Method, device and equipment for controlling receiving-end voltage of flexible direct current transmission system

Abstract

The present invention relates to the field of power system protection and control, and discloses a method, device and equipment for controlling the voltage at the receiving end of a flexible direct current transmission system. The receiving-end converter station of the flexible direct current transmission system collects the AC bus voltage, active and reactive currents before and after the fault, and calculates the equivalent complex impedance and potential vector after the receiving-end power grid fault; calculates and compares the maximum AC current phase angle of the receiving-end converter station that avoids DC transient overvoltage of the flexible direct current transmission system and the AC current phase angle of the receiving-end converter station required for maximizing the voltage at the receiving-end grid connection point of the flexible direct current transmission system, and calculates the maximum feasible value of the voltage at the receiving-end grid connection point of the flexible direct current transmission system under the premise of avoiding DC transient overvoltage; calculates and adjusts the active and reactive current reference values ​​of the receiving-end converter station of the flexible direct current transmission system, and controls the voltage at the receiving-end grid connection point of the flexible direct current transmission system to the maximum feasible value under the grid fault. The present invention quantifies and realizes the maximum voltage increase at the receiving-end grid connection point under the DC voltage safety condition of the flexible direct current transmission system under the grid fault by coordinating the active and reactive currents of the receiving-end converter station of the flexible direct current transmission system.

Description

Technical Field

[0001] The present invention relates to the field of power system protection and control, and in particular to a method, device and equipment for controlling the receiving-end voltage of a flexible direct current transmission system. Background Art

[0002] Flexible HVDC transmission systems enable decoupled control of active and reactive currents without the risk of commutation failure, and have been widely used for onshore and offshore wind power transmission. When a three-phase short-circuit fault occurs in the receiving grid, the AC busbar voltage at the receiving converter station of the Flexible HVDC system drops. To prevent overcurrent shutdown of power devices, the receiving converter station is equipped with an AC current limiter. Consequently, during a grid fault, the active power at the receiving converter station often fails to recover to its initial value, resulting in an active power surplus in the Flexible HVDC transmission system and an increase in DC voltage. This can cause the Flexible HVDC transmission system to shut down due to DC voltage exceeding the limit. During a receiving grid fault, the Flexible HVDC transmission system must provide AC voltage support. However, under the maximum allowable AC current limit, the supporting effect of the receiving converter station on the receiving grid voltage is far weaker than that of a synchronous motor. This can further reduce the active power at the receiving converter station and exacerbate DC voltage fluctuations. Therefore, ensuring DC voltage safety and effectively supporting the receiving grid voltage during grid faults has become a hot topic in Flexible HVDC transmission control.

[0003] The reference value for reactive current at the receiving-end converter station during a grid fault is mostly determined based on grid-connection specifications for renewable energy sources such as wind farms. However, simply increasing the reactive current at the receiving-end converter station linearly based on the degree of AC voltage drop fails to fully utilize the AC voltage support capability of HVDC Flexible transmission. Existing researchers have proposed a method for maximizing reactive power control at the receiving-end converter station based on the effective short-circuit ratio, effectively improving the receiving-end grid voltage. Other researchers have achieved maximum voltage support at the receiving-end grid connection point by matching the phase of the receiving-end converter station's AC current with the grid's short-circuit current, but this requires communication. According to instantaneous power theory, raising the AC voltage helps increase the active power at the receiving-end converter station during a grid fault. However, the reactive current at the receiving-end converter station required for AC voltage support also squeezes the feasible range of the receiving-end converter station's active current, potentially exacerbating the active power surplus. While AC and DC voltage control in HVDC Flexible transmission during grid faults interact, research on this topic is limited. Technicians have quantitatively analyzed the function of the maximum allowable power imbalance time and the active and reactive currents required for AC voltage support at the receiving-end converter station, using energy dissipation devices as a means of DC voltage control. However, the dynamic characteristics of DC voltage under constant DC voltage control at the sending-end converter station differ significantly from those of energy dissipation devices. Currently, no research has examined the effect of AC voltage support on the maximum DC voltage deviation under constant DC voltage control. This makes it difficult to quantify the maximum AC voltage support capability of HVDC Flexible Transmission for the receiving-end grid while ensuring DC voltage safety. Furthermore, this could result in transient DC overvoltages or poor AC voltage rise during faults in the receiving-end grid. Summary of the Invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides a method, device and apparatus for controlling the receiving-end voltage of a flexible direct current transmission system. The method comprises the following steps:

[0005] In the first aspect of the present application, the present invention proposes a method for controlling the voltage at the receiving end of a flexible direct current transmission system. The receiving end converter station collects the AC bus voltage, active and reactive currents before and after the fault, and calculates the equivalent complex impedance and potential vector under the receiving end grid fault; calculates and compares the maximum AC current phase angle of the receiving end converter station to avoid DC transient overvoltage, and the AC current phase angle of the receiving end converter station required to maximize the voltage at the receiving end grid connection point of the flexible direct current transmission system, and calculates the maximum feasible value of the voltage at the receiving end grid connection point of the flexible direct current transmission system under the premise of avoiding DC transient overvoltage; adjusts the active and reactive current reference values ​​of the receiving end converter station, and raises the voltage at the receiving end grid connection point of the flexible direct current transmission system to the maximum feasible value under the grid fault. The present invention quantifies and realizes the maximum increase in the receiving end grid voltage based on the DC voltage safety of the flexible direct current transmission under the grid fault through the coordination of the active and reactive currents of the receiving end converter station. Specifically, it includes the following steps:

[0006] S101: Collects the AC bus voltage, active and reactive currents of the receiving-end converter station of the flexible DC transmission before and after the grid fault;

[0007] S102: Calculate the equivalent complex impedance and potential vector under the receiving-end power grid fault;

[0008] S103: Calculate the maximum AC current phase angle at the receiving-end converter station to avoid DC transient overvoltage and the AC current phase angle at the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system;

[0009] S104: comparing the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage with the AC current phase angle of the receiving-end converter station required for maximizing the voltage at the receiving-end grid-connection point of the HVDC Flexible system. If the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage is less than the AC current phase angle of the receiving-end converter station required for maximizing the voltage at the receiving-end grid-connection point of the HVDC Flexible system, executing S105; otherwise, executing S106.

