A voltage control method for a flexible HVDC system based on cooperation between sending and receiving converter stations

Abstract

The present application relates to the field of power system protection and control, in particular to a kind of voltage control method of flexible HVDC system based on sending and receiving end converter station cooperation, comprising: real-time monitoring the AC bus voltage of receiving end converter station, when receiving end converter station AC bus voltage drops, start control;According to only adjusting receiving end converter station can ensure the longest time of DC voltage safety and expected fault fastest clearance time, power rebalancing time and maximum allowed sending end converter station adjustment time, the control start time of sending end converter station, active power adjustment coefficient and control exit time are calculated;By linearly reducing the active power of sending end converter station and cooperating with receiving end converter station, the control can be implemented;The present application can avoid DC overvoltage while minimizing the active power adjustment amount of sending end converter station, and weaken the influence of receiving end power grid fault as far as possible.

Description

Technical Field

[0001] This invention relates to the field of power system protection and control, and specifically to a voltage control method for a flexible DC transmission system based on the coordination of sending and receiving end converter stations. Background Technology

[0002] Flexible DC transmission systems based on voltage source converters have no requirements on the voltage strength of the connected systems, making them an ideal means of future AC grid interconnection and renewable energy transmission. The safety of the DC voltage is the cornerstone of ensuring flexible power control and normal operation of the flexible DC transmission system. However, when a fault occurs in the receiving-end grid, the active power of the receiving-end converter station decreases, leading to unbalanced power within the flexible DC transmission system and a gradual increase in DC voltage. DC overvoltage may induce damage to power semiconductor devices or even lockout of the flexible DC transmission system, threatening the stability of both the sending and receiving end grids. Therefore, ensuring the safety of the voltage in the flexible DC transmission system under receiving-end grid faults is an urgent problem to be solved.

[0003] During receiving-end grid faults, increasing the active power of the receiving-end converter station can reduce the overvoltage level of the flexible DC transmission system. However, during faults, the receiving-end converter station often needs to continuously provide reactive power to promote voltage recovery in the receiving-end grid and ensure grid safety and stability. In recent years, the idea of ​​changing the control reference value of the sending-end converter station to regulate the active power of the sending-end grid, thereby ensuring DC voltage safety, has attracted attention. Technicians have proposed introducing a DC voltage deviation term into the outer loop of the AC voltage control of the sending-end converter station, forcing renewable energy power plants to enter low-voltage ride-through by reducing the grid connection point voltage, thereby reducing the active power of the renewable energy power plants. Some technicians have also proposed a DC voltage-sending-end grid frequency mapping method, which autonomously transmits the rise in DC voltage after a receiving-end grid fault to the renewable energy power plants, causing the renewable energy power plants to reduce active power in response to frequency changes. However, the voltage reduction method or frequency increase method causes the renewable energy power plants to deviate from their normal operating state, which may lead to problems such as response overshoot or synchronization instability.

[0004] The dynamic rise in DC voltage is caused by the accumulation of unbalanced power. The safety margin of DC voltage depends not only on the difference in active power between the sending-end and receiving-end converter stations, but also on the duration of the power imbalance. Existing DC voltage control methods aim to achieve instantaneous balance of active power between the sending-end and receiving-end converter stations by initiating active power regulation or unloading at a fixed threshold. However, because the dynamic rise of DC voltage is not considered and is independent of the reactive power control at the receiving-end converter station, existing methods may lead to unnecessary active power regulation at the sending-end converter station due to neglecting the DC voltage safety margin under the influence of the power imbalance time. Furthermore, the inability to accurately assess the changes in the feasible range of active power at both the sending and receiving-end converter stations under different reactive power controls at the receiving-end converter station may result in increased active power regulation at the sending-end converter station, exacerbating the impact on the sending-end power grid.

[0005] In summary, how to characterize the impact of power imbalance time and reactive power control of the receiving-end converter station on DC voltage safety margin, and how to formulate necessary action criteria for active power regulation of the sending-end converter station; and how to coordinate the active power regulation of the sending-end converter station with the active and reactive power of the receiving-end converter station to minimize the scope and extent of the impact of grid faults at the receiving end have become urgent problems for those skilled in the art. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention proposes a voltage control method for a flexible DC transmission system based on the coordination of sending and receiving end converter stations. This method includes:

[0007] S1: Real-time monitoring of the AC bus voltage of the receiving-end converter station;

[0008] S2: Set a threshold voltage. Compare the AC bus voltage with the set threshold voltage. If the AC bus voltage is less than the threshold voltage, a fault occurs in the receiving end of the power grid, and step S3 is executed. Otherwise, return to step S1.

