can autonomously and quickly reduce the active power input on the machine side, thereby maintaining power balance and stabilizing the DC voltage within the allowable range. This avoids the drawbacks of existing technologies that require control mode switching (such as switching to virtual synchronization control or fixed frequency control) or rely on complex crowbar circuit designs, eliminates the transient impact risk that may be caused by mode switching, and ensures that the core inertial synchronization control does not fail throughout the fault period, thus enhancing the transient stability of the system.

[0032] (2) This invention achieves comprehensive and safe suppression of transient overcurrents, avoiding the risks of overmodulation and secondary impacts. This invention addresses the overcurrent problem through a dual strategy:

[0033] ① Suppressing steady-state periodic components: By simultaneously considering the modulation physical constraints and current-limiting capability constraints of the converter, the equivalent internal potential is reconstructed to ensure that it operates within the intersection of the voltage and current feasible domains. This fundamentally solves the problem that traditional current-limiting methods may cause overmodulation, leading to voltage waveform distortion or even instability, maximizing the performance of the converter while ensuring equipment safety.

[0034] ② Suppression of transient DC components: By designing an adaptive virtual impedance, utilizing the zero-crossing surge characteristic and boundedness of the arctangent function, the virtual resistance can be rapidly, accurately, and adaptively adjusted according to the overcurrent limit and voltage drop depth. This not only greatly improves the ability to suppress transient DC components in the early stages of a fault but also ensures that the virtual impedance does not become excessive due to inrush current, avoiding the risk of exacerbating system instability. After the suppression effect is completed, the virtual resistance can smoothly and flexibly withdraw, avoiding the secondary impact that may be caused by the hard removal of traditional virtual impedance.

[0035] In summary, this invention proposes a fault ride-through strategy that does not require switching control modes. It can collaboratively solve the two core problems of DC voltage instability and transient overcurrent under inertial synchronous control, significantly improving the transient stability and grid connection reliability of permanent magnet wind turbines in weak grid environments, and has high engineering practical value.

[0036] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0037] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0038] Figure 1 Machine-side control block diagram for improving transient control strategy for inertial synchronization control;

[0039] Figure 2 This is a schematic diagram for the analysis of the work angle curve;

[0040] Figure 3 Diagram showing the internal potential constraint of the converter output;

[0041] Figure 4 Design a schematic diagram for virtual impedance;

[0042] Figure 5 This is to assess the transient overcurrent suppression effect when the voltage drops to 0.8 pu;

[0043] Figure 6 This is the effect of suppressing transient overcurrent when the voltage drops to 0.2 pu. Detailed Implementation

[0044] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0045] This invention provides an improved transient control strategy based on additional DC voltage control and adaptive virtual impedance. On one hand, the control coefficients are set considering the DC voltage fluctuation limit constraints and the power angle curve characteristics, enabling the active load shedding on the generator side to maintain DC voltage stability and ensure that the inertial synchronization control does not fail. On the other hand, the equivalent internal potential is reconstructed based on the dual constraints of modulation and converter current limiting to suppress the steady-state periodic component of transient overcurrent. Furthermore, based on the current over-limit boundary, an adaptive virtual impedance is designed in conjunction with the arctangent function to suppress the transient DC component during voltage drop.

[0046] Example 1:

[0047] Please see Figure 1 This embodiment presents an improved transient control method for inertial synchronous grid connection of permanent magnet wind turbine units, specifically including the following steps:

[0048] S1: DC voltage U output from the converter on the auxiliary machine sidedc Voltage regulation under control. An additional U is used. dc Control, constructing an active power and U in the active power control loop on the machine side. dc The droop relationship changes the active power reference value in U dc When the power is raised, the active power input on the generator side is reduced to achieve power balance.

