Flexible direct current power transmission system receiving end voltage control method, device and equipment

Abstract

The invention relates to the field of power system protection and control, and discloses a flexible direct current power transmission system receiving end voltage control method, device and equipment. A receiving-end converter station of the flexible direct-current power transmission system collects alternating-current bus voltage, active current and reactive current before and after a fault, and calculates equivalent complex impedance and a potential vector after the fault of a receiving-end power grid; calculating and comparing the maximum alternating current phase angle of the receiving end converter station for avoiding the direct current transient overvoltage of the flexible direct current power transmission system with the alternating current phase angle of the receiving end converter station required for maximizing the voltage of the receiving end grid-connected point of the flexible direct current power transmission system; calculating the maximum feasible value of the grid-connected point voltage of the receiving end of the flexible direct-current power transmission system on the premise of avoiding the direct-current transient overvoltage; and calculating and adjusting active and reactive current reference values of the receiving end converter station of the flexible direct-current power transmission system, and controlling the voltage of the receiving end grid-connected point of the flexible direct-current power transmission system to the maximum feasible value under the power grid fault. According to the method, active and reactive currents of the receiving end converter station of the flexible direct-current power transmission system are coordinated, so that the maximum voltage rise of the receiving end grid-connected point under the direct-current voltage safety condition of the flexible direct-current power transmission under the power grid fault is quantified and realized.

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 DC transmission systems can decouple active and reactive currents and have no risk of commutation failure. They have been widely used in onshore and offshore wind power transmission. When a three-phase short circuit fault occurs in the receiving power grid, the AC bus voltage of the receiving converter station of the flexible DC transmission system drops. To prevent overcurrent blocking of power devices, the receiving converter station is equipped with an AC current limiting link. Therefore, the active power of the receiving converter station is often difficult to recover to the initial value under power grid faults, resulting in a surplus of active power of flexible DC transmission, an increase in DC voltage, and the flexible DC transmission may be blocked due to over-limit DC voltage. During the receiving power grid fault, flexible DC transmission needs to provide AC voltage support. However, under the maximum allowable AC current limit, the supporting effect of the receiving converter station on the receiving power grid voltage is much weaker than that of the synchronous motor, and it will cause the active power of the receiving converter station to further decrease, exacerbating the DC voltage change. Therefore, how to ensure the safety of DC voltage and effectively support the receiving power grid voltage during power grid faults has become a hot issue in flexible DC transmission control.

[0003] The reference value of reactive current at the receiving-end converter station during a grid fault is mostly determined according to the grid connection specifications of new energy sources such as wind farms. However, the AC voltage support capacity of flexible DC transmission cannot be fully utilized by simply increasing the reactive current of the receiving-end converter station linearly according to the degree of AC voltage drop. Existing technicians have proposed a method for maximizing reactive power control at the receiving-end converter station based on the effective short-circuit ratio, which effectively improves the voltage of the receiving-end grid; some technicians have also achieved the maximum support of the voltage at the receiving-end grid connection point by matching the AC current of the receiving-end converter station and the short-circuit current phase of the grid, but it depends on communication. According to the instantaneous power theory, raising the AC voltage is conducive to increasing the active power of the receiving-end converter station during a grid fault, but the reactive current of the receiving-end converter station required for AC voltage support also squeezes the feasible range of the active current of the receiving-end converter station, which may aggravate the active power surplus. The AC and DC voltage control of flexible DC transmission during grid faults affect each other, but there is little related research. The technicians quantitatively analyzed the function of the maximum allowable power imbalance time and the active and reactive currents of the receiving-end converter station required for AC voltage support, and used energy-consuming devices as a means of DC voltage control. However, the dynamic characteristics of the DC voltage under the fixed DC voltage control of the sending-end converter station are very different from those of the energy-consuming devices. Currently, there is no research on the maximum DC voltage deviation under the fixed DC voltage control by AC voltage support. It is difficult to quantify the maximum AC voltage support capacity of the receiving-end power grid under the premise of DC voltage safety of flexible DC transmission, and it may also lead to DC transient overvoltage or poor AC voltage rise of flexible DC transmission during faults in the receiving-end power grid. Summary of the invention

