A method for calculating transient overvoltage of a grid-connection type wind power LCC-HVDC transmission system

Abstract

The application provides a kind of network type wind power LCC-HVDC sending out system transient overvoltage calculation method, comprising: according to network type wind power and thermal power unit bundling equivalent model of DC external transmission system, the system power flow equation is established;According to the high penetration characteristics of network type wind power unit and the active frequency characteristics, the active and reactive power of wind turbine under fault is calculated;According to the power characteristics under commutation failure fault of UHVDC transmission system, the DC transmission active power, DC consumption reactive power and rectifier side filter output reactive power are calculated;According to the system power flow equation and the aforementioned calculation results, the power flowing to AC system under fault is calculated;According to the power flowing to AC system under fault, the sending end commutation bus transient overvoltage is calculated.The application can quickly calculate the network type wind-fire bundling system sending end commutation bus transient overvoltage under commutation failure scenario, and can quickly judge the network type wind turbine off-grid risk.

Description

Technical Field

[0001] The present invention relates to a method for calculating transient overvoltage of a grid-type wind power transmission system via LCC-HVDC, and belongs to the technical field of power systems and automation thereof. Background Art

[0002] Under the "dual carbon" initiative, the power system is gradually evolving toward a high-proportion power electronics and AC / DC hybrid configuration. Large-scale wind and solar power are being transmitted via UHVDC. Short-circuit faults on the inverter side can easily trigger DC commutation failures. On the one hand, a large active power surplus on the DC sending side during a commutation failure can cause an increase in the AC system frequency at the sending end, potentially triggering high-frequency generator tripping and jeopardizing the safe and stable operation of the system. On the other hand, during a commutation failure, DC reactive power demand decreases. However, due to the long filter cut-off time, the reactive power output from the filter is fed back to the AC system, causing the commutation bus voltage to rise. Due to the insufficient voltage withstand capability of power electronic components, the risk of wind farms near the sending end disconnecting from the grid increases, further triggering cascading failures. Therefore, it is urgent to study quantitative calculation methods for transient overvoltages on the commutation bus on the rectifier side during commutation failure scenarios.

[0003] Currently, calculations of transient overvoltages on the rectifier-side AC busbar during UHVDC commutation failures often fail to consider the impact of wind turbine integration, making this approach unsuitable for calculating transient overvoltages in wind-thermal bundled DC transmission systems. A few studies have considered the impact of wind turbine integration, but these studies employ grid-following control strategies and fail to consider transient overvoltage calculations for grid-connected wind turbines in UHVDC transmission systems. With the increasing integration of grid-connected wind turbines, the transient overvoltage mechanisms in the DC transmission system are changing. Therefore, there is an urgent need to investigate transient overvoltage calculation methods for grid-connected wind power in DC transmission systems to rapidly assess the risk of grid-connected wind turbines disconnecting from the grid. Summary of the Invention

[0004] The present invention provides a method for calculating transient overvoltage of a grid-connected wind power transmission system via LCC-HVDC, which solves the problems disclosed in the background art.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] A method for calculating transient overvoltage of a grid-connected wind power transmission system via LCC-HVDC includes:

[0007] Based on the equivalent model of the grid-connected wind power and thermal power generation bundled with DC transmission system, the system power flow equation is established;

[0008] Calculating the active and reactive power of the wind turbine under fault conditions based on the high-speed run-through characteristics and active frequency characteristics of the grid-type wind turbine generator set; the high-speed run-through characteristics and active frequency characteristics of the grid-type wind turbine generator set include the reactive current coefficient during the high-speed run-through period, the virtual inertia coefficient, and the virtual damping coefficient of the grid-type wind turbine generator set;

[0009] Based on the power characteristics of the UHVDC transmission system under commutation failure fault, calculate the DC transmission active power, DC consumption reactive power and rectifier-side filter output reactive power;

[0010] The power flowing to the AC system during a fault is calculated based on the system power flow equation, the active and reactive power of the wind turbine during a fault, the DC transmission active power, the DC consumption reactive power, and the rectifier-side filter output reactive power.

[0011] The transient overvoltage of the commutation bus at the sending end is calculated based on the power flowing to the AC system under the fault.

