LCC-HVDC operation range optimization method and system considering transient overvoltage constraint

Abstract

The invention discloses an LCC-HVDC operation range optimization method and system considering transient overvoltage constraint, and belongs to the field of power system automation. The method comprises the following steps: calculating the capacity of a matched capacitor based on the power system topology and the LCC-HVDC transmission power; carrying out load flow calculation in combination with a capacitor scheme, initializing equipment parameters, establishing a quasi-steady-state model of each equipment, and solving a quasi-steady state at the moment of maximum reactive power consumption during commutation failure; based on the model and the result, solving the quasi-steady state at the initial moment of the commutation failure recovery stage; and calculating the node voltage of the rectifier station, optimizing the LCC-HVDC power and equipment parameters along the gradient if the node voltage exceeds the limit, and iterating until the transient voltage safety is met, otherwise, determining that the current scheme is an operation scheme. The system comprises a corresponding function module and a data storage module. The method can be applied to an actual hybrid system, the transient overvoltage is accurately evaluated, the LCC-HVDC safety power range is determined, the power grid safety is guaranteed, and the new energy consumption efficiency is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power systems and their automation, and particularly relates to a LCC-HVDC operating range optimization method and system considering transient overvoltage constraints. BACKGROUND

[0002] The installed capacity of clean energy in the power system continues to increase, and large-scale high-proportion new energy units are continuously connected to the power grid. At the same time, the construction and operation of multi-direct current transmission lines represented by ultra-high voltage direct current (UHVDC) transmission make it possible to send large-capacity clean energy across regions. Under the new pattern of the power system, direct current transmission technology is of great significance to promote efficient consumption of renewable energy, alleviate the tension of traditional transmission corridors, and improve the flexibility and safety of power transmission.

[0003] Among many direct current transmission technologies, the high-voltage direct current transmission system (LCC-HVDC) based on line commutated converter (LCC) has been widely used in international and domestic projects due to its mature technology route and advantages of large-capacity and long-distance power transmission. However, with the coupling of multiple LCC-HVDC transmission systems, large-scale new energy bases, and complex alternating current power grids, new operation risks and technical challenges of the power system have emerged.

[0004] Under the condition of deep coupling of LCC-HVDC system with high-proportion new energy power sources and multiple direct current channels, the operating state of the LCC-HVDC system is no longer a simple linear relationship in a single scenario, but presents complex characteristics of multi-time scale and multi-device coordination. When a short-circuit fault occurs in the alternating current system on the inverter side of the LCC-HVDC system, causing commutation failure of the direct current transmission system, the commutation failure and recovery stage dynamic process will cause transient overvoltage in the direct current near zone of the sending end power grid, which may cause potential damage to the equipment, such as insulation breakdown, valve group over-stress, etc. The traditional LCC-HVDC operating range optimization method often only focuses on the control dynamic process of the direct current system itself, and ignores the further aggravation of the system transient overvoltage phenomenon caused by the fault ride-through control switching characteristics of the new energy equipment in the direct current near zone. Therefore, the existing technical route often underestimates the degree of transient overvoltage in the sending end power grid caused by direct current commutation failure, thereby threatening the safe operation of the power system. SUMMARY

[0005] The technical problems to be solved by the present application are to provide a LCC-HVDC operating range optimization method and system considering transient overvoltage constraints, without considering the equivalence of the system to a single-machine system, which can be applied to the LCC-HVDC operating range optimization of an actual operating AC-DC hybrid power system containing a high proportion of new energy equipment, and used to solve the technical problem that the influence of the interaction between the LCC-HVDC itself and the new energy equipment on the transient overvoltage of the power system is ignored while only considering the dynamic characteristics of the LCC-HVDC itself.

[0006] The present application adopts the following technical solutions: A LCC-HVDC operating range optimization method considering transient overvoltage constraints, comprising the following steps: S1, based on the network topology structure of the power system to be studied, calculating the supporting capacitor capacity according to the transmission power of the current LCC-HVDC system; S2, performing AC-DC hybrid power system power flow calculation according to the obtained supporting capacitor capacity, initializing the parameters of each device in the system according to the power flow calculation results, and establishing a quasi-steady state model of each device in the power system, and solving the quasi-steady state of the power system at the moment when the reactive power consumption is maximum during the commutation failure of the LCC-HVDC system; S3, based on the established quasi-steady state model of each device and the solved quasi-steady state results of the power system during the commutation failure, solving the quasi-steady state of the power system at the initial moment of the commutation failure recovery stage of the LCC-HVDC system; S4, calculating the node voltage of the rectifier station of the LCC-HVDC system at the initial moment of the commutation failure recovery stage, and if the voltage exceeds the transient overvoltage limit value, optimizing the transmission power of the LCC-HVDC system and the supporting device parameters along the gradient direction of the node voltage to the DC steady-state power, and repeating the above steps until the node voltage is within the safety threshold; if not, the current LCC-HVDC system transmission power and supporting device parameters are taken as the actual power grid operation scheme.

[0007] Preferably, step S1 is specifically: According to the LCC-HVDC system primary side system equation, the reactive power consumed by the rectifier and inverter stations is calculated; the supporting capacitor is configured to emit reactive power equal to the reactive power consumed by the converter station.

[0008] Preferably, the supporting capacitor is:

[0009] Wherein, , Gss and Gsd are the admittance of the reactive power capacitors matched with the converter stations on both sides of the LCC-HVDC system; , Pss and Psd are the transmission power of the LCC-HVDC system on both sides at the steady state moment. , respectively are the power factor angles of the LCC-HVDC system at the steady state moment at the two sides of the converter station; , respectively are the node voltages of the LCC-HVDC system at the steady state moment at the two sides of the converter station.

[0010] Preferably, in step S2, the specific process of solving the quasi-steady state of the power system at the moment of maximum reactive power consumption during the commutation failure of the LCC-HVDC system is as follows: First, the definition of the quasi-steady state of the power system is determined, that is, the controllers of the power electronic devices of the whole system are controlled to reach a constant value, the synchronous generator is in the initial stage of the electromechanical transient process, and the high-voltage direct-current transmission system, the new energy device, and the load all belong to the category of power electronic devices; Then, the iterative equation for solving the quasi-steady state of the power system is established; Finally, based on the above iterative equation, combined with the quasi-steady state model of each device, the quasi-steady state of the power system at the moment of maximum reactive power consumption during the commutation failure is solved.

[0011] Preferably, the step of establishing the quasi-steady state model of the commutation failure LCC-HVDC system comprises: According to the equivalent circuit during the commutation failure, the maximum DC current and the trigger advance angle corresponding to the moment are calculated; Based on the maximum DC current and the trigger advance angle, the d-axis and q-axis currents injected by the LCC-HVDC system into the power grid are calculated; The specific way to establish the quasi-steady state model of the normally operating LCC-HVDC system is as follows: Due to the influence of the commutation failure of the LCC-HVDC system, the voltage of the rectifier station decreases, in order to maintain the DC voltage, the trigger advance angle of the rectifier side decreases, and the inverter side is in the constant extinction angle control mode; Based on the above operating characteristics, combined with the electrical parameters of the LCC-HVDC system, the d-axis and q-axis currents injected by the rectifier station of the normally operating LCC-HVDC system into the power grid are determined, and the quasi-steady state model is established.

