Analysis method for frequency change characteristics of direct-driven fan

Abstract

The invention relates to a method for analyzing frequency change characteristics of a direct-driven fan, and belongs to the technical field of new energy power generation. The method comprises the following steps: firstly, according to a simplified equivalent circuit of the direct-drive fan, combining a relation between active power and a phase angle of a single infinite system of the direct-drive fan to obtain phase change characteristics of three-phase internal potential of the direct-drive fan after a fault occurs; and then combining the three-phase internal potential of the direct-drive fan, the grid-connected port voltage relational expression and the direct-drive fan injection power grid current expression to analyze the frequency change characteristics of the direct-drive fan. And finally, considering the influence of the phase locking difference on the voltage phase of the grid-connected port, and analyzing the frequency change characteristic of the direct-driven fan when the phase locking difference exists. The invention provides the method for analyzing the frequency change characteristics of the direct-driven fan during the fault, and a theoretical basis is provided for subsequent research.

Description

Technical Field

[0001] The present invention relates to an analysis method for the frequency variation characteristics of a direct-drive wind turbine, belonging to the technical field of new energy power generation. Background Art

[0002] With the global energy transformation, new energy power generation has been vigorously developed. As of 2024, the installed wind power capacity in China reached 520 million kW, with a year-on-year increase of 18%. The permanent magnet direct-drive wind turbine is connected to the AC grid through a back-to-back converter, which has the function of AC-DC isolation and can enhance the grid connection stability. Therefore, the direct-drive wind turbine has become one of the widely used wind turbine types.

[0003] Compared with traditional synchronous generators, due to the large rotational inertia of synchronous machines, the frequency change caused by faults is very small. However, after a large number of direct-drive wind turbines are connected to the grid, due to the presence of a large number of power electronic devices, there are problems such as low inertia and weak anti-interference ability. When a fault occurs in the outgoing line of a wind farm, it may cause a large frequency change. Traditional relay protection is mainly set according to the fault characteristics of synchronous machines. If a large frequency change is caused by a line fault, it will inevitably affect the action characteristics of existing relay protection. Therefore, in order to ensure the safe and stable operation of wind turbine units and power systems, it is of great significance to deeply analyze the frequency variation characteristics of direct-drive wind turbines under line fault conditions.

[0004] At present, the analysis of the fault characteristics of direct-drive wind turbines mainly includes two aspects. On the one hand, it is the analysis of the fault current characteristics of direct-drive wind turbines. Existing research has analyzed the influencing factors, variation laws of short-circuit currents, and the transient characteristics of fault currents of direct-drive wind turbines under various control strategies and influencing factors by deriving the expression of fault currents. On the other hand, regarding the analysis of frequency characteristics under fault conditions, the research object of existing literature is the change of the grid frequency containing direct-drive wind turbines, rather than the frequency response characteristics of the direct-drive wind turbine itself. Existing research mainly analyzes the impact of direct-drive wind turbines on the system frequency under line fault conditions and the use of direct-drive wind turbines to participate in system frequency modulation.

[0005] In summary, the existing publicly available research on the fault characteristics of direct-drive wind turbines under the condition of a fault in the outgoing line of a wind farm lacks a specific analysis method for the frequency variation characteristics of the direct-drive wind turbine itself after a fault occurs. Summary of the Invention

[0006] The purpose of the present invention is to provide an analysis method for the frequency variation characteristics of a direct-drive wind turbine, aiming to solve the technical problem that the existing technology lacks a specific analysis of the frequency variation characteristics of the direct-drive wind turbine itself.