[0012] Furthermore, according to the equivalent system model of wind-fire bundled grid-type DC transmission, a system power flow equation is established. The established system power flow equation is:

[0013] P s +jQ s +P GFM +jQ GFM +jQ c =P d +jQ d

[0014] Among them, P s The output active power of the thermal power unit, Q s is the reactive power output of the thermal power unit, P GFM The output active power of the grid-connected wind turbine is Q GFM The reactive power output of the grid-connected wind turbine is Q c P is the reactive power output by the DC sending filter, d It transmits active power to the DC sending end, Q d It consumes reactive power at the DC sending end.

[0015] Furthermore, the calculation of the active and reactive power of the wind turbine under fault conditions based on the high-power penetration characteristics and active frequency characteristics of the grid-connected wind turbine generator system specifically includes:

[0016] The reactive power of the fan under fault conditions is calculated according to the following formula:

[0017] Q GFM =K2U T I N (U T -1.1)

[0018] Among them, QGFM is the reactive power of the wind turbine under fault, K2 is the reactive current coefficient of the wind turbine during high-voltage run-through, U T is the wind turbine grid connection point voltage, I N is the rated current of the fan;

[0019] The active power of the fan under fault conditions is calculated according to the following formula:

[0020]

[0021] Among them, P GFM is the active power of the wind turbine under fault conditions, P ref is the reference value of the steady-state active power of the wind turbine, D is the virtual damping coefficient of the grid-type wind turbine, J is the virtual inertia coefficient of the grid-type wind turbine, Δω is the angular frequency deviation, and ω is the angular frequency.

[0022] Furthermore, the calculation of the DC transmission active power, DC consumption reactive power, and rectifier-side filter output reactive power based on the power characteristics of the UHVDC transmission system in a commutation failure scenario specifically includes:

[0023] Obtain the current limit value of the UHVDC low-voltage current limiting link and calculate the DC transmission active power and reactive power consumption in the commutation failure scenario based on the current limit value;

[0024] The formula for calculating the active power transmitted by DC during a fault is:

[0025] P df =I dlow U dN

[0026] Among them, P df is the active power transmitted during DC commutation failure, I dlow is the current limit value of the DC low-voltage current limiting link, U dN is the DC rated voltage;

[0027] Calculate the DC no-load voltage on the rectifier side using the following formula:

[0028]

[0029] Among them, U d0 is the DC no-load voltage on the rectifier side, N is the number of 6-pulse commutation bridges, T is the ratio of the converter transformer, U LN is the rated value of the AC bus voltage;

[0030] The DC reactive power consumed in the commutation failure scenario is calculated based on the DC no-load voltage on the rectifier side. The calculation formula is:

[0031]

[0032] Among them, Q df is the reactive power consumed during DC commutation failure;

[0033] The calculation formula for the reactive power output by the rectifier side filter is:

[0034]

[0035] Among them, Q cf is the reactive power output by the filter on the rectifier side when DC commutation fails, U Lf For rectifier side cross

[0036] Current bus transient voltage, Q cN It is the steady-state output reactive power of the filter.

[0037] Furthermore, the power flowing to the AC system under fault conditions is calculated based on the system power flow equation, and the calculation formula is:

[0038]

[0039] Among them, P sf is the active power flowing to the AC system in the commutation failure scenario, Q sf is the reactive power flowing into the AC system in the commutation failure scenario.

[0040] Furthermore, the calculation of the transient overvoltage of the sending-end commutation bus according to the power flowing to the AC system under the fault specifically includes:

[0041] The calculation formula for the transient overvoltage of the sending-end commutation bus is:

[0042]

[0043] Among them, U s is the equivalent AC system voltage, X s is the equivalent reactance between the equivalent AC system and the commutation busbar on the rectifier side;

[0044] The forward-backward substitution method is used to iteratively solve the transient overvoltage of the sending-end commutation bus.

[0045] Furthermore, the iterative solution of the transient overvoltage of the sending-end commutation busbar using the forward-backward substitution method specifically includes:

[0046] Given the initial value of transient bus overvoltage U Lf0 ;

[0047] Will U Lf0 Substitute this into the formula for calculating the power flowing to the AC system under the commutation failure scenario to obtain the initial value of the power flowing to the AC system S sf0 ;

[0048] Combined with ULf0 and S sf0 Calculate the equivalent power S injected into the AC system si The power S consumed by the equivalent impedance of the AC system zi ;

[0049] Using S si Perform back substitution and calculate U Lfi , according to U Lfi Update the power S injected into the AC system in the commutation failure scenario sfi ;

[0050] According to U Lfi and S sfi Calculate the equivalent power supply potential U si and the initial potential U of the equivalent power supply s Compare and if the difference is less than the set value, stop the iterative calculation, otherwise proceed to the next iteration until the difference meets the requirements.

