A method and system for analyzing safety and stability margin of an extra-high voltage direct current receiving end power grid

Abstract

The application relates to the technical field of power systems, and discloses a method and system for analyzing the safety and stability margin of an ultra-high voltage direct current receiving-end power grid, wherein the method comprises the following steps: generating a first evaluation index for evaluating the voltage stability margin of the ultra-high voltage direct current receiving-end power grid according to the correlation rule between voltage stiffness and dynamic reactive power compensation devices; generating a second evaluation index for evaluating the power angle stability margin of the ultra-high voltage direct current receiving-end power grid according to the correlation rule between channel strength and non-synchronous motor power sources; generating a third evaluation index for evaluating the frequency stability margin of the ultra-high voltage direct current receiving-end power grid according to the correlation rule between frequency support strength and non-synchronous motor power sources; evaluating the power grid safety and stability margin according to the first evaluation index, the second evaluation index and the third evaluation index, and generating corresponding improvement measures, so that the technical problems of evaluating the safety and stability margin of the ultra-high voltage direct current receiving-end power grid and improving the safety and stability margin of the power grid based on the evaluation are solved.

Description

Technical Field

[0001] This application relates to the field of power system technology, and in particular to a method and system for analyzing the safety and stability margin of an ultra-high voltage direct current receiving-end power grid. Background Technology

[0002] With the rapid development and engineering practice of ultra-high voltage direct current (UHVDC) large-capacity power transmission and new energy sources, the number of receiving-end power grids to DC landing points is increasing, and the DC landing points are becoming more densely distributed. This expands the fault propagation range of regional power grids, and the risks to power grid safety and stability are constantly increasing. To improve the safety and stability level of the receiving-end power grid under the background of multiple centralized DC feeds, it is crucial to study a safety and stability margin analysis method for UHVDC receiving-end power grids based on the correlation laws of multiple types of power sources. Summary of the Invention

[0003] This application provides a method and system for analyzing the safety and stability margin of an ultra-high voltage direct current (UHVDC) receiving-end power grid, which solves the technical problems of evaluating the safety and stability margin of the UHVDC receiving-end power grid and improving the safety and stability margin of the power grid based on the evaluation.

[0004] To achieve the above objectives, the main technical solutions adopted in this application include:

[0005] In a first aspect, embodiments of this application provide a method for analyzing the safety and stability margin of an ultra-high voltage direct current (UHVDC) receiving-end power grid. The method comprises: generating a first evaluation index for assessing the voltage stability margin of the UHVDC receiving-end power grid based on the correlation between voltage stiffness and dynamic reactive power compensation devices, including voltage stiffness; generating a second evaluation index for assessing the power angle stability margin of the UHVDC receiving-end power grid based on the correlation between channel strength and asynchronous power sources, including damping ratio and maximum generator power angle difference; generating a third evaluation index for assessing the frequency stability margin of the UHVDC receiving-end power grid based on the correlation between frequency support strength and asynchronous power sources, including DC feed-in ratio, maximum single DC feed-in ratio, and critical single DC feed-in ratio; and evaluating the power grid safety and stability margin based on the first, second, and third evaluation indices, and generating corresponding improvement measures to enhance the power grid safety and stability margin.

[0006] This application proposes a method for analyzing the safety and stability margin of an ultra-high voltage direct current (UHVDC) receiving-end power grid. Based on the correlation between voltage stiffness and dynamic reactive power compensation devices, a first evaluation index is generated to assess the voltage stability margin of the UHVDC receiving-end power grid, including voltage stiffness. Based on the correlation between channel strength and asynchronous power sources, a second evaluation index is generated to assess the power angle stability margin of the UHVDC receiving-end power grid, including damping ratio and maximum generator power angle difference. Based on the correlation between frequency support strength and asynchronous power sources, a third evaluation index is generated to assess the frequency stability margin of the UHVDC receiving-end power grid, including DC feed-in ratio, maximum single DC feed-in ratio, and critical single DC feed-in ratio. Based on the first, second, and third evaluation indices, the safety and stability margin of the power grid is evaluated, and corresponding improvement measures are generated to enhance the safety and stability margin of the power grid. This method solves the technical problems of evaluating the safety and stability margin of the UHVDC receiving-end power grid and improving the safety and stability margin based on the evaluation. It can provide a reference for the planning and operation of AC / DC hybrid power grids, ensuring the safe and stable operation of the system.

[0007] Optionally, the expression for the voltage stiffness is:

[0008]

[0009] In the above formula, It is absolute voltage stiffness. It is relative voltage stiffness. These are the equipment impedance, additional impedance, and system impedance in the equivalent circuit for practical calculation of voltage stiffness.

[0010] Optionally, the step of generating a second evaluation index for assessing the power angle stability margin of the UHVDC receiving-end grid based on the correlation between channel strength and asynchronous power supply includes the damping ratio and the maximum power angle difference of the generator. Specifically, this includes: establishing a simple two-machine system model powered by an LCC-HVDC system connected to the PCC bus; establishing system equations during normal operation, including the generator rotor motion equation and the active power transmission equation of the AC side network; establishing the active power transmission equation after DC power loss, and the generator power angle change equation after DC blocking; generating a second-order differential equation for the change in generator power angle difference, calculating the damping ratio and the maximum power angle difference of the generator; and evaluating the power angle stability margin of the UHVDC receiving-end grid based on the damping ratio and the maximum power angle difference of the generator.

[0011] Optionally, the expression for the damping ratio is:

[0012]

[0013] In the above formula, For the damping ratio, The coefficients of the first-order terms, , These are the moments of inertia of the first and second generators in a simple two-machine system, respectively. , These are the impedances of the first transformer and the second transformer, respectively. It is the impedance on the connecting line;

[0014] The expression for the maximum power angle difference between the first generator and the second generator is:

[0015]

[0016] in,

[0017]

[0018]

[0019] In the above formula, This represents the maximum power angle difference between the first and second generators. The power angle difference between the first generator and the second generator when the system reaches the peak head-swing; The power angle difference between the first generator and the second generator in the system under steady state; , These are the initial power angles of the first and second generators, respectively. It is the overshoot of the system's step response. A1 and A2 are the electrical distance between the first and second generators; A1 and A2 are coefficients.

[0020] Optionally, the step of generating a third evaluation index for assessing the frequency stability margin of the UHVDC receiving-end grid based on the correlation between frequency support strength and asynchronous power supply includes the DC feed-in ratio, the maximum single DC feed-in ratio, and the critical single DC feed-in ratio. Specifically, this includes: establishing a frequency stability model for a high-proportion renewable energy receiving-end grid; calculating the wind farm power generation penetration rate, the DC feed-in ratio, and the maximum single DC feed-in ratio; calculating the frequency deviation factor based on the wind farm power generation penetration rate and the DC feed-in ratio; calculating the critical single DC feed-in ratio based on the frequency deviation factor; and evaluating the frequency stability margin of the UHVDC receiving-end grid based on the DC feed-in ratio, the maximum single DC feed-in ratio, and the critical single DC feed-in ratio.

