A PCC transient voltage instability identification method for a water-wind coupling DC power transmission system

Abstract

This invention discloses a method for identifying PCC transient voltage instability in a hydro-wind-solar coupled DC transmission system, belonging to the field of power system safety and stability analysis technology. The method includes collecting offline parameters and constructing an electromagnetic transient simulation model. Typical fault transient simulations are performed on the system to determine voltage stability. Typical fault transient simulations and voltage stability determination are conducted under different hydrological periods and different hydropower unit output conditions. The short-circuit ratio at the PCC is calculated under different hydrological periods, and the critical short-circuit ratio of the system under fault conditions is determined based on voltage stability. Online system parameters at the PCC are collected, and the online short-circuit ratio and voltage over-limit margin are calculated. Based on the online short-circuit ratio, the system critical short-circuit ratio, and the voltage over-limit margin, the method determines whether transient voltage instability has occurred and outputs the identification result. This invention achieves accurate online identification of PCC transient voltage instability in complex hydro-wind-solar coupled DC transmission systems, providing important reference for subsequent voltage stability and safety control measures.

Description

Technical Field

[0001] This invention belongs to the field of power system safety and stability analysis technology, and specifically relates to a method for judging PCC transient voltage instability in a hydro-wind-solar coupled DC transmission system. Background Technology

[0002] With the transformation of the global energy structure and the advancement of the "dual carbon" goal, the proportion of new energy power generation, represented by wind power and photovoltaics, continues to increase. In the resource-rich western and northern regions of my country, large-scale wind and photovoltaic power sources are often constructed in conjunction with hydropower stations to form a hydro-wind-solar complementary system, which transmits clean energy over long distances in large capacity through high-voltage direct current (HVDC) transmission channels. Under current technological conditions, the point of common coupling (PCC) of the hydro-wind-solar coupled DC transmission system is highly susceptible to transient low voltage or transient overvoltage when subjected to severe fault disturbances. If the voltage trajectory after the fault exceeds the low voltage crossing boundary or high voltage crossing boundary of the new energy units, the wind turbines and photovoltaic inverters may trigger protection actions and disconnect from the grid, leading to a redistribution of active and reactive power, further deteriorating local voltage support, and ultimately evolving into transient voltage instability. Currently, the industry largely relies on offline simulation analysis or a single voltage threshold for identifying transient voltage instability. This approach fails to simultaneously reflect system structural vulnerability, differences in fault types, and the impact of renewable energy breakthroughs on instability evolution. Particularly when changes in hydropower output lead to dynamic variations in the equivalent grid strength at the point of common coupling, a single criterion struggles to balance linearity, accuracy, and engineering feasibility. Therefore, there is an urgent need for a method that can both reflect the mechanism of grid strength changes and enable real-time early warning and instability confirmation based on online measurements. Summary of the Invention

[0003] To address the aforementioned issues, this invention provides a PCC transient voltage instability discrimination method for hydro-wind-solar coupled DC transmission systems. This method addresses the problem that existing technologies often rely on offline simulation analysis or use only a single voltage threshold for judging transient voltage instability, making it difficult to simultaneously reflect the system's structural vulnerability, differences in fault types, and the impact of new energy source over-limit on instability evolution.

[0004] A method for determining PCC transient voltage instability in a hydro-wind-solar coupled DC transmission system includes: Offline system parameters of a hydro-wind-solar coupled DC transmission system were collected and an electromagnetic transient simulation model was constructed. Based on the electromagnetic transient simulation model, typical fault transient simulations of the system are performed, and the voltage stability of the simulation results is judged to obtain the first voltage stability result. Typical fault transient simulations of the system were repeated under different hydrological periods and different hydropower unit output conditions, and the voltage stability of the simulation results was judged to obtain the second voltage stability result. The short-circuit ratio at the common junction is calculated based on the simulation results under different hydrological periods, and the critical short-circuit ratio of the system under fault is determined based on the first voltage stability results, the second voltage stability results and the short-circuit ratio. Collect online system parameters at the point of common coupling and calculate the online short-circuit ratio and voltage over-limit margin of the system at the point of common coupling; The system determines whether transient voltage instability has occurred based on the online short-circuit ratio, the system critical short-circuit ratio, and the voltage over-limit margin, and outputs the judgment result.

[0005] According to a specific embodiment of the present invention, the process of collecting offline system parameters of a hydro-wind-solar coupled DC transmission system and constructing an electromagnetic transient simulation model further includes: In offline mode, offline system parameters of hydropower units, wind power units, photovoltaic power stations and high-voltage direct current transmission are collected; An electromagnetic transient simulation model of a hydro-wind-solar coupled DC transmission system was constructed based on offline system parameters.

[0006] According to a specific embodiment of the present invention, a typical fault transient simulation of the system is performed based on an electromagnetic transient simulation model, and the voltage stability of the simulation results is determined to obtain a first voltage stability result, which specifically includes: Based on the electromagnetic transient simulation model, fault transient simulations of the system were performed for single-phase short circuit, three-phase short circuit, commutation failure, and single-pole blocking, respectively. Based on the simulation results, the transient voltage stability evaluation criteria were used to determine the transient voltage stability under single-phase short circuit, three-phase short circuit, commutation failure, and single-pole blocking, respectively, and the first voltage stability result was obtained.

[0007] According to a specific embodiment of the present invention, the system undergoes repeated typical fault transient simulations under different hydrological periods and different hydropower unit output conditions, and the voltage stability of the simulation results is determined to obtain a second voltage stability result, which further includes: Using the typical scenario induction method, the output of hydropower units was changed under different hydrological periods, and the transient simulation of typical faults of the system was repeated. Based on the simulation results, the transient voltage stability of typical faults under different hydrological periods was determined using the transient voltage stability evaluation standard, and the second voltage stability result was obtained.

