Method and system for calculating equivalent of ac three-phase short circuit current provided by flexible dc transmission

Abstract

The present application relates to a kind of equivalent method and system of calculating flexible ac transmission provides three-phase short circuit current, it includes: according to the amplitude and phase angle of the voltage before fault, input node equation is calculated, obtains the current time converter bus voltage;According to the current time converter bus voltage, and flexible dc fault before operation mode, fault ride-through control mode and corresponding limit condition, determine the converter bus voltage of next time;Determine whether the converter bus voltage of next time meets preset condition, meet the amplitude and phase angle of corresponding equivalent voltage of next time output, to superimposed calculation flexible ac three-phase short circuit current and the short circuit current provided by flexible dc.The present application can determine the current injected by flexible dc to system during fault, realize the fast, accurate calculation of flexible dc providing three-phase short circuit current.The present application can be applied in the field of flexible dc transmission.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible DC power transmission, in particular to an equivalent method and system for calculating three-phase short-circuit current provided by flexible DC power transmission. BACKGROUND

[0002] Flexible DC power transmission technology can realize active and reactive decoupling independent control, rapid and flexible regulation, and can supply power to weak systems and island systems, which has significant advantages in improving system stability and power transmission capacity, and has been widely used in engineering applications. Compared with conventional DC, flexible DC has the advantages of small occupation area and no commutation failure, which makes it more suitable for deep load center to improve power flow distribution.

[0003] Flexible DC can realize fault ride-through and does not block during short-circuit fault, thereby avoiding the cumbersome process of restarting after fault clearing and reducing the recovery time of the system. Therefore, after the short-circuit fault of the AC system, the flexible DC still injects current into the system, thereby affecting the system short-circuit current level. The load center generally has a compact main grid structure and dense load, and faces the problem of exceeding the short-circuit current. With the increasing voltage level and capacity of the connected flexible DC, the short-circuit current level of the receiving end system approaches or exceeds the breaking limit of the circuit breaker, which makes it difficult to clear the short-circuit fault and poses a serious threat to the safe and stable operation of the system. The influence of flexible DC on the short-circuit current of the AC system cannot be ignored, and the provided short-circuit current value has become an important concern of power planning and operation departments.

[0004] Currently, the methods for improving the accuracy of calculating the three-phase short-circuit current provided by flexible DC power transmission mainly include iterative calculation based on vector superposition and electromagnetic transient simulation. Iterative calculation is to calculate the output current characteristics of flexible DC at the grid connection point after a fault, obtain the vector superposition of the flexible DC short-circuit current component and the AC side short-circuit current component, and then correct the output current of flexible DC according to the calculated grid connection point voltage. After repeated iterative calculation, the short-circuit current is obtained; electromagnetic transient simulation is to build a model of flexible DC power transmission and AC power grid in electromagnetic transient simulation software, and obtain the three-phase short-circuit current of AC through real-time simulation. Both methods can effectively improve the calculation accuracy of AC short-circuit current considering flexible DC power transmission, but the premise is to accurately depict the fault ride-through characteristics of flexible DC under the condition of considering the calculation speed and accuracy, so as to realize accurate engineering practical calculation. SUMMARY

[0005] In view of the above problems, the purpose of the present application is to provide an equivalent method and system for calculating three-phase short-circuit current provided by flexible DC power transmission, which can determine the current injected by flexible DC into the system during a fault, and realize rapid and accurate calculation of three-phase short-circuit current provided by flexible DC.

[0006] To achieve the above object, in a first aspect, the application adopts the following technical solution: An equivalent method for calculating AC three-phase short-circuit current provided by flexible DC power transmission, comprising: inputting a node equation according to the amplitude and phase angle of the pre-fault HVDC equivalent voltage to perform fault calculation, to obtain the current time converter bus voltage; determining the next time converter bus voltage according to the current time converter bus voltage, the pre-fault HVDC operation mode, the fault ride-through control mode and the corresponding limit conditions; determining whether the next time converter bus voltage meets the preset condition, and if so, outputting the amplitude and phase angle of the corresponding equivalent voltage at the next time to superimpose and calculate the AC three-phase short-circuit current of the HVDC and the short-circuit current provided by the flexible DC.

