Improved inertia synchronous transient control method for permanent magnet wind turbine generator

Abstract

The invention relates to an improved permanent magnet wind turbine generator inertial synchronous transient control method, and belongs to the field of new energy grid connection. The method comprises the following steps: S1, adopting additional DC voltage control, constructing a droop relationship between active power and DC voltage in a machine-side active power control loop, and adaptively reducing an active power reference value output by a machine-side converter when the DC voltage rises; s2, transient overcurrent suppression: reconstructing an equivalent internal potential based on the modulation constraint of the output voltage of the converter and the current limiting constraint of the converter, and controlling the internal potential in a feasible region determined by the dual constraints to suppress the steady-state periodic component of the transient overcurrent; and self-adaptive virtual impedance is designed, and the value of the virtual impedance is adaptively adjusted according to the out-of-limit amount of the transient current and the drop depth of the power grid voltage so as to suppress the transient direct current component of the transient overcurrent. According to the invention, fault ride-through of inertia synchronous control of the permanent magnet wind turbine generator can be realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of new energy grid connection, and relates to an improved inertial synchronous transient control method for a permanent magnet wind turbine. BACKGROUND

[0002] With the increasing demand for renewable energy worldwide, offshore wind power, as a clean and efficient form of energy, is experiencing a rapid increase in installed capacity. In offshore wind power systems, the large number of permanent magnet synchronous generator (PMSG) wind turbines need to be connected to the power system through grid-connected converters, forming a significant feature of the sending system with a high proportion of power electronic equipment. Therefore, the operating characteristics of grid-connected converters, especially their control strategies, have become a key factor affecting the stability and safety of the entire power system.

[0003] Offshore wind farms are usually far from land and need to be connected to the power grid through long-distance cables. In addition, their penetration rate in the power grid is constantly increasing, resulting in a generally low short-circuit ratio (SCR) at the grid connection point and a weakened grid strength. The traditional Grid-Following (GFL) control strategy relies on a Phase-Locked Loop (PLL) to synchronize with the grid voltage, which has poor adaptability in a weak grid environment and is prone to PLL lockout and subsynchronous oscillation, exacerbating the operational risks of the system.

[0004] To address the above challenges, the Grid-Forming (GFM) control strategy has emerged. Grid-forming converters can simulate the behavior of synchronous generators and actively provide voltage, frequency, and inertia support for the grid, thereby enhancing the stability of the grid. Inertial Synchronization Control is a promising grid-forming control technology. This technology controls the DC-side voltage by simulating the motion equation of the rotor of a synchronous generator, thereby achieving self-synchronization of the converter and the grid without relying on a phase-locked loop, effectively avoiding the instability problems of the phase-locked loop in a weak grid. At the same time, it avoids the complex coordination problem between the machine-side and grid-side control strategies in conventional permanent magnet wind turbines, improving the engineering practicability.

[0005] However, despite the many advantages of inertial synchronization control, it still faces two major technical bottlenecks in transient operating conditions such as grid faults:

[0006] (1) Transient DC voltage instability and synchronization failure: During severe faults such as voltage sag, the drastic fluctuation of energy exchange will lead to the instability of DC bus capacitor voltage. To solve this problem, the existing technology proposes to temporarily switch the control mode from inertia synchronous control to other synchronization methods. One solution is to switch to virtual synchronous control (Virtual Synchronous Generator, VSG), but this solution requires the Crowbar resistance of the unit to be redesigned to stabilize the DC voltage, which not only increases the design complexity of the Crowbar circuit, but also introduces the risk of transient impact during mode switching. Another solution is to switch to a mode with a fixed output grid rated frequency (such as 50Hz) during faults, which also relies on the Crowbar circuit to stabilize the DC voltage. Although this method can achieve fault ride-through when the grid frequency and phase are constant, actual grid faults are often accompanied by frequency drop and phase jump, and the fixed frequency output characteristic makes it unable to perceive and adapt to the actual frequency and phase changes of the grid, limiting the application scenario.

[0007] (2) Transient overcurrent suppression problem: Inertia synchronous control presents voltage source characteristics, which is difficult to effectively limit its output current during grid short-circuit faults, and is prone to overcurrent, causing damage to power electronic devices such as converters. To solve this problem, one method is to switch the control mode from grid-forming to grid-following during faults, using the current source characteristics of grid-following control to achieve fault current limiting. However, this mode switching not only may reduce system stability due to the introduction of a phase-locked loop in a weak grid, but also may cause transient impact during the two switching processes (when the fault occurs and when the fault is recovered). Another method is to introduce a current limiting element or virtual impedance while maintaining the grid-forming control characteristics to suppress overcurrent. However, this method often fails to fully consider the physical constraints of the converter modulation element. If the current reference value is simply limited in the control algorithm, the calculated modulation signal amplitude may exceed the maximum modulation capacity of the converter, causing over-modulation, which in turn leads to output voltage waveform distortion and may even endanger equipment safety.