[0012] Preferably, the specific way to establish the quasi-steady state model of the new energy device is as follows: The new energy device is divided into normal control mode and fault ride-through control mode according to the voltage amplitude of the grid-connected point, and a control word is introduced to represent its control state. When the grid-connected point voltage is less than the ride-through voltage threshold, the device enters the fault ride-through control state; when the grid-connected point voltage is greater than or equal to the ride-through voltage threshold, the device is in the normal control mode; Determine the injection current of the new energy equipment based on active power and reactive power reference values in normal control mode, and the injection current based on fault ride-through active control coefficient, fault ride-through current maximum limit value and fault ride-through reactive control coefficient in fault ride-through control mode, respectively. Finally, the injection current expression of the new energy equipment to the power grid is obtained, and the quasi-steady state model is established,

[0013] Among them, is the control state control word of the new energy equipment, when the voltage of the grid-connected point of the new energy equipment is less than the ride-through voltage threshold , it indicates that the new energy equipment enters the fault ride-through control state, when is greater than or equal to , it indicates that the equipment is in normal control mode. are the injection currents of the new energy equipment in normal control mode and fault ride-through control mode to the power grid, respectively. are the active power and reactive power reference values of the new energy equipment in normal control mode, respectively. is the fault ride-through active control coefficient of the new energy equipment, is the fault ride-through current maximum limit value of the new energy equipment, is the fault ride-through reactive control coefficient of the new energy equipment.

[0014] Preferably, the specific way of establishing the load quasi-steady state model is: The ZIP load model is adopted, which is only related to the voltage amplitude of the node where it is located. The active power and reactive power expressions of the load at the fault moment are:

[0015] Among them, respectively represent the current amplitude and the initial amplitude of the voltage of the node where the load is located; represent the initial active power and reactive power of the load; are the actual active power and reactive power of the load at the current voltage level; and are the constant impedance, constant current and constant power proportion coefficients of the active power and reactive power of the load, respectively, and the active power and reactive power proportion coefficients need to satisfy: , ; Based on the above power expression, the injection current of the load to the power grid is derived, and the quasi-steady state model is established.

[0016] Preferably, the specific way of establishing the synchronous generator quasi-steady state model is: Considering that the time for power electronic equipment to reach quasi-steady state is 20-40 ms, at which time the sub-transient process of the synchronous generator has ended and the transient process is in the initial stage, a dual-axis model is used for modeling; To exclude the influence of the swing of the power angle of the synchronous generator, the initial power angle level before the fault is used in combination with the direct-axis and quadrature-axis potential of the synchronous generator to determine the injection current of the synchronous generator to the power grid, and the quasi-steady state model is established, and the injection current of the synchronous generator to the power grid is:

[0017] wherein, are the direct-axis and quadrature-axis potential of the synchronous generator, respectively.

[0018] Preferably, the step of optimizing the transmission power of the LCC-HVDC includes: calculating the partial derivative of the node voltage to the capacitor current; updating the DC transmission power in the gradient direction according to the partial derivative result, and recalculating the parameters of the matching capacitor, specifically: when the node voltage of the rectifier station of the commutation failure LCC-HVDC system is greater than the transient overvoltage limit value, the LCC-HVDC system transmission power is updated in the gradient decreasing direction, when the node voltage of the rectifier station of the commutation failure LCC-HVDC system is much smaller than the transient overvoltage limit value, the LCC-HVDC system transmission power is updated in the gradient increasing direction.

[0019] In the second aspect, the embodiments of the present application provide an LCC-HVDC operating range optimization system considering transient overvoltage constraints, which includes: The capacity module calculates the capacity of the matching capacitor based on the network topology of the power system to be studied according to the transmission power of the current LCC-HVDC system; The construction module performs AC-DC hybrid power system power flow calculation according to the obtained capacity of the matching capacitor, initializes the parameters of each device in the system according to the power flow calculation result, and establishes the quasi-steady state model of each device in the power system, and solves the quasi-steady state of the power system at the moment when the reactive power consumption is maximum during the commutation failure of the LCC-HVDC system; The evaluation module solves the quasi-steady state of the power system at the initial moment of the commutation failure recovery stage of the LCC-HVDC system based on the established quasi-steady state model of each device and the solved quasi-steady state result of the power system during the commutation failure; The output module calculates the node voltage of the rectifier station of the LCC-HVDC system at the initial time of the commutation failure recovery stage, and when the voltage exceeds the transient overvoltage limit value, the LCC-HVDC system transmission power and the matching equipment parameters are optimized in the gradient direction of the node voltage to the direct-current steady-state power, and the above steps are repeated until the node voltage is within the safety threshold; if not, the current LCC-HVDC system transmission power and the matching equipment parameters are taken as the actual power grid operation scheme.

[0020] In a third aspect, a computer device includes a memory, a processor, and a computer program stored in the memory and executable on the processor, and the processor implements the steps of the LCC-HVDC operation range optimization method considering transient overvoltage constraints when executing the computer program.

[0021] In a fourth aspect, an embodiment of the present application provides a computer readable storage medium including a computer program, and the computer program implements the steps of the LCC-HVDC operation range optimization method considering transient overvoltage constraints when executed by a processor.

[0022] In a fifth aspect, a chip includes a memory, a processor, and a computer program stored in the memory and executable on the processor, and the processor implements the steps of the LCC-HVDC operation range optimization method considering transient overvoltage constraints when executing the computer program.

[0023] In a sixth aspect, an embodiment of the present application provides an electronic device including a computer program, and the computer program implements the steps of the LCC-HVDC operation range optimization method considering transient overvoltage constraints when executed by the electronic device.

[0024] Compared with the prior art, the present application has at least the following beneficial effects: The application discloses a LCC-HVDC operation range optimization method considering transient overvoltage constraints, which can accurately calculate and configure the matching capacitor capacity based on the power system network topology and the current LCC-HVDC transmission power, ensure that the voltage and reactive power of the system are effectively compensated during normal operation, match the reactive power consumption of the LCC-HVDC matching capacitor and the converter station on both sides, prevent low-voltage operation of the power system, and improve the stability of the system; under the specific fault condition of DC commutation failure, the key two stages of the dynamic response process of the LCC-HVDC system during the fault are captured. The detailed models of the commutation failure LCC-HVDC, normal LCC-HVDC, new energy equipment, load and synchronous generator are covered, so that the dynamic behavior of each device during the fault can be accurately reflected. The power system does not need to be equivalently processed, and is suitable for a multi-loop DC transmission complex AC system containing a high proportion of new energy equipment; the key time section at the initial moment of the commutation failure recovery stage of the LCC-HVDC is selected, the node voltage of the commutation failure recovery stage is calculated, and the LCC-HVDC transmission power and the matching equipment are optimized along the power gradient direction, so that the transient overvoltage does not exceed the safety threshold; the dynamic interaction between the LCC-HVDC and the new energy equipment in the AC-DC hybrid system can be accurately reflected, and the accurate evaluation and optimization capability of the transient overvoltage in the commutation failure and recovery process is highlighted. Through multi-time scale modeling and gradient optimization, the overvoltage underestimation problem caused by ignoring the response of the new energy equipment in the traditional method is effectively avoided, and the safety and new energy consumption capacity of the system are improved.