[0007] Further, the calculation of the amplitude and phase angle of the pre-fault HVDC equivalent voltage comprises:

[0008] The pre-fault flexible DC converter bus voltage is set as an initial value, and the amplitude and phase angle of the equivalent voltage source are solved according to the reference value of the current component on the d-axis and q-axis before the fault.

[0009] Further, the acquisition of the next time converter bus voltage comprises:

[0010] The reference value of the current component on the d-axis and q-axis at the current time is determined according to the pre-fault HVDC operation mode, the fault ride-through control mode and the corresponding limit conditions;

[0011] The amplitude and phase angle of the equivalent voltage at the current time are calculated according to the current time converter bus voltage and the reference value of the current component on the d-axis and q-axis at the current time;

[0012] The amplitude and phase angle of the equivalent voltage at the current time are input into the node equation to perform fault calculation, to obtain the next time converter bus voltage.

[0013] Further, the preset condition is to determine whether the absolute value of the difference between the next time converter bus voltage and the current time converter bus voltage is less than a set threshold.

[0014] In a second aspect, the application adopts the following technical solution: An equivalent system for calculating AC three-phase short-circuit current provided by flexible DC power transmission, comprising: a first processing module for inputting a node equation according to the amplitude and phase angle of the pre-fault HVDC equivalent voltage to perform fault calculation, to obtain the current time converter bus voltage; a second processing module for determining the next time converter bus voltage according to the current time converter bus voltage, the pre-fault HVDC operation mode, the fault ride-through control mode and the corresponding limit conditions; and a calculation module for determining whether the next time converter bus voltage meets the preset condition, and if so, outputting the amplitude and phase angle of the corresponding equivalent voltage at the next time to superimpose and calculate the AC three-phase short-circuit current of the HVDC and the short-circuit current provided by the flexible DC.

[0015] Further, in the first processing module, the calculation of the amplitude and phase angle of the pre-fault DC equivalent voltage comprises:

[0016] The pre-fault flexible DC bus voltage is set as an initial value, and the amplitude and phase angle of the equivalent voltage source are solved according to the reference value of the current component on the d-axis and q-axis.

[0017] Further, in the second processing module, the obtaining of the converter bus voltage at the next time comprises:

[0018] According to the pre-fault flexible DC operation mode, the fault ride-through control mode and the corresponding limit condition, the reference value of the current component on the d-axis and q-axis at the current time is determined;

[0019] According to the converter bus voltage at the current time and the reference value of the current component on the d-axis and q-axis at the current time, the amplitude and phase angle of the equivalent voltage at the current time are calculated;

[0020] The amplitude and phase angle of the equivalent voltage at the current time are input into a node equation for fault calculation to obtain the converter bus voltage at the next time.

[0021] Further, in the calculation module, the preset condition is that whether the absolute value of the difference between the converter bus voltage at the next time and the converter bus voltage at the current time is less than a set threshold value.

[0022] In a third aspect, the present application adopts the following technical solution: a computer readable storage medium storing one or more programs, the one or more programs including instructions that, when executed by a computing device, cause the computing device to perform any of the above methods.

[0023] In a fourth aspect, the present application adopts the following technical solution: a computing device comprising one or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, and the one or more programs include instructions for executing any of the above methods.