[0010] S105: Calculating the maximum feasible voltage at the receiving-end grid connection point of the HVDC Flexible system under a power grid fault based on the maximum AC current phase angle at the receiving-end converter station to avoid DC transient overvoltage. Furthermore, calculating the active and reactive currents at the receiving-end converter station that will enable the voltage at the receiving-end grid connection point of the HVDC Flexible system to reach the maximum feasible value. These values ​​are used as reference values ​​for controlling the active and reactive currents at the receiving-end converter station, and control is implemented.

[0011] S106: Based on the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid-connected point of the Flexible DC transmission system, the maximum feasible voltage at the receiving-end grid-connected point of the Flexible DC transmission system under a power grid fault is calculated, and then the active and reactive currents of the receiving-end converter station that make the voltage at the receiving-end grid-connected point of the Flexible DC transmission system reach the maximum feasible value are calculated, and the active and reactive currents of the receiving-end converter station are used as reference values ​​for active and reactive current control of the receiving-end converter station to implement control.

[0012] In a second aspect of the present application, the present invention provides a receiving-end voltage control device for a flexible direct current transmission system, comprising:

[0013] An acquisition module is used to collect the AC bus voltage, active and reactive currents of the receiving-end converter station of the flexible DC transmission before and after the grid fault;

[0014] The first calculation module is used to calculate the equivalent complex impedance and potential vector under the receiving end power grid fault;

[0015] The second calculation module is used to calculate the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage and the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system;

[0016] a comparison module, configured to compare a maximum AC current phase angle at the receiving-end converter station for avoiding DC transient overvoltage with the AC current phase angle at the receiving-end converter station required for maximizing the voltage at the receiving-end grid-connection point of the flexible DC transmission system; if the maximum AC current phase angle at the receiving-end converter station for avoiding DC transient overvoltage is less than the AC current phase angle at the receiving-end converter station required for maximizing the voltage at the receiving-end grid-connection point of the flexible DC transmission system, then calling the third calculation module; otherwise, calling the fourth calculation module;

[0017] a third calculation module, configured to calculate the maximum feasible voltage at the receiving-end grid connection point of the flexible HVDC transmission system under a power grid fault based on the maximum AC current phase angle at the receiving-end converter station that avoids DC transient overvoltage, further calculate the active and reactive currents at the receiving-end converter station that enable the voltage at the receiving-end grid connection point of the flexible HVDC transmission system to reach the maximum feasible value, and invoke the first control module;

[0018] a fourth calculation module, configured to calculate the maximum feasible voltage at the receiving-end grid-connection point of the Flexible HVDC transmission system under a power grid fault based on the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid-connection point of the Flexible HVDC transmission system, further calculate the active and reactive currents of the receiving-end converter station required to maximize the voltage at the receiving-end grid-connection point of the Flexible HVDC transmission system, and invoke the second control module;

[0019] a first control module, configured to implement control using the active and reactive currents of the receiving-end converter station calculated by the third calculation module so as to enable the voltage at the receiving-end grid connection point of the flexible HVDC transmission system to reach a maximum feasible value as reference values ​​for controlling the active and reactive currents of the receiving-end converter station;

[0020] The second control module is used to implement control by using the active and reactive currents of the receiving-end converter station calculated by the fourth calculation module so that the voltage at the receiving-end grid connection point of the flexible DC transmission system reaches the maximum feasible value as the active and reactive current control reference values ​​of the receiving-end converter station.

[0021] In the third aspect of the present application, the present invention proposes a receiving-end voltage control device for a flexible direct current transmission system, comprising a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor. When the processor executes the computer program, it implements the steps of the receiving-end voltage control method for a flexible direct current transmission system as described in the first aspect of the present application.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. Existing technologies assess the impact of flexible DC transmission systems on normal power grids using short-circuit ratios and improved indices. However, there is no quantitative method for characterizing the AC voltage rise effect under grid faults, taking into account the influence of flexible DC transmission control. This makes it difficult to guide the coordination of fixed values ​​and timing of grid safety and stability control devices, potentially leading to malfunction of reactive power compensation devices and even AC voltage instability. This invention calculates and compares the maximum AC current phase angle at the receiving converter station to avoid DC transient overvoltages and the AC current phase angle at the receiving converter station required to maximize the voltage at the receiving point of the flexible DC transmission system. It also calculates the maximum feasible value of the voltage at the receiving point of the flexible DC transmission system while avoiding DC transient overvoltages, quantifying the maximum AC voltage support capability of the flexible DC transmission system for a faulty grid under the assumption of DC voltage.

[0024] 2. Most of the existing technologies are aimed at the sending-end converter station with AC voltage or active power as the control object. There is no research on the influence of AC voltage support on the maximum DC voltage deviation under the fixed DC voltage control of the sending-end converter station, which may cause the flexible DC transmission to be locked due to DC transient overvoltage, or provide poor support for the receiving-end grid voltage. When the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage is less than the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system, the present invention makes the active and reactive current reference values ​​of the receiving-end converter station equal to the active and reactive currents corresponding to the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage; otherwise, the active and reactive current reference values ​​of the receiving-end converter station are equal to the active and reactive currents corresponding to the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system, thereby achieving maximum support for the receiving-end grid voltage under the premise of ensuring DC voltage safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to make the purpose, technical solutions and advantages of the invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0026] Figure 1 This is a flow chart of a method for controlling voltage at a receiving end of a flexible DC transmission system in an embodiment of the present application;

[0027] Figure 2 This is a structural diagram of a receiving-end voltage control device for a flexible DC transmission system in an embodiment of the present application;

[0028] Figure 3 This is a schematic diagram of the structure of the receiving-end voltage control device of the flexible DC transmission system in an embodiment of the present application;

[0029] Figure 4 This is a structural diagram of flexible DC transmission under grid fault;

[0030] Figure 5 These are effect diagrams under the embodiments of the present invention, wherein Figure (a) is a diagram showing the relationship between time and DC voltage, Figure (b) is a diagram showing the relationship between time and the voltage at the receiving-end grid connection point of the flexible DC transmission system, Figure (c) is a diagram showing the relationship between time and the active power of the receiving-end converter station, Figure (d) is a diagram showing the relationship between time and the reactive power of the receiving-end converter station, and Figure (e) is a diagram showing the relationship between time and the AC current phase angle of the receiving-end converter station. DETAILED DESCRIPTION

[0031] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0032] Based on the analysis of background technology, how to determine the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage, the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid-connected point of the flexible DC transmission system, calculate the maximum feasible value of the receiving-end grid-connected point of the flexible DC transmission system under the premise of avoiding DC transient overvoltage, and quantify the maximum AC voltage support capacity of the receiving-end power grid under the premise of DC voltage safety of flexible DC transmission; how to coordinate the active and reactive currents of the receiving-end converter station when using fixed DC voltage control at the sending-end converter station, and support the receiving-end power grid voltage as much as possible under the premise of ensuring the DC voltage safety of flexible DC transmission, has become a problem that technical personnel in this field urgently need to solve.