[0008] According to a specific embodiment of the present invention, the formula for calculating the short-circuit ratio is as follows:

[0009] Wherein, SCR is the short-circuit ratio at the point of common coupling. For the short-circuit capacity at the common junction, This represents the total rated installed capacity of wind power and photovoltaic power.

[0010] According to a specific embodiment of the present invention, the online system parameters include: three-phase voltage, three-phase current, hydropower unit operating status information, wind turbine unit operating status information, photovoltaic unit operating status information, and fault and protection action signals.

[0011] According to a specific embodiment of the present invention, the formula for calculating the online short-circuit ratio is as follows:

[0012]

[0013]

[0014] in, For online short-circuit ratio, For the online short-circuit capacity at the common junction point, Let be the sum of the rated capacities of wind power and photovoltaic power at time t in grid-connected state. As the system's baseline capacity, The Thevenin equivalent impedance of the AC system at the point of common coupling, Ω wind (t) represents the set of wind turbine units in grid-connected state at time t, Ω PV(t) Let be the set of photovoltaic units in grid-connected state at time t. The rated capacity of the wind turbine unit. This refers to the rated capacity of the photovoltaic unit.

[0015] According to a specific embodiment of the present invention, the method for calculating the voltage over-limit margin is as follows: The equivalent fault crossing boundary after aggregation for wind power and photovoltaic power plants is defined as follows:

[0016]

[0017] in, For low-voltage equivalent fault crossing the boundary, For the low-voltage crossover boundary of the i-th type of grid-connected renewable energy, For high-voltage equivalent fault crossing the boundary, For the high-voltage crossover boundary of the i-th type of grid-connected renewable energy; When a fault occurs, in the preset monitoring window [ tf , tf+Tw Within this range, a low voltage over-limit margin is constructed based on the measured voltage at the point of common coupling. and high voltage over-limit margin The calculation formula is as follows:

[0018] in, This is the measured voltage at the point of common coupling.

[0019] According to a specific embodiment of the present invention, determining whether transient voltage instability has occurred based on the online short-circuit ratio, the system critical short-circuit ratio, and the voltage over-limit margin, and outputting the determination result further includes: When the online short-circuit ratio is lower than the system's critical short-circuit ratio, and the measured voltage at the point of common coupling exceeds the corresponding voltage over-limit margin, the system is determined to have experienced transient voltage instability. The determination result of undervoltage disconnection type transient voltage instability or overvoltage disconnection type transient voltage instability is based on the direction of voltage exceeding the limit.

[0020] According to a specific embodiment of the present invention, when the online short-circuit ratio is lower than the system's critical short-circuit ratio and the measured voltage at the point of common coupling exceeds the corresponding voltage over-limit margin, determining that the system has experienced transient voltage instability further includes: When the online short-circuit ratio of the AC system at the common connection point meets the requirement If the low voltage exceeds the limit margin If the value is greater than 0, the system is determined to have experienced undervoltage disconnection-type transient voltage instability. If the high voltage exceeds the limit margin... If the value is greater than 0, the system is determined to have experienced overvoltage disconnection-type transient voltage instability. When the online short-circuit ratio of the AC system at the common connection point meets the requirement If the low voltage exceeds the limit margin If the value is greater than 0, the system is determined to have experienced undervoltage disconnection-type transient voltage instability. When the online short-circuit ratio of the AC system at the common connection point meets the requirement If the fault is cleared, it is determined that the system will not experience transient voltage instability after the fault ends. and Let be the critical short-circuit ratio of the system, and < , The online short-circuit ratio of the communication system at the common connection point.

[0021] Compared with the prior art, the PCC transient voltage instability discrimination method for a water-wind-solar coupled DC transmission system provided by the present invention has the following advantages: 1. The project is highly feasible and has the capability for online application.

[0022] The input quantities required for the discrimination method proposed in this invention (such as PCC three-phase voltage, current, unit operating status, fault signals, etc.) can all be obtained online by existing energy management systems (EMS), supervisory control systems (SCADA), synchronous phasor measurement units (PMU), and protection devices, or obtained through simple online estimation. This method does not rely on complex offline simulation iterations, is fully adaptable to online risk assessment and early warning platforms of actual power grids, and possesses extremely high engineering application value and operability.

[0023] 2. The discrimination mechanism is improved, enabling accurate classification of instability types.

[0024] Existing technologies often employ a single voltage threshold, making it difficult to differentiate the nature of faults. This invention constructs a two-layer screening mechanism based on "grid strength-voltage boundary," which can not only accurately identify the occurrence of transient voltage instability but also effectively distinguish between undervoltage-induced grid disconnection transient voltage instability caused by three-phase short circuits and overvoltage-induced grid disconnection transient voltage instability caused by unipolar blocking. This classification and discrimination capability provides a crucial basis for subsequently developing targeted safety control strategies (such as reactive power compensation during undervoltage and DC reduction during overvoltage).

[0025] 3. It has strong adaptability and reflects the dynamic evolution of power grid strength.

[0026] To address the challenge of dynamic changes in grid strength (short-circuit ratio) caused by significant fluctuations in hydropower output under varying hydrological conditions in hydro-wind-solar coupled systems, this invention introduces the concept of online short-circuit ratio. By taking into account real-time changes in the number of operating hydropower units and the actual grid-connected capacity of renewable energy sources, the equivalent impedance and short-circuit capacity at the PCC are dynamically calculated, solving the problem of poor adaptability and susceptibility to misjudgment in scenarios with varying operating modes using traditional static criteria.