[0025] Further, the reactive power consumption of the converter station on both sides of the LCC-HVDC can be accurately quantified, and the low-voltage operation of the system caused by insufficient reactive power compensation can be avoided. The capacitor susceptance formula derived based on the reactive power balance principle ensures accurate configuration of the matching capacitor capacity, so that the reactive power compensation demand can be met without excessive capacity, and resources are not wasted, thereby laying a reliable foundation for subsequent AC-DC hybrid system power flow calculation and quasi-steady state analysis, and improving the steady-state operation stability of the system.

[0026] Further, by focusing on the key working condition of the maximum reactive power consumption during commutation failure, the most dangerous voltage trough moment of the system can be accurately captured, and the optimistic evaluation result caused by ignoring the extreme working condition can be avoided. The clear definition of the quasi-steady state and the definition of the device category unify the boundary conditions of modeling and solving, ensure that the establishment of the quasi-steady state model of each device has a consistent time scale and analysis benchmark, reduce the calculation error caused by fuzzy boundaries, and improve the accuracy and reliability of the transient characteristic evaluation of the system.

[0027] Further, the association of power electronic devices with the injection current and node voltage, and the association of synchronous generator injection current with power angle, accurately reflect the device characteristics, enabling the equation to truly simulate the electrical characteristics of the system in the quasi-steady state. Based on the equation solution, each device model can be efficiently integrated, avoiding the disconnection of results caused by dispersed calculation, and improving the efficiency and accuracy of quasi-steady state solution.

[0028] Further, the quasi-steady state model is constructed one by one according to the device type, achieving fine and full coverage of modeling, and avoiding distortion of system characteristics due to missing or excessive simplification of part of the device model. The modeling scheme is customized according to the operating characteristics of different devices to ensure that the model can accurately reflect the true behavior of the device in the transient process. The supplement of the device model at the initial time of the recovery phase improves the modeling of the whole cycle of "phase failure-recovery", covers the key recovery nodes after the fault, and improves the integrity and comprehensiveness of the evaluation.

[0029] Further, the introduction of control words enables the model to automatically switch control modes, accurately simulating the different operating states of new energy devices under normal and fault voltage conditions, and avoiding errors caused by fixed control modes. The introduction of various parameters in the current expression quantifies the influence of new energy device fault ride-through strategies on injection current, enabling the model to reflect the transient response differences under different device parameters or control strategies. This model provides accurate basis for subsequent evaluation of the aggravating effect of new energy devices on transient overvoltage, improving the pertinence of the evaluation.

[0030] Further, the ZIP model takes into account the constant impedance, constant current and constant power characteristics of the load, which is more consistent with the actual voltage response law of the load than the single characteristic model, avoiding calculation bias of load power due to model simplification. The introduction of various parameters in the fault instantaneous power formula quantifies the influence of voltage changes on load power, and the proportionality coefficient constraint ensures the reasonableness of the model parameters and the integrity of the physical meaning. Based on the power-derived injection current, the association between load power and system electrical quantities is established, enabling the load model to be accurately integrated into the system quasi-steady state solution, improving the overall modeling accuracy of the system.

[0031] Further, the explicit 20-40ms time scale enables the time scale of the synchronous generator model to match that of other power electronic device models, avoiding distortion of the transient process simulation due to time scale misalignment. The two-axis model accurately reflects the transient electrical characteristics of the synchronous generator, which is more consistent with the actual situation than the simplified model. The fixed initial power angle before the fault eliminates the interference of power angle swing on the transient overvoltage evaluation, focuses on the inherent electrical response of the synchronous generator in the quasi-steady state, and makes the evaluation results more focused on the coupling effect of LCC-HVDC and new energy devices, improving the pertinence and accuracy of the evaluation.

[0032] Further, the gradient is determined by partial derivative calculation, which quantifies the influence degree of DC power change on node voltage, provides accurate direction guidance for power optimization, and avoids blind adjustment. The optimization strategy of over-limit reduction and large increase of margin ensures that the voltage can be quickly reduced to the safety threshold, maximizes the LCC-HVDC transmission power under the premise of safety, improves the new energy consumption capacity, and balances the system safety and economy. Recalculating the parameters of the matching capacitor ensures that the reactive power compensation can still match the system demand after power optimization, avoids reactive power imbalance caused by power change, and ensures the steady-state stability of the optimized system.

[0033] It can be understood that the beneficial effects of the above-mentioned second aspect to the sixth aspect can be referred to the related description in the above-mentioned first aspect, which will not be repeated here.

[0034] In summary, the application accurately calculates and configures the capacity of the matching capacitor, simulates the dynamic response of the system under specific fault conditions such as commutation failure, and optimizes the LCC-HVDC transmission power range to meet the transient overvoltage safety constraint; through detailed fault simulation and quasi-steady-state analysis, combined with multi-device model and iterative optimization process, it can effectively prevent and control transient overvoltage phenomenon, and improve the operation safety and stability of the power system under the environment of high proportion of new energy access and coexistence of multiple LCC-HVDC transmission groups.

[0035] The technical solutions of the application will be further described in detail below with the help of the drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 A flow chart of an LCC-HVDC operating range optimization method considering transient overvoltage constraints is provided for the embodiments of the application. Figure 2 A schematic diagram of an LCC-HVDC normal operating state equivalent circuit topology is provided for the embodiments of the application. Figure 3 A schematic diagram of an LCC-HVDC commutation failure operating state equivalent circuit topology is provided for the embodiments of the application. Figure 4 A schematic diagram of a practical LCC-HVDC control system in China is provided for the embodiments of the application. Figure 5 A schematic diagram of a practical LCC-HVDC constant current control topology in China is provided for the embodiments of the application. Figure 6 A schematic diagram of a CSEE-FS test system is provided for the embodiments of the application. Figure 7 A curve of LCC-HVDC transmission power with increasing iteration number; Fig. 8 is two DC simultaneous commutation failure node voltage curves before and after optimization, wherein (a) is B10 node voltage curve, (b) is B12 node voltage curve, and the curve dotted line in the figure is before optimization, and the solid line is after optimization; Figure 9 A schematic diagram of a computer device provided by an embodiment of the present application is shown in Fig. 1. Figure 10 A block diagram of a chip provided by an embodiment of the present application is shown in Fig. 2.

[0037] In the figure, 60 is a computer device; 61 is a processor; 62 is a memory; 63 is a computer program; 600 is an electronic device; 610 is a processing unit; 620 is a storage unit; 6201 is a random access storage unit; 6202 is a cache storage unit; 6203 is a read-only storage unit; 6204 is a program / utility; 6205 is a program module; 630 is a bus; 640 is a display unit; 650 is an input / output interface; 660 is a network adapter; and 700 is an external device. DETAILED DESCRIPTION

[0038] The present application provides a LCC-HVDC operating range optimization method considering transient overvoltage constraints, based on multi-time scale dynamic analysis, the transient overvoltage characteristics of high new energy penetration rate AC-DC hybrid system are finely modeled and quantitatively evaluated, so as to maximize the transmission capacity and operation flexibility of the system while ensuring the safe operation of the LCC-HVDC system and related equipment. Through the optimization method, scientific basis can be provided for power grid dispatching and operation decision, and the safety and stability and efficient operation level of the power grid under the condition of multi-DC group and high proportion of new energy access can be effectively improved.

