Transient overvoltage evaluation method for dc sending-end system considering new energy distribution characteristics

Abstract

The application provides a DC sending end system transient overvoltage evaluation method considering new energy distribution characteristics, including obtaining active power deviation of a rectifier according to operation parameters; considering reactive power characteristics of a new energy unit, reactive power characteristics of the rectifier and reactive power characteristics of a reactive power compensation device, obtaining reactive power deviation of the rectifier according to operation parameters, system parameters and control parameters; obtaining system transient voltage according to system parameters, reactive power deviation and active power deviation; obtaining overvoltage safety margin according to the system transient voltage and a voltage tolerance limit to judge the risk of disconnection of the collection bus. The method can intuitively and quantitatively evaluate whether the DC sending end system has the risk of disconnection caused by transient overvoltage, provide a key early warning basis for the DC sending end system, guarantee the global operation stability of the multi-source interconnected system, and provide scientific theoretical support and guidance for subsequent development of transient overvoltage suppression strategies.

Description

Technical Field

[0001] This application relates to the field of power system fault monitoring and protection technology, specifically to a transient overvoltage assessment method for DC sending-end systems that takes into account the distribution characteristics of new energy sources. Background Technology

[0002] With the continuous increase in my country's installed wind power capacity and grid-connected capacity, large-scale wind power transmission via ultra-high-voltage direct current (UHVDC) has become an effective way to achieve large-scale and wide-ranging energy allocation. When large-capacity UHVDC power experiences significant disturbances—including commutation failure, AC short circuits near the sending end, and DC blocking—both the sending-end converter station and the wind farm experience a large surplus of reactive power, causing transient overvoltage problems in the near-field wind farms and increasing the risk of large-scale renewable energy units (wind turbines) disconnecting from the grid. In recent years, multiple cascading grid disconnection incidents involving renewable energy units have occurred in large-scale wind power bases in Northwest and North my country.

[0003] Existing analytical methods mainly focus on the causes of transient overvoltages in renewable energy units, the risks of grid disconnection accidents caused by DC faults, and corresponding countermeasures. However, research on the quantitative calculation of transient overvoltages in renewable energy units is relatively limited, and there is a lack of high-precision analytical and quantitative assessment methods, making it impossible to effectively address the cascading grid disconnection accidents of renewable energy units. Summary of the Invention

[0004] This application addresses the problems existing in the prior art by providing a transient overvoltage assessment method for DC sending-end systems that considers the distribution characteristics of new energy sources. When a fault occurs in the DC sending-end system, it comprehensively considers the dynamic reactive power response characteristics of new energy units in low-voltage ride-through mode, the reactive power characteristics of the rectifier converter during the fault due to a surge in reactive power consumption, and the reactive power characteristics of the reactive power compensation device fluctuating with the voltage of the collecting bus during the fault. This achieves high-precision analysis and accurate quantitative calculation of the system transient voltage. Furthermore, it analyzes the relationship between the system transient voltage and the voltage withstand limit to obtain the overvoltage safety margin, a margin index for measuring the safety level of the transient voltage in the DC sending-end system during a fault. This method can intuitively and quantitatively assess whether there is a risk of grid disconnection caused by transient overvoltage in the DC sending-end system, providing crucial early warning information for the DC sending-end system, ensuring the global operational stability of multi-source interconnected systems, and effectively assessing the risk of grid disconnection of the collecting bus and / or new energy units in the DC sending-end system. It provides scientific theoretical support and guidance for the subsequent development of transient overvoltage suppression strategies.

[0005] To achieve the above objectives, the technical solution adopted in this application is as follows: A transient overvoltage assessment method for a DC sending-end system considering the distribution characteristics of new energy sources. The DC sending-end system includes an AC system, at least one new energy generator unit, a rectifier converter, and a DC system. The AC system and at least one new energy generator unit are connected in parallel to the input terminal of the rectifier converter. The output terminal of the rectifier converter is connected to the DC system. The rectifier converter includes a busbar and a reactive power compensation device. The assessment method includes: Obtain the operating parameters, system parameters, and control parameters of the DC sending-end system; obtain the voltage withstand limit value; The active power deviation of the rectifier converter is obtained based on the operating parameters; Considering the total reactive power characteristics, the reactive power deviation of the rectifier converter is obtained based on the operating parameters, system parameters, and control parameters. The total reactive power characteristics include the reactive power characteristics of each new energy unit, the reactive power characteristics of the rectifier converter, and the reactive power characteristics of the reactive power compensation device. The system transient voltage is obtained based on system parameters, reactive power deviation, and active power deviation. The overvoltage safety margin is obtained based on the system transient voltage and voltage withstand limit. The risk of the DC sending system disconnecting from the grid is determined based on the overvoltage safety margin.