[0024] The present application has the following advantages due to the above technical solutions:

[0025] The equivalent method and system for calculating the three-phase short-circuit current of the flexible HVDC power transmission system are adopted in the application, when the short-circuit fault occurs in the power system, the flexible HVDC is equivalent to the controllable voltage source of the grid-connected voltage control according to the specific control strategy and the setting parameters of the flexible HVDC, and the three-phase short-circuit current of the flexible AC power transmission is calculated. The application is suitable for various flexible HVDC fault ride-through control strategies and system operation modes, accurately describes the fault ride-through characteristics of the flexible power transmission, improves the accuracy of calculating the short-circuit current of the fault node, ensures the calculation speed, and has engineering practical value. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a flexible HVDC power transmission system control schematic diagram in the embodiment of the application;

[0027] Figure 2a is a flexible HVDC power transmission system outer loop controller taking the fixed active power and fixed reactive power control limit effect schematic diagram in the embodiment of the application;

[0028] Figure 2b is a flexible HVDC power transmission system outer loop controller taking the fixed active power and fixed AC voltage control limit effect schematic diagram in the embodiment of the application;

[0029] Figure 2c is a flexible HVDC power transmission system outer loop controller taking the fixed DC voltage and fixed reactive power control limit effect schematic diagram in the embodiment of the application;

[0030] Figure 3 is a flowchart of the equivalent voltage source applied to the iterative calculation method for calculating the short-circuit current in the embodiment of the application;

[0031] Figure 4 is a schematic diagram of the equivalent voltage source control principle in the embodiment of the application;

[0032] Figure 5 is a simulation waveform diagram of the equivalent voltage source applied to the vector superposition method for calculating the short-circuit current in the embodiment of the application. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the embodiments of the application clearer, the technical scheme of the embodiments of the application will be described clearly and completely below with reference to the drawings of the embodiments of the application. Obviously, the described embodiments are part of the embodiments of the application, rather than all the embodiments. Based on the described embodiments of the application, all other embodiments obtained by those skilled in the art belong to the scope of protection of the application.

[0034] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments in accordance with the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, steps, operations, elements, components, and / or groups thereof, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and / or groups thereof.

[0035] In order to solve the problem that the calculation speed and accuracy of the three-phase short-circuit current provided by the flexible DC power supply cannot be considered, an equivalent method and system for calculating the three-phase short-circuit current provided by the flexible DC power supply are provided. The application comprises: according to the fault ride-through strategy and parameter setting of the flexible DC, the output characteristics of the flexible DC during the fault are determined, the equivalent voltage source model of the grid-connected point voltage control is determined, and the three-phase short-circuit current is calculated based on the electromagnetic transient simulation tracking calculation or the superposition principle iterative calculation according to the system operation mode. The application can reasonably calculate the short-circuit current level of the power system connected with the flexible DC power supply, and further provide a reference for analyzing the influence of the flexible DC on the short-circuit current under different control modes and the influence of the flexible DC on the short-circuit current of the system under different operation modes. According to the specific fault ride-through control strategy of the flexible DC, the current injected by the flexible DC to the system during the fault is determined, and the fast and accurate calculation of the three-phase short-circuit current provided by the flexible DC is realized.

[0036] The overall control structure of the flexible DC converter adopted by the application is as shown in Figure 1 The main control is double-loop decoupling control in dq coordinate system, which is divided into inner loop control and outer loop control. The inner loop control is direct current control, which can obtain excellent dynamic response performance. On the one hand, the dq axis current components are not coupled, and each forms an independent control link. On the other hand, the current is converted from alternating current to direct current, which can be quickly and accurately tracked through PI control. Through the negative feedback of state variables i d and i q , the control input V d and V q are adjusted, so that the state variables i d and i q quickly track the reference values i * d and i * q . The outer loop control can be set as constant active power control or constant DC voltage control according to the control target d-axis, and can be set as constant reactive power control or constant AC voltage control according to the q-axis. According to the control target, the flexible DC control system can be divided into four control modes, as shown in Table 1. The PI links of d and q axes are set to have amplitude limits, and a current limiting link is set to avoid overcurrent. Among them, I dmax is the amplitude limit of the d-axis PI link output, and I qmaxThe amplitude limiting for the q-axis PI link output, I max The amplitude limiting for the current limiting link output.

