Method for calculating short-circuit current of AC / DC hybrid power transmission system containing MMC

Abstract

The invention discloses a method for calculating short-circuit current of an alternating-current and direct-current hybrid power transmission system containing an MMC. The method comprises the following steps: establishing a model of the alternating-current and direct-current hybrid power transmission system; the model comprises detailed parameters and topological structures of an alternating current network, a direct current network and the MMC; an MMC fault model is decoupled and simplified into a current source, according to short-circuit fault type classification, MMC output short-circuit current calculation formulas of each classification are obtained by considering the internal inductance and capacitance of the MMC and dynamic characteristics of a control strategy of the MMC, and an overall fault model is constructed; a nodal analysis method is combined with a time domain simulation technology, and a solving equation of the short-circuit current of the AC / DC hybrid power transmission system containing the MMC is constructed. According to the method, the problem that a traditional short-circuit current calculation method is difficult to adapt to a complex current path in an MMC-containing system can be solved; the accuracy of short-circuit current calculation can be effectively improved, and a reliable basis is provided for fault protection of a system.

Description

Technical Field

[0001] The present invention relates to the technical field of power system planning and operation, and in particular to a method for calculating short-circuit current of an AC / DC hybrid power transmission system containing an MMC. Background Art

[0002] With the growth of energy demand and the large-scale access of renewable energy (such as wind and solar energy), the application of AC / DC hybrid transmission system in power transmission is gradually increasing. The traditional pure AC transmission system and pure DC transmission system have their own advantages and disadvantages, while the AC / DC hybrid power system can better meet the needs of complex power networks and improve the efficiency and stability of energy transmission by flexibly combining the advantages of both.

[0003] As an advanced power electronics technology, modular multilevel converter (MMC) plays a key role in AC / DC hybrid power transmission systems. MMC has the advantages of high efficiency, low loss, and strong voltage regulation, making it the main voltage source, current source, and power converter in modern power systems. MMC can achieve efficient energy conversion and voltage regulation between AC and DC grids, and adapt to the power flow requirements under complex network conditions.

[0004] In traditional power systems, short-circuit current calculations are usually based on simplified models and assumptions, and are applicable to a single type of power network structure (such as pure AC or pure DC systems). However, with the emergence of AC / DC hybrid transmission systems, especially those containing MMCs, traditional short-circuit current calculation methods can no longer meet the requirements for accurate analysis of the complex dynamic characteristics of the system. In power system design and operation, short-circuit current calculation is a key indicator for evaluating equipment tolerance and system protection settings. For AC / DC hybrid power systems containing MMCs, it is necessary to accurately predict the current distribution of each node and module during a short-circuit fault to ensure the safe and stable operation of the system during a fault. There is an urgent need for a solution method that can comprehensively consider the dynamic characteristics of AC / DC networks, MMCs, and their mutual influence with power electronic equipment. This method should be able to find a balance between calculation accuracy and calculation efficiency to cope with complex practical engineering application scenarios. Summary of the invention

[0005] 1. Technical problems to be solved:

[0006] In view of the above technical problems, the present invention provides a method for calculating the short-circuit current of an AC / DC hybrid power transmission system containing MMC, and a solution method that provides a guiding basis for the planning and operation of the power system.

[0007] 2. Technical solution:

[0008] A method for calculating short-circuit current of an AC / DC hybrid power transmission system containing MMC, characterized in that it comprises the following steps:

[0009] Step 1: Establish a model of the MMC-HVDC AC / DC hybrid transmission system; the model of the MMC-HVDC AC / DC hybrid transmission system couples the circuit topology of the DC grid and the AC grid included in the sending-end substation to the receiving-end substation, and corresponding PCC nodes are set at the output end of the sending-end substation and the input end of the receiving-end substation; at this time, the MMC acts as a core converter to realize DC power transmission and conversion between power grids; the PCC node is a node connecting the MMC to the public power grid.

[0010] Step 2: Decouple the MMC fault model in the MMC-HVDC AC / DC hybrid transmission system and simplify it into a current source. Classify it according to the type of short-circuit fault, and obtain the calculation formula of the short-circuit current output by the MMC corresponding to each classification, and then construct the overall fault model of the MMC-HVDC AC / DC hybrid transmission system.

