Analysis method for influence of virtual inertia of wind turbine during commutation failure on overvoltage of sending end

Abstract

The application provides an analysis method for influence of fan virtual inertia on overvoltage of a sending end during commutation failure, and the method comprises the following steps: considering influence of a direct current control system and fan virtual inertia control, a whole process of transient voltage change of the sending end during the commutation failure is given; a quantitative relationship between a transient overvoltage formation time and a size of fan active output is established; and influence of virtual inertia control and each parameter size on the overvoltage peak value is analyzed. The method can effectively guide selection of virtual inertia control parameters, on the basis of considering transient reactive characteristics of a rectifier, from the perspective of fan active output change, and adding influence analysis of the virtual inertia control on the transient overvoltage.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power systems, in particular to a method for analyzing the influence of virtual inertia of a wind turbine during commutation failure on overvoltage of a sending end. BACKGROUND

[0002] In response to the national energy sustainable development strategy, new energy represented by wind power has made great progress in China. China is gradually forming a double-high power system with high proportion of new energy and high proportion of power electronic equipment. Since the wind turbine is connected to the grid through a converter, the frequency disturbance resistance of the system is greatly reduced. Under this background, virtual inertia technology emerged as the times require and effectively improved the system frequency stability problem through application in wind turbines. However, another important factor affecting the stable operation of wind turbines is the overvoltage problem under the fault of AC-DC transmission system. During commutation failure, the sending end AC system will produce overvoltage due to the large decrease in reactive power consumption of the rectifier. Since the wind turbine will be disconnected from the grid when the terminal voltage exceeds 1.3pu, and the amplitude of the overvoltage caused by commutation failure is proportional to the DC transmission capacity and inversely proportional to the short-circuit ratio of the sending end AC system, the overvoltage problem caused by commutation failure seriously restricts the improvement of AC-DC transmission capacity.

[0003] At present, the mechanism analysis of the sending end transient overvoltage during commutation failure is mainly carried out from the transient reactive power characteristics of the rectifier. Some scholars have analyzed the transient reactive power characteristics of the rectifier during commutation failure and fault recovery and the reactive power exchange between the DC and the sending and receiving end system on the basis of deducing the reactive power consumption equation of the sending and receiving rectifiers; some scholars have described the sensitivity relationship of each parameter of the DC control link to the peak value of the sending end transient overvoltage, and proposed an optimization scheme for the DC control parameters under the premise of ensuring the normal operation of the DC system; and some scholars have given a formula for calculating the peak value of the sending end overvoltage by analyzing the change characteristics of the DC voltage and current, and given an overvoltage suppression method by optimizing the reactive power characteristics of the rectifier.

[0004] However, after the commutation failure occurs, the sending end AC system will not only suffer from reactive power impact due to the change in reactive power consumption of the rectifier; due to the short-circuiting of the inverter side during commutation failure, the active power transmitted by the DC will decrease rapidly under the action of the DC control system, thereby causing a sudden increase in the frequency of the sending end AC system. Therefore, the virtual inertia control of the wind farm in response to the frequency change will also act, and the change in active power output of the wind turbine will inevitably affect the sending end transient overvoltage, thereby affecting the safe and stable operation of the wind turbine. And with the increasing capacity of wind power connected to the grid, this influence will inevitably become more and more obvious. However, the influence of virtual inertia control of wind turbines on the sending end transient overvoltage during commutation failure has not been fully considered and analyzed. SUMMARY

[0005] In order to further illustrate the generation mechanism of the sending end transient overvoltage in the AC / DC power transmission system, the application provides a method for analyzing the influence of the virtual inertia control of the wind turbine on the sending end transient overvoltage.

[0006] In order to achieve the above object, the application provides the following scheme.

[0007] The method for analyzing the influence of the virtual inertia of the wind turbine on the sending end overvoltage during the commutation failure has the following steps.

[0008] Step 100: Under the condition of stable operation of the system, the static relationship between the rectifier side bus voltage and the active and reactive power flowing into the node is established.