[0006] In some embodiments, the system transient voltage includes the collector bus voltage; the overvoltage safety margin includes a first overvoltage safety margin, which characterizes the overvoltage safety margin that prevents the collector bus from disconnecting from the grid; the disconnection risk of the DC sending system includes the disconnection risk of the collector bus. The overvoltage safety margin is obtained based on the system transient voltage and voltage withstand limit. The risk of the DC sending-end system disconnecting from the grid is then assessed based on this overvoltage safety margin, including: The ratio of the difference between the bus voltage and the voltage withstand limit to the voltage withstand limit is used as the first overvoltage safety margin. The risk of the busbar disconnecting from the grid is determined based on the first overvoltage safety margin.

[0007] In some embodiments, determining the risk of the busbar disconnecting from the grid based on a first overvoltage safety margin includes: Determine whether the safety margin for the first overvoltage is greater than 0; When the first overvoltage safety margin is greater than 0, the busbar is at risk of disconnection from the grid. When the safety margin of the first overvoltage is not greater than 0, there is no risk of the busbar disconnecting from the grid.

[0008] In some embodiments, the bus voltage is: ; in, To collect bus voltage, To collect the rated value of the bus voltage, This refers to the active power deviation of the rectifier converter. This refers to the reactive power deviation of the rectifier converter. This refers to the short-circuit capacity at the busbar.

[0009] In some embodiments, the system transient voltage further includes: the voltage at the new energy grid connection point; the overvoltage safety margin further includes a second overvoltage safety margin, which represents the overvoltage safety margin for the corresponding new energy unit not to disconnect from the grid; the grid disconnection risk of the DC transmission system includes the grid disconnection risk of the new energy unit; The overvoltage safety margin is obtained based on the system transient voltage and the voltage withstand limit value. The risk of the DC sending-end system disconnecting from the grid is determined based on the overvoltage safety margin, including: The ratio of the difference between the voltage at the new energy grid connection point and the voltage withstand limit to the voltage withstand limit is used as the second overvoltage safety margin. The risk of grid disconnection for the corresponding new energy generating units is determined based on the second overvoltage safety margin.

[0010] In some embodiments, determining the grid disconnection risk of the corresponding new energy unit based on the second overvoltage safety margin includes: Determine whether the safety margin for the second overvoltage is greater than 0; When the second overvoltage safety margin is greater than 0, the corresponding new energy unit is at risk of being disconnected from the grid. When the safety margin of the second overvoltage is not greater than 0, the corresponding new energy unit has no risk of being disconnected from the grid.

[0011] In some embodiments, the voltage at the new energy grid connection point is: ; in, For the first The voltage at the new energy grid connection point of each new energy unit This refers to the serial number of the new energy generating unit. For the first The reactive power generated by each new energy unit For the first Equivalent reactance from each new energy unit to the collection bus.

[0012] In some embodiments, the voltage withstand limit ranges from 0.9. -1.5 , This indicates the per-unit value.

[0013] In some embodiments, the voltage withstand limit is 1.3. .

[0014] In some embodiments, the active power deviation characterizes the difference between the rated active power transmitted from the DC sending-end system to the rectifier side of the DC system and the active power transmitted by the rectifier converter. The reactive power deviation characterizes the difference between the sum of the reactive power generated by each new energy unit, the reactive power generated by the reactive power compensation device, and the rated reactive power transmitted from the DC sending system to the rectifier side of the DC system, and the reactive power transmitted by the rectifier converter.

[0015] Compared with the prior art, this application has the following advantages: 1. This application comprehensively considers the dynamic reactive power response characteristics of new energy units in low voltage ride-through mode during faults, the reactive power characteristics of rectifier converters during faults with surges in reactive power consumption, and the reactive power characteristics of reactive power compensation devices fluctuating with the voltage of the collecting bus during faults to obtain the system transient voltage. This enables high-precision analysis and accurate quantitative calculation of the system transient voltage. Furthermore, it analyzes the relationship between the system transient voltage and the voltage withstand limit to obtain the overvoltage safety margin, an indicator for measuring the safety level of the transient voltage of the DC sending-end system. This allows for a direct and quantitative assessment of whether the DC sending-end system has a risk of grid disconnection caused by transient overvoltage. Thus, it provides key early warning information for the DC sending-end system before a "secondary transient voltage rise" and chain reaction are triggered by the grid disconnection of a single new energy unit, ensuring the overall operational stability of the multi-source interconnection system, effectively avoiding the occurrence of a whole-scale grid disconnection accident of the collecting bus, and avoiding the occurrence of a chain disconnection accident of new energy units.