[0021] Optionally, the expression for the power generation penetration rate of the wind farm is:

[0022]

[0023] In the above formula, To provide power to wind turbines; K represents the total output of the system's generators. PCON Power factor ratio; It is equivalent to the capacity of a traditional generator set;

[0024] The expression for the DC feed ratio is:

[0025]

[0026] In the above formula, This refers to the DC feed ratio; This represents the DC feed power of the i-th line;

[0027] The expression for the maximum single DC feed ratio is:

[0028]

[0029] The expression for the frequency deviation factor is:

[0030]

[0031] In the above formula, R is the frequency deviation factor; eq It is the equivalent droop factor for all conventional generators; D L The active power frequency regulation coefficient of the system load;

[0032] The expression for the critical single DC feed ratio is:

[0033]

[0034] In the above formula, The critical single DC feed ratio; This is the frequency deviation factor; The frequency deviation limit under normal operating conditions of the power system. The rated frequency of the power system.

[0035] Optionally, the step of evaluating the safety and stability margin of the power grid based on the first evaluation index, the second evaluation index, and the third evaluation index, and generating corresponding improvement measures to improve the safety and stability margin of the power grid, specifically includes: determining whether the second evaluation index has reached a second set threshold; if not, optimizing the power transmission curve to improve the power angle stability margin of the power grid; if yes, determining whether the third evaluation index has reached a third set threshold; if not, optimizing the power transmission curve to improve the frequency stability margin of the power grid; if yes, determining whether the first evaluation index has reached a first set threshold; if not, optimizing the power transmission curve to improve the voltage stability margin of the power grid.

[0036] Secondly, embodiments of this application provide a safety and stability margin evaluation system for an ultra-high voltage direct current (UHVDC) receiving-end power grid. The system includes: a voltage stability margin evaluation module, used to generate a first evaluation index for evaluating the voltage stability margin of the UHVDC receiving-end power grid based on the correlation between voltage stiffness and dynamic reactive power compensation devices, including voltage stiffness; a power angle stability margin evaluation module, used to generate a second evaluation index for evaluating the power angle stability margin of the UHVDC receiving-end power grid based on the correlation between channel strength and asynchronous power supply, including damping ratio and maximum generator power angle difference; a frequency stability margin evaluation module, used to generate a third evaluation index for evaluating the frequency stability margin of the UHVDC receiving-end power grid based on the correlation between frequency support strength and asynchronous power supply, including DC feed-in ratio, maximum single DC feed-in ratio, and critical single DC feed-in ratio; and a comprehensive evaluation module, used to evaluate the safety and stability margin of the power grid based on the first, second, and third evaluation indices, and generate corresponding improvement measures to enhance the safety and stability margin of the power grid.

[0037] Thirdly, embodiments of this application provide a computer device, including: a memory and a processor, wherein the memory and the processor are communicatively connected to each other, the memory stores computer instructions, and the processor executes the above-described method by executing the computer instructions.

[0038] Fourthly, embodiments of this application provide a computer-readable storage medium storing computer instructions for causing a computer to perform the methods described above.

[0039] Fifthly, embodiments of this application provide a computer program product, including computer instructions, which are used to cause a computer to perform the above-described method. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0041] Figure 1 A flowchart illustrating the method for analyzing the safety and stability margin of the UHVDC receiving-end power grid provided in this application embodiment;

[0042] Figure 2 A graph showing the variation of voltage stiffness with phase shifter capacity under strong system conditions provided in this application embodiment;

[0043] Figure 3A graph showing the variation of voltage stiffness with phase shifter capacity in a weak system, provided as an embodiment of this application.

[0044] Figure 4 A graph showing the variation of voltage stiffness with STATCOM capacity under strong system conditions provided in this application embodiment;

[0045] Figure 5 A graph showing the variation of voltage stiffness with STATCOM capacity in a weak system, provided in an embodiment of this application;

[0046] Figure 6 A frequency stability model diagram of a high-proportion renewable energy receiving-end power grid provided in this application embodiment;

[0047] Figure 7 A model diagram of the governor-prime mover of a reheat thermal power unit provided in the embodiments of this application;

[0048] Figure 8 A model diagram of the governor-prime mover of a hydroelectric generator provided in the embodiments of this application;

[0049] Figure 9 The curves showing the penetration rate of new energy power generation and the steady-state critical single DC feed-in ratio of the receiving-end power grid are provided for embodiments of this application.

[0050] Figure 10 A graph showing the calculation results of the transient critical single DC feed ratio of a wind farm without virtual inertia, provided in the embodiments of this application;

[0051] Figure 11 The following is a graph showing the influence of the wind farm output ratio on the transient critical single DC feed ratio provided in the embodiments of this application;

[0052] Figure 12 The curve showing the influence of the virtual inertia gain of the wind farm on the transient critical single DC feed ratio is provided in the embodiments of this application.

[0053] Figure 13 A diagram illustrating the safety and stability margin evaluation system for the UHVDC receiving-end power grid provided in this application embodiment;

[0054] Figure 14 A schematic diagram of the structure of a computer device provided in an embodiment of this application. Detailed Implementation

[0055] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0056] Compared to high-voltage AC transmission, line-commutated converter-based high-voltage direct current (LCC-HVDC) transmission has been widely used in power transmission projects due to its advantages such as the absence of synchronous operation stability issues, ease of interconnection with asynchronous AC grids, and rapid controllability of active power. With the development of DC transmission technology, the number and capacity of DC systems have increased dramatically, leading to the formation of multi-infeed DC systems where multiple LCC-HVDC lines converge at the same receiving-end AC grid. The centralized infeeding of conventional ultra-high-voltage DC lines poses significant safety risks to the receiving-end grid. Complex interactions exist between AC and DC lines, as well as between DC lines themselves, in multi-infeed DC systems, making voltage stability and commutation failure issues particularly prominent. This poses a significant threat to the safe and stable operation of the entire AC / DC system: on the one hand, simultaneous commutation failures of multiple DC lines can subject the receiving-end grid to power / energy surges, increasing safety risks; on the other hand, the single-point power input of conventional DC lines is enormous, resulting in significant frequency and voltage surges to the local 500kV grid during faults, requiring substantial power flow transfer capabilities and placing high demands on the adaptability of the receiving-end grid.

[0057] In densely populated AC / DC hybrid power grids, a short-circuit fault in the 500kV grid can easily cause multiple DC lines to fail to commutate simultaneously. Commutation failure leads to a drop in DC voltage, an increase in DC current, and a decrease in DC power. If multiple commutation failures occur in the DC system, it may lead to problems such as AC system instability and DC blocking. In multi-infeed DC systems, if the electrical distance between the multiple DC system landing points is close, a single short-circuit fault may cause multiple DC lines to fail to commutate simultaneously. Both simultaneous and multiple DC commutation failures seriously threaten the safe and stable operation of the power grid. With the rapid development and engineering practice of ultra-high-voltage DC high-capacity transmission and new energy sources, the number of DC landing points from the receiving-end grid is increasing, and the DC landing points are becoming more densely packed. The fault propagation range of the regional power grid is expanding, and the risks to the safety and stability of the power grid are constantly increasing.

[0058] To improve the safety and stability of the receiving-end power grid under the background of multiple centralized DC feeds, it is very important to study a safety and stability margin analysis method for UHVDC receiving-end power grid based on the correlation law of multiple types of power sources.

[0059] According to an embodiment of this application, an embodiment of a method for analyzing the safety and stability margin of an ultra-high voltage direct current receiving-end power grid is provided. It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions. Furthermore, although a logical order is shown in the flowchart, in some cases, the steps shown or described may be executed in a different order than that shown here.