[0051] Furthermore, the method further comprises the steps of:

[0052] The wind turbine off-grid risk result is determined based on the transient overvoltage of the sending-end commutation bus.

[0053] Furthermore, the determination of the wind turbine off-grid risk result based on the transient overvoltage of the sending-end commutation busbar specifically includes:

[0054] Determine whether the calculated transient overvoltage of the sending-end commutation bus exceeds 1.3pu. If it exceeds 1.3pu, it is determined that the wind turbine is at risk of disconnection from the grid.

[0055] The beneficial effects achieved by the present invention are as follows: the present invention can quickly calculate the transient overvoltage of the commutation bus at the sending end of the grid-type wind-fire bundling system under the commutation failure scenario, quantitatively analyze the influence of the grid-type control parameters on the transient overvoltage of the commutation bus at the sending end, and quickly determine the risk of grid-type wind turbines being disconnected from the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 This is a flow chart of a method for calculating transient overvoltage in a grid-connected wind power transmission system via LCC-HVDC according to an embodiment of the present invention;

[0057] Figure 2 is the equivalent circuit diagram of the DC sending end system;

[0058] Figure 3 It is the control strategy of grid-type direct-drive wind turbine;

[0059] Figure 4 Transient overvoltage simulation results. Specific implementation plan

[0060] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention.

[0061] like Figure 1 As shown, an embodiment of the present invention provides a method for calculating transient overvoltage of a grid-connected wind power transmission system via LCC-HVDC, comprising the following steps:

[0062] Step 1: Establish the system power flow equation based on the equivalent model of the DC transmission system for bundled wind power and thermal power units;

[0063] Step 2: Calculate the active and reactive power of the wind turbine under fault conditions based on the high-speed run-through characteristics and active-frequency characteristics of the grid-type wind turbine, wherein the high-speed run-through characteristics and active-frequency characteristics of the grid-type wind turbine include the reactive current coefficient during the high-speed run-through period, the virtual inertia coefficient, and the virtual damping coefficient of the grid-type wind turbine;

[0064] Step 3: Calculate the DC transmission active power, DC consumption reactive power, and rectifier-side filter output reactive power based on the power characteristics of the UHVDC system under commutation failure.

[0065] Step 4: Calculate the power flowing to the AC system during the fault based on the system power flow equation obtained in step 1, the active and reactive power of the wind turbine during the fault obtained in step 2, the DC transmission active power, DC consumption reactive power, and rectifier-side filter output reactive power obtained in step 3.

[0066] Step 5: Calculate the transient overvoltage of the commutation bus at the sending end according to the power flowing to the AC system under the fault;

[0067] Step 6: determining the wind turbine off-grid risk result based on the transient overvoltage of the sending-end commutation bus;

[0068] Example 1

[0069] Figure 2 The equivalent model of the grid-type wind-fire bundled LCC-HVDC transmission system provided by the present invention is as follows: Figure 2 As shown, the system power flow equation is established:

[0070] P s +jQ s +P GFM +jQ GFM +jQ c =P d +jQ d (1)

[0071] Among them, P s The output active power of the thermal power unit, Q s is the reactive power output of the thermal power unit, PGFM The output active power of the grid-connected wind turbine is Q GFM Output reactive power for grid-type wind turbines.

[0072] Q c P is the reactive power output by the DC sending filter, d It transmits active power to the DC sending end, Q d It consumes reactive power at the DC sending end.

[0073] Rectifier side commutation bus voltage U L and the equivalent AC system voltage U s The relationship is:

[0074]

[0075] Among them, ΔU line is the longitudinal component of the line voltage drop, δU line is the transverse component of the line voltage drop, U s is the equivalent AC system voltage, U L is the commutation bus voltage on the rectifier side, R s is the equivalent resistance between the equivalent AC system and the commutation busbar on the rectifier side, X s is the equivalent reactance between the equivalent AC system and the commutation bus on the rectifier side.

[0076] Figure 2 PMSG is a direct-drive fan with a grid structure. w is the equivalent resistance between the fan and the commutation busbar on the rectifier side, X w It is the equivalent reactance between the wind turbine and the commutation busbar on the rectifier side.

[0077] Since the high voltage line impedance often presents inductive reactance characteristics, the line resistance can be ignored, and U can be obtained L with U s The relationship is:

[0078]

[0079] The equivalent AC system is replaced by an infinite power source, and its transient period U s Remain unchanged, and analyze the commutation bus voltage U of the sending end system under commutation failure L When considering the changes in P s , Q s changes.