[0009] S3: Adjust the active power control reference value and reactive power control reference value of the receiving-end converter station, and determine the control start time of the sending-end converter station based on the active power control reference value and reactive power control reference value;

[0010] S4: Determine the difference between the current time and the control start time of the sending-end converter station. If the current time is greater than the control start time of the sending-end converter station, then collect the DC voltage of the flexible DC transmission system and execute step S5; otherwise, repeat step S4.

[0011] S5: Calculate the active power regulation coefficient of the sending-end converter station, and calculate the active power control reference value of the sending-end converter station based on the active power regulation coefficient.

[0012] S6: Calculate the control exit time of the sending-end converter station;

[0013] S7: Determine the difference between the current time and the control exit time of the sending-end converter station. If the current time is greater than the control exit time of the sending-end converter station, proceed to step S8; otherwise, repeat step S7.

[0014] S8: Set the active power control reference value of the sending-end converter station to the active power at the time of control exit of the sending-end converter station.

[0015] Preferably, determining the control start-up time of the sending-end converter station includes:

[0016]

[0017] in, To control the start-up time of the sending-end converter station, ts T is the time when the fault occurs. cut T is the expected fastest time to clear the fault. MSR This is the longest time that can guarantee the safety of DC voltage when only the receiving-end converter station is adjusted.

[0018] Furthermore, the formula for calculating the longest time that can guarantee DC voltage safety under conditions where only the receiving-end converter station is adjusted is as follows:

[0019]

[0020]

[0021]

[0022]

[0023]

[0024]

[0025]

[0026] Among them, P SEC0 This represents the initial active power of the power-sending converter station before the fault. For the reactive power of the receiving-end converter station, C eq U is the equivalent capacitance of the flexible DC transmission system. DC.N K is the rated DC voltage. U K is the maximum allowable voltage coefficient. I I is the maximum allowable AC current coefficient of the receiving-end converter station. REC.N R is the rated AC current of the receiving-end converter station. f X represents the equivalent fault transition resistance parameter. T To connect the equivalent reactance of the transformer, X S The equivalent reactance of the receiving-end power grid is given by v, p, q, A, B, and C, which are all intermediate parameters.

[0027] Preferably, the expression for calculating the active power regulation coefficient of the sending-end converter station is:

[0028]

[0029]

[0030]

[0031]

[0032] Among them, K ref C is the active power regulation coefficient of the sending-end converter station.eq K is the equivalent capacitance of the flexible DC transmission system. U U is the maximum allowable voltage coefficient. DC.SEC P is the voltage of the flexible DC transmission system at the start of active power regulation at the sending-end converter station. SEC0 This represents the initial active power of the power-sending converter station before the fault. The reactive power of the receiving-end converter station is represented by a, b, c, A, B, and C, which are all intermediate parameters.

[0033] Preferably, the active power control reference values ​​for the sending-end converter station include:

[0034] P REC.f =-K ref t+P SEC.0

[0035] Among them, P REC.f P represents the active power control reference value for the sending-end converter station, t represents the active power adjustment time for the sending-end converter station, and P represents the active power control reference value for the sending-end converter station. SEC0 This represents the initial active power of the converter station before the fault.

[0036] Preferably, the calculation of the control exit time of the sending-end converter station includes:

[0037]

[0038] in, T is used to control the exit time of the sending-end converter station. reb For power rebalancing time, This is the maximum allowable adjustment time for the sending-end converter station.

[0039] Furthermore, the expression for the power rebalancing time is:

[0040]

[0041] Among them, C eq K is the equivalent capacitance of the flexible DC transmission system. U U is the maximum allowable voltage coefficient. DC.SEC K is the voltage of the flexible DC transmission system at the start of active power regulation at the sending-end converter station. ref This is the active power regulation coefficient of the sending-end converter station.

[0042] Furthermore, calculating the maximum permissible settling time of the sending-end converter station includes: constructing an inequality based on the settling time of the sending-end converter station according to the active power regulation coefficient of the sending-end converter station; solving the inequality using Newton's method to obtain the maximum permissible settling time of the sending-end converter station; its expression is:

[0043]

[0044]

[0045]

[0046]

[0047]

[0048] Among them, f max T is the maximum permissible frequency of the sending-end power grid. G and R G These represent the turbine time constant and primary frequency regulation coefficient of the synchronous turbine group, respectively, where f0 is the rated frequency of the sending-end power grid, and F... G H is the equivalent boiler reheat coefficient for a synchronous turbine group. N and K N These are the equivalent virtual inertia coefficient and droop coefficient of the new energy power station, respectively. D represents the penetration rate of renewable energy power plants into the sending-end power grid. S H is the equivalent damping coefficient of the sending-end power grid. S Let x1, x2, η, κ, and λ be the equivalent inertia coefficients of the sending-end power grid, where x1, x2, η, κ, and λ are all intermediate parameters.