[0007] To achieve the above object, the technical solution of the present invention is as follows: Based on the direct-drive wind turbine single-machine infinite system, starting from the relationship between the active power and phase angle of the direct-drive wind turbine single-machine infinite system, considering the phase-locked difference generated during the dynamic response process of the phase-locked loop, an analysis method for the frequency change characteristics of the direct-drive wind turbine is proposed. The specific steps are as follows:

[0008] Step1: According to the topology of the direct-drive wind turbine grid-connected system, simplify and equivalent the direct-drive wind turbine grid-connected system to obtain a simplified equivalent circuit of the direct-drive wind turbine. Use the mathematical model of the grid-side converter of the direct-drive wind turbine grid-connected system to obtain the voltage-oriented double-loop control strategy of the grid-side converter of the direct-drive wind turbine;

[0009] Step2: Based on the relationship between the active power and phase angle of the direct-drive wind turbine single-machine infinite system, use the simplified equivalent circuit of the direct-drive wind turbine, and combine with the voltage-oriented double-loop control strategy of the grid-side converter of the direct-drive wind turbine to obtain the phase change characteristics of the three-phase internal potential of the direct-drive wind turbine and the output power expression of the direct-drive wind turbine;

[0010] Step3: Use the equivalent circuit of the direct-drive wind turbine to obtain the relationship between the three-phase internal potential of the direct-drive wind turbine and the grid connection port voltage and the expression of the current injected by the direct-drive wind turbine into the grid. Combine with the obtained phase change characteristics of the three-phase internal potential of the direct-drive wind turbine to obtain the phase change phasors of the three-phase internal potential of the direct-drive wind turbine, the grid connection port voltage, and the current injected by the direct-drive wind turbine into the grid;

[0011] Step4: According to the phase change phasors of the three-phase internal potential of the direct-drive wind turbine, the grid connection port voltage, and the current injected by the direct-drive wind turbine into the grid, obtain the frequency change characteristics of the direct-drive wind turbine;

[0012] Step5: Analyze the output expression of the phase-locked loop of the direct-drive wind turbine, and combine with the transfer function of the typical basic structure of the phase-locked loop to obtain the electrical phasor change when there is a phase-locked difference in the phase-locked loop;

[0013] Step6: According to the electrical phasor change when there is a phase-locked difference in the phase-locked loop, and combine with the obtained phase change phasors of the three-phase internal potential of the direct-drive wind turbine, the grid connection port voltage, and the current injected by the direct-drive wind turbine into the grid, obtain the output frequency change characteristics of the direct-drive wind turbine grid-connected system when there is a phase-locked difference in the phase-locked loop.

[0014] The specific content of Step2 is as follows:

[0015] The relationship between the active power and phase angle of the direct-drive wind turbine single-machine infinite system is:

[0016]

[0017] In the formula, P is the actual output power of the direct-drive wind turbine, U g is the equivalent internal potential of the direct-drive synchronous wind generator, U b is the voltage at the grid connection point of the unit, x is the reactance connected by the converter, and δ is Ug Phase angle difference with U b ;

[0018] According to the simplified equivalent circuit of the direct-drive wind turbine, when the output power of the direct-drive wind turbine changes, the three-phase equivalent internal potential of the direct-drive wind turbine undergoes a phase change under the action of the voltage-oriented double-loop feedback control of the grid-side converter of the direct-drive wind turbine, and the output power expression of the direct-drive wind turbine is obtained as follows:

[0019]

[0020] In the formula, P set is the active power target value of the direct-drive wind turbine, and U' g is the equivalent internal potential of the direct-drive synchronous wind generator after the phase change occurs.

[0021] Specifically, the direct-drive wind turbine converter is controlled as a constant power control, and the active power target value P set is set. When the output power of the direct-drive wind turbine changes, the actual output power of the direct-drive wind turbine is not the power target value P set . In order for the output of the direct-drive wind turbine to reach the set power target value P set , it is necessary to adjust the actual output power P of the direct-drive wind turbine. Based on the relationship formula (1) between the active power and phase angle of the direct-drive wind turbine single-machine infinite system, when the output power P is increased, the phase angle difference between U g and U b increases, and when the output power P is decreased, the phase angle difference between U g and U b decreases. The three-phase equivalent internal potential U g of the direct-drive wind turbine will undergo a phase change to U' g under the action of a series of feedback controls of the grid-side converter of the wind turbine in the control system. At this time, the measured value U b of the grid connection point voltage and the current value U' g of the internal potential satisfy formula (2).