[0033] Based on the above problems, this application calculates and compares the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage of the flexible DC transmission system and the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid-connected point of the flexible DC transmission system under the premise of avoiding DC transient overvoltage, calculates the maximum feasible value of the voltage at the receiving-end grid-connected point of the flexible DC transmission system under the premise of avoiding DC transient overvoltage, calculates and adjusts the active and reactive current reference values ​​of the receiving-end converter station, and controls the voltage at the receiving-end grid-connected point of the flexible DC transmission system to the maximum feasible value under the grid fault.

[0034] like Figure 1 As shown, the present invention proposes a method for controlling the receiving-end voltage of a flexible DC transmission system, comprising the following steps:

[0035] S101: Collects the AC bus voltage, active and reactive currents of the receiving-end converter station of the flexible DC transmission before and after the grid fault;

[0036] S102: Calculate the equivalent complex impedance and potential vector under the receiving-end power grid fault;

[0037] S103: Calculate the maximum AC current phase angle at the receiving-end converter station to avoid DC transient overvoltage and the AC current phase angle at the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system;

[0038] S104: comparing the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage with the AC current phase angle of the receiving-end converter station required for maximizing the voltage at the receiving-end grid-connection point of the HVDC Flexible system. If the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage is less than the AC current phase angle of the receiving-end converter station required for maximizing the voltage at the receiving-end grid-connection point of the HVDC Flexible system, executing S105; otherwise, executing S106.

[0039] S105: Calculating the maximum feasible voltage at the receiving-end grid connection point of the HVDC Flexible system under a power grid fault based on the maximum AC current phase angle at the receiving-end converter station to avoid DC transient overvoltage. Furthermore, calculating the active and reactive currents at the receiving-end converter station that will enable the voltage at the receiving-end grid connection point of the HVDC Flexible system to reach the maximum feasible value. These values ​​are used as reference values ​​for controlling the active and reactive currents at the receiving-end converter station, and control is implemented.

[0040] S106: Based on the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid-connected point of the Flexible DC transmission system, the maximum feasible voltage at the receiving-end grid-connected point of the Flexible DC transmission system under a power grid fault is calculated, and then the active and reactive currents of the receiving-end converter station that make the voltage at the receiving-end grid-connected point of the Flexible DC transmission system reach the maximum feasible value are calculated, and the active and reactive currents of the receiving-end converter station are used as reference values ​​for active and reactive current control of the receiving-end converter station to implement control.

[0041] In a flexible HVDC transmission system, when a fault occurs in the receiving grid, it is crucial to collect the AC bus voltage, active and reactive currents at the receiving converter station. This helps to quickly locate the fault, assess its impact, and take appropriate control measures.

[0042] Based on this, in some embodiments of the present application, in step S101, it is necessary to collect the AC bus voltage, active and reactive currents of the receiving converter station of the flexible direct current transmission system before and after the grid fault. The AC bus voltage is one of the important parameters of the receiving converter station in the flexible direct current transmission system. When the receiving grid fails, the AC bus voltage may change, such as dropping or fluctuating. The purpose of collecting the AC bus voltage is to monitor the status of the receiving grid in real time, promptly detect abnormal AC bus voltage, and thus take appropriate control measures. The collection method generally includes using a voltage transformer (PT) to convert the high voltage into a low voltage signal, and then converting the analog signal into a digital signal through an analog-to-digital converter (ADC) for subsequent processing and analysis. Active current refers to the AC current component that can generate actual power (i.e., active power). In the flexible direct current transmission system, active current is used to transmit electrical energy and may change when the grid fails. The purpose of collecting active current is to monitor the transmission of electrical energy and promptly detect problems such as active power imbalance or transmission anomalies. The acquisition method typically involves using a current transformer (CT) to convert a large current into a small current signal, and then converting the analog signal into a digital signal using an ADC. Furthermore, an active power measurement device can be used to directly measure the active power generated by the active current, and the magnitude of the active current can be inferred from this. Reactive current refers to the current component that does not generate electrical energy but affects the grid voltage quality. In a flexible DC transmission system, reactive current is used to regulate the receiving grid voltage and provide reactive power support. In the event of a grid fault, the reactive current at the receiving converter station may also change. The method for acquiring reactive current is similar to that for acquiring active current, and a CT is typically used to convert the large current into a small current signal, which is then digitized using an ADC. Furthermore, a reactive power measurement device can be used to directly measure the reactive power, and the magnitude of the reactive current can be inferred from this.

[0043] In some embodiments of the present application, in step S102, the equivalent complex impedance and potential vector under the receiving-end power grid fault are determined respectively according to the following methods:

[0044] The equivalent reactance of the receiving-end power grid during normal operation is calculated based on the AC bus voltage, active and reactive currents of the receiving-end converter station, the rated voltage of the receiving-end power grid, and the equivalent reactance of the connecting transformer during normal operation.

[0045] The virtual short-circuit fault transition resistance of the receiving-end power grid is calculated based on the equivalent reactance of the receiving-end power grid during normal operation, the equivalent reactance of the connecting transformer, the active and reactive currents of the receiving-end converter station under power grid fault conditions, and their control parameters.

[0046] The virtual short-circuit fault transient reactance of the receiving-end power grid is calculated based on the equivalent reactance of the receiving-end power grid during normal operation, the equivalent reactance of the connecting transformer, the active and reactive currents of the receiving-end converter station under power grid fault conditions, and their control parameters.