[0009] Step 200: During the commutation failure, the transient change process of the rectifier side DC voltage and current is analyzed according to the action logic of the DC control system, including the rectifier side control and the inverter side control.

[0010] Step 300: According to the transient change of the reactive power consumption of the rectifier and the active power output of the wind turbine, the whole process of the transient voltage change of the sending end during the commutation failure is analyzed.

[0011] Step 400: The vector relationship between the virtual inertia control and the peak value of the sending end transient overvoltage is established.

[0012] According to the specific embodiments provided by the application, the following technical effects are disclosed.

[0013] Through the analysis, it is shown that the peak value of the sending end transient overvoltage caused by the commutation failure is negatively correlated with the size of the active power generated by the wind turbine at the current time. Increasing the virtual inertia damping coefficient will obviously slow down the active power recovery rate of the wind turbine after the commutation failure, and thus deteriorate the sending end overvoltage. The application comprehensively considers the DC control system and the virtual inertia control, and can comprehensively and accurately analyze the influence of the virtual inertia of the wind turbine on the sending end overvoltage during the commutation failure. BRIEF DESCRIPTION OF DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed in the embodiments. Obviously, the drawings in the following description only only some embodiments of the application, and for those skilled in the art, other drawings can also be obtained without creative labor.

[0015] Figure 1 The flow chart of the method for analyzing the influence of the virtual inertia of the wind turbine on the sending end overvoltage during the commutation failure is provided for the embodiments of the application.

[0016] Figure 2 The equivalent model diagram of the sending end wind-fire bundled in the embodiment 1 of the application.

[0017] Figure 3 DC control system logic diagram in embodiment 1 of the present application;

[0018] Figure 4 DC operation curve diagram during commutation failure in embodiment 1 of the present application;

[0019] Figure 5 Fan virtual inertia control block diagram in embodiment 1 of the present application;

[0020] Figure 6 Sending end converter bus voltage comparison vector diagram before and after in embodiment 1 of the present application;

[0021] Figure 7 (a) Active power output change curve diagram of thermal power unit during commutation failure in embodiment 1 of the present application;

[0022] Figure 7 (b) Reactive power output change curve diagram of thermal power unit during commutation failure in embodiment 1 of the present application;

[0023] Figure 7 (c) Rectifier side converter bus voltage change curve diagram during commutation failure in embodiment 1 of the present application;

[0024] Figure 8 Sending end converter bus transient voltage change during commutation failure before and after adding virtual inertia control in embodiment 1 of the present application;

[0025] Figure 9 Fan active power output under different virtual inertia parameters in embodiment 1 of the present application; DETAILED DESCRIPTION

[0026] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0027] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application will be further described in detail below with reference to the drawings and specific embodiments.

[0028] Figure 1 The flowchart of the analysis method of the influence of fan virtual inertia on sending end overvoltage during commutation failure in the present application is shown as follows: Figure 1 The method includes the following steps:

[0029] Step 100: in the case of stable operation of the system, the static relationship between the rectifier side bus voltage and the active and reactive power flowing into the node is established, specifically including:

[0030] Figure 2 For the equivalent model of the wind and thermal power bundled and sent out by the DC transmission system, P1 and Q1 are the active and reactive power generated by the thermal power unit respectively; P2, P3, Q2 and Q3 are the active and reactive power transmitted from the thermal power unit and the wind power unit to the sending end of the DC system respectively; P dr is the active power transmitted by the DC system; Q dr is the reactive power consumed by the rectifier; Q c is the reactive power generated by the AC filter system of the rectifier station; U dr is the DC voltage at the rectifier side; I d is the DC current; U g is the bus voltage at the rectifier side; U1 and U2 are the grid-connected voltages of the thermal and wind power units respectively; R1 and X1 are the equivalent resistance and reactance between the thermal power unit and the DC system respectively; R2 and X2 are the equivalent resistance and reactance between the wind power unit and the DC system respectively.