[0037] Table 1 Mode classification of different control objectives of d-axis and q-axis of flexible DC transmission control system

[0038]

[0039] The output characteristics during the fault of the flexible DC are closely related to the control mode. The output characteristics under different control modes, different fault ride-through strategies and current limiting link settings are different. For example, the current limiting link can take equal amplitude limiting, active power priority, reactive power priority, etc. Taking the equal amplitude limiting of the current limiting link as an example, the output characteristics of the flexible DC during the fault under different control modes are analyzed.

[0040] I. Control mode 1

[0041] When the flexible DC takes control mode 1, the components I d , I q of the output current of the flexible DC on the d-axis and the q-axis are respectively:

[0042]

[0043]

[0044] In the formula, V is the voltage of the flexible DC bus, P ref , Q ref are the set values of active power and reactive power respectively.

[0045] During the fault, the system voltage drops. If the set values of active power and reactive power P ref , Q ref are unchanged, according to the degree of decline of the voltage V of the converter bus, the I d , I q required for the flexible DC to maintain the set power unchanged during the fault can be calculated. Comparing it with the amplitude I dmax , I qmax set by the outer loop PI link and the amplitude I max set by the current limiting link, the reference values of the components of the output current of the flexible DC transmission on the d-axis and the q-axis after limiting during the fault are respectively Specifically as shown in Table 2.

[0046] Table 2 d-axis and q-axis current reference values of control mode 1 output by the current limiting link

[0047]

[0048] It is assumed that the flexible DC injects active power and absorbs reactive power into the system before the fault, Pref >0, Q ref <0, which is I d >0、I q >0. According to I d , I q The limit I set by the outer loop PI link dmax , I qmax And the limit I set by the current limiting link max After the comparison relationship is obtained, the limit In the specific distribution areas of the first quadrant, such as Figure 2a As shown in Table 2, it corresponds to Table 2. For example, after a fault occurs, the active power exceeds the limit but the reactive power does not. At this time, the calculation shows that I is in area ③, and the flexible DC output current after the limit is I'.

[0049] 2. Control Mode 2

[0050] When the flexible DC transmission adopts control mode 2, the commutation bus voltage V drops after the fault, and the constant AC voltage control adopted by the q-axis will uniformly increase the reactive power output until it is limited. After the limit, the reference values ​​of the output current components of the flexible DC transmission on the d-axis and q-axis during the fault are The details are shown in Table 3.

[0051] Table 3 Reference values ​​of d-axis and q-axis currents output by the current limiting link in control mode 2

[0052]

[0053] It can also be based on I d , I q The limit I set by the outer loop PI link dmax , I qmax And the limit I set by the current limiting link max After the comparison relationship is obtained, the limit In the specific distribution areas of the first quadrant, such as Figure 2b As shown, corresponding to Table 3. q When the limit is not reached, it indicates that the reactive power generated by the flexible DC is sufficient to compensate the grid connection point voltage to the reference value. At this time, the fault point is far away from the flexible DC grid connection point, the V drop is limited, and the short-circuit current provided by the flexible DC is small. Assuming I q =I qmax The calculated results are also conservative and only slightly overestimated.

[0054] 3. Control Mode 3

[0055] When the flexible DC takes the control mode 3, the bus voltage V of the converter drops after the fault, the output power of the converter station instantaneously decreases, the output power of the converter station is unbalanced, and then the DC voltage decreases, the d-axis takes the constant DC voltage control and increases uniformly until the limit value. Specifically, as shown in Table 4.

[0056] Table 4: d, q-axis current reference values output by the current limiting link in control mode 3

[0057]

[0058] Similarly, according to the comparison relationship of I d , I q and the limit value I dmax , I qmax and the limit value I max set by the current limiting link, the reference values of the output current of the flexible DC in the d-axis and the q-axis during the fault after the limit value are obtained. The specific distribution area in the first quadrant is shown in Table 4. In a special case, if the active power of the flexible DC transmission is 0 before the fault occurs, the DC voltage does not change after the fault, and the flexible DC only outputs the reactive power. Figure 2c

[0059] Four, control mode 4

[0060] When the flexible DC takes the control mode 4, according to the above analysis, the reference values of the output current of the flexible DC in the d-axis and the q-axis during the fault after the limit value are obtained. Specifically, as shown in Table 5.