[0027] 4. The criteria are highly robust, balancing safety and economy.

[0028] This invention effectively avoids the jitter and malfunctions near the critical stability point caused by the single threshold method by combining offline calibration of the critical short-circuit ratio with online voltage over-limit margin for collaborative discrimination. This method minimizes intervention in normal control while ensuring system safety and stability (preventing large-scale grid disconnection), thus improving the system's operational economy.

[0029] 5. It has a wide range of applications and high promotional value.

[0030] This invention is specifically designed for the complex characteristics of multi-energy complementary DC transmission systems involving water, wind, and solar power, and is suitable for weak grid environments with a high proportion of power electronic equipment connected. Its core technical concept—based on a discrimination framework of "offline critical parameter calibration + online intensity and boundary collaborative verification"—can also be extended to transient voltage stability monitoring in other types of new energy bases (such as pure wind and solar bases, desert and Gobi bases, etc.). Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of this disclosure or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this disclosure. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a flowchart of a PCC transient voltage instability discrimination method for a water-wind-solar coupled DC transmission system according to an embodiment of the present invention.

[0033] Figure 2 This is a flowchart of a method for collecting offline system parameters of a hydro-wind-solar coupled DC transmission system and constructing an electromagnetic transient simulation model according to an embodiment of the present invention.

[0034] Figure 3 This is a flowchart of a method for performing typical fault transient simulation of a system and determining the voltage stability of the simulation results, according to an embodiment of the present invention.

[0035] Figure 4 This is a flowchart of a method for performing transient simulations of typical faults in a system under different hydrological periods and different hydropower unit output conditions, and for determining the voltage stability of the simulation results, according to an embodiment of the present invention.

[0036] Figure 5 This is a flowchart of a method for determining whether transient voltage instability has occurred based on online short-circuit ratio, system critical short-circuit ratio, and voltage over-limit margin, according to an embodiment of the present invention, and outputting the determination result.

[0037] Figure 6 This is a schematic diagram of the transient voltage instability judgment process at the common connection point of a water-wind-solar coupled DC transmission system according to an embodiment of the present invention.

[0038] Figure 7 This is a schematic diagram of a hydro-wind-solar DC power transmission system according to an embodiment of the present invention.

[0039] Figure 8 This is a schematic diagram of a three-phase short-circuit fault on the AC side of a water-wind-solar coupled DC transmission system according to an embodiment of the present invention.

[0040] Figure 9 This is a common coupling point voltage waveform under a three-phase short circuit fault when a hydropower unit with an output of 2000MW occurs, according to an embodiment of the present invention.

[0041] Figure 10This is the wind turbine terminal voltage waveform under a three-phase short circuit fault when the output of a hydropower unit is 2000MW, according to an embodiment of the present invention.

[0042] Figure 11 This is a common coupling point voltage waveform under single-pole blocking when the output of a hydropower unit is 1000MW, according to an embodiment of the present invention.

[0043] Figure 12 This is the wind turbine terminal voltage waveform under single-pole blocking when the output of a hydropower unit is 1000MW, according to an embodiment of the present invention.

[0044] Figure 13 This is the photovoltaic inverter terminal voltage waveform under single-pole blocking when the output of a hydropower unit is 1000MW, according to an embodiment of the present invention. Detailed Implementation

[0045] To enable those skilled in the art to more clearly understand the concepts and ideas of the present invention, the present invention is described in detail below with reference to specific embodiments. It should be understood that the embodiments given herein are only a part of all possible embodiments of the present invention. Those skilled in the art, after reading this specification, are capable of making improvements, modifications, or substitutions to parts or the entirety of the following embodiments, and such improvements, modifications, or substitutions are also included within the scope of protection claimed by the present invention.

[0046] In this document, the terms “announcement,” “arrival,” and other similar words are not intended to imply any order, quantity, or importance, but are merely used to distinguish different elements. The terms “one,” “a,” and other similar words are not intended to indicate the existence of only one thing, but rather that the description pertains to only one of the things, which may have one or more. The terms “contains,” “includes,” and other similar words are intended to indicate a logical relationship, not a spatial relationship. For example, “A includes B” means that logically B belongs to A, not that spatially B is located inside A. Furthermore, the meanings of the terms “contains,” “includes,” and other similar words should be considered open-ended, not closed. For example, “A includes B” means that B belongs to A, but B does not necessarily constitute all of A; A may also include other elements such as C, D, and E.

[0047] In this document, the terms "embodiment," "this embodiment," "an embodiment," and "one embodiment" do not imply that the description applies only to one specific embodiment, but rather that such description may also be applicable to one or more other embodiments. Those skilled in the art will understand that any description made herein with respect to one embodiment can be substituted, combined, or otherwise combined with the descriptions in one or more other embodiments. New embodiments resulting from such substitutions, combinations, or other combinations are readily conceived by those skilled in the art and fall within the scope of protection of this invention.

[0048] Example 1 Additional aspects and advantages of embodiments of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of embodiments of the invention. Figures 1-13 This invention provides a method for determining PCC transient voltage instability in a hydro-wind-solar coupled DC transmission system, comprising: S1: Collect offline system parameters of the water-wind-solar coupled DC transmission system and construct an electromagnetic transient simulation model.

[0049] S2: Based on the electromagnetic transient simulation model, perform typical fault transient simulations of the system and determine the voltage stability of the simulation results to obtain the first voltage stability result.

[0050] S3: Repeatedly simulate typical fault transients of the system under different hydrological periods and different hydropower unit output conditions, and determine the voltage stability of the simulation results to obtain the second voltage stability result.

