Fault Ride-Through Control Method for Flexible DC Transmission System Based on Active and Reactive Power Coordination

Abstract

The present invention discloses a fault ride-through control method for a flexible DC transmission system based on active and reactive power coordination. After detecting the voltage dip of the AC bus at the receiving-end converter station, the fault ride-through is initiated. The amplitude and phase of the AC bus voltage at the receiving-end converter station, the active power and reactive power of the receiving-end converter station at the moment of fault are collected to locate the fault position and calculate the fault transition resistance; then, the maximum allowable power imbalance time of the flexible DC transmission system, the active current and reactive current of the power coordination control point at the receiving-end converter station are calculated; finally, the power outer-loop controller of the receiving-end converter station is blocked, and the fault ride-through can be implemented by changing the current control reference value. The present invention can realize the coordination of DC voltage and AC voltage during the fault ride-through through the coordination of active power and reactive power at the receiving-end converter station, and strongly support the fault grid voltage while minimizing the load shedding or power regulation actions of the sending-end power grid.

Description

Technical Field

[0001] The present invention relates to the field of power system protection and control, and particularly to a fault ride-through control method for a flexible DC transmission system based on active and reactive power coordination. Background Art

[0002] The flexible DC transmission system uses controllable turn-off devices, fundamentally avoiding the problems of reactive power compensation and commutation failure in conventional DC transmission, and is widely regarded as an effective way for regional power grid interconnection and large-scale new energy grid connection. However, the flexible DC transmission system is very sensitive to the change of the AC grid voltage. When a fault occurs in the receiving-end grid, the AC bus voltage drops, the power output of the receiving-end converter station is limited, and the surplus power causes the DC voltage to rise rapidly, which may damage the power semiconductor devices and threaten the safe and stable operation of the regional power grid. The safe operation of the flexible DC transmission system under the fault of the receiving-end grid has become the focus of attention in the industry.

[0003] Configuring a load dump on the DC side is the main method for the fault ride-through of the flexible DC transmission system. Technical personnel have proposed a new energy-consuming method by combining a centralized load dump topology and a decentralized load dump topology. However, the load dump causes a DC pole-to-ground short circuit or an AC phase-to-ground fault, and the input or withdrawal of the load dump will impact the AC-DC system. Using the voltage reduction method and the frequency increase method to quickly reduce the power fed from the sending-end grid into the flexible DC transmission system is also a common method for the fault ride-through of the flexible DC transmission system. Technical personnel trigger the low-voltage ride-through of new energy by reducing the voltage of the sending-end grid to reduce the power fed into the flexible DC transmission system; or introduce a frequency deviation term into the outer loop of the new energy control, and reduce the active power output of the new energy by increasing the grid connection point frequency. However, the voltage reduction method and the frequency increase method force the new energy at the sending end to enter special states such as low-voltage ride-through and frequency response, which may threaten the safety of the unit and even cause a series of problems such as synchronous instability and overcurrent.

[0004] The receiving-end converter station is often used to provide reactive power to support the grid voltage during the fault ride-through. Since the AC current of the receiving-end converter station cannot exceed a certain limit, the active power of the receiving-end converter station is restricted by the reactive power, that is, the DC voltage is closely related to the reactive power control strategy under the fault of the receiving-end grid. A large number of researchers have carried out research on the reactive power control strategy of the receiving-end converter station under the fault of the receiving-end grid. However, the existing research has ignored the impact of the reactive power control strategy of the receiving-end converter station on the DC voltage safety. In addition, the rise of the DC voltage is caused by the continuous accumulation of surplus power. The safety margin of the DC voltage depends not only on the difference between the DC power and the active power of the receiving-end converter station, but also on the power imbalance time. The existing load dump or sending-end power regulation does not consider the impact of the power imbalance time and is independent of the reactive power control strategy, which may not only cause unnecessary actions of the load dump or the sending-end grid power regulation, but also may not be able to effectively support the grid voltage.

[0005] In summary, to minimize the impact and scope of faults as much as possible, unloading or power regulation actions at the sending-end power grid should be avoided to the greatest extent on the premise of DC voltage safety. Therefore, how to characterize the coupling relationship between DC voltage safety margin, power imbalance time, and reactive power control, and strongly support the voltage of the faulty power grid while avoiding unloading or power regulation actions at the sending-end power grid as much as possible during fault ride-through has become an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0006] In view of the above deficiencies of the prior art, the present invention provides a fault ride-through control method for a flexible DC transmission system based on active and reactive power coordination, effectively solving the problem of characterizing the coupling relationship between DC voltage safety margin, power imbalance time, and reactive power control, and being able to strongly support the voltage of the faulty power grid while avoiding unloading or power regulation actions at the sending-end power grid as much as possible during fault ride-through, with high reliability and practicability.