[0047] The equivalent complex impedance of the receiving-end power grid under the fault is calculated based on the virtual short-circuit fault transition resistance and reactance of the receiving-end power grid and the equivalent reactance of the receiving-end power grid during normal operation;

[0048] According to the virtual short-circuit fault transition resistance and reactance of the receiving-end power grid and the equivalent potential vector and equivalent reactance of the receiving-end power grid during normal operation, the equivalent potential vector of the receiving-end power grid under fault is calculated.

[0049] When implementing the present invention, the equivalent complex impedance and potential vector under a receiving-end power grid fault are determined as follows:

[0050]

[0051] Where: Z R.f and e R.f are the equivalent complex impedance and potential vector under the receiving-end power grid fault; e R.N is the equivalent potential vector of the receiving end power grid during normal operation; X R.N is the equivalent reactance of the receiving end power grid during normal operation, R V.f and X V.f are the virtual short-circuit fault transition resistance and reactance of the receiving-end power grid, respectively, and are calculated as follows:

[0052]

[0053] Where: U GEC.N E is the AC bus voltage of the receiving converter station during normal operation; R.N is the rated voltage of the receiving grid; I N.d and I N.q are respectively the active and reactive currents of the receiving converter station during normal operation; X T is the equivalent reactance of the connected transformer; I f.d and I f.q are the active and reactive currents of the receiving converter station under power grid fault respectively; the parameters p and q are calculated as follows:

[0054] q=U GEC.f +I f.q (X T +X R.N )-E R.N cos(δ R.N -δ GEC.f )

[0055] p=I f.d (X T +XR.N )+E R.N sin(δ R.N -δ GEC.f )

[0056] Where: δ R.N U is the potential phase angle of the receiving end power grid during normal operation; GEC.f and δ GEC.f are the AC bus voltage amplitude and phase angle of the receiving-end converter station under power grid fault, respectively; p represents the active current control parameter of the receiving-end converter station under power grid fault, and q represents the reactive current control parameter of the receiving-end converter station under power grid fault.

[0057] In some embodiments of the present application, in step S103, the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage is determined as follows:

[0058] The first parameter is calculated based on the AC bus voltage of the sending-end converter station, the integral coefficient of the DC voltage control outer loop of the sending-end converter station, and the rated DC voltage and equivalent DC capacitance of the flexible DC transmission system.

[0059] The second parameter is calculated based on the first parameter and the proportional coefficient of the DC voltage control outer loop of the sending-end converter station;

[0060] Calculate a third parameter based on the first parameter and the second parameter;

[0061] Calculate the fourth parameter based on the second parameter and the third parameter;

[0062] The fifth parameter is calculated based on the active power of the receiving-end converter station during normal operation, the maximum allowable DC voltage deviation of the flexible DC transmission, the rated DC voltage, the equivalent DC capacitance, and the first parameter, the second parameter, the third parameter, and the fourth parameter.

[0063] Calculating a first control parameter based on the fifth parameter, the maximum allowable AC current coefficient and the rated AC current of the receiving-end converter station, and the equivalent resistance under a receiving-end power grid fault;

[0064] Calculating a second control parameter based on the fifth parameter, the maximum allowable AC current coefficient and the rated AC current of the receiving-end converter station, and the equivalent resistance and reactance under a receiving-end power grid fault;

[0065] The third control parameter is calculated based on the fifth parameter, the maximum allowable AC current coefficient and the rated AC current of the receiving-end converter station, and the equivalent resistance and reactance under the receiving-end power grid fault;

[0066] Calculating a first current parameter according to the first control parameter, the second control parameter, and the third control parameter;

[0067] Calculating a second current parameter according to the maximum allowable AC current coefficient of the receiving-end converter station, the rated AC current, and the first current parameter;

[0068] The maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage is calculated based on the inverse tangent function of the ratio of the first current parameter to the second current parameter.

[0069] When implementing the present invention, the maximum AC current phase angle at the receiving-end converter station to avoid DC transient overvoltage is determined as follows:

[0070]

[0071] Where: δ MAP To avoid DC transient overvoltage at the receiving converter station, the maximum AC current phase angle; parameters and are the first current parameter and the second current parameter respectively.

[0072] When implementing the present invention, the first current parameter and the second current parameter are calculated as follows:

[0073]

[0074] Where: K I and I GEC.N are the maximum allowable AC current coefficient and rated AC current of the receiving converter station respectively; the first control parameter a, the second control parameter b and the third control parameter c are calculated as follows:

[0075] a=M 2 +[(K I I GEC.N ) 2 +R R.f ] 2

[0076]

[0077] Where R R.f and X R.f are the equivalent resistance and reactance under the receiving-end power grid fault respectively; the fifth parameter M is calculated as follows:

[0078]

[0079] Where: P GEC.N is the active power of the receiving converter station during normal operation; ΔU DC.per 、U DC.N and C DC are the maximum allowable DC voltage deviation, rated DC voltage and equivalent DC capacitance of flexible DC transmission respectively; the first parameter ω, the second parameter ξ, the third parameter γ and the fourth parameter TDC.max They are:

[0080] ω 2 =U SEC k i / (U DC.N C DC )

[0081] ξ=k p ω / (2k i )

[0082]

[0083] Where: U SEC k is the AC bus voltage of the sending-end converter station, which is approximately the rated value; i and k p are the integral coefficient and proportional coefficient of the DC voltage control outer loop of the sending-end converter station respectively.

[0084] In some embodiments of the present application, in step S103, the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible HVDC transmission system is determined as follows:

[0085] The first phase angle parameter is calculated based on the virtual short-circuit fault transition resistance and reactance of the receiving-end power grid and the equivalent reactance of the receiving-end power grid during normal operation;

[0086] The second phase angle parameter is calculated based on the inverse tangent function of the ratio of the equivalent reactance and resistance under the receiving-end power grid fault;

[0087] According to the difference between the first phase angle parameter and the second phase angle parameter, the AC current phase angle of the receiving converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system is calculated.

[0088] In the specific implementation of the present invention, the AC current phase angle at the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible HVDC transmission system is determined as follows:

[0089] δ PRS =α-β

[0090] Where: δ PRS is the AC current phase angle of the receiving converter station required to maximize the voltage at the receiving-end grid connection point of the flexible HVDC transmission system; α and β are the first phase angle parameter and the second phase angle parameter, respectively.