[0008] In summary, the existing inertia synchronous control strategy is not perfect in terms of fault ride-through, and it is difficult to simultaneously consider DC voltage stability and effective overcurrent suppression during the transient process. Therefore, there is an urgent need to propose an improved transient control strategy to solve the above problems. SUMMARY

[0009] Therefore, the purpose of the present application is to provide an improved inertia synchronous transient control method for permanent magnet wind turbines, which realizes safe and reliable fault ride-through of inertia synchronous control for permanent magnet wind turbines through DC voltage stabilization control and adaptive virtual impedance method considering modulation and converter current limiting constraints, and maintains DC voltage stability and effectively suppresses 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 Umax is the maximum allowed U dc Umax is the maximum allowed U

[0023] Further, in step S2, the modulation constraint is specifically that 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 output internal potential voltage, and exceeding the area will cause over-modulation.

[0024] Further, in step S2, the current-limiting constraint is specifically that when the grid voltage drops, the transient overcurrent will cause a voltage drop I t X g The constraint under the current-limiting of the converter is I t X g ≤mI N X g , wherein I t is the output current of the grid-side inverter, I N is the rated current, m is the overcurrent limit of the converter, and X g is the total reactance between the grid-side converter and the grid.

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

[0026] Further, in step S2, the designed adaptive virtual impedance is to set the virtual reactance value to zero and only use a virtual resistor for current limiting.

[0027] Further, in step S2, the calculation formula of the virtual resistor is:

[0028]

[0029] In the formula, I F is the current at the fault moment, I Flimit is the steady-state periodic overcurrent limit, that is, I Flimit =min(I max ,mI N ), wherein I N is the rated current, m is the overcurrent limit of the converter, and I max is the maximum current under the modulation constraint; U t0 is the rated value of the grid voltage, U s is the grid voltage drop value, and R v is the virtual resistor.

[0030] The beneficial effects of the present application are:

[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] In order to make the objects, technical solutions and advantages of the present application clearer, the preferred embodiments of the present application will be described in detail below with reference to the drawings, in which:

[0038] Figure 1 Control block diagram of the improved transient control strategy for inertia synchronous control of the generator side;

[0039] Figure 2 Schematic diagram for power angle curve analysis;

[0040] Figure 3 Internal potential constraint diagram for converter output;

[0041] Figure 4 Principle diagram for virtual impedance design;

[0042] Figure 5 Transient overcurrent suppression effect when the voltage drops to 0.8 p.u;

[0043] Figure 6 Transient overcurrent suppression effect when the voltage drops to 0.2 p.u. DETAILED DESCRIPTION

[0044] The embodiments of the present application will be described in detail below with reference to specific examples. Those skilled in the art can easily understand other advantages and effects of the present application from the content disclosed in the specification. The present application can also be implemented or applied in other different embodiments, and various modifications or changes can be made to the details in the specification based on different views and applications without departing from the spirit of the present application. It should be noted that the diagrams provided in the following examples only illustrate the basic concept of the present application in a schematic manner, and the following examples and features in the examples can be combined with each other without conflict.

[0045] The present application provides an improved transient control strategy based on additional DC voltage control and adaptive virtual impedance. On the one hand, considering the DC voltage fluctuation limit constraint and the power angle curve characteristic, the control coefficient is set to make the generator side active load shedding maintain the stability of the DC voltage and keep the inertia synchronous control from being invalid. On the other hand, based on the modulation and converter current limiting double constraint, the equivalent internal potential is reconstructed to suppress the steady-state periodic component of the transient overcurrent, and further according to the current limit boundary, the adaptive virtual impedance is designed by combining the inverse tangent function to suppress the transient DC component at the voltage drop moment.

[0046] Embodiment 1:

[0047] Please refer to Figure 1 The present embodiment is an improved transient control method for inertia synchronous grid connection of a permanent magnet wind turbine, which specifically includes the following steps:

[0048] S1: additional DC voltage U of the generator side converter outputdc Controlled voltage stabilization. Additional U dc Control, active power in the machine side of the active power control loop is built in the active power and U dc droop relationship, change the active power reference value, U dc rise to reduce the machine side of the active input, achieve power balance.

