Transient overvoltage calculation method for network construction type wind power output system through LCC-HVDC

Abstract

The invention provides a method for calculating transient overvoltage of grid-forming type wind power through an LCC-HVDC delivery system, and the method comprises the steps: building a system power flow equation according to an equivalent model of the grid-forming type wind power and thermal power generating unit bundled direct current delivery system; calculating the active power and reactive power of the fan under the fault according to the high-penetration characteristic and the active frequency characteristic of the grid-forming type wind turbine generator; calculating DC transmission active power, DC consumption reactive power and rectification side filter output reactive power according to the power characteristics of the extra-high voltage DC power transmission system under the commutation failure fault; calculating the power flowing to the alternating current system under the fault according to a system power flow equation and the calculation result; and according to the power flowing to the alternating current system under the fault, calculating the transient overvoltage of the transmitting-end conversion bus. According to the method, the transient overvoltage of the sending end commutation bus of the network construction type wind-thermal bundling system in the commutation failure scene can be quickly calculated, and the network construction type fan off-network risk can be quickly judged.

Description

Technical Field

[0001] The 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 system and automation thereof. Background Art

[0002] Under the background of "dual carbon", the power system has gradually evolved into a high-proportion power electronics and AC / DC hybrid form. Large-scale wind and solar power are transmitted via UHV DC, and short-circuit faults on the inverter side are prone to cause DC commutation failure. On the one hand, under the commutation failure fault, a large amount of active power surplus is at the DC sending end, resulting in an increase in the frequency of the AC system at the sending end, which may cause high-frequency machine cutting action and endanger the safe and stable operation of the system. On the other hand, under the commutation failure fault, the DC reactive power demand is reduced. Due to the long filter removal time, the reactive power output by the filter is sent back to the AC system, which will cause the commutation bus voltage to rise. Due to the insufficient voltage resistance of power electronic devices, the risk of wind power disconnection in the vicinity of the sending end increases, further causing chain failures. Therefore, it is urgent to study the quantitative calculation method of the transient overvoltage of the commutation bus on the rectifier side under the commutation failure scenario.

[0003] At present, the calculation of transient overvoltage of the AC busbar on the rectifier side under the UHV DC commutation failure fault often does not consider the impact of wind turbine access, which is obviously not suitable for the calculation of transient overvoltage of wind-fire bundled DC transmission system. A few studies have considered the impact of wind turbine access, but the wind turbines in these studies adopt a grid-following control strategy, and the calculation of transient overvoltage of grid-type wind turbines through the UHV DC transmission system is not considered. With the large-scale access of grid-type wind turbines, the transient overvoltage mechanism of the DC sending-end system has changed. Therefore, it is urgent to study the calculation method of transient overvoltage of grid-type wind power through the DC transmission system to quickly determine the risk of grid-type wind turbines at the sending end being disconnected 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 technology.

[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, comprising:

[0007] According to the equivalent model of the grid-connected wind power and thermal power units bundled through DC transmission system, the system power flow equation is established;

[0008] According to the high-throughput characteristics and active frequency characteristics of the grid-type wind turbine generator set, the active and reactive power of the wind turbine under fault conditions are calculated; the high-throughput characteristics and active frequency characteristics of the grid-type wind turbine generator set include the reactive current coefficient during the high-throughput period, the virtual inertia coefficient and the virtual damping coefficient of the grid-type wind turbine generator set;

[0009] According to the power characteristics of the UHV DC transmission system under commutation failure fault, the DC transmission active power, DC consumption reactive power and rectifier side filter output reactive power are calculated;

[0010] The power flowing to the AC system under fault is calculated based on the system power flow equation, the active and reactive power of the fan under 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 according to the power flowing to the AC system under the fault.

[0012] Furthermore, according to the equivalent system model of wind-fire bundling with direct current transmission, a system power flow equation is established, and 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 is the active power output 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, Q c is the reactive power output by the DC sending filter, P d It transmits active power to the DC sending end, Q d Consumes reactive power at the DC sending end.