[0031] The analysis of the steady-state amplitude of the rectifier side bus voltage is to give the mathematical relationship between the amplitude of the rectifier side bus voltage and the active and reactive power of the node during the steady state. When operating in steady state, the rectifier side bus node satisfies the following power balance relationship:

[0032]

[0033] P dr and Q dr can be expressed as:

[0034]

[0035] where is the power factor angle of the rectifier, U dr0 is the no-load DC voltage at the rectifier side. The relationship between the grid-connected voltage U1 of the thermal power unit and the rectifier side bus voltage U g is:

[0036]

[0037] where ΔU and δU are the vertical and horizontal components of the line voltage drop respectively. By ignoring the line resistance, the relationship between U1 and U g is:

[0038]

[0039] If the thermal power unit is considered as an infinite power source, i.e. U1 is basically unchanged during the transient state, then the rectifier side bus voltage Ug When considering the changes in P1 and Q1 according to equation (4), the characteristics of these changes should be given special attention. Traditional thermal power units generate voltage and frequency at various nodes of the power system, while wind turbines adjust their output power at their grid connection nodes based on the voltage and frequency of those nodes. Therefore, during the analysis of commutation failure, U... g When considering transient changes, the impact of changes in the active power output of the wind turbine on the output P1 of the thermal power unit will be discussed.

[0040] Step 200: During the commutation failure period, based on the DC control system's operating logic, including rectifier-side control and inverter-side control, analyze the transient changes in rectifier-side DC voltage and current. The specific details are as follows:

[0041] The DC control system is as follows Figure 3 This is a logic block diagram of a DC control system under the CIGRE-HVDC standard model. Wherein, I... d I is the direct current; α is the rectifier firing angle; I d-order This is the DC current command value transmitted from the DC main control stage; in the PSCAD standard DC model, its value is 1 pu. dref This is the DC current reference value output by the low-voltage current limiting device to the rectifier / inverter side for constant current control; U di γ is the DC voltage on the inverter side; γ is the minimum turn-off angle of the inverter; γ ref This is the reference value for the inverter-side turn-off angle; β i The inverter side constant current control output leads the firing angle; β v The inverter-side turn-off angle control output leads the trigger angle; β is the inverter-side lead trigger angle, and its value is β. i β v The maximum value in.

[0042] The DC control system includes rectifier-side control and inverter-side control. The rectifier side is equipped with a constant α... min Control and constant current control, where constant α min Control by Figure 3 The PI limiting element in the rectifier-side control is implemented, and its external characteristic equation is:

[0043] U dr =1.3E r cosα min -(3 / π)X r I d (5)

[0044] U dr E is the DC voltage on the rectifier side. r X represents the effective value of the open-circuit voltage on the valve side of the rectifier-converter transformer. r This is the equivalent commutation reactance on the rectifier side.

[0045] The DC current reference value of the rectifier side constant current control is provided by the inverter side low voltage current limiting device, and the reference value is determined by the DC voltage of the inverter side, and the relationship is:

[0046]

[0047] The rectifier side constant current control compares the size of I d and I d-order , adjusts the alpha angle, and controls the DC current.

[0048] The inverter side is provided with constant current control, constant turn-off angle control and current deviation control. In the steady state, the inverter side operates in the constant turn-off angle control mode, and the constant turn-off angle control is intended to stabilize the turn-off angle γ at about 15° to prevent commutation failure. The external characteristic equation is:

[0049] U di =1.3E i cosγ-(3 / π)X i I d (7)

[0050] The current reference value of the inverter side constant current control is 0.1 pu smaller than that of the rectifier side, and because the PI loop of the inverter side constant current control has a minimum amplitude limiting parameter, when the DC current is greater than the constant current reference value, the constant current control cannot respond. Only when the DC current is less than the constant current control current reference value, the inverter increases the beta angle to reduce the inverter side DC voltage U di , and then increases the DC current.

[0051] The current deviation control is mainly used to realize the smooth switching between the inverter side constant current control and the constant turn-off angle control. When the DC current is between the DC side constant current control current reference values, the current deviation control will increase the gamma angle according to the current deviation to increase the DC current. The external characteristic equation is:

[0052] γ=γ ref +K(I dref -I d ) / I dref (8)

[0053] Where K is a positive real coefficient.