[0011] Step 3: When a short circuit fault occurs in the AC / DC system containing MMC, based on the voltage source characteristics of the synchronous machine, the sub-transient potential will not change suddenly after the short circuit fault occurs in the AC / DC system containing MMC, and the verification current for verifying the short circuit current of the AC / DC hybrid transmission system containing MMC is determined.

[0012] Step 4: PQ decouple the voltage at the PCC from the short-circuit current output by the MMC, decouple the MMC fault model and simplify it into a current source model to obtain a multi-port network model based on hybrid parameters (H parameters):

[0013] Step 5: Calculate the short-circuit current of the MMC based on the equivalent simplified multi-port network model.

[0014] Step 6: Input the parameters of the AC / DC hybrid transmission system to obtain the short-circuit current at the fault point and the voltage at the PCC.

[0015] Furthermore, step 2 specifically includes:

[0016] S21: If the voltage of the PCC node is less than the preset voltage drop threshold when a fault occurs, the short-circuit current I output by the MMC is MMC As shown in formula (1):

[0017]

[0018] In the above formula: is the effective value of the positive sequence voltage at the PCC; is the positive sequence voltage phase at PCC; and They are respectively the active power and reactive power command values ​​of MMC; and They are active current and reactive current command values ​​respectively;

[0019] S22: If the voltage of the PCC node is greater than the preset voltage drop threshold when a fault occurs, the current limiting link of the outer loop controller is used to limit the active current and reactive current command values ​​in proportion. At this time, the command value of the MMC As shown in formula (2);

[0020]

[0021] In the above formula, i vdref 、i vqref The d-axis and q-axis components of the current command value output by the PI controller current limiting link of the outer loop controller respectively; i vdmax 、i vqmax They are the limit value I of the PI controller limit link vmax The d and q axis components of the MMC; at this time, the short-circuit current output by the MMC is shown in formula (3):

[0022]

[0023] S23: If the distance between the fault and the PCC node is greater than the preset electrical distance threshold, the reactive power generated by the MMC cannot be accurately determined at this time. The reactive current output by the MMC is approximately determined by the electrical distance between the PCC node and the fault point. The short-circuit current I output by the MMC at this time MMC As shown in formula (4):

[0024]

[0025] S24: If the distance between the fault and the PCC node is less than the preset electrical distance threshold, and the voltage at the PCC cannot reach the command value, under the action of the current limiting link, the MMC outputs active power according to the active power command value, and the remaining current capacity outputs reactive power, thereby achieving voltage support for the commutation bus; at this time, the MMC adopts a constant AC voltage control strategy, and the short-circuit current output by the MMC is shown in formula (5):

[0026]

[0027] S25: If the distance between the fault and the PCC node is less than the preset electrical distance threshold, and the active power output by the MMC and the voltage at the PCC cannot reach the corresponding command value, the short-circuit current output by the MMC is as shown in the following formula (6):

[0028] The short-circuit current output by the MMC is:

[0029]

[0030] Furthermore, in step 3, when a short-circuit fault occurs in the AC / DC system containing the MMC, based on the voltage source characteristics of the synchronous machine, the sub-transient potential will not change suddenly after the fault, and the initial sub-transient current I″ is taken as the short-circuit current I output by the synchronous machine. short To verify the short-circuit calculation mechanism of the AC / DC hybrid transmission system containing MMC, as shown in formula (7):

[0031] I short =I″=E″ / x′ d ′ (7)

[0032] Where: x′ d ' is the subtransient reactance, I' is the subtransient current, and E' is the subtransient voltage.

[0033] Furthermore, step four specifically includes:

[0034] S41: for a power system including a voltage source and a controlled current source, the power flow calculation result during normal steady-state operation, the equivalent voltage source at the fault point, the change in output current before and after the MMC fault, and the equivalent voltage source corresponding to the change in output current before and after the MMC failure are superimposed with the relevant quantities during normal steady-state operation to obtain the voltage and current components after the MMC failure;

[0035] S42: Since the influence of the output current variable before and after the MMC failure on the node voltage in the remote area of ​​the converter station can be ignored compared with the synchronous machine power supply in the power system with a relatively large short circuit, ignoring the voltage change at the PCC, the node voltage equation can be obtained:

[0036]

[0037] Where: Z MM ΔU is the self-impedance of the node M to which the MMC is connected; PCC1 and ΔU PCC2 are the voltage change at PCC when the equivalent voltage source at the fault point and the change in output current before and after the MMC fault act respectively; U PCC is the approximate calculation result of the voltage at PCC based on the above assumptions; U D0 And is obtained by formula (9):