[0039] Referring to Figure 1 , the LCC-HVDC operating range optimization method considering transient overvoltage constraints comprises the following steps: S1, based on the network topology structure of the studied power system, the capacitor capacity is calculated based on the current LCC-HVDC transmission power; The system equivalent circuit of the LCC-HVDC in normal operation is as shown in Figure 2 Therefore, the system equation of the primary side of the LCC-HVDC can be written as: (1) Wherein, , are the no-load DC voltages of the rectifier side and the inverter side of the LCC-HVDC respectively; , are the DC voltages of the rectifier side and the inverter side of the LCC-HVDC respectively; , Equivalent commutation reactance of LCC-HVDC rectifier side and inverter side converter transformer respectively Triggering advance angle of LCC-HVDC rectifier side , Triggering delay angle and arc extinction angle of LCC-HVDC inverter side , Transformer ratio of LCC-HVDC rectifier side and inverter side converter transformer respectively DC line current of LCC-HVDC DC line resistance of LCC-HVDC , Power factor angle of LCC-HVDC rectifier side and inverter side respectively , Active power transmission of LCC-HVDC rectifier side and inverter side respectively , Reactive power consumption of LCC-HVDC rectifier side and inverter side respectively. At this time, in order to compensate the reactive power consumption of LCC-HVDC on both sides to prevent the low voltage operation of the power system, the corresponding capacity capacitor needs to be configured at the converter station on both sides. At this time, the reactive power generated by the capacitors on both sides should be equal to the power consumption of the converter station, so the LCC-HVDC matching capacitor susceptance can be represented by the LCC-HVDC primary side system equation as follows: (2) Wherein, , LCC-HVDC matching reactive power capacitor susceptance on both sides respectively , LCC-HVDC transmission power on both sides at steady state , LCC-HVDC power factor angle on both sides at steady state respectively , LCC-HVDC node voltage on both sides at steady state respectively

[0040] S2, according to the LCC-HVDC and matching capacitor scheme calculated in step S1, carry out AC / DC hybrid power system power flow calculation, and initialize each device parameter according to the power flow calculation result; First, calculate the maximum reactive power consumption point during commutation failure of LCC-HVDC to solve the system voltage at the lowest time during the commutation failure phase shift process of the system quasi-steady state. The quasi-steady state here is that the controller of the whole system power electronic device is controlled to reach the set value, while the synchronous generator is still in the initial stage of the electromechanical transient process. The high-voltage direct current transmission system, new energy device and load described in the embodiment belong to this category.

[0041] The iteration equation for solving the quasi-steady state of the power system is: (3) wherein, is the admittance matrix of the nodes of the power system; , are the voltage amplitude and phase angle of each node of the power system respectively, is a diagonal matrix operator; are the d-axis and q-axis currents injected by the i-th power electronic device into the power grid, which represent that the magnitude of the currents injected by the power electronic device into the power grid is a function of the voltage amplitude of the connected public coupling node in the process of quasi-steady state modeling and solving, represents the current injected by the synchronous generator into the power grid, and the value is dependent on the power angle level of the synchronous generator.

[0042] The quasi-steady state models of all devices in the power system are established in the following order: S201, for the commutation failure LCC-HVDC, since the thyristor cannot be normally turned off, the system inverter side is continuously turned on, which causes an approximate short circuit phenomenon, and the equivalent circuit is shown in Figure 2 The normal operation mode is changed to the commutation failure equivalent topology circuit shown in Figure 3 At this time, the DC current of the primary side of the LCC-HVDC in formula (1) is expressed as: (4) Further, the reactive power consumed by the rectifier side of the LCC-HVDC and its derivative are expressed as: (5) The LCC-HVDC control system topology diagram is shown in Figure 4 For the rectifier side trigger lead angle phase shift process triggered by the commutation failure, the main control link is still used to generate the current control reference value to the constant current control link, so that the rectifier side trigger lead angle is increased to suppress the rapid growth of the DC line current and the continuous decline of the rectifier side DC voltage When is 0, the DC line circuit reaches the maximum value , and the derivative is still positive, when is 0, the derivative is negative, which indicates that the DC line current reaches the inflection point before the DC voltage.

[0043] Further, since the LCC-HVDC continuously satisfies the following equation: (6) Therefore, the application takes the maximum value of the DC line current and the voltage at the zero-crossing point to estimate the maximum reactive power consumed by the power system during the commutation failure process of the LCC-HVDC. The maximum point of the DC current and the triggering advance angle at the time can be calculated by the following equations: (7) wherein, is the rectifier-side triggering advance angle at the time when the maximum reactive power is consumed during the commutation failure of the LCC-HVDC, , As shown in the LCC-HVDC constant current control topology diagram, Kp and Ki are the proportional and integral controller gains of the constant current PI controller, Figure 5 is the steady-state DC line current calculated by the LCC-HVDC power flow, At this time, the DC current injected by the commutation failure LCC-HVDC into the system is (8) wherein, , are the d-axis and q-axis currents injected by the LCC-HVDC into the system at the maximum reactive power point during the commutation failure.

[0044] S202, for the normally operating LCC-HVDC, generally, there is a certain electrical distance between the DCs, and the commutation failure DC will cause the voltage of the other normally operating DC rectifier station to partially decrease. At this time, in order to maintain the DC voltage, the current of the other DC will decrease the rectifier-side triggering advance angle to a certain extent Since the normally operating DC inverter side can still normally turn on and turn off and the inverter side is in the constant extinction angle control mode, at this time, the current injected by the normally operating DC into the system is represented as: (9) wherein, is the inverter-side extinction angle control reference value of the normally operating LCC-HVDC, , are the d-axis and q-axis currents injected by the normally operating LCC-HVDC rectifier station into the system.

[0045] S203, for the power system containing new energy equipment, the new energy equipment is divided into a normal control mode and a fault ride-through control mode according to the voltage amplitude of the grid-connected point, and the currents injected by the two control modes into the system are: (10) wherein, is the control state control word of the new energy equipment, and when the voltage of the grid-connected point of the new energy equipment is less than the ride-through voltage threshold The time table indicates that the new energy equipment enters the fault ride-through control state, and when greater than or equal to indicates that the equipment is in a normal control mode; respectively, the new energy equipment normal control mode and the fault ride-through control mode inject current into the power grid; respectively, the new energy equipment normal control mode active and reactive power reference value; is the new energy equipment fault ride-through active control coefficient, is the new energy equipment fault ride-through current maximum limit value, is the new energy equipment fault ride-through reactive control coefficient.

[0046] S204, for the load in the power system, the domestic dispatching system generally uses the ZIP load model for modeling and simulation, the ZIP load is only related to the voltage amplitude of the node, so its time domain simulation model is its quasi-steady state model, and the ZIP load power level at the fault instant under this model is: (11) wherein, respectively, the load at the current amplitude and the initial amplitude of the node voltage; represent the initial active power and reactive power of the load; is the actual active power and reactive power of the load at the current voltage level; and are the constant impedance, constant current, and constant power proportionality coefficients of the load active and reactive power, and the active and reactive power proportionality coefficients need to satisfy: , .