[0051] S4: Calculate the short-circuit ratio at the common junction under different hydrological periods based on simulation results, and determine the critical short-circuit ratio of the system under fault conditions based on the first voltage stability result, the second voltage stability result, and the short-circuit ratio.

[0052] S5: Collect online system parameters at the point of common coupling and calculate the online short-circuit ratio and voltage over-limit margin of the system at the point of common coupling.

[0053] S6: Determine whether transient voltage instability has occurred based on online short-circuit ratio, system critical short-circuit ratio, and voltage over-limit margin, and output the judgment result.

[0054] Specifically, step S1, which involves collecting offline system parameters of the water-wind-solar coupled DC transmission system and constructing an electromagnetic transient simulation model, further includes: S11: In offline mode, collect offline system parameters of hydropower units, wind power units, photovoltaic power stations and high-voltage direct current transmission.

[0055] S12: Construct an electromagnetic transient simulation model of a water-wind-solar coupled DC transmission system based on offline system parameters.

[0056] In a specific embodiment of the present invention, the invention first collects and summarizes key system parameters of hydropower units, wind power units, photovoltaic power plants, and high-voltage direct current transmission in an offline state, and then performs impedance equivalence analysis of the nearby power grid as a conventional voltage source. Finally, it uses PSCAD software to establish an electromagnetic transient simulation model of the hydro-wind-solar coupled DC transmission system. The power structure diagram of this hydro-wind-solar coupled DC transmission system is shown below. Figure 7 As shown, the system connects hydropower units, wind power units, and photovoltaic power plants to the sending-end AC system via a Point of Common Coupling (PCC), and then transmits power externally through a high-voltage direct current (HVDC) transmission channel. The installed capacity of wind power is 1000MW, the installed capacity of photovoltaic power is 3000MW, the total rated capacity of new energy sources is 4000MW, the power transmitted through the HVDC transmission channel is 8000MW, and the local load of the system is 200MW. The electromagnetic transient simulation model established in this invention provides a high-fidelity electromagnetic transient simulation environment that realistically reflects the dynamic characteristics of the hydro-wind-solar coupled HVDC transmission system, providing a necessary tool for subsequent offline scanning to obtain the critical short-circuit ratio.

[0057] Specifically, step S2 involves performing typical fault transient simulations of the system based on the electromagnetic transient simulation model and determining the voltage stability of the simulation results to obtain the first voltage stability result, which specifically includes: S21: Based on the electromagnetic transient simulation model, the system is subjected to fault transient simulations for single-phase short circuit, three-phase short circuit, commutation failure, and single-pole blocking, respectively.

[0058] In a specific embodiment of the present invention, in order to reproduce the dynamic response of the hydro-wind-solar coupled DC transmission system under extreme operating conditions through high-precision electromagnetic transient simulation, and to define the stability boundary of the system according to national standards, the present invention systematically sets up and simulates four typical severe faults that the hydro-wind-solar coupled DC transmission system may encounter in actual operation: single-phase short circuit, three-phase short circuit, commutation failure, and single-pole blocking. During the simulation, the voltage and current time-domain waveforms of the point of common coupling (PCC) and the grid connection points (POIs) of each renewable energy plant need to be recorded in detail.

[0059] S22: Based on the simulation results, the transient voltage stability evaluation criteria are used to determine the transient voltage stability of single-phase short circuit, three-phase short circuit, commutation failure and single-pole blocking, respectively, and the first voltage stability result is obtained.

[0060] In a specific embodiment of the present invention, the present invention uses transient voltage stability evaluation criteria to determine the voltage stability of the system based on the simulation results of each fault scenario in step S21. The determination results of different fault types (single-phase short circuit, three-phase short circuit, commutation failure, single-pole blocking) under different system operating modes are summarized to obtain the first voltage stability result, which provides data support for subsequent calculation of the critical short circuit ratio.

[0061] Specifically, step S3 involves repeating typical fault transient simulations of the system under different hydrological periods and different hydropower unit output conditions, and determining the voltage stability of the simulation results to obtain a second voltage stability result, which further includes: S31: Using the typical scenario induction method, the output of hydropower units is changed under different hydrological periods, and the transient simulation of typical faults of the system is repeated.

[0062] In a specific embodiment of the present invention, the invention employs a typical scenario induction method, varying the output of hydropower units under different hydrological periods, and repeatedly performing transient simulations of the system based on the four typical faults defined in step S2 (single-phase short circuit, three-phase short circuit, commutation failure, and single-pole blockage). Based on historical hydrological data and reservoir scheduling plans, the present invention divides the system operating conditions into three typical periods: high-water season, normal-water season, and low-water season. Each period represents a different adjustable hydropower capacity and start-up mode. Specifically, within each hydrological period, a gradient for the change in hydropower unit output is set. In this embodiment of the invention, the output of the hydropower unit is gradually increased from a minimum of 1000MW to 8000MW (e.g., 1000MW, 2000MW, 3000MW, 4000MW, 6000MW, 8000MW). As the output changes, the excitation current and terminal voltage of the hydropower generator change accordingly, which in turn causes a significant change in the equivalent impedance of the external power grid connected to the PCC. For each specific combination of "hydrological period - output level", the simulation of single-phase short circuit, three-phase short circuit, commutation failure and single-pole blocking fault in step S21 is repeated, and the voltage drop / rise depth, recovery time and grid disconnection status of the new energy unit at the PCC are recorded in detail for each combination, forming multi-dimensional simulation data.

[0063] S32: Based on the simulation results, the transient voltage stability of typical faults under different hydrological periods is judged by the transient voltage stability evaluation standard, and the second voltage stability result is obtained.