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

[0016] The reactive power of the fan under fault 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 fan under fault, K2 is the reactive current coefficient of the fan 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 is calculated according to the following formula:

[0020]

[0021] Among them, P GFM is the active power of the fan under fault, 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 according to the power characteristics of the UHV DC transmission system in the commutation failure scenario specifically includes:

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

[0028]

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

[0030] The reactive power consumed by the DC 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 the DC commutation failure;

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

[0034]

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

[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 according to 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 It is the reactive power flowing to the AC system in the commutation failure scenario.

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

[0041] The calculation formula for transient overvoltage of the commutation busbar at the sending end 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 by using the forward-backward substitution method specifically includes:

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

[0047] Will U Lf0 Substitute it into the calculation formula of the power flowing to the AC system in 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 Find the equivalent power supply potential U si and with 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 requirement.

[0051] Further, 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 wind turbine off-grid risk result is determined according to the transient overvoltage of the commutation bus at the sending end, specifically including:

[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 being disconnected 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 can 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 of a grid-connected wind power transmission system via LCC-HVDC according to an embodiment of the present invention;

[0057] Figure 2 It 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 This is the transient overvoltage simulation result. 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 solution of the present invention, and cannot be used to limit the protection scope 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 according to the equivalent model of the grid-connected wind power and thermal power units bundled through the DC transmission system;

[0063] Step 2: Calculate the active and reactive power of the wind turbine under fault conditions according to the high-throughput characteristics and active frequency characteristics of the grid-type wind turbine generator set, wherein the high-throughput characteristics and active frequency characteristics of the grid-type wind turbine generator set include the reactive current coefficient during the high-throughput period, the virtual inertia coefficient and the virtual damping coefficient of the grid-type wind turbine generator set;

[0064] Step 3: Calculate the active power transmitted by direct current, the reactive power consumed by direct current, and the reactive power output by the filter on the rectifier side according to the power characteristics of the ultra-high voltage direct current transmission system under the commutation failure fault;

[0065] Step 4, calculating the power flowing to the AC system under the fault condition according to the system power flow equation obtained in step 1, the active and reactive power of the fan under the fault condition obtained in step 2, the DC transmission active power, DC consumption reactive power and the rectifier-side filter output reactive power obtained in step 3;

[0066] Step 5, calculating 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 six, determining the wind turbine off-grid risk result according to the transient overvoltage of the sending-end commutation bus;

[0068] Embodiment 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 is the active power output 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-connected wind turbines.

[0072] Q c is the reactive power output by the DC sending filter, P d It transmits active power to the DC sending end, Q d 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 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 It is the equivalent reactance between the equivalent AC system and the commutation bus on the rectifier side.

[0076] Figure 2 PMSG is a grid-type direct-drive fan, R 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 an inductive reactance characteristic, 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 flowing to the AC system, it is important to consider s , Q s changes.

[0080] Analysis of the impact of wind turbine grid control on transient overvoltage of commutation busbar on the rectifier side: Since traditional grid-connected direct-drive wind turbines often use fixed power control, they cannot respond to changes in grid voltage and frequency and actively change output power to support the grid. The grid-connected 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 fan under fault, 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-connected point change, the grid-type control strategy can adjust the wind turbine output active and reactive power to suppress frequency and voltage fluctuations of the power grid.

[0088] In the case of a commutation failure, the active power transmitted by the DC will be greatly reduced, the frequency of the AC system will increase, and the active power output of the fan will be reduced under the action of the active frequency link of the direct-drive fan. In addition, the reactive power consumed by the DC will be greatly reduced, and the reactive power surplus of the filter will cause the AC bus voltage at the sending end to rise, and the fan will enter a high voltage ride-through state, absorbing part 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 adopted by the wind turbine 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] Firstly, the current limit value of the UHV DC low-voltage current limiting link is obtained, and the DC transmission active power 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 the 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 formula:

[0097]

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

[0099] The reactive power consumed by the DC 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 the DC commutation failure;

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

[0103]

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

[0105] The power flowing to the AC system under fault conditions 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 It is the reactive power flowing to 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 of the DC bus overvoltage at the sending end under commutation failure fault is obtained as follows:

[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 between the sending-end AC system and the commutation busbar is X 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 the transient period, it can be seen that the increase of the virtual damping coefficient, the virtual inertia coefficient and the reactive voltage coefficient during the high-speed run-through period are all 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 built on the PSCAD / EMTDC simulation platform. The system parameters are shown in the table above. A three-phase ground short circuit fault was set on the inverter side commutation bus. The transition resistance was 1Ω. The fault occurred at 3.3s and lasted for 0.1s. During the fault, the UHV DC transmission system failed to commutate. The commutation bus on the rectifier side showed a low-to-high characteristic, such as Figure 4 The influence of different high-throughput reactive current coefficients on the overvoltage peak is shown in Table 2.