[0038]

[0039] Where: Solve the short-circuit current equation under different conditions in step 2;

[0040] The voltage at the PCC after the fault is obtained by the above method, and then according to The short-circuit current output by the MMC can be obtained. At this time, the MMC is regarded as a current source to participate in the calculation of the short-circuit current of the AC power grid;

[0041] S43: Eliminate irrelevant connection nodes in the network to simplify the topological structure of the power grid, and make it equivalent to a multi-port network based on hybrid parameters (H parameters); in the multi-port network, the current source branch is defined as a first-class port, the voltage source branch is defined as a second-class port, and the required current and voltage of the branch are equivalent to the current source or voltage source branch through the substitution principle and are classified as first-class or second-class ports.

[0042] Furthermore, step five specifically includes:

[0043] S51: Number the branches, with the port branch placed in front:

[0044]

[0045] In the above formula, I c and U v are the current of the current source branch and the voltage of the voltage source branch respectively; H 11 , H 12 , H 21 , H 22 is the H parameter;

[0046] The voltage U of the current source branch is obtained c and the current I in the voltage source branch v for:

[0047]

[0048] In the above formula, I c and U v are the current of the current source branch and the voltage of the voltage source branch respectively; H 11 , H 12 , H 21 , H 22 is the H parameter;

[0049] S52: Determine the relationship between the branch voltage and the node voltage as follows:

[0050] U b =A T U n (12)

[0051] In the above formula, U n represents the voltage vector of n nodes, A represents the network association matrix;

[0052] S53: From the node voltage equation and Kirchhoff's voltage law, we can get:

[0053]

[0054] Where: Ic and U v are the current of the current source branch and the voltage of the voltage source branch, Jn is the current source vector injected into n nodes, Y n is the n-node admittance matrix, Y b is the admittance matrix of b branches, U n is the voltage vector of n nodes;

[0055] S54: Substitute equations (12) and (13) into equation (11) to derive the H parameter, as shown in equation (14):

[0056]

[0057]

[0058] Furthermore, in step 6, if there are M synchronous power sources and N flexible DC transmission systems connected in the system, the short-circuit current I D and the voltage at the PCC where the i-th flexible DC transmission system is located As shown in the following formula (15):

[0059]

[0060] In the above formula, H 11(i,k) , H 12(i,k) represents the i-th row and k-th column element in the corresponding H parameter; H 21(D,k) , H 22(D,k) Indicates the Dth row and kth column in the corresponding H parameter; I D Indicates the short-circuit current at the fault point; U vk is the voltage of the kth DC system current source branch; I MMCk is the current in the kth voltage source path of the DC system.

[0061] Furthermore, for designing an AC / DC hybrid power transmission system containing MMC, the design process includes: establishing a model of the MMC-HVDC AC / DC hybrid power transmission system in MATLAB or SIMULINK according to the short-circuit current calculation method according to claim 6; by adjusting the control mode and parameter combination of MMC-HVDC in the simulation model, the calculation results of the short-circuit current under different control modes and parameter combinations of MMC-HVDC are obtained, thereby realizing the design of the AC / DC hybrid power transmission system containing MMC.

[0062] 3. Beneficial effects:

[0063] (1) This method provides a method for calculating the short-circuit current of an AC / DC hybrid transmission system containing MMC. A model is constructed for the MMC-HVDC AC / DC hybrid transmission system. In the model, the MMC is used as the core converter, and the PCC node where the MMC is connected to the public power grid is used to facilitate the calculation of the short-circuit current.

[0064] (2) In the method for calculating the short-circuit current of an AC / DC hybrid transmission system containing MMC provided by this method, the types of short-circuit faults are classified according to the complex dynamic characteristics of the system when a short-circuit fault occurs, and a detailed mathematical model under the fault state, namely, an overall fault model, is established. The MMC AC side fault model is simplified to a current source model by approximately solving the voltage at the grid connection point (PCC) to achieve decoupling between the voltage at the PCC and the output current of the MMC. The dynamic characteristics of the MMC are fully considered, including the internal inductance of the MMC, the capacitance of the MMC, and the control strategy of the MMC.

[0065] (3) The method provides a method for calculating the short-circuit current of an AC / DC hybrid power transmission system containing MMC, which provides a solution process for constructing the short-circuit current of the AC / DC hybrid power transmission system containing MMC to achieve the final calculation of the short-circuit current.