[0004] In view of the above-mentioned deficiencies of the prior art, the present invention provides a method, device and equipment for controlling the voltage at the receiving end 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 power 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 under the power grid fault to the maximum feasible value. The present invention quantifies and realizes the maximum increase in the receiving-end power grid voltage on the basis of the DC voltage safety of the flexible direct current transmission under the power 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: Collecting the AC bus voltage, active and reactive current of the receiving-end converter station of the flexible DC transmission before and after the power 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 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;

[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-connected point of the flexible DC transmission 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-connected point of the flexible DC transmission system, executing S105, otherwise executing S106;

[0010] S105: Calculate the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under power grid fault according to the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage, and then calculate the active and reactive currents of the receiving-end converter station that make the voltage of the receiving-end grid connection point of the flexible DC transmission system reach the maximum feasible value, and use them as reference values ​​for active and reactive current control of the receiving-end converter station to implement control;

[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, calculate the maximum feasible voltage at the receiving-end grid-connected point of the flexible DC transmission system under power grid fault, and then calculate 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, and use them 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 proposes a receiving-end voltage control device of a flexible direct current transmission system, comprising:

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

[0016] A comparison module is used to compare 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-connected point of the flexible DC transmission 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-connected point of the flexible DC transmission system, the third calculation module is called, otherwise the fourth calculation module is called;

[0017] The third calculation module is used to calculate the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under the power grid fault according to the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage, and then calculate the active and reactive currents of the receiving-end converter station that make the voltage of the receiving-end grid connection point of the flexible DC transmission system reach the maximum feasible value, and call the first control module;

[0018] The fourth calculation module is used to calculate the maximum feasible voltage of the receiving-end grid-connected point of the flexible direct current transmission system under a power grid fault according to the AC current phase angle of the receiving-end converter station required for maximizing the voltage of the receiving-end grid-connected point of the flexible direct current transmission system, and then calculate the active and reactive currents of the receiving-end converter station that make the voltage of the receiving-end grid-connected point of the flexible direct current transmission system reach the maximum feasible value, and call the second control module;

[0019] A first control module is used to use the active and reactive currents of the receiving-end converter station calculated by the third calculation module so that the voltage at the receiving-end grid connection point of the flexible DC power transmission system reaches the maximum feasible value as the active and reactive current control reference values ​​of the receiving-end converter station, and implement control;

[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 to make the voltage at the receiving-end grid connection point of the flexible DC power transmission system reach 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, wherein the processor 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 when executing the computer program.

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

[0023] 1. The existing technology evaluates the impact of the flexible DC transmission system on the normal power grid through the short-circuit ratio and its improved index. There is no quantitative characterization method for the AC voltage raising effect under the power grid fault taking into account the influence of the flexible DC transmission control. It is difficult to guide the coordination of the setting value and timing of the power grid safety and stability control device, which may cause the reactive power compensation device to malfunction and even AC voltage instability. The present invention 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 DC transmission system, calculates the maximum feasible value of the voltage at the receiving-end grid connection point of the flexible DC transmission system under the premise of avoiding DC transient overvoltage, and quantifies the maximum AC voltage support capacity of the faulty power grid under the premise of the DC voltage of the flexible DC transmission.

[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 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, so as to achieve 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 solution 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 schematic diagram of the structure of a receiving-end voltage control device of a flexible DC power transmission system in an embodiment of the present application;

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

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

[0030] Figure 5 It is a diagram showing the effect of an embodiment 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 be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. 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 creative work 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 fixed DC voltage control is adopted 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 need to solve urgently.

[0033] Based on the above problems, the present 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 condition of a receiving-end grid fault, 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 voltage at the receiving end of a flexible direct current transmission system, comprising the following steps:

[0035] S101: Collecting the AC bus voltage, active and reactive current of the receiving-end converter station of the flexible DC transmission before and after the power 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 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;

[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-connected point of the flexible DC transmission 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-connected point of the flexible DC transmission system, executing S105, otherwise executing S106;

[0039] S105: Calculate the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under power grid fault according to the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage, and then calculate the active and reactive currents of the receiving-end converter station that make the voltage of the receiving-end grid connection point of the flexible DC transmission system reach the maximum feasible value, and use them as reference values ​​for active and reactive current control of the receiving-end converter station to implement control;

[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, calculate the maximum feasible voltage at the receiving-end grid-connected point of the flexible DC transmission system under power grid fault, and then calculate 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, and use them as reference values ​​for active and reactive current control of the receiving-end converter station to implement control.