[0047] Further, based on the load power level expression of formula (11), the load current injected into the power grid is: (12) S205, for the synchronous generator contained in the power system, since the power electronic equipment needs 20-40 ms to reach the quasi-steady state, at this time, for the electromechanical transient of the synchronous machine, the secondary transient process has ended, and the transient process is in the initial stage, therefore, for the quasi-steady state of the power system at the fault instant, a double-axis model is used for modeling the synchronous machine, further, in order to exclude the influence of the synchronous generator power angle swing on the equipment to be evaluated, the synchronous generator power angle uses the initial power angle level before the fault at this time, the synchronous generator current injected into the power grid is: (13) wherein, respectively, the direct-axis and quadrature-axis potential of the synchronous generator.

[0048] S206, for the DC system supporting capacitor, at this time the capacitor injects power into the power grid: (14) In summary, by simultaneously solving equations (3), (8), (9), (10), (12), (13) and (14), the impact of the maximum reactive power absorption during the commutation failure process of LCC-HVDC on the power system can be estimated. Next, the quasi-steady state of the system when the trigger angle of the inverter side of LCC-HVDC can be re-conducted after the commutation failure is calculated. At this time, due to the low voltage current limiting restriction of the transmission power of LCC-HVDC after commutation failure or the closed state waiting for re-unlocking, the DC system will transmit very low power or no power, and the reactive power consumption is also at the lowest value. To ensure the robustness of the scheme, for the initial moment of commutation failure recovery of LCC-HVDC, the DC is considered to be in a closed state at this time, and the current injected by the DC into the system is: (15) S207, for the new energy equipment at the initial moment of the commutation failure recovery stage, since its reactive power can be immediately withdrawn and restored to the steady state level before the fault, its active power still needs to be slowly recovered from the power level during the fault ride-through period to the pre-fault level, therefore the current injected by the new energy equipment into the power grid at the initial moment of the commutation failure recovery stage is: (16) wherein, is the current injected by the new energy equipment into the power grid during the fault ride-through period, the control state control word of the new energy equipment is the recorded result of the quasi-steady state new energy equipment at the commutation failure moment.

[0049] S3, further, by simultaneously solving equations (3), (15), (9), (16), (12), (13) and (14) obtained in step S2, the quasi-steady state of the power system at the initial moment of the commutation failure recovery stage can be solved; At this time, since the LCC-HVDC after commutation failure is still in a closed state and thus does not consume reactive power, while the capacitor bank of the LCC-HVDC converter station is still in a normal working state, making the reactive power output by the DC system converter station much greater than the consumed reactive power, thus causing a local transient overvoltage phenomenon in the system, at this time the voltage Vrec of the LCC-HVDC converter station node is generally the most serious node of the transient overvoltage phenomenon in the system, therefore by calculating whether the node voltage Vrec of the commutation failure LCC-HVDC rectifier station at the initial moment of commutation failure recovery is greater than the transient overvoltage limit value Vmax, the current system can be evaluated whether it has a risk of transient overvoltage.

[0050] S4, calculate the voltage gradient of the node under the corresponding fault, if the voltage Vrec exceeds the safety constraint, optimize the LCC-HVDC transmission power and the supporting equipment in the gradient direction, if the voltage Vrec is within the safety threshold, it means that the system operation configuration scheme meets the transient overvoltage constraint, that is, the LCC-HVDC transmission power level can use this scheme as the actual power grid operation scheme.

[0051] For formula (3), its small signal model is further expanded as: (17) Wherein, , , respectively, the synchronous generator injects the direct and quadrature axis current into the grid, , , represents the voltage amplitude and phase angle increment of each node of the power system, is the current increment injected by each node of the power system into the grid.

[0052] Therefore, the partial derivative of the voltage of the power system node to the current is: (18) Wherein , , , is the partial derivative coefficient matrix of the voltage of the power system node to the current, which is specifically represented as: (19) Wherein, , , respectively, the real part and the imaginary part of the admittance matrix Y of the power system. Therefore, the partial derivative of the voltage Vrec of the commutation failure LCC-HVDC rectifier station node to the capacitor current is: (20) Wherein, the subscript rec represents the corresponding element of the matrix corresponding to the node containing the commutation failure LCC-HVDC rectifier side.

[0053] Further, the partial derivative of the rectifier station reactive capacitor of the commutation failure LCC-HVDC node to the system injected current to the voltage is: (21) Therefore, when the commutation failure DC rectifier station is greater than the safety threshold set by the dispatching system at the initial time of the commutation failure recovery stage, the corresponding commutation station transmission DC power needs to be updated in the direction of reducing the corresponding gradient: (22) Wherein, is the updated corresponding DC system transmission power, to update the step length.

[0054] When the initial time of the commutation failure DC rectifier station in the commutation failure recovery stage is much smaller than the safety threshold set by the dispatching system, it indicates that the current system overvoltage level still has a large safety margin, and the corresponding DC power transmission of the corresponding converter station can be updated along the corresponding gradient increase direction to increase the new energy output power in the system and reduce the power supply pressure of the receiving end power grid: (23) In summary, after updating the DC and supporting capacitor parameters according to the new DC transmission power according to formula (9) and (2), the step S2 and step S3 are repeated to check whether the node voltage Vrec of the LCC-HVDC rectifier station node under the corresponding fault satisfies the transient overvoltage constraint, and after multiple iterations, the convergence condition Vmax-Vrec<ε is met, where ε is the convergence error value of the algorithm, and when the condition is met, it indicates that the current LCC-HVDC transmission power is the maximum DC power that the sending end power grid can transmit under the condition of satisfying the system transient overvoltage constraint. Through this method, the interactive coupling influence of the LCC-HVDC DC power transmission system and the new energy equipment can be fully considered to optimize the system configuration scheme so that the power system can reliably operate within the safe voltage range.

[0055] In another embodiment of the present application, a LCC-HVDC operating range optimization system considering transient overvoltage constraints is provided, which can be used to implement the above-mentioned LCC-HVDC operating range optimization method considering transient overvoltage constraints. Specifically, the LCC-HVDC operating range optimization system considering transient overvoltage constraints includes a capacity module, a construction module, an evaluation module and an output module.

[0056] The capacity module calculates the capacity of the supporting capacitor based on the network topology of the power system to be studied according to the transmission power of the current LCC-HVDC system. The construction module performs AC / DC hybrid power system power flow calculation according to the obtained supporting capacitor capacity, initializes the parameters of each device in the system according to the power flow calculation result, and establishes a quasi-steady state model of each device in the power system, and solves the quasi-steady state of the power system at the moment of maximum reactive power consumption during the commutation failure of the LCC-HVDC system. The evaluation module solves the quasi-steady state of the power system at the initial time of the commutation failure recovery stage of the LCC-HVDC system based on the established quasi-steady state model of each device and the solved quasi-steady state result of the power system during the commutation failure. The output module calculates the node voltage of the rectifier station of the LCC-HVDC system at the initial time of the commutation failure recovery stage, and when the voltage exceeds the transient overvoltage limit value, the LCC-HVDC system transmission power and the matching equipment parameters are optimized along the gradient direction of the node voltage to the direct-current steady-state power, and the above steps are repeated until the node voltage is within the safety threshold; if not, the current LCC-HVDC system transmission power and the matching equipment parameters are taken as the actual power grid operation scheme.