[0054] Step 300: According to the transient changes of the rectifier reactive power consumption and the fan active power output, the whole process of the sending end transient voltage change during commutation failure is analyzed, which specifically includes:

[0055] According to formula (2), the relationship between the reactive power consumed by the rectifier and the DC voltage U dr and the DC current I d of the rectifier side is:

[0056]

[0057] It can be seen that the reactive power Q consumed by the rectifier during the commutation failure period dr with U dr I d Closely related. Based on the characteristics of DC voltage, current, and active power transmission changes during commutation failure, the transient voltage change process of the rectifier-side AC bus can be divided into four stages. Figure 4 The following is a DC operating curve during a single commutation failure, where U is shown in the figure. di This is the DC voltage on the inverter side.

[0058] Stage 1: The DC system operates at point A in steady state. When a short-circuit fault occurs on the inverter side, the voltage drop at the inverter-side converter bus causes the inverter to fail to commutate. At this time, the DC inverter side is essentially short-circuited, therefore U di It will rapidly decrease to 0, and the rectifier-side constant current control will not be able to act in time, resulting in I d Increase, according to Figure 4 The DC system will then operate to transient operating point B. According to equation (8), during this process, Q... dr This will continue to increase, leading to an increase in Q1. Simultaneously, during commutation failure, the DC active power transmission P... dr As the frequency of the power generation unit decreases, its active power output P1 also decreases accordingly. Simultaneously, due to the sudden increase in the frequency of the sending-end system, the virtual inertial control will reduce the active power output P2 of the wind turbine. According to equation (4), the rectifier-side converter bus voltage U during this stage... Lr It will continue to decrease.

[0059] Stage 2: Due to U di When the current is reduced to 0, according to equation (5), the reference value I of the DC current output by the VDCOL control is... dref It will be reduced to the minimum limit, but the current DC current I d Greater than I dref If the inverter-side constant current control fails to respond, the rectifier-side constant current control will activate. The rectifier side will increase angle α and decrease U. dr , making I d As the current decreases and approaches the DC reference value, the system operates from point B to point C. According to equation (9), Q... dr This will decrease significantly, leading to a decrease in Q1. Meanwhile, with U... dr with I d The reduction in DC transmission active power P dr As it continues to decrease to near 0, the active power output P1 of the thermal power unit decreases accordingly to its minimum value. Therefore, during this stage, U Lr It will decrease to a minimum value and then gradually increase.

[0060] The phase 3: when the direct current is reduced to a certain level, the inverter restores normal commutation, U di is rapidly increased, which leads to I d is further reduced. The rectifier side constant current control increases U dr to suppress the reduction of I d . If I d is reduced to less than the inverter side constant current control reference current, the inverter will switch from the constant off-angle control to the constant current control. In this process, the system will run from the point C to the point D, the rectifier side direct current voltage U dr is increased, the direct current I d is reduced, and according to the formula (9), the reactive power Q dr consumed by the rectifier will continue to reduce, and Q1 is correspondingly reduced. At the same time, the direct current active transmission starts to recover, P dr is gradually increased, and the active output P1 of the thermal power unit at the sending end and the active output P2 of the wind power unit are correspondingly recovered. According to the formula (4), in this phase, U Lr will continue to increase and reach the maximum value.

[0061] The phase 4: U di continues to recover, and with the action of the rectifier side and the inverter side constant current control, the direct current I d starts to gradually increase, when the direct current is increased to equal the inverter side constant current control current reference value, the inverter side enters the current deviation control, and transits to the constant off-angle control, and finally the direct current system returns to the stable operation point A. In this phase, the reactive power Q dr consumed by the rectifier and the direct current transmission active P dr are gradually recovered to the steady state operation condition. Therefore, in this phase, U Lr will gradually recover to the steady state with the recovery of the direct current system.