[0061] Table 5: d, q-axis current reference values output by the current limiting link in control mode 4

[0062]

[0063] When the flexible DC takes different control modes, different fault ride-through strategies and current limiting link settings, the output characteristics of the flexible DC can be obtained according to the drop degree of the grid-connected point voltage, the current injected by the flexible DC to the system during the fault is determined, and the AC short-circuit current calculation considering the flexible DC is realized.

[0064] In an embodiment of the present application, an equivalent method for calculating the AC three-phase short-circuit current provided by the flexible DC transmission is provided. As shown in Figure 3 the equivalent method for calculating the AC three-phase short-circuit current provided by the flexible DC transmission includes the following steps:

[0065] ​1) According to the amplitude E0 and phase angle δ0 of the flexible DC equivalent voltage before the fault, the node equation is input to perform fault calculation and the current commutation bus voltage V is obtained. n ; Let n = 1;

[0066] Among them, the node equation refers to the node voltage equation in the circuit written with the node bus voltage as the variable, which can obtain the node admittance matrix of the grid structure. Traditional short-circuit current calculation is based on the node equation for fault calculation. It is expressed as a node equation in this field and in power system analysis textbooks.

[0067] 2) According to the current commutation bus voltage V n , as well as the flexible DC fault pre-fault operation mode, fault ride-through control mode and corresponding constraints, determine the commutation bus voltage V at the next moment n+1 ;

[0068] 3) Determine the commutation bus voltage V at the next moment n+1 Whether the preset conditions are met and the amplitude E and phase angle δ of the equivalent voltage corresponding to the next moment are output, the AC three-phase short-circuit current of the flexible DC and the short-circuit current provided by the flexible DC are superimposed and calculated.

[0069] The equivalent method for calculating the AC three-phase short-circuit current provided by flexible direct current transmission of the present invention can be applied to various AC three-phase short-circuit current calculation methods, such as the iterative calculation method of three-phase short-circuit current using the superposition principle.

[0070] In the above step 1), the amplitude and phase angle of the flexible DC equivalent voltage before the fault are calculated as follows: the flexible DC bus voltage before the fault is set to the initial value V0, and the reference values ​​of the current components on the d-axis and q-axis before the fault are calculated. Solve for the amplitude E0 and phase angle δ0 of the equivalent voltage source.

[0071] In this embodiment, the flexible DC converter uses a voltage source converter, but due to the use of dq decoupling control, it must rely on the grid voltage for reference. Its inner loop uses direct current control, and the output characteristics are the same as those of a current source. Therefore, the flexible DC converter can be equivalent to a current-controlled voltage source with grid-connected point voltage feedback, such as Figure 4 As shown. Figure 4 The selected calculation controller shown can obtain the amplitude and phase angle of the equivalent voltage source as follows:

[0072]

[0073]

[0074] Where, is the phase angle between the flexible DC output current I and the commutation bus voltage V, X Lis the equivalent impedance from the flexible DC converter to the grid connection point, specifically:

[0075]

[0076]

[0077] Among them, L t Commutation reactance, L arm is the converter arm reactance.

[0078] In the above step 2), obtaining the commutation bus voltage at the next moment includes the following steps:

[0079] 2.1) According to the pre-fault operation mode of the flexible DC system, the fault ride-through control mode and the corresponding constraints, determine the reference values ​​of the current components on the d-axis and q-axis at the current moment.

[0080] 2.2) Based on the current bus voltage and the reference values ​​of the current components on the d-axis and q-axis at the current moment Calculate the current equivalent voltage amplitude E n and phase angle δ n ;

[0081] 2.3) Input the current equivalent voltage amplitude and phase angle into the node equation for fault calculation to obtain the commutation bus voltage V at the next moment. n+1 .