[0057] The application provides a terminal device, which comprises a processor and a memory, the memory is used for storing a computer program, the computer program comprises program instructions, and the processor is used for executing the program instructions stored in the computer storage medium. The processor can be a central processing unit (CPU), and can also be other general-purpose processors, graphics processing units (GPUs), tensor processing units (TPUs), digital signal processors (DSPs), application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components and the like, which are the computing core and control core of the terminal, and are suitable for implementing one or more instructions, and are specifically suitable for loading and executing one or more instructions to implement a corresponding method flow or a corresponding function; the processor in the embodiments of the application can be used for the operation of the LCC-HVDC operation range optimization method considering the transient overvoltage constraint, comprising: Based on the network topology of the power system to be studied, the capacity of the matching capacitor is calculated according to the transmission power of the current LCC-HVDC system; the power flow of the AC-DC hybrid power system is calculated according to the obtained matching capacitor capacity, the parameters of each device in the system are initialized according to the results of the power flow calculation, and the quasi-steady state model of each device in the power system is established to solve the quasi-steady state of the power system at the moment when the reactive power consumption is maximum during the commutation failure of the LCC-HVDC system; based on the established quasi-steady state model of each device and the quasi-steady state results of the power system during the commutation failure, the quasi-steady state of the power system at the initial moment of the commutation failure recovery stage of the LCC-HVDC system is solved; the node voltage of the rectifier station of the LCC-HVDC system at the initial moment of the commutation failure recovery stage is calculated, and when the voltage exceeds the transient overvoltage limit value; the transmission power of the LCC-HVDC system and the parameters of the matching equipment are optimized along the gradient direction of the node voltage to the DC steady-state power, and the above steps are repeated until the node voltage is within the safety threshold; if not, the current LCC-HVDC system transmission power and matching equipment parameters are taken as the actual power grid operation scheme.

[0058] Please refer to Figure 9 , the terminal device is a computer device, the computer device 60 of the embodiment includes: a processor 61, a memory 62, and a computer program 63 stored in the memory 62 and executable on the processor 61, and the computer program 63 realizes the method for estimating the concentration of radioactive iodine species in the containment after an accident in the embodiment when executed by the processor 61, in order to avoid repetition, which will not be described here. Alternatively, the computer program 63 realizes the functions of each model / unit in the LCC-HVDC operation range optimization system considering the transient overvoltage constraint when executed by the processor 61, in order to avoid repetition, which will not be described here.

[0059] The computer device 60 can be a desktop computer, a notebook computer, a palm computer, and a cloud server, etc. The computer device 60 can include, but is not limited to, a processor 61 and a memory 62. Those skilled in the art can understand that Figure 9 The computer device 60 is only an example and does not constitute a limitation on the computer device 60, and can include more or fewer components than shown, or combine certain components, or different components, for example, the computer device can also include an input / output device, a network access device, a bus, etc.

[0060] The processor 61 can be a central processing unit (CPU), and can also be other general-purpose processors, graphics processing units (GPUs), tensor processing units (TPUs), digital signal processors (DSPs), application specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs) or other programmable logic devices, discrete gates or transistor logic, discrete hardware components, etc. The general-purpose processor can be a microprocessor or the processor can also be any conventional processor.

[0061] The memory 62 can be an internal storage unit of the computer device 60, such as a hard disk or a memory of the computer device 60. The memory 62 can also be an external storage device of the computer device 60, such as a plug-in hard disk, a smart media card (SMC), a secure digital (SD) card, a flash card, etc. equipped on the computer device 60.

[0062] Further, the memory 62 can include both an internal storage unit and an external storage device of the computer device 60. The memory 62 is used to store computer programs and other programs and data required by the computer device. The memory 62 can also be used to temporarily store data that has been output or will be output.

[0063] Please refer to Figure 10 , the terminal device is an electronic device 600, and the electronic device 600 is in the form of a general-purpose computing device. The components of the electronic device can include but are not limited to: at least one processing unit 610, at least one storage unit 620, a bus 630 connecting different platform components (including the storage unit 620 and the processing unit 610), a display unit 640, etc.

[0064] Among them, the storage unit stores program code, and the program code can be executed by the processing unit 610, so that the processing unit 610 executes the steps of various exemplary embodiments according to the present application described in the method part of the specification. For example, the processing unit 610 can execute the steps as shown in Figure 1 .

[0065] Storage 620 can include a readable medium in the form of volatile memory, such as random access memory (RAM) 6201 and / or cache memory 6202, and can further include non-volatile memory, such as read only memory (ROM) 6203.

[0066] Storage 620 can also include program / utility 6204 having a set of programs / modules 6205, including operating system, one or more application programs, other program modules, and program data, each of which or a combination thereof, can include implementation of a network environment, for example.

[0067] Bus 630 can represent one or more of several types of bus structures, including a storage bus or bus controller, peripheral bus, graphics bus, processor or local bus using any of a variety of bus architectures.

[0068] Electronic device 600 can also communicate with one or more external devices 700 such as a keyboard or pointing device, using one or more communication interfaces 650. Communication interfaces 650 can also enable electronic device 600 to communicate with one or more devices that enable user interaction with electronic device 600 (for example, remote control or weaponry) and / or communication with one or more other computing devices. Such communication can be facilitated, for example, by an input / output interface 650. Electronic device 600 can also communicate with one or more networks (for example, a local area network, a wide area network, and / or the public switched telephone network) through a network adapter 660. Network adapter 660 can communicate with the other components of electronic device 600 through bus 630. It will be appreciated that other hardware and / or software modules can be used in conjunction with electronic device 600, including but not limited to microcode, device drivers, redundant processing units, external disk drive arrays, RAID systems, tape drives, and data archival storage systems, etc.

[0069] Example 4 The present application further provides a storage medium, specifically a computer readable storage medium, which is a memory device in the terminal device, and is used to store programs and data. It can be understood that the computer readable storage medium herein can include the built-in storage medium in the terminal device, and of course can include the expansion storage medium supported by the terminal device, and can be any tangible medium containing or storing programs, which can be used by or in combination with an instruction execution system, device or apparatus. The computer readable storage medium provides a storage space, which stores the operating system of the terminal. Moreover, one or more instructions suitable for being loaded and executed by the processor are also stored in the storage space, and the instructions can be one or more computer programs (including program codes). It should be noted that more specific examples of the computer readable storage medium include an electrical connection having one or more wires, a portable disk, a hard disk, a random access memory, a read-only memory, an erasable programmable read-only memory, an optical fiber, a portable compact disk read-only memory, an optical storage device, a magnetic storage device, or any suitable combination of the above.

[0070] The computer readable storage medium further includes a data signal carried in baseband or propagated as a carrier wave, in which readable program codes are borne. Such a propagated data signal can take various forms, including but not limited to electro-magnetic signal, optical signal or any suitable combination of the above. The readable storage medium can also be any readable medium other than the readable storage medium, which can send, propagate or transmit programs for use by or in combination with an instruction execution system, device or apparatus. The program codes contained in the readable storage medium can be transmitted by any suitable medium, including but not limited to wireless, wired, optical cable, radio frequency, etc., or any suitable combination of the above.

[0071] The program codes for executing the operation of the present application can be written in any combination of one or more programming languages, including object-oriented programming languages such as Java, C++, etc., and conventional procedural programming languages such as "C" language or similar programming languages. The program codes can be executed entirely on the user computing device, partially on the user device, as an independent software package, partially on the user computing device and partially on the remote computing device, or entirely on the remote computing device or server. In the case involving remote computing device, the remote computing device can be connected to the user computing device through any kind of network, including local area network or wide area network, or can be connected to external computing device (for example, connected through the Internet by using an Internet service provider).