[0064] In a specific embodiment of the present invention, based on the simulation data generated in step S31, the present invention again applies the transient voltage stability evaluation standard to determine the voltage stability of the system, and summarizes the determination results of fault types (single-phase short circuit, three-phase short circuit, commutation failure, single-pole blocking) under different hydrological periods to obtain a second voltage stability result.

[0065] Specifically, step S4 calculates the short-circuit ratio at the common junction under different hydrological periods based on simulation results, and determines the critical short-circuit ratio of the system under fault conditions based on the first voltage stability results, the second voltage stability results, and the short-circuit ratio. The formula for calculating the short-circuit ratio is: (1) in, SCR The short-circuit ratio at the common junction. For the short-circuit capacity at the common junction, This represents the total rated installed capacity of wind power and photovoltaic power.

[0066] In one specific embodiment of the present invention, the short-circuit ratio is used to characterize the strength of the AC system at the point of common coupling. SCR As a criterion, this invention defines the short-circuit ratio at the point of common coupling as the ratio of the PCC short-circuit capacity to the total rated capacity of new energy sources, i.e. In this embodiment of the invention, the total rated installed capacity of wind power and photovoltaic power is... The short-circuit capacity at the point of common coupling (PCC) is calculated using the following formula: (2) in, (3) Finally, the formula for calculating the short-circuit ratio at PCC can be obtained as follows: (4) In the formula, As the system's baseline capacity, The equivalent impedance of the AC system at the fault point. The longitudinal axis transient reactance of a water turbine. The number of turbines connected to the AC system. This represents the equivalent impedance between the Southwest Power Grid, the transmission lines, and the PCC.

[0067] Based on the above calculations, the short-circuit ratio of the point of common coupling (PCC) is obtained under different output periods of the hydropower unit. Combined with the fault stability results obtained in steps S2 and S3, the system stability under different output periods of the hydropower unit is shown in Table 1.

[0068] Table 1 System stability under different power output conditions of hydropower units at different times

[0069] Based on the offline simulation results in Table 1, the critical short-circuit ratio of the system in this embodiment of the invention is determined to be... , The critical short-circuit ratio is a parameter obtained by calibrating the system structure, control parameters, and simulation conditions of the embodiments of the present invention.

[0070] Specifically, in step S5, online system parameters at the point of common coupling are collected, and the online short-circuit ratio and voltage over-limit margin of the system at the point of common coupling are calculated. The online system parameters include three-phase voltage, three-phase current, hydropower unit operating status information, wind turbine unit operating status information, photovoltaic unit operating status information, and fault and protection action signals.

[0071] In a specific embodiment of the present invention, the present invention calculates the Thevenin equivalent impedance of the AC system at the PCC online based on switch status information, hydropower unit operating status information, and network parameters, and further calculates the online short-circuit capacity and online short-circuit ratio of the PCC. The formula for calculating the online short-circuit ratio is as follows: (5) In the formula, For online short-circuit ratio, For the online short-circuit capacity at the common junction point, It represents the sum of the rated capacities of wind power and photovoltaic power at time t in the grid-connected state.

[0072] When all new energy sources are connected to the grid Degraded to a fixed rated total capacity P N Therefore, the online short-circuit capacity at PCC satisfies: (6) In the formula, As the system's baseline capacity, The Thevenin equivalent impedance for the AC system at the point of common coupling.

[0073] The online Thevenin equivalent impedance of the AC system satisfies any of the following forms: When the parameters of all hydro-generators are consistent: (7) When the parameters of each hydro-generator are not completely consistent: (8) in, The online equivalent impedance of the external AC system and transmission lines referred to the PCC. For the longitudinal axis transient reactance of a single hydro-generator, The transient reactance of the longitudinal axis of the m-th hydro-generator is... This represents the current number of operating hydroelectric generators. It is the imaginary unit.

[0074] To accommodate the possibility of new energy units being disconnected from the grid or having their capacity reduced, the sum of the rated capacities of wind power and photovoltaic power currently still connected to the grid is defined as: (9) In the formula, Ω wind (t) represents the set of wind turbine units in grid-connected state at time t, Ω PV(t) Let be the set of photovoltaic units in grid-connected state at time t. The rated capacity of the wind turbine unit. This refers to the rated capacity of the photovoltaic unit.

[0075] In a specific embodiment of the present invention, in addition to the online short-circuit ratio Furthermore, the actual voltage response after a fault needs to be included in the criterion. This invention constructs a voltage over-limit margin based on the measured PCC voltage within a preset monitoring window, including low voltage over-limit margin or high voltage over-limit margin. The calculation method for the voltage over-limit margin is as follows: The equivalent fault crossing boundary after aggregation for wind power and photovoltaic power plants is defined as follows: (10) (11) in, For low-voltage equivalent fault crossing the boundary, For the low-voltage crossover boundary of the i-th type of grid-connected renewable energy, For high-voltage equivalent fault crossing the boundary, For the i-th type of grid-connected new energy, the high-voltage crossover boundary is defined.

[0076] When there is a significant voltage drop between the renewable energy power station and the PCC (Power Control Center), the fault crossing boundary is recalculated to the PCC side before comparison. When a fault occurs, within the preset monitoring window... tf , tf+Tw Within this range, a low voltage over-limit margin is constructed based on the measured voltage at the point of common coupling. and high voltage over-limit margin The calculation formula is as follows: (12) (13) in, The measured voltage at the point of common coupling, when When >0, it indicates that the PCC voltage has fallen below the equivalent low voltage crossing boundary; when When the value is greater than 0, it indicates that the PCC voltage exceeds the equivalent high voltage crossing boundary.