[0066] In summary, this method utilizes the complex dynamic characteristics and control logic of the MMC module, combines the node analysis method and time domain simulation technology, and can more accurately simulate and calculate the current response of the system under short-circuit faults. By considering the actual operating status of the MMC module and the dynamic characteristics of the circuit, the accuracy of short-circuit current calculation is effectively improved, providing a reliable basis for the safety assessment of the power system and the design of protection devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 is a schematic diagram of a model of an MMC-HVDC hybrid AC / DC transmission system in this method;

[0068] Figure 2 Schematic diagram of the current and voltage limiting link of the MMC outer loop controller in this method;

[0069] Figure 3 Flowchart of this method. DETAILED DESCRIPTION

[0070] The present invention will be described in detail below with reference to the accompanying drawings.

[0071] This method is used to solve the problem of short-circuit current level verification of the modular multilevel converter (MMC) of the AC / DC transmission system in consideration of the operation mode and control system of the MMC. First, the MMC AC side fault model is established through the obtained parameters; according to the established fault model, the control method and parameters of the MMC-HVDC are further adjusted. The MMC AC side fault model is simplified to a current source model, the relationship between the voltage and the MCC output current at the PCC is decoupled, and then the short-circuit current is calculated. According to the calculated short-circuit current, the short-circuit current with less risk is selected, and then the system control method and parameters with the lowest risk of short-circuit fault are obtained.

[0072] As attached Figure 3 As shown, a method for calculating short-circuit current of an AC / DC hybrid power transmission system including MMC is characterized by comprising the following steps:

[0073] Step 1: Establish a model of the MMC-HVDC AC / DC hybrid transmission system; the model of the MMC-HVDC AC / DC hybrid transmission system couples the circuit topology of the DC grid and the AC grid included in the sending-end substation to the receiving-end substation, and corresponding PCC nodes are set at the output end of the sending-end substation and the input end of the receiving-end substation; at this time, the MMC acts as a core converter to realize DC power transmission and conversion between power grids; the PCC node is a node connecting the MMC to the public power grid.

[0074] like Figure 1 The figure shows the overall structure of the MMC-HVDC transmission system involved in the method, including the AC network, the DC network and the detailed parameters and topology of the MMC. In the transmission system, the converters at each end can usually independently control the active and reactive physical quantities, in the two-terminal MMC-HVDC transmission system using vector control. The active physical quantities include active power and DC voltage, while the reactive physical quantities include AC voltage and reactive power, wherein the reactive physical quantities include DC voltage and DC voltage, and the reactive physical quantities include DC voltage and DC voltage. If various types of short-circuit faults occur at various locations of the AC system during system operation, the voltage at the PCC will be affected to varying degrees and may decrease. The amount of power injected by the MMC system in the AC system is determined according to the voltage at the PCC end. In order to establish the AC side model of the MMC under fault conditions, the modeling analysis is carried out based on the degree of voltage reduction at the PCC. All modeling processes are carried out under per-unit conditions, and then the steady-state model of the MMC-HVDC AC / DC hybrid transmission system is constructed.

[0075] Step 2: Decouple the MMC fault model in the MMC-HVDC AC / DC hybrid transmission system and simplify it into a current source. Classify it according to the type of short-circuit fault, obtain the calculation formula of the short-circuit current output by the MMC corresponding to each classification, and then construct the overall fault model of the MMC-HVDC AC / DC hybrid transmission system;

[0076] When a short-circuit fault occurs, the dynamic characteristics of the system are complex, and the fault process needs to be classified through electromagnetic transient analysis. During the classification process, the dynamic characteristics of the internal inductance and capacitance of the MMC and the control strategy must be fully considered, and a detailed mathematical model including multiple short-circuit faults must be established.

[0077] Step 3: When a short circuit fault occurs in the AC / DC system containing MMC, based on the voltage source characteristics of the synchronous machine, the sub-transient potential will not change suddenly after the short circuit fault occurs in the AC / DC system containing MMC, and the verification current for verifying the short circuit current of the AC / DC hybrid transmission system containing MMC is determined.

[0078] Step 4: PQ decouple the voltage at the PCC and the short-circuit current output by the MMC, decouple the MMC fault model and simplify it into a current source model to obtain a multi-port network model based on hybrid parameters (H parameters).