[0060] Figure 1 This is a flowchart of the UHVDC receiving-end power grid safety and stability margin analysis method provided in the embodiments of this application, such as... Figure 1 As shown, the process includes the following steps:

[0061] Step S1: Based on the correlation between voltage stiffness and dynamic reactive power compensation device, generate a first evaluation index for evaluating the voltage stability margin of the UHVDC receiving-end grid, including voltage stiffness.

[0062] The expression for the voltage stiffness of the receiving-end power grid is:

[0063]

[0064] In the above formula, It is absolute voltage stiffness. It is relative voltage stiffness. These are the equipment impedance, additional impedance, and system impedance in the equivalent circuit for practical calculation of voltage stiffness.

[0065] Currently, there are two main types of dynamic reactive power compensation devices commonly used in power grids: synchronous condensers and STATCOMs. The former is in the form of a traditional synchronous machine, while the latter is in the form of a power electronic converter. Both target the control terminal voltage under normal conditions. Under normal conditions, thanks to the dynamic reactive power compensation device, the DC landing voltage does not change due to small fluctuations in DC power, and the voltage stiffness can be considered infinite. However, commutation failure and time-lapse recovery issues fall under the research scope of power grids in fault conditions. Under fault conditions, the impact of synchronous condensers and STATCOMs on voltage stiffness is drastically different.

[0066] This application first analyzes the impact of the synchronous condenser and STATCOM on voltage stiffness under fault conditions, and then analyzes the impact of the synchronous condenser and STATCOM on the time-lapse recovery strategy.

[0067] (1) Analysis of the effect of the synchronous condenser on voltage stiffness: Under fault conditions, the synchronous condenser can be equivalent to the Thevenin equivalent circuit of the voltage source impedance. Under fault conditions, the addition of the synchronous condenser will have a certain impact on the voltage stiffness of both DC systems. Generally speaking, the larger the capacity of the synchronous condenser, the smaller its equivalent reactance. The influence of the synchronous condenser capacity on the voltage stiffness of the two DC systems is analyzed by changing the value of the synchronous condenser capacity. For example... Figure 2 As shown, changing the capacity of the synchronous condenser will change the equivalent reactance of the generator set. Keeping the equivalent electromotive force Ed constant, we can obtain the changes in the voltage stiffness of the two DC systems as the synchronous condenser capacity changes. From... Figure 2 As can be seen, with the increase in the capacity of the synchronous condenser, the voltage stiffness of both DC systems also increases, but the increase is very small. Since the external AC system is inherently a stronger system than the DC system, the addition of the synchronous condenser has a limited effect on improvement. Figure 3 As shown, if the external system strength is reduced, and the equivalent reactances X1 and X2 of the generator set are reduced from 0.2 pu to 0.8 pu, the change in voltage stiffness of the two DC systems with the synchronous condenser capacity is recalculated: from Figure 3 As can be seen, if the strength of the external system weakens, the effect of the synchronous condenser on improving voltage stiffness becomes very significant.

[0068] (2) Analysis of the effect of STATCOM on voltage stiffness: Under fault conditions, STATCOM will reach current limiting and can be equivalent to a constant current source circuit. Under fault conditions, the addition of STATCOM will also have a certain impact on the voltage stiffness of the two DC systems, mainly affecting the no-load voltage of the two DC systems, but having a small impact on the equivalent impedance of the system. The effect of STATCOM on the voltage stiffness of the two DC systems is analyzed by changing the STATCOM capacity value. For example... Figure 4 As shown, changing the capacity of the STATCOM will change the maximum current of the STATCOM, revealing how the voltage stiffness of the two DC systems changes with the STATCOM capacity. From... Figure 4 As can be seen, with the increase of STATCOM capacity, the voltage stiffness of both DC systems also increases, albeit by a small margin. In strong systems, the effect of STATCOM on improving voltage stiffness is almost negligible. Figure 5 As shown, if the external system strength is reduced, and the equivalent reactances X1 and X2 of the unit are reduced from 0.2 pu to 0.8 pu, the voltage stiffness of the two DC systems as a function of the STATCOM capacity is recalculated. Figure 5 As can be seen, if the strength of the external system weakens, the effect of STATCOM on improving voltage stiffness becomes very significant. (Comparison) Figure 3 and Figure 5It can be observed that, under the same capacity of synchronous condenser and STATCOM, the synchronous condenser has a more significant effect on improving DC voltage stiffness.

[0069] Based on the above analysis of the impact of synchronous condensers and STATCOMs on voltage stiffness, the following conclusions can be drawn: 1) In strong systems, dynamic reactive power compensation devices, represented by synchronous condensers or STATCOMs, have little impact on the fault-state voltage stiffness of the DC system. The voltage stiffness of the DC system mainly depends on the external AC system. Therefore, in strong systems, dynamic reactive power compensation devices have little impact on the time-lapse recovery strategy. 2) In weak systems, dynamic reactive power compensation devices, represented by synchronous condensers or STATCOMs, can significantly improve the voltage stiffness of the DC system, thereby accelerating the recovery speed of commutation failures in various DC systems and reducing energy losses during the commutation failure recovery process. 3) Compared with STATCOMs of the same capacity, synchronous condensers have a more significant effect on improving the fault-state voltage stiffness of the DC system, thus having a better effect on accelerating the recovery speed of commutation failures in the DC system.

[0070] Analyze the impact of synchronous condensers and STATCOMs on the fault recovery strategy: Consider increasing the capacity of synchronous condensers and STATCOMs, or replacing STATCOMs with synchronous condensers of equal capacity, so as to better improve the voltage stiffness of DC system fault states and more significantly accelerate the recovery speed of DC system commutation failure.

[0071] Step S3: Based on the correlation between channel strength and asynchronous power supply, a second evaluation index is generated to assess the power angle stability margin of the UHVDC receiving-end grid, including damping ratio and maximum generator power angle difference, specifically including:

[0072] Step S301: Establish a simple two-generator system model as a receiving-end AC system, powered by an LCC-HVDC system connected to the PCC bus. For simplicity, the load on the second bus is considered a constant impedance load. For the two generators on the AC side, a second-order model considering only the rotor motion equations is adopted, and it is assumed that the two generators can maintain a constant transient electromotive force E' when the generator capacity and excitation current meet the requirements.

[0073] Step S303: Establish the system equations for normal operation, including the generator rotor motion equations and the active power transmission equations of the AC side network;

[0074] The rotor motion equations for the first and second generators are as follows:

[0075] ,

[0076]

[0077] In the above formula, , These are the power angles of the first generator and the second generator, respectively. , These are the rotational speeds of the first generator and the second generator, respectively. , These are the moments of inertia of the first and second generators in a simple two-machine system, respectively. , The damping coefficient; , These are the electromagnetic powers of the first generator and the second generator, respectively. , These are the mechanical power of the first generator and the second generator, respectively.

[0078] The electromagnetic power outputs of the first generator and the second generator are respectively:

[0079] ,

[0080]

[0081] In the above formula, , These are the transient electromotive forces of the first generator and the second generator, respectively. , These are the voltage amplitudes at the first busbar and the second busbar, respectively. , The reactances of the first generator and the second generator are respectively. , These are the phase angles of the first generator and the second generator, respectively. , These are the transient reactances of the first generator and the second generator, respectively.

[0082] Active power transmission equations for AC-side networks:

[0083]

[0084] In the above formula, The active power of the AC-side network. This refers to the reactance of the AC-side network.