[0022] The specific content of Step3 is as follows:

[0023] According to the equivalent circuit of the direct-drive wind turbine and combining the phase change characteristics of the three-phase internal potential of the direct-drive wind turbine, the relationship formula between the three-phase internal potential of the direct-drive wind turbine and the grid connection port voltage is obtained as follows:

[0024] U g =U b +jωL g ·I(3)

[0025] In the formula, I is the current injected by the direct-drive wind turbine into the power grid, ω is the angular velocity of the generator at power frequency, j is the imaginary part of the impedance, and L g is the filter inductor of the direct-drive wind turbine grid connection system;

[0026] When the phases of the three-phase internal electromotive forces of the direct-drive wind turbine change, the phase of the grid connection point voltage of the direct-drive wind turbine also changes under the condition that the inductance and current remain unchanged. The expression for the current injected by the direct-drive wind turbine into the power grid is obtained as follows:

[0027]

[0028] Specifically, according to the equivalent circuit of the direct-drive wind turbine and combining with the characteristics of the phase change of the three-phase internal electromotive forces of the direct-drive wind turbine, from the relationship formula (3) between the three-phase internal electromotive forces and the grid connection point voltage of the direct-drive wind turbine, when the phase of the internal electromotive force U of the direct-drive wind turbine g changes, the phase of the grid connection point voltage U b also changes under the condition that the inductance and current remain unchanged and becomes U' b . At this time, after one round of feedback control output, the voltage U' b of the grid connection point and the current value U' g of the current internal electromotive force of the direct-drive wind turbine no longer satisfy formula (2), and then enter the next round of feedback control. From the expression (4) of the current injected by the direct-drive wind turbine into the power grid, when the phases of the three-phase internal electromotive forces U g of the direct-drive wind turbine and the grid connection point voltage U b of the direct-drive wind turbine both change, the phase of the current I injected by the direct-drive wind turbine into the power grid is also constantly changing.

[0029] Specifically, Step4 is as follows:

[0030] The three-phase equivalent internal electromotive force U g of the direct-drive wind turbine changes its phase to U' set g because the actual output power P of the direct-drive wind turbine needs to reach the set power target value P g . At the same time, due to the relationship between the internal electromotive force U b of the direct-drive wind turbine and the grid connection point voltage U b in formula (3), the grid connection point voltage U b also changes its phase and becomes U' g . Similarly, due to the current expression (4) injected by the direct-drive wind turbine into the power grid, the phase of the current I injected by the direct-drive wind turbine into the power grid also becomes I' with the phase changes of U b . The phases of U g , U b and I will change continuously due to the constant power control of the direct-drive wind turbine converter. The phase changes within a certain period of time are cumulatively equivalent to the change in frequency;

[0031] Specifically, Step6 is as follows:

[0032] ​Due to the dynamic response process of the phase-locked loop in the grid-connected control strategy of the direct-drive wind turbine, the phase-locked loop cannot detect the actual voltage phase at the grid connection port in a timely and accurate manner. There is a deviation between the output phase and the actual phase, resulting in a phase-locked difference Δθ. pll , the grid-connected port voltage tracked by the direct-drive wind turbine is not the actual grid-connected port voltage, and the dq-axis components of the grid-connected port voltage become formula (5). When the phase-locked difference is positive, it means that the voltage phase output by the phase-locked loop leads the actual phase of the grid-connected port voltage. The voltage phase output after feedback control by the inverter is also advanced accordingly. When the phase-locked difference is negative, it means that the voltage phase output by the phase-locked loop lags behind the actual phase of the grid-connected port voltage. The voltage phase output after feedback control by the inverter is also lagged accordingly.