[0041] In a flexible DC transmission system, when a fault occurs in the receiving power grid, it is crucial to collect the AC bus voltage, active and reactive currents of the receiving converter station, which helps to quickly locate the fault, evaluate the impact of the fault, and take corresponding 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 current of the receiving-end converter station of the flexible DC transmission before and after the grid fault. The AC bus voltage is one of the important parameters of the receiving-end converter station in the flexible DC transmission system. When the receiving-end 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 state of the receiving-end grid in real time, to promptly detect abnormal AC bus voltage, and to take corresponding control measures. The collection method generally includes using a voltage transformer (PT) to convert a 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 DC transmission system, active current is used to transmit electric energy and may change when the grid fails. The purpose of collecting active current is to monitor the transmission of electric energy and promptly detect problems such as active power imbalance or transmission abnormality. The acquisition method usually includes 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 through an ADC. In addition, the active power generated by the active current can be directly measured by an active power measurement device, and the size of the active current can be calculated based on this. Reactive current refers to the current component that does not generate electrical energy but affects the quality of the grid voltage. In a flexible DC transmission system, reactive current is used to regulate the receiving grid voltage and provide reactive support. In the event of a grid fault, the reactive current of the receiving converter station may also change. The method of collecting reactive current is similar to that of collecting active current. CT is usually used to convert a large current into a small current signal and digitize it through an ADC. In addition, the reactive power can be directly measured by a reactive power measurement device, and the size of the reactive current can be calculated based on 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] According to 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 equivalent reactance of the receiving-end power grid during normal operation is calculated;

[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 and their control parameters;

[0046] According to 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 and their control parameters, the virtual short-circuit fault transition reactance of the receiving-end power grid is calculated;

[0047] According to 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 equivalent complex impedance under the receiving-end power grid fault is calculated;

[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 under the receiving-end power grid fault is calculated.

[0049] When the present invention is specifically implemented, the equivalent complex impedance and potential vector under the 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 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 I is the rated voltage of the receiving grid; 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 connecting transformer; I f.d and I f.q are respectively the active and reactive currents of the receiving converter station under power grid fault; 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: 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; 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 obtained by calculation according to 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;

[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] According to the second parameter and the third parameter, a fourth parameter is calculated;

[0062] The fifth parameter is calculated according to the active power of the receiving-end converter station during normal operation, the maximum allowable DC voltage deviation of 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] The first 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 under the receiving-end power grid fault;

[0064] The second 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;

[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] The second current parameter is calculated 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 the present invention is specifically implemented, the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage is determined as follows:

[0070]

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

[0072] When the present invention is specifically implemented, the first current parameter and the second current parameter are calculated according to the following formulae:

[0073]

[0074] Where: K I and I GEC.N are the maximum allowable AC current coefficient and rated AC current of the receiving end 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] In the formula, 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 They are respectively the integral coefficient and proportional coefficient of the DC voltage control outer loop of the sending-end converter station.

[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 DC power transmission system is determined by the following method:

[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 according to 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-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system is calculated.

[0088] When the present invention is specifically implemented, 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 power transmission system is determined as follows:

[0089] δ PRS =α-β

[0090] Where: PRS is 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; 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 according to the following formula:

[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 for maximizing the voltage of the receiving-end grid connection point of the flexible DC transmission system, the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under a power grid fault is determined as follows:

[0094] The first voltage parameter is calculated according to 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] According to the equivalent resistance and reactance under the receiving-end power 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, the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under the power grid fault is calculated.

[0096] In the specific implementation of the present invention, 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 of the receiving-end grid connection point of the flexible DC transmission system, the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under power grid fault is determined as follows:

[0097]

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

[0099]

[0100] In the 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 of 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] According to the second current parameter, a reference value of active current of the receiving-end converter station is calculated;

[0102] According to the first current parameter, a reactive current reference value of the receiving-end converter station is calculated.