[0062] Step 400: establishing the vector relationship between the virtual inertia control and the peak value of the transient overvoltage at the sending end, specifically comprising:

[0063] The wind turbine virtual inertia control block diagram is shown in Figure 5 , wherein the asterisk represents the reference value of the corresponding variable; ω s represents the rotor angular velocity of the wind turbine; I sq , I sd respectively represent the active current and the reactive current; Δi q is the active current control increment output by the virtual inertia control; K p is the damping coefficient of the virtual inertia control; K d is the inertia coefficient of the virtual inertia control.

[0064] In steady state, the direct-drive wind turbine operates in maximum power point tracking (MPPT) mode, and the reactive current reference value is typically set to 0 to ensure the turbine operates in unity power factor mode. Figure 5 Under the control logic, the active and reactive power of the direct-drive wind turbine connected to the grid in steady state satisfies equation (10):

[0065]

[0066] Where P w Q w These represent the active and reactive power of the wind turbine connected to the grid, respectively.

[0067] During commutation failure, due to the surplus active power in the sending-end system, the virtual inertial control of the wind turbine will adjust the active current I accordingly based on changes in the grid connection frequency. sq The size of the fan can be adjusted to change the active power output of the fan.

[0068] During a commutation failure, the rectifier-side commutator bus voltage will undergo the aforementioned four stages of change. Overvoltage occurs in stages 2 and 3, especially in stage 3 when the inverter has just resumed normal commutation, resulting in high rectifier reactive power consumption Q. dr The decrease in power output and the increase in active power output P1 of thermal power units will simultaneously lead to U Lr The increase of U. This invention focuses on the size of P1 in stage 3. According to equation (4), the larger P1 is in this stage, the more it will lead to U. Lr The larger the value, the greater the impact. Since P1 satisfies equation (1), and in stages 1 and 2, the active power output P2 of the fan is reduced to suppress the increase in the sending-end frequency, the active power output of the fan cannot recover in stage 3, which will lead to an increase in the active power output P1 of the thermal power unit, thereby worsening the transient overvoltage at the sending end. The effect of the virtual inertial control of the fan on the peak value of the sending-end overvoltage is as follows: Figure 6 As shown.

[0069] The Figure 6 In the diagram, P1' represents the reactive power output of the thermal power unit after virtual inertial control is applied to the wind turbine; U g 'The voltage vector of the rectifier-side converter bus after adding virtual inertial control to the wind turbine.' Specific implementation examples:

[0071] Based on the CIGRE-HVDC standard DC test system on the well-known PSCAD platform, the correctness of the above theoretical analysis method is verified, specifically including:

[0072] Based on the CIGRE-HVDC standard DC model under the PSCAD platform, a direct-drive wind farm is added at the sending end, and the turbine / grid-side converter adopts... Figure 5 The control strategy shown is as follows: the active power output of the wind turbine is 200MW in steady state, and the DC steady-state transmission power is 1000MW.

[0073] A three-phase short circuit is applied at the converter bus node on the inverter side, with a transition resistance of 150Ω. The fault occurs at 2.5 seconds and lasts for 0.1 seconds. The virtual inertial control proportional coefficient is set to 40, and the derivative coefficient is set to 10. Figure 7 (a)~(c) represent the active power output P1, reactive power output Q1, and rectifier-side converter bus voltage U of the thermal power unit during the commutation failure period, respectively. Lr Simulated waveform. (From...) Figure 7 It can be seen that the rectifier-side converter bus voltage U Lr The simulation results of transient changes during commutation failure are consistent with the theoretical analysis regarding the relationship between the output power P1 and Q1 of the thermal power unit.

[0074] To verify the effect of virtual inertia on the transient overvoltage at the sending end, virtual inertial control was added to the direct-drive wind farm, with the proportional element coefficient set to 40 and the derivative element coefficient set to 10. Figure 8 To determine the transient voltage U of the sending-end converter bus during commutation failure before and after the addition of virtual inertial control. g The changes. From Figure 8 It can be seen that after adding virtual inertial control, when the system fails to commutate, the virtual inertial control of the direct-drive wind turbine will worsen the transient overvoltage of the rectifier-side converter bus. The simulation results are consistent with the theoretical analysis.