[0049] The machine-side control framework is as follows: Figure 1 As shown. The reference value for the turbine-side power setpoint is P. ref =P mref -k·ΔU dc , where P ref For additional U dc Control the adjusted active power reference value, P mref The reference power is the maximum power point tracking power, and k is the additional U. dc The control adjustment factor, or droop factor for short. To meet the grid connection requirements of the converter, the DC voltage fluctuation must meet the ΔU... dc ≤ΔU dcmax , where ΔU dcmax For the maximum allowed U dc Fluctuations. Therefore, to maintain transient stability, the power adjustment must be less than the maximum power point tracking power P. mref k must satisfy k≤(P) mref -P g ) / ΔU dcmax When the grid voltage drops, the amplitude of the power angle curve shifts downward, changing from curve P0 to P1. ref0 This is the power reference value before the fault. As the droop coefficient k decreases, it changes with U... dc The power adjustment amount of adaptive regulation decreases, which is equivalent to the power reference value P after the fault. ref Increase, such as Figure 2 The power angle curve analysis in the figure shows P ref1 Change to P ref2 At this point, the intersection of the power reference value and the power angle curve shifts upward (points C and D move towards points A and B), and the transient stability margin decreases. Therefore, it is necessary to combine the maximum U... dc The droop coefficient k is selected based on the fluctuation limit and the characteristics of the power angle curve.

[0050] S2: Overcurrent steady-state periodic component suppression strategy considering modulation constraints and internal potential reconstruction. To maximize DC capacitor voltage utilization, space vector pulse width modulation (SVPWM) is adopted, and the radius of the inscribed circle of the converter output voltage is... This region represents the feasible region of the converter's output internal potential and voltage; exceeding this region will cause overmodulation. When the grid voltage drops, transient overcurrent will cause a voltage drop I across the impedance. t X g The constraint under converter current limiting is I.t X g ≤mI N X g , among which, I N Where is the rated current, and m is the converter overcurrent limit. To ensure stable operation, the converter output internal potential must operate within the overlap region of the converter's current-limiting constraint current feasible region and voltage modulation feasible region, such as... Figure 3 As shown.

[0051] S3: Overcurrent transient DC component suppression strategy based on adaptive virtual impedance. The design principle of virtual impedance is as follows: Figure 4 As shown, where I F I represents the current at the moment of the fault. Flimit For steady-state periodic overcurrent limiting, i.e., I Flimit =min(I max ,mI N In the formula, I N Where m is the rated current, I is the converter overcurrent limit, and m is the rated current. max The maximum current under modulation constraint; ΔI (ΔI = I F -I Flimit K represents the change in fault current. R For the adaptive virtual impedance coefficient, U t0 U is the rated voltage of the power grid. s R is the voltage drop value of the power grid. v This is a virtual resistor. The larger the proportion of resistance in the impedance, the smaller the system's time constant, and the faster the transient DC component decays. Therefore, the virtual reactance in the virtual impedance is set to zero, and only the virtual resistance is used for current limiting. The transient adaptive virtual impedance, acting before and after a voltage dip and recovery from the mains voltage, can flexibly exit after suppressing the transient DC component. To avoid the impact caused by the virtual impedance switching off, it is designed to follow the transient overcurrent according to the arctangent function, and its amplitude is designed to be linearly related to the dip depth. Specifically, this can be expressed as the following formula:

[0052]

[0053] When the transient overcurrent exceeds the limit, the virtual resistance will increase rapidly according to the arctangent function. Furthermore, due to the boundedness of the arctangent function, the virtual resistance will not become too large and exacerbate the risk of transient instability if there is an impact overcurrent. This realizes that the virtual impedance can adaptively adjust the virtual resistance value according to the transient overcurrent and the drop depth.

[0054] Example 2:

[0055] When the grid voltage drops to 0.8 pu, current limiting is achieved using both the internal potential reconstruction method and the adaptive virtual impedance method. The experimental results are as follows: Figure 5As shown. When only the internal potential reconstruction method is used, a transient inrush current with an amplitude of 1.375 pu is generated at the instant of grid voltage drop and quickly decays to 1.28 pu. When combined with adaptive virtual impedance, the transient inrush component is suppressed, and the transient overcurrent remains within 1.28 pu at the instant of grid voltage drop, without exceeding the limit. When the grid voltage drops to 0.2 pu, current limiting is applied using both the internal potential reconstruction method and the adaptive virtual impedance method. At this time, the modulation constraint is greater than the converter current limit value, so IFlimit is set to 1.5 pu, and the results are as follows. Figure 6 As shown in the figure. The results show that without the addition of adaptive virtual impedance, a transient inrush current of nearly 2 p.u. will be generated during the grid dip, which seriously threatens the normal and stable operation of the converter. When adaptive virtual impedance is added, the transient inrush current during the dip can be effectively suppressed, ensuring that the transient overcurrent does not exceed the limit.