[0033]

[0034] In the formula, u d , u q are the dq-axis components of the actual grid-connected port voltage, and u' d , u' q are the dq-axis components of the grid-connected port voltage output by the phase-locked loop, and Δθ pll is the deviation between the phase output by the phase-locked loop and the actual phase.

[0035] Specifically, the existence of the phase-locked difference causes the current value U g of the three-phase equivalent internal potential of the direct-drive wind turbine output after the double-loop control of the converter to change irregularly in phase, thereby causing the voltage U b at the grid connection point and the phase of the current I injected by the direct-drive wind turbine into the grid to change accordingly. Since the response time of the current inner loop is very short and the response speed is fast, when the power is not the set power, the phase of the grid voltage and the current injected by the direct-drive wind turbine into the grid is constantly changing, and the frequency is constantly changing. During this time, the phase change caused by the phase-locked difference makes the frequency change of the direct-drive wind turbine more complex. Assuming that when the power is not the set power and the voltage phase is constantly changing, the phase-locked difference is positive, that is, the voltage phase output by the phase-locked loop leads the actual phase of the grid-connected port voltage, the phase-locked difference accelerates the frequency change. And assuming that when the power is not the set power and the voltage phase is constantly changing, the phase-locked difference is negative, that is, the voltage phase output by the phase-locked loop lags behind the actual phase of the grid-connected port voltage, the phase-locked difference delays the frequency change, thereby causing the frequency to change again on the original basis.

[0036] The beneficial effects of the present invention are:

[0037] (1) According to the relationship between the active power and phase angle of the direct-drive wind turbine single-machine infinite system, the present invention analyzes the phase change characteristics of the three-phase internal potential of the direct-drive wind turbine in combination with the control strategy of the grid-connected inverter;

[0038] (2) Based on the equivalent circuit of the grid-connected system of the direct-drive wind turbine, the present invention analyzes the voltage and current phase changes of the direct-drive wind turbine during line faults, and combines the relationship between the phase change and the frequency to analyze the frequency change characteristics of the direct-drive wind turbine;

[0039] (3) Considering the dynamic characteristics of the phase-locked loop, the present invention analyzes the frequency change characteristics of the direct-drive wind turbine in the presence of a phase-locked difference, providing a basis for the analysis and research of the output characteristics of the direct-drive wind turbine. Description of the Drawings

[0040] Figure 1 is the flowchart of the present invention;

[0041] Figure 2 is the schematic diagram of the fault of the outgoing line of the direct-drive wind turbine of the present invention;

[0042] Figure 3 is the relationship curve diagram of the active power and phase angle of the direct-drive wind turbine single-machine infinite system of the present invention;

[0043] Figure 4 is the equivalent circuit diagram of the direct-drive wind turbine of the present invention;

[0044] Figure 5 is the present invention's U g 、U b and the phasor diagram of the phase change of the injected current I;

[0045] Figure 6 is the electrical phasor diagram when there is a phase-locked difference in the present invention;

[0046] Figure 7 is the frequency change characteristic diagram of the direct-drive wind turbine of the present invention;

[0047] Figure 8 is the schematic diagram of the phase-locked difference of the PMSG during the fault of the present invention.

[0048] Figure 9 is the frequency change characteristic diagram of the direct-drive wind turbine when there is a phase-locked difference in the present invention. Detailed Embodiment

[0049] To make the above objects, effects, and advantages of the present invention more obvious and understandable, the following will describe in detail the specific implementation method of the present invention with reference to the drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, not all embodiments. Based on the embodiments in the present invention, all embodiments obtained by those of ordinary skill in the art without creative work shall fall within the scope of protection of the present invention.