[0082] In the above step 3), the preset condition is: to determine the commutation bus voltage V at the next moment n+1 and the current commutation bus voltage V n Is the absolute value of the difference ε less than the set threshold? If it is less than, the preset condition is met, otherwise return to step 2) and recalculate.

[0083] In this embodiment, ε can be set according to actual calculation conditions, for example, it can be set to 0.1.

[0084] In summary, the equivalent method of the present invention can better simulate the external characteristics of flexible direct current transmission during a fault, and can improve the calculation accuracy of the AC three-phase short-circuit current provided by flexible direct current transmission in the three-phase short-circuit calculation process based on the node equation.

[0085] The equivalent method of calculating the AC three-phase short-circuit current provided by flexible direct current transmission in this invention can also be applied to the calculation of AC three-phase short-circuit current in electromagnetic transient simulation. An electromagnetic transient simulation model of flexible direct current access to IEEE39 node system is established in PSCAD / EMTDC. Simulations are performed under three modes: no consideration of flexible direct current, flexible direct current detailed control, and flexible direct current equivalent voltage source. The short-circuit current comparison at the same fault point is shown in the following figure. Figure 5 As shown in the figure, the simulation results show that the AC short-circuit currents calculated using the Flexible DC detailed model and the Flexible DC equivalent voltage source model are exactly the same, with the Flexible DC providing a short-circuit current of approximately 1 kA. Compared to calculating short-circuit current using Flexible DC detailed control to establish a steady-state state and then set a fault simulation, using Flexible DC equivalent voltage source feedback tracking simulation to calculate short-circuit current significantly improves the simulation speed of electromagnetic transient three-phase AC short-circuits, reducing calculation time by at least 90%.

[0086] In one embodiment of the present invention, an equivalent system for calculating an AC three-phase short-circuit current provided by a flexible direct current transmission system is provided, comprising:

[0087] The first processing module inputs the node equation to perform fault calculation based on the amplitude and phase angle of the flexible DC equivalent voltage before the fault, and obtains the commutation bus voltage at the current moment;

[0088] The second processing module determines the commutation bus voltage at the next moment based on the current commutation bus voltage, the flexible DC system pre-fault operation mode, the fault ride-through control mode, and corresponding constraints;

[0089] The calculation module determines whether the commutation bus voltage at the next moment meets the preset conditions and outputs the amplitude and phase angle of the equivalent voltage corresponding to the next moment, so as to superimpose the calculation of the AC three-phase short-circuit current of the flexible direct current and the short-circuit current provided by the flexible direct current.

[0090] In the above-mentioned first processing module, the amplitude and phase angle of the flexible DC equivalent voltage before the fault are calculated as follows: the flexible DC bus voltage before the fault is set as the initial value, and the amplitude and phase angle of the equivalent voltage source are solved according to the reference values ​​of the current components on the d-axis and q-axis before the fault.

[0091] In the second processing module, obtaining the commutation bus voltage at the next moment includes:

[0092] Determine the reference values ​​of the current components on the d-axis and q-axis at the current moment according to the pre-fault operation mode, fault ride-through control mode and corresponding constraints of the flexible DC system;

[0093] The current value and phase angle of the equivalent voltage at the current moment are calculated based on the current bus voltage and the reference values ​​of the current components on the d-axis and q-axis at the current moment;

[0094] The amplitude and phase angle of the equivalent voltage at the current moment are input into the node equation to perform fault calculation to obtain the converter bus voltage at the next moment.

[0095] In the calculation module, the preset condition is whether the absolute value of the difference between the converter bus voltage at the next moment and the converter bus voltage at the current moment is less than a set threshold.

[0096] The system provided by the embodiment is used for executing the above-mentioned method embodiments, and the specific process and detailed content are referred to the above-mentioned embodiments, which will not be repeated here.