[0016] 2. This application can effectively assess the grid disconnection risk of the busbar and / or new energy units through the first overvoltage safety margin and / or the second overvoltage safety margin, providing scientific theoretical support and guidance for the subsequent formulation of transient overvoltage suppression strategies. Attached Figure Description

[0017] Figure 1 This is a flowchart illustrating the transient overvoltage assessment method for a DC power transmission system that considers the distribution characteristics of new energy sources in an embodiment of this application. Figure 2 This is a schematic diagram of the equivalent circuit model of the DC sending-end system in the embodiments of this application; Figure 3 A diagram showing the terminal voltage of a new energy generating unit after it has been disconnected from the grid. Figure 4 This is a diagram of the generator terminal voltage of a new energy unit that does not disconnect from the grid after adopting the transient overvoltage assessment method for DC sending-end systems that considers the distribution characteristics of new energy sources proposed in this application.

[0018] The attached diagrams are labeled as follows: 100, AC system; 200, new energy unit; 300, rectifier converter; 310, collecting bus; 320, reactive power compensation device; 400, DC system. Detailed Implementation

[0019] When commutation failure causes significant voltage fluctuations in the DC sending-end system, the control strategy of the renewable energy units will switch accordingly. The dynamically increasing reactive power generated by these units during low-voltage ride-through, coupled with electrical and control aspects with the rectifier side, may contribute to overvoltage. Therefore, the impact of renewable energy should be considered when quantifying the transient voltage of the DC sending-end system. Current research has not yet proposed methods for calculating transient voltage amplitudes considering the influence of renewable energy, nor has it analyzed the comprehensive influencing factors.

[0020] To clearly illustrate the technical features of this solution, the implementation methods of this application will be described in detail below with reference to the accompanying drawings and embodiments. This will allow for a full understanding and implementation of how this application uses technical means to solve technical problems and achieve corresponding technical effects. The embodiments of this application and the various features within them can be combined with each other without conflict, and the resulting technical solutions are all within the protection scope of this application.

[0021] See Figure 1 This application proposes a transient overvoltage assessment method for a DC sending-end system considering the distribution characteristics of new energy sources. The DC sending-end system includes an AC system 100, at least one new energy generator unit 200, a rectifier converter 300, and a DC system 400. The AC system 100 and at least one new energy generator unit 200 are connected in parallel to the input terminal of the rectifier converter 300. The output terminal of the rectifier converter 300 is connected to the DC system 400. The rectifier converter 300 includes a busbar 310 and a reactive power compensation device 320. The equivalent circuit model of the DC sending-end system is shown below. Figure 2 .

[0022] Figure 2 The CCP has two new energy power units with a capacity of 200. The active power fed into the DC system from the sending-end AC system. The reactive power fed into the DC system from the sending-end AC system. This refers to the active power generated by the first new energy unit. The reactive power generated by the first new energy unit This refers to the active power generated by the second renewable energy unit. The reactive power generated by the first new energy unit The active power transmitted by the rectifier converter. The reactive power consumed by the rectifier converter. To collect bus voltage, The reactive power provided to the reactive power compensation device in the rectifier converter. The DC voltage on the rectifier side of the DC system. This refers to the DC current on the rectifier side of the DC system. In steady state, the active and reactive power of the DC sending-end system satisfy the following: ; in, This refers to the serial number of the new energy generating unit. This represents the total number of new energy generating units. For the first The active power generated by each new energy unit For the first The reactive power generated by each new energy unit.

[0023] Typically, voltage amplitude is strongly coupled with reactive power; therefore, the voltage amplitude of the DC sending-end system is mainly related to the reactive power flowing into the AC system. Under normal operating conditions, reactive power consumption in the DC sending-end system is approximately 40%-60% of the active power transmission level. This reactive power is almost entirely provided by the reactive power compensation device at the collector bus in the rectifier converter, and the reactive power exchange between the DC and AC systems is close to zero. During commutation failure, changes in the reactive power characteristics of the DC sending-end system disrupt its reactive power balance, causing transient voltage problems at the collector bus in the rectifier converter.