[0091] When the present invention is specifically implemented, the first phase angle parameter and the second phase angle parameter are calculated as follows:

[0092]

[0093] In some embodiments of the present application, in step S105, when the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage is less than the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the HVDC Flexible system, the maximum feasible voltage at the receiving-end grid connection point of the HVDC Flexible system under a grid fault is determined as follows:

[0094] Calculating a first voltage parameter based on the rated voltage of the receiving-end power grid, the virtual short-circuit fault transition resistance and reactance of the receiving-end power grid, and the equivalent reactance of the receiving-end power grid during normal operation;

[0095] The maximum feasible voltage at the receiving-end grid connection point of the flexible DC transmission system under grid fault is calculated based on the equivalent resistance and reactance under grid fault, the maximum allowable AC current coefficient of the receiving-end converter station and its rated AC current, the first current parameter, the second current parameter and the first voltage parameter.

[0096] In a specific implementation of the present invention, when the maximum AC current phase angle at the receiving-end converter station for avoiding DC transient overvoltage is less than the AC current phase angle at the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the HVDC Flexible system, the maximum feasible voltage at the receiving-end grid connection point of the HVDC Flexible system under a grid fault is determined as follows:

[0097]

[0098] Where: The first voltage parameter N is calculated as follows:

[0099]

[0100] In this embodiment of the present invention, in step S105, when the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage is less than the AC current phase angle of the receiving-end converter station required for maximizing the voltage at the receiving-end grid connection point of the flexible DC transmission system, the active and reactive current reference values ​​of the receiving-end converter station are determined as follows:

[0101] Calculating a reference value of the active current of the receiving-end converter station according to the second current parameter;

[0102] A reactive current reference value of the receiving-end converter station is calculated based on the first current parameter.

[0103] In the specific implementation of the present invention, when the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage is less than the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system, the active and reactive current reference values ​​of the receiving-end converter station are determined according to the following method to achieve maximum support for the receiving-end grid voltage while ensuring DC voltage safety:

[0104]

[0105] Where: and are the reference values ​​of active and reactive currents at the receiving converter station respectively.

[0106] In some embodiments of the present application, in step S106, when the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage is greater than the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the HVDC Flexible system, the maximum feasible voltage at the receiving-end grid connection point of the HVDC Flexible system under a grid fault is determined as follows:

[0107] Based on the equivalent reactance of the receiving-end grid during normal operation, the equivalent resistance and reactance under a receiving-end grid fault, the maximum allowable AC current coefficient and rated AC current of the receiving-end converter station, the maximum feasible voltage at the receiving-end grid connection point of the flexible DC transmission system under a grid fault is calculated.

[0108] In a specific implementation of the present invention, when the maximum AC current phase angle at the receiving-end converter station that avoids DC transient overvoltage is greater than the AC current phase angle at the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the Flexible DC transmission system, the maximum feasible voltage at the receiving-end grid connection point of the Flexible DC transmission system under a grid fault is determined as follows:

[0109]

[0110] In a specific implementation of the present invention, when the maximum AC current phase angle at the receiving-end converter station for avoiding DC transient overvoltage is greater than the AC current phase angle at the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system, the reference values ​​of the active and reactive currents at the receiving-end converter station are determined as follows, so as to raise the voltage at the receiving-end grid connection point of the flexible DC transmission system to a maximum value while always maintaining the DC voltage deviation below the maximum allowable value:

[0111]

[0112] It is understood that in this embodiment of the present invention, when the maximum AC current phase angle at the receiving-end converter station to avoid DC transient overvoltage is less than the AC current phase angle at the receiving-end converter station required to maximize the voltage at the receiving-end grid-connection point of the flexible DC transmission system, the receiving-end grid voltage is supported to the maximum extent possible while ensuring DC voltage safety. When the maximum AC current phase angle at the receiving-end converter station to avoid DC transient overvoltage is greater than the AC current phase angle at the receiving-end converter station required to maximize the voltage at the receiving-end grid-connection point of the flexible DC transmission system, the voltage at the receiving-end grid-connection point of the flexible DC transmission system is raised to its maximum value while always maintaining the DC voltage deviation below the maximum allowable value.

[0113] In some embodiments of the present application, Figure 2 As shown, a receiving-end voltage control device for a flexible direct current transmission system includes:

[0114] An acquisition module is used to collect the AC bus voltage, active and reactive currents of the receiving-end converter station of the flexible DC transmission before and after the grid fault;

[0115] The first calculation module is used to calculate the equivalent complex impedance and potential vector under the receiving end power grid fault;

[0116] The second calculation module is used to calculate the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage and the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system;

[0117] a comparison module, configured to compare a maximum AC current phase angle at the receiving-end converter station for avoiding DC transient overvoltage with the AC current phase angle at the receiving-end converter station required for maximizing the voltage at the receiving-end grid-connection point of the flexible DC transmission system; if the maximum AC current phase angle at the receiving-end converter station for avoiding DC transient overvoltage is less than the AC current phase angle at the receiving-end converter station required for maximizing the voltage at the receiving-end grid-connection point of the flexible DC transmission system, then calling the third calculation module; otherwise, calling the fourth calculation module;

[0118] a third calculation module, configured to calculate the maximum feasible voltage at the receiving-end grid connection point of the flexible HVDC transmission system under a power grid fault based on the maximum AC current phase angle at the receiving-end converter station that avoids DC transient overvoltage, further calculate the active and reactive currents at the receiving-end converter station that enable the voltage at the receiving-end grid connection point of the flexible HVDC transmission system to reach the maximum feasible value, and invoke the first control module;

[0119] a fourth calculation module, configured to calculate the maximum feasible voltage at the receiving-end grid-connection point of the Flexible HVDC transmission system under a power grid fault based on the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid-connection point of the Flexible HVDC transmission system, further calculate the active and reactive currents of the receiving-end converter station required to maximize the voltage at the receiving-end grid-connection point of the Flexible HVDC transmission system, and invoke the second control module;

[0120] a first control module, configured to implement control using the active and reactive currents of the receiving-end converter station calculated by the third calculation module so as to enable the voltage at the receiving-end grid connection point of the flexible HVDC transmission system to reach a maximum feasible value as reference values ​​for controlling the active and reactive currents of the receiving-end converter station;

[0121] The second control module is used to implement control by using the active and reactive currents of the receiving-end converter station calculated by the fourth calculation module so that the voltage at the receiving-end grid connection point of the flexible DC transmission system reaches the maximum feasible value as the active and reactive current control reference values ​​of the receiving-end converter station.