[0049] The beneficial effects of this invention are:

[0050] This invention proposes a necessary action criterion for active power regulation of the sending-end converter station by comparing the longest time that ensures DC voltage safety when only the receiving-end converter station is adjusted and the fastest expected fault clearance time. This can minimize active power regulation of the sending-end converter station while ensuring DC voltage safety. Furthermore, by comparing power rebalancing time and the maximum allowable adjustment time of the sending-end converter station, this invention proposes a linear active power regulation method for the sending-end converter station. This can minimize the amount of active power regulation at the sending-end converter station while ensuring the frequency safety of the sending-end power grid. The algorithm of this invention is simple in principle, easy to implement, and highly practical. Attached Figure Description

[0051] Figure 1 This is a flowchart of the voltage control method for a flexible DC transmission system based on the coordination of sending and receiving end converter stations according to the present invention;

[0052] Figure 2 This is a schematic diagram of the flexible DC transmission system under receiving-end grid faults according to the present invention;

[0053] Figure 3 This is a rendering of the invention. Detailed Implementation

[0054] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0055] This invention discloses a voltage control method for a flexible DC transmission system based on coordinated operation of sending and receiving end converter stations. Control is initiated when the AC bus voltage at the receiving end converter station drops. The method calculates the control initiation time, active power regulation coefficient, and control exit time of the sending end converter station based on the longest time that ensures DC voltage safety under the condition of only adjusting the receiving end converter station, the expected fastest fault clearance time, the power rebalancing time, and the maximum allowable adjustment time of the sending end converter station. Control is implemented by linearly reducing the active power of the sending end converter station and coordinating with the receiving end converter station. This invention can minimize the active power regulation of the sending end converter station while avoiding DC overvoltage, thus mitigating the impact of receiving end grid faults as much as possible.

[0056] A specific implementation method for a voltage control method of a flexible DC transmission system based on the coordination of sending and receiving end converter stations, such as... Figure 1 As shown, the method includes:

[0057] S1: Real-time monitoring of the AC bus voltage of the receiving-end converter station;

[0058] S2: Set a threshold voltage. Compare the AC bus voltage with the set threshold voltage. If the AC bus voltage is less than the threshold voltage, a fault occurs in the receiving end of the power grid, and step S3 is executed. Otherwise, return to step S1.

[0059] S3: Adjust the active power control reference value and reactive power control reference value of the receiving-end converter station, and determine the control start time of the sending-end converter station based on the active power control reference value and reactive power control reference value;

[0060] S4: Determine the difference between the current time and the control start time of the sending-end converter station. If the current time is greater than the control start time of the sending-end converter station, then collect the DC voltage of the flexible DC transmission system and execute step S5; otherwise, repeat step S4.

[0061] S5: Calculate the active power regulation coefficient of the sending-end converter station, and calculate the active power control reference value of the sending-end converter station based on the active power regulation coefficient.

[0062] S6: Calculate the control exit time of the sending-end converter station;

[0063] S7: Determine the difference between the current time and the control exit time of the sending-end converter station. If the current time is greater than the control exit time of the sending-end converter station, proceed to step S8; otherwise, repeat step S7.

[0064] S8: Set the active power control reference value of the sending-end converter station to the active power at the time of control exit of the sending-end converter station.

[0065] In this embodiment, the voltage threshold set in step S2 is 0.95 times the rated voltage. That is, when the AC bus voltage drops below 0.95 times the rated voltage, it is determined that a fault has occurred in the receiving-end power grid.

[0066] In this embodiment, in step S3, the reactive power control reference value of the receiving-end converter station is adjusted to the minimum reactive power required by the receiving-end power grid, and the active power control reference value of the receiving-end converter station is adjusted to the maximum active power under the maximum allowable AC current limit.