[0024] The evaluation methods include: The system acquires the operating parameters, system parameters, and control parameters of the DC transmission system. When a fault occurs in the DC transmission system, it comprehensively acquires real-time data of all operating parameters, system parameters, and control parameters through a monitoring system. Simultaneously, it acquires the voltage withstand limit value. In some embodiments, the voltage withstand limit value ranges from 0.9. -1.5 , This indicates the per-unit value. Optionally, the voltage withstand limit is 1.3. .

[0025] The active power deviation of the rectifier converter is obtained based on the operating parameters; Considering the total reactive power characteristics, the reactive power deviation of the rectifier converter is obtained based on the operating parameters, system parameters, and control parameters. The total reactive power characteristics include the reactive power characteristics of each new energy unit, the reactive power characteristics of the rectifier converter, and the reactive power characteristics of the reactive power compensation device. In some embodiments, the active power deviation characterizes the difference between the rated active power transmitted from the DC sending-end system to the rectifier side of the DC system and the active power transmitted by the rectifier converter; the active power deviation is: ; in, This refers to the active power deviation of the rectifier converter. The rated active power absorbed by the rectifier converter. The rated active power transmitted from the DC sending-end system to the rectifier side of the DC system; Reactive power deviation represents the difference between the sum of the reactive power generated by each new energy unit, the reactive power generated by the reactive power compensation device, and the rated reactive power transmitted from the DC sending-end system to the rectifier side of the DC system, and the reactive power transmitted by the rectifier converter. The reactive power deviation is: ; in, This refers to the reactive power deviation of the rectifier converter. For the first The reactive power generated by each new energy unit The rated reactive power transmitted from the DC sending-end system to the rectifier side of the DC system; The system transient voltage is obtained based on system parameters, reactive power deviation, and active power deviation. The risk of the DC sending system disconnecting from the grid is determined based on the overvoltage safety margin.

[0026] Beneficially, by comprehensively considering the dynamic reactive power response characteristics of new energy units in low voltage ride-through mode during faults, the reactive power characteristics of rectifier converters experiencing a surge in reactive power consumption during faults, and the reactive power characteristics of reactive power compensation devices fluctuating with the voltage of the collecting bus during faults, the system transient voltage can be obtained. This enables high-precision analysis and accurate quantitative calculation of the system transient voltage. Furthermore, by analyzing the relationship between the system transient voltage and the voltage withstand limit, the overvoltage safety margin, an indicator for measuring the safety level of the transient voltage of the DC sending-end system, can be obtained. This allows for a direct and quantitative assessment of whether the DC sending-end system has a risk of grid disconnection caused by transient overvoltage. Thus, it provides crucial early warning information for the DC sending-end system before a "secondary transient voltage rise" and chain reaction are triggered by the grid disconnection of a single new energy unit, ensuring the overall operational stability of the multi-source interconnection system, effectively preventing the occurrence of a whole-scale grid disconnection accident of the collecting bus, and preventing the occurrence of a chain disconnection accident of new energy units.

[0027] The system transient voltage includes the collector bus voltage; the overvoltage safety margin includes the first overvoltage safety margin, which characterizes the overvoltage safety margin that prevents the collector bus from disconnecting from the grid; the grid disconnection risk of the DC sending-end system includes the grid disconnection risk of the collector bus. The overvoltage safety margin is obtained based on the system transient voltage and voltage withstand limit. The risk of the DC sending-end system disconnecting from the grid is then assessed based on this overvoltage safety margin, including: The ratio of the difference between the bus voltage and the withstand voltage limit to the withstand voltage limit is used as the first overvoltage safety margin; the first overvoltage safety margin is: ; in, For the first overvoltage safety margin, This is the voltage withstand limit value.

[0028] The risk of the busbar disconnecting from the grid is determined based on the first overvoltage safety margin.

[0029] The equivalent reactance of the DC sending-end system can be expressed as: ; in, The equivalent reactance of the DC sending-end system. This refers to the short-circuit capacity at the busbar. according to Figure 2 It can be seen that the bus voltage can also be expressed as: (1) in, To collect the rated value of the bus voltage, To collect the transverse component of the bus voltage, To collect the longitudinal component of the bus voltage; (2) Substituting equation (2) into equation (1), we obtain the bus voltage as follows: (3) Substituting the short-circuit ratio expression into equation (3), we obtain the bus voltage as follows: (4) In steady state, the longitudinal component of active power usually has a small effect on voltage, and equation (4) can be simplified to: (5) in, This is to collect the per-unit value of the bus voltage.