[0085] Injected power balance equations at the first and second busbars:

[0086]

[0087]

[0088] In the above formula, The power injected into the first bus, This is the power output from the second bus to the load.

[0089] DC power loss can cause power angle fluctuations between generators. If the power angle difference between two generators reaches a certain value during this fluctuation, power angle instability between the two generators may occur, requiring a second line of defense to disconnect a certain capacity of the generators to bring the system back to the stable region. For ease of formula derivation, the following analysis is based on the following assumptions: The voltage at each node on the AC side is approximately 1; during the transient process after DC power loss, there will be no large voltage fluctuations on the AC side, and the voltage values ​​at each node will remain at 1, so the load size remains unchanged; the power angle difference between the two generators is small before and after the fault, and can be used to replace the sine value of the power angle difference, i.e., calculations can be performed using DC power flow. Although this assumption has a larger error after the power angle fluctuation, this study mainly aims to investigate the mechanism and influencing factors of the transient process, without precise calculations, so this assumption is considered acceptable; when studying the power angle fluctuation problem between the two generators, it is assumed that the speed governor has not yet had time to activate, and the mechanical power remains unchanged.

[0090] Based on the above assumptions, the load is a constant power model, the active load size remains unchanged, the resistance in the network is ignored, and there is no network loss. The sum of the output of the first generator and the second generator is the active power of the load.

[0091] Step S305: Establish the active power transmission equation after DC power loss, and the generator power angle change equation after DC blocking.

[0092] The active power transfer equation at time t after DC power loss is:

[0093]

[0094]

[0095]

[0096] The expression for the DC blocking time θ2 is as follows:

[0097]

[0098] in,

[0099] Substitute the expression for θ2 into P e1 (t) and P e2 From the expression of (t), we obtain P. e1 (t) and P e2 (t) New expression:

[0100]

[0101]

[0102] Let the initial powers of the first and second generators before DC blocking be P, respectively. e1 (0 - ) and P e2 (0 - And satisfy the following formula:

[0103]

[0104] At any given moment after DC blocking, the change in electromagnetic power of the first and second generators is:

[0105]

[0106]

[0107] Initial output power of the first and second generators , and connecting line L 12 Transmit initial power It can be represented in the following form:

[0108]

[0109]

[0110]

[0111] Combining the above three equations, we can solve for the outputs of the first and second generators under steady-state conditions before DC blocking:

[0112]

[0113]

[0114] After sorting, we can obtain:

[0115]

[0116]

[0117] The equations for the change in power angle of the first and second generators after DC blocking are as follows:

[0118]

[0119]

[0120] in,

[0121] ,

[0122] Further orders:

[0123]

[0124]

[0125] In the above formula, This represents the change in the power angle difference between the first and second generators. These are the coefficients of the first-order term.

[0126] Step S307: Generate the second-order differential equation for the change in generator power angle difference, and calculate the damping ratio and the maximum power angle difference of the generator;

[0127] By using the equations for the change in power angle of the first and second generators after DC blocking, the second-order differential equation for the change in the power angle difference between the first and second generators can be obtained:

[0128]

[0129] The above equation represents a second-order system, and its corresponding characteristic equation is:

[0130]

[0131] The characteristic equation of a second-order system has the following standard form:

[0132]

[0133] In the above formula, ξ is the damping ratio, ω n This is the natural oscillation frequency.

[0134] According to the time-domain analysis theory of second-order linear systems, when the damping ratio ξ satisfies 0 < ξ < 1, the system belongs to an underdamped second-order linear system, h(∞) is the steady-state value of the response of the second-order linear system, and t p h(t) is the time it takes for the system to reach the peak value of the head swing. p () represents the peak value of the step response of a second-order linear system.

[0135] σ% is defined as the overshoot of the step response of a second-order linear system, and we have:

[0136]

[0137] Overshoot is only related to the system's damping ratio. For the system described above, the damping ratio is:

[0138]

[0139] As can be seen from the above formula, the damping ratio is only related to the inherent parameters of the generator and the line parameters, and has nothing to do with the DC connection location or the blocking capacity. That is, in a large system, once the two groups of generators of interest are determined, the damping ratio of their electromechanical oscillations is determined, regardless of which DC line or its capacity. DC power loss will not change the damping ratio of the electromechanical oscillations between the two groups of generators.

[0140] Besides the damping ratio, another important factor for power angle stability is the maximum power angle difference between the two generators during transient processes. If the maximum power angle difference is too large, it will cause the power angle between the two generators to diverge and become unstable. For a second-order linear system, the peak value of the change in power angle difference depends on the steady-state overshoot of the response.

[0141]

[0142]

[0143]

[0144] In the above formula, This represents the maximum power angle difference between the first and second generators. The power angle difference between the first generator and the second generator when the system reaches the peak head-swing; The power angle difference between the first generator and the second generator under steady-state system conditions; , These are the initial power angles of the first and second generators, respectively. It is the overshoot of the system's step response. A1 and A2 are the electrical distance between the first and second generators; A1 and A2 are coefficients.

[0145] Once two generator units are selected as the research objects, the electrical distance between the two units, generator parameters, damping ratio during electromechanical transients, and initial power angle difference are determined. The change in power angle difference then depends only on... The magnitudes of A1 and A2 are determined by the electrical distance between the generator set and the locked converter station, as well as the capacity of the locked DC power supply. For example, if the electrical distance between the first generator and the LCC converter station is closer than that of the second generator, then A1 will be greater than A2. If the inertial time constant of the first generator is smaller than that of the second generator, A1 will be further increased. The greater the difference in electrical distance between the first generator, the second generator, and the converter station, the smaller the rotor inertia time constant of the closer unit and the larger the rotor inertia time constant of the farther unit. In the transient process after DC blocking occurs, the peak value of the power angle difference between the two units will be greater.

[0146] The rotor inertia time constant of a generator varies depending on the type of generator set. The normal range for the rotor inertia time constant of hydropower and thermal power units is as follows:

[0147] Table 1 Normal Range of Rotor Inertia Time Constant of Unit

[0148]

[0149] The rotor inertia time constant of a generator is usually given as a reference value based on the generator's rated capacity. During calculations, the rotor inertia time constants of both generators need to be converted to values ​​under the same reference value. Under the same reference value, the inertia time constant of the smaller capacity unit is much smaller than that of the larger capacity unit. Therefore, in a transient process after DC blocking, the largest change in power angle difference usually occurs between the small unit near the blocked converter station and the large unit far away.

[0150] Based on the above analysis, the following conclusions can be drawn: When asynchronous power sources, represented by new energy sources and DC, experience power loss, the power flow shifts, causing a change in the power angle difference between the generators. During this electromechanical transient process, the damping ratio of the response to the change in the power angle difference between the two generators does not change with the capacity and location of the asynchronous power source, but is determined by the parameters of the two generators themselves and the electrical distance. This is based on the overshoot, damping ratio, and electrical distance between the two generators. The overshoot is less affected by electrical distance, while the steady-state power angle difference change is related to electrical distance. Proportional, therefore the electrical distance between the two machines The larger the value, the greater the change in power angle difference during the transient process. During the transient process following the loss of power from the asynchronous generator, the greater the electrical distance difference between the two units and the asynchronous generator, the smaller the rotor inertia time constant of the closer unit and the larger the rotor inertia time constant of the farther unit. This results in a larger peak value for the change in power angle difference between the two units. The trend of the power angle difference is that the power angle of the farther unit leads that of the closer unit.