[0049] Machine side control framework as follows Figure 1 shown. Wherein, the machine side power given reference value is P ref =P mref -k·ΔU dc , wherein, P ref is the additional U dc control adjusted active power reference value, P mref is the maximum power tracking reference power, k is the additional U dc control adjustment coefficient, referred to as the droop coefficient. To meet the grid-connected requirements of the converter, the DC voltage fluctuation needs to meet ΔU dc ≤ΔU dcmax , wherein, ΔU dcmax is the maximum U dc fluctuation allowed. Therefore, in order to maintain transient stability, the power adjustment amount needs to be less than the maximum power tracking power P mref , k needs to meet k≤(P mref -P g ) / ΔU dcmax When the grid voltage drops, the power angle curve amplitude moves down, from the curve P0 to P1, P ref0 is the power reference value before the fault. When the droop coefficient k decreases, the power adjustment amount adjusted adaptively with U dc changes decreases, which is equivalent to the increase of the power reference value P ref after the fault, as shown in the power angle curve analysis in Figure 2 P ref1 changes to P ref2 , at this time the intersection of the power reference value and the power angle curve moves up (C, D point moves to A, B point), the transient stability margin decreases. Therefore, it is necessary to select the droop coefficient k in combination with the maximum U dc fluctuation limit and the characteristics of the power angle curve.

[0050] S2: Overcurrent steady-state periodic component suppression strategy of internal potential reconstruction considering modulation constraints. In order to maximize the utilization rate of DC capacitor voltage, space vector pulse width modulation (SVPWM modulation) is adopted, and the output voltage of the converter is inscribed circle radius This domain is the feasible region of the output internal potential voltage of the converter, and overmodulation will occur beyond this region. When the grid voltage drops, transient overcurrent will cause voltage drop I t X g on the impedance under the current limit of the convertert X g ≤mI N X g , where I N is the rated current, and m is the current limit of the converter. To ensure stable operation of the converter output internal potential, the converter needs to operate in the overlapping part of the current limit constraint current feasible region and the voltage modulation feasible region, as shown in Figure 3 .

[0051] S3: Overcurrent transient DC component suppression strategy based on adaptive virtual impedance. The design principle of virtual impedance is shown in Figure 4 , where I F is the current at the time of fault; I Flimit is the steady-state period overcurrent limit, that is, I Flimit = min(I max , mI N ), where I N is the rated current, m is the current limit of the converter, and I max is the maximum current under modulation constraints; ΔI (ΔI = I F - I Flimit ) is the change in fault current, K R is the adaptive virtual impedance coefficient, U t0 is the rated value of the grid voltage, U s is the grid voltage drop value, and R v is the virtual resistance. When the resistance proportion in the impedance is larger, the time constant of the system is smaller, and the transient DC component decays faster. The virtual reactance value in the virtual impedance used is set to zero, and only the virtual resistance is used for current limiting. The transient adaptive virtual impedance before and after the grid voltage drop and recovery acts flexibly after suppressing the transient DC component. To avoid the impact caused by the virtual impedance cutting out, the amplitude is designed to change according to the inverse tangent function law, and the amplitude is designed to be linearly related to the drop depth. Specifically, it can be expressed as follows:

[0052]

[0053] When the transient overcurrent exceeds the limit, the virtual resistance will increase rapidly according to the inverse tangent function, and due to the boundedness of the inverse tangent function, if there is an impact overcurrent, it will not cause the virtual resistance to be too large and aggravate the risk of transient instability, realizing the adaptive adjustment of 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 p.u., the current is limited by the internal potential reconstruction method only and combined with the adaptive virtual impedance current limiting, respectively, and the experimental results are as follows Figure 5It can be seen that the application proposes a transient control improvement strategy of inertia synchronous control of the permanent magnet wind turbine, uses additional DC voltage control and adaptive virtual impedance to realize a fault ride-through strategy under transient conditions. The main advantages are embodied in two aspects: Figure 6 As shown in the figure. When only the internal potential reconstruction method is used, a transient impact current with an amplitude of 1.375 p.u. is generated at the moment of grid voltage drop and quickly decays to 1.28 p.u.; when combined with adaptive virtual impedance, the transient impact component is suppressed, and the transient overcurrent at the moment of grid voltage drop is kept within 1.28 p.u., without exceeding the limit. When the grid voltage drops to 0.2 p.u., the current limiting is respectively limited by the internal potential reconstruction method and combined with adaptive virtual impedance. At this time, the modulation constraint is greater than the current limiter value, so IFlimit takes 1.5 p.u., and the results are as shown in the figure. Figure 6 As shown in the figure. The results show that when the adaptive virtual impedance is not added, a transient impact current of nearly 2 p.u. is generated at the moment of grid drop, which seriously threatens the normal and stable operation of the converter, and when the adaptive virtual impedance is added, the transient impact current at the moment of drop can be effectively suppressed, and the transient overcurrent is not over-limited.