[0097] The computing device structure provided in the embodiment of the application can be a terminal, which can include a processor, a communications interface, a memory, a display screen and an input device. The processor, the communications interface and the memory complete mutual communication through a communication bus. The processor is used for providing computing and control capability. The memory includes a nonvolatile storage medium and an internal memory. The nonvolatile storage medium stores an operating system and a computer program. The computer program is executed by the processor to implement an equivalent method for providing AC three-phase short-circuit current of flexible DC power transmission. The internal memory provides an environment for running the operating system and the computer program in the nonvolatile storage medium. The communications interface is used for wired or wireless communication with an external terminal. The wireless communication can be realized through WIFI, a management network, NFC (near field communication) or other technologies. The display screen can be a liquid crystal display screen or an electronic ink display screen. The input device can be a touch layer overlaid on the display screen, or a key, a trackball or a touchpad arranged on the shell of the computing device, or an external keyboard, a touchpad or a mouse, etc. The processor can call logical instructions in the memory.

[0098] In addition, the logical instructions in the above-mentioned memory can be implemented in the form of a software functional unit and can be stored in a computer-readable storage medium when sold or used as an independent product. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or the part of the technical solution, can be embodied in the form of a software product. The computer software product is stored in a storage medium and includes several instructions for enabling a computer device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the method described in each embodiment of the present invention. The aforementioned storage medium includes: various media that can store program codes, such as a USB flash drive, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk or an optical disk.

[0099] In one embodiment of the present invention, a computer program product is provided, which includes a computer program stored on a non-transitory computer-readable storage medium, and the computer program includes program instructions. When the program instructions are executed by a computer, the computer can execute the methods provided by the above-mentioned method embodiments.

[0100] In one embodiment of the present invention, a non-transitory computer-readable storage medium is provided. The non-transitory computer-readable storage medium stores server instructions. The computer instructions enable a computer to execute the methods provided in the above embodiments.

[0101] The above embodiment provides a computer-readable storage medium, whose implementation principle and technical effects are similar to those of the above method embodiment, and will not be repeated here.

[0102] The present invention is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the above-mentioned embodiments. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of the processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 a process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0103] These computer program instructions can also be stored in a computer readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instructions which implement the flow Figure 1 The functions specified in the flow or flows and / or blocks Figure 1 The functions specified in the flow or flows and / or blocks

[0104] These computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flow Figure 1 The functions specified in the flow or flows and / or blocks Figure 1 The functions specified in the flow or flows and / or blocks

[0105] Finally, it should be noted that the above-mentioned embodiments are merely used to illustrate the technical solutions of the present application, rather than limit the same; even though the above-mentioned embodiments have been described in detail, those skilled in the art should understand: the technical solutions recorded in the above-mentioned embodiments can still be modified, or some technical features can be replaced by equivalent; and these modifications or replacements do not make the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. An equivalent method for calculating the AC three-phase short-circuit current provided by flexible direct current transmission, characterized in that: include: According to the amplitude and phase angle of the flexible DC equivalent voltage before the fault, the node equation is input to perform fault calculation and obtain the commutation bus voltage at the current moment; Determine the commutation bus voltage at the next moment based on the current commutation bus voltage, as well as the flexible DC system pre-fault operation mode, fault ride-through control mode, and corresponding constraints. Determine whether the commutation bus voltage at the next moment meets the preset conditions. If so, output the amplitude and phase angle of the equivalent voltage corresponding to the next moment to superimpose the calculation and take into account the AC three-phase short-circuit current of the flexible direct current; The calculation of the amplitude and phase angle of the flexible DC equivalent voltage before the fault includes: The flexible DC converter uses a voltage source converter, which treats the flexible DC as a current-controlled voltage source with grid-connection point voltage feedback. The pre-fault flexible DC bus voltage is set as the initial value, and the amplitude and phase angle of the equivalent voltage source are calculated based on the reference values ​​of the current components on the d-axis and q-axis before the fault. The amplitude and phase angle of the equivalent voltage source are: ， Where E is the amplitude; is the phase angle; is the flexible DC output current I and commutation bus voltage V The phase angle, X L is the equivalent impedance from the flexible DC converter to the grid connection point, specifically: ， in, L t commutation reactance; L arm is the converter arm reactance; are the reference values ​​of the components of the output current of the flexible DC transmission on the d-axis and q-axis during the fault period, respectively.