[0151] Step S309: Evaluate the power angle stability margin of the UHVDC receiving-end grid based on the damping ratio and the maximum power angle difference of the generator.

[0152] The damping ratio characterizes the degree of electromechanical oscillation in a system. The damping ratio depends only on the inherent parameters of the generators and the line parameters, and is independent of the DC connection location and blocking capacity. Therefore, DC power loss will not change the damping ratio of the electromechanical oscillations between the two generator groups. Maximum power angle difference between generators. The power angle stability margin characterizes the system; the smaller the maximum power angle difference, the better the system's power angle stability. During the transient process after the loss of power from the asynchronous generator, the greater the difference in electrical distance between the two units and the asynchronous generator, the smaller the rotor inertia time constant of the closer unit and the larger the rotor inertia time constant of the farther unit. This results in a larger peak value of the change in power angle between the two units and a worse system power angle stability.

[0153] For example, if the damping ratio and the maximum power angle difference of the generator do not reach the second set threshold, in areas with abundant power sources, it is advisable to reduce the amount of electricity transmitted from new energy sources and conventional units through the same AC cross section, increase the output of new energy units at the load center, thereby avoiding the increase in the penetration rate of new energy sources from crowding out the transmission channels of conventional units, effectively alleviating the transmission pressure on the grid, and improving the power angle stability of some conventional units.

[0154] Step S5: Based on the correlation between frequency support strength and asynchronous power supply, a third evaluation index is generated to assess the frequency stability margin of the UHVDC receiving-end grid. This index includes the DC feed-in ratio, the maximum single DC feed-in ratio, and the critical single DC feed-in ratio. Specifically, it includes:

[0155] Establish a frequency stability model for the power grid with a high proportion of renewable energy receiving end, such as... Figure 6 As shown;

[0156] Calculate the wind farm's power generation penetration rate, DC feed-in ratio, and maximum single DC feed-in ratio;

[0157] The power generation penetration rate of a wind farm is calculated using the following formula. :

[0158] ,

[0159] in,

[0160]

[0161]

[0162]

[0163]

[0164]

[0165] In the above formula, P SYS S represents the total output of the system's generators. SYS For total installed capacity, P CON S CON These represent the equivalent output and capacity of a conventional generator set, P. W S WThese represent the wind turbine's output and capacity, respectively, in K. PCON The power factor ratio is the ratio of the active power output to the installed capacity of all conventional generators.

[0166] The DC feed ratio is calculated using the following formula. and maximum single DC feed ratio :

[0167]

[0168]

[0169] in, Let be the DC feed power of the i-th line.

[0170] The frequency deviation factor is calculated based on the power generation penetration rate and the DC feed-in ratio; the formula for calculating the frequency deviation factor is as follows:

[0171]

[0172]

[0173] in, R is the frequency deviation factor. eq D is the equivalent droop factor for all conventional generators. L R is the active power frequency regulation coefficient of the system load; both are per-unit values. i P Ni These are the droop coefficient and rated active power of the i-th generator, respectively.

[0174] The critical single DC feed ratio is calculated based on the frequency deviation factor; the formula for calculating the critical single DC feed ratio is as follows:

[0175]

[0176] in, The critical single DC feed ratio, The frequency deviation limit under normal operating conditions of the power system. The rated frequency of the power system.

[0177] The frequency stability margin of the UHVDC receiving-end grid is evaluated based on the DC feed-in ratio, maximum single DC feed-in ratio, and critical single DC feed-in ratio. The frequency intensity of the receiving-end grid containing asynchronous generator power sources is comprehensively evaluated using the following criteria: frequency deviation factor. Used to characterize the steady-state frequency support strength of the system A larger value indicates a stronger steady-state frequency support capability of the system. Under the constraints of the steady-state and transient frequency indices of the receiving-end power grid, the corresponding critical single DC feed-in ratios of the system are calculated as follows: and , and The larger the value, the stronger the system's ability to support steady-state and transient frequencies.

[0178] In some embodiments, the steady-state critical single DC feed-in ratio is analyzed under different new energy power generation penetration rates, specifically including:

[0179] The power supply model primarily considers thermal and hydropower units, with wind turbines representing new energy sources. The frequency stability model is as follows: Figure 6 As shown. Figure 6 As an equivalent conventional generator, a system frequency response model is adopted, assuming a uniform frequency across the entire grid. This allows the generator rotor motion equations to be equivalently aggregated into a single rotating generator model to simulate frequency variation characteristics. The equivalent synchronous generator rotor equations are:

[0180]

[0181] In the above formula, T M T E D represents the mechanical torque, electromagnetic torque, and damping coefficient, respectively, and H represents the damping coefficient. s The inertial time constant of the synchronous generator. H represents the rotational speed of the synchronous generator. When the mechanical torque and electromagnetic torque of a synchronous generator are unbalanced, the rotor accelerates or decelerates. s In this embodiment, a typical value of 4.0s is used.

[0182] The hybrid governor-prime mover model consists of reheat power plants (such as...) Figure 7 (as shown) and hydropower units (such as) Figure 8 The speed governor and prime mover models (shown) together constitute the mechanical power output P. M1 and P M2 This value is determined by the output ratio of the corresponding type of unit in the system, and typical parameters are shown in Table 2.

[0183] Table 2 Simple System Parameters

[0184]

[0185] Under three scenarios—single DC feed at the receiving end, multiple DC feeds at the receiving end with a ratio of 10%, and multiple DC feeds at the receiving end with a ratio of 20%—the steady-state critical single DC feed ratio is examined for different renewable energy power generation penetration rates. The calculation results are as follows: Figure 9 The lower frequency regulation curve is shown. Wind turbines can adjust their output through pitch angle control, but they typically operate at maximum wind energy capture, at which point the pitch angle cannot be further reduced. In the receiving-end grid, they are considered a constant power source. As wind power penetration increases, the system can withstand the steady-state critical single DC feed ratio. Linear decrease; the medium- to long-term role of DC feed-in into the receiving-end grid is equivalent to a power source, which is essentially consistent with the aforementioned impact of new energy power generation on the system's steady-state frequency. When the DC feed-in ratio γ increases, the output ratio of traditional generators decreases, and the primary frequency regulation capability decreases, such as... Figure 9 As shown in the figure. The calculation results show that, relying solely on the system's primary frequency regulation capability, the receiving-end grid has the lowest steady-state critical single DC feed-in ratio, which is only around 7%.

[0186] In some embodiments, the transient frequency characteristics of a receiving-end power grid with wind power and multiple DC feeds are analyzed after a DC bipolar blocking fault, specifically including:

[0187] To analyze in detail the transient frequency characteristics of a receiving-end power grid with wind power and multiple DC feeds after a DC bipolar blocking fault, a model was built in PSCAD / EMTDC software as follows: Figure 6 The system shown uses typical values ​​for the DFIG parameters.

[0188] First, we conduct a scenario analysis of wind farms without inertia support.

[0189] The scenario of a wind farm without inertia support includes the following three situations: the DFIG is not equipped with a virtual inertia auxiliary controller; the wind turbine is already operating at full power when the wind speed is at or above the rated wind speed; and the virtual inertia auxiliary controller is not functioning due to operational reasons. In the scenario of a wind farm without inertia support, firstly, when the traditional units in the receiving-end grid are reheat turbine units, the transient critical single DC feed-in ratio is assessed under single DC feed-in and multiple DC feed-in ratios of 20% and 30%, respectively. like Figure 10 As shown.