[0056] It can be seen that the application proposes a transient control improvement strategy of inertia synchronous control of the permanent magnet wind turbine, uses additional DC voltage control and adaptive virtual impedance to realize a fault ride-through strategy under transient conditions. The main advantages are embodied in two aspects:

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

[0058] (2) The steady-state periodic component of the transient overcurrent is described by considering the modulation constraint and the converter overcurrent capability constraint, and the adaptive virtual impedance coefficient is designed by using the zero-crossing surge characteristics and boundedness advantage of the arctangent function to improve the suppression ability of the transient overcurrent DC component.

[0059] Finally, it should be pointed out that the above examples are only used to illustrate the technical solutions of the application and not to limit it, although the application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the application can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions, which should be covered in the scope of the claims of the application.

Claims

1. An improved permanent magnet wind turbine inertia synchronous transient control method, characterized in that, The method comprises the following steps: S1: additional DC voltage control; With the additional DC voltage control, the active power and DC voltage droop relationship is constructed in the active power control loop at the generator side. When the DC voltage rises, the active power reference value output by the generator side converter is adaptively reduced to maintain power balance and DC voltage stability in the transient process; 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 electromotive force is reconstructed, and the internal electromotive force is controlled in the feasible region determined by the double constraints to suppress the steady-state periodic component of the transient overcurrent; Transient DC component suppression: an adaptive virtual impedance is designed, and the value of the virtual impedance is adaptively adjusted according to the over-limit amount of the transient current and the drop depth of the grid voltage to suppress the transient DC component of the transient overcurrent.

2. The improved inertia synchronous transient control method of permanent magnet wind turbine generator as claimed in claim 1 wherein, In step S1, the active power and DC voltage droop relationship constructed is: P ref = P mref -k ΔU dc Wherein, U dc is a direct current voltage, P ref is an additional U dc control adjusted active power reference value, P mref is a maximum power tracking reference power; ΔU dc is a direct current voltage fluctuation amount; k is the additional U dc The control adjustment coefficient, referred to as the droop coefficient, is set in combination with the maximum U dc The fluctuation limit and the power angle curve characteristics of the converter output are determined, and the following conditions are met: k < (P mref - P g ) / ΔU dcmax where ΔU dcmax is the maximum U dc fluctuation allowed.

3. The improved inertia synchronous transient control method of permanent magnet wind turbine generator as claimed in claim 1 wherein, In step S2, the modulation constraint is specifically that 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 output voltage, and exceeding the region will cause overmodulation.

4. The improved inertia synchronous transient control method of permanent magnet wind turbine generator as claimed in claim 1 wherein, In step S2, the current limiting constraint is specifically: when the grid voltage drops, the transient overcurrent will cause a voltage drop I t X g The constraint under the current limiting of the converter is I t X g ≤mI N X g , wherein I t is the output current of the grid-side inverter, I N is the rated current, m is the overcurrent limit of the converter, and X g is the total reactance between the grid-side converter and the grid.

5. The improved inertia synchronous transient control method of permanent magnet wind turbine as claimed in claim 1, 3 or 4 wherein, In step S2, the reconstructed equivalent internal electromotive force refers to controlling the operating point of the internal electromotive force in the overlapping part of the current limiting constraint current feasible region and the voltage modulation feasible region of the converter.

6. The improved permanent magnet wind turbine inertia synchronous transient control method of claim 1, wherein, In step S2, the adaptive virtual impedance designed is to set the virtual reactance value to zero and only use virtual resistance for current limiting.

7. The improved inertia synchronous transient control method of permanent magnet wind turbine generator as claimed in claim 6 wherein, In step S2, the calculation formula of the virtual resistance is: where I F is the current at the fault moment, I Flimit is the steady-state period overcurrent limit, i.e. I Flimit = min(I max , mI N ), where I N is the rated current, m is the converter overcurrent limit, I max is the maximum current under modulation constraints; U t0 is the grid voltage rated value, U s is the grid voltage dip value, R v is the virtual resistance.

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