[0122] In some embodiments of the present application, Figure 3As shown, the present application provides a receiving-end voltage control device for a flexible direct current transmission system, comprising a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor. When the processor executes the computer program, the receiving-end voltage control method for the flexible direct current transmission system is implemented.

[0123] The present disclosure may be a system, method and / or computer program product. The computer program product may include a computer-readable storage medium carrying computer-readable program instructions for causing a processor to implement various aspects of the present disclosure.

[0124] Computer-readable storage media can be a tangible device that can hold and store the instructions used by the instruction execution device. Computer-readable storage media can be, for example, but not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination thereof. More specific examples (non-exhaustive list) of computer-readable storage media include: a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanical encoding device, for example, a punch card or a raised structure in a groove on which instructions are stored, and any suitable combination thereof. Computer-readable storage media used herein is not interpreted as a transient signal itself, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagated by waveguides or other transmission media (for example, light pulses by fiber optic cables), or electrical signals transmitted by wires.

[0125] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to each computing / processing device, or downloaded to an external computer or external storage device via a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network can include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. The network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards the computer-readable program instructions to be stored in the computer-readable storage medium in each computing / processing device.

[0126] The computer program instructions for performing the disclosed operation can be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-related instructions, microcode, firmware instructions, state setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, and conventional procedural programming languages ​​such as "C" language or similar programming languages. Computer-readable program instructions can be executed completely on a user's computer, partially on a user's computer, executed as an independent software package, partially on a user's computer and partially on a remote computer, or completely on a remote computer or server. In the case of a remote computer, the remote computer can be connected to the user's computer via any type of network, including a local area network (LAN) or a wide area network (WAN), or can be connected to an external computer (such as, for example, utilizing an Internet service provider to connect via the Internet). In certain embodiments, by utilizing the state information of computer-readable program instructions to personalize an electronic circuit, such as a programmable logic circuit, a field programmable gate array (FPGA), or a programmable logic array (PLA), the electronic circuit can execute computer-readable program instructions, thereby realizing various aspects of the present disclosure.

[0127] Various aspects of the present disclosure are described herein with reference to flowcharts and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the present disclosure. It should be understood that each block of the flowcharts and / or block diagrams, and combinations of blocks in the flowcharts and / or block diagrams, can be implemented by computer-readable program instructions.

[0128] These computer-readable program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, or other programmable data processing device, thereby producing a machine, so that when these instructions are executed by the processor of the computer or other programmable data processing device, a device is generated that implements the functions / actions specified in one or more blocks in the flowchart and / or block diagram. These computer-readable program instructions can also be stored in a computer-readable storage medium, where these instructions cause the computer, programmable data processing device, and / or other device to operate in a specific manner. Thus, the computer-readable medium storing the instructions comprises an article of manufacture that includes instructions for implementing various aspects of the functions / actions specified in one or more blocks in the flowchart and / or block diagram.

[0129] Computer-readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device so that a series of operational steps are performed on the computer, other programmable data processing apparatus, or other device to produce a computer-implemented process, thereby causing the instructions executed on the computer, other programmable data processing apparatus, or other device to implement the functions / actions specified in one or more blocks in the flowchart and / or block diagram.

[0130] The flow charts and block diagrams in the accompanying drawings show the possible architecture, functions and operations of the systems, methods and computer program products according to multiple embodiments of the present disclosure. In this regard, each box in the flow chart or block diagram can represent a part of a module, program segment or instruction, and the part of the module, program segment or instruction contains one or more executable instructions for realizing the prescribed logical function. In some alternative implementations, the functions marked in the box can also occur in a sequence different from that marked in the accompanying drawings. For example, two consecutive boxes can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved. It should also be noted that each box in the block diagram and / or flow chart, and the combination of the boxes in the block diagram and / or flow chart can be implemented by a dedicated hardware-based system that performs the prescribed function or action, or can be implemented by a combination of dedicated hardware and computer instructions.

[0131] In order to verify the effectiveness of the present invention, Figure 4 The structural diagram of flexible DC transmission under power grid fault is shown as an example for analysis. Figure 4 As shown, in this embodiment, the receiving-end grid adopts an IEEE-39 busbar system, and Flexible DC transmission is connected to bus 14. Flexible DC transmission uses symmetrical monopole connection, with a rated DC voltage of ±800 kV and a rated transmission capacity of 900 MW. During normal operation, the receiving-end converter station uses active power control with a control reference value of 1.0 pu; the sending-end converter station uses DC voltage control with a control reference value of 1.0 pu.

[0132] This example uses a scenario where a three-phase short circuit fault occurs on bus 6 at 2.15 seconds and lasts for 200 milliseconds. To verify the effectiveness of the proposed method for controlling voltage at the receiving end of a flexible DC transmission system, a comparison method for maximizing reactive power at the receiving converter station based on the effective short-circuit ratio was used. The DC voltage, the voltage at the receiving end of the flexible DC transmission system's grid connection point, the active power and reactive power at the receiving converter station, and the AC current phase angle were recorded and analyzed.

[0133] According to the aforementioned step S103, the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage and the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system are -44 degrees and -42 degrees respectively.

[0134] According to the active and reactive current reference values ​​of the receiving-end converter station in the aforementioned step S105 being 0.89 pu and 0.81 pu respectively, the maximum feasible voltage of the receiving-end grid connection point of the flexible HVDC transmission system under a grid fault is 0.67 pu.

[0135] Figure 5 (a) to (e) are waveforms of DC voltage, voltage at the receiving-end grid connection point of the flexible DC transmission system, active power, reactive power, and AC current phase angle at the receiving-end converter station under grid fault conditions, respectively. Figure 5 The horizontal axis represents time, and the vertical axes represent DC voltage, voltage at the receiving-end grid connection point of the flexible DC transmission system, active power, reactive power, and AC current phase angle of the receiving-end converter station. The solid line represents the curve of the flexible DC transmission system receiving-end voltage control method proposed in the present invention, and the dotted line represents the curve of the comparison group.