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

[0120]

[0121] It can be seen from the data in Table 2 that the transient overvoltage calculation value and the simulation value are highly consistent, and the transient overvoltage calculation method of the grid-type wind power through DC transmission system proposed in the present invention has high accuracy. As the high-throughput reactive current coefficient of the grid-type wind turbine increases, the transient overvoltage shows a clear downward trend.

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

Claims

1. A method for calculating transient overvoltage of a grid-connected wind power transmission system via LCC-HVDC, characterized in that: include: According to the equivalent model of the grid-connected wind power and thermal power units bundled through DC transmission system, the system power flow equation is established; According to the high-throughput characteristics and active frequency characteristics of the grid-type wind turbine generator set, the active and reactive power of the wind turbine under fault conditions are calculated; the high-throughput characteristics and active frequency characteristics of the grid-type wind turbine generator set include the reactive current coefficient during the high-throughput period, the virtual inertia coefficient and the virtual damping coefficient of the grid-type wind turbine generator set; According to the power characteristics of the UHV DC transmission system under commutation failure fault, the DC transmission active power, DC consumption reactive power and rectifier side filter output reactive power are calculated; The power flowing to the AC system under fault is calculated based on the system power flow equation, the active and reactive power of the fan under 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 according to 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 bundling with direct current transmission in the grid-type, a system power flow equation is established, and the established system power flow equation is: P s +jQ s +P GFM +jQ GFM +jQ c =P d +jQ d Among them, P s is the active power output 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, Q c is the reactive power output by the DC sending filter, P d It transmits active power to the DC sending end, Q d 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 according to the high penetration characteristics and active frequency characteristics of the grid-connected wind turbine generator set specifically includes: The reactive power of the fan under fault is calculated according to the following formula: Q GFM =K2U T I N (U T -1.1) Among them, Q GFM is the reactive power of the fan under fault, K2 is the reactive current coefficient of the fan 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 is calculated according to the following formula: Among them, P GFM is the active power of the fan under fault, 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.

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 according to the power characteristics of the UHV DC transmission system in the commutation failure scenario specifically includes: Obtain the current limit value of the UHV DC low-voltage current limiting link, and calculate the DC transmission active power and reactive power consumption in the commutation failure scenario based on the limit value; The formula for calculating the active power transmitted by DC during a fault is: P df =I dlow IN dN Among them, P df is the active power transmitted during the 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 formula: Among them, U d0 is the DC no-load voltage on the rectifier side, N is the number of 6-pulse converter bridges, T is the ratio of the converter transformer, U LN is the rated value of the AC bus voltage; The reactive power consumed by the DC 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 the DC commutation failure; The calculation formula of the reactive power output by the filter on the rectifier side is: Among them, Q cf is the reactive power output by the filter on the rectifier side when the DC commutation fails, U Lf For rectifier side 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 according to the system power flow equation, and the calculation formula is: Among them, P sf is the active power flowing to the AC system in the commutation failure scenario, Q sf It is the reactive power flowing to 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 bus at the sending end according to the power flowing to the AC system under fault conditions specifically includes: The calculation formula for transient overvoltage of the commutation busbar at the sending end 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 the transient overvoltage of the sending-end commutation bus according to the power flow equation under fault conditions according to claim 6, characterized in that: The iterative solution of the transient overvoltage of the sending-end commutation busbar by the forward-backward substitution method specifically includes: Given the initial value of transient bus overvoltage U Lf0 ; Will U Lf0 Substitute it into the calculation formula of the power flowing to the AC system in 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 Find the equivalent power supply potential U si and with 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 requirement.

8. 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: 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 1 or 8, characterized in that: The determination of the wind turbine off-grid risk result according to the transient overvoltage of the commutation bus at the sending end specifically includes: 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 being disconnected from the grid.

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