[0030] When a commutation failure occurs in the DC sending system, the DC current and firing angle increase simultaneously, causing a surge in reactive power consumption of the rectifier converter. This results in transient low voltage in the converter station and its vicinity. As the firing angle continues to increase, the DC current rapidly decreases to zero, and the reactive power consumption of the rectifier converter also drops to zero, which in turn causes transient overvoltage in the converter station and its vicinity.

[0031] Meanwhile, when a commutation failure occurs in the DC sending-end system, considering that the reactive power control of the renewable energy units will respond to changes in the voltage amplitude at the renewable energy grid connection point, and that there is a lag in control switching, its lagging reactive power characteristics may exacerbate the fluctuations in the voltage amplitude at the renewable energy grid connection point. During the commutation failure period in the DC sending-end system, the bus voltage of the converter station where the rectifier converter is located will drop to 0.6. Around this time, a large number of surrounding renewable energy units entered low-voltage ride-through. During low-voltage ride-through, the active power output of these renewable energy units decreased significantly, while the reactive power output increased significantly. , This represents the change in reactive power. The active power characteristics of low-voltage ride-through in renewable energy units will result in a change in the surplus reactive power on the renewable energy unit side. The reactive power characteristics of low-voltage ride-through in renewable energy units will significantly increase, leading to a substantial increase in their reactive power output. This will further raise the voltage on the renewable energy unit side and the voltage of the converter station where the rectifier is located after a fault, meaning that the voltage at the renewable energy grid connection point and the voltage of the collecting bus will both increase further.

[0032] As can be seen from the above analysis, after the commutation failure of the DC sending-end system causes a large number of new energy units to enter the low voltage ride-through stage, the low voltage ride-through characteristics of the new energy units will make the new energy power station another "reactive source" in addition to the converter station where the rectifier converter is located. The combined effect of the two "reactive sources" will further aggravate the transient overvoltage in the near-field of the sending end during the commutation failure of the DC sending-end system.

[0033] The above analysis shows that the power injected into the AC grid from the DC sending-end system during a fault consists of three parts: the power related to the renewable energy units, the power related to the rectifier converter, and the power related to the reactive power compensation device in the rectifier converter. The reactive power characteristics of the renewable energy units, rectifier converter, and reactive power compensation device during a fault are described below.

[0034] Reactive power characteristics of new energy units during a fault: When the voltage at the grid connection point of the renewable energy source is detected to be lower than the voltage threshold for low-voltage ride-through control, the renewable energy unit will switch from the normal operation control mode to the low-voltage ride-through control mode. During the low-voltage ride-through control period, the active current reference value of the renewable energy unit is: (6) in, This is a reference value for the active current of new energy generating units. For the active current of new energy generating units; The reference value for reactive current of new energy generating units is: (7) in, For the first Reference values ​​for reactive current of each new energy unit For the first Reactive power adjustment coefficient for low voltage ride-through of a new energy unit For the first The voltage at the new energy grid connection point of each new energy unit For the first The rated current of each new energy unit; The total reactive power generated by all new energy generating units is: (8) in, This refers to the total reactive power generated by all new energy generating units. The total capacity of all new energy generating units; Equation (8) reflects the dynamic change of the total reactive power generated by the new energy generating units during the fault period with the voltage of the bus.

[0035] Reactive power characteristics of the rectifier converter during a fault: The relationship between the reactive power consumed by the rectifier converter and the active power transmitted by the rectifier converter can be expressed as: (9) in, The power factor angle, This refers to the no-load DC voltage on the rectifier side of the DC system. The number of 6-pulse rectifier converters per pole. The transformer turns ratio within the converter station. Pi; From equation (9), we can obtain: (10) in, The firing angle is the rectifier side firing angle of the DC system. Angular frequency, This is the commutation reactance.