[0067] In this embodiment, the formula for determining the control start-up time of the sending-end converter station based on the active power control reference value and the reactive power control reference value in step S3 is as follows:

[0068]

[0069] in, To control the start-up time of the sending-end converter station; t s The time when the fault occurred; T cut The expected fastest fault clearing time is equal to the operating time of the main protection system of the receiving-end power grid; T MSR This is the longest time that can guarantee the safety of DC voltage when only the receiving-end converter station is adjusted.

[0070] The method to ensure DC voltage safety for the longest time by adjusting only the receiving-end converter station is as follows:

[0071]

[0072] The intermediate parameters v, p, and q are calculated as follows:

[0073]

[0074]

[0075]

[0076] In the formula: P SEC0 The initial active power of the converter station at the sending end before the fault occurred; C represents the reactive power of the receiving-end converter station, which is equal to the minimum reactive power required by the receiving-end power grid. eq U is the equivalent capacitance of a flexible DC transmission system. DC.NThe rated DC voltage; K U The maximum allowable voltage coefficient; the intermediate parameters A, B, and C are calculated as follows:

[0077]

[0078] Where: K I I is the maximum allowable AC current coefficient of the receiving-end converter station. REC.N R is the rated AC current of the receiving-end converter station. f X represents the equivalent fault transition resistance parameter. T To connect the equivalent reactance of the transformer, X S This is the equivalent reactance of the receiving-end power grid.

[0079] In this embodiment, the process of calculating the active power regulation coefficient of the sending-end converter station in step S5 includes:

[0080]

[0081] Where: K ref The active power regulation coefficient of the sending-end converter station; the calculation formulas for intermediate parameters a, b, and c are as follows:

[0082]

[0083] Among them, U DC.SEC The voltage of the flexible DC transmission system at the start of active power regulation at the sending-end converter station. The process of calculating the active power control reference value for the sending-end converter station includes:

[0084] P REC.f =-K ref t+P SEC.0

[0085] In the formula: P REC.f t represents the active power control reference value of the sending-end converter station, and t represents the active power adjustment time of the sending-end converter station.

[0086] In this embodiment, the specific process of calculating the control exit time of the sending-end converter station in step S6 includes: obtaining the control start time of the sending-end converter station; calculating the power rebalancing time and the maximum allowable adjustment time of the sending-end converter station; selecting the shortest time between the power rebalancing time and the maximum allowable adjustment time of the sending-end converter station; summing the control start time of the sending-end converter station with the shortest time; and using the sum as the control exit time of the sending-end converter station. The formula is:

[0087]

[0088] In the formula: T is used to control the exit time of the sending-end converter station. reb For power rebalancing time, This is the maximum allowable adjustment time for the sending-end converter station.

[0089] Specifically, the formula for calculating the power rebalancing time is:

[0090]

[0091] The maximum allowable settling time for the sending-end converter station is calculated using the following formula obtained by Newton's method, where T is... SEC Maximum value:

[0092]

[0093] In the formula: f max T is the maximum permissible frequency of the sending-end power grid. G and R G Here, x1 and x2 are the turbine time constant and primary frequency regulation coefficient of the synchronous generator group, respectively, and f0 is the rated frequency of the sending-end power grid; the intermediate parameters x1 and x2 are calculated according to the following formulas:

[0094]

[0095] In the formula, the intermediate parameters η, κ, and λ are respectively:

[0096]

[0097] In the formula: F G H is the equivalent boiler reheat coefficient for a synchronous turbine group. N and K N These are the equivalent virtual inertia coefficient and droop coefficient of the new energy power station, respectively. D represents the penetration rate of renewable energy power plants into the sending-end power grid. S H is the equivalent damping coefficient of the sending-end power grid. S Let be the equivalent inertia coefficient of the sending-end power grid.

[0098] To verify the effectiveness of the method of the present invention, as follows: Figure 2 The following analysis uses a schematic diagram of a flexible DC transmission system under a fault in the receiving-end grid as an example. The flexible DC transmission system adopts a pseudo-bipolar structure with a rated voltage of ±100kV, a rated transmission capacity of 200MVA, a maximum allowable voltage coefficient of 1.1, and a maximum allowable current coefficient of 1.2. During normal operation, the sending-end converter station uses constant active and reactive power control, with an active power control reference value of 200MW and a reactive power control reference value of 0Mvar; the receiving-end converter station uses constant DC voltage and reactive power control, with a DC voltage control reference value of 100kV and a reactive power control reference value of 0Mvar. The receiving-end grid uses longitudinal differential protection as the main protection, with an operating time of 150ms. The maximum allowable frequency deviation of the sending-end grid is 0.5Hz.