[0050] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may be practiced in other ways different from those described herein. Persons skilled in the art may make similar extensions without departing from the spirit of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0051] Embodiment 1: As Figure 1 shown, a method for analyzing the frequency variation characteristics of a direct-drive wind turbine, the specific steps are as follows:

[0052] Step1: According to the topology of the direct-drive wind turbine grid-connected system, simplify and equivalent the direct-drive wind turbine grid-connected system to obtain a simplified equivalent circuit of the direct-drive wind turbine. Use the mathematical model of the grid-side converter of the direct-drive wind turbine grid-connected system to obtain the voltage-oriented double-loop control strategy of the grid-side converter of the direct-drive wind turbine.

[0053] Specifically, as Figure 2 shown, the embodiment is to analyze the frequency variation characteristics of the direct-drive wind turbine when a short-circuit fault occurs at the midpoint of the 110 kV outgoing line of the wind farm. According to the topology of the direct-drive wind turbine grid-connected system, simplify and equivalent the direct-drive wind turbine grid-connected system to obtain a simplified equivalent circuit of the direct-drive wind turbine.

[0054] Step2: As Figure 3 shown, based on the relationship between the active power and phase angle of the direct-drive wind turbine single-machine infinite system, use the simplified equivalent circuit of the direct-drive wind turbine, and combine the voltage-oriented double-loop control strategy of the grid-side converter of the direct-drive wind turbine to obtain the phase change characteristics of the three-phase internal electromotive force of the direct-drive wind turbine and the output power expression of the direct-drive wind turbine.

[0055] Specifically, when a short-circuit fault occurs in the outgoing line of the wind farm, the grid voltage drops, and the grid-connected port voltage U b decreases, and the actual output power P of the direct-drive wind turbine decreases. Because the converter of the direct-drive wind turbine is controlled as constant power control, a power target value P set is set. At this time, the actual output power of the direct-drive wind turbine cannot reach the power target value P set . In order to reach the set power target value P set , the direct-drive wind turbine needs to increase the actual output power P. According to the curve of the relationship between the active power and phase angle of the direct-drive wind turbine single-machine infinite system, when the output power P is increased, the phase angle difference between U g and U b increases. The phase of the grid-connected port voltage U b remains unchanged. In order to increase the phase angle difference between U g and U b to increase the power generation, the three-phase equivalent internal electromotive force U g of the direct-drive wind turbine will change from phase lead to U' g . The output power expression of the direct-drive wind turbine is:

[0056]

[0057] Step3: As shown in Figure 4 , using the equivalent circuit of the direct-drive wind turbine, the relationship between the three-phase internal electromotive force and the grid connection port voltage of the direct-drive wind turbine and the expression of the current injected by the direct-drive wind turbine into the grid are obtained. Combining the obtained phase change characteristics of the three-phase internal electromotive force of the direct-drive wind turbine, the phase change phasors of the three-phase internal electromotive force, the grid connection port voltage, and the current injected by the direct-drive wind turbine into the grid are obtained.

[0058] Specifically, according to the equivalent circuit of the direct-drive wind turbine, the relationship between the three-phase internal electromotive force, the grid connection port voltage of the direct-drive wind turbine, and the expression of the current injected by the direct-drive wind turbine into the grid:

[0059] U g =U b +jωL g ·I(2)

[0060]

[0061] As shown in Figure 5 , it can be seen that when the phase of U g leads, the phase of U b also leads under the condition that the inductance and the current do not change, becoming U' b . Similarly, due to the relationship between U g , U b and I, the phase of the current I injected by the direct-drive wind turbine into the grid also leads as the phases of U g and U b lead, becoming I'. Therefore, the phases of U g , U b and I will continuously lead due to the constant power control of the direct-drive wind turbine converter.

[0062] Step4: According to the phase change phasors of the three-phase internal electromotive force, the grid connection port voltage, and the current injected by the direct-drive wind turbine into the grid, the frequency change characteristics of the direct-drive wind turbine are obtained.