[0072] The one or more instructions stored in the computer readable storage medium can be loaded and executed by the processor to implement the corresponding steps of the LCC-HVDC operation range optimization method considering the transient overvoltage constraint in the above embodiments; the one or more instructions stored in the computer readable storage medium are loaded and executed by the processor to implement the following steps: Based on the network topology of the power system to be studied, the capacity of the matching capacitor is calculated according to the transmission power of the current LCC-HVDC system; the power flow calculation of the AC-DC hybrid power system is performed according to the obtained matching capacitor capacity, the parameters of each device in the system are initialized according to the power flow calculation result, and the quasi-steady state model of each device in the power system is established to solve the quasi-steady state of the power system at the moment when the reactive power consumption is maximum during the commutation failure of the LCC-HVDC system; based on the established quasi-steady state model of each device and the obtained quasi-steady state result of the power system during the commutation failure, the quasi-steady state of the power system at the initial moment of the commutation failure recovery stage of the LCC-HVDC system is solved; the node voltage of the rectifier station of the LCC-HVDC system at the initial moment of the commutation failure recovery stage is calculated, and when the voltage exceeds the transient overvoltage limit value; the transmission power of the LCC-HVDC system and the matching device parameters are optimized along the gradient direction of the node voltage to the DC steady-state power, and the above steps are repeated until the node voltage is within the safety threshold; if not, the current LCC-HVDC system transmission power and matching device parameters are taken as the actual power grid operation scheme.

[0073] The database involved in each embodiment provided in the present application can include at least one of a relational database and a non-relational database. The non-relational database can include a distributed database based on a blockchain, and the like, without being limited thereto. The processor involved in each embodiment provided in the present application can be a general-purpose processor, a central processing unit, a graphics processing unit, a digital signal processor, a programmable logic device, a data processing logic device based on quantum computing, and the like, without being limited thereto.

[0074] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. All other embodiments obtained by a person of ordinary skill in the art without creative labor based on the embodiments in the present application belong to the scope of protection of the present application.

[0075] To verify that the method proposed in this invention can optimize the operating range of LCC-HVDC so that the power system meets transient overvoltage constraints to ensure safe and stable system operation, this invention uses... Figure 6 The CSEE-FS test system, which incorporates a high proportion of new energy sources, is shown as a specific embodiment for algorithm verification. The network topology of the test system is as follows: Figure 6 As shown in Table 1, the initial start-up mode of the unit is based on an 80% renewable energy penetration rate. The start-up mode of the traditional unit is shown in Table 1. Table 1 Synchronous Generator Parameters of CSEE-VS Test System

[0076] The remaining power sources in the system are provided by the WT01, WT03, and WT05 wind turbine clusters and the PV01, PV03, and PV05 photovoltaic power stations, with a total power output of 6900MW. The low voltage ride-through threshold for renewable energy units is 0.9pu. There are a total of three LCC-HVDC DC systems in the system, of which B08 LCC-HVDC transmits 1000MW and B10 and B12 LCC-HVDC transmit 800MW. After examining the recovery of two geographically close LCC-HVDC systems after a short circuit failure on the inverter side, which caused commutation failure, the voltage at nodes B10 and B12 reached about 1.18pu under the initial operating scheme, which exceeds the high voltage ride-through threshold of 1.1pu for renewable energy. This may cause renewable energy to disconnect from the grid, affecting the safe and stable operation of the power grid.

[0077] The algorithm provided in this invention is used to reduce the transmission power of two LCC-HVDC lines so that Vrec < the high-voltage ride-through threshold Vmax of the new energy power station (1.1 p.u). The curve of transmission power increasing with the number of iterations is shown below. Figure 7 As shown in Figure 8, the B10 DC transmission power decreases faster due to its closer proximity to the renewable energy power station, indicating that the low-voltage ride-through process of the renewable energy also affects the overvoltage level of the power system. This results in the LCC-HVDC having the same initial transmission power but different gradient magnitudes, ultimately converging the transmission power of both LCC-HVDCs to around 460MW and 500MW. Figure 8 shows the voltage curves of the nodes where both DCs simultaneously fail to commutate before and after optimization. It can be seen that when the LCC-HVDC power is limited to the aforementioned limits, the system transient overvoltage is successfully limited to below 1.1 pu. Therefore, the maximum transmission power of the two DCs is the final result obtained by the algorithm proposed in this invention. Therefore, the operating range of the LCC-HVDC proposed in this invention can effectively meet the system transient overvoltage constraint requirements while maximizing the absorption of power generated by local renewable energy power stations.

[0078] In summary, the LCC-HVDC operating range optimization method and system considering transient overvoltage constraints effectively determines the operating range of the LCC-HVDC that can transmit power under the constraint of the transient overvoltage of the power system caused by fault disturbance, and the voltage of the power system does not exceed the overvoltage protection limit value in the initial stage of fault removal and recovery after the occurrence of an AC system short-circuit fault or a DC system commutation failure fault in the transmission power interval range, so as to prevent the new energy station and other equipment from being disconnected from the grid due to transient overvoltage, thereby threatening the safe and stable operation of the power system. The LCC-HVDC operating range optimization analysis method provided by the present application can effectively improve the power consumption level of the new energy station at the sending end of the power grid while ensuring the safe and stable voltage of the power grid.

[0079] The above is only used to illustrate the technical idea of the present application, and cannot limit the protection scope of the present application. Any modification made according to the technical idea of the present application on the basis of the technical scheme falls within the protection scope of the claims of the present application.

Claims

1. A method for LCC-HVDC operating range optimization considering transient overvoltage constraints, characterized in that, The method comprises the following steps: S1, based on the network topology of the power system to be studied, the capacity of the matching capacitor is calculated according to the transmission power of the current LCC-HVDC system; S2, according to the obtained matching capacitor capacity, the AC-DC hybrid power system power flow calculation is carried out, the system equipment parameters are initialized according to the power flow calculation result, and the quasi-steady state model of each equipment in the power system is established, and the quasi-steady state of the power system at the moment of maximum reactive power consumption during commutation failure of the LCC-HVDC system is solved; S3, based on the established quasi-steady state model of each equipment and the quasi-steady state result of the power system during commutation failure obtained by solving, the quasi-steady state of the power system at the initial moment of the commutation failure recovery stage of the LCC-HVDC system is solved; S4, the node voltage of the rectifier station of the LCC-HVDC system at the initial moment of the commutation failure recovery stage is calculated, and when the voltage exceeds the transient overvoltage limit value; the transmission power of the LCC-HVDC system and the matching equipment parameters are optimized along the gradient direction of the node voltage to the direct-current steady-state power, and the above steps are repeated until the node voltage is within the safety threshold; if not, the current LCC-HVDC system transmission power and matching equipment parameters are taken as the actual power grid operation scheme.

2. The LCC-HVDC operating range optimization method considering transient overvoltage constraints according to claim 1, characterized in that, Step S1 is specifically: The reactive power consumed by the rectifier station and the inverter station of the LCC-HVDC system is calculated according to the primary side system equation of the LCC-HVDC system; the matching capacitor is configured to generate reactive power equal to the reactive power consumed by the rectifier station.