[0136] like Figure 5 As shown by the dotted lines (a) to (e), the comparison group causes the active power of the receiving converter station to change by nearly 1.0 pu before and after the receiving grid fault. The DC voltage increases sharply to 1.25 pu before the active power of the sending converter station decreases to balance with the active power of the receiving converter station. This is much larger than the maximum allowable DC voltage deviation of 0.1 pu, which may cause the flexible DC transmission to be locked.

[0137] Depend on Figure 5 As can be seen from the solid lines (a) to (e), the present invention takes into account the limitations of DC voltage safety on AC voltage support. The AC current phase angle at the receiving-end converter station is less than the maximum AC current phase angle required to avoid DC transient overvoltage. The maximum DC voltage deviation during the grid fault is only 0.09 pu, which not only avoids DC transient overvoltage but also raises the voltage at the grid connection point of the flexible DC transmission system to 0.67 pu.

[0138] This demonstrates that during a grid fault, the receiving-end converter station only provides reactive current, ignoring the fault transition impedance. This results in a sudden change in the equivalent complex impedance of the receiving-end grid before and after the fault, failing to effectively support the receiving-end grid voltage. In contrast, the present invention supports AC voltage by coordinating the active and reactive currents of the receiving-end converter station. While providing only 0.56 pu of reactive power at the receiving-end converter station, it ensures that the voltage drop across the equivalent complex impedance of the receiving-end grid is equal to the phase angle of the equivalent potential under the receiving-end grid fault, resulting in an optimal voltage boost at the receiving-end grid connection point of the flexible DC transmission system.

[0139] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for controlling the receiving-end voltage of a flexible direct current transmission system, characterized in that: The following steps are involved: S101: Collects the AC bus voltage, active and reactive currents of the receiving-end converter station of the flexible DC transmission before and after the grid fault; S102: Calculate the equivalent complex impedance and potential vector under the receiving-end power grid fault; S103: Calculate the maximum AC current phase angle at the receiving-end converter station to avoid DC transient overvoltage and the AC current phase angle at the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system; S104: comparing the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage with the AC current phase angle of the receiving-end converter station required for maximizing the voltage at the receiving-end grid-connection point of the HVDC Flexible system. If the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage is less than the AC current phase angle of the receiving-end converter station required for maximizing the voltage at the receiving-end grid-connection point of the HVDC Flexible system, executing S105; otherwise, executing S106. S105: Calculating the maximum feasible voltage at the receiving-end grid connection point of the HVDC Flexible system under a power grid fault based on the maximum AC current phase angle at the receiving-end converter station to avoid DC transient overvoltage. Furthermore, calculating the active and reactive currents at the receiving-end converter station that will enable the voltage at the receiving-end grid connection point of the HVDC Flexible system to reach the maximum feasible value. These values ​​are used as reference values ​​for controlling the active and reactive currents at the receiving-end converter station, and control is implemented. S106: Based on the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid-connected point of the Flexible DC transmission system, the maximum feasible voltage at the receiving-end grid-connected point of the Flexible DC transmission system under a power grid fault is calculated, and then the active and reactive currents of the receiving-end converter station that make the voltage at the receiving-end grid-connected point of the Flexible DC transmission system reach the maximum feasible value are calculated, and the active and reactive currents of the receiving-end converter station are used as reference values ​​for active and reactive current control of the receiving-end converter station to implement control.

2. The method for controlling the receiving-end voltage of a flexible DC transmission system according to claim 1, wherein: In step S102, the equivalent complex impedance and potential vector under the receiving-end power grid fault are determined as follows: The equivalent reactance of the receiving-end power grid during normal operation is calculated based on the AC bus voltage, active and reactive currents of the receiving-end converter station, the rated voltage of the receiving-end power grid, and the equivalent reactance of the connecting transformer during normal operation. The virtual short-circuit fault transition resistance of the receiving-end power grid is calculated based on the equivalent reactance of the receiving-end power grid during normal operation, the equivalent reactance of the connecting transformer, the active and reactive currents of the receiving-end converter station under power grid fault conditions, and their control parameters. The virtual short-circuit fault transient reactance of the receiving-end power grid is calculated based on the equivalent reactance of the receiving-end power grid during normal operation, the equivalent reactance of the connecting transformer, the active and reactive currents of the receiving-end converter station under power grid fault conditions, and their control parameters. The equivalent complex impedance of the receiving-end power grid under the fault is calculated based on the virtual short-circuit fault transition resistance and reactance of the receiving-end power grid and the equivalent reactance of the receiving-end power grid during normal operation; According to the virtual short-circuit fault transition resistance and reactance of the receiving-end power grid and the equivalent potential vector and equivalent reactance of the receiving-end power grid during normal operation, the equivalent potential vector of the receiving-end power grid under fault is calculated.

3. The receiving-end voltage control method of a flexible DC transmission system according to claim 1, characterized in that: In step S103, the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible HVDC transmission system is determined as follows: The first phase angle parameter is calculated based on the virtual short-circuit fault transition resistance and reactance of the receiving-end power grid and the equivalent reactance of the receiving-end power grid during normal operation; The second phase angle parameter is calculated based on the inverse tangent function of the ratio of the equivalent reactance and resistance under the receiving-end power grid fault; According to the difference between the first phase angle parameter and the second phase angle parameter, the AC current phase angle of the receiving converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system is calculated.