[0079] In this step, the node analysis method is combined with time domain simulation technology to solve the short-circuit current of the AC / DC hybrid transmission system with various short-circuit faults containing MMC.

[0080] Step 5: Calculate the short-circuit current of the MMC based on the equivalent simplified multi-port network model;

[0081] Step 6: Input the parameters of the AC / DC hybrid transmission system to obtain the short-circuit current at the fault point and the voltage at the PCC.

[0082] Furthermore, step 2 specifically includes:

[0083] S21: If the voltage of the PCC node is less than the preset voltage drop threshold when a fault occurs, the short-circuit current I output by the MMC is MMC As shown in formula (1):

[0084]

[0085] In the above formula: is the effective value of the positive sequence voltage at the PCC; is the positive sequence voltage phase at PCC; and They are respectively the active power and reactive power command values ​​of MMC; and They are active current and reactive current command values ​​respectively;

[0086] S22: If the voltage of the PCC node is greater than the preset voltage drop threshold when a fault occurs, the current limiting link of the outer loop controller is used to limit the active current and reactive current command values ​​in proportion. At this time, the command value of the MMC As shown in formula (2);

[0087]

[0088] In the above formula, i vdref 、i vqref The d-axis and q-axis components of the current command value output by the PI controller current limiting link of the outer loop controller respectively; ivdmax 、i vqmax They are the limit value I of the PI controller limit link vmax The d and q axis components of the MMC; at this time, the short-circuit current output by the MMC is shown in formula (3):

[0089]

[0090] S23: If the distance between the fault and the PCC node is greater than the preset electrical distance threshold, the reactive power generated by the MMC cannot be accurately determined at this time. The reactive current output by the MMC is approximately determined by the electrical distance between the PCC node and the fault point. The short-circuit current I output by the MMC at this time MMC As shown in formula (4):

[0091]

[0092] S24: If the distance between the fault and the PCC node is less than the preset electrical distance threshold, and the voltage at the PCC cannot reach the command value, under the action of the current limiting link, the MMC outputs active power according to the active power command value, and the remaining current capacity outputs reactive power, thereby achieving voltage support for the commutation bus; at this time, the MMC adopts a constant AC voltage control strategy, and the short-circuit current output by the MMC is shown in formula (5):

[0093]

[0094] S25: If the distance between the fault and the PCC node is less than the preset electrical distance threshold, and the active power output by the MMC and the voltage at the PCC cannot reach the corresponding command value, the short-circuit current output by the MMC is as shown in the following formula (6):

[0095] The short-circuit current output by the MMC is:

[0096]

[0097] Furthermore, in step 3, when a short-circuit fault occurs in the AC / DC system containing the MMC, based on the voltage source characteristics of the synchronous machine, the sub-transient potential will not change suddenly after the fault, and the initial sub-transient current I″ is taken as the short-circuit current I output by the synchronous machine. short To verify the short-circuit calculation mechanism of the AC / DC hybrid transmission system containing MMC, as shown in formula (7):

[0098] I short =I″=E″ / x″ d (7)

[0099] Where: x″ d is the subtransient reactance, I″ is the subtransient current, and E″ is the subtransient voltage.

[0100] Furthermore, step four specifically includes:

[0101] S41: for a power system including a voltage source and a controlled current source, the power flow calculation result during normal steady-state operation, the equivalent voltage source at the fault point, the change in output current before and after the MMC fault, and the equivalent voltage source corresponding to the change in output current before and after the MMC failure are superimposed with the relevant quantities during normal steady-state operation to obtain the voltage and current components after the MMC failure;

[0102] S42: Since the influence of the output current variable before and after the MMC failure on the node voltage in the remote area of ​​the converter station can be ignored compared with the synchronous machine power supply in the power system with a relatively large short circuit, ignoring the voltage change at the PCC, the node voltage equation can be obtained:

[0103]

[0104] Where: Z MM ΔU is the self-impedance of the node M to which the MMC is connected; PCC1 and ΔU PCC2 are the voltage change at PCC when the equivalent voltage source at the fault point and the change in output current before and after the MMC fault act respectively; U PCC is the approximate calculation result of the voltage at PCC based on the above assumptions; U D0 And is obtained by formula (9):

[0105]

[0106] Where: Solve the short-circuit current equation under different conditions in step 2;

[0107] The voltage at the PCC after the fault is obtained by the above method, and then according to The short-circuit current output by the MMC can be obtained. At this time, the MMC is regarded as a current source to participate in the calculation of the short-circuit current of the AC power grid;

[0108] S43: Eliminate irrelevant connection nodes in the network to simplify the topological structure of the power grid, and make it equivalent to a multi-port network based on hybrid parameters (H parameters); in the multi-port network, the current source branch is defined as a first-class port, the voltage source branch is defined as a second-class port, and the required current and voltage of the branch are equivalent to the current source or voltage source branch through the substitution principle and are classified as first-class or second-class ports.