[0007] To solve the above technical problems, the present invention adopts the following technical solutions, including the following steps:

[0008] S101: Real-time monitor the AC bus voltage of the receiving-end converter station. 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, and the fault ride-through control of the flexible DC transmission system is started, and S102 is executed; otherwise, return to S101.

[0009] S102: Collect the amplitude and phase of the AC bus voltage of the receiving-end converter station at the moment of the fault, and the active power and reactive power of the receiving-end converter station.

[0010] S103: Locate the fault position and calculate the fault transition resistance.

[0011] S104: Calculate the maximum allowable power imbalance time of the flexible DC transmission system.

[0012] S105: Calculate the active current and reactive current of the power coordination control point of the receiving-end converter station.

[0013] S106: Block the power outer-loop controller of the receiving-end converter station, set the active and reactive currents of the power coordination control point as the current control reference value of the receiving-end converter station, and implement the control.

[0014] Preferably, in the step S103, the fault position is determined as follows:

[0015]

[0016] In the formula, α is the ratio of the distance from the fault point to the connection transformer to the total length of the AC transmission line; U rec.0is the AC bus voltage amplitude of the receiving-end converter station at the moment of fault; X T is the equivalent reactance of the connecting transformer; X L is the equivalent reactance of the AC transmission line; The parameter γ is calculated by the following formula:

[0017] γ = E S.0 ηU rec.0 sin(θ S.0 - θ rec.0 ) - η(X T + X L + X S )P rec.0 - Q rec.0

[0018] In the formula, X S is the equivalent reactance of the receiving-end power grid; P rec.0 is the active power of the receiving-end converter station at the moment of fault; Q rec.0 is the reactive power of the receiving-end converter station at the moment of fault; θ rec.0 is the AC bus voltage phase of the receiving-end converter station at the moment of fault; E S.0 is the amplitude of the equivalent electromotive force of the receiving-end power grid, θ S.0 is the phase angle of the equivalent electromotive force of the receiving-end power grid, and the electromotive force of the receiving-end power grid remains unchanged before and after the fault; The parameter η is calculated by the following formula:

[0019]

[0020] Preferably, in the step S103, the fault transition resistance is calculated as follows:

[0021]

[0022] Preferably, in the step S104, the maximum allowable power imbalance time is calculated as follows:

[0023]

[0024] In the formula, is calculated by the following formula:

[0025]

[0026] In the formula, P dc is the DC power; C eq is the equivalent capacitance on the DC side; U dcN is the rated DC voltage; K U is the maximum allowable voltage coefficient; The parameters a′, b+, p1, p2 are respectively:

[0027]

[0028] In the formula, X′L = αX L + X T ; I recN is the rated AC current of the receiving-end converter station; K I is the maximum allowable current coefficient; the parameters A, B, C, and M are calculated according to the following formulas respectively:

[0029]

[0030] where X″ L ″ = (1 - α)X L + X S ; E S.0 is the amplitude of the equivalent potential of the receiving-end power grid.

[0031] is the maximum value of T in the following formula, which can be solved by the Newton method:

[0032]

[0033] where the parameters τ, are calculated according to the following formulas respectively:

[0034]

[0035] Preferably, in the step S105, the power coordination control point is the intersection of the DC voltage constraint curve and the AC current constraint curve when the power imbalance time is the maximum allowable power imbalance time. When the receiving-end converter station operates at the power coordination control point, it can provide strong voltage support for the receiving-end power grid while avoiding DC voltage over-limit to the greatest extent. The active current and reactive current at the power coordination control point are calculated according to the following formulas:

[0036]

[0037] where is the active current at the coordination control point; is the reactive current at the coordination control point; the parameters are respectively:

[0038]

[0039]

[0040]

[0041] Preferably, the DC voltage constraint curve is determined by the following function:

[0042]

[0043] where Irec.d is the d-axis component of the AC current of the receiving-end converter station; I rec.q is the q-axis component of the AC current of the receiving-end converter station.