2. The equivalent method for calculating the AC three-phase short-circuit current provided by flexible direct current transmission according to claim 1, characterized in that: The acquisition of the commutation bus voltage at the next moment includes: Determine the reference values ​​of the current components on the d-axis and q-axis at the current moment according to the pre-fault operation mode of the flexible DC system, the fault ride-through control mode, and the corresponding constraints; The current value and phase angle of the equivalent voltage at the current moment are calculated based on the current bus voltage and the reference values ​​of the current components on the d-axis and q-axis at the current moment; The amplitude and phase angle of the equivalent voltage at the current moment are input into the node equation for fault calculation to obtain the commutation bus voltage at the next moment.

3. The equivalent method for calculating the AC three-phase short-circuit current provided by flexible direct current transmission according to claim 1, characterized in that: The preset condition is: judging whether the absolute value of the difference between the commutation bus voltage at the next moment and the commutation bus voltage at the current moment is less than a set threshold.

4. An equivalent system for calculating the three-phase AC short-circuit current provided by flexible direct current transmission, used to implement the equivalent method for calculating the three-phase AC short-circuit current provided by flexible direct current transmission as claimed in any one of claims 1 to 3, characterized in that: include: The first processing module inputs the node equation to perform fault calculation based on the amplitude and phase angle of the flexible DC equivalent voltage before the fault, and obtains the commutation bus voltage at the current moment; The second processing module determines the commutation bus voltage at the next moment based on the current commutation bus voltage, the flexible DC system pre-fault operation mode, the fault ride-through control mode, and corresponding constraints; The calculation module determines whether the commutation bus voltage at the next moment meets the preset conditions. If it does, it outputs the amplitude and phase angle of the equivalent voltage corresponding to the next moment to superimpose the calculation and take into account the AC three-phase short-circuit current of the flexible direct current.

5. The equivalent system for calculating the AC three-phase short-circuit current provided by flexible direct current transmission according to claim 4, characterized in that: In the first processing module, the calculation of the amplitude and phase angle of the flexible DC equivalent voltage before the fault includes: The VDC bus voltage before the fault is set as the initial value, and the amplitude and phase angle of the equivalent voltage source are solved according to the reference values ​​of the current components on the d-axis and q-axis before the fault.

6. The equivalent system for calculating the AC three-phase short-circuit current provided by flexible direct current transmission according to claim 4, characterized in that: In the second processing module, obtaining the commutation bus voltage at the next moment includes: Determine the reference values ​​of the current components on the d-axis and q-axis at the current moment according to the pre-fault operation mode of the flexible DC system, the fault ride-through control mode, and the corresponding constraints; The current value and phase angle of the equivalent voltage at the current moment are calculated based on the current bus voltage and the reference values ​​of the current components on the d-axis and q-axis at the current moment; The amplitude and phase angle of the equivalent voltage at the current moment are input into the node equation for fault calculation to obtain the commutation bus voltage at the next moment.

7. The method for calculating an equivalent system for providing three-phase AC short-circuit current for flexible direct current transmission according to claim 4, wherein: In the calculation module, the preset condition is: judging whether the absolute value of the difference between the commutation bus voltage at the next moment and the commutation bus voltage at the current moment is less than a set threshold.

8. A computer-readable storage medium storing one or more programs, characterized in that: The one or more programs include instructions that, when executed by a computing device, cause the computing device to perform any one of the methods of claims 1 to 3 .

9. A computing device, characterized in that include: One or more processors, a memory, and one or more programs, wherein the one or more programs are stored in the memory and configured to be executed by the one or more processors, and the one or more programs include instructions for executing any one of the methods according to claims 1 to 3.

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