[0080] Analysis of the impact of wind turbine grid-type control on transient overvoltage of the commutation busbar on the rectifier side: Since traditional grid-type direct-drive wind turbines often use constant power control, they cannot respond to changes in grid voltage and frequency and actively change output power to support the grid. The grid-type control strategy can effectively solve this problem. The present invention applies the active frequency characteristics and reactive voltage characteristics of the synchronous generator to the grid-side converter of the direct-drive wind turbine. Figure 3 This is a grid-type control block diagram of the direct-drive wind turbine grid-side converter provided by the present invention.

[0081] During the overvoltage period of commutation failure, the grid-connected direct-drive wind turbine enters the high voltage ride-through state. The reactive power of the wind turbine under fault is:

[0082] Q GFM =K2U T I N (U T -1.1) (4)

[0083] Among them, Q GFM is the reactive power of the fan under fault, K2 is the reactive current coefficient of the fan high-voltage run-through, U T is the wind turbine grid connection point voltage, I N is the rated current of the fan.

[0084] Due to the effect of the fan active frequency link, the active power of the fan under fault is:

[0085]

[0086] Among them, P GFM is the active power of the wind turbine under fault conditions, P ref is the reference value of the steady-state active power of the wind turbine, D is the virtual damping coefficient of the grid-type wind turbine, J is the virtual inertia coefficient of the grid-type wind turbine, Δω is the angular frequency deviation, and ω is the angular frequency;

[0087] When a direct-drive wind turbine adopts a grid-type control strategy, when the frequency and voltage of the wind turbine grid connection point change, the grid-type control strategy can adjust the wind turbine output active and reactive power to suppress the frequency and voltage fluctuations of the power grid.

[0088] In the event of a commutation failure, the active power transmitted by the DC will be significantly reduced, the frequency of the AC system will increase, and the active power output of the direct-drive wind turbine will be reduced due to the active frequency link. In addition, the reactive power consumed by the DC will be greatly reduced. The reactive power surplus of the filter will cause the AC bus voltage at the sending end to increase, and the wind turbine will enter a high voltage ride-through state, absorbing some of the reactive power surplus.

[0089] Therefore, the direct-drive wind turbine adopting the grid-type control strategy can reduce the active and reactive power fed back to the AC system in the scenario of DC commutation failure.

[0090] According to UL with U s It can be seen from the relationship between and that the grid-type control strategy for wind turbines can reduce the transient overvoltage of the commutation bus at the sending end.

[0091] according to Figure 2 The equivalent model of the LCC-HVDC transmission system for medium-grid wind turbines is used to calculate the DC transmission active power, DC consumption reactive power, and rectifier-side filter output reactive power:

[0092] First, the current limit value of the UHVDC low-voltage current limiting link is obtained, and the DC active power transmission and reactive power consumption in the commutation failure scenario are calculated based on the limit value.

[0093] The calculation formula of DC transmission active power during fault period is:

[0094] P df =I dlow U dN (6)

[0095] Among them, P df is the active power transmitted during DC commutation failure, I dlow is the current limit value of the DC low-voltage current limiting link, U dN is the DC rated voltage.

[0096] Calculate the DC no-load voltage on the rectifier side using the following formula:

[0097]

[0098] Among them, U d0 is the DC no-load voltage on the rectifier side, N is the number of 6-pulse commutation bridges, T is the ratio of the converter transformer, U LN is the rated value of the AC bus voltage;

[0099] The DC reactive power consumed in the commutation failure scenario is calculated based on the DC no-load voltage on the rectifier side. The calculation formula is:

[0100]

[0101] Among them, Q df is the reactive power consumed during DC commutation failure;

[0102] The calculation formula for the reactive power output by the rectifier side filter is:

[0103]

[0104] Among them, Q cf is the reactive power output by the filter on the rectifier side when DC commutation fails, U Lf is the AC bus transient voltage on the rectifier side, Q cNIt is the steady-state output reactive power of the filter.

[0105] The power flowing to the AC system during a fault is calculated based on the system power flow equation. The calculation formula is:

[0106]

[0107] Among them, P sf is the active power flowing to the AC system in the commutation failure scenario, Q sf is the reactive power flowing into the AC system in the commutation failure scenario.