[0077] Specifically, step S6 determines whether transient voltage instability has occurred based on the online short-circuit ratio, the system critical short-circuit ratio, and the voltage over-limit margin, and outputs the judgment result, which further includes: S61: When the online short-circuit ratio is lower than the system's critical short-circuit ratio, and the measured voltage at the point of common coupling exceeds the corresponding voltage over-limit margin, the system is determined to have experienced transient voltage instability, specifically including: When the online short-circuit ratio of the AC system at the common connection point meets the requirement If the low voltage exceeds the limit margin If the value is greater than 0, the system is determined to have experienced undervoltage disconnection-type transient voltage instability. If the high voltage exceeds the limit margin... If the value is greater than 0, the system is determined to have experienced overvoltage disconnection-type transient voltage instability. When the online short-circuit ratio of the AC system at the common connection point meets the requirement If the low voltage exceeds the limit margin If the value is greater than 0, the system is determined to have experienced undervoltage disconnection-type transient voltage instability. When the online short-circuit ratio of the AC system at the common connection point meets the requirement If the fault is cleared, it is determined that the system will not experience transient voltage instability after the fault ends. and Let be the critical short-circuit ratio of the system, and < , The online short-circuit ratio of the communication system at the common connection point.

[0078] S62: Output the judgment result of undervoltage disconnection type transient voltage instability or overvoltage disconnection type transient voltage instability according to the voltage over-limit direction.

[0079] In a specific embodiment of the present invention, the present invention establishes a two-layer screening mechanism of "grid strength-voltage boundary" to comprehensively analyze the real-time grid strength index (online short-circuit ratio) obtained from online monitoring and the voltage safety index (voltage over-limit margin), thus overcoming the limitations of a single threshold criterion and achieving accurate and rapid classification and identification of transient voltage instability. Specifically, at the time of fault occurrence... and the subsequent preset monitoring window Internally, the online short-circuit ratio and voltage over-limit margin calculated in step S5 are comprehensively called, and the following hierarchical logic is used to make a judgment: 1. When the online short-circuit ratio is lower than the system's critical short-circuit ratio, and the measured voltage at the point of common coupling exceeds the corresponding voltage over-limit margin, the system is deemed to have experienced transient voltage instability, specifically including the following three scenarios: 1) When the online short-circuit ratio of the AC system at the point of common coupling (PCC) meets the following conditions: This indicates that the AC grid strength at the point of common coupling (PCC) is extremely weak, and the system's ability to withstand large disturbances is severely insufficient. If the low voltage exceeds the limit margin... If the voltage is greater than 0 (i.e., the PCC voltage drops below the equivalent low voltage crossing boundary), then the system is determined to have experienced undervoltage disconnection-type transient voltage instability. This type of instability is usually caused by serious faults such as three-phase short circuits, leading to the protection tripping of new energy units due to excessively low voltage. If the high voltage exceeds the limit margin... If the value is greater than 0 (i.e., the PCC voltage exceeds the equivalent high voltage crossing boundary), then the system is determined to have experienced overvoltage disconnection-type transient voltage instability. This type of instability is usually caused by faults such as DC single-pole blocking that lead to excess active power, resulting in overvoltage and causing the unit to disconnect from the grid.

[0080] 2) When the online short-circuit ratio of the AC system at the point of common connection meets the following conditions: This indicates that the power grid strength in this area is moderate, and the system can withstand overvoltage surges, but its tolerance to deep undervoltage faults remains limited. If the undervoltage exceedance margin... If the value is greater than 0, the system is determined to have experienced undervoltage disconnection transient voltage instability.

[0081] 3) When the online short-circuit ratio of the AC system at the point of common coupling (PCC) meets the following requirements: When the system has sufficient short-circuit capacity and voltage support capability, it indicates that the system will not experience transient voltage instability after the fault ends, and no subsequent safety control measures need to be triggered, regardless of how the voltage fluctuates during the fault.

[0082] 2. When a transient voltage instability is detected in the system, the system outputs a judgment result indicating whether the transient voltage instability is undervoltage-to-grid disconnection or overvoltage-to-grid disconnection, based on the direction of the voltage exceeding the limit. This judgment result can be directly uploaded to the energy management system (EMS) or the stabilization device. For example, when the system is judged to be "undervoltage-to-grid disconnection instability," it can automatically trigger the emergency reactive power compensation device to activate or disconnect a portion of the DC power; when the system is judged to be "overvoltage-to-grid disconnection instability," it can trigger a rapid reduction in DC power or activate the dynamic reactive power absorption device, thereby achieving targeted prevention and emergency control.

[0083] In a specific embodiment of the present invention, such as Figure 8 The diagram shows a three-phase short-circuit fault set on the sending-end AC system line. The fault occurs at 1.0s and the faulty line is disconnected after 100ms.

[0084] When the hydropower unit output is 2000MW, the calculated short-circuit ratio at the point of common coupling is 2.151, which is less than the critical short-circuit ratio. After the fault occurred, the voltage at the point of common coupling (PCC) dropped rapidly, and the voltage at the wind turbine terminals decreased synchronously. The voltages at the PCC and the wind turbine terminals were as follows: Figure 9 and Figure 10 As shown.

[0085] Based on the online judgment process, the following conditions are met before the fault occurs. Furthermore, the fault process further satisfies If the voltage drops below the fault ride-through requirement, the system is determined to have experienced undervoltage grid disconnection transient voltage instability. In this embodiment, the wind turbine's terminal voltage is lower than the fault ride-through requirement, triggering the grid disconnection protection, indicating that the system has experienced undervoltage grid disconnection transient voltage instability.