[0063] Specifically, when a fault occurs in the outgoing line of the direct-drive wind turbine, due to the constant power control of the grid-side converter of the direct-drive wind turbine, the actual output power of the direct-drive wind turbine cannot reach the set power target value. In order to make the output power reach the set value, the phases of the three-phase equivalent internal electromotive force U g , the grid connection point voltage U b and the current I injected by the direct-drive wind turbine into the grid continuously lead. During the time before the protection action, the accumulation of this phase lead is equivalent to an increase in frequency. When a short-circuit fault occurs in the outgoing line of the direct-drive wind turbine, the output frequency of the direct-drive wind turbine keeps increasing during the fault duration, and the growth rate is relatively fast. As shown in Figure 7As shown, it has changed by nearly 1.6 Hz in a short time. The large frequency change will affect the relay protection and may cause incorrect operation of the relay protection.

[0064] Step5: As Figure 6 shown, analyze the output expression of the PLL of the direct-drive wind turbine, and combine it with the transfer function of the typical basic structure of the PLL to obtain the electrical phasor change when there is a phase difference in the PLL.

[0065] Specifically, when the grid-connected port voltage drops instantaneously and the voltage phase jumps, due to the dynamic response process of the PLL, the voltage phase output after being controlled by the PLL cannot follow the actual grid-connected port voltage phase in time. This results in a deviation between the phase output by the PLL and the actual phase, which is called the phase difference. As Figure 8 shown, when a short-circuit fault occurs in the outgoing line of the direct-drive wind turbine, the PLL has a phase difference, and the phase difference is negative. At this time, the voltage phase output by the PLL lags behind the actual phase of the grid-connected port voltage. The internal potential U g of the direct-drive wind turbine output after being feedback-controlled by the inverter also lags in phase; at the same time, the phase of the grid-connected point voltage U b and the current I injected by the direct-drive wind turbine into the grid also lags. The existence of the phase difference causes the electrical quantity to lag after being generally advanced, delaying the phase advance of the electrical quantity.

[0066] Step6: According to the electrical phasor change when there is a phase difference in the PLL, and combine it with the obtained phasors of the three-phase internal potential of the direct-drive wind turbine, the grid-connected port voltage, and the phase change of the current injected by the direct-drive wind turbine into the grid to obtain the output frequency change characteristics of the direct-drive wind turbine grid-connected system when there is a phase difference in the PLL.

[0067] Specifically, since the response time of the current inner loop is very short and the response speed is fast, during the time when the power reaches the set power and the voltage phase continuously advances and the frequency continuously increases, the phase change caused by the phase difference affects the speed of frequency change. When the phase difference is negative, the existence of the phase difference delays the increase of the frequency. As Figure 9 shown, the fault frequency of the direct-drive wind turbine considering the phase difference is lower than that without considering the phase difference at the same time.

[0068] The specific implementation manners of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the above implementation manners. Various changes can be made without departing from the spirit of the present invention within the knowledge scope of those of ordinary skill in the art.