[0190] from Figure 10 It is evident that under single DC infeed, as the wind farm power generation penetration rate increases, the system's equivalent inertial time constant gradually decreases because the wind farm cannot provide inertial support to the system, leading to a decrease in the transient critical single DC infeed ratio. With multiple DC infeeds, without considering DC power modulation, the multiple DC infeed ratio... The impact on system frequency stability is essentially the same as that of wind farm power generation penetration: in When improving, The corresponding decrease. Furthermore, in the scenario of a wind farm without inertial support, simulation results show... It has a quasi-linear relationship with η.

[0191] Secondly, an analysis of the virtual inertia support capacity of the wind farm was conducted.

[0192] The virtual inertia control of wind turbines modifies the active power output value based on the comprehensive frequency deviation input of the receiving-end power grid, and temporarily releases or absorbs the energy stored in the wind turbine rotor. Therefore, its inertia support level for the system is not only related to the penetration rate of wind power generation, but also closely related to the installed capacity of the wind farm and the frequency change rate gain coefficient K.

[0193] First, taking a single DC-fed receiving-end grid as an example, with a frequency change rate gain coefficient K=10, we examine the wind farm output ratio P. W / S W Under scenarios of 40%, 60%, 80%, and 100%, the impact of wind farm power generation penetration rate on the transient critical single DC feed value is as follows: Figure 11 As shown. Under the same wind farm power generation penetration rate, different output ratios of wind farms correspond to different wind power installed capacities. Figure 11 Simulation results show that the larger the installed capacity of a wind farm, the stronger its ability to support transient frequency inertia, a characteristic similar to that of traditional generators. When the wind farm's output ratio is 100%, it can no longer provide inertial support due to the overcurrent limitation of the wind turbine.

[0194] Second, in the wind farm output ratio P W / S W Based on 60%, the influence of the gain coefficient K on the rate of change of frequency is examined separately. Figure 12 As shown. The previous analysis theoretically examined the frequency change rate gain coefficient K and the equivalent inertial time constant of the wind farm. However, in reality, due to variations in wind farm output and rotor speed, the actual values ​​did not reach the theoretical values. From... Figure 12 It is evident that the gain coefficient K has a significant impact on improving the inertia level of the receiving-end power grid. When K=20, the system inertia level is greatly improved.

[0195] Step S7: Based on the first evaluation index, the second evaluation index, and the third evaluation index, evaluate the safety and stability margin of the power grid and generate corresponding improvement measures to enhance the safety and stability margin of the power grid, specifically including:

[0196] If the second evaluation index value reaches the second set threshold, then optimize the power transmission curve to improve the power angle stability margin of the power grid; if it does, then optimize the third evaluation index value to improve the frequency stability margin of the power grid; if it does, then optimize the first evaluation index value to improve the voltage stability margin of the power grid.

[0197] In some embodiments, after obtaining the voltage, power angle, and frequency stability margin indices, a comprehensive evaluation is conducted on the safety and stability margins of the UHVDC receiving-end power transmission curve in Zhejiang Province before and after optimization. The specific steps are as follows:

[0198] Based on the actual needs of the project, the primary requirement is to ensure the power angle stability of the receiving-end power grid, meaning the maximum power angle difference between the system units must not exceed 180°. Building upon this, the system's voltage and frequency stability are analyzed, aiming to minimize the frequency response drop after a fault to no more than 0.5Hz and reduce the voltage recovery time as much as possible. When both voltage and frequency stability requirements are not met, the primary goal of power transmission curve optimization is to improve frequency stability, mitigating the frequency drop under DC blocking conditions. Secondly, improving the system's voltage recovery characteristics is also considered.

[0199] Before power transmission curve optimization, the fault characteristics of the regional power grid system under seven operating modes are summarized in Table 3:

[0200] Table 3 Summary of fault characteristics before power supply curve optimization

[0201]

[0202] First, determine the power angle stability margin: Under the above 7 operating modes, the maximum power angle difference of the system units does not exceed 180° and gradually decreases over time, so the power angle stability of the system meets the requirements.

[0203] Based on the fault characteristics summary in Table 3, the following conclusions are drawn:

[0204] Under both the low winter and mid-summer operating modes of 2025, the system's voltage recovery time does not exceed 700ms and the frequency response drop does not exceed 0.5Hz, indicating good overall system response characteristics. Therefore, only minor optimization of the power delivery curve is needed to improve the system's stability margin.

[0205] Under bipolar blocking conditions, the frequency drop exceeds 0.5 Hz in the three operating modes of 2025 (high winter, low summer, and low flood season), indicating poor frequency characteristics and failure to meet the frequency stability margin requirements. Therefore, the primary goal of power transmission curve optimization is to improve frequency stability to mitigate the frequency drop under DC blocking conditions. Secondly, in the 2025 low summer and low flood season operating modes, voltage recovery exceeds 800ms, indicating poor voltage recovery characteristics. Power transmission curve optimization must also consider improving the system voltage recovery characteristics.

[0206] Under both the 2025 summer high-voltage and 2027 operating modes, the DC voltage recovery time under fault conditions exceeded 800ms, indicating poor voltage recovery characteristics. However, the system's frequency stability characteristics were good, with frequency drops within 0.5Hz under various fault conditions. Therefore, the optimization of the power transmission curve mainly considers improving the system's voltage recovery characteristics.

[0207] The data comparison above revealed that the voltage stability of the 2025 summer low-voltage and 2025 flood low-voltage operating modes was worse than the other five operating modes. Therefore, curve optimization was focused on these two operating modes to improve the system's safety and stability. Table 4 shows the power output of each DC circuit before and after power delivery curve optimization.

[0208] Table 4 DC power before and after power delivery curve optimization

[0209]

[0210] After optimizing the power transmission curve, the fault characteristics of the regional power grid system under these two operating modes are summarized in Table 5:

[0211] Table 5 Summary of Fault Characteristics After Power Supply Curve Optimization

[0212]

[0213] Similarly, under the above two operating modes, the maximum power angle difference of the system units does not exceed 180° and gradually decreases over time, so the power angle stability of the system meets the requirements.

[0214] Comparing the fault characteristics summaries in Tables 3 and 5 under the same operating mode, the following conclusions are drawn:

[0215] Under the low-temperature operation mode in summer 2025, the voltage level and DC power recovery speed at the moment of short-circuit fault clearance of Binjin DC and Baizhe DC were improved, and the effect of power transmission curve optimization was very obvious, while the recovery speed of Lingshao DC remained basically unchanged. The system frequency response characteristics improved when Binjin DC was single-pole blocked, slightly decreased when Lingshao DC was single-pole blocked, and significantly improved when Baizhe DC was single-pole blocked. After power transmission curve optimization, the overall system frequency response characteristics improved. The frequency drop of Binjin DC and Baizhe DC single-pole blocking was reduced after curve optimization, further improving the system frequency characteristics, and the overall system frequency characteristics were good.

[0216] Under the 2025 flood season low-voltage operation mode, the voltage levels at the moment of short-circuit fault clearance on all three DC lines improved, the power recovery speed of Binjin DC and Baizhe DC improved, while the recovery speed of Lingshao DC remained basically unchanged. The frequency drop of the single-pole blocking on Baizhe DC decreased, but compared with other operation modes, the overall system frequency characteristics were still relatively poor.