[0075] Depend on Figure 9 It can be seen that, in the scenario of single commutation failure, increasing the virtual inertial control differential coefficient K... d This accelerates the active power response speed of the wind turbine, but since the power output amplitude of the wind turbine is almost unaffected, the differential coefficient K... d The magnitude of the voltage has little effect on the amplitude of the overvoltage at the sending end; however, increasing the virtual inertial control proportional coefficient K... p To suppress sudden increases in the sending-end frequency, the active power output of the wind turbine during commutation failure will be significantly reduced. Furthermore, when the inverter resumes normal commutation, the active power output of the wind turbine cannot recover quickly, leading to K... p The larger the value, the smaller the active power output P2 of the wind turbine when the overvoltage peak is formed, which in turn leads to a larger value for P1, which will make the overvoltage at the sending end more severe.

[0076] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the systems disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple; relevant parts can be referred to the method section.

[0077] The principles and implementation manners of the present application are described by using specific examples in the present application, and the above examples are only used to help understand the method of the present application and its core idea; meanwhile, for the general technical personnel in the art, the specific implementation manners and application ranges will be changed according to the idea of the present application. In conclusion, the content of the present specification should not be understood as the limitation of the present application.

Claims

1. A method for analyzing the influence of the virtual inertia of the wind turbine on the overvoltage at the sending end during commutation failure, characterized in that, Combining the DC control system and the virtual inertial control of the wind turbine, this paper analyzes the transient voltage change mechanism of the rectifier-side converter bus during commutation failure in a wind-fire bundled DC power transmission system scenario, including: Under stable system operation, establish the static relationship between the voltage of the rectifier-side converter bus and the active and reactive power flowing into this node; During commutation failure, based on the DC control system's operating logic, including rectifier-side control and inverter-side control, the transient change process of DC voltage and current on the rectifier side is analyzed to obtain the external characteristic equations of the rectifier side and the inverter side. Based on the transient changes in rectifier reactive power consumption and wind turbine active power output, the static relationship between the rectifier-side converter bus voltage and the active and reactive power flowing into this node, and the external characteristic equations of the rectifier side and the inverter side, the entire process of the transient voltage change at the sending end during commutation failure is analyzed. The entire process of the transient voltage change at the sending end during commutation failure is divided into four stages. In stage 3, the active power output of the wind turbine cannot recover in time, which will lead to an increase in the active power output of the thermal power unit, thereby worsening the transient overvoltage at the sending end. The four stages include stage 1, stage 2, stage 3, and stage 4. Stage 1: DC system steady state Operating at point A, when a short-circuit fault occurs on the inverter side, the DC system will operate to transient operating point B. In stage 1, the rectifier-side converter bus voltage continuously decreases. In stage 2, the DC system operates from point B to point C. In stage 2, the rectifier-side converter bus voltage will decrease to a minimum and then gradually increase. In stage 3, the DC system will operate from point C to point D. In stage 3, the rectifier-side converter bus voltage will continue to increase and reach its maximum value. In stage 4, the DC system returns to stable operating point A. In stage 4, the rectifier-side converter bus voltage will gradually recover to steady state as the DC system recovers. For stage 3, a vector relationship is established between virtual inertial control and the peak value of transient overvoltage at the sending end, and the influence of the virtual inertial control of the wind turbine on the peak value of overvoltage at the sending end is determined based on the vector relationship.