[0108] Substituting equations (4), (5), (6), (8), and (9) into equation (10), the power flowing to the AC system under commutation failure is obtained as follows:

[0109]

[0110] Substituting equation (11) into equation (3), the calculation expression for the DC bus overvoltage at the sending end under commutation failure fault is:

[0111]

[0112] According to the above formula, for a certain wind-fire bundled DC transmission system, the AC system voltage at the sending end is U s , the reactance X between the sending-end AC system and the commutation bus s , commutation bus rated voltage U LN , DC low voltage current limiting link current limit value I dlow The virtual inertia J, virtual damping D, and high-throughput reactive voltage coefficient K2 of the grid-type direct-drive wind turbine are all known quantities, and the forward-backward substitution method can be used to iteratively solve the transient overvoltage of the sending-end bus.

[0113] According to the output power of the grid-type wind turbine during transient period, it can be seen that the increase of virtual damping coefficient, virtual inertia coefficient and reactive voltage coefficient during high-voltage run-through period is beneficial to reducing the transient overvoltage peak of the commutation bus at the sending end.

[0114] Figure 2 The main parameters of the DC transmission system of the grid-type fan are shown in Table 1.

[0115] Table 1 Main parameters of the system

[0116]

[0117]

[0118] A grid-type wind power transmission system model was constructed on the PSCAD / EMTDC simulation platform. The system parameters are shown in the table above. A three-phase ground fault was set on the inverter-side commutation busbar, with a transition resistance of 1Ω, a fault occurrence time of 3.3s, and a fault duration of 0.1s. During the fault, the UHVDC transmission system failed to commutate, and the commutation busbar on the rectifier side showed a characteristic of first low and then high, as shown in the figure below. Figure 4 The effects of different high-voltage reactive current coefficients on the overvoltage peak are shown in Table 2.

[0119] Table 2 Comparison of transient overvoltage simulation and calculation values ​​at different K2

[0120]

[0121] The data in Table 2 shows that the calculated transient overvoltage values ​​are highly consistent with the simulated values. The proposed method for calculating transient overvoltage in a grid-connected wind power system via DC transmission has high accuracy. As the high-throughput reactive current coefficient of the grid-connected wind turbine increases, the transient overvoltage shows a clear downward trend.

[0122] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for calculating transient overvoltage in a grid-connected wind power transmission system via LCC-HVDC, characterized in that: include: Based on the equivalent model of the grid-connected wind power and thermal power generation bundled with DC transmission system, the system power flow equation is established; Calculating the active and reactive power of the wind turbine under fault conditions based on the high-speed run-through characteristics and active frequency characteristics of the grid-type wind turbine generator set; the high-speed run-through characteristics and active frequency characteristics of the grid-type wind turbine generator set include the reactive current coefficient during the high-speed run-through period, the virtual inertia coefficient, and the virtual damping coefficient of the grid-type wind turbine generator set; Based on the power characteristics of the UHVDC transmission system under commutation failure fault, calculate the DC transmission active power, DC consumption reactive power and rectifier-side filter output reactive power; The power flowing to the AC system during a fault is calculated based on the system power flow equation, the active and reactive power of the wind turbine during a fault, the DC transmission active power, the DC consumption reactive power, and the rectifier-side filter output reactive power. The transient overvoltage of the commutation bus at the sending end is calculated based on the power flowing to the AC system under the fault.

2. The method for calculating transient overvoltage of a grid-connected wind power transmission system via LCC-HVDC according to claim 1 is characterized in that: According to the equivalent system model of wind-fire bundled grid-type DC transmission, a system power flow equation is established. The established system power flow equation is: ； Among them, P s The output active power of the thermal power unit, Q s is the reactive power output of the thermal power unit, P GFM The output active power of the grid-connected wind turbine is Q GFM The reactive power output of the grid-connected wind turbine is Q c P is the reactive power output by the DC sending filter, d It transmits active power to the DC sending end, Q d It consumes reactive power at the DC sending end.

3. The method for calculating transient overvoltage of a grid-connected wind power transmission system via LCC-HVDC according to claim 2 is characterized in that: The calculation of the active and reactive power of the wind turbine under fault conditions based on the high-voltage penetration characteristics and active frequency characteristics of the grid-connected wind turbine generator system specifically includes: The reactive power of the fan under fault conditions is calculated according to the following formula: ； Among them, Q GFM is the reactive power of the wind turbine under fault, K2 is the reactive current coefficient of the wind turbine during high-voltage run-through, U T is the wind turbine grid connection point voltage, I N is the rated current of the fan; The active power of the fan under fault conditions is calculated according to the following formula: ； Among them, P GFM is the active power of the wind turbine under fault conditions, P ref is the reference value of the steady-state active power of the wind turbine, D is the virtual damping coefficient of the grid-type wind turbine, J is the virtual inertia coefficient of the grid-type wind turbine, is the angular frequency deviation, is the angular frequency.