[0109] In this method, in the process of calculating the short-circuit current, directly analyzing the relationship between the voltage at the PCC (common coupling point) and the short-circuit current output by the MMC (modular multilevel converter) may increase the computational complexity, thereby affecting the analysis efficiency. Therefore, by decoupling the above steps, the voltage at the PCC is separated from the short-circuit current output by the MMC, which can greatly simplify the entire analysis process. By approximately solving the voltage at the PCC, the complex electromagnetic response model of the MMC under a short-circuit fault can be converted into a more manageable current source model. This method not only makes the originally highly coupled system easier to understand and analyze, but also improves the accuracy and efficiency of short-circuit current calculation in practical applications. After being simplified to a current source model, researchers can more directly analyze and predict the current response of the system under a short-circuit fault, thereby providing stronger support for system protection design.

[0110] Furthermore, step five specifically includes:

[0111] S51: Number the branches, with the port branch placed in front:

[0112]

[0113] In the above formula, I c and U v are the current of the current source branch and the voltage of the voltage source branch respectively; H 11 , H 12 , H 21 , H 22 is the H parameter;

[0114] The voltage U of the current source branch is obtained c and the current I in the voltage source branch v for:

[0115]

[0116] In the above formula, I c and U v are the current of the current source branch and the voltage of the voltage source branch respectively; H 11 , H 12 , H 21 , H 22 is the H parameter;

[0117] S52: Determine the relationship between the branch voltage and the node voltage as follows:

[0118] U b =A T U n (12)

[0119] In the above formula, U nrepresents the voltage vector of n nodes, A represents the network association matrix;

[0120] S53: From the node voltage equation and Kirchhoff's voltage law, we can get:

[0121]

[0122] Where: Ic and U v are the current of the current source branch and the voltage of the voltage source branch, J n is the current source vector injected into n nodes, Y n is the n-node admittance matrix, Y b is the admittance matrix of b branches, U n is the voltage vector of n nodes;

[0123] S54: Substitute equations (12) and (13) into equation (11) to derive the H parameter, as shown in equation (14):

[0124]

[0125]

[0126] Furthermore, in step 6, if there are M synchronous power sources and N flexible DC transmission systems connected in the system, the short-circuit current I D and the voltage at the PCC where the i-th flexible DC transmission system is located As shown in the following formula (15):

[0127]

[0128] In the above formula, H 11(i,k) , H 12(i,k) represents the i-th row and k-th column element in the corresponding H parameter; H 21(D,k) , H 22(D,k) Indicates the Dth row and kth column in the corresponding H parameter; I D Indicates the short-circuit current at the fault point; U vk is the voltage of the kth DC system current source branch; I MMCk is the current in the kth voltage source path of the DC system.

[0129] Furthermore, for designing an AC / DC hybrid power transmission system containing MMC, the design process includes: establishing a model of the MMC-HVDC AC / DC hybrid power transmission system in MATLAB or SIMULINK according to the short-circuit current calculation method according to claim 6; by adjusting the control mode and parameter combination of MMC-HVDC in the simulation model, the calculation results of the short-circuit current under different control modes and parameter combinations of MMC-HVDC are obtained, thereby realizing the design of the AC / DC hybrid power transmission system containing MMC.

[0130] In order to plan and design an AC / DC hybrid power transmission system containing a modular multilevel converter (MMC), a short-circuit fault model simulation model of an AC / DC hybrid power system containing a modular multilevel converter (MMC) can be established in MATLAB or SIMULINK according to the short-circuit current calculation method of the present invention. By adjusting the control mode and parameter combination of MMC-HVDC in the simulation model, the calculation results of the short-circuit current under different control modes and parameter combinations of MMC-HVDC can be obtained conveniently and quickly, thereby guiding the planning and design of an AC / DC hybrid power transmission system containing a modular multilevel converter (MMC).