[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 of the receiving-end grid connection point of the flexible DC power transmission system, the active and reactive current reference values ​​of the receiving-end converter station are determined according to the following method, so as to achieve maximum support for the receiving-end grid voltage under the premise of ensuring DC voltage safety:

[0104]

[0105] Where: and They are the reference values ​​of active and reactive currents of 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 for maximizing the voltage of the receiving-end grid connection point of the flexible DC transmission system, the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under a power grid fault is determined as follows:

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

[0108] 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 greater than the AC current phase angle of the receiving-end converter station required to maximize the voltage of the receiving-end grid connection point of the flexible DC transmission system, the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under power grid fault is determined as follows:

[0109]

[0110] 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 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 flexible DC power transmission system, the active and reactive current reference values ​​of the receiving-end converter station are determined according to the following method, while raising the voltage at the receiving-end grid connection point of the flexible DC power transmission system to the maximum value, the DC voltage deviation is always kept less than the maximum allowable value:

[0111]

[0112] It can be understood that in the embodiment 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 for maximizing the voltage at the receiving-end grid-connected point of the flexible DC transmission system, it is achieved to support the receiving-end grid voltage to the maximum extent under the premise of ensuring the safety of the DC voltage. When the maximum AC current phase angle of the receiving-end converter station to avoid 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-connected point of the flexible DC transmission system, while raising the voltage at the receiving-end grid-connected point of the flexible DC transmission system to the maximum value, the DC voltage deviation is always kept less than the maximum allowable value.

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

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

[0117] A comparison module is used to compare 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-connected point of the flexible DC transmission 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-connected point of the flexible DC transmission system, the third calculation module is called, otherwise the fourth calculation module is called;

[0118] The third calculation module is used to calculate the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under the power grid fault according to the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage, and then calculate the active and reactive currents of the receiving-end converter station that make the voltage of the receiving-end grid connection point of the flexible DC transmission system reach the maximum feasible value, and call the first control module;

[0119] The fourth calculation module is used to calculate the maximum feasible voltage of the receiving-end grid-connected point of the flexible direct current transmission system under a power grid fault according to the AC current phase angle of the receiving-end converter station required for maximizing the voltage of the receiving-end grid-connected point of the flexible direct current transmission system, and then calculate the active and reactive currents of the receiving-end converter station that make the voltage of the receiving-end grid-connected point of the flexible direct current transmission system reach the maximum feasible value, and call the second control module;

[0120] A first control module is used to use the active and reactive currents of the receiving-end converter station calculated by the third calculation module so that the voltage at the receiving-end grid connection point of the flexible DC power transmission system reaches the maximum feasible value as the active and reactive current control reference values ​​of the receiving-end converter station, and implement control;

[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 to make the voltage at the receiving-end grid connection point of the flexible DC power transmission system reach 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, wherein the processor implements the receiving-end voltage control method for the flexible direct current transmission system when executing the computer program.

[0123] The present disclosure may be a system, a method and / or a 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 medium can be a tangible device that can hold and store instructions used by an instruction execution device. Computer readable storage medium 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 lists) of computer readable storage medium include: portable computer disks, hard disks, random access memories (RAM), read-only memories (ROM), erasable programmable read-only memories (EPROM or flash memory), static random access memories (SRAM), portable compact disk read-only memories (CD-ROM), digital versatile disks (DVD), memory sticks, floppy disks, mechanical encoding devices, such as punch cards or convex structures in grooves on which instructions are stored, and any suitable combination thereof. Computer readable storage medium used here 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 (e.g., light pulses by optical fiber 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, optical fiber transmissions, wireless transmissions, 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 for storage in the computer-readable storage medium in each computing / processing device.

[0126] The computer program instructions for performing the disclosed operation may 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++, etc., and conventional procedural programming languages, such as "C" language or similar programming languages. Computer-readable program instructions may be executed completely on a user's computer, partially on a user's computer, as an independent software package, partially on a user's computer, partially on a remote computer, or completely on a remote computer or server. In the case of a remote computer, the remote computer may 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 may be connected to an external computer (e.g., using an Internet service provider to connect via the Internet). In some embodiments, an electronic circuit, such as a programmable logic circuit, a field programmable gate array (FPGA), or a programmable logic array (PLA), may be personalized by utilizing the state information of the computer-readable program instructions, and the electronic circuit may execute the computer-readable program instructions, thereby realizing various aspects of the disclosed invention.