4. The method for controlling the receiving-end voltage of a flexible DC transmission system according to claim 1, wherein: In step S103, the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage is determined as follows: The first parameter is calculated based on the AC bus voltage of the sending-end converter station, the integral coefficient of the DC voltage control outer loop of the sending-end converter station, and the rated DC voltage and equivalent DC capacitance of the flexible DC transmission system. The second parameter is calculated based on the first parameter and the proportional coefficient of the DC voltage control outer loop of the sending-end converter station; Calculate a third parameter based on the first parameter and the second parameter; Calculate the fourth parameter based on the second parameter and the third parameter; The fifth parameter is calculated based on the active power of the receiving-end converter station during normal operation, the maximum allowable DC voltage deviation of the flexible DC transmission, the rated DC voltage, the equivalent DC capacitance, and the first parameter, the second parameter, the third parameter, and the fourth parameter. Calculating a first control parameter based on the fifth parameter, the maximum allowable AC current coefficient and the rated AC current of the receiving-end converter station, and the equivalent resistance under a receiving-end power grid fault; Calculating a second control parameter based on the fifth parameter, the maximum allowable AC current coefficient and the rated AC current of the receiving-end converter station, and the equivalent resistance and reactance under a receiving-end power grid fault; The third control parameter is calculated based on the fifth parameter, the maximum allowable AC current coefficient and the rated AC current of the receiving-end converter station, and the equivalent resistance and reactance under the receiving-end power grid fault; Calculating a first current parameter according to the first control parameter, the second control parameter, and the third control parameter; Calculating a second current parameter according to the maximum allowable AC current coefficient of the receiving-end converter station, the rated AC current, and the first current parameter; The maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage is calculated based on the inverse tangent function of the ratio of the first current parameter to the second current parameter.

5. The method for controlling the receiving-end voltage of a flexible DC transmission system according to claim 4, characterized in that: In step S105, when the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage is less than the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the HVDC Flexible system, the maximum feasible voltage at the receiving-end grid connection point of the HVDC Flexible system under the power grid fault is determined as follows: Calculating a first voltage parameter based on the rated voltage of the receiving-end power grid, the virtual short-circuit fault transition resistance and reactance of the receiving-end power grid, and the equivalent reactance of the receiving-end power grid during normal operation; The maximum feasible voltage at the receiving-end grid connection point of the flexible DC transmission system under grid fault is calculated based on the equivalent resistance and reactance under grid fault, the maximum allowable AC current coefficient of the receiving-end converter station and its rated AC current, the first current parameter, the second current parameter and the first voltage parameter.

6. The method for controlling the receiving-end voltage of a flexible DC transmission system according to claim 4, characterized in that: In step S105, when the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage is less than the AC current phase angle of the receiving-end converter station required for maximizing the voltage at the receiving-end grid connection point of the flexible DC transmission system, the active and reactive current reference values ​​of the receiving-end converter station are determined as follows: Calculating a reference value of the active current of the receiving-end converter station according to the second current parameter; A reactive current reference value of the receiving-end converter station is calculated based on the first current parameter.

7. The method for controlling the receiving-end voltage of a flexible DC transmission system according to claim 1, wherein: In step S106, when the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage is greater than the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the HVDC Flexible system, the maximum feasible voltage at the receiving-end grid connection point of the HVDC Flexible system under the power grid fault is determined as follows: The maximum feasible voltage at the receiving-end grid connection point of the flexible DC transmission system under grid fault conditions is calculated based on the equivalent reactance of the receiving-end grid during normal operation, the equivalent resistance and reactance of the receiving-end grid under fault conditions, the maximum allowable AC current coefficient, and the rated AC current of the receiving-end converter station.

8. The method for controlling the receiving-end voltage of a flexible DC transmission system according to claim 1, wherein: In step S106, when the maximum AC current phase angle of the receiving-end converter station for avoiding DC transient overvoltage is greater than the AC current phase angle of the receiving-end converter station required for maximizing the voltage at the receiving-end grid connection point of the flexible DC transmission system, the active and reactive current reference values ​​of the receiving-end converter station are determined as follows: The active current reference value of the receiving-end converter station is calculated based on the cosine value of the AC current phase angle at the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system, the product of the maximum allowable AC current coefficient at the receiving-end converter station and the rated AC current; The reactive current reference value of the receiving-end converter station is calculated based on the sine value of the AC current phase angle at the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system, the product of the maximum allowable AC current coefficient of the receiving-end converter station and the rated AC current.

9. A receiving-end voltage control device for a flexible DC transmission system, characterized in that: include: An acquisition module is used to collect the AC bus voltage, active and reactive currents of the receiving-end converter station of the flexible DC transmission before and after the grid fault; The first calculation module is used to calculate the equivalent complex impedance and potential vector under the receiving end power grid fault; The second calculation module is used to calculate the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage and the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system; a comparison module, configured to compare a maximum AC current phase angle at the receiving-end converter station for avoiding DC transient overvoltage with the AC current phase angle at the receiving-end converter station required for maximizing the voltage at the receiving-end grid-connection point of the flexible DC transmission system; if the maximum AC current phase angle at the receiving-end converter station for avoiding DC transient overvoltage is less than the AC current phase angle at the receiving-end converter station required for maximizing the voltage at the receiving-end grid-connection point of the flexible DC transmission system, then calling the third calculation module; otherwise, calling the fourth calculation module; a third calculation module, configured to calculate the maximum feasible voltage at the receiving-end grid connection point of the flexible HVDC transmission system under a power grid fault based on the maximum AC current phase angle at the receiving-end converter station that avoids DC transient overvoltage, further calculate the active and reactive currents at the receiving-end converter station that enable the voltage at the receiving-end grid connection point of the flexible HVDC transmission system to reach the maximum feasible value, and invoke the first control module; a fourth calculation module, configured to calculate the maximum feasible voltage at the receiving-end grid-connection point of the Flexible HVDC transmission system under a power grid fault based on the AC current phase angle of the receiving-end converter station required to maximize the voltage at the receiving-end grid-connection point of the Flexible HVDC transmission system, further calculate the active and reactive currents of the receiving-end converter station required to maximize the voltage at the receiving-end grid-connection point of the Flexible HVDC transmission system, and invoke the second control module; a first control module, configured to implement control using the active and reactive currents of the receiving-end converter station calculated by the third calculation module so as to enable the voltage at the receiving-end grid connection point of the flexible HVDC transmission system to reach a maximum feasible value as reference values ​​for controlling the active and reactive currents of the receiving-end converter station; The second control module is used to implement control by using the active and reactive currents of the receiving-end converter station calculated by the fourth calculation module so that the voltage at the receiving-end grid connection point of the flexible DC transmission system reaches the maximum feasible value as the active and reactive current control reference values ​​of the receiving-end converter station.

10. A receiving-end voltage control device for a flexible DC transmission system, characterized in that: The method comprises a processor, a memory, and a computer program stored in the memory and configured to be executed by the processor, wherein the processor implements the method for controlling the receiving terminal voltage of a flexible direct current transmission system according to any one of claims 1 to 8 when executing the computer program.

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