[0044] Preferably, the AC current constraint curve is determined by the following function:

[0045]

[0046] In a fault ride-through control method for a flexible DC power transmission system based on active and reactive power coordination disclosed by the present invention, after detecting a voltage dip at the AC bus of the receiving-end converter station, fault ride-through is initiated. The amplitude and phase of the AC bus voltage of the receiving-end converter station, the active power and reactive power of the receiving-end converter station at the moment of the fault are collected, the fault location is located and the fault transition resistance is calculated; then the maximum allowable power imbalance time of the flexible DC power transmission system, the active current and reactive current of the power coordination control point of the receiving-end converter station are calculated; finally, the power outer-loop controller of the receiving-end converter station is blocked, and the current control reference value is changed to implement fault ride-through. The present invention can realize the coordination of DC voltage and AC voltage during fault ride-through through the coordination of active power and reactive power of the receiving-end converter station, and strongly support the fault grid voltage while avoiding load shedding or power regulation actions of the sending-end power grid as much as possible.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] The prior art avoids DC voltage over-limit during grid faults through load shedding and sending-end power regulation, but load shedding causes DC pole-to-ground short circuit or AC phase-to-ground fault, and both the input and output of load shedding will impact the AC-DC system. Moreover, the safety risks and cost-benefits of load shedding cannot be ignored, and its application limitations are constantly emerging. The present invention considers the dynamic change of the DC voltage safety margin with the power imbalance time, and at the same time suppresses the rise of DC voltage through the coordinated control of active and reactive power, and can avoid the input of load shedding or the regulation of sending-end power as much as possible.

[0049] The prior art supports the voltage of the receiving-end power grid through reactive power control, but ignores the mutual influence between active and reactive power control, which may not only lead to unnecessary actions of load shedding or sending-end power grid power regulation, but also be difficult to accurately describe the reactive power capacity of the converter station, resulting in limited support for grid voltage. The present invention considers the coupling of the DC voltage safety margin with the power imbalance time and the reactive power control strategy, and uses the active current and reactive current of the receiving-end converter station to describe the feasible range of avoiding DC overvoltage and AC overcurrent, and can provide strong reactive power support for the receiving-end power grid while avoiding DC overvoltage.

[0050] The prior art realizes the fault ride-through of the flexible DC transmission system by adding hardware or communication systems, etc. The present invention only needs to add a small amount of software algorithms to the inner-loop current controller of the converter station of the flexible DC transmission system to achieve it. The principle is simple, easy to implement, and has strong practicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to make the objectives, technical solutions, and advantages of the invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings, where:

[0052] Figure 1 is a flowchart of a fault ride-through control method for a flexible DC transmission system based on active and reactive power coordination;

[0053] Figure 2 is a schematic structural diagram of a fault ride-through control device for a flexible DC transmission system based on active and reactive power coordination;

[0054] Figure 3 is a schematic structural diagram of a flexible DC transmission system under a receiving-end grid fault;

[0055] Figure 4 is an effect diagram under an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0056] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0057] As Figure 1 shown, the fault ride-through control method for a flexible DC transmission system based on active and reactive power coordination disclosed by the present invention includes the following steps:

[0058] S101: Real-time monitor the AC bus voltage of the receiving-end converter station. 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 grid, and the fault ride-through control of the flexible DC transmission system is started, and S102 is executed; otherwise, return to S101.

[0059] S102: Collect the amplitude and phase of the AC bus voltage of the receiving-end converter station at the moment of the fault, as well as the active power and reactive power of the receiving-end converter station.

[0060] S103: Locate the fault position and calculate the fault transition resistance.

[0061] S104: Calculate the maximum allowable power imbalance time of the flexible DC transmission system.

[0062] S105: Calculate the active current and reactive current of the power coordination control point of the receiving-end converter station;

[0063] S106: Block the power outer-loop controller of the receiving-end converter station, set the active and reactive currents of the power coordination control point as the current control reference value of the receiving-end converter station, and implement the control;

[0064] During specific implementation, in step S103, the fault location is determined as follows:

[0065]

[0066] In the formula, α is the ratio of the distance from the fault point to the connection transformer to the total length of the AC transmission line; U rec.0 is the amplitude of the AC bus voltage of the receiving-end converter station at the moment of the fault; X T is the equivalent reactance of the connection transformer; X L is the equivalent reactance of the AC transmission line; the parameter γ is calculated according to the following formula:

[0067] γ = E S.0 ηU rec.0 sin(θ S.0 -θ rec.0 ) - η(X T + X L + X S )P rec.0 - Q rec.0

[0068] In the formula, X S is the equivalent reactance of the receiving-end power grid; P rec.0 is the active power of the receiving-end converter station at the moment of the fault; Q rec.0 is the reactive power of the receiving-end converter station at the moment of the fault; θ rec.0 is the phase of the AC bus voltage of the receiving-end converter station at the moment of the fault; E S.0 is the amplitude of the equivalent electromotive force of the receiving-end power grid, θ S.0 is the phase angle of the equivalent electromotive force of the receiving-end power grid, and the electromotive force of the receiving-end power grid remains unchanged before and after the fault; the parameter η is calculated according to the following formula:

[0069]

[0070] During specific implementation, in step S103, the fault transition resistance is calculated as follows:

[0071]

[0072] During specific implementation, in step S104, the maximum allowable power imbalance time is calculated as follows:

[0073]

[0074] In the formula, is calculated by the following formula:

[0075]

[0076] In the formula, P dc is the DC power; C eq is the equivalent capacitance on the DC side; U dcN is the rated DC voltage; K U is the maximum allowable voltage coefficient; the parameters a′, b′, p1, and p2 are respectively:

[0077]

[0078] In the formula, X′ L =αX L +X T ; I recN is the rated AC current of the receiving-end converter station; K I is the maximum allowable current coefficient; the parameters A, B, C, and M are calculated according to the following formulas respectively:

[0079]

[0080] In the formula, X″ L =(1-α)X L +X S ; E S.0 is the amplitude of the equivalent electromotive force of the receiving-end power grid.

[0081] is the maximum value of T in the following formula, which can be solved by the Newton method:

[0082]

[0083] In the formula, the parameters τ, are calculated according to the following formulas respectively:

[0084]

[0085] In specific implementation, in step S105, the power coordination control point is the intersection of the DC voltage constraint curve and the AC current constraint curve when the power imbalance time is the maximum allowable power imbalance time. When the receiving-end converter station operates at the power coordination control point, it can provide strong voltage support for the receiving-end power grid while avoiding DC voltage over-limit to the greatest extent. The active current and reactive current at the power coordination control point are calculated by the following formulas:

[0086]

[0087] In the formula, is the active current at the coordination control point; Reactive current for the coordinated control point; parameter They are respectively:

[0088]

[0089]

[0090]

[0091] During specific implementation, the DC voltage constraint curve is determined by the following function:

[0092]

[0093] In the formula, I rec.d is the d-axis component of the AC current of the receiving-end converter station; I rec.q is the q-axis component of the AC current of the receiving-end converter station.

[0094] During specific implementation, the AC current constraint curve is determined by the following function:

[0095]

[0096] In the embodiment of the present invention, this embodiment also provides a fault ride-through control device for a flexible DC transmission system based on active and reactive power coordination, Figure 2 is a schematic structural diagram of a fault ride-through control device for a flexible DC transmission system based on active and reactive power coordination according to an embodiment of the present invention, as Figure 2 shown. The device includes an inner-loop current controller of the converter station of the flexible DC transmission system. The inner-loop current controller includes a first calculation module, a second calculation module, a third calculation module, and a control module. The first calculation module locates the fault position and calculates the fault transition resistance by using the amplitude and phase of the AC bus voltage of the receiving-end converter station at the moment of the fault, the active power and reactive power of the receiving-end converter station. The first calculation module is connected to the second calculation module and transmits the fault position and the fault transition resistance to the second calculation module. The second calculation module calculates the maximum allowable power imbalance time of the flexible DC transmission system by using the fault position and the fault transition resistance. The second calculation module is connected to the third calculation module and transmits the maximum allowable power imbalance time to the third calculation module. The third calculation module calculates the active current and reactive current of the power coordination control point of the receiving-end converter station by using the maximum allowable power imbalance time. The third calculation module is connected to the control module and transmits the active current and reactive current of the coordination control point to the control module. The control module is used to block the power outer-loop controller of the receiving-end converter station, set the active and reactive currents of the power coordination control point as the current control reference values of the receiving-end converter station, and implement control.