[0056] As can be seen, this invention proposes an improved transient control strategy for the inertial synchronization control of permanent magnet wind turbines, utilizing additional DC voltage control and adaptive virtual impedance to achieve a fault ride-through strategy under transient operating conditions. The main advantages are reflected in two aspects:

[0057] (1) By adding DC voltage control, the DC voltage is coupled with the output power of the machine side to form a droop relationship, and the droop coefficient is set by the DC voltage fluctuation limit and the converter output power angle curve to ensure the transient stability of DC voltage and the effectiveness of inertial synchronous control.

[0058] (2) Considering the modulation constraints and the overcurrent capacity constraints of the converter, the feasible region of the steady-state periodic component of the transient overcurrent is characterized, and the zero-crossing surge characteristics and boundedness advantages of the arctangent function are used to design an adaptive virtual impedance coefficient to improve the suppression capability of the DC component of the transient overcurrent.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. An improved inertial synchronization transient control method for permanent magnet wind turbine generators, characterized in that, The method includes the following steps: S1: Additional DC voltage control; An additional DC voltage control is adopted to establish a relationship between active power and DC voltage droop in the active power control loop on the machine side. When the DC voltage rises, the active power reference value output by the machine side converter is adaptively reduced to maintain power balance and DC voltage stability during the transient process. The relationship between the constructed active power and DC voltage droop is as follows: in, U dc DC voltage P ref For additional U dc Control the adjusted active power reference value. P mref The reference power is the maximum power tracking power. This refers to the DC voltage fluctuation. k For additional U dc The control adjustment factor, or droop factor for short, is set by combining the maximum... U dc Fluctuation limits are determined by the power angle curve characteristics of the converter output, specifically satisfying the following conditions: In the formula, Δ U dcmax For the maximum allowed U dc Volatility; S2: Transient overcurrent suppression, including steady-state periodic component suppression and transient DC component suppression; Steady-state periodic component suppression: Based on the modulation constraint of the converter output voltage and the current limiting constraint of the converter, the equivalent internal potential is reconstructed and controlled within the feasible region determined by the dual constraints to suppress the steady-state periodic component of transient overcurrent. Transient DC component suppression: An adaptive virtual impedance is designed, specifically by setting the virtual reactance value to zero and using only a virtual resistance for current limiting; the value of the virtual impedance is adaptively adjusted according to the over-limit of the transient current and the drop depth of the grid voltage to suppress the transient DC component of the transient overcurrent. The formula for calculating the virtual resistance is: In the formula, I F The current at the moment of the fault. I Flimit For steady-state periodic overcurrent limiting, i.e. ,in, I N Rated current, m For converter overcurrent limiting, I max This represents the maximum current value under modulation constraints. U t0 This is the rated voltage of the power grid. U s This represents the voltage drop across the power grid. R v This is a virtual resistor.

2. The improved inertial synchronization transient control method for permanent magnet wind turbine generators according to claim 1, characterized in that, In step S2, the modulation constraint specifically refers to: the radius of the inscribed circle of the converter output voltage is... The area covered by the inscribed circle is the feasible region of the converter's output internal potential and voltage. Exceeding this region will cause overmodulation.

3. The improved inertial synchronization transient control method for permanent magnet wind turbine generators according to claim 1, characterized in that, In step S2, the current limiting constraint specifically means that when the grid voltage drops, the transient overcurrent will cause a voltage drop across the impedance. I t X g The constraint under converter current limiting is: ,in, I t For the grid-side inverter output current, I N Rated current, m For converter overcurrent limiting, X g This represents the total reactance between the grid-side converter and the power grid.

4. The improved inertial synchronization transient control method for permanent magnet wind turbine generators according to claim 1, 2, or 3, characterized in that, In step S2, the reconstructed equivalent internal potential refers to controlling the operating point of the internal potential within the overlapping portion of the current-limiting constraint current feasible region and the voltage modulation feasible region of the converter.

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