[0131] Although the present invention has been disclosed as above in terms of preferred embodiments, they are not intended to limit the present invention. Anyone skilled in the art can make various changes or modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection defined by the claims of this application.

Claims

1. A method for calculating short-circuit current of an AC / DC hybrid power transmission system containing MMC, characterized in that: The following steps are involved: Step 1: Establish a model of the MMC-HVDC AC / DC hybrid transmission system; the model of the MMC-HVDC AC / DC hybrid transmission system couples the circuit topology of the DC grid and the AC grid from the sending-end substation to the receiving-end substation, and sets corresponding PCC nodes at the output end of the sending-end substation and the input end of the receiving-end substation; at this time, the MMC acts as a core converter to realize DC power transmission and conversion between power grids; the PCC node is a node connecting the MMC to the public power grid; Step 2: Decouple the MMC fault model in the MMC-HVDC AC / DC hybrid transmission system and simplify it into a current source. Classify it according to the type of short-circuit fault, obtain the calculation formula of the short-circuit current output by the MMC corresponding to each classification, and then construct the overall fault model of the MMC-HVDC AC / DC hybrid transmission system; Step 3: When a short-circuit fault occurs in the AC / DC system containing the MMC, based on the voltage source characteristics of the synchronous machine, the sub-transient potential will not change suddenly after the short-circuit fault occurs in the AC / DC system containing the MMC, and the calibration current for calibrating the short-circuit current of the AC / DC hybrid transmission system containing the MMC is determined; Step 4: PQ decouple the voltage at the PCC from the short-circuit current output by the MMC, decouple the MMC fault model and simplify it into a current source model to obtain a multi-port network model based on hybrid parameters (H parameters): Step 5: Calculate the short-circuit current of the MMC based on the equivalent simplified multi-port network model; Step 6: Input the parameters of the AC / DC hybrid transmission system to obtain the short-circuit current at the fault point and the voltage at the PCC.

2. The method for calculating short-circuit current of an AC / DC hybrid power transmission system containing MMC according to claim 1, characterized in that: Step 2 specifically includes: S21: If the voltage of the PCC node is less than the preset voltage drop threshold when a fault occurs, the short-circuit current I output by the MMC is MMC As shown in formula (1): In the above formula: is the effective value of positive sequence voltage at PCC; is the positive sequence voltage phase at PCC; P s * and They are the active power and reactive power command values ​​of MMC respectively; and They are active current and reactive current command values ​​respectively; S22: If the voltage of the PCC node is greater than the preset voltage drop threshold when a fault occurs, the current limiting link of the outer loop controller is used to limit the active current and reactive current command values ​​in proportion. At this time, the command value of the MMC As shown in formula (2); In the above formula, i vdref 、i vqref The d-axis and q-axis components of the current command value output by the PI controller current limiting link of the outer loop controller respectively; i vdmax 、i vqmax They are the limit value I of the PI controller limit link vmax The d and q axis components of the MMC; at this time, the short-circuit current output by the MMC is shown in formula (3): S23: If the distance between the fault and the PCC node is greater than the preset electrical distance threshold, the reactive power generated by the MMC cannot be accurately determined at this time. The reactive current output by the MMC is approximately determined by the electrical distance between the PCC node and the fault point. The short-circuit current I output by the MMC at this time MMC As shown in formula (4): S24: If the distance between the fault and the PCC node is less than the preset electrical distance threshold, and the voltage at the PCC cannot reach the command value, under the action of the current limiting link, the MMC outputs active power according to the active power command value, and the remaining current capacity outputs reactive power, thereby achieving voltage support for the commutation bus; at this time, the MMC adopts a constant AC voltage control strategy, and the short-circuit current output by the MMC is shown in formula (5): S25: If the distance between the fault and the PCC node is less than the preset electrical distance threshold, and the active power output by the MMC and the voltage at the PCC cannot reach the corresponding command value, the short-circuit current output by the MMC is as shown in the following formula (6): The short-circuit current output by the MMC is:

3. The method for calculating short-circuit current of an AC / DC hybrid power transmission system containing MMC according to claim 2, characterized in that: In step 3, when a short circuit occurs in the AC / DC system containing the MMC, based on the voltage source characteristics of the synchronous machine, the sub-transient potential will not change suddenly after the fault, and the starting sub-transient current I ″ As the short-circuit current I of the synchronous machine output short To verify the short-circuit calculation mechanism of the AC / DC hybrid transmission system containing MMC, as shown in formula (7): Where: x′ d ' is the subtransient reactance, I' is the subtransient current, and E' is the subtransient voltage.