[0127] Various aspects of the present disclosure are described herein with reference to the flowcharts and / or block diagrams of the methods, devices (systems) and computer program products according to the embodiments of the present disclosure. It should be understood that each box in the flowchart and / or block diagram and the combination of each box in the flowchart and / or block diagram 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 that implements the functions / actions specified in one or more boxes in the flowchart and / or block diagram is generated. These computer-readable program instructions can also be stored in a computer-readable storage medium, and these instructions cause the computer, programmable data processing device, and / or other equipment to work in a specific manner, so that the computer-readable medium storing the instructions includes a manufactured product, which includes instructions for implementing various aspects of the functions / actions specified in one or more boxes 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 operating 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 boxes in the flowchart and / or block diagram.

[0130] The flow chart and block diagram in the accompanying drawings show the possible architecture, function and operation of the system, method and computer program product according to multiple embodiments of the present disclosure. In this regard, each square box in the flow chart or block diagram can represent a part of a module, program segment or instruction, and a part of the module, program segment or instruction includes one or more executable instructions for realizing the specified logical function. In some alternative implementations, the function marked in the square box can also occur in a sequence different from that marked in the accompanying drawings. For example, two continuous square boxes can actually be executed substantially in parallel, and they can sometimes be executed in reverse order, depending on the functions involved. It should also be noted that each square box in the block diagram and / or flow chart, and the combination of the square boxes in the block diagram and / or flow chart can be implemented with a dedicated hardware-based system that performs the specified function or action, or can be implemented with a combination of special 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 power grid adopts the IEEE-39 node system, and the flexible DC transmission is connected to the bus 14. The flexible DC transmission adopts symmetrical monopole connection, the rated DC voltage is ±800kV, and the rated transmission capacity is 900MW. During normal operation, the receiving-end converter station adopts active power control, and the control reference value is 1.0pu; the sending-end converter station adopts DC voltage control, and the control reference value is 1.0pu.

[0132] This embodiment takes the scenario that a three-phase short circuit fault occurs on bus 6 at 2.15s and lasts for 200ms. In order to verify the effect of the voltage control method of the flexible DC transmission system receiving end proposed in the present invention, the comparison group is the control method of maximizing reactive power of the receiving end converter station based on the effective short circuit ratio. The DC voltage, the voltage of the receiving end grid connection point of the flexible DC transmission system, the active power, reactive power and AC current phase angle of the receiving end converter station are 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 DC transmission system under power grid fault is 0.67 pu.

[0135] Figure 5 (a) to (e) are waveforms of DC voltage, grid connection point voltage of the flexible DC transmission system, active power, reactive power and AC current phase angle of the receiving converter station under grid fault. Figure 5 The horizontal axis is time, and the vertical axes are 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 is the curve of the receiving-end voltage control method of the flexible DC transmission system proposed in the present invention, and the dotted line is 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-end converter station to change by nearly 1.0 pu before and after the receiving-end power grid fault, and the DC voltage increases sharply to 1.25 pu before the active power of the sending-end converter station decreases to balance with the active power of the receiving-end converter station, which is much larger than the maximum allowable DC voltage deviation of 0.1 pu and may cause the flexible DC transmission to be locked.

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

[0138] This shows that the receiving-end converter station only provides reactive current under power grid fault, and ignores the fault transition impedance, resulting in a sudden change in the equivalent complex impedance of the receiving-end power grid before and after the fault, which cannot effectively support the receiving-end power grid voltage. On the contrary, the present invention supports the AC voltage by coordinating the active and reactive currents of the receiving-end converter station. Although it only provides 0.56pu of reactive power of the receiving-end converter station, it can make the voltage drop on the equivalent complex impedance of the receiving-end power grid equal to the equivalent potential phase angle under the receiving-end power grid fault, and the voltage raising effect of the receiving-end grid connection point of the flexible DC transmission system is optimal.

[0139] Although 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 the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for controlling voltage at a receiving end of a flexible direct current transmission system, characterized in that: The following steps are involved: S101: Collecting the AC bus voltage, active and reactive current of the receiving-end converter station of the flexible DC transmission before and after the power 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 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; 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-connected point of the flexible DC transmission 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-connected point of the flexible DC transmission system, executing S105, otherwise executing S106; S105: Calculate the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under power grid fault according to the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage, and then calculate the active and reactive currents of the receiving-end converter station that make the voltage of the receiving-end grid connection point of the flexible DC transmission system reach the maximum feasible value, and use them as reference values ​​for active and reactive current control of the receiving-end converter station to implement control; 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, calculate the maximum feasible voltage at the receiving-end grid-connected point of the flexible DC transmission system under power grid fault, and then calculate 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, and use them 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, characterized in that: 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: According to 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 equivalent reactance of the receiving-end power grid during normal operation is calculated; 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 and their control parameters; According to 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 and their control parameters, the virtual short-circuit fault transition reactance of the receiving-end power grid is calculated; According to 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 equivalent complex impedance under the receiving-end power grid fault is calculated; 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 under the receiving-end power grid fault is calculated.