[0099] This invention uses a three-phase short-circuit fault with an equivalent fault transition resistance of 8.5Ω occurring at point f at 2.15s, and clearing the fault 150ms later as a scenario to verify the effectiveness of the voltage control method for flexible DC transmission systems based on the coordination of sending and receiving end converter stations disclosed in this invention. The minimum reactive power provided by the receiving end converter station under receiving end grid fault conditions is determined based on the degree of AC voltage drop, and the reactive current reference value is taken as 1.5(0.9-U). REC )I REC.N Comparison Group 1 achieved DC voltage control by step-reducing the active power of the sending-end converter station after the DC voltage rose to 1.06 pu. Comparison Group 2 implemented load shedding when the DC voltage rose to 1.1 pu. The DC voltage, active power of the sending-end converter station, active power of the receiving-end converter station, frequency deviation of the sending-end grid, reactive power of the receiving-end converter station, and AC bus voltage of the receiving-end converter station were recorded and analyzed.

[0100] In this embodiment, the active power regulation start-up time of the sending-end converter station is set to 2.15s; the active power regulation coefficient of the sending-end converter station is 4.63pu / s; and the active power regulation exit time of the sending-end converter station is set to 2.202s.

[0101] Figure 3 Figures (a) to (f) show the waveforms of DC voltage, active power at the sending-end converter station, active power at the receiving-end converter station, frequency deviation of the sending-end grid, reactive power at the receiving-end converter station, and AC bus voltage at the receiving-end converter station in a flexible DC transmission system under a three-phase short-circuit fault. The horizontal axis represents time, and the vertical axes represent DC voltage, active power at the sending-end converter station, active power at the receiving-end converter station, frequency deviation of the sending-end grid, reactive power at the receiving-end converter station, and AC bus voltage at the receiving-end converter station, respectively. Solid lines represent curves using the active and reactive power coordination-based voltage control method for flexible DC transmission systems disclosed in this invention; dashed lines represent curves in comparison group 1; and dotted lines represent curves in comparison group 2.

[0102] Depend on Figure 3As shown in (a) to (d), in comparison group 1, DC voltage stability was achieved at the expense of the sending-end grid frequency safety. The change in active power at the sending-end converter station before and after the fault reached 0.36 pu, and the maximum frequency deviation of the sending-end grid reached 0.55 Hz, exceeding the maximum allowable frequency deviation. This invention effectively coordinates DC voltage with the safety of the sending-end grid. The sending-end grid frequency continuously increases as the active power of the sending-end converter station decreases, reaching the maximum allowable frequency precisely within the maximum allowable adjustment time of the sending-end converter station, verifying the necessity of adaptive exit of active power adjustment at the sending-end converter station. This invention minimizes the active power of the sending-end converter station while avoiding sending-end grid frequency exceeding limits, delaying the DC voltage exceedance time (i.e., the first unloading and reconnection time) by 27 ms compared to comparison group 2. The relatively smaller unbalanced power also reduces the number of unloading and switching operations during the fault duration. The reactive power and AC bus voltage of the receiving-end converter station under this invention are basically consistent with the comparison group, indicating that this invention delays the DC voltage exceedance time while ensuring the safety of the sending-end grid and also maximizes the support for the receiving-end grid voltage. Figure 3 (e) and (f).

[0103] The above-described embodiments further illustrate the purpose, technical solution, and advantages of the present invention. It should be understood that the above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made to the present invention within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A voltage control method for a flexible DC transmission system based on the coordination of sending and receiving end converter stations, characterized in that, The method includes: S1: Real-time monitoring of the AC bus voltage of the receiving-end converter station; S2: Set a threshold voltage. Compare the AC bus voltage with the set threshold voltage. If the AC bus voltage is less than the threshold voltage, a fault occurs in the receiving end of the power grid, and step S3 is executed. Otherwise, return to step S1. S3: Adjust the active power control reference value and reactive power control reference value of the receiving-end converter station, and determine the control start time of the sending-end converter station based on the active power control reference value and reactive power control reference value; S4: Determine the difference between the current time and the control start time of the sending-end converter station. If the current time is greater than the control start time of the sending-end converter station, then collect the DC voltage of the flexible DC transmission system and execute step S5; otherwise, repeat step S4. S5: Calculate the active power regulation coefficient of the sending-end converter station, and calculate the active power control reference value of the sending-end converter station based on the active power regulation coefficient. S6: Calculate the control exit time of the sending-end converter station; S7: Determine the difference between the current time and the control exit time of the sending-end converter station. If the current time is greater than the control exit time of the sending-end converter station, proceed to step S8; otherwise, repeat step S7. S8: Set the active power control reference value of the sending-end converter station to the active power at the time of control exit of the sending-end converter station.