[0086] In another embodiment, a single-pole blocking control signal is applied to the DC transmission system. The single-pole blocking occurs at 1.0s, and 0.2s after the blocking occurs, half of the AC filter and half of the parallel compensation capacitor at the sending end are disconnected. At this time, the original active power transmission and reactive power balance of the system is rapidly disrupted, and the voltage at the point of common coupling rises rapidly.

[0087] When the hydropower unit output is 1000MW, the offline calculation shows that the short-circuit ratio at the point of common coupling is 1.327, which is less than the critical short-circuit ratio. After the fault occurred, the voltage at the wind turbine and photovoltaic power station terminals rose rapidly, exceeding their respective fault ride-through limits. The voltage at the PCC and the voltage at the wind turbine and photovoltaic inverter terminals were as follows: Figure 11 , Figure 12 and Figure 13 As shown.

[0088] Based on the online judgment process, the following conditions are met before the fault occurs. Furthermore, the fault process further satisfies If the voltage exceeds the fault ride-through limit, the system is determined to have experienced overvoltage grid disconnection transient voltage instability. In this embodiment, the new energy units triggered grid disconnection protection because the voltage at the wind turbine and photovoltaic power station terminals exceeded the fault ride-through limit, indicating that the system has experienced overvoltage grid disconnection transient voltage instability.

[0089] In summary, the PCC transient voltage instability discrimination method for a water-wind-solar coupled DC transmission system provided by this invention has the following advantages: 1. The project is highly feasible and has the capability for online application.

[0090] The input quantities required for the discrimination method proposed in this invention (such as PCC three-phase voltage, current, unit operating status, fault signals, etc.) can all be obtained online by existing energy management systems (EMS), supervisory control systems (SCADA), synchronous phasor measurement units (PMU), and protection devices, or obtained through simple online estimation. This method does not rely on complex offline simulation iterations, is fully adaptable to online risk assessment and early warning platforms of actual power grids, and possesses extremely high engineering application value and operability.

[0091] 2. The discrimination mechanism is improved, enabling accurate classification of instability types.

[0092] Existing technologies often employ a single voltage threshold, making it difficult to differentiate the nature of faults. This invention constructs a two-layer screening mechanism based on "grid strength-voltage boundary," which can not only accurately identify the occurrence of transient voltage instability but also effectively distinguish between undervoltage-induced grid disconnection transient voltage instability caused by three-phase short circuits and overvoltage-induced grid disconnection transient voltage instability caused by unipolar blocking. This classification and discrimination capability provides a crucial basis for subsequently developing targeted safety control strategies (such as reactive power compensation during undervoltage and DC reduction during overvoltage).

[0093] 3. It has strong adaptability and reflects the dynamic evolution of power grid strength.

[0094] To address the challenge of dynamic changes in grid strength (short-circuit ratio) caused by significant fluctuations in hydropower output under varying hydrological conditions in hydro-wind-solar coupled systems, this invention introduces the concept of online short-circuit ratio. By taking into account real-time changes in the number of operating hydropower units and the actual grid-connected capacity of renewable energy sources, the equivalent impedance and short-circuit capacity at the PCC are dynamically calculated, solving the problem of poor adaptability and susceptibility to misjudgment in scenarios with varying operating modes using traditional static criteria.

[0095] 4. The criteria are highly robust, balancing safety and economy.

[0096] This invention effectively avoids the jitter and malfunctions near the critical stability point caused by the single threshold method by combining offline calibration of the critical short-circuit ratio with online voltage over-limit margin for collaborative discrimination. This method minimizes intervention in normal control while ensuring system safety and stability (preventing large-scale grid disconnection), thus improving the system's operational economy.

[0097] 5. It has a wide range of applications and high promotional value.

[0098] This invention is specifically designed for the complex characteristics of multi-energy complementary DC transmission systems involving water, wind, and solar power, and is suitable for weak grid environments with a high proportion of power electronic equipment connected. Its core technical concept—based on a discrimination framework of "offline critical parameter calibration + online intensity and boundary collaborative verification"—can also be extended to transient voltage stability monitoring in other types of new energy bases (such as pure wind and solar bases, desert and Gobi bases, etc.).

[0099] The concepts, principles, and ideas of the present invention have been described in detail above with reference to specific embodiments (including examples and instances). Those skilled in the art should understand that the embodiments of the present invention are not limited to those given above. After reading this application, those skilled in the art can make any possible improvements, substitutions, and equivalents to the steps, methods, systems, and components in the above embodiments. These improvements, substitutions, and equivalents should be considered to fall within the scope of the present invention, and the scope of protection of the present invention is limited to the claims.

Claims

1. A method for determining PCC transient voltage instability in a hydro-wind-solar coupled DC transmission system, characterized in that, include: Offline system parameters of a hydro-wind-solar coupled DC transmission system were collected and an electromagnetic transient simulation model was constructed. Based on the electromagnetic transient simulation model, typical fault transient simulations are performed on the system, and the voltage stability of the simulation results is judged to obtain the first voltage stability result. Typical fault transient simulations of the system were repeated under different hydrological periods and different hydropower unit output conditions, and the voltage stability of the simulation results was judged to obtain the second voltage stability result. The short-circuit ratio at the common junction is calculated based on the simulation results under different hydrological periods, and the critical short-circuit ratio of the system under fault is determined based on the first voltage stability results, the second voltage stability results and the short-circuit ratio. Collect online system parameters at the point of common coupling and calculate the online short-circuit ratio and voltage over-limit margin of the system at the point of common coupling; The system determines whether transient voltage instability has occurred based on the online short-circuit ratio, the system critical short-circuit ratio, and the voltage over-limit margin, and outputs the judgment result.