[0036] Simplifying, the reactive power consumed by the rectifier converter is: (11) Reactive power characteristics of the reactive power compensation device during a fault: DC sending-end systems typically use AC filter banks or static capacitors as reactive power compensation devices. During faults in DC sending-end systems, the reactive power output of the reactive power compensation device will vary with fluctuations in the collecting bus voltage. Simplifying the reactive power compensation device as an equivalent capacitor, the reactive power output of the equivalent capacitor will also increase during transient overvoltage periods. Since the reactive power generated by the capacitor in the AC bus reactive power compensation should be proportional to the square of the AC bus voltage, the relationship between the reactive power provided by the reactive power compensation device and the collecting bus voltage can be expressed by the following formula: (12) in, This is the equivalent capacitance of the reactive power compensation device. The rated reactive power provided to the reactive power compensation device; Simplifying equation (12), we obtain the reactive power provided by the reactive power compensation device as follows: (13) The above describes the reactive power characteristics of new energy generating units, rectifier converters, and reactive power compensation devices during fault periods.

[0037] Taking into account the dynamic reactive power response characteristics of new energy units in low voltage ride-through (LVRT) mode during the fault, the reactive power characteristics of the rectifier converter during the fault with the surge in reactive power consumption, and the reactive power characteristics of the reactive power compensation device during the fault with the fluctuation of the collection bus voltage, and obtaining the reactive power deviation based on the total reactive power characteristics, the collection bus voltage is obtained according to equation (4), realizing high-precision analysis and accurate quantitative calculation of the collection bus voltage. On the basis of obtaining the quantitative result of the collection bus voltage, the relationship between the collection bus voltage and the voltage withstand limit is further analyzed to obtain the first overvoltage safety margin, which is a margin index for measuring the safety level of the collection bus of the DC sending system. It can intuitively and quantitatively assess whether there is a risk of grid disconnection caused by transient overvoltage of the collection bus, provide key early warning basis for the DC sending system, ensure the global operation stability of the multi-source interconnection system, effectively avoid the overall grid disconnection accident of the collection bus, and avoid the chain grid disconnection accident of new energy units.

[0038] In some embodiments, determining the risk of the busbar disconnecting from the grid based on a first overvoltage safety margin includes: Determine whether the safety margin for the first overvoltage is greater than 0; When the first overvoltage safety margin is greater than 0, i.e. At this time, the busbar may be disconnected from the grid; When the first overvoltage safety margin is not greater than 0, i.e. At that time, there is no risk of the busbar disconnecting from the grid.

[0039] In some embodiments, the system transient voltage further includes: the voltage at the new energy grid connection point; the overvoltage safety margin further includes a second overvoltage safety margin, which represents the overvoltage safety margin for the corresponding new energy unit not to disconnect from the grid; the grid disconnection risk of the DC transmission system includes the grid disconnection risk of the new energy unit; The overvoltage safety margin is obtained based on the system transient voltage and the voltage withstand limit value. The risk of the DC sending-end system disconnecting from the grid is determined based on the overvoltage safety margin, including: The ratio of the difference between the new energy grid connection point voltage and the voltage withstand limit value to the voltage withstand limit value is used as the second overvoltage safety margin; the second overvoltage safety margin is: ; in, For the first The second overvoltage safety margin for each new energy unit.

[0040] The risk of grid disconnection for the corresponding new energy generating units is determined based on the second overvoltage safety margin.

[0041] The relationship between the grid connection point voltage of the new energy unit and the collection bus voltage is as follows: (14) In the formula, For the first Equivalent reactance from each new energy unit to the collection bus. For the first The current output by each new energy unit The imaginary unit; The reactive power generated by the new energy generating units is: (15) In the formula, For the first The reactive current output by each new energy unit; From equations (14) and (15), we can see that: (16) Analysis reveals that the grid connection voltage of the new energy unit is: (17) Equation (17) describes the nonlinear relationship between the grid connection point voltage of the new energy unit and the equivalent reactance from the new energy unit to the busbar. As the equivalent reactance from the new energy unit to the busbar increases, the grid connection point voltage of the new energy unit gradually rises.

[0042] Taking into account the dynamic reactive power response characteristics of the new energy generating units in the low voltage ride-through (LVRT) mode during the fault, the reactive power characteristics of the rectifier converter during the fault with the surge in reactive power consumption, and the reactive power characteristics of the reactive power compensation device fluctuating with the voltage of the collecting bus during the fault, and obtaining the voltage of the collecting bus, the grid connection point voltage of the new energy generating units can also be obtained according to Equation (17), realizing high-precision analysis and accurate quantitative calculation of the grid connection point voltage of the new energy generating units. Based on the quantitative results of the grid connection point voltage of the energy generating units, the relationship between the grid connection point voltage of the new energy generating units and the voltage withstand limit value is further analyzed to obtain the second overvoltage safety margin, which is a margin index for measuring the safety level of the new energy generating units in the DC sending system. It can intuitively and quantitatively assess whether there is a risk of grid disconnection caused by transient overvoltage of the new energy generating units. Thus, before the "secondary transient voltage rise" and chain reaction caused by the grid disconnection of a single new energy generating unit, it provides a key early warning basis for the DC sending system, ensures the global operational stability of the multi-source interconnection system, and effectively avoids grid disconnection accidents of new energy generating units and chain disconnection accidents of new energy generating units.