2. The voltage control method for a flexible DC transmission system based on the coordination of sending and receiving end converter stations according to claim 1, characterized in that, Determining the control start-up time of the sending-end converter station includes: in, To control the start-up time of the sending-end converter station, t s T is the time when the fault occurs. cut T is the expected fastest time to clear the fault. MSR This is the longest time that can guarantee the safety of DC voltage when only the receiving-end converter station is adjusted.

3. The voltage control method for a flexible DC transmission system based on the coordination of sending and receiving end converter stations according to claim 2, characterized in that, The formula for calculating the longest time that can guarantee DC voltage safety when only the receiving-end converter station is adjusted is: Among them, P SEC0 This represents the initial active power of the power-sending converter station before the fault. For the reactive power of the receiving-end converter station, C eq U is the equivalent capacitance of the flexible DC transmission system. DC.N K is the rated DC voltage. U K is the maximum allowable voltage coefficient. I I is the maximum allowable AC current coefficient of the receiving-end converter station. REC.N R is the rated AC current of the receiving-end converter station. f X represents the equivalent fault transition resistance parameter. T To connect the equivalent reactance of the transformer, X S The equivalent reactance of the receiving-end power grid is given by v, p, q, A, B, and C, which are all intermediate parameters.

4. The voltage control method for a flexible DC transmission system based on the coordination of sending and receiving end converter stations according to claim 1, characterized in that, The expression for calculating the active power regulation coefficient of the sending-end converter station is: Among them, K ref C is the active power regulation coefficient of the sending-end converter station. eq K is the equivalent capacitance of the flexible DC transmission system. U U is the maximum allowable voltage coefficient. DC.SEC P is the voltage of the flexible DC transmission system at the start of active power regulation at the sending-end converter station. SEC0 This represents the initial active power of the power-sending converter station before the fault. The reactive power of the receiving-end converter station is represented by a, b, c, A, B, and C, which are all intermediate parameters.

5. The voltage control method for a flexible DC transmission system based on the coordination of sending and receiving end converter stations according to claim 1, characterized in that, The calculation of active power control reference values ​​for the sending-end converter station includes: P REC.f =-K ref t+P SEC.0 Among them, P REC.f P represents the active power control reference value for the sending-end converter station, t represents the active power adjustment time for the sending-end converter station, and P represents the active power control reference value for the sending-end converter station. SEC0 This represents the initial active power of the converter station before the fault.

6. The voltage control method for a flexible DC transmission system based on the coordination of sending and receiving end converter stations according to claim 1, characterized in that, The calculation of the control exit time of the sending-end converter station includes: in, T is used to control the exit time of the sending-end converter station. reb For power rebalancing time, This is the maximum allowable adjustment time for the sending-end converter station.

7. A voltage control method for a flexible DC transmission system based on the coordination of sending and receiving end converter stations according to claim 6, characterized in that, The expression for the power rebalancing time is: Among them, C eq K is the equivalent capacitance of the flexible DC transmission system. U U is the maximum allowable voltage coefficient. DC.SEC K is the voltage of the flexible DC transmission system at the start of active power regulation at the sending-end converter station. ref This is the active power regulation coefficient of the sending-end converter station.

8. A voltage control method for a flexible DC transmission system based on the coordination of sending and receiving end converter stations according to claim 6, characterized in that, The calculation of the maximum allowable settling time of the sending-end converter station includes: constructing an inequality based on the active power regulation coefficient of the sending-end converter station; solving the inequality using Newton's method to obtain the maximum allowable settling time of the sending-end converter station; its expression is: i = 1 or 2 Among them, f max T is the maximum permissible frequency of the sending-end power grid. G and R G These represent the turbine time constant and primary frequency regulation coefficient of the synchronous turbine group, respectively, where f0 is the rated frequency of the sending-end power grid, and F... G H is the equivalent boiler reheat coefficient for a synchronous turbine group. N and K N These are the equivalent virtual inertia coefficient and droop coefficient of the new energy power station, respectively. D represents the penetration rate of renewable energy power plants into the sending-end power grid. S H is the equivalent damping coefficient of the sending-end power grid. S Let x1, x2, η, κ, and λ be the equivalent inertia coefficients of the sending-end power grid, where x1, x2, η, κ, and λ are all intermediate parameters.

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