4. The method for calculating short-circuit current of an AC / DC hybrid power transmission system containing MMC according to claim 3, characterized in that: Step 4 specifically includes: S41: for a power system including a voltage source and a controlled current source, the power flow calculation result during normal steady-state operation, the equivalent voltage source at the fault point, the change in output current before and after the MMC fault, and the equivalent voltage source corresponding to the change in output current before and after the MMC failure are superimposed with the relevant quantities during normal steady-state operation to obtain the voltage and current components after the MMC failure; S42: Since the influence of the output current variable before and after the MMC failure on the node voltage in the remote area of ​​the converter station can be ignored compared with the synchronous machine power supply in the power system with a relatively large short circuit, ignoring the voltage change at the PCC, the node voltage equation can be obtained: Where: Z MM ΔU is the self-impedance of the node M to which the MMC is connected; PCC1 and ΔU PCC2 are the voltage change at PCC when the equivalent voltage source at the fault point and the change in output current before and after the MMC fault act respectively; U PCC is the approximate calculation result of the voltage at PCC based on the above assumptions; U D0 And is obtained by formula (9): Where: Solve the short-circuit current equation under different conditions in step 2; The voltage at the PCC after the fault is obtained by the above method, and then according to The short-circuit current output by the MMC can be obtained. At this time, the MMC is regarded as a current source to participate in the calculation of the short-circuit current of the AC power grid; S43: Eliminate irrelevant connection nodes in the network to simplify the topological structure of the power grid, and make it equivalent to a multi-port network based on hybrid parameters (H parameters); in the multi-port network, the current source branch is defined as a first-class port, the voltage source branch is defined as a second-class port, and the required current and voltage of the branch are equivalent to the current source or voltage source branch through the substitution principle and are classified as first-class or second-class ports.

5. The method for calculating short-circuit current of an AC / DC hybrid power transmission system containing MMC according to claim 1, characterized in that: Step 5 specifically includes: S51: Number the branches, with the port branch placed in front: In the above formula, I c and U v are the current of the current source branch and the voltage of the voltage source branch respectively; H 11 , H 12 , H 21 , H 22 is the H parameter; Get the voltage U of the current source branch c and the current I in the voltage source branch v for: In the above formula, I c and U v are the current of the current source branch and the voltage of the voltage source branch respectively; H 11 , H 12 , H 21 , H 22 is the H parameter; S52: Determine the relationship between the branch voltage and the node voltage as follows: U b =A T U n (12) In the above formula, U n represents the voltage vector of n nodes, A represents the network association matrix; S53: From the node voltage equation and Kirchhoff's voltage law, we can get: Where: Ic and U v are the current of the current source branch and the voltage of the voltage source branch, J n is the current source vector injected into n nodes, Y n is the n-node admittance matrix, Y b is the admittance matrix of b branches, U n is the voltage vector of n nodes; S54: Substitute equations (12) and (13) into equation (11) to derive the H parameter, as shown in equation (14):

6. The method for calculating short-circuit current of an AC / DC hybrid power transmission system containing MMC according to claim 5, characterized in that: In step 6, if there are M synchronous power sources and N flexible DC transmission systems connected in the system, the short-circuit current I D and the voltage u at the PCC where the i-th flexible DC transmission system is located pcci As shown in the following formula (15): In the above formula, H 11(i,k) , H 12(i,k) represents the i-th row and k-th column element in the corresponding H parameter; H 21(D,k) , H 22(D,k) Indicates the Dth row and kth column in the corresponding H parameter; I D Indicates the short-circuit current at the fault point; U vk is the voltage of the kth DC system current source branch; I MMCk is the current in the kth voltage source path of the DC system.

7. The method for calculating short-circuit current of an AC / DC hybrid power transmission system containing MMC according to claim 6, characterized in that: The invention is used for designing an AC / DC hybrid power transmission system containing MMC. The design process includes: establishing a model of the MMC-HVDC AC / DC hybrid power transmission system according to the short-circuit current calculation method according to claim 6 in MATLAB or SIMULINK; obtaining the calculation results of the short-circuit current under different control modes and parameter combinations of MMC-HVDC by adjusting the control mode and parameter combination of MMC-HVDC in the simulation model, thereby realizing the design of the AC / DC hybrid power transmission system containing MMC.

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