[0043] In some embodiments, the risk of grid disconnection of the corresponding new energy unit is determined based on the second overvoltage safety margin, including: Determine whether the safety margin for the second overvoltage is greater than 0; When the second overvoltage safety margin is greater than 0, i.e. At that time, the corresponding new energy unit is the first Some new energy generating units are at risk of being disconnected from the grid; When the second overvoltage safety margin is not greater than 0, i.e. At that time, the corresponding new energy unit is the first Each new energy generating unit has no risk of being disconnected from the grid.

[0044] Figure 3 , Figure 4 The figures show the generator terminal voltages of new energy generating units during a DC sending-end system failure, using the traditional transient overvoltage assessment method, and using the transient overvoltage assessment method proposed in this application.

[0045] Figure 3 In China, transient voltage fluctuations caused by DC sending-end system faults can easily lead to grid disconnection accidents for renewable energy units. After grid disconnection, the short-circuit capacity of renewable energy units will decrease significantly, thereby exacerbating transient undervoltage or transient overvoltage phenomena, forming a "secondary transient voltage rise". This process may cause renewable energy units that have not been disconnected to reach their voltage cross-limits, thereby triggering a chain of grid disconnection accidents and increasing the risk of a global system failure caused by a single fault.

[0046] After renewable energy units are disconnected from the grid, the lack of power weakens the DC transmission system's ability to withstand faults and transient impacts, further worsening its transient voltage response. Large-scale disconnection of renewable energy units not only directly leads to a lack of active power in the DC transmission system, but more importantly, it significantly reduces the system's equivalent short-circuit capacity. This weakening of physical strength increases voltage sensitivity, causing a deterioration in the coupling relationship between reactive power and the terminal voltage of renewable energy units when facing residual transient impacts. This triggers a "secondary transient voltage rise" phenomenon, ultimately causing the remaining renewable energy units to reach the cross-limit.

[0047] Figure 4 By adopting the transient overvoltage assessment method proposed in this application, the risk of grid disconnection can be monitored in real time and corresponding strategies can be implemented to ensure that the new energy units do not disconnect from the grid, thereby avoiding the phenomenon of "secondary transient voltage rise".

[0048] Those skilled in the art will understand that embodiments of this application can be provided as methods, apparatus (systems), or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. The solutions in the embodiments of this application can be implemented in various computer languages, such as the object-oriented programming language Java and the interpreted scripting language JavaScript.

[0049] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce an instruction that executes via the processor of the computer or other programmable data processing apparatus to create an instruction for implementing the flowchart. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0050] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1The function specified in one or more boxes.

[0051] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0052] Finally, it should be noted that the above content is only used to illustrate the technical solution of this application, and is not intended to limit the scope of protection of this application. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of this application shall not depart from the substance and scope of the technical solution of this application.

Claims

1. A transient overvoltage assessment method for a DC sending-end system considering the distribution characteristics of new energy sources, wherein the DC sending-end system includes an AC system, at least one new energy generator unit, a rectifier converter, and a DC system, wherein the AC system and the at least one new energy generator unit are connected in parallel to the input terminal of the rectifier converter, the output terminal of the rectifier converter is connected to the DC system, and the rectifier converter includes a collecting bus and a reactive power compensation device, characterized in that, The evaluation method includes: Obtain the operating parameters, system parameters, and control parameters of the DC transmission system; obtain the voltage withstand limit value; The active power deviation of the rectifier converter is obtained based on the operating parameters. Considering the total reactive power characteristics, the reactive power deviation of the rectifier converter is obtained based on the operating parameters, the system parameters, and the control parameters. The total reactive power characteristics include the reactive power characteristics of each of the new energy units, the reactive power characteristics of the rectifier converter, and the reactive power characteristics of the reactive power compensation device. The system transient voltage is obtained based on the system parameters, the reactive power deviation, and the active power deviation; The overvoltage safety margin is obtained based on the system transient voltage and the voltage withstand limit value; The risk of the DC sending system disconnecting from the grid is determined based on the overvoltage safety margin.