[0097] To verify the effectiveness of the method of the present invention, asFigure 3 Taking the structural schematic diagram of the flexible DC transmission system under the fault of the receiving-end power grid as an example for analysis. The receiving-end power grid adopts a 2-region 4-machine system. The flexible DC transmission system adopts a pseudo-bipolar structure, with a rated voltage of ±100 kV, a rated transmission capacity of 200 MVA, a maximum allowable voltage coefficient of 1.1, and a maximum allowable current coefficient of 1.2. The sending-end converter station adopts constant active power and reactive power control, with an active power reference value of 200 MW and a reactive power reference value of 0 Mvar. During normal operation, the receiving-end converter station adopts constant DC voltage and reactive power control, with a DC voltage reference value of 100 kV and a reactive power reference value of 0 Mvar. The starting value of unloading is 1.1 p.u. The detailed parameters of the system are shown in Table 1.

[0098] Table 1 System parameters

[0099]

[0100]

[0101] In the present invention, a three-phase short-circuit fault occurs at point f on the line at 2.2 s, and the fault is cleared after 100 ms, to verify the effect of the fault ride-through control method for the flexible DC transmission system based on active and reactive power coordination disclosed in the present invention. The first comparison group refers to the new energy grid connection regulations, and the receiving-end converter station is controlled to provide reactive power to the power grid according to the degree of AC voltage drop, and the reference value of the reactive current control is taken as (U rec -0.9)I rec.N ; The second comparison group is constant reactive current control, and the reference value of the reactive current is taken as 0.5 p.u.. Record and analyze the DC voltage, the AC bus voltage of the receiving-end converter station, the active power and reactive power of the receiving-end converter station, and the active current and reactive current of the receiving-end converter station.

[0102] In this specific embodiment, in step S103, the ratio of the distance from the fault point to the connection transformer to the total length of the line is 0.14, and the fault transition resistance is 20.6 Ω.

[0103] In this specific embodiment, in step S104, the maximum allowable power imbalance time of the flexible DC transmission system is 109 ms.

[0104] In this specific embodiment, in step S105, the active current and reactive current of the power coordination control point are 1.17 p.u. and 0.25 p.u. respectively.

[0105] Figure 4 (a) to (f) are respectively the waveform diagrams of the DC voltage, the reactive current and active current of the receiving-end converter station, the active power and reactive power of the receiving-end converter station, and the AC bus voltage of the receiving-end converter station under the three-phase short-circuit fault. Figure 4The abscissa represents time, and the ordinates represent DC voltage, reactive current, active current, active power, reactive power, and the AC bus voltage of the converter station respectively. The solid line is the curve of the fault ride-through control method for the flexible DC transmission system based on the coordination of active and reactive power disclosed in the present invention, the dashed line is the curve of Comparative Group 1, and the dotted line is the curve of Comparative Group 2. As Figure 4 shown by the solid line in (a) to (e), under the present invention, the reactive current of the receiving-end converter station is 0.21 p.u., the active current is 1.17 p.u., the active power is 171.4 Mvar, the reactive power is 59.4 MW, and the DC voltage does not reach the action value of unloading. As Figure 4 shown by the dashed line and the dotted line in (a) to (e), although the reactive current and reactive power of the receiving-end converter station in Comparative Group 1 and Comparative Group 2 are greater than those of the present invention, the active power output decreases significantly at the same time, being 157.3 MW and 164.7 MW respectively. The surplus power in Comparative Group 1 and Comparative Group 2 is larger, making the rising slope of the DC voltage greater than that of the present invention. The DC voltage exceeds the limit at 78 ms after the fault occurs in Comparative Group 1, and the DC voltage reaches a maximum of 1.16 p.u. in Comparative Group 2. As Figure 4 shown in (f), during the fault, the AC bus voltages of the receiving-end converter stations in the present invention, Comparative Group 1, and Comparative Group 2 are basically the same, all rising to around 0.7 p.u. This shows that the present invention does not lose the voltage support for the power grid while avoiding the DC voltage from exceeding the limit.

[0106] In the description of the present invention, it should be understood that the orientation or positional relationships indicated by the terms "coaxial", "bottom", "one end", "top", "middle", "the other end", "upper", "one side", "top", "inner", "outer", "front", "center", "both ends", etc. are based on the orientation or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be construed as a limitation on the present invention.