2. The method for analyzing the influence of the virtual inertia of the wind turbine on the overvoltage at the sending end during commutation failure as described in claim 1, characterized in that, The establishment of the static relationship between the rectifier-side converter bus voltage and the active and reactive power flowing into the node specifically includes: During steady-state operation, the rectifier-side converter bus nodes satisfy the power balance relationship: Where P2 is the active power transmitted from the wind turbine to the DC system sending end, and P3 is the active power transmitted from the thermal power unit to the DC system sending end. dr Q1 represents the active power transmitted to the DC system; Q2 represents the reactive power transmitted from the wind turbine to the DC system's sending end; Q3 represents the reactive power transmitted from the thermal power unit to the DC system's sending end. c Q represents the reactive power generated by the AC filter system of the rectifier station. dr The reactive power consumed by the rectifier; P dr With Q dr The relationship is represented as: in U is the rectifier power factor angle. dr0 U is the unloaded DC voltage on the rectifier side; dr I is the DC voltage on the rectifier side. d It is the direct current; the grid-connected voltage U1 of the thermal power unit and the converter bus voltage U on the rectifier side. g The relationship is: Where ΔU and δU are the longitudinal and transverse components of the line voltage drop, respectively; P1 is the active power generated by the thermal power unit, Q1 is the reactive power generated by the thermal power unit, and R1 is the equivalent resistance between the thermal power unit and the DC system. Neglecting the line resistance, we obtain U1 and U... g The relationship is:

3. The method for analyzing the influence of the virtual inertia of the wind turbine on the overvoltage at the sending end during commutation failure as described in claim 1, characterized in that, Analyze the operating logic of the DC control system, specifically the rectifier-side control: The rectifier side is equipped with a fixed α min Control and constant current control, where constant α min Control is achieved by a PI limiting circuit in the rectifier-side control, and its external characteristic equation is: The dr =1.3E r what min -(3 / π)X r I d Among them U dr E is the DC voltage on the rectifier side. r X represents the effective value of the open-circuit voltage on the valve side of the rectifier-converter transformer. r I is the equivalent commutation reactance on the rectifier side. d It is direct current; The reference value of the DC current for constant current control on the rectifier side is provided by the low-voltage current limiting device on the inverter side, and the magnitude of the reference value is determined by the DC voltage on the inverter side.

4. The method for analyzing the influence of the virtual inertia of the wind turbine on the overvoltage at the sending end during commutation failure as described in claim 1, characterized in that, Analyze the operating logic of the DC control system, specifically the inverter-side control: The inverter side is equipped with constant current control, constant turn-off angle control, and current deviation control. In steady state, the inverter side operates in constant turn-off angle control mode. To prevent commutation failure, the constant turn-off angle control aims to stabilize the turn-off angle γ at approximately 15°. Its external characteristic equation is: IN di =1.3E i cosγ-(3 / π)X i AND d Among them, I d Where is the DC current, γ is the minimum turn-off angle of the inverter, and E i X represents the effective value of the no-load voltage on the valve side of the inverter-side converter transformer. i This is the equivalent commutation reactance on the inverter side; The current reference value for constant current control on the inverter side is 0.1 pu smaller than that on the rectifier side. Furthermore, because the PI circuit under constant current control on the inverter side has a minimum limiting parameter, constant current control cannot respond when the DC current exceeds the constant current reference value. Only when the DC current is less than the constant current control current reference value does the inverter reduce the inverter-side DC voltage U by increasing the β angle. di This increases the DC current; β is the inverter-side lead firing angle. Current deviation control is used to achieve smooth switching between constant current control and constant turn-off angle control on the inverter side. When the DC current is between the reference values ​​of the constant current control on both sides of the DC current, the current deviation control will increase the γ angle according to the current deviation to increase the DC current. Its external characteristic equation is: γ=γ ref +K(I dref -I d ) / I dref Where K is a positive real coefficient, γ ref I is the reference value for the inverter-side turn-off angle. dref This is the reference value of the DC current output by the low-voltage current limiting device for constant current control on the rectifier / inverter side.

5. The method for analyzing the influence of the virtual inertia of the wind turbine on the overvoltage at the sending end during commutation failure as described in claim 1, characterized in that, The analysis of the entire process of transient voltage change at the sending end during commutation failure specifically includes: Based on the occurrence, duration, termination, and recovery of inverter commutation failure, the transient voltage change at the sending end is divided into four stages. Based on the transient reactive power characteristics of the rectifier and the active power output characteristics of the wind turbine in each stage, the voltage change of the converter bus at the sending end in each stage is analyzed.

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