Claims

1. A method for analyzing the frequency variation characteristics of a direct-drive wind turbine, characterized in that: Step1: According to the topological structure of the direct-drive wind turbine grid-connected system, simplify and equivalent the direct-drive wind turbine grid-connected system to obtain a simplified equivalent circuit of the direct-drive wind turbine, and use the mathematical model of the grid-side converter of the direct-drive wind turbine grid-connected system to obtain the voltage-oriented double-loop control strategy of the grid-side converter of the direct-drive wind turbine; Step2: Based on the relationship between the active power and phase angle of the direct-drive wind turbine single-machine infinite system, use the simplified equivalent circuit of the direct-drive wind turbine, and combine the voltage-oriented double-loop control strategy of the grid-side converter of the direct-drive wind turbine to obtain the phase change characteristics of the three-phase internal electromotive force of the direct-drive wind turbine and the output power expression of the direct-drive wind turbine; Step3: Use the equivalent circuit of the direct-drive wind turbine to obtain the relationship between the three-phase internal electromotive force and the grid connection port voltage of the direct-drive wind turbine and the expression of the current injected into the grid by the direct-drive wind turbine. Combine the obtained phase change characteristics of the three-phase internal electromotive force of the direct-drive wind turbine to obtain the phase change phasors of the three-phase internal electromotive force, the grid connection port voltage, and the current injected into the grid by the direct-drive wind turbine; Step4: According to the phase change phasors of the three-phase internal electromotive force, the grid connection port voltage, and the current injected into the grid by the direct-drive wind turbine, obtain the frequency variation characteristics of the direct-drive wind turbine; Step5: Analyze the output expression of the direct-drive wind turbine phase-locked loop, and combine the transfer function of the typical basic structure of the phase-locked loop to obtain the electrical phasor change when there is a phase-locked difference in the phase-locked loop; Step6: According to the electrical phasor change when there is a phase-locked difference in the phase-locked loop, and combine the obtained phase change phasors of the three-phase internal electromotive force, the grid connection port voltage, and the current injected into the grid by the direct-drive wind turbine, obtain the output frequency variation characteristics of the direct-drive wind turbine grid-connected system when there is a phase-locked difference in the phase-locked loop.

2. The analysis method for the frequency change characteristics of a direct drive fan according to claim 1, characterized in that The specific content of Step2 is as follows: The relationship between the active power and phase angle of the direct-drive wind turbine single-machine infinite system is: Wherein, P is the actual output power of the direct-drive wind turbine, and U g is the equivalent internal electromotive force of the direct-drive synchronous wind generator, and U b is the grid connection point voltage of the unit, x is the converter connection reactance, and δ is the phase angle difference between U g and U b ; According to the simplified equivalent circuit of the direct-drive wind turbine, when the output power of the direct-drive wind turbine changes, the three-phase equivalent internal electromotive force of the direct-drive wind turbine undergoes a phase change under the action of the voltage-oriented double-loop feedback control of the grid-side converter of the direct-drive wind turbine, and the output power expression of the direct-drive wind turbine is obtained as: Wherein, P set is the active power target value of the direct-drive wind turbine, and U' g is the equivalent internal electromotive force of the direct-drive synchronous wind turbine after the phase change occurs.

3. The analysis method for the frequency change characteristics of a direct-drive fan according to claim 1, wherein The specific content of Step3 is as follows: According to the equivalent circuit of the direct-drive wind turbine, combined with the phase change characteristics of the three-phase internal electromotive force of the direct-drive wind turbine, the relationship between the three-phase internal electromotive force and the grid connection port voltage of the direct-drive wind turbine is obtained as: U g = U b + jωL g · I(3) Where, I is the current injected by the direct-drive wind turbine into the grid, ω is the angular velocity of the generator at power frequency, j is the imaginary part of the impedance, and L g is the filter inductance of the direct-drive wind turbine grid-connected system; When the phase of the three-phase internal electromotive force of the direct-drive wind turbine changes, the phase of the grid connection point voltage of the direct-drive wind turbine also changes under the condition that the inductance and current do not change, and the expression of the current injected into the grid by the direct-drive wind turbine is obtained as: When the phases of the three-phase internal electromotive force of the direct-drive wind turbine and the grid connection point voltage of the direct-drive wind turbine both change, the phase of the current injected into the grid by the direct-drive wind turbine also changes.

4. The analysis method for the frequency variation characteristics of a direct-drive fan according to claim 1, wherein The specific content of Step6 is as follows: The dq-axis components of the grid connection port voltage tracked by the direct-drive wind turbine become the following formula, and the phase of the three-phase internal electromotive force of the direct-drive wind turbine output after the inverter feedback control changes: Wherein, u d , u q are the dq-axis components of the actual grid-connected port voltage, u' d , u' q are the dq-axis components of the grid-connected port voltage output by the phase-locked loop, and Δθ pll is the deviation between the phase output by the phase-locked loop and the actual phase.

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