[0107] In the present invention, unless otherwise clearly defined and limited, the terms "installed", "set", "connected", "fixed", "rotated", etc. should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the internal communication of two elements or the interaction relationship between two elements. Unless otherwise clearly defined, for those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0108] Although embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A fault ride-through control method for a flexible DC transmission system based on active and reactive power coordination, characterized in that It includes the following steps: S101: Monitor the AC bus voltage of the receiving-end converter station in real time. 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, and the fault ride-through control of the flexible DC transmission system is started, and S102 is executed; otherwise, return to S101; S102: Collect the amplitude and phase of the AC bus voltage of the receiving-end converter station at the moment of the fault, as well as the active power and reactive power of the receiving-end converter station; S103: Locate the fault position and calculate the fault transition resistance; in step S103, the fault transition resistance is calculated according to the following method: where α is the ratio of the distance from the fault point to the connecting transformer to the total length of the AC line; X S is the equivalent reactance of the receiving-end power grid; X T is the equivalent reactance of the connecting transformer; X L is the equivalent reactance of the AC transmission line; η represents the second parameter; the second parameter η is calculated by the following formula: Wherein, P rec.0 is the active power of the receiving-end converter station at the moment of fault; Q rec.0 is the reactive power of the receiving-end converter station at the moment of fault; E S.0 is the amplitude of the equivalent electromotive force of the receiving-end power grid; θ rec.0 is the phase of the AC bus voltage of the receiving-end converter station at the moment of fault; θ S.0 is the phase angle of the equivalent electromotive force of the receiving-end power grid, and the electromotive force of the receiving-end power grid remains unchanged before and after the fault; U rec.0 is the amplitude of the AC bus voltage of the receiving-end converter station at the moment of fault; S104: Calculate the maximum allowable power imbalance time of the flexible DC transmission system; S105: Calculate the active current and reactive current of the power coordination control point of the receiving-end converter station; S106: Block the power outer-loop controller of the receiving-end converter station, set the active and reactive currents of the power coordination control point as the current control reference value of the receiving-end converter station, and implement the control.

2. The fault ride-through control method for a flexible DC transmission system based on active and reactive power coordination according to claim 1, characterized in that In step S103, the fault position is determined according to the following method: In the formula, γ represents the first parameter.

3. The fault ride-through control method for a flexible DC power transmission system based on active and reactive power coordination according to claim 2, wherein The first parameter γ is calculated according to the following formula: γ = E S.0 ηU rec.0 sin(θ S.0 - θ rec.0 ) - η(X T + X L + X S )P rec.0 - Q rec.0 where E S.0 is the amplitude of the equivalent electromotive force of the receiving-end power grid.

4. The fault ride-through control method for a flexible DC transmission system based on active and reactive power coordination according to claim 1, wherein In step S104, the maximum allowable power imbalance time is calculated in the following manner: In the formula, represents the third parameter; represents the fourth parameter.

5. The fault ride-through control method for a flexible DC power transmission system based on active and reactive power coordination according to claim 4, wherein The third parameter is calculated by the following formula: Wherein, P dc is the DC power; C eq is the equivalent capacitance on the DC side; U dcN is the rated DC voltage; K U is the maximum allowable voltage coefficient; the parameters a′, b′, p1, and p2 are calculated according to the following formulas respectively: where X′ L = αX L + X T ; I recN is the rated AC current of the receiving-end converter station; K I is the maximum allowable current coefficient; the parameters A, B, C, and M are calculated according to the following formulas respectively: where X′ L ′ = (1 - α)X L + X S ; R f is the fault transition resistance; Fourth parameter Is the maximum value of T in the following formula solved by the Newton method: wherein, parameters τ, are calculated according to the following formulas respectively:

6. The fault ride-through control method for a flexible DC transmission system based on active and reactive power coordination according to claim 5, characterized in that In step S105, the power coordination control point is the intersection of the DC voltage constraint curve and the AC current constraint curve when the power imbalance time is the maximum allowable power imbalance time; the active current and reactive current of the power coordination control point are calculated according to the following formula: Wherein, is the active current of the coordinated control point; is the reactive current of the coordinated control point; The parameters are respectively:

7. The fault ride-through control method for a flexible DC transmission system based on active and reactive power coordination according to claim 6, characterized in that The DC voltage constraint curve is determined by the following function: where I rec.d is the d-axis component of the AC current of the receiving-end converter station; I rec.q is the q-axis component of the AC current of the receiving-end converter station.

8. The fault ride-through control method for a flexible DC power transmission system based on active and reactive power coordination according to claim 6, characterized in that The AC current constraint curve is determined by the following function: where I rec.d is the d-axis component of the AC current of the receiving-end converter station; I rec.q is the q-axis component of the AC current of the receiving-end converter station.

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