2. The transient overvoltage assessment method for DC sending-end systems considering the distribution characteristics of new energy sources according to claim 1, characterized in that, The system transient voltage includes the collector bus voltage; the overvoltage safety margin includes a first overvoltage safety margin, which characterizes the overvoltage safety margin that prevents the collector bus from disconnecting from the grid; the grid disconnection risk of the DC sending-end system includes the grid disconnection risk of the collector bus. The overvoltage safety margin is obtained based on the system transient voltage and the voltage withstand limit value. The risk of the DC sending-end system disconnecting from the grid is determined based on the overvoltage safety margin, including: The ratio of the difference between the bus voltage and the voltage withstand limit to the voltage withstand limit is used as the first overvoltage safety margin. The risk of the busbar disconnecting from the grid is determined based on the first overvoltage safety margin.

3. The transient overvoltage assessment method for DC sending-end systems considering the distribution characteristics of new energy sources according to claim 2, characterized in that, Determining the risk of disconnection of the busbar based on the first overvoltage safety margin includes: Determine whether the first overvoltage safety margin is greater than 0; When the first overvoltage safety margin is greater than 0, the busbar is at risk of disconnection from the grid. When the first overvoltage safety margin is not greater than 0, the busbar has no risk of being disconnected from the grid.

4. The transient overvoltage assessment method for DC sending-end systems considering the distribution characteristics of new energy sources according to claim 2, characterized in that, The voltage of the bus is: ； in, To collect bus voltage, To collect the rated value of the bus voltage, This refers to the active power deviation of the rectifier converter. This refers to the reactive power deviation of the rectifier converter. This refers to the short-circuit capacity at the busbar.

5. The transient overvoltage assessment method for DC sending-end systems considering the distribution characteristics of new energy sources according to claim 4, characterized in that, The system transient voltage also includes the voltage at the new energy grid connection point; the overvoltage safety margin also includes a second overvoltage safety margin, which represents the overvoltage safety margin for the corresponding new energy unit to remain connected to the grid; the grid disconnection risk of the DC transmission system includes the grid disconnection risk of the new energy unit. The overvoltage safety margin is obtained based on the system transient voltage and the voltage withstand limit value. The risk of the DC sending-end system disconnecting from the grid is determined based on the overvoltage safety margin, including: The ratio of the difference between the new energy grid connection point voltage and the voltage withstand limit value to the voltage withstand limit value is used as the second overvoltage safety margin; The risk of the corresponding new energy unit being disconnected from the grid is determined based on the second overvoltage safety margin.

6. The transient overvoltage assessment method for DC sending-end systems considering the distribution characteristics of new energy sources according to claim 5, characterized in that, The risk of grid disconnection of the corresponding new energy unit is determined based on the second overvoltage safety margin, including: Determine whether the second overvoltage safety margin is greater than 0; When the second overvoltage safety margin is greater than 0, the corresponding new energy unit is at risk of being disconnected from the grid. When the second overvoltage safety margin is not greater than 0, the corresponding new energy unit has no risk of being disconnected from the grid.

7. The transient overvoltage assessment method for DC sending-end systems considering the distribution characteristics of new energy sources according to claim 5, characterized in that, The voltage at the new energy grid connection point is: ； in, For the first The voltage at the new energy grid connection point of each new energy unit This refers to the serial number of the new energy generating unit. For the first The reactive power generated by each new energy unit For the first Equivalent reactance from each new energy unit to the collection bus.

8. The transient overvoltage assessment method for DC sending-end systems considering the distribution characteristics of new energy sources according to any one of claims 1-7, characterized in that, The voltage withstand limit range includes 0.

9. -1.5 , This indicates the per-unit value.

9. The transient overvoltage assessment method for DC sending-end systems considering the distribution characteristics of new energy sources according to claim 8, characterized in that, The voltage withstand limit is 1.

3. .

10. The transient overvoltage assessment method for DC sending-end systems considering the distribution characteristics of new energy sources according to claim 1, characterized in that: The active power deviation represents the difference between the rated active power transmitted from the DC sending-end system to the rectifier side of the DC system and the active power transmitted by the rectifier converter. The reactive power deviation represents the difference between the reactive power generated by each of the new energy generating units, the reactive power generated by the reactive power compensation device, and the sum of the rated reactive power transmitted from the DC sending-end system to the rectifier side of the DC system, and the reactive power transmitted by the rectifier converter.

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