4. The method for calculating transient overvoltage of a grid-connected wind power transmission system via LCC-HVDC according to claim 3 is characterized in that: The calculation of the DC transmission active power, DC consumption reactive power, and rectifier-side filter output reactive power based on the power characteristics of the UHVDC transmission system in a commutation failure scenario specifically includes: Obtain the current limit value of the UHVDC low-voltage current limiting link and calculate the DC transmission active power and reactive power consumption in the commutation failure scenario based on the current limit value; The formula for calculating the active power transmitted by DC during a fault is: ； Among them, P df is the active power transmitted during DC commutation failure, I dlow is the current limit value of the DC low-voltage current limiting link, U dN is the DC rated voltage; Calculate the DC no-load voltage on the rectifier side using the following formula: ； Among them, U d0 is the DC no-load voltage on the rectifier side, N is the number of 6-pulse commutation bridges, T is the ratio of the converter transformer, U LN is the rated value of the AC bus voltage; The DC reactive power consumed in the commutation failure scenario is calculated based on the DC no-load voltage on the rectifier side. The calculation formula is: ； Among them, Q df is the reactive power consumed during DC commutation failure; The calculation formula for the reactive power output by the rectifier side filter is: ； Among them, Q cf is the reactive power output by the filter on the rectifier side when DC commutation fails, U Lf For rectifier side cross Current bus transient voltage, Q cN It is the steady-state output reactive power of the filter.

5. The method for calculating transient overvoltage of a grid-connected wind power transmission system via LCC-HVDC according to claim 4 is characterized in that: The power flowing to the AC system under fault conditions is calculated based on the system power flow equation. The calculation formula is: ； Among them, P sf is the active power flowing to the AC system in the commutation failure scenario, Q sf is the reactive power flowing into the AC system in the commutation failure scenario.

6. The method for calculating transient overvoltage of a grid-connected wind power transmission system via LCC-HVDC according to claim 5 is characterized in that: The calculation of the transient overvoltage of the commutation busbar at the sending end according to the power flowing to the AC system under the fault specifically includes: The calculation formula for the transient overvoltage of the sending-end commutation bus is: ； Among them, U s is the equivalent AC system voltage, X s is the equivalent reactance between the equivalent AC system and the commutation busbar on the rectifier side; The forward-backward substitution method is used to iteratively solve the transient overvoltage of the sending-end commutation bus.

7. The method for calculating transient overvoltage of a grid-connected wind power transmission system via LCC-HVDC according to claim 6 is characterized in that: The iterative solution of the transient overvoltage of the sending-end commutation busbar using the forward-backward substitution method specifically includes: Given the initial value of transient bus overvoltage U Lf0 ; Will U Lf0 Substitute this into the formula for calculating the power flowing to the AC system under the commutation failure scenario to obtain the initial value of the power flowing to the AC system S sf0 ; Combined with U Lf0 and S sf0 Calculate the equivalent power S injected into the AC system si The power S consumed by the equivalent impedance of the AC system zi ; Using S si Perform back substitution and calculate U Lfi , according to U Lfi Update the power S injected into the AC system in the commutation failure scenario sfi ; According to U Lfi and S sfi Calculate the equivalent power supply potential U si and the initial potential U of the equivalent power supply s Compare and if the difference is less than the set value, stop the iterative calculation, otherwise proceed to the next iteration until the difference meets the requirements.

8. The method for calculating transient overvoltage of a grid-connected wind power transmission system via LCC-HVDC according to claim 1, characterized in that: Also includes steps; The wind turbine off-grid risk result is determined based on the transient overvoltage of the sending-end commutation bus.

9. The method for calculating transient overvoltage of a grid-connected wind power transmission system via LCC-HVDC according to claim 8, characterized in that: The wind turbine off-grid risk result is determined based on the transient overvoltage of the sending-end commutation busbar, specifically including: Determine whether the calculated transient overvoltage of the sending-end commutation bus exceeds 1.3pu. If it exceeds 1.3pu, it is determined that the wind turbine is at risk of disconnection from the grid.

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