[0217] Simulation analysis shows that the fault characteristics of the system under various operating modes have been improved to a certain extent after the power supply curve optimization. After the power supply curve optimization, the recovery time under AC N-1 and N-2 faults under various operating modes has reached within 700ms. Except for the 2025 flood low mode, the frequency drop of the system under DC bipolar blocking is within 0.5Hz.

[0218] It is worth noting that optimizing the power transmission curve can only improve the system's fault characteristics to a certain extent. The overall system response trend remains consistent with that before the fault, which is determined by the structure of the receiving-end power grid and the start-up mode of the generating units. When the start-up mode of the receiving-end power grid is not reasonable enough, simply optimizing the DC power transmission curve cannot fully improve system stability (such as the low-flood-season operation mode in 2025). In this case, it is necessary to consider optimizing the start-up mode of the receiving-end power grid and ensuring that the system has sufficient reserves.

[0219] In summary, the UHVDC receiving-end power grid safety and stability margin analysis method proposed in this application generates a first evaluation index for evaluating the voltage stability margin of the UHVDC receiving-end power grid based on the correlation between voltage stiffness and dynamic reactive power compensation devices, including voltage stiffness; a second evaluation index for evaluating the power angle stability margin of the UHVDC receiving-end power grid based on the correlation between channel strength and asynchronous power supply, including damping ratio and generator maximum power angle difference; and a third evaluation index for evaluating the frequency stability margin of the UHVDC receiving-end power grid based on the correlation between frequency support strength and asynchronous power supply, including DC feed-in ratio, maximum single DC feed-in ratio, and critical single DC feed-in ratio. Based on the first, second, and third evaluation indices, the power grid safety and stability margin is evaluated, and corresponding improvement measures are generated to enhance the power grid safety and stability margin. This method solves the technical problems of evaluating the safety and stability margin of the UHVDC receiving-end power grid and improving the safety and stability margin based on the evaluation. It can provide a reference for the planning and operation of AC / DC hybrid power grids, ensuring the safe and stable operation of the system.

[0220] Please refer to Figure 13This application provides a system for evaluating the safety and stability margin of an ultra-high voltage direct current (UHVDC) receiving-end power grid. The system includes: a voltage stability margin evaluation module, used to generate a first evaluation index for evaluating the voltage stability margin of the UHVDC receiving-end power grid based on the correlation between voltage stiffness and dynamic reactive power compensation devices, including voltage stiffness; a power angle stability margin evaluation module, used to generate a second evaluation index for evaluating the power angle stability margin of the UHVDC receiving-end power grid based on the correlation between channel strength and asynchronous power supply, including damping ratio and maximum generator power angle difference; a frequency stability margin evaluation module, used to generate a third evaluation index for evaluating the frequency stability margin of the UHVDC receiving-end power grid based on the correlation between frequency support strength and asynchronous power supply, including DC feed-in ratio, maximum single DC feed-in ratio, and critical single DC feed-in ratio; and a comprehensive evaluation module, used to evaluate the safety and stability margin of the power grid based on the first, second, and third evaluation indices, and generate corresponding improvement measures to enhance the safety and stability margin of the power grid.

[0221] In this embodiment, the UHVDC receiving-end power grid safety and stability margin evaluation system is presented in the form of functional units. Here, a unit refers to an ASIC (Application Specific Integrated Circuit) circuit, a processor and memory that execute one or more software or fixed programs, and / or other devices that can provide the above functions.

[0222] Please see Figure 14 , Figure 14 This is a schematic diagram of the structure of a computer device provided in an embodiment of this application, such as... Figure 14 As shown, the computer device includes one or more processors 10, memory 20, and interfaces for connecting the components, including high-speed interfaces and low-speed interfaces. The components communicate with each other via different buses and can be mounted on a common motherboard or otherwise installed as needed. The processors can process instructions executed within the computer device, including instructions stored in or on memory to display graphical information of a GUI on external input / output devices (such as display devices coupled to the interfaces). In some alternative implementations, multiple processors and / or multiple buses can be used with multiple memories and multiple memory modules, if desired. Similarly, multiple computer devices can be connected, each providing some of the necessary operations (e.g., as a server array, a group of blade servers, or a multiprocessor system). Figure 14 Take a processor 10 as an example.

[0223] Processor 10 may be a central processing unit, a network processor, or a combination thereof. Processor 10 may further include a hardware chip. The hardware chip may be an application-specific integrated circuit (ASIC), a programmable logic device (PLD), or a combination thereof. The programmable logic device may be a complex programmable logic device (CAMP), a field-programmable gate array (FPGA), a general-purpose array logic (GDA), or any combination thereof.

[0224] The memory 20 stores instructions executable by at least one processor 10 to cause the at least one processor 10 to perform the method shown in the above embodiments.

[0225] The memory 20 may include a program storage area and a data storage area. The program storage area may store the operating system and applications required for at least one function; the data storage area may store data created based on the use of the computer device. Furthermore, the memory 20 may include high-speed random access memory and may also include non-transitory memory, such as at least one disk storage device, flash memory device, or other non-transitory solid-state storage device. In some alternative embodiments, the memory 20 may optionally include memory remotely located relative to the processor 10, and these remote memories may be connected to the computer device via a network. Examples of such networks include, but are not limited to, the Internet, intranets, local area networks, mobile communication networks, and combinations thereof.

[0226] The memory 20 may include volatile memory, such as random access memory; the memory may also include non-volatile memory, such as flash memory, hard disk or solid-state drive; the memory 20 may also include a combination of the above types of memory.

[0227] The computer device also includes a communication interface 30 for communicating with other devices or communication networks.

[0228] This application also provides a computer-readable storage medium. The methods described in this application can be implemented in hardware or firmware, or implemented as recordable on a storage medium, or implemented as computer code downloaded over a network and originally stored on a remote storage medium or a non-transitory machine-readable storage medium and subsequently stored on a local storage medium. Thus, the methods described herein can be processed by software stored on a storage medium using a general-purpose computer, a dedicated processor, or programmable or dedicated hardware. The storage medium can be a magnetic disk, optical disk, read-only memory, random access memory, flash memory, hard disk, or solid-state drive, etc.; further, the storage medium can also include combinations of the above types of memory. It is understood that computers, processors, microprocessor controllers, or programmable hardware include storage components capable of storing or receiving software or computer code. When the software or computer code is accessed and executed by the computer, processor, or hardware, the methods shown in the above embodiments are implemented.

[0229] This application provides a computer program product including computer instructions stored in a computer-readable storage medium. A processor of a computer device reads the computer instructions from the computer-readable storage medium and executes the computer instructions, causing the computer device to perform the method of any embodiment of this application.

[0230] Although embodiments of this application have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of this application, and all such modifications and variations fall within the scope defined by the appended claims.

[0231] The systems or modules described in the above embodiments can be implemented by computer chips or entities, or by products with certain functions. A typical implementation device is a computer. Specifically, a computer can be, for example, a personal computer, laptop computer, cellular phone, camera phone, smartphone, personal digital assistant, media player, navigation device, email device, game console, tablet computer, wearable device, or any combination of these devices.

[0232] For ease of description, the above devices are described separately by function as various units. Of course, in implementing this application, the functions of each unit can be implemented in one or more software and / or hardware.