2. The PCC transient voltage instability discrimination method for a water-wind-solar coupled DC transmission system according to claim 1, characterized in that, The process of collecting offline system parameters of the hydro-wind-solar coupled DC transmission system and constructing an electromagnetic transient simulation model further includes: In offline mode, offline system parameters of hydropower units, wind power units, photovoltaic power stations and high-voltage direct current transmission are collected; An electromagnetic transient simulation model of a hydro-wind-solar coupled DC transmission system was constructed based on offline system parameters.

3. The PCC transient voltage instability discrimination method for a water-wind-solar coupled DC transmission system according to claim 1, characterized in that, The step of performing typical fault transient simulations on the system based on the electromagnetic transient simulation model and determining the voltage stability of the simulation results to obtain the first voltage stability result specifically includes: Based on the electromagnetic transient simulation model, fault transient simulations of the system were performed for single-phase short circuit, three-phase short circuit, commutation failure, and single-pole blocking, respectively. Based on the simulation results, the transient voltage stability evaluation criteria were used to determine the transient voltage stability under single-phase short circuit, three-phase short circuit, commutation failure, and single-pole blocking, respectively, and the first voltage stability result was obtained.

4. The PCC transient voltage instability discrimination method for a water-wind-solar coupled DC transmission system according to claim 1, characterized in that, The process of repeatedly performing transient simulations of typical faults on the system under different hydrological periods and different hydropower unit output conditions, and determining the voltage stability of the simulation results to obtain a second voltage stability result, further includes: Using the typical scenario induction method, the output of hydropower units was changed under different hydrological periods, and the transient simulation of typical faults of the system was repeated. Based on the simulation results, the transient voltage stability of typical faults under different hydrological periods was determined using the transient voltage stability evaluation standard, and the second voltage stability result was obtained.

5. The PCC transient voltage instability discrimination method for a water-wind-solar coupled DC transmission system according to claim 1, characterized in that, The formula for calculating the short-circuit ratio is: Wherein, SCR is the short-circuit ratio at the point of common coupling. For the short-circuit capacity at the common junction, This represents the total rated installed capacity of wind power and photovoltaic power.

6. The PCC transient voltage instability discrimination method for a water-wind-solar coupled DC transmission system according to claim 1, characterized in that, The online system parameters include: three-phase voltage, three-phase current, hydropower unit operating status information, wind turbine unit operating status information, photovoltaic unit operating status information, and fault and protection action signals.

7. The PCC transient voltage instability discrimination method for a water-wind-solar coupled DC transmission system according to claim 1, characterized in that, The formula for calculating the online short-circuit ratio is: in, For online short-circuit ratio, For the online short-circuit capacity at the common junction point, Let be the sum of the rated capacities of wind power and photovoltaic power at time t in grid-connected state. As the system's baseline capacity, The Thevenin equivalent impedance of the AC system at the point of common coupling, Ω wind (t) represents the set of wind turbine units in grid-connected state at time t, Ω PV(t) Let be the set of photovoltaic units in grid-connected state at time t. The rated capacity of the wind turbine unit. This refers to the rated capacity of the photovoltaic unit.

8. The PCC transient voltage instability discrimination method for a water-wind-solar coupled DC transmission system according to claim 1, characterized in that, The method for calculating the voltage over-limit margin is as follows: The equivalent fault crossing boundary after aggregation for wind power and photovoltaic power plants is defined as follows: in, For low-voltage equivalent fault crossing the boundary, For the low-voltage crossover boundary of the i-th type of grid-connected renewable energy, For high-voltage equivalent fault crossing the boundary, For the high-voltage crossover boundary of the i-th type of grid-connected renewable energy; When a fault occurs, in the preset monitoring window [ tf , tf+Tw Within this range, a low voltage over-limit margin is constructed based on the measured voltage at the point of common coupling. and high voltage over-limit margin The calculation formula is as follows: in, This is the measured voltage at the point of common coupling.

9. The PCC transient voltage instability discrimination method for a water-wind-solar coupled DC transmission system according to claim 1, characterized in that, The method of determining whether transient voltage instability has occurred based on online short-circuit ratio, system critical short-circuit ratio, and voltage over-limit margin, and outputting the judgment result, further includes: When the online short-circuit ratio is lower than the system's critical short-circuit ratio, and the measured voltage at the point of common coupling exceeds the corresponding voltage over-limit margin, the system is determined to have experienced transient voltage instability. The determination result of undervoltage disconnection type transient voltage instability or overvoltage disconnection type transient voltage instability is based on the direction of voltage exceeding the limit.

10. The PCC transient voltage instability discrimination method for a water-wind-solar coupled DC transmission system according to claim 9, characterized in that, The determination that the system has experienced transient voltage instability when the online short-circuit ratio is lower than the system's critical short-circuit ratio and the measured voltage at the point of common coupling exceeds the corresponding voltage over-limit margin further includes: When the online short-circuit ratio of the AC system at the common connection point meets the requirement If the low voltage exceeds the limit margin If the value is greater than 0, the system is determined to have experienced undervoltage disconnection-type transient voltage instability. If the high voltage exceeds the limit margin... If the value is greater than 0, the system is determined to have experienced overvoltage disconnection-type transient voltage instability. When the online short-circuit ratio of the AC system at the common connection point meets the requirement If the low voltage exceeds the limit margin If the value is greater than 0, the system is determined to have experienced undervoltage disconnection-type transient voltage instability. When the online short-circuit ratio of the AC system at the common connection point meets the requirement If the fault is cleared, it is determined that the system will not experience transient voltage instability after the fault ends. and Let be the critical short-circuit ratio of the system, and < , The online short-circuit ratio of the communication system at the common connection point.

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