3. The method for controlling the receiving-end voltage 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 DC 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 according to 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-end 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, characterized in that: 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 obtained by calculation according to 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; 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; According to the second parameter and the third parameter, a fourth parameter is calculated; The fifth parameter is calculated according to the active power of the receiving-end converter station during normal operation, the maximum allowable DC voltage deviation of 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; The first 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 under the receiving-end power grid fault; The second 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; 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; The second current parameter is calculated 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 for maximizing the voltage of the receiving-end grid connection point of the flexible DC transmission system, the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under power grid fault is determined as follows: The first voltage parameter is calculated according to 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; According to the equivalent resistance and reactance under the receiving-end power 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, the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under the power grid fault is calculated.

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 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 as follows: According to the second current parameter, a reference value of active current of the receiving-end converter station is calculated; According to the first current parameter, a reactive current reference value of the receiving-end converter station is calculated.

7. The method for controlling the receiving-end voltage of a flexible DC transmission system according to claim 1, characterized in that: 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 of the receiving-end grid connection point of the flexible DC transmission system, the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under power grid fault is determined as follows: Based on the equivalent reactance of the receiving-end power grid during normal operation, the equivalent resistance and reactance under the receiving-end power grid fault, the maximum allowable AC current coefficient and the rated AC current of the receiving-end converter station, the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under power grid fault is calculated.

8. The method for controlling the receiving-end voltage of a flexible DC transmission system according to claim 1, characterized in that: In step S106, when the maximum AC current phase angle of the receiving-end converter station to avoid 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 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 of the receiving-end converter station required for maximizing the voltage at the receiving-end grid connection point of the flexible DC transmission system and the product of the maximum allowable AC current coefficient of 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 of the receiving-end converter station required to maximize the voltage at the receiving-end grid connection point of the flexible DC transmission system and 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 direct current transmission system, characterized in that: include: An acquisition module is used to collect the AC bus voltage, active and reactive current of the receiving-end converter station of the flexible DC transmission before and after the power 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 of the receiving-end grid connection point of the flexible DC transmission system; A comparison module is used to compare 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-connected point of the flexible DC transmission 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-connected point of the flexible DC transmission system, the third calculation module is called, otherwise the fourth calculation module is called; The third calculation module is used to calculate the maximum feasible voltage of the receiving-end grid connection point of the flexible DC transmission system under the power grid fault according to the maximum AC current phase angle of the receiving-end converter station to avoid DC transient overvoltage, and then calculate the active and reactive currents of the receiving-end converter station that make the voltage of the receiving-end grid connection point of the flexible DC transmission system reach the maximum feasible value, and call the first control module; The fourth calculation module is used to calculate the maximum feasible voltage of the receiving-end grid-connected point of the flexible direct current transmission system under a power grid fault according to the AC current phase angle of the receiving-end converter station required for maximizing the voltage of the receiving-end grid-connected point of the flexible direct current transmission system, and then calculate the active and reactive currents of the receiving-end converter station that make the voltage of the receiving-end grid-connected point of the flexible direct current transmission system reach the maximum feasible value, and call the second control module; A first control module is used to use the active and reactive currents of the receiving-end converter station calculated by the third calculation module so that the voltage at the receiving-end grid connection point of the flexible DC power transmission system reaches the maximum feasible value as the active and reactive current control reference values ​​of the receiving-end converter station, and implement control; 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 to make the voltage at the receiving-end grid connection point of the flexible DC power transmission system reach the maximum feasible value as the active and reactive current control reference values ​​of the receiving-end converter station.

10. A voltage control device for a receiving end of 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 receiving-end voltage control method of the flexible direct current transmission system according to any one of claims 1 to 8 when executing the computer program.

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