[0233] Those skilled in the art will understand that embodiments of this application can be provided as methods, 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 embodied 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.

[0234] This application is described with reference to flowchart illustrations and / or block diagrams of methods, 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 a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions 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.

[0235] 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 1 The function specified in one or more boxes.

[0236] 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.

[0237] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0238] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments.

[0239] The above description is merely an embodiment of this application and is not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

[0240] Although embodiments of this application have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of this application, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A method for analyzing safety and stability margin of an extra-high voltage direct current receiving end power grid, characterized in that, The method comprises: According to the correlation law of voltage stiffness and dynamic reactive power compensation device, a first evaluation index for evaluating the voltage stability margin of the UHV DC receiving end power grid is generated, including voltage stiffness; According to the correlation law of channel strength and non-synchronous power supply, a second evaluation index for evaluating the power angle stability margin of the UHV DC receiving end power grid is generated, including damping ratio and maximum generator power angle difference; According to the correlation law of frequency support strength and non-synchronous power supply, a third evaluation index for evaluating the frequency stability margin of the UHV DC receiving end power grid is generated, including DC feed-in ratio, maximum single DC feed-in ratio and critical single DC feed-in ratio; According to the first evaluation index, the second evaluation index and the third evaluation index, the safety and stability margin of the power grid is evaluated, and corresponding improvement measures are generated to improve the safety and stability margin of the power grid, wherein evaluating the safety and stability margin of the power grid comprises: judging whether the second evaluation index reaches a second set threshold, if not, optimizing the power transmission curve to improve the power angle stability margin of the power grid; if yes, judging whether the third evaluation index reaches a third set threshold, if not, optimizing the power transmission curve to improve the frequency stability margin of the power grid; if yes, judging whether the first evaluation index reaches a first set threshold, if not, optimizing the power transmission curve to improve the voltage stability margin of the power grid; Evaluating the safety and stability margin of the power grid specifically comprises: first, ensuring the power angle stability of the receiving end power grid, requiring that the maximum power angle difference of the system unit does not exceed 180°; on this basis, analyzing the voltage stability and frequency stability of the system, expecting that the frequency response drop of the system after a fault does not exceed 0.5Hz; when the second evaluation index and the third evaluation index do not meet the conditions, the primary goal of power transmission curve optimization is to improve the frequency stability, so that the frequency drop degree is alleviated in the case of DC block, and then the voltage recovery characteristics of the system are considered to improve, so that the voltage recovery time does not exceed 700ms; The expression of the voltage stiffness is: In the above formula, is the absolute voltage stiffness, is the relative voltage stiffness, , , are the device impedance, the additional impedance, and the system impedance in the practical calculation equivalent circuit of the voltage stiffness, respectively. By increasing the capacity of the phase modifier and the STATCOM, or replacing the STATCOM with a phase modifier of the same capacity to improve the voltage stiffness of the DC system in fault state; In the case where the damping ratio and the maximum generator power angle difference do not reach the second set threshold, in the power-rich area, reduce the power transmitted by the new energy and the conventional unit through the same AC section to the outside, and increase the output of the new energy unit in the load center; According to the correlation law of channel strength and non-synchronous power supply, a second evaluation index for evaluating the power angle stability margin of the UHV DC receiving end power grid is generated, including damping ratio and maximum generator power angle difference, specifically comprising: A simple two-machine system model is established, which is powered by a LCC-HVDC system connected to the PCC bus; The system equation under normal operation is established, including the generator rotor motion equation and the active power transmission equation of the AC side network; The active power transmission equation after the loss of DC power and the power angle change equation of the generator after the DC block are established; The second-order differential equation of the generator power angle difference change is generated, and the damping ratio and the maximum generator power angle difference are calculated. According to the damping ratio and the maximum power angle difference of the generator, the power angle stability margin of the UHV DC receiving end power grid is evaluated; The third evaluation index for evaluating the frequency stability margin of the UHV DC receiving end power grid is generated according to the correlation law of the frequency support strength and the non-synchronous electromechanical power supply, and includes a DC feed-in ratio, a maximum single DC feed-in ratio and a critical single DC feed-in ratio, and specifically includes: A frequency stability model of the high-proportion new energy receiving end power grid is established; The wind farm generation penetration, the DC feed-in ratio and the maximum single DC feed-in ratio are calculated; According to the wind farm generation penetration and the DC feed-in ratio, the frequency deviation factor is calculated; According to the frequency deviation factor, the critical single DC feed-in ratio is calculated; According to the DC feed-in ratio, the maximum single DC feed-in ratio and the critical single DC feed-in ratio, the frequency stability margin of the UHV DC receiving end power grid is evaluated.

2. The method of claim 1, wherein, The expression of the damping ratio is: In the above formulae, is the damping ratio, is the first-order term coefficient, , are the moments of inertia of the first generator and the second generator in the simple two-machine system, respectively, , are the impedances of the first transformer and the second transformer, respectively; is the impedance on the tie line; The expression of the maximum power angle difference of the first generator and the second generator is: Wherein, In the above formula, is the maximum power angle difference of the first and second generators; is the power angle difference of the first and second generators when the system reaches the peak of the head swing; is the power angle difference of the first and second generators when the system is in steady state; , are the initial power angles of the first and second generators, respectively; is the overshoot of the step response of the system, is the electrical distance between the first and second generators; A1 and A2 are coefficients.

3. The method of claim 1, wherein, The expression of the wind farm generation penetration is: In the above formula, is the wind turbine output; is the total system generator output; K PCON is the power factor ratio; is the equivalent conventional generator capacity; The expression of the DC feed-in ratio is: In the above formulae, is the direct current feed-in ratio; is the i-th direct current feed-in power; The expression of the maximum single DC feed-in ratio is: The expression of the frequency deviation factor is: In the above formula, is the frequency deviation factor; R eq is the equivalent droop coefficient of all conventional generators; D L is the active power frequency regulation coefficient of the system load; The expression of the critical single DC feed-in ratio is: In the above formula, is the critical single DC feed-in ratio; is the frequency deviation factor; Af max is the frequency deviation limit value under normal operating conditions of the power system, is the rated frequency of the power system.

4. A system for evaluating the safety and stability margin of an extra-high voltage direct current receiving end power grid, characterized in that, The system capable of realizing the UHV DC receiving end power grid safety and stability margin analysis method of any one of claims 1-3 comprises: A voltage stability margin evaluation module for generating a first evaluation index for evaluating the voltage stability margin of the UHV DC receiving end power grid according to the correlation law of the voltage stiffness and the dynamic reactive power compensation device, including voltage stiffness; A power angle stability margin evaluation module for generating a second evaluation index for evaluating the power angle stability margin of the UHV DC receiving end power grid according to the correlation law of the channel strength and the non-synchronous electromechanical power supply, including damping ratio and generator maximum power angle difference; A frequency stability margin evaluation module for generating a third evaluation index for evaluating the frequency stability margin of the UHV DC receiving end power grid according to the correlation law of the frequency support strength and the non-synchronous electromechanical power supply, including DC feed-in ratio, maximum single DC feed-in ratio and critical single DC feed-in ratio; A comprehensive evaluation module for evaluating the safety and stability margin of the power grid according to the first evaluation index, the second evaluation index and the third evaluation index, and generating corresponding improvement measures to improve the safety and stability margin of the power grid.

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