3. The LCC-HVDC operating range optimization method considering transient overvoltage constraints according to claim 2, characterized in that, The matching capacitor is: wherein, , are the reactive capacitor susceptances of the LCC-HVDC system at both sides respectively; , are the transmission powers of the LCC-HVDC system at steady state respectively; , are the power factor angles of the LCC-HVDC system at steady state respectively; , are the node voltages of the LCC-HVDC system at steady state respectively.

4. The LCC-HVDC operating range optimization method considering transient overvoltage constraints according to claim 1, characterized in that, In step S2, the specific process of solving the quasi-steady state of the power system at the moment of maximum reactive power consumption during commutation failure of the LCC-HVDC system is: Firstly, the definition of the quasi-steady state of the power system is determined, that is, the state of the system in which the controller of the power electronic equipment is controlled to reach the control reference value and the synchronous generator is in the initial stage of the electromechanical transient process, and the high-voltage direct-current transmission system, new energy equipment and load belong to the category of power electronic equipment; Then, the iteration equation for solving the quasi-steady state of the power system is established; Finally, based on the above iteration equation, combined with the quasi-steady state model of each equipment, the quasi-steady state of the power system at the moment of maximum reactive power consumption during commutation failure is solved.

5. The LCC-HVDC operating range optimization method considering transient overvoltage constraints according to claim 4, characterized in that, The steps for establishing the quasi-steady state model of the commutation failure LCC-HVDC system include: According to the equivalent circuit during commutation failure, the maximum DC current and the corresponding trigger advance angle at that moment are calculated; Based on the maximum DC current and the trigger advance angle, the d, q axis current injected into the power grid by the LCC-HVDC system is calculated; The specific way to establish the quasi-steady state model of the normal operation LCC-HVDC system is: Due to the influence of commutation failure on the normal operation LCC-HVDC system, the rectifier station voltage decreases, in order to maintain the DC voltage, the trigger advance angle of the rectifier side will decrease, and the inverter side is in the fixed extinction angle control mode; Based on the above operating characteristics, combined with the electrical parameters of the LCC-HVDC transmission system, the d, q axis current injected into the power grid by the rectifier station of the normal operation LCC-HVDC system is determined, and the quasi-steady state model is established.

6. The LCC-HVDC operating range optimization method considering transient overvoltage constraints according to claim 4, characterized in that, The specific way to establish the quasi-steady state model of the new energy equipment is: The new energy equipment is divided into normal control mode and fault ride-through control mode according to the voltage amplitude of the grid-connected point, and a control word is introduced to represent the control state of the equipment; when the voltage of the grid-connected point is less than the ride-through voltage threshold, the equipment enters the fault ride-through control state; When the voltage of the grid-connected point is greater than or equal to the ride-through voltage threshold, the equipment is in the normal control mode; The injection current of the new energy equipment in the normal control mode based on active and reactive power reference values and the injection current of the new energy equipment in the fault ride-through control mode based on fault ride-through active control coefficient, fault ride-through current maximum limit value and fault ride-through reactive control coefficient are determined respectively; Finally, the injection current expression of the new energy equipment to the power grid is obtained, and the quasi-steady state model of the new energy equipment is established, wherein, is the control word of the control state of the new energy device, when the grid-connected point voltage of the new energy device is less than the ride-through voltage threshold , it indicates that the new energy device enters the fault ride-through control state, when greater than or equal to , it indicates that the device is in a normal control mode; respectively, are the grid injection currents of the new energy device in the normal control mode and the fault ride-through control mode; respectively, are the active and reactive power reference values of the new energy device in the normal control mode; is the active control coefficient of the new energy device in the fault ride-through, is the maximum limit value of the current of the new energy device in the fault ride-through, is the reactive control coefficient of the new energy device in the fault ride-through.

7. The LCC-HVDC operating range optimization method considering transient overvoltage constraints according to claim 4, characterized in that, The specific way to establish the quasi-steady state model of the load is: The ZIP load model is adopted, which is only related to the voltage amplitude of the node, and the active power and reactive power expressions of the load at the fault moment are: wherein, P0and Q0represent the initial active and reactive power of the load at the initial voltage level; P0and Q0represent the initial active and reactive power of the load at the initial voltage level; P0and Q0represent the initial active and reactive power of the load at the initial voltage level; P0and Q0represent the initial active and reactive power of the load at the initial voltage level; P0and Q0represent the initial active and reactive power of the load at the initial voltage level; P0and Q0represent the initial active and reactive power of the load at the initial voltage level; P0and Q0represent the initial active and reactive power of the load at the initial voltage level; Based on the above power expressions, the injection current of the load to the power grid is derived, and the quasi-steady state model of the load is established.

8. The LCC-HVDC operating range optimization method considering transient overvoltage constraints according to claim 4, characterized in that, The specific way to establish the quasi-steady state model of the synchronous generator is: Considering that the time for the power electronic equipment to reach the quasi-steady state is 20-40 ms, at this time the sub-transient process of the synchronous generator has ended and the transient process is in the initial stage, a two-axis model is adopted for modeling; In order to exclude the influence of the synchronous generator power angle swing, the initial power angle level before the fault is adopted, and the injection current of the synchronous generator to the power grid is determined based on the direct-axis and quadrature-axis potential of the synchronous generator, and the quasi-steady state model is established, and the injection current of the synchronous generator to the power grid is: wherein are the direct and quadrature axis potentials of the synchronous generator, respectively.

9. The LCC-HVDC operating range optimization method considering transient overvoltage constraints according to claim 1, characterized in that, The steps for optimizing the transmission power of LCC-HVDC include: Calculating the partial derivative of the node voltage to the capacitor current; updating the DC transmission power in the gradient direction according to the partial derivative result, and recalculating the supporting capacitor parameters, specifically: When the node voltage of the rectifier station of the commutation failure LCC-HVDC system is greater than the transient overvoltage limit value, the transmission power of the LCC-HVDC system is updated in the gradient decreasing direction, When the node voltage of the rectifier station of the commutation failure LCC-HVDC system is much smaller than the transient overvoltage limit value, the transmission power of the LCC-HVDC system is updated in the gradient increasing direction.

10. A system for LCC-HVDC operating range optimization considering transient overvoltage constraints, characterized in that, It includes: The capacity module calculates the capacity of the supporting capacitor based on the network topology of the power system to be studied and the transmission power of the current LCC-HVDC system; The construction module performs AC-DC hybrid power system power flow calculation based on the obtained supporting capacitor capacity, initializes the parameters of each device in the system according to the power flow calculation result, and establishes the quasi-steady state model of each device in the power system to solve the quasi-steady state of the power system at the moment when the reactive power consumption is maximum during the commutation failure of the LCC-HVDC; The evaluation module solves the quasi-steady state of the power system at the initial moment of the commutation failure recovery stage of the LCC-HVDC system based on the established quasi-steady state model of each device and the solved quasi-steady state result of the power system during the commutation failure. The output module calculates the node voltage of the rectifier station of the LCC-HVDC system at the initial time of the commutation failure recovery stage, and when the voltage exceeds the transient overvoltage limit value, the LCC-HVDC transmission power and the matching equipment parameters are optimized along the gradient direction of the node voltage to the direct-current steady-state power, and the above steps are repeated until the node voltage is within the safety threshold; if not, the current LCC-HVDC transmission power and the matching equipment parameters are taken as